Balanced Structural Cross Section of the Western Salt Range and Potwar Plateau
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AN ABSTRACT OF THE THESIS OF Michael R. Leathers for the degree of Master of Science in Geology presented on May 28.1 987. Title: Balanced Structural Cross Section of the Western Salt Range and Potwar Plateau. Pakistan: Deformation Near the Strike-SliD Terminus of an Overthrust Sheet. Signature redacted for privacy. Abstract approved: - 7 RobertJ. Lillie The Salt Range and Potwar Plateau of northern Pakistan are part of the thin-skinned, active thrust system related to the ongoing collision of the Asian and Indian continental blocks. Platform rocks and orogenic molasse of the Indo-Pakistani shield are deformed in south-verging thrusts and folds, relative to a northward-converging basement. This results in three different styles of deformation along a north-south, balanced cross section in the western Potwar Plateau and Salt Range. This cross section, as well as structure and isopach maps for the entire Salt Range! Potwar Plateau region, were constructed using surface geologic maps, seismic reflection, drillhole, and gravity data. In the Northern Potwar Deformed Zone (NPDZ), shortening of 68% iS accomodated by a trailing imbricate stack of high ad low-angle, south- verging faults. These faults are rooted in a master decollement within an Eocambrian evaporite sequence (Salt Range Fm.), which overlies a north- dipping basement surface at a depth of approximately 8 km. Southward progression of regional deformation with time appears to be interrupted by overstep, or back-break, thrusting. The low-angle faults, or a possible basement uplift, may represent the ramp for a protracted interval of imbricate faulting that began in middle to late Miocene (15-10 mybp). Later, the basal detachment propagated rapidly, transporting the NPDZ sequence and the developing Soan Syncline (southern Potwar Plateau) southward, without major internal deformation within the Syncline. Within the last 2.0 my, the older units overlying the Salt Range Fm. have been exposed along the Salt Range, where the basal detachment ramped to the surface. A north-facing, subsurface normal fault, that offsets basement and controls frontal ramping along the central portion of the Salt Range, is not present in the western Salt Range. Rather, the older platform rocks and overlying molasse are carried southward up a basement monocline. This monocline is a protrusion on the northeast flank of the northwest-trending Sargodha Ridge, a buried basement arch that may represent a flexural response to Himalayan tectonic loading. Although the footwall, underlying the thrust plane, has not been drilled in the Salt Range, seismic control and two-dimensional gravity modeling suggest the presence of autochthonous sedimentary rocks, overlain by a thickened layer of evaporites, In the western Salt Range, the leading edge of the Salt Range Thrust (SRI) overrode its footwail block about 34 km. Additional shortening may have been accomodated by a series of imbricate thrusts in the Salt Range Fm., or the footwall strata may be imbricated. This would add northward taper to the basement dip, allowing the SRI to be exposed at the surface. An average shortening rate of approximately 1.3 cm/yr is comparable to the shortening rate for the NPDZ, if thrusting there occurred over a 7-8 my interval, Overthrust shortening along the SRT in the central Salt Range has been estimated at between 1 9 and 23 km. The larger amount of overthrusting in the western Salt Range, as compared to the central Salt Range, is consistent with the structurally-salient range front along this western edge. Minimum overall shortening, from the southern edge of the Peshawar Basin to the undeformed Jhelum Plain, is estimated at 21 3 km, a shortening of about 49%. This is comparable to the amount of fold and thrust shortening in other deformed forelands. Balanced Structural Cross Section of the Western Salt Range and Potwar Plateau, Pakistan: Deformation Near the Strike-Slip Terminus of an Overthrust Sheet by Michael R. Leathers A THESIS submitted to Oregon State University in partial fulfillment of the requirements f or the degree of Master of Science Completed May 28, 1987 Commencement June 1 988 APPROVED: Signature redacted for privacy. Assistant Professor of Geology in charge of major Signature redacted for privacy. aman of the Dpartient of Geology Signature redacted for privacy. Dean of the Graduate School Date thesis is presented May 28. 1987 Typed by Michael R. Leathers ACKNOWLEDGEMENTS This thesis is an outgrowth of work done as a research assistant under Dr. Robert Lillie and Dr. Robert Yeats. It is part of a larger project at OSU, in cooperation with various Pakistani organizations and individuals, to study aspects of both the foreland and hinterland of the Pakistani portion of the Himalayan Mountain belt. It was through the efforts of Drs. Yeats and Lillie that the data for this thesis were assembled. As principle investigators of the National Science Foundation (NSF) proposal for foreland studies, their guidance, and contributions to the conclusions of this project are greatly appreciated. Professors involved in other aspects of the overall cooperative effort, and whose helpful assistance is acknowledged, are Dr. Robert Lawrence of OSU and Dr. Larry Snee, presently of the USGS in Denver. Coordination and assistance came from the Oil and Gas Development Corporation of Pakistan (OGDC), and the Ministry of Petroleum and Natural Resources of Pakistan, to whom we are grateful for the use of the subsurface data used in this study. Also greatly appreciated is the assistance of several representatives from OGDC and the Geological Survey of Pakistan, especially Mohammed Yousuf and Ahmad Hussain, during their visits to 05U. The OSU project is supported by NSF grants tNT 8 1-18403, EAR 83- 18194, INT 86-09914, and EAR 86-08224, supplemented by funds from Conoco, Inc., Chevron International, and Amoco Production Company. Gravity modeling was performed with interactive software developed by Environmental Simulations Laboratory at Southern Filinois University. Interactive cooperation among students working in Pakistan has been enjoyable, and essential to the coordination of the different portions of this OSU project. Discussions with Jim McDougall, along with his slides from his field area along the Kalabagh Fault, have helped visualize the area. The section on Regional Geology benefited from discussions with Steve Carter, Joe DiPietro, Jim LaFortune, Ian Madin, Kevin Pogue, and Phillip Verplank. Students involved in the study of the foreland area are Dan Baker, Yannick Duroy, Steve Jaume and Ned Pennock. The ability to work cooperatively, yet independently, has resulted in theses that compliment each other well, and yet avoid serious redundancy. Discussions with each 0 these people have been essential to my understanding of the geology and tectonics of the foreland area. More important, though, is their help in developing and refining techniques in analyzing the geologic and geophysical data. Yannick Duroy was also helpful in teaching me the use of the interactive gravity modeling software, as well as gravity theory. The cooperation between myself, Bob Lillie and Ned Pennock that allowed me to be a teaching assistant for two of Dr. Lillies geophysics classes was greatly appreciated. The time was well spent both learning more about seismic reflection theory and gravity methods, as well as a fun and challenging exposure to teaching. Computer expertise was provided by Dana Willis in preparing the computer graphics, and in printing the thesis. Without his help, I, and many other Macintosh users would have been left out in the cold. Completion of this thesis would not have been possible without the friendship and companionship of several people; many whom are inhabitants of the basement dungeon at Wilkinson Hall. My many thanks for an enjoyable stay at OSU and Corvallis. TABLE OF CONTENTS INTRODUCTION 1 Overview 1 Description or thesis area 4 Objectives 5 Geologic problems 7 Description or data sources 7 Previous and significant related work 14 REGIONAL SETTING 19 Tectonic 19 Structure 24 Strattgraphy 37 Sedimentology 42 DISCUSSION OF CROSS SECTION 54 Seismic reflection lines and interpretation 54 Choosing the line or section 56 Depth section 57 Balancing and restoration 61 DEFORMATION STYLE OF WESTERN POT WAR PLATEAU AND SALT RANGE 65 Salt Range 65 Southern Potwar Plateau 72 Northern Potwar Derormed Zone (NPDZ) 73 Kalabagh Fault and Reentrant 80 COMPARISON OF STRUCTURAL STYLES 86 Kohat Plateau/Kalabagh Reentrant/Sargodha Ridge 86 Attock Cherat Range/Westernmost Salt Range 89 Central Potwar Plateau/Salt Range 89 Eastern Potwar Plateau 94 Indian Foreland 96 RAMPING MECHANISMS 99 Basement faults 99 Western Salt Range 1 04 Influence of Sargodha Ridge 107 Lithologic and fades control 116 Eastern Salt Range 118 TIMING OF DEFORMATION 124 Methods 1 24 Northern Potwar Deformed Zone (NPDZ) 1 25 Salt Range 128 Kalabagh Fault 129 Western Potwar Plateau and Bannu Basin 130 AMOUNT AND RATE OF SHORTENING 132 Restored cross section C-C, 132 Definition of parameters 135 Calculated shortening 135 Rate of shortening and displacement 143 CONCLUSIONS 150 61 BL I OGRAPHY 155 APPEND I CES Methodology 178 Velocity analysis and comparison of techniques 1 78 Gravity modeling 180 Seismic reflection lines 190 Restored portions of cross section C-C, 210 Isopach and structure maps 226 Three-dimensional evolutionary diagrams 258 LIST OF FIGURES Figure Page Geographical location map of Pakistan showing significant 2 rivers and cities. Generalized tectonic map of northern Pakistan 3 showing location of lines of section. Landsat Image showing major structural provinces and 6 geomorphic features of northern Pakistan fold and thrust belt. Index maps of seismic reflection data, and structural 8 and gravity cross section locations. Composite geologic maps of northern Pakistan 11 fold and thrust belt. Geology maps used to compile composite geologic maps. 13 Production and exploration wells of northern Pakistan 15 fold and thrust belt for which information is available, Simplified Bouguer gravity map of northern Pakistan fold 16 and thrust belt.