Tectonically Active Landscapes E-Book

William B. Bull

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 735

Veröffentlichungsjahr: 2011

Beschreibung

This book explores how mountainous landscapes respond to tectonic deformation. It integrates previously unpublished concepts and ideas with recent articles about hills and streams. Readers will learn which landforms change quickly in response to uplift, which parts of the landscape are slowest to adjust to tectonic perturbations, and which landform characteristics are most useful for describing tectonically active and inactive terrains. Study areas include diverse landscapes and tectonic settings: seacoasts, soil-mantled hills, and lofty mountains. The humid Southern Alps of New Zealand change quickly because of rapid uplift and erosion. The semiarid Panamint Range of southeastern California has such miniscule annual stream power that tectonic landforms persist for millions of years. Tectonically Active Landscapes addresses diverse key topics about tectonics and topography. It is essential reading for research geologists and advance-level undergraduate and graduate students in the earth sciences.

Leseprobe

Contents

Preface

Chapter 1 Tectonic Settings f the Study Regions

1.1 Introduction

1.2 North America–Pacific Plate Boundary

1.3 Australia–Pacific Plate Boundary

1.4 India–Asia Plate Collision

1.5 Aegean Transtension

1.6 Summary

Chapter 2 Drainage Basins

2.1 Hydraulic Coordinates

2.2 Basin Shapes

2.3 Divide Migration and Stream Capture

2.4 Tectonically Translocated Watersheds

2.5 Summary

Chapter 3 Hillslopes

3.1 Hillslope Model Boundaries

3.2 Late Quaternary Tectonic Deformation of the Diablo Range

3.3 Sediment Flux and Denudation Rates

3.4 Ridgecrests

3.5 Canyonlands

3.6 Cross-Valley Shapes

3.7 Tectonic Signatures in Hillslopes

3.8 Summary

Chapter 4 Sediment Yield and Landslides

4.1 Sediment Yield

4.2 Mass Movements

4.3 Summary

Chapter 5 A Debate About Steady State

5.1 A Century of Conceptual Models

5.2 Hillslope Degradation

5.3 Erosion of Mountain Ranges

5.4 NonSteady-State Erosion of Fluvial Systems

Summary

Chapter 6 Erosion and Tectonics

6.1 Exfoliation Joints

6.2 Ridgecrest Spreading

6.3 Erosional Controls of Fault Zone Partitioning

6.4 Consequences of Erosion Induced by Long-Term Plate Collision

6.5 Summary

Chapter 7 Fault-Propagation Landscapes

7.1 Normal Faulting

7.2 Thrust Faulting

7.3 Transtensional Faulting

7.4 Summary

Chapter 8 Tectonic Geomorphology of a Plate Boundary

8.1 Walker Lane–Eastern California Shear Zone

8.2 Sierra Nevada Microplate

8.3 Mendocino Triple Junction

8.4 Summary

Glossary

Appendix A

References Cited

Index

This edition first published 2009, © 2009 by William B. Bull

Blackwell Publishing was acquired by John Wiley & Sons in February 2007. Blackwell’s publishing program has been merged with Wiley’s global Scientific, Technical and Medical business to form Wiley-Blackwell.

Registered office: John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK

Editorial offices: 9600 Garsington Road, Oxford, OX4 2DQ, UK

The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK

111 River Street, Hoboken, NJ 07030-5774, USA

For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com/wiley-blackwell

The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.

Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought.

Library of Congress Cataloguing-in-Publication Data

Bull, William B., 1930-

Tectonically active landscapes / William B. Bull.

p. cm.

Includes bibliographical references and index.

ISBN 978-1-4051-9012-1 (hardcover: alk. paper)

1. Morphotectonics. 2. Landscape changes. I. Title.

QE511.44.B853 2009

551.41--dc22

2008052159

ISBN: 978-1-4051-9012-1

A catalogue record for this book is available from the British Library.

Preface

This book is about tectonic controls on landscape shapes and processes, with streams as the essential connecting link between upstream and downstream reaches of fluvial systems. Uplift on a range-bounding fault increases relief, changes rates of geomorphic processes, and modifies the shapes of hills and streams. Streams respond to rock uplift by deepening their valley floors, thus changing the base levels for adjacent hillslopes. We seek to learn more about landscape responses to rock uplift as a function of:

1) Rate, style, location of tectonic deformation,

2) Rock mass strength,

3) Climatic controls on erosion and deposition.

Intriguing diverse challenges include:

1) To determine of how quickly nearby and distant parts of hilly landscapes change in consecutive reaches upstream from an active structure.

2) Recognition of how cumulative rock uplift over Late Quaternary time spans creates landscape assemblages with distinctive topographic signatures. Tectonically active and inactive landscapes are very different.

3) Assessment of whether or not a landform, or landscape, achieves a time-independent configuration after tectonic and climatic perturbations change geomorphic processes on hills and along streams.

4) How does the arrival of a propagating tip of a normal, thrust, or strike-slip fault change geomorphic processes?

5) How do fluvial systems of mountain ranges respond to regional extension?

6) How quickly do streams of small watersheds react to a pulse of uplift created by rising hot asthenosphere associated with a migrating triple junction?

7) What have been the geomorphic consequences of 30 million years of uplift and intrusion of India into the Tibetan Plateau on the rivers of southern Asia? Is areal variation in summer monsoon rainfall important?

8) How are hills and streams affected by coastal erosion during times of sea-level highstands?

9) How do rising ground-water tables cause landslides?

10) Do fluvial systems have sufficient continuity to fully adjust to rock uplift?

These topics are indicative of the diversity of subject matter in “Tectonically Active Landscapes”. Geomorphic processes are discussed for time spans ranging from 1,000 to 80,000,000 years. The sizes of landforms studied here ranges from a tectonic microplate to a hillside gully.

My intent is to develop basic principles of landscape evolution by examining hills, streams, and piedmonts of selected mountain ranges. These were chosen to include arid to extremely humid climates, inactive to highly active tectonic settings, and fractured, soft to massive, tough rock types. Structural styles include normal, thrust, strike-slip, and listric faults. I explore and test concepts so you can better understand the landscape evolution of your favorite mountains.

Tectonic settings of the specific study areas are summarized in Chapter 1. Most are in the broad San Andreas transform boundary between the North America and Pacific Plates, so we return to this tectonic setting in every subsequent chapter. This large, diverse region is arid to humid, includes the normal and strikeslip faults of the Basin and Range Province and the thrust faults of the Coast Ranges. It includes the landscapes of the Sierra Nevada microplate and the Mendocino triple junction. The Southern Alps of New Zealand are an essential study area because of their rapid uplift, high erosion rates, and extremely humid climate. The Himalayas provide a setting of maximum relief and a very long time span of tectonic deformation and landscape changes.

Chapters 2–4 about how hillslopes adjust to the static or changing valley floors to which they are graded. Small narrow drainage basins that characterize youthful scarps become larger and more circular over the long time spans needed to create a mountain range (Chapter 2). Stream capture events are important. Chapter 3 defines topographic signatures in hillslopes from the standpoint of rates of base-level change. Chapter 4 examines mass movements in tectonically active regions in the context of how rain changes ground-water levels, seepage forces, and hillslope stability.

Important issues in tectonics and topography are the focus of the concluding four chapters. The common presumption of the past 50 years that entire landscapes achieve unchanging (steady state) configurations is tested in Chapter 5. Chapter 6 examines a much newer subject. How does erosion change crustal loading and influence styles and rates of fracturing, faulting, and exhumation of deep crustal materials? How streams respond to propagation of the tips of active faults is explored in Chapter 7. Concluding Chapter 8 has a plate-tectonics emphasis. It discusses how hills and streams of a granitic microplate and a migrating triple junction respond to temporary uplift caused by upwelling of hot asthenosphere.

These essays are written for scientists and students with an interest in geomorphic processes, landscape evolution, and tectonics of plate boundaries. This book is appropriate for upper division and graduate-level courses in active tectonics, tectonic geomorphology, physical geography and geomorphology, plate-tectonics geology, engineering geology, and environmental studies.

This project began in 1975 when Luna Leopold encouraged me to write books as a career development tool. Fieldwork was done in many countries. “Geomorphic Responses to Climatic Change” (Bull, 1991) reveals pervasive Late Quaternary impacts on geomorphic processes of arid and humid regions. Tectonic Geomorphology of Mountains – a New Approach to Paleoseismology (Bull, 2007) has an applied theme. Tectonically Active Landscapes could just as well be entitled Landscape Evolution.

I appreciate the formal reviews of the entire book manuscript by Ed Keller and an anonymous reviewer. Their suggestions improved clarity of writing and completeness of thought. I received valuable suggestions about hydrogeologic discussions in Chapter 4 from Leo Leonhart of Hargis & Associates, Gray Wilson, and Erik Nelson of Engineering Analytics. Lewis Owen reviewed an earlier version of Chapter 8.

Correspondence with colleagues provided essential insights:

Bodo Bookhagen, Himalayan monsoons,

John Bradshaw, New Zealand greywackes,

Bill Dickinson, volcanism, triple junctions,

Kurt Frankel, Fish Lake fault zone,

Roger Hooke, Panamint Range tilt,

Oliver Korup, Southern Alps landslides,

Kurt Frankel, Fish Lake fault zone,

Peter Koons, Southern Alps tectonics,

Les McFadden, soils and landscapes,

Jim McKean, hillslope processes,

Dorothy Merritts, triple-junction landscapes,

Jarg Pettinga, Southern Alps tectonics,

Fred Phillips, Owens Valley–Sierra faulting,

Tom Sawyer, Sierra, Diablo Range tectonics,

Greg Stock, Sierra Nevada rivers incisement,

Kirk Vincent, triple-junction landscapes,

John Wakabayashi, Sierra Nevada tectonics.

Generous Earth scientists provided essential technical illustrations:

Bodo Bookhagen: Figures 6.20–6.25,

Malcolm Clark: Figures 8.3, 8.6,

Dykstra Eusden: Figure 4.1,

Ryan Elley: Figure 2.18,

Tom Farr: Figure 8.4,

Kurt Frankel: Figures 7.21–7.23,

Bernard Hallet: Figures 6.16–6.18,

Oliver Korup: Figures 5.9–5.11,

Scott Miller: Figures 7.15A, 7.16,

Fred Phillips: Figure 8.17,

Gerald Roberts: Figures 7.1, 7.3–7.5,

Tom Sawyer, Jeff Unruh: Figures 2.13–2.15,

Jamie Schulmeister: Figure 4.3,

Greg Stock: Figure 8.21.

Photographs illustrate many landscape complexities allowing readers to make their own interpretations.

Tim Davies: Figure 6.9,

Peter Haeussler: Figure 6.2,

Ed Keller: Chapter 7 banner photo,

Peter Kresan: Figures 2.3, 3.15, 3.20,

Jim McCalpin: Figure 6.3,

Karl Mueller: Figure 7.20,

Bill Tucker: Figures 3.13A–D, 3.14,

Alistair Wright: Chapter 2 banner photo, Figure 4.19A.

Images are important for landscape analysis and portrayal. Tom Farr helped locate NASA-Jet Propulsion Laboratory images used here; Figures 1.8, 1.9, 1.13, 3.20, 4.14, 6.15, 8.4. Environment Canterbury, New Zealand made the map of alluvial-fan flooding used as Figure 2.18. Malcolm Clark, U. S. Geological Survey supplied the high altitude photos in Figures 8.3 and 8.6.

I thank you all for adding depth to discussions and for illustrations that added much diversity to this book.

It was a pleasure to work with the production staff at Blackwell Publishing including Ian Francis, Delia Sandford, and especially Rosie Hayden.

Essential financial and logistical support for this work was supplied by the U.S. National Science Foundation, National Earthquake Hazards Reduction Program of the U.S. Geological Survey, National Geographic Society, University of Canterbury in New Zealand, Hebrew University of Jerusalem, Royal Swedish Academy of Sciences, and Cambridge University in the United Kingdom.

Chapter 1

Tectonic Settings f the Study Regions

This tectonically active landscape is rising out of the sea on the transpressional boundary between the Pacific and Australian plates. The landforms reflect three basic controls on landscape evolution – lithology, climate, and rock uplift.

The long pair of parallel ridges of the Eocene Amuri Formation is an erosionally truncated syncline in the Puhi Puhi valley of New Zealand. The limestone rock mass strength is higher than the fractured sandstone in the middleground, so differential erosion has made the syncline higher in the landscape.

Folding and thrust faulting here have resulted in ~10 km of local crustal shortening (Crampton et al., 2003). Uplift rates increase abruptly from 1 to 6 m/ky across the Jordan thrust fault in the foreground. They also appear to increase towards the left side of the view; note the parallism of the limestone ridges and the skylined ridgecrest underlain by fractured sandstone.

Steep braided rivers at the lower-right edge of this view reflect rapid erosion of the fractured greywacke sandstone of the Torlesse Formation underlying the 2,500 m high Seaward Kaikoura Range. Flat grassy ridgecrests in the broad valley are stream terraces that record tectonic and climatic controls on this rapidly changing landscape. Late Quaternary climatic changes caused pulses of aggradation that briefly raised valley floors. Long-term tectonically induced downcutting has preserved previous valley floors as the steps and risers in flights of stream terraces.

The format of this book is to explore wide-ranging, important, topics in tectonic geomorphology. I mainly use the exceptionally diverse landscapes and tectonic settings of southwestern North America. Lengthy discussions presented in Chapters 2, 6, and 7 use sites in tectonically young New Zealand and Greece, and in the Himalayas where time spans of tectonic deformation are much longer. United States mountainous landscapes investigated in Chapters 2 through 8 are influenced by regional tectonic shifts of the adjacent Pacific and North American plates. So, this diverse tectonic setting is summarized first.

Chapter 1 summarizes regional tectonic settings. This format underscores the interlinked nature of recent tectonic activity of southwestern North America in one place. Tectonic settings of this region include the Basin and Range Province, Walker Lane–Eastern California shear zone, Sierra Nevada microplate, Diablo Range adjacent to the San Andreas fault, and the migrating Mendocino triple junction. Chapters 2–4 study hillslope processes and responses to tectonic base-level fall of adjacent streams. Chapters 5–8 are a series of essays that explore steady-state premises, consider how erosion influences fracturing and faulting, examines drainage-net responses to propagation of fault tips, and discuss how upwelling asthenosphere affects geomorphic processes.

1.1 Introduction

Rising mountains are different than tectonically inactive landscapes. Just being high and lofty may not tell us if mountain-building forces are still active. Instead, we should scrutinize landforms and the geomorphic processes that create them. Streams deepen their valleys in response to increases of watershed relief, and this changes the length, slope, and curvature of the adjacent hills.

Uplift may be regional and isostatic, but often is concentrated on range-bounding faults and folds. Such local tectonic deformation may be regarded as a perturbation (change in a variable of fluvial systems) that steepens stream gradients in the mountain-front reach. The effects of such tectonic displacements emanate from the range-bounding fault. Long time spans may pass before the consequences of renewed uplift arrive at distant upstream reaches of the drainage basin. Streams are the essential connecting link between different sections of a drainage basin. Tectonically induced downcutting along a trunk stream channel steepens the footslopes of the adjacent hillsides and increases hillslope area too. Notable results include increases of stream power and hillslope sediment yield. Concurrent changes in climate also alter types and rates of geomorphic processes.

Changes in climate and rock uplift operate separately – they are independent variables – to alter geomorphic processes and landscape characteristics. Both variables originate outside of the drainage basins that comprise mountainous landscapes.

Response times for Late Quaternary climate-change perturbations are much shorter than those for mountain-front uplift. Climate change impacts all of a watershed with minimal time lag. The resulting changes in the size and amount of stream-channel bedload influence rates of tectonically induced downcutting. Climate-change induced impacts on geomorphic processes, such as changing valley-floor deposition to erosion, are pervasive. An aggradation event temporarily overwhelms the influence of concurrent tectonic displacements for a reach of a stream tending to maintain steady state valley-floor erosion (unchanging longitudinal profile underlain by a strath).

A good example is the Charwell River basin in the South Island of New Zealand. Repeated surface ruptures on a range-bounding fault created a 40 m high sub-alluvial fault scarp during a climatechange-induced aggradation event. Aggradation disrupted the continuity of an erosional fluvial system and prevented the tectonic perturbation from migrating upstream. The mountain range was raised 42 m between 26 and 9 ka1, but climatic controls delayed the upstream transmission of the tectonic perturbation for 17 ky. Sediment flux modeling (Gasparini et al., 2007) illustrates the strength of climatic controls on bedrock incision rates. “Geomorphic Responses to Climatic Change” (Bull, 1991) elucidates the effects of Late Quaternary climatic and tectonic perturbations in mountain ranges whose present climates range from extremely arid to extremely humid.

We have much to learn about the possible impacts associated with the present acceleration of human-induced global climate changes. Geomorphic impacts may resemble the consequences of the Late Quaternary climate changes, but the types, rates, and magnitudes of change in geomorphic processes could be quite different.

Both the Pleistocene and Holocene styles of climate change are now history, having been replaced by the Anthropocene (Crutzen and Stoermer, 2000). Intensification of human activities with the onset of the industrial revolution now influences the climate of planet Earth. Amounts of temperature change seem modest, but rates of change are more than an order of magnitude faster than during the Quaternary. Models based on slowly changing Quaternary climates may not be appropriate now.

We could be entering significantly new territory with local demise of permafrost (Cheng and Wu, 2007; Gruber and Haeberli, 2007), and eradication of so much tropical rain forest (Aitken and Leigh, 1992). But human impacts on forests are not a recent development (Bjorse and Bradshaw, 2000; Ruddiman, 2003).

Clearing of forests for agriculture began at 8 ka causing a modest increase in atmospheric carbon dioxide, CO2 (Ruddiman, 2003 and the polar ice cores (Ferretti et al., 2005) record the increase of methane, CH4, at the time of major increase of Asian rice paddies at 5 ka. Such human impacts in the early Anthropocene, with major acceleration since the onset of the industrial revolution, have more than offset the Holocene decrease of solar radiation at 65° N that is a function of Milankovitch orbital parameters (Ruddiman, 2003).

A key aspect will be crossings of the geomorphic threshold separating erosional and depositional modes of stream processes. Local uplift that changes slope of stream channels may put a given geomorphic process closer to, or further from, a threshold separating contrasting styles of landscape behavior.

This book emphasizes hills and their relation to streams. Streams catch our attention with visually impressive flows of water and debris, the power of boulder-transport processes, and as being sources of water, electric power, and fertile land. Tectonically induced changes in the altitude of a reach of a stream affect the base level to which the adjacent hillslopes are graded. Hills yield water and sediment to valley floors. Mountain ranges are nearly all hills, but their shape is dependent on the behavior of streams.

The landforms of tectonically active landscapes have definitive characteristics that reflect local rates of rock uplift, climate, and rock mass strength. This book is about the nature of these signatures and how the hills and streams of fluvial systems change with the passage of time. The three blocks of Figure 1.1 diagram the flow of topics presented in the three parts of the book. A discourse that focuses mainly on streams is worthy of a separate book, perhaps “Fluvial Tectonic Geomorphology”.

We use tectonic signatures in landforms to explore several topics in tectonics and topography. I present data and analyses from study areas in quite different tectonic, climatic, and lithologic settings. Insight gained from study sites in the southwestern United States, Greece, New Zealand, and the Himalayas should aid you in resolving similar problems in your favorite geographical settings. Study site locations of this book are shown in Figures 1.2, 1.9, and 1.13. The captions for these three figures contain links to chapter section numbers. A fluvial emphasis here excludes permafrost and glaciers, sand seas, lacustrine and tidal settings, and active volcanoes – but includes marine terraces.

I use a variety of geomorphic concepts, and presume that you know these key principles. A broad base of essential tools lets us explore diverse approaches in tectonic geomorphology. They include a sensitive erosional–depositional threshold, time lags of response to perturbations, type 1 and type 2 dynamic equilibrium, local and ultimate base levels, impediments to the continuity of fluvial systems, and the process of tectonically induced downcutting to the base level of erosion. See the Glossary for basic definitions. Chapter 2 in a companion book is devoted to defining and discussing these essential tenets; see “Tectonic Geomorphology of Mountains” (Bull, 2007). Chapter 1 of that book assesses the nuances of scrunch and stretch bedrock uplift.

Quaternary temporal terms (Table 1.1) have been assigned conventional ages. The 12-ka age assignment for the beginning of the Holocene is arbitrary and is preceded by the transition between full-glacial and interglacial climatic conditions. Unless specifically noted, radiocarbon ages are conventional (using the old 5,568 year half-life allows comparison with dates in the older literature) and have been corrected for isotope fractionation. The term “radiocarbon age” means that the correct 5,730 year half-life is used and that variations in atmospheric 14C have been accounted for, using the techniques of Stuiver et al. (1998). Calibration of radiocarbon ages (Bard et al., 1990) shows that the peak of full-glacial conditions may be 22 ka instead of the conventional radiocarbon age estimate of 18 ka. The 125- and 790-ka ages are radiometric and paleomagnetic ages that have been fine-tuned using the astronomical clock (Johnson, 1982; Edwards et al., 1987a, b). The 1,650-ka age is near the top of the Olduvai reversed polarity event (Berggren et al., 1995).

Figure 1.1 Flow chart of major topics of this book by chapter.

1.2 North America–Pacific Plate Boundary

The San Andreas transform boundary between the North America and Pacific plates in the southwestern United States is a 200–800 km wide transition zone extending from Pacific Ocean coastal fault zones far into the Basin and Range Province. Two primary components control many secondary tectonic features of the transition zone. The following synopsis emphasizes coincidences of timing of important tectonic events in the boundary between the Pacific and North American plates. The San Andreas fault is the primary plate-boundary fault zone at the present time. This right-lateral continental transform fault slices through batholithic complexes to create the Peninsular and Transverse Ranges of southern California. It then continues through the central and northern Coast Ranges, and turns west at Cape Mendocino to join the Mendocino fracture zone (Fig. 1.3).

The Sierra Nevada microplate is an equally important component. This 650 km long mountain range was created by batholithic intrusions in the Mesozoic. The mountain range is long but the microplate is immense because it also includes the adjacent Central Valley of California (Fig. 1.3). This tectonic block has minimal internal deformation but the eastern side of the Sierra Nevada was raised abruptly at about 4 ka (Jones et al., 2004; Saleeby and Foster, 2004). An impressive escarpment now rises 1,000 m in the north and 2,000 m in the south (Fig. 1.4).

Table 1.1 Assigned ages of Quaternary temporal terms, in thousands of years before present (ka).

AgekaHolocene Late 0–4 Middle 4–8 Early 8–12 Pleistocene Latest 12–22 Late 12–125 Middle 125–790 Early 790–1,650

Figure 1.2 Locations and study sites in the western United States and their book section numbers [3.5]. BP, Baboquivari Peak [3.6.1]; CM, Cape Mendocino [1.2.4, 4.2.2, 8.3]; CP Colorado Plateau [3.5]; DG, Downpour Gulch [3.4.2]; DM, Diamond and Fort Sage Mountains [8.1]; DR, Diablo Range sites including the Ciervo Hills, Dormant Hollow, Kettleman Hills, Laguna Seca Hills, Panoche Hills, Tumey Hills, and Vigorous Vale [1.2.3, 2.3.2.2, 3.2, 3.2, 3.7, 5.1, 5.4]; FB, Fort Bragg [8.3]; GC, Grand Canyon [3.5]; GM, Gila Mountains [3.6]; KR, Kings River [8.2]; MDe, Mojave Desert [1.2.1, 8.1]; FR, Feather River and Mount Lassen and the southern end of the subduction related Cascade volcanoes [8.2.2]; L, The Loop abandoned meander [3.5.1]; MD, Mount Diablo [2.3.2.2]; OCR, Oregon Coast Ranges; [3.4, 3.7]; OV Owens Valley[1.2.1, 8.1, 8.2]; PR, Panamint Range and Death Valley, [1.2.1.1, 2.2.1, 8.1.1]; SC, Santa Catalina Mountains [2.3.1]; SF, San Francisco Bay region [2.3.2.2, 4.2.1.2.2]; SGM, San Gabriel Mountains [3.6.2]; SJ, San Juan River [3.5.1]; SJV San Joaquin Valley [1.2.3, 3.4.1, 7.5.2]; SN, Sierra Nevada microplate [1.2.2, 5.3.2, 8.2]; ST, Salton Trough [1.2.1]; TR, Tobin Range [7.1.1]; WM, White Mountains [1.2.1, 7.3]; WR, Wheeler Ridge [7.2.2]; YO, Yosemite National Park [6.1]. Digital topography courtesy of Richard J. Pike, US Geological Survey.

Figure 1.3 Tectonic setting of southwestern United States. Numbered lines show the northward migration of the south edge of the Gorda plate (from Atwater and Stock, 1998) to its present position at the Mendocino triple junction. Mendocino fracture zone extends northwest, and the Cascadia subduction zone north, from the triple junction. The Sierra Nevada microplate was wider at 5 Ma because it extended west almost to the San Andreas fault, and extended east into the Basin and Range Province. Area within the solid line outlines the area of accelerated extensional faulting since 5 Ma (Jones et al., 2004, figure 1). This area and the area inside the dashed line approximate the province of the Walker Lane–Eastern California shear zone, that has accommodated >10% of the plate-boundary dextral shear since 5 Ma. Areas of sinestral shear act as transrotational domains that facilitate tectonic linking within the plate boundary (Dickinson and Wernicke, 1997).

A major tectonic event – detachment of the changed important tectonic processes elsewhere in Sierra Nevada batholithic root – (called delamination) the San Andreas transform boundary. Crustal extenresulted in a pulse of uplift of the microplate, and sion accelerated into the eastern margin of the Sierra Nevada, thus making the Basin and Range Province ever broader. See Figure 1.3. The San Andreas fault dextral shear zone split to create the seemingly diffuse Walker Lane–Eastern California shear zone in eastern California and western Nevada. Parts of the central Coast Ranges were created since the delamination event. The following summaries are made in the context of how this batholithic-root detachment affected the rates and styles in each distinct tectonic province.

Geomorphic features of hills and streams in of each study area tell us more about present and past tectonic activity. The rates and styles of tectonic activity, briefly noted in this introductory chapter, will be deciphered and discussed further when the tectonic geomorphology is presented for each study area.

1.2.1 Walker Lane–Eastern California Shear Zone

About 10 to 20% of the dextral shear between the Pacific and North American plates split off from the San Andreas fault at 3 to 5 Ma. Unlike the San Andreas fault, movements along this much younger shear zone seem dispersed. Diffuse, and appearing spatially intermittent, this is called the “Eastern California shear zone” in the south and the “Walker Lane shear zone” in the north. Two prominent dextral active faults in the south are separated by 100 km – the 310 km long Death Valley–Fish Lake and the 110 km long Owens Valley fault zones (Reheis and Dixon, 1996). Two active dextral fault zones in the north are separated by 60 km – Honey Lake and Mohawk Valley (Wills and Borchardt, 1993). Field studies and seismic analyses reveal additional widespread dextral faulting.

The importance of the San Andreas as a plate boundary fault is obvious because of its 300 km of cumulative displacement since 30 Ma (Dickinson, 1996). Magnitude Mw >7 earthquakes occurred on this fault in A.D. 1812, 1838, 1857, and 1906.

In marked contrast the Walker Lane–Eastern California shear zone initially appeared so intermittent and diffuse that it has taken a century to recognize its importance. Nevertheless, historic earthquakes attest to its tectonic significance. It too has had four Mw >7 dextral earthquakes since A.D. 1800 including the magnitude Mw 7.6 Owens Valley earthquake of 1872, the Mw 7.2 Cedar Mountain event of 1932 in western Nevada, the Mw 7.3 Landers earthquake in 1992 and Mw 7.1 Hector Mine earthquake in 1999; both recent events were in the central Mojave Desert.

Geodetic measurements indicate right-lateral shear between the Pacific and North American plates of about 10 to 14 m/ky (Bennett et al., 1999; Dixon et al., 2000; Argus and Gordon, 2001). The Sierra Nevada microplate has been rotated counterclockwise in response to deformation of adjacent tectonic provinces.

Extension of the distinctive Basin and Range Province began at 35 Ma in the north and had propagated to the south by 20 Ma (Dilles and Gans, 1995). Such normal faulting migrated westward during the late Cenozoic by encroaching into the Sierra Nevada microplate. The microplate was much wider just 5 My ago (Fig 1.3).

Uplift and eastward tilting of the lofty White Mountains as a separate block (Guth, 1997) began at 12 Ma (Stockli et al., 2003), which is the same time as encroachment near Reno created the Carson Range (Henry and Perkins, 2001). This early episode of synchronous normal faulting occurred along nearly 300 km of the Sierra Nevada–Basin and Range boundary. The Panamint Range also may have been created at this time.

Range-bounding normal faulting after ~3.5 Ma created the eastern escarpment of the Sierra Nevada (Fig. 1.4) and increased relief of the White and Inyo Mountains on the other side of Owens Valley. Neither synchronous episode of tectonic rejuvenation was related to the continued migration of the Mendocino triple junction (Unruh, 1991).

This encroachment has reduced the width of the Sierra Nevada microplate. Detached batholithic rocks are now Basin and Range Province mountain ranges. Examples include the plutonic rocks of the Panamint and Argus Ranges in the south and the Diamond and Fort Sage Mountains in the north (Dilles and Gans, 1995; Wakabayashi and Sawyer, 2001; Surpless et al., 2002). Encroachment began before onset of the Walker Lane–Eastern California shear zone dextral shearing, and is continuing.

Encroachment of Walker Lane–Eastern California shear zone near Reno, Nevada was associated with pulses of volcanic activity at 12 and 3 Ma (Henry and Perkins, 2001). Putirka and Busby (2007) suggest that high K2O volcanism in the central Sierra Nevada was synchronous with the onset of Walker Lane–Eastern California shear zone transtensional tectonic deformation. In an interesting conclusion they state “We speculate that high-K2O lavas in the southern Sierra are similarly related to the onset of transtensional stresses, not delamination”.

Diverse recent studies provide a clearer tectonics picture for the Coast Ranges and the Sierra Nevada, but tectonic deformation of the western Basin and Range Province is not as well understood. Local changes from oblique-slip extension to nearly pure strike-slip faulting at Honey Lake and Owens Valley (Monastero et al., 2005) underscore changes in the importance of dextral faulting. Rates of strike-slip displacement on individual faults of 5 to 9 m/ky (Klinger, 1999) are not visually obvious if mountain fronts are not created.

Figure 1.4 Eastern escarpment of the Sierra Nevada microplate. Aerial view west across Owens Valley. Oblique right-lateral movement on the Owens Valley fault zone ruptured through the town of Lone Pine in the left foreground in 1872 (Mw 7.6). The eastern escarpment of the Sierra Nevada rises from the range-bounding fault zone to summit of Mount Whitney, MW (altitude 4,418 m). TC, Tuttle Creek; LP Lone Pine Creek; GC, George Creek; BC, Bairs Creek; SH Shepherd Creek; SY Symmes Creek. A pediment surface, P in the Alabama Hills, AH, extends under the adjacent piedmont alluvium. 1955 Photograph GS–OAI–5–13, courtesy of U. S. Geological Survey.

The change from regional extension to regional transtension east of the Sierra Nevada microplate suggests early stages of a new plate-boundary shear zone. This interesting possibility is based on seismic (Wallace, 1984), geologic (Wesnousky, 1986, Dokka and Travis, 1990 a; b; Putirka and Busby, 2007), and geodetic studies (Sauber et al., 1986, 1994; Savage et al., 1993). Wallace speculated that future earthquakes would fill gaps in historical seismicity. The subsequent north-trending dextral surface ruptures of the Landers earthquake of 1992 and Hector Mine earthquake of 1999 corroborate his 1984 prediction. Some workers include these surface-rupture events in a model of a new plate-boundary fault zone (Nur et al., 1993; Sauber et al., 1994; Du and Aydin, 1996).

This introduction reveals the subtle, but important, nature of this part of a changing plate boundary. The tectonics of the Walker Lane–Eastern California shear zone, as augmented by tectonic geomorphology input, is worthy of a more far-reaching discourse presented in Section 8.1.

1.2.1.1 Panamint Range

The raised, tilted Panamint Range block was chosen as a study area within the Walker Lane–Eastern California shear zone because of its diverse tectonic landforms. The Range rises 3,400 m above adjacent Death Valley whose low point has sunk to an altitude of – 89 m. Granitic and gneissic rocks, quartzite, dolomite, and argillite are common lithologies. Granitic rocks of the Panamint Range are part of the former Sierran batholithic arc; several plutons were emplaced at about 70 to 100 Ma (Hodges et al., 1990; Mahood et al., 1996). From west to east, present exposure of granitic rocks decreases from 99% in the Sierra Nevada, to >80% in the Argus Range, to <40% in the Panamint Range. This trend suggests an eastward increase in thickness of sedimentary rock cover. Since 5 Ma, Basin and Range Province extensional faulting has topographically separated the Panamint and Argus Ranges from the Sierra Nevada.

Large, exhumed fault surfaces in the Black Mountains flanking the east side of Death Valley are referred to as “turtlebacks”. Realizing that these antiformal-shaped fault planes are the result of late Cenozoic normal faulting led to further work (Wright et al., 1974) including articles about the magnitude, timing, and style of extension (Burchfiel and Davis, 1981; Stewart, 1983; Wernicke et al., 1988; and Burchfiel et al., 1995). Cichanski’s study (2000) shows that similar planar to curviplanar low-angle faults were a common and recent style of tectonic extension in many mountain ranges of the region. Accordant ridgecrests in a highly rilled bedrock surface that is dipping 15° to 35° characterize exposures of these faults, but locally these low-angle faults dip as much as 35° to 40°.

Changes in style of range-front faulting in Panamint Valley and nearby ranges (Loomis and Burbank, 1988) occurred during the Late Cenozoic. Changing plate-boundary tectonics includes initiation of more than 64 km of left-lateral displacement on the Garlock fault since about 10 Ma (Burbank and Whistler, 1987; Dawson et al., 2000, 2003). It marks the southwestern extent of the Basin and Range Province (Davis and Burchfiel, 1973). Panamint and Saline Valleys appear to be genetically linked rhombochasms that began to open at the same time (Zellmer, 1980; Burchfiel et al., 1987). Such wrench-fault tectonics has been important (Densmore and Anderson, 1997) and may have set the stage for low-angle range-front faulting.

Cichanski describes a recent change from low-angle to vertical range-bounding faulting on the west side of the Panamint Range. Zhang et al. (1990) suggest that such faulting began in the Holocene, but soil profiles on the older faulted fan remnants indicate a Late Pleistocene age to me. A key conclusion by Cichanski is that prolonged low-angle normal faulting, not the presently active oblique-dextral Panamint Valley fault zone, is responsible for the present range-front topography on the west side of the Panamint Range. Dextral faulting at ~0.5 m/ky is occurring on the Panamint Valley fault, which is on the Argus Range side of Panamint Valley (Densmore and Anderson, 1997).

Varying rates of range-front uplift have resulted in tectonic blocks being raised and tilted. The Panamint Range block is being tilted eastward (Maxson, 1950; Green, in Hunt and Mabey, 1966; Hooke, 1972) to help create the topographic depression of Death Valley, which was formed by a complex mix of E–W crustal extension and N–S transtension. Pleistocene tilting about a N–S axis west of the valley trough has resulted in a 20 to 30 m downward displacement of ~120 ka to ~180 ka lacustrine shorelines of Late Quaternary age at the toe of the Panamint Range piedmont relative to patches of shoreline tufas clinging to the escarpment of the Black Mountains on the east side of Death Valley (Hooke, 1972).

1.2.2 Sierra Nevada

The Sierra Nevada microplate is a major tectonic element of western North America. The 650 km long Sierra Nevada was created by enormous batholithic intrusions during the Mesozoic. The majestic landscapes of these mountains are the focus of conceptual models in landscape evolution in Chapters 5 and 8.

Mesozoic subduction beneath the North American plate produced granitoid melts that rose above their heavier denser ultramafic residues, which were held in place by the subducting slab. Eventual failure of the slab allowed the ultramafic root to detach from the granitic microplate and sink into the mantle. This major tectonic event – detachment of the Sierra Nevada batholithic root, delamination – affected tectonic elements in much of the broad transform boundary. Ongoing synchronous subduction removed most of the oceanic Farallon plate, creating the Mendocino triple junction; both processes influenced microplate and transform-boundary tectonics. This introduction summarizes apparent coincidences of timing of several changes of tectonic style between the Pacific and North American tectonic plates.

Late Cenozoic rock uplift rates of the Sierra Nevada were slow to moderate. Unruh’s 1991 long-term estimate is based on initiation of late Cenozoic uplift at roughly 5 Ma and maximum possible uplift of the range crest of 2500 m. This conservative estimate, <0.5 m/ky, is fairly slow because of the assumption that uplift was uniformly distributed throughout the 5 My time span. A nonuniform uplift model, presented later in Section 8.2.2, suggests that short-term uplift rates were faster, being comparable to the present uplift of the Southern Alps of New Zealand, Taiwan, and parts of the Himalayas.

Diverse mechanisms have been proposed for late Cenozoic renewed uplift of the Sierra Nevada. These include flexural isostasy (Chase and Wallace, 1986), support by a buoyant asthenosphere (Liu and Shen, 1998), isostatic erosional unloading in the mountains combined with depositional loading in the San Joaquin Valley (Small and Anderson, 1995), and extension of the adjacent Basin and Range Province (Wernicke et al., 1996; McQuarrie and Wernicke, 2005). Tectonic reconstruction for southwestern North America shows that the Sierra Nevada microplate has moved ~235 km N78°W with respect to the Colorado Plateau since ~ 16 Ma (Geological Society of America Penrose Conference, 21–26 April 2005, “Kinematics and Geodynamics of Intraplate Dextral Shear in Eastern California and Western Nevada”). Manley et al. (2000) suggest that foundering of the eclogitic root occurred as recently as 3.5 Ma. To these potential uplift mechanisms I would add a flexural uplift component associated with accelerated deposition on the western edge of the microplate as a result of major increases in the sediment yield from the Coast Ranges.

Distinctive early Sierran landscapes were eroded during times of tectonic quiescence. Using the chronology of Wakabayashi and Sawyer (2001), uplift related to batholithic intrusion was complete by 99 Ma. Erosion had reduced the northern Sierra Nevada to a tectonically inactive landscape by 52 Ma with remnants of mountains rising as much as 800 m above adjacent pediment base levels. Virtually no uplift occurred between 57 Ma and 5 Ma. Renewed volcanism during this interval covered all but a few scattered basement highs with the Mehrten Formation. This volcanic activity ceased at about 5 Ma as the Mendocino triple junction migrated ever farther to the north. Eocene-to-Pliocene tectonically induced downcutting by Sierra Nevada rivers was miniscule. Wakabayashi and Sawyer, using altitudes of dated stream gravels, estimated valley floor degradation to be less than 0.007 m/1000 years. The southern Sierra Nevada, although greatly eroded, still had local relief of 2,000 m that was inherited from uplift that occurred between 99 Ma and 57 Ma.

Tectonic quiescence ended at about 3.5 Ma with initiation of uplift along range bounding normal faults on the eastern flank of the Sierra Nevada. Encroachment by normal faults continued, perhaps at an accelerated pace in the northern Sierra Nevada. Part of the impressive relief of the Sierra Nevada eastern escarpment (Fig. 1.4) may be the result of dropping of adjacent the Owens Valley as extensional processes promoted collapse of formerly higher terrain in much same way as the valleys between the transition ranges in Arizona fell away from the Colorado Plateau (Lucchitta, 1979; Mayer, 1979). The Yuba and Stanislaus Rivers were beheaded as a new range crest developed. Similar beheading of the San Joaquin River dropped at least 40 km of its ancestral headwaters into what is now Owens Valley (Huber, 1981). Valley floor lava flows of the past 10 Ma (Huber, 1990) were tilted and incised to become meandering flat-topped ridgecrests. These non-steady-state landscapes document recent bedrock uplift along the east-side range-bounding fault.

The effect of tilting caused by uplift of only ~0.1m/ky was enhanced incision by glaciers and large rivers that now flow through spectacular canyons in plutonic rocks. Wakabayashi and Sawyer point out that late Cenozoic tilting raised the entire length of the Sierra Nevada, with uplift ranging from 1,710 to 1,930 m. Rock uplift and surface uplift of the Sierra Nevada crest were about the same because Late Quaternary erosion rates are very low (Small et al., 1997; House et al., 2001; Stock et al., 2005). We need to further examine the causes of this recent uplift.

Uplift at >1m/ky may have been a consequence of the crustal delamination. Detachment of the crustal root of the Sierra Nevada batholith followed by upwelling of hot asthenosphere would initiate a sudden pulse of rock uplift that would fade away as adjustments between the new set of crustal mountain-building forces stabilized. I find the diverse evidence for a delamination event fairly convincing.

Data from a geophysical transect extending from the San Joaquin Valley across the Sierra Nevada and Panamint Ranges constrains tectonic models (Wernicke et al., 1996; Liu and Shen, 1998; Fliedner et al., 2000). The Cretaceous batholith had a thick residual root, but a crustal root is no longer present to support the Sierra Nevada.

Xenolith (pieces of older rock engulfed by rising magma) composition changes (Ducea and Saleeby, 1996, 1998) indicate that a former dense root beneath the Sierra Nevada crest was convectively removed before 3 Ma. Saleeby and Foster (2004) say “the sharp contrast in peridotite mantle facies fields between the mid-Miocene and Pliocene–Quaternary xenolith suites, and a Pliocene change in the composition of lavas erupted in the region to more primitive compositions are interpreted to indicate the removal of the sub-batholith mantle lithosphere and its replacement by asthenosphere. Removal of dense (3.5 g/cm3) eclogite and replacement with lower density peridotite (3.3 g/cm3) may have increased buoyancy sufficiently to account for as much as 2 km of range-crest uplift (Liu and Shen, 1998). Similar delamination-magmatic pulses may have occurred beneath the Puna Plateau of Argentina and the Tibetan Plateau (Glazner, 2003).

Mantle lithosphere is now abnormally thin beneath the Sierra Nevada and Panamint Ranges (Jones et al., 1994; Wernicke et al., 1996; Ruppert et al., 1998). They seem to be largely supported by a buoyant upwelling of hot asthenosphere. Modeling by Liu and Shen (1998) indicates that mantle upwelling beneath the Basin and Range extensional province caused ductile material within the lithosphere to flow southwestward. Zandt (2003) refers to this flow as “mantle wind” that shifted this part of the detached Sierra Nevada batholithic root – a mantle drip – to the southwest.

Xenolith studies suggest recent removal of garnet-bearing rocks triggered a brief (3.5 ± 0.25 Ma) pulse of potassium-rich basaltic vulcanism (Fig. 1.5) in a 200 km diameter circular area in the central Sierra Nevada (Manley et al., 2000; Farmer et al., 2002; Jones et al., 2004). This event indicates the most likely timing and location of the main delamination event. Zandt (2003) points out that the highest peaks of the Sierra Nevada occur in the area where the delamination event began – maximum strength of the geophysical perturbation coincides with maximum uplift. I like the interpretation that recent uplift of the Sierra Nevada also could be this young, and that volcanism and uplift resulted from a common delamination-triggering mechanism. Section 8.2.2 discusses the landscape-evolution consequences of the “Post-4-Ma uplift event”, focusing mainly on this same part of the Sierra Nevada, which includes the Kings River, Owens Valley, and the highest part of the east-side escarpment.

The interval between 5 and 3 Ma fits a model for regional synchronous changes in many plateboundary tectonic processes, and of landscapes that record changes in tectonic base-level controls. Geophysical detection of the lack of a crustal root beneath the lofty Sierra Nevada has led to much creative thinking about tectonics of the region.

Figure 1.5 Times of volcanism north of the Garlock fault. Dashed line outlines area of 4 to 3 Ma volcanism and solid line outlines area of highly potassic lavas. OV is Owens Valley and PV is Panamint Valley. From Manley et al., 2000.

A brilliant synthesis by Jones et al. (2004) recognizes the late Pliocene foundering of the Sierra Nevada crustal root as a perturbation that changed regional plate tectonics. They reason that this crustal delamination event increased the total gravitational potential energy of the lithosphere (Jones et al., 1996), thus increasing both extensional strain rates in the western Basin and Range Province and the altitude of the eastern Sierra Nevada. They also conclude “an increase in extensional displacement rates must be accommodated by a decrease in rates of extension or an increase in rates of shortening somewhere in the vicinity of the Sierra Nevada” (Jones et al., 2004, p. 1411). The timing of recent thrust faulting that created the Coast Ranges bordering the Central Valley coincides nicely (Wentworth and Zoback, 1989; Namson et al., 1990; Lettis and Hanson, 1991; Wakabayashi and Smith, 1994). And, “Lithospheric removal may also be responsible for shifting of the distribution of transform slip from the San Andreas Fault system to the Eastern California shear zone” (Jones et al., 2004, p. 1408). Details and confirmation of their model of synchronous regional changes will be tested by future work. Initial geomorphic tests of the model presented in Section 8.2 support uplift of the Sierra Nevada at about 3.5 Ma, concurrent with changes in tectonic style of the Coast Ranges and formation of the Walker Lane–Eastern California shear zone.

Cenozoic tectonism of western North America is related to the Mendocino plate boundary triple junction, which migrated northward creating the San Andreas zone. This is a continental right-lateral transform fault system (Atwater, 1970; Atwater and Stock, 1998) that presently includes the Maacama and Bartlett Springs faults. They conclude that no discernible change in rates of motion of the Pacific–North American plate boundary has occurred since 8 Ma. This is a key assumption of the Jones et al. model where horizontal velocities across the Sierra before and after the delamination event match the boundary condition of Pacific–North American plate motion. The assumption is supported by the work of Argus and Gordon (2001). They say “the Sierran microplate changed motion relative to North America at the same time (~ 8 to 6 Ma) as did the Pacific plate. Local acceleration of extensional encroachment in a belt east of the Sierra Nevada may match increased rates of Coast Ranges shortening to the west. The Sierra Nevada microplate has become narrower as a result of both tectonic processes.

Interactions between plate-boundary kinematics and locally derived forces generated by the foundering of Sierra Nevada eclogitic lithosphere appear pervasive. Although not as nicely constrained as the 3.5 ± 0.25 Ma brief pulse of potassic vulcanism associated with recent Sierra Nevada uplift, key plate tectonics and geomorphic events seem to be regional consequences of the delamination event. The degree of synchroneity is a bit fuzzy because 1) there is minimal information about the time span needed for the delamination process to occur in different parts of the microplate, and 2) establishment of times of tectonic events elsewhere in the plate boundary may be rather crude where dating uncertainties are large.

Vertical and horizontal displacements commonly do not occur on the same fault. Instead they may be partitioned between adjacent faults. Partitioning refers to the common situation where vertical and horizontal components of displacement occur on separate faults within a fault zone. Partitioning styles are defined later in this chapter (Fig. 1.10). A parallel style of partitioning is common in the Walker Lane–Eastern California shear zone. Examples include dextral fault zones in Owens and Death Valleys and adjacent range-bounding normal faults. Parallel partitioning was the result of dextral faulting being introduced into an area of pre-existing normal faulting.

How significant were the processes of extensional faulting, dextral faulting, and crustal-root delamination in the formation of Sierra Nevada and nearby mountain ranges? What is the present rate of uplift on the Sierra Nevada range-bounding fault? How important is Late Quaternary strike-slip faulting relative to normal faulting? How did the large rivers draining the western flank of the Sierra Nevada respond to pulsatory uplift? How long are the ridgecrest response times to stream-channel downcutting? These are some of the questions addressed in Sections 5.3.2 and 8.2.

1.2.3 Diablo Range

The Coast Ranges border the west side of the Sierra Nevada microplate, the only gap being where rivers leave the central valley and flow into San Francisco bay (the estuary by the 10 Ma notation in Figure 1.3). The south half of the Central Valley of California is the San Joaquin Valley, and this book discusses sites on the west side from Mt. Diablo at the north end (Section 2.3.2.2) to central districts (Sections 3.2, 3.4, 4.2.1.2.2., 4.2.2, 5.4). Wheeler Ridge (Fig. 1.2 and Section 7.2.2) at the south end of the San Joaquin Valley is another key study site.

The western edge of the central San Joaquin Valley has deceptive, but fascinating, mountain fronts. I say deceptive because I now realize that some of these low, largely barren hills are so tectonically active that they have only recently risen to modest altitudes of 300 to 900 m. With the advantage of plate-tectonics hindsight we now realize that, although low, the hills flanking the San Joaquin Valley are both rising and seismogenic.

Plate-boundary tectonics near the right-lateral oblique-slip San Andreas fault (Fig. 1.3) has created a fold-and-thrust belt (Wentworth, and Zoback, 1990; Yerkes, 1990) that is encroaching into the adjacent San Joaquin Valley. West-dipping thrust faults border about 600 km of the eastern margin of the Coast Ranges, in both the Sacramento and San Joaquin Valleys (Wentworth and Zoback, 1990; Wakabayashi and Smith, 1994). Both strike-slip and thrust faults are present locally (LaForge and Lee, 1982: Lettis, 1985). Lettis and Hanson (1991) use a regional strain-partitioning model that relates the kinematics of the fold-and-thrust belt to the San Andreas fault.

Historical earthquakes reveal how active the young foothill belt is. The first of a northwest-to-southeast sequence of earthquakes on the same 100-km long hidden thrust fault (Stein and Yeats, 1989; Stein and Ekstrom, 1992; Lin and Stein, 2006) was the Mw magnitude 5.4 New Idria earthquake of 1982 on an internal fault. The Mw magnitude 6.5 Coalinga earthquake in 1983 (Hill, 1984; Rymer and Ellsworth, 1990) emanated from beneath Anticline Ridge, and the Mw magnitude 6.1 earthquake of 1985 from beneath the north end of the Kettleman Hills (Wentworth et al., 1983). Both are range-bounding structures. The crest of Anticline Ridge rose abruptly about 500 mm during the 1983 earthquake (King and Stein, 1983). In hindsight, I now realize that comparison of first-order level-line surveys across Anticline Ridge in March, 1962 and March, 1963 recorded a precursor event to the destructive Coalinga earthquake. Benchmarks rose as much as 24 mm in a pattern that mimicked the topographic profile of Anticline Ridge (Bull, 1975b, fig. 4). These “hidden earthquakes” (Stein and Yeats, 1989) underscore the continuing active nature of thrust faults in the cores of these folds. The landscape evolution this reflects is rapid, continuing uplift. But how does this thrust faulting fit into the Jones et al. (2004) model of regional tectonic activity as influenced by the delamination of the Sierra Nevada batholithic crustal root?

Sedimentary basins can contain a wealth of information about the geomorphic consequences of tectonic events in their source mountain ranges (Dickinson, 1974; Angevine et al., 1990; Frostick, and Steel, 1994; Emery and Myers, 1996; Cloetingh et al., 1998; Busby and Ingersoll, (1995). Geomorphologists look at the consequences of cumulative erosion on landscapes. Sedimentologists have access to sequences of outputs from fluvial systems. In the example used here, the distributions of thick Quaternary basin fill in the Central Valley of California may be regarded as a competition for a finite amount of space for newly arriving deposits from Sierra Nevada and Coast Ranges source areas. A tectonic story is revealed by the depositional contact between basin-fill derived from these two opposing source areas.

Geologic logs of many water wells in San Joaquin Valley provide a wealth of information about this contest for space. The granitic rocks of the Sierra Nevada to the east supply distinctive micaceous arkosic sands to the basin. Sierran watersheds and rivers are large. Terrains and rock types along the west side of the San Joaquin Valley are much different. These mountains consist of a main range and a younger foothill belt. Water-laid sediments derived from the main Diablo Range consist primarily of silty to clayey sand transported by fairly large streams, such as Panoche Creek, through gaps in the foothill belt. Clay contents are high in the poorly sorted water-laid and debris flow alluvial-fan deposits derived from the foothill belt watersheds. These small drainage basins are underlain by mudstone, diatomaceous shale, and soft sandstone.

Rates of deposition in the San Joaquin Valley have accelerated during the Quaternary. Sierra Nevada sediment yield was increased by tectonically induced downcutting of deep canyons in response to 1 to 2 km of post-delamination uplift of the range crest and by glacial erosion. Rapid, continuing uplift of the Diablo Range foothill belt created new sources of sediment. Mountain-front encroachment reduced the area of the large San Joaquin Valley depositional basin. All three factors accelerated rates of accumulation of valley fill, but did either source area win the contest for depositional space?

Competition between Diablan and Sierran alluvial fans for space in the San Joaquin Valley can be described by cross sections such as Figure 1.6 (also see Lettis, 1985). Alluvial fans occupy space that is proportional to the amount of sediment deposited on each fan (Hooke, 1968). Contacts between components of basin fill are vertical where aggradation rates are similar, and sloping where rates are dissimilar. Amounts of sediment derived from Diablan and Sierran sources varied with Late Quaternary climatic change and with uplift of the ranges. I assume that the overall Sierra Nevada sediment yield has not changed during the past 600 ky. Of course there were temporary and large increases during times of glacier advances and decreases during interglacial times (Lettis and Unruh, 1991). Glacial induced sediment yield increases may be responsible for temporary advances of Sierran alluvium toward the Diablo Range (Fig. 1.6). It is further assumed that the proportion of sediment flushed out to the Pacific Ocean has not changed.

The gently sloping contact between Diablan and Sierran alluvium (Fig. 1.6) records major changes in relative source-area sediment yields. The area of the Diablo Range piedmont has doubled. Progradation of Diablan alluvial-fan deposits over Sierran alluvium during the past 620 ky since deposition of the Corcoran Lake Clay indicates that alluvium derived from the Diablo Range was deposited at progressively faster rates, relative to rates of accumulation of arkosic alluvium derived from the Sierra Nevada. Transgression of Diablo Range basin fill over Sierran fill continued despite the depression of the basin that resulted from tectonic loading of the rising Coast Ranges. Much of the Corcoran lake clay is now far below sea level.

Mean rates of aggradation of basin fill above the Corcoran and above the much younger ‘A’ lake clay increased from about 0.30 m/ky for the time span since 620 ka to about 0.55 m/ky for the time span since 27 ka (Croft, 1968, 1972). These increases in area and rate of deposition suggest a three-to six-fold increase in sediment yield from the rising Diablo Range. Diablo Range denudation rates may have increased from roughly 0.1 to 0.5 m/ky. Rates of sedimentation were sufficiently rapid to exceed tectonic subsidence (maximum rates are along the western margin of the valley) and shift the axis of deposition eastward.

The consequences of the delamination event for the crustal root of the Sierra Nevada batholith caused rapid uplift of the Sierra Nevada that occurred before 2 Ma. But, using the model of Jones et al. (2004), the regional consequences were perhaps even more profound for the California Coast Ranges. Compression near the San Andreas fault created a fold-and-thrust belt that continues to encroach northeastward into the microplate. The present eastern edge of the Coast Ranges was not formed by a synchronous pulse of uplift, as was the eastern front of the Sierra Nevada. Uplift progressed from southeast to northwest. This migration of thrust-faulted mountain fronts profoundly influenced landscape evolution and has continued to the present.

Figure 1.6 Cross section of valley fill deposited since the 620 ka Corcoran Clay Member of the Tulare Formation. From Magleby and Klein, 1965, plate 4, and Croft, 1968, 1972). Vertical exaggeration is 40:1

1.2.4 Mendocino Triple Junction

Innovative plate-tectonic analyses by Wilson (1965), McKenzie and Morgan (1969), and Atwater (1970) deciphered the consequences of a spreading ridge between the Farallon and Pacific plates moving into the North American–Farallon subduction trench at –30 Ma. These plate movements were relative. North America might have progressively overridden parts of a Pacific plate oceanic spreading center, and the ratio of movement between the two plates probably changed with time. This arrival of the Pacific plate split the Farallon plate into the now distant Cocos and Juan de Fuca plates (Fig. 1.7B, C). Right-lateral continental transform faults formed – represented at present by the San Andreas fault. The southern part of the Juan de Fuca plate is named the Gorda plate.

The oceanic ridge of upwelling new crust met the North American plate at an oblique angle, but continued to spread out to both sides. The compressional side of the ridge moved off to the southeast as a triple junction between the underthrusting side of the ridge, the subduction trench in front of it and a newly created right-lateral and ever-lengthening transform fault extending behind. This was the birth of a system of transform faults. The San Andreas fault is one of the more recent faults. This ridge–transform–trench Rivera triple junction now is off the coast of Mexico.

Figure 1.7 Plate-tectonic evolution of the San Andreas continental transform boundary. Paired arrows indicate dextral relative motion of transform faults. Double-headed arrows are ocean ridge spreading centers. Teeth are on the upper plate of subduction thrusts. Triple-junction locations are shown by the filled circles. From Dickinson, 1981.A. Pacific plate approaches the North America plate at about 30 Ma.B. At about 20 Ma. Rivera triple junction, RTJ, is moving southeast. Mendocino triple junction, MTJ, is moving northwest.C. Plate boundary at the present time. San Andreas fault ends at string of rhombochasms at the head of the Gulf of California, which did not exist before 5 Ma. Rivera and Cocos plates are at far right. Gorda–Juan de Fuca plate is at far left.

Migration to the northwest also formed part of the ever-lengthening right-lateral San Andreas transform fault. Being a transform–transform–trench tectonic system, the types of plate boundary interactions were different for this triple junction. New lithosphere generated on the north flank of the spreading ridge moved northward along the North American plate margin (Furlong and Govers, 1999; Furlong and Schwartz, 2004). They conclude that the Pacific–North America relative plate motion was subparallel to the North America plate margin. Subduction did not play a role. Instead, northwest migration of the Mendocino triple junction created a crustal void – a slab window – into which hot asthenosphere flowed. These processes thickened the crust ahead of the triple junction and thinned the crust in the wake of the migrating crustal welt. Rapid uplift rates decreased as the Mendocino triple junction continued its northwest migration, allowing the new crustal materials created by asthenospheric inflow to become cooler.

Uplift was not the result of convergence or underthrusting processes for the transform–transform–trench system. In part it resulted from delayed upwelling of hot asthenosphere into the crustal abyss created by the northwest movement of the Gorda plate beneath a presumably rigid North American plate (Dickinson and Snyder, 1978; Lachenbruch and Sass, 1980; Zandt and Furlong, 1982; Furlong and Schwartz, 2004). Concurrent lengthening of the San Andreas continental transform has continued to the present.