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Pohn U.S. GEOLOGICAL SURVEY BULLETIN 2163 Lateral ramps in the subsurface can be detected from changes in surface folds. Such changes occur in fold-and-thrust belts worldwide and probably indicate lateral ramps. U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Charles G. Groat, Director UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 2000 Published in the Eastern Region, Reston, Va. Manuscript approved for publication August 10, 1998 Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government For sale by U.S. Geological Survey Information Services Box 25286, Federal Center Denver, CO 80225 or call the toll-free telephone number 1-888-ASK-USGS Library of Congress Cataloging-in-Publication Data Pohn, Howard A. Lateral ramps in the folded Appalachians and in overthrust belts worldwide— A fundamental element of thrust-belt architecture / by Howard A. Pohn. p. cm. — (U.S. Geological Survey bulletin ; 2163) Includes bibliographical references. Supt. of Docs. no.: I19.3:2163 1. Thrust faults (Geology)—Appalachian Mountains. I. Title. II. Series. QE75.B9 no. 2163 [QE606.5.I5] 557.3 s—dc21 98-48485 [551.8′72′0974] CIP CONTENTS Abstract........................................................................................................................... 1 Introduction ....................................................................................................................1 Purpose and Scope.................................................................................................. 2 Acknowledgments .................................................................................................. 2 Geometric Considerations of Lateral Ramps.................................................................. 4 Overview ................................................................................................................ 4 The Basic Model..................................................................................................... 5 Field Examples and Supporting Data ............................................................................. 7 Susquehanna Lateral Ramp .................................................................................... 7 Stratigraphic and Structural Setting................................................................ 7 Seismic Evidence............................................................................................ 10 Dip-Line Section..................................................................................... 10 Strike-Line Section................................................................................. 11 Pennsylvania-Maryland-West Virginia Lateral Ramp ........................................... 11 Mathias Lateral Ramp ............................................................................................ 11 Surface Evidence ............................................................................................ 11 Seismic Evidence............................................................................................ 11 Highland County Lateral Ramp.............................................................................. 19 Surface and Geophysical Evidence ................................................................ 19 Seismic Evidence............................................................................................ 19 Igneous Intrusions Associated with Lateral Ramps ............................................... 19 Additional Lateral Ramps in the Central Appalachians ................................................. 19 Wilkes-Barre Lateral Ramp.................................................................................... 19 Seven Mountains Lateral Ramp ............................................................................. 26 Tyrone-Mount Union Lateral Ramp....................................................................... 27 Bedford Lateral Ramp ............................................................................................ 27 Lexington Lateral Ramp......................................................................................... 27 Roanoke Lateral Ramp ........................................................................................... 27 Fincastle Conglomerate: An Indicator of the Roanoke Lateral Ramp .......... 30 Changes in Unit Thickness and Lithologies................................................... 31 Coarse-Pebble Conglomerates Related to Other Ramps ........................................ 31 Lateral Ramps in the Southern Appalachians................................................................. 31 Lineament Swarms of Eastern Tennessee and Western North Carolina........................ 37 Lateral Ramps and Tectonic Windows........................................................................... 37 Lateral Ramps and Frequency of Disturbed Zones ........................................................ 46 Lateral Ramps and Possible Basement Faulting............................................................. 49 Movement Along Lateral Ramps Through Time ........................................................... 50 Observed Association of Lateral Ramps and Giant Landslides ..................................... 51 Possible Relationship of Lateral Ramps to Offshore Transform Faults......................... 51 The Presence of Lateral Ramps Worldwide................................................................... 52 Lateral