AN ABSTRACT OF THE THESIS OF
Peter M. Powers for the degree of Master of Science in Geology presented on June 7.
1996. Title: Structure and Shortening of the Kangra and Dehra Dun Reentrants, Sub-
Himalaya. India.
Signature redacted for privacy. Abstract approved:
Robert J. Lillie
Surface-geology, oil-well, seismic-reflection, and magnetostratigraphic data are integrated to evaluate structural style and shortening rate at the Himalayan front (Sub- Himalaya) of northwest India. The Sub-Himalaya, between the Main Boundary thrust and the Himalayan Frontal fault, is the actively deforming front of the Himalaya. At certain locations, the Himalayan Frontal fault is a blind thrust beneath anticlines of Siwalik (Tertiary) molasse, parallel to the Himalayan arc. The Main Boundary thrust, in contrast, is sinuous, so that the width of the Sub-Himalaya ranges from 30 to 80 km. Where the Sub-Himalaya is narrow (Nahan salient), Tertiary rocks are exposed in imbricate thrust sheets; where the Sub-Himalaya is broad (Kangra and Dehra Dun reentrants), alluvium fills wide synclinal valleys (duns). Seismic-reflection data reveal that surface anticlines form in association with south-vergent thrusts that root in a d&ollement at the base of the Tertiary section. Reflection profiles and well data also indicate that the basement lithology changes northward from Precambrian crystalline rocks beneath the Indo-Gangetic plains to Precambrian and Cambrian metasedimentary rocks beneath the Sub-Himalaya. The Sub-Himalayan décollement dips 2.5° northward beneath the Kangra reentrant, but it is steeper at 6° beneath the Dehra Dun reentrant. The Kangra and Dehra Dun reentrants are characterized by fault-propagation folds with steep limbs in the north and by broad anticlines with gently north-dipping limbs in the south. A balanced cross section of the Kangra reentrant shows that a minimum of 23 km shortening has occurred since 1.9-1.5 Ma, yielding a shortening rate of 14±2 mm/yr. Shortening has occurred at a rate of 7-15 mm/yr across the Dehra Dun reentrant. These data compare with other published shortening rates and indicate that -25% of the total India-Eurasia convergence is accommodated within the Sub- Himalaya of northwest India. ©Copyright by Peter M. Powers June 7, 1996 All Rights Reserved Structure and Shortening of the Kangra and Dehra Dun Reentrants,
Sub-Himalaya, India
by
Peter M. Powers
A THESIS
submitted to
Oregon State University
in partial fulfillment of the requirements for the degree of
Master of Science
Completed June 7, 1996 Commencement June 1997 Master of Science thesis of Peter M. Powers presented on June 7, 1996
APPROVED:
Signature redacted for privaày.
Major Professor, representing Geology
Snatureredacted for privacy.
Chair of Department of Geosciences
Signature redacted for privacy.
Dean of Gradu&School
I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request.
Signature redacted for privacy.
Peter M. Powers, Author ACKNOWLEDGMENTS
This study was part of a collaborative project between Oregon State University and the Oil and Natural Gas Corporation, Dehra Dun, India, which was made possible by National Science Foundation (NSF) grant EAR-9303577. I am grateful to the members of the Oil and Natural Gas Corporation who made this project a success: Messrs. V.N. Misra, A.K. Srivastava, and G.C. Naik. I would also like to thank Bob Lillie for his input and guidance throughout the course of my stay in Corvallis. Special thanks are due to Bob Yeats, who gave me a wonderful introduction to India during the fall of 1994. Thanks are also in order for Firoze and Sucheta Dotiwala who taught me a great deal about Indian culture and life. Lastly, I thank my parents, who have been forever supportive of my interests and aspirations. TABLE OF CONTENTS
Page
INTRODUCTION 1
TECTONIC AND STRUCTURAL FRAMEWORK 5
STRATIGRAPHIC SETTING 9
STRUCTURAL CROSS SECTIONS 12 General structural divisions 12 Kangra reentrant (section A-A') 13 Southern structures 13 Central structures 15 Northern structures 17 Dehra Dun reentrant (section B-B') 19 Southern structures 19 Northern structures 19
RESTORED CROSS SECTIONS AND SHORTENING AMOUNTS 28
SHORTENING RATES 29
DISCUSSION 33 Basement lithology 33 Basement warps and offsets 35 Dip of décollement 36 Implication and Comparison of shortening rates 36 Mechanics of thrusting 39 Hydrocarbon prospects 41 Earthquake hazards 42
CONCLUSIONS 45
REFERENCES CITED 47
APPENDICES 52 Appendix A Velocity analysis 53 Appendix B Kangra reentrantnotes and seismic profiles 54 Appendix C Dehra Dun reentrantnotes 60 Appendix D Nahan salientnotes, seismic profile, and cross section 62 LIST OF FIGURES
Figure
Regional tectonic map of the western Himalaya, showing the principal structural elements of the collision 2
Schematic crustal-scale section of the Himalayan collision zone 3
Geologic map of the Potwar Plateau region, showing locations of balanced cross sections used to constrain shortening rates in the Pakistan foreland 7
Geologic map of part of the Sub-Himalaya, showing balanced sections, seismic profiles, and drill holes 8
Generalized stratigraphy of the Himalayan foreland in India 10
Balanced and restored structural cross section (line A-A') 14
Unmigrated seismic-reflection profile Kangra-2 16
Unmigrated seismic-reflection profile Kangra-4 18
Balanced and restored structural cross section (line B-B') 20
Migrated seismic-reflection profile Doon-S 21
Unmigrated seismic-reflection profile Doon-N 24
Magnetic-polarity stratigraphy used to constrain an average sedimentation rate at the southern margin of the Himalayan foreland 30
Long-term sedimentation rates (in m/kyr) from the foreland of India and Pakistan, derived from magnetic-polarity stratigraphic correlations 31
Map showing structure contours of the décollement beneath the Sub- Himalaya of northwest India 34
Diagram comparing shortening rates determined by various methods 37
Diagram showing velocity vectors of the Indian plate 40
Plot of well pressures vs. depth from reported mudweights 43 LIST OF APPENDIX FIGURES
Figure Page
Geologic map of the Sub-Himalaya of northwest India showing location of seismic lines and structure sections discussed in text and appendices 55
Unmigrated seismic-reflection profile Kangra-1 56
Unmigrated seismic-reflection profile Kangra-3 57
Diagram illustrating the complications of projecting exploratory wells in zones of complex structure 58
Unmigrated seismic-reflection profile Nahan-1 63
Structural cross section c-c' across the Nahan salient 66 STRUCTURE AND SHORTENING OF THE KANGRA AND DEHRA DUN REENTRANTS, SUB-HIMALAYA, INDIA
INTRODUCTION
The Himalayan foreland fold-and-thrust belt (Sub-Himalaya) delineates a zone of structural deformation that reflects the ongoing convergence of India and Eurasia
(Fig. 1). Numerous moderate(5.5 7) and a few great (M 8) thrust earthquakes have occurred in the region, but the distribution of seismic activity is not well constrained, because accurate data have only been collected since the 1960s (Molnar, 1984). Records for the past 2 centuries, although not precise by modern standards, delineate seismic gaps where earthquakes have not occurred (Seeber et al., 1981; Yeats et al., 1992). Given the consistent structural style of the Himalayan front between the eastern and western syntaxes in India, the long-term slip rate on faults along the front should be roughly equal over an extended period of time, such as the late Neogene. Therefore, the seismic gaps of the Himalayan front are interpreted as regions where accumulated strain has yet to be released in the form of one or more great earthquakes. When earthquakes do occur on the plate-boundary décollement (Main Himalayan thrust, Fig. 2), they are expressed as shortening in the Sub-Himalaya. In order to evaluate potential hazards from such earthquakes, knowledge of the long-term slip rate on the décollement is necessary. To this end, seismic-reflection, exploratory- well, and surface-geologic data were used to construct balanced cross sections of the Kangra and Dehra Dun structural reentrants of northwest India. The reentrants are ideal for balanced sections because they record the largest amount of late Neogene shortening in the Sub-Himalaya. Outside the reentrants, where imbricate thrusting predominates, only minimum hanging-wall cutoffs can be identified, resulting in poorly constrained section restorations. The reentrants also provide indirect evidence for the position of the Main Himalayan thrust (terminology of Zhao et al., 1993) beneath the Lesser Himalaya, where no direct evidence exists other than the depth to earthquake hypocenters. Specific structural problems addressed include (1) the changing décollement depth beneath deformed Sub-Himalayan strata; (2) features on the basement surface, such as warps and offsets; (3) the configuration of structures within the overlyingstrata, HimalayanFigure 1. Regional Frontal tectonicfault crops map out of discontinuouslythe western Himalaya, along theshowing southern the marginprincipal of structural the Sub-Himalaya elements of(stippled the collision. pattern). The press,Nagrota,MagnetostratigraphicMMT=MainThe map and PH=Pabbiareas Yeats Mantle for et Figs.al., Hills, thrust, section 1992). 3 andPU=Parmandal-Utterbeni, PP=Potwar results 4 are highlightedused Plateau, in this bystudySRT=Salt boxes. R=Rohtas, (filled MBT=Main Range circles): SM=Samba-Mansar thrust, H=Haritalyangar, Boundary ITSZ=Indus-Tsangpo thrust, (adapted J=Jawalamukhi, MCT=Main from suture Jaswal Central zone. JN=Jammu- et thrust,al., in Indo-Ganqetic Plains - - - Sub-Himalaya - ______M Lesser Him. High Himalaya Sr 0 - -s -' - - -' - / / S '-.z------:-- -' - - KILOMETERS INDIAN CRUST:1, I, I - - -r - 50 (VE 1:1) Undeformed foreland50 strata :' MOHO100 150 Indian crystalline basement 200 :" 250 300 Figure 2. Schematic crustal-scale section of the Himalayan collision zone. Principal Himalayan tectonostratigraphic units Deformed-passiveActive foreland fold-and-thrust margin Strata belt Earthquake hypocenters marginHimalayan(MHT)(MCT).are bounded (Vindhyan) ofEarthquake Zhao slab by istheet adaptedal. strata Himalayanhypocenters (1993) atop from may the FrontalSchelling and cutIndian nodalinto fault Shield andbasement planes (HFF), Arita and (Ni (1991)beneaththe and Main Barazangi, and the Boundary Jackson Sub-Lesser and1984) Himalaya. andthrust Lesser Bilhamsuggest (MBT), Himalaya. The (1994).that geometryand the theNote Main Main ofautochthonous Himalayan the Central Lesser thrust thrust passive 4 such as fault-propagation folds, backthrusts, and trianglezones; and (4) the potential involvement of basement in compressional deformation. The cross sections developed in this study suggest that shortening in the Kangra reentrant occurs above a northeast-dipping detachment atop crystalline and metasedimentary basement of the Indian shield. The detachment steepensto the southeast in the Dehra Dun reentrant, where south-vergent thrustingmay involve metasedimentary basement. Structures within the deforming wedgeare steep to overturned in the northern parts of the reentrants; in southern parts, closer to the foredeep, Tertiary strata are gently deformed, and backthrusting iscommon. Imbricate thrusting predominates in narrow parts of the Sub-Himalaya (the Nahan salient and southeast of Dehra Dun). Warps and offsetson the basement surface, observed on seismic profiles and inferred from section balancing, probably affect the location of thrust ramps. The cross sections provide minimum estimates of late Neogene shortening accommodated on faults within the Himalayan frontal zone. Available magnetostratigraphic data (Johnson et al., 1979, 1983; Raynolds and Johnson, 1985; Ranga Rao et al., 1988; Ranga Rao, 1989; Meigs, 1995) have been usedto determine the timing of structural events and to constrain estimates of the long-term sliprate. 5 TECTONIC AND STRUCTURAL FRAMEWORK
The Himalaya represent one of the few places on Earth where continental crust is attempting to subduct beneath continental crust. The Indus-Tsangpo suture zone of Tibet and India marks the Paleogene collision of India with Eurasia (Gansser, 1964; Thakur, 1992). Farther to the south (Figs. 1, 2), Precambrian basement (High Himalaya) and a relatively complete cover of Phanerozoic rocks (Tethys Himalaya) were thrust southward over a discontinuous sedimentary sequence along the Main Central thrust, which was active about 21 Ma (Hubbard and Harrison, 1989). The Precambrian and younger rocks of the High and Tethys Himalaya were originally part of India's northern passive margin. South of the Main Central thrust, Proterozoic and younger rocks of the Lesser Himalaya are thrust southward over the Miocene to Pleistocene Siwalik group, along the Main Boundary thrust. Deformation on the Main Boundary thrust began before 10 Ma (Meigs et al., 1995) and continues today (Valdiya, 1992). At present, the Indian craton is moving north-northeast at a rate of 50 mm/yr relative to the Eurasian plate (Minister and Jordan, 1978; DeMets et al., 1990), with most of this convergence accommodated on faults north of the Himalayan arc (Avouac and Tapponnier, 1993). Molasse of the Siwalik Group, derived from the growing Himalaya, occupies the actively deforming Himalayan front (Karunakaran and Ranga Rao, 1976; Molnar, 1990). Numerous faults crop out south of the Main Boundary thrust, the southernmost termed the Himalayan Frontal fault (Nakata, 1989). The Himalayan Frontal Fault locally cuts Siwalik strata at the surface, but for the most part, it is blind; strain release is thus expressed as anticline growth (Stein and Yeats, 1989; Yeats and Lillie, 1991; Yeats et al., 1992). Structures within the Sub-Himalaya (the zone between the Main Boundary thrust and Himalayan Frontal fault) are consistent with thin-skinned fold-and-thrust deformation above a gently dipping detachment (Seeber et al., 1981). The detachment does not extend southward beneath the undeformed Indo-Gangetic plains, however, as suggested by Seeber et al. (1981). Alternate models, which incorporate basement wrench faults, have also been proposed (Raiverman et al., 1993, 1994b). As the Indian plate underthrusts the Himalaya, it warps down in response to an advancing orogenic load. Given the great size of the Himalaya, and the high rate of erosion due to monsoonal precipitation, there is no shortage of sediment with which to fill the resultant basin. The sediments are time transgressive and have progressively lapped onto the Indian craton with continued convergence (Lyon-Caen and Molnar, 6 1985). Exposure of these strata in the Sub-Himalaya and drill hole data facilitate reconstruction of the Himalayan foreland as it has evolved during the late Neogene. Seismic-reflection profiles and well data from the Sub-Himalaya of northwest India and Pakistan (this study; Lillieet al., 1987) show that south-vergent deformation of platform and foreland strata occurs abovea north-dipping d&ollement. This detachment separates allochthonous sedimentary rocks from autochthonousstrata and basement. In Pakistan, Paleozoic and Mesozoic (platform) through Tertiary (foreland molasse) strata are involved in thrusting; in India, Paleozoic and Mesozoicstrata are absent. The Sub-Himalaya in Pakistan is very broad (-100 km), and structures trend east-northeast (Fig. 3). The width is great because the décollement lies within Eocambrian evaporites (Lillie et al., 1987). Givena weak detachment beneath the central Potwar Plateau, thrusting propagates far into the foreland, with little internal deformation of the overriding thrust wedge (Bakeret al, 1988). Although the eastern Potwar Plateau is also broad, the frequency of faults and folds within the thrust wedge increases (Pennock et al., 1989); this increase is attributed to a decreased basement dip, such that greater topography is required to maintaina critical taper of the thrust wedge. East of the Jhelum syntaxis in India (Fig. 4), the Sub-Himalaya trends southeast and is narrow (30-80 km). The detachment is steeper, and salt is not present at the décollement level; thrust-wedge taper anglesare thus greater (Davis and Lillie, 1994). Structural style in the Sub-Himalayan reentrants of northwest India is similar to that of the Sub-Himalaya in Pakistan. The Kangra and Dehra Dun structuralreentrants, defined by bends in the Main Boundary thrust, exhibit broad anticlines with gently- dipping limbs at their southern margins (Fig. 4). Conversely, fault-propagation folds with steeply-dipping limbs characterize the northern regions of thereentrants. In Pakistan, these variations in style are reflected in the transition from the moderately- deformed Potwar Plateau to the highly deformed north Potwar deformedzone (Fig. 3). 7
710 730 720 74. w
N.P.D.
3 + 90a POTWAR PLATEAU z .y e\t Punjab Plains
Sargodha9 5D 32° DNeogene molasse W + KM PaIeocene to Eocene fPermian to Jurassic Precambrian to Cambrian sedimentary rocks Precambrian lçdian Shield
Figure 3. Geologic map of the Potwar Plateau region, showing locations of balanced cross sections used to constrain shortening rates in the Pakistan foreland: W-'W' (Leathers, 1987), X-X' (Jaswal et al., in press), Y-Y' (Baker, 1987), Z-Z' (Pennocket al., 1989). Note the two southernmost anticlinesat the eastern terminus of the deformation front (PH=Pabbi Hills, R=Rohtas); these anticlinesare useful for comparison with other young structures, such as the Mohand anticline in India. NPDZ=northern Porwar deformed zone, MBT=Main Boundary thrust, SRT=Salt Range thrust. 8
32
, HoshiarpUr Adampur
31- + Sullej R / Rupar q
Siwalik (Lower,Middle,Upper)
Dharmsala
Subathu DS Lesser Himalaya metasediment / Bilaspur Ls. -/ 30 High (crystalline) and Tethys (mstasedimentary) Himalay- Saharanp Seismic-reflection profile o qo B1 Exploratory well KILOMETERS 76 77 78 Figure 4. Geologic map of part of the Sub-Himalaya (location on Fig. 1), showing balanced sections, seismic profiles, and drill holes. Note the great variation in the width of the Sub-Himalaya (MBT to HFF) due in large part to the sinuous surface trace of the MBT. Structures: BA=Balh anticline, BGT=Bihmgoda thrust, BrT=Barsar thrust, BS=Balaru syncline, BT=Bilaspur thrust, DU=Dumkhar syncline, HFF=Himalayan Frontal fault, JMT=Jawalamukhi thrust, LS=Lambargaon syncline, MBT=Main Boundary thrust, MA=Mohand anticline, MCT=Main Central thrust, PA=Paror anticline, PT=Palampur thrust, SA=Sarkaghat anticline, SAN=Santaurgarh anticline, SMA=Suruin-Mastgarh anticline, ST=Soan thrust. Seismic-reflection profiles: DN=Doon- N, DS=Doon-S, K2=Kangra-2, K4=Kangra-4, N1=Nahan-1. Adapted from Karunakaran and Ranga Rao (1976) and from unpublished Oil and Natural Gas Corporation maps. 9 STRATIGRAPHIC SETTING
Foreland stratigraphy coarsens upward from shallow marine strata through conglomeratic molasse (Fig. 5). The base of the section rests unconformablyon Proterozoic rocks of the Vindhyan, Delhi, and Aravalli Groups, whichcrop out on the Indian Shield to the south (Gansser, 1964). Metasediment of the Vindhyan Group isan Indian craton equivalent of Lesser Himalayan rocks; the Delhi and Aravalli Groupsmay correlate with Precambrian crystalline rocks of the High Himalaya (Thakur, 1992). A thin (50 m) section of limestone, the Singtali Formation, rests unconformably on Precambrian rocks of the Lesser Himalaya. Najman et al. (1993) interpret the Singtali Formation to have been deposited in response to initial flexing of the Indian craton, during the Late Cretaceous to early Paleocene. Although these limestones are scarce within the area of this study, they crop out east of the Main Boundary thrust in the vicinity of Simla, where they are overlain by Subathu younger marine strata. The upper Paleocene to upper Eocene Subathu Group is composed of mudstone with minor limestone and sandstone lenses (Karunakaran and Ranga Rao, 1976; Thakur, 1992). These sedimentary rocks reflect further marine transgression, with an influx of detritus from the Himalaya. North of the Main Boundary thrust, the Subathu Group either overlies the Singtali Formation possibly unconformably (Najman et aL, 1993), or rests unconformably on Precambrian to Cambrian rocks of the Lesser Himalaya (Karunakaran and Ranga Rao, 1976). Beneath the Sub-Himalaya (this study), the Subathu Group rests unconformably on the Vindhyan Group. Najman et aI. (1993) report, however, that the Subathu Group thins toward the north, contradicting foreland- basin models that predict northward thickening; they attribute this restricted sedimentation to reactivation of basement faults within the Subathu basin. The Dharmsala Group conformably overlies the Subathu Group and is the stratigraphic equivalent of the Murree Group in Pakistan (Karunakaran and Ranga Rao, 1976). Lower Dharmsala strata (marine to continental sand, clay, and siltstone) reflect final infilling of the shallow marine basin due to increased convergence and uplift of the Himalaya, and a shift from an arid to humid climate (Najman, 1993). Upper Dharmsala strata are mostly sandstone, reflecting deposition by braided rivers on an alluvial plain (Thakur, 1992). The Miocene to Pleistocene Siwalik Group, famous for its abundant vertebrate fossil assemblages, conformably overlies the Dharmsala Group. Siwalik molasse coarsens upward from siltstone with sandstone-clay alternations (Lower Siwalik), to sandstone 10
AGE LITHOLOGY FM DESCRIPTiON VELOCITYTHICKNESS (mis) (m) o conglomerates of plains andduns;nverterraces Ne al and glacial deposits of foothills oS 0 0 000 0 0 conglomerate 0 00 0 0 with increasing sandstone o Upper O'..00 away from MBT; some ...0%° c, Siwalik 2500 2300 0 calcite cementation; clay and siltstone interbeds 0 sandstone with minor claystone; Middle conglomerates prominent Siwalik 3500 2000 .. closer to MBT; appear- ance of kyanite as marker
altemations of sand- Lower stone and claystone : 4 13 00 . .. Siwalik with minor siltstone and pebble horizons
I III Upper greenish grey Dharmsala sandstone with minor 4100 1300 111111 claystone OLU jj - Lower* purple clay and S2 : . :. () Dharmsala siltstone with minor 4300 1300 r sandstone red and green o Subathu nummulitic shales Group 4300 1500 ., minor limestone and sandstone -J LUo
Fm limestone 50+ oa: Ui0 Vindhyan sandstone, siltstone, and Group* limestone, some weakly 4400 metamorphosed
th E Delhi and calc and gamet schist, -1 Aravalli arkosic sandstone, - Groups and marble
Figure 5. Generalized stratigraphy of the Himalayan foreland in India. Velocities are interval velocities used for conversion of seismic profiles from time to depth. Thicknesses are maxima in meters; stratigraphic descriptions are from Karunakaran and Ranga Rao (1976), Thakur (1992), and Najman et al. (1993). An asterisknext to a formation name indicates hydrocarbon source-rock potential (Srivastava et al., 1983; Agarwal et al., 1994; Biswas, 1994). Patterns shown in the lithology column are the same as those used in cross sections A-A' (Fig. 6) and B-B' (Fig. 9). 11 with minor siltstone, claystone, and conglomerate (Middle), to sandstone and conglomerate (Upper)(Karunakaran and Ranga Rao, 1988; Thakur, 1992). Middle and Upper Siwalik sediments differ laterally across the foreland, exhibiting an increase in conglomerate toward the Main Boundary thrust in the northeast (Raiverman, 1983; Thakur, 1992). Lower Siwalik and older strataare typically well indurated, whereas that of the Middle and Upper Siwalikare normally friable. The only exception is in the middle part of the Upper Siwalik, where strata are cemented by calcite (Raiverman, 1983). The Siwalik Group is overlain by Quaternary conglomerate deposited in broad synclines (dun valleys) and in the Indo-Gangetic plains, south of the Himalayan Frontal fault. The Quaternary dun section also includes thick alluvial fans, the heads of which terminate against, or are offset by, the Main Boundary thrust (Sah and Srivastava, 1992; Nossin, 1971). 12 STRUCTURAL CROSS SECTIONS
GENERAL STRUCTURAL DIVISIONS Along-strike variation in structural style of the Sub-Himalaya corresponds with changes in the width of the belt. In the vicinity of Jammu, the Sub-Himalaya contains two distinct structural zones. The northern zone consists of folds and imbricate thrust sheets of Dharmsala/Murree rocks, with minor Siwalik molasse. 'Where the Chenab river leaves the Lesser and High Himalaya, the bases of thrust sheets expose Precambrian Sirban Limestone (equivalent to Bilaspur Limestone?). In contrast, the southernzone exposes only Siwalik rocks that crop out on the flanks of the Suruin-Mastgarh anticline. The northern zone abruptly terminates near the Ravi River, but the structures of the southern zone persist along strike, with the Suruin-Mastgarh anticline representing the northwest extension of the Balh anticline (Fig. 4). Southeast of Jammu (Figs. 4, 6), the Sub-Himalaya widens (to -P80 km) in the Kangra structural reentrant. Widening accompanies a shift in orientation of the Main Boundary thrust (from southeast to east), and a southwestward (forelandward) jump in the Himalayan Frontal fault, relative to the Suruin-Mastgarh anticline. Within the reentrant, fault traces and fold axes are parallel to the Himalayan front. At the northern margin of the reentrant, however, fault traces mimic the trace of the Main Boundary thrust. Contrasting styles of deformation in the Kangra reentrant permit a threefold division of structures (southern, central, and northern). Southeast of the Kangra reentrant in the Nahan salient, where the Sub-Himalaya is narrower (-30 km), the Bilaspur thrust separates folds and imbricate thrusts of Subathu and Dharmsala strata from the less deformed Siwaliks to the south. Oil and Natural Gas Corporation geologic maps show Precambrian Bilaspur Limestone in the hanging wall of the Bilaspur thrust. A seismic-reflection profile across the central part of the Nahan salient (Nahan- 1, Fig. 4, Appendix D) suggests that the imbricate thrust blocks between the Bilaspur and Main Boundary thrusts are cored by highly-reflective Bilaspur Limestone. This thrust stack rests above a detachment within Middle Siwalik strata. Deformation south of the Bilaspur thrust is attributed to slip on a d&ollement above Vindhyan strata; no deep wells, however, constrain this interpretation. In either case, a buried wedge of relatively undeformed Siwalik strata extends at least as far north as the surface trace of the Main Boundary thrust. The Dehra Dun reentrant is similar to the Kangra reentrant, with a tightly folded northern zone (Santaurgarh anticline) and a broad (Mohand) anticline in the 13 south, at the deformation front (Figs. 4, 9). Southeast of Dehra Dun, the Sub-Himalaya is narrow, and imbricate thrusting predominates.
KANGRA REENTRANT (SECTION A-A')
Southern structures Adjacent to the Indo-Gangetic plains, the south-vergent Himalayan Frontal fault and Soan thrust bring Upper Siwalik rocks to the surface in broad anticlines and synclines with little internal deformation (unpublished Oil and Natural Gas Corp. maps). The trace of the Janauri anticline is discontinuous, offset by small faults oriented perpendicular to regional strike; at the southeast end of the anticline, the faultsmerge with small backthrusts. Southwest of the Janauri anticline, the Adampur well penetrates Precambrian quartzite at a depth of 2260 m (left side, Fig. 6). Siwalik units thicken toward the northeast in the Hoshiarpur well, which does not penetrate basement. The Janauri-2 well penetrates 1800 m of Dharmsala strata before reaching Precambrian marble at a depth of 4220 m. From the Indo-Gangetic plains to the Janauri anticline, the Adampur and Janauri-2 wells constrain a basement dip of 2.5°. The Janauri structure does not balance, however, if one assumesa northward continuation of basement with the same dip and stays within the constraints provided by the Janauri wells and surface geology. One could argue that the top of the Dharmsala occurs just below the base of the Hoshiarpur well, which might make balancing possible. This would require anomalous northward thinning of Lower Siwalik strata, however, which is unlikely. It thereforeseems that a minimum basement offset of 400 m occurs north of the Janauri-2 well, north of which the basement continues with shallow dip. Such an offset would be similar to that observed in the Doon-S profile (discussed in next section), where Dharmsala strata are observed onlapping Precambrian crystalline basement. Thickening of Dharmsala strata is also accommodated by a second thrust, which does not break the surface, but which is encountered at 3300 m in the Janauri-1 well. Slip on this thrust is likely responsible for formation ofa second anticline at the northwest end of the Janauri structure. Although the Janauri-1 well gives no indication of thrust vergence, a backthrust is favored,as north-vergent faults cut the southeast end of the Janauri structure; likewise, thereare no dip variations on the line of section that support a south-vergent thrust. 14
.5, Kanqra-2 I-ot Kangra-4 (projected) 10ei Dhauladhar(High Range Himalaya) 2 Adampur INDO-GANGETIC PLAINS Hoshiarpur '9p. Sow, RWe/ _.___.'1_ . 5.,,, Rw., O 5/ 0 Lambargaon Syndine04o A' -2 0 ... 23.4 km shortenn. Palampur Thrust to Janaun Anticline) 264 10 DISTANCE (KM) -ar i6o 108 Q Sw2 NE-2 w - --. :- :;:. -:--- :-... ---4 10 10 0 50 AlluviumUpper Siwalik 100 (VindhyanPrecambrian(DelhiPrecambrian and Group) and Aravalli crystalline Cambrian Groups) basementmetasedimentary rocks Cenozoicmolasse 1::: :1 LowerMiddle SiwalikSiwalik -i Seismic reflection profile shownVindhyan/crystalline4).FigureBoundary Note above6. the Balanced offsetthrust. the unrestored in (top) thecontact. andbasement section. restoredLocations surface HFF=Himalayan (bottom) of seismic-reflectionbeneath structural the FrontalJanauri cross profiles fault, anticlinesection MBT=Main and (line and wells A-A',the areinferred Fig. - riUpper Dharmsala Lower Dharmsala (with thin Paleocene to Eocene Subathu at base) A Exploratory well 15 Where no seismic or well constraints exist, foreland structuresare modeled as fault-propagation (Suppe and Medwedeff, 1990)or fault-bend folds (Suppe, 1983). A fault-bend fold model is used for deformed strata above the Soan thrust, where Upper Siwalik rocks show, in northward progression, gentle (15°-25°) north dips, shallow (00 50) north andsouth dips, then gentle north dips. Slip on a 23° north-dipping thrust, steepening upward to 350, produces the observed surface geometry. Alternatively, the Soan structures could have developedas fault-propagation folds above two more steeply inclined thrusts (Biswas, 1994). Sucha geometry compares with that of the Janauri anticline and similar frontal Siwalikstructures in Nepal (Schelling and Arita, 1991). The Soan thrust, on the other hand, brings only Upper Siwalik and minor Middle Siwalik rocks to the surface, yet cuts the surface along much of its length (Yeatset al., 1992). If they are fault-propagation folds, it is unlikely that the Soan thrust would be exposed. Likewise, surface dips do not define steeply dipping forelimbs, whichare characteristic of fault-propagation folds (Suppe and Medwedeff, 1990).
Central structures North of the Soan Thrust, the Siwalik strata exhibit complex deformation in the right-stepping Dera Gopipur anticline, Balaru syncline, and Balh anticline. The Balh anticline is truncated on its northern limb by the Jawalamukhi thrust. Seismic profile Kangra-2 (Fig. 7), integrated with oil exploratory well data, shows that deformation is restricted to the Tertiary section abovea series of high-amplitude and relatively flat lying reflectors at 3.0 to 3.3 s. Depth conversion indicates that the décollement liesat a minimum depth of 6 km and thatwarps in the subthrust reflectors result from velocity pullup and are not due to a basement offset. Subthrust reflectors showa dip discordance, with more steeply north-dipping high-amplitude reflectors below 3.3s. Although none of the wells penetrate pre-Tertiary rocks, the high-amplitude reflectors below 3.3 s are not characteristic of the Tertiary section andare likely pre-Tertiary rocks of Vindhyan affinity. The reflectors between 3.0 and 3.3s may also represent strata of Viridhyan age, although they could indicate lower Subathuor Singtali strata. Low-amplitude reflectors of Middle and Lower Siwalik strata in the Balaru syncline are visible at the southwest end of Kangra-2. Surface geology shows the Jhor fault cutting and truncating the Balh anticline. From southeastto northwest, the vergence of the Jhor fault switches from south to north, then back to south. Additional profiles northwest and southeast of Kangra-2 show that the Jhor fault is actually the surface expression of two oppositely verging faults, boundinga subsurface triangle zone. 16
A SW JMI-B Tçogrp0y(.rox f1=1550n) (TO=6720rn) JM12 NE (TD$O60n (T0=5O47n 500rn or Jwamuh tu1 ASL :0 75
I-
a
B .JMI-B 0 1 2 3 4 ToogropEy (o,rOx to, 1= 1550 rn) I (TD=6720 ' JMI-2 Dalorn= Bt1 nnbdo, (10=3068 A j(TD=0047m) 500n, BIouyo JIo,Irn,n Jwnr0khH AOL 06001 0
S M S,wk LS:wa5k
UDbaIa
-3
PRO TERTIARY V:ndhogn
Figure 7. Unmigrated seismic-reflection profile Kangra-2 (line K2, Fig. 4; A: uninterpreted, B: interpreted) Line crosses the Jawalamukhi thrust along the A-A' line of section (Figs. 4, 6). Surface outcrop and dip information from unpublished Oil and Natural Gas Corp. maps. Total depths (TD) in wellsare relative to sea level. The high amplitude reflectors below 3.0 s mark thetop of pre-Tertiary strata (-6 km depth). These reflectors are depressed at the southwest end of the section dueto low velocity, Middle Siwalik strata in the Balaru syncline. The line below the décollementat the southwest end of B marks a dip discordance (unconformity?) within the subthrust reflectors. 17 Across Kangra-2, where the Jhor fault is north-vergent, older Middle Siwalik strata overthrust younger Middle Siwalik strata; the Jhor fault thus continues into the subsurface as a north-vergent thrust. Thickening and complex deformation of Upper Dharmsala and Lower Siwalik strata in the footwall of the Jawalamukhi thrust indicate that the south-vergent part of the Jhor fault is laterally persistent, but with differing amounts of slip. Farther to the northwest, the Jhor fault is also south vergent. Here, tight folding of the Balaru syncline and Balh anticline givesway to imbricate thrusting (Appendix B). A similar transition occurs to the southeast along the Dera Gopipur structure, where the anticline gives way to the Barsar backthrust. Steep dips in the bottom 2500 m of the JMI-B well are interpretedto result from fracturing of Dharmsala and Subathu strata and do not represent the true dip of these formations at depth. Likewise, those reflectors not disrupted by noise suggest dips of only 20°-40°. In the central part of the Kangra reentrant, seismic and well data (Fig. 7) constrain a gently north-dipping décollement and subthrust surface. Steeply dipping strata on the limbs of the Dera Gopipur anticline formed through growth of a fault- propagation fold. The fault at the core of this fold, however, roots in a flat within Upper Dharmsala strata. Although the Barsar backthrust representsa major change in structural geometry, it brings Upper Dharmsala strata to the surface, also suggesting a flat in the Upper Dharmsala. It is likely that the Balh anticline is coeval with the Dera Gopipur anticline, having formed over a ramp that roots in the basal décollement. Subsequentto formation of the Dera Gopipur and Balh anticlines,a duplex formed which uplifted the intervening Balaru syncline.
Northern structures The Lambargaon syncline, north of the Jawalamukhi thrust, isan asymmetric fold, exhibiting little internal deformation. The steeply dipping north limb of the syncline forms the forelimb of the Paror anticline. Although seismic profile Kangra-4 (Fig. 8) is 10 km northwest of the A-A' line of section, it nonetheless constrains the subsurface geometry of the Paror anticline and the décollement's depth and dip. The profile shows that undulating, pre-Tertiary basement reflectors deepento 3.8 seconds (-7 km, yielding a décollement dip of 2.5°) beneath the northern part of the Kangra reentrant; deformation is restricted to Tertiary rocks above this horizon. Depth conversion shows that the undulating reflectors represent smooth and gently north- dipping basement. The profile also displaysa certain amount of vertical exaggeration, due to lower formation velocities in theupper 2.5 s. 18
ASW Togrp0y(ppro,. t1=1500n) NE Dturn= PeoF di90 PoJ9por 0WUF 50 900 ,, 70 ASL 0
0 2 3 4 ITop grapEy(appror. for 1rl000m) B OLOMETEkS Dalum= Parc, arr0dirro Palampor Eccot 50k- 000 ,,, 70 ASL U D,msaja
LDharrnsafo
0
F--
SEoc0 Uncen,n
-L0harn,sScDathu. I Dh ralarSobatho
4 PRETERTIARY Vndhy TiDecoiIement
Figure 8. Unmigrated seismic-reflection profile Kangra-4 (line K4, Fig. 4; A: uninterpreted, B: interpreted). Line crosses the Paror anticline in the northernpart of the Kangra reentrant, 10 km northwest ofcross section A-A' (Figs. 4, 6). Surface outcrop and dip information from unpublished Oil and Natural Gas Corp.maps. Note that high amplitude pre-Tertiary reflectorsare at a depth of3.7 s (-7km). North-dipping reflectors on the backlimb of the Paror anticline donot reflect surface dips because the profile is unmigrated. 19 The Paror anticline is a fault-propagation fold, exhibiting anticline breakthrough (Suppe and Medwedeff, 1990). The anticline, which dies outto the southeast, is similar in geometry to the tightly folded Sarkaghat anticline in theeastern part of the reentrant. The fault that cuts the axis of the Paror anticline, however, isnot continuous with the fault at the crest of the Sarkaghat anticline. The Palampur thrust (right side, Fig. 6) is the southernmost in a series of north-dipping imbricate thrusts that bring Dharmsalaage and younger rocks to the surface. The Palampur thrusttruncates the Paror anticline and is itself cut by the Main Boundary thrust.
DEHRA DUN REENTRANT (SECTION B-B')
Southern structures South of the Mohand anticline (Fig. 9), seismic profile Doon-S (Fig. 10) shows undeformed flat-lying Siwalik and Dharmsala strata beneath the Indo-Gangetic plains, above an undulating basement surface. There is also a marked change in the amplitude of pre-Tertiary reflectors, likely revealing a change in basement lithology; Siwalik and Dharmsala reflectors deepen about 0.4 s across this transition. Beneath the Mohand exploratory well, which penetrates Vindhyan strata at 4600 m depth, Siwalik and Dharmsala formation contacts are 0.4 s higher; this apparent rise is not the result of velocity pullup effects. The Himalayan Frontal fault, which brings Middle Siwalik rocks to the surface, truncates Siwalik strata in the northern part of the profile. To the southwest, the Himalayan Frontal fault is north-vergent and breaks the crest of the Mohand anticline as the Bhimgoda backthrust (unpublished Oil and Natural Gas Corp. maps).
Northern structures An apparent unconformity at 2.75 s on the south end of the Doon-N profile (Fig. 11) marks the décollement which dips 6° beneath the Doon Valley. At the northern end of the profile, steep to overturned dips of the Santaurgarh anticline result in poor data quality, precluding subsurface interpretation. The overturned Santaurgarh anticline was modeled as a fault-propagation fold, but the deeper geometry of the structure is uncertain. Two faults cut up from the décollement and deform Dharmsala and Lower Siwalik strata in the central portion of the profile. North-dipping reflectors between 2.8 and 3.5 s in the hanging wall of the northern fault suggest that the décollement steepens Saharanpur Doon-S Doon-N B' (Lesser Himalaya)Mussoorie Hills 2 20 0: oo0%oo00 C0°° ooOO:00% INDO-GANGETIC PLAINS Y/_Y MohaBZr&line 20 ______864 DISTANCE (KM) 50 NE 02 62 6 Alluvium (VindhyanPrecambrian Group) and Cambrian metasedimentary rocks 8 Cenozoicmolasse :1 Lower Siwalik MiddleUpper SiwalikSiwalik Seismic(DelhiPrecambrian reflection and Aravalli crysa1fine profile Groups) basement 5profilesFigure km. MBT=Main 9. and Balanced weils are Boundary(top) shown and aboverestoredthrust. the (bottom) unrestored structural section. cross Total section shortening (line acrossB-B', Fig. the 4).Himalayan Locations Frontal of seismic-reflection fault (HFF) is Dharmsala 4 Exploratory well 21
Figure 10. Migrated seismic-reflection profile Doon-S (line DS, Fig. 4; A: uninterpreted, B: interpreted) Line extends from the Indo-Gangetic plains to the Mohand anticline along the B-B' line of section (Figs. 4, 7). Surface outcrop and dip information from unpublished Oil and Natural Gas Corp. maps. Total depth (TD) in the Mohand well is from sea level. Approximately 6.5 kilometers of seismic profile has been cut out to highlight uplift of Siwalik strata beneath the Mohand anticline and resultant growth strata. Note the difference in seismic signature of pre-Tertiary units between the north and south ends of the profile. Dharmsala strata onlap crystalline rocks across this basement contact. HFF=Himalayan Frontal fault. 22
Sea LevelDatum= SW Topography (approx. for I s = 1500 m) 6.5 KM Mohand (TD=5264 m) 28 40 25 25NE
Figure lOa 0I 1 I 2 I KILOMETERS 3 4
24
Figure 11. Unmigrated seismic-reflection profile Doon-N (line DN, Fig. 4; A: uninterpreted, B: interpreted). Line crosses the Doon valley along the B-B' line of section (Figs. 4, 7). Note the dip discordance at 2.7 s marking the décollement on the south end of the profile.
Sea LevelDatum= B Topography (approx. loris = 1500 n) 90
Figure lib 0 2 KILOMETERS 3 4 5 27 beneath the northern part of the Doon Valley. Likewise, Rao (1986)notes that alluvial fans in the northern part of the reentrant have been uplifted 800m relative to coeval deposits in the Doon Valley. Given 5 km of slip (Fig. 9), the décollement beneath the Santaurgarh anticline may dip as muchas 15°. 28 RESTORED CROSS SECTIONS AND SHORTENING AMOUNTS
When restored to their prethrust configurations, balanced cross sections of foreland fold-and-thrust belts can provide estimates of tectonic shortening,as has been demonstrated in the Sub-Himalaya of Pakistan (Pennock et al., 1989; Bakeret al., 1988; Leathers, 1987). Balanced sectionsassume a transport direction parallel to the line of section, with little or no movement of material in or out of the section. This assumption is warranted for the Sub-Himalaya of India as there isno documented evidence of long- term arc-parallel strike-slip faulting. Cross section A-A' (Fig. 6) was selected in order to maximize seismic and exploratory well coverage within the Kangra reentrant. The line terminates at the Palampur thrust for two reasons: first, the northernmost datableexposures of Upper Siwalik strata occur north and south of the Paror anticline, and second, there isno hanging wall cutoff for the Palampur thrust. Line length and area balancing of cross section A-A' shows that a minimum of 23 km (22% of 106 km) shortening has occurred across the Kangra reentrant. This agrees with a value of 22% derived from a total volume balance of the restored section (22% material eroded) Cross section B-B' (Fig. 9)was constructed along the lines of the Doon-S and Doon-N seismic profiles, and the Mohand well was projected -10 km northwestonto the line of section (Appendix C). Although structural complications exist (for example, uplift of Siwalik strata beneath the Mohand anticline), they representa negligible amount of shortening. Line-length balancing along Lower Siwalik strata shows that 11 km (26% of 42 km) shortening has occurred along section B-B', with 5 km accommodated across the Mohand anticline. 29 SHORTENING RATES
Although balanced and restored cross sections provide reliable shortening amounts across fold-and-thrust belts, these values are of little use unless constrained by a period of time over which the shortening took place. In the foreland of Pakistan, detailed magnetostratigraphic studies provide timing constraints on foreland deformation, but in India, there are few such constraints. Two measured and dated sections (Fig. 12), one near Jawalamukhi (Meigs et aI., 1995) and another at Haritalyangar (Johnson et al., 1983), provide ages for Lower, Middle and Upper Siwalik strata on the southern limb of the Lambargaon syncline; poor exposures of the Upper Siwalik section in the core of the syncline, however, preclude dating of youngest Siwalik strata. On the other hand, an age can be estimated if the full thickness of Upper Siwalik strata and representative sedimentation rates are known. Balanced cross section A-A' (Fig. 6) shows that Upper Siwalik strata in the core of the Lambargaon syncline are at least 2300 m thick. This value agrees with a reported thickness of 2300 m for the Upper Siwalik near the Sutlej River in the southwest part of the reentrant (Karunakaran and Ranga Rao, 1976). It is clear from the restored section that all Tertiary formations thicken toward the north, thus this value serves as a minimum estimate. Average sedimentation rates from Pakistan and India vary from 0.36 to 0.53 m/kyr (Fig. 13). These extreme values, however, are derived from sections that span only 3 Myr and therefore may not be representative of longer term rates. The 0.36 m/kyr value, from the Pabbi Hills section (Raynolds and Johnson, 1985) represents an average of low (-0.23 m/kyr) and high (-0.50 m/kyr) rates which result from interactions between foredeep migration and tectonic deformation. Likewise, the Pabbi Hills section is at the axis of the Jhelum reentrant where the sedimentation rate was generally higher (Raynolds and Johnson, 1985). The high 0.53 m/kyr value from Haritalyangar may also result from deposition along the axis of a trunk river between the margins of the Kangra reentrant (Meigs et al., 1995). Raynolds and Johnson (1985) demonstrate in the Jhelum reentrant that the sedimentation rate was higher south of the deformation front. Only when structures began to attain surface expression did the sedimentation rate slow dramatically. Taking this into account and disregarding extreme values, the sedimentation rate along the Himalayan front for the past 6 Myr is on the order of 0.42-0.46 mlkyr. 30
NW0 SE PH JNSM R
cri 4 J U)
Cl) ci) Us 6 E >
2- aS 800 C.) 9 .. U) KM C 0) 10 10aS LS<.... E 11 11
12 12 -Normal ri Reversed 13 13
Figure 12. Magnetic-polarity stratigraphy (locations, Fig. 1) used to constrain an average sedimentation rate at the southern margin of the Himalayan foreland. Stratigraphies are tied to the magnetic polarity time scale of Cande and Kent (1992). Dashed lines showa southeastward thickening of Middle Siwalik strata, which may be due to the more proximal locations of the Jawalamukhi and Haritalyangar sections to the Main Boundary thrust. Stars mark dated tuff beds; tie lines represent published correlations. Lithostratigraphy: LS=Lower Siwalik, MS=Middle Siwalik, US=Upper SiwalikDated sections:R=Rohtas (Johnson et al., 1979), PH=Pabbi Hills (Johnson et al., 1979), JN=Jammu-Nagrota (Ranga Rao et al., 1988), PU=Parmandal-Utterbeni (Ranga Rao et al., 1988), SM=Samba-Mansar (Ranga-Rao, 1989), J=Jawalamukhi (Meigs et aL, 1995), H=Haritalyangar (Johnson et al, 1983). Pakistan Foreland 6 Jammu Kangra 3 5 4 Age (Ma)3 2 0 2- 12 10 Age (Ma)8 6 4 2 0 0 14 12 Age (Ma) 10 8 6 4 discussion.polarityFigure 13. stratigraphic Long-term correlations sedimentation in Fig. rates 12. (in Section m/kyr) name from abbreviations the foreland ofare India the same and Pakistan,as those in derived Fig. 12. from See magnetictext for 32
The magnetostratigraphic section at Jawalamukhi (Meigs et al., 1995)covers 1025 m of Upper Siwalik strata leaving 1275 m of unmeasured section. Allowing fora potential error of ±25 m and assuming no change in sediment accumulationrate, this yields a 1.5-1.9 Ma minimumage for the youngest Siwalik strata in the northern part of the Kangra reentrant. Incorporatinga shortening amount of 23.4±0.5 km and assuming all shortening post dates 1.5-1.9 Ma givesa shortening rate of 14±2 mm/yr. This rate is a minimum, as it does not account for an unknown amount of shortening that may have occurred on the Main Boundary thrust and other faults in the northernpart of the reentrant since 2 Ma. The timing of structural events across section B-B' is not well constrained, and thus shortening rates can be determined only through comparison withages of Siwalik strata and structures from other locations. In the vicinity of Jammu, Ranga Rao et al. (1988) dated Upper Siwalik strata on the south flank of the Suruin-Mastgarh anticlineas 0.7-0.8 Ma (Parmandal-Utterbeni section, Fig. 12); if all the shortening in the Dehra Dun reentrant occurred since that time, then the shortening rate is 14±1 mm/yr. This rate could be higher or lower, however, because growth of the Santaurgarh anticline may have begun prior to this time, andan unknown amount of shortening has also occurred on the Main Boundary thrust. Alternatively, one can compare the Mohand anticline to the Pabbi Hills anticline, the southernmost structure in the eastern Potwar Plateau (Fig. 3). Initial deformation of the Pabbi Hills structure occurred at 0.7 Ma, with attainment of surface expression at 0.4 Ma (Johnsonet aI., 1979). Applying this age range to the 5 km shortening accommodated across the Mohand anticline yieldsa shortening rate of 7-12 mm/yr. 33
DISCUSSION
Structural style within the Sub-Himalaya of northwest India is highly variable. Where the Sub-Himalaya is wide, the southernmost structures are typically broad anticlines, whereas northern structures are fault-propagation folds with steep to overturned limbs. This geometry may be due to increased internal strength and age of the rocks at the base of the Tertiary section within the northern parts of the Dehra Dun and Kangra reentrants. Such rheologic differences cause faults in northern areas to cut up steeply from the décollement. In the southern parts of the reentrants, the Tertiary sequence consists only of Siwalik rocks, which are much weaker, and faults propagate to the surface at a lower angle. The broad zones contrast with narrow parts of the Sub- Himalaya where imbricate thrusting is common.
BASEMENT LITHOLOGY The basement (subthrust) lithology changes beneath the Sub-Himalaya, and it is possible to map the southern margin of the Precambrian Vindhyan basin (Fig. 14). The location of the contact between the crystalline and Vindhyan metasedimentary units is known from the Doon-S seismic profile (Fig. 10), on the basis of contrasting seismic signatures. To the northwest, the contact must lie somewhere between the Janauri-2 well and the Jawalamukhi thrust. Because balancing of the Janauri anticline requires a basement warp, the contact is placed at the warp, as such an offset is associated with the contact south of the Mohand anticline. Although there is a parallelism between the basement contact location and the surface trace of the Himalayan Frontal fault, their relationship is circumstantial. With continued convergence, the Himalayan Frontal fault likely will shift to the south of the basement contact. The two are related in the vicinity of the Janauri anticline in as much as an inferred offset across the basement contact caused ramping ofthe décollement. These data are important because balanced cross sections of the entire Himalayan collision (Schelling and Arita, 1991; Srivastava and Mitra, 1994) often rely upon stratigraphic thicknesses within the Lesser Himalaya to predict the geometryof Vindhyan strata beneath the foreland. In Nepal, this may be a safe assumption, given that the Vindhyan basin is much more extensive. In northwest India, however, it is unlikely that Vindhyan strata thicken from 0 to 10 km across the width of the Sub- Himalaya. 34
, ,,, , !',, , ',,,,,,,,,,,,,, , , , ,
\\fl.' \\\\\\\'%\\\... , , , , , S.lI'7 ,,,,,, ,, , , ,,,,,,,,, , , , , , , 32° , ,l-, , S.'. S.'. S.'.'. %\\\\\'v,,,_l.,,,,,'.'.'.S.'.'.\\'.'. v '.\'.'.,,,,,, '.'.'.'..-_..S.''. '.'.'.'.'.'. S.''''.'.S.S.S.S.S.'.'. ' .°.!. .'-V S. S.'.
+ 1- ' -- - .1-''- 31° ,,,,J ___S.<
;4. f 'S
+4%,:. l;..2 Indo-Gangetic Plains . 30° 9,.. iqo KM \ - I I t 76° 77° 78°
Figure 14. Map showing structure contours of the décollement beneath the Sub- Himalaya of northwest India (solid lines; contours relative to sea level). The discontinuous trace of the Himalayan Frontal fault (dashed line with solid triangles) marks the zero contour, where the d&ollement ramps to the surface. Well tops and seismic profiles (heavy gray lines) are shown where they were used to constrain a depth to the d&ollement; wells which penetrate basement are filled. Structure contours reflect the simplest interpretation of the available data; thin black lines on the 6000 and 7000 contours mark an alternate interpretation that would support a lateral ramp beneath the Lesser Himalaya. The dotted black line marks the subsurface contact between crystalline (Deihi/Aravalli) and metasedimentary (Vindhyan) basement. HFF=Himalayan Frontal fault, MBT=Main Boundary thrust, MCT=Main Central thrust. 35
BASEMENT WARPS AND OFFSETS Basement offsets likely have a significant effect on the location of structures within the Sub-Himalaya, as they do in other mountain belts (Wiltschko and Eastman, 1983). A lack of continuous seismic profiles across the Kangra reentrant, however, precludes such interpretations, except where they are required by section balancing (Janauri anticline). Raiverman et al. (1994) report several down-to-the-north normal faults south of the Mohand anticline. These structures were mapped on seismic profiles from the Indo-Gangetic plains and are consistent with observations from the Sub- Himalaya in Pakistan (Baker et al., 1988). In addition, Vindhyan strata are offset beneath the Bilaspur thrust in the Nahan salient (Appendix D). This feature likely formed in response to normal faulting, as similar offsets within Vindhyan strata to the east are common (Karunakaran and Ranga Rao, 1976; Srivastava et aI., 1983). Normal offsets within the Vindhyan basin may relate to Paleozoic rifting, as the faults do not seem to affect overlying Siwalik strata. The zone of intense deformation south of the Jawalamukhi thrust is similar to the northern Potwar deformed zone. Both regions exhibit north- and south-vergent thrusts and duplexing, and persist for many kilometers along strike. Given these similarities and the presence of a basement warp beneath the northern Potwar deformed zone (Jaswal et al., in press), it is possible that the structures of the central Kangra reentrant also formed above a perturbation in the basement surface. A curious feature of the Doon-S seismic profile is the apparent southward tilting and uplift of Siwalik strata beneath the Mohand well, which cannot be attributed solely to velocity pullup. Growth strata south of the Mohand anticline (Fig. 10) suggest that this feature formed during deposition of the upper part of the Upper Siwalik. The observed uplift may be the result of down-to-the-south normal faulting, related to flexure (Duroy eta1., 1989), butno noticeable offset at the base of the Tertiary section supports this interpretation. Alternatively, gentle south dips beneath the Himalayan Frontal fault may correlate with the south dips observed at the surface where the fault is expressed as the Bhimgoda backthrust; this interpretation, however, would require a tip line within Vindhyan strata, as no fault is encountered near the base of the Mohand well. Either interpretation requires deformation of Vindhyan strata beneath the Mohand structure, which contrasts with the Kangra reentrant where deformation is restricted to the Tertiary section. 36 DIP OF DECOLLEMENT The active décollement beneath the Sub-Himalaya of northwest India steepens from 2.5° in the Kangra reentrant to 6° in the Dehra Dun reentrant (Fig. 14). These findings have important implications for modeling the entire collision zone from the Indo-Gangetic plains to the High Himalaya. The observed change in dip of the décollement may reflect a change in the geometry of the Main Himalayan thrust beneath the Lesser Himalaya here. The structure contour map was compiled from the two cross sections presented here and other seismic data (Appendix B, D), but a lack of control points in the eastern part of the Kangra reentrant complicates interpretation. Contours may instead parallel the trace of the Main Boundary thrust where it strikes northward. If so, the sinuous trace of the thrust could be related to a subsurface lateral ramp, as suggested by Molnar (1987) in his studies of Middlemiss' (1910) account of the Kangra earthquake.
IMPLICATION AND COMPARISON OF SHORTENING BATES Although little knowledge of the history of activity on the Main Boundary thrust in the area of this study exists, the 14±2 mm/yr Indo-Himalayan shortening rate implies that the Main Boundary thrust has not been very active during the past 2 Myr. If a large amount of shortening had occurred, then the total Indo-Himalayan convergence rate (Sub-Himalaya pius Main Boundary thrust shortening) would likely be too high. This conclusion agrees with other data on the Main Boundary thrust from other parts of the Himalaya. In Pakistan, Burbank and Raynolds (1986) demonstrate that the Main Boundary thrust was active at 2.1-1.8 Ma but has been inactive since that time. In Nepal, the Main Boundary thrust has been active recently but exhibits only down-to-the-north normal and right-lateral strike-slip motion (Nakata, 1989). In order to evaluate earthquake hazards and constrain a recurrence interval of great earthquakes, knowledge of an average frontal shortening rate is necessary. For this reason I reevaluate rates determined from balanced cross sections of the Potwar Plateau and compare them with rates from this and other studies (Fig. 15). Under ideal conditions, the locus of foreland deformation and the age of resultant unconformities are precisely known. In the Himalayan foreland of Pakistan, however, where out-of-sequence thrusting is common, shortening rates are minimum estimates and may not represent the total Indo-Himalayan convergence. In the western Potwar Plateau (Fig. 3), Leathers (1987) determined a shortening rate of 13 mm/yr based on 38 km shortening between the Soan syncline and Salt Range thrust over a 25-30- 1020-15- ConvergenceAverageRateHimalayan = 14±4 Indo- mm/yr 5- 1 3 5 6 0 I 2I I 4 I P I I I I I 7 1 Figure1987; 15. (2) Diagram Baker, 1987; comparing (3) Pennock shortening et al., rates 1989; determined (4) Jaswal byet al.,various in press; methods. (5) Lyon-Caen See and Molnar, 1985; (6) Avouac /'f text for discussion. (1) Leathers, publishedproducedand Tapponnier, thevalues values 1993;and for the (7) the dashed Jackson Salt Range lines and mark thrustBilham, (SRT) 1994. Combinations of Baker (1987) and Jaswal/4,:, error bars. NPDZ=northern Potwar deformed zone, SRIPP=Salt to Main Boundary thrust (MBT). The black rectangles mark et al. (in press) data Range/Potwar Plateau. , " , 38 3 Myr period. Termination of folding in the Soan syncline at 1.9 Ma (Johnson et al., 1986) in the eastern Potwar Plateau could also constrain the timing of deformation, suggesting that shortening rates may be as high as 20 mm/yr. In the central Potwar Plateau, Baker et al. (1988) determined a rate of 9-14 mm/yr based on shortening between the Soan syncline and the Salt Range thrust. In the eastern Potwar Plateau, Pennock et al., (1989) determined a rate of 7 mm/yr based on 17.8 km shortening since 2.5 Ma, between the Domeli anticline and the Pabbi Hills structure. The 2.5 Ma age (Johnson et al., 1986), however, marks unroofing of Eocene strata on the Domeli thrust and not "surface expression." Thus total frontal shortening since 2.5 Ma is -10 km, yielding a shortening rate (4 mm/yr) that is clearly too low. One can also derive a rate for the eastern Potwar Plateau by subtracting shortening before 2.1 Ma (age of Soan syncline folding) from the total (24 km). However, the rate remains -7 mm/yr. Alternatively, a shortening rate can be determined by considering only the Pabbi Hills and Rohtas anticlines, which attained surface expression at 0.4 Ma (Johnson et al., 1979). In as much as the age of surface expression marks the onset of measurable shortening, then rates within the eastern Potwar Plateau may be as high as 12 mm/yr (4.8 km since 0.4 Ma). All of the above estimates do not include shortening after 2.1 Ma north of the lines of section between the Soan syncline and Main Boundary and Attock thrusts (Burbank and Raynolds, 1988). Jaswal et al. (in press) determined a large shortening rate of 22 mm/yr within the northern Potwar deformed zone on the basis of 69 km shortening between the Soan syncline and Main Boundary thrust from 5.1-2.0 Ma. Poor seismic-reflection data in the northern part of this section suggest that shortening amounts may be smaller. An earlier estimate of shortening across the northern Potwar deformed zone by Baker (1987) suggested a shortening amount of 45±15 km. Taking 30 km as a minimum amount of shortening yields a rate as low as 10 mm/yr. A combination of data from Baker et al. (1988) and Jaswal et al. (in press) yields the best range of shortening rates, which likely bracket the true shortening rate. Assuming that 50 to 93 km shortening occurred since deposition of the youngest Siwalik strata in the central Potwar plateau and northern deformed zone at 5.1 Ma (Johnson et al., 1986), then the Indo-Himalayan convergence rate falls between 10 and 18 mm/yr. Indo-Himalayan convergence rates have also been determined by methods other than balanced cross sections. Lyon-Caen and Molnar (1985) determined a rate of 10-15 mm/yr (and possibly as high as 20 mm/yr) based on the rate at which Tertiary 39 sediments transgressed the Indian craton. Avouac and Tapponnier (1993) calculated a residual convergence rate of 18 mm/yr by subtracting all shortening accommodated on faults north of the High Himalayas from the total Indo-Eurasian convergence rate. Lastly, Jackson and Bilham (1994) determined a rate of 9-18 mm/yr based on uplift rates of the High and Lesser Himalaya in Nepal. Indo-Himalayan convergence rates range from 7 to 22 mm/yr (Fig. 15). These end-member rates are likely extreme values, however, as they are both based on poorly constrained shortening amounts. Rather, a more likely average rate is 14±4 mm/yr, which brackets most of the values summarized here. It is interesting to note that thrust fronts within the Sub-Himalaya of northwest India are highly oblique (45°-70°) to the Indo-Eurasian convergence direction (Fig.16a). Given this geometry, there is surprisingly little evidence of temporally persistent strike- slip faulting. Nakata (1989) identifies right-lateral strike-slip faults in the Sub-Himalaya of Nepal, but these lie where the Himalayan front strikes eastward. Raiverman et al. (1993) relate right-stepping anticlines south of the Jawalamukhi thrust to basement wrench faulting, but these structures more likely result from minor right-lateral transpression within the thrust wedge. In addition, Molnar (1990) demonstrates that P- axes and fault plane solution azimuths are perpendicular to the Himalayan front. These observations suggest that strain partitioning occurs in the Himalayan collision, as it does in oceanic subduction zones. In the Sub-Himalaya of northwest India, all material south of the Indus-Tsangpo suture is overthrusting the Indian plate perpendicular to the arc (Fig 16b, c). The required strike-slip component is thus accommodated about 300 km inboard of the deformation front, along the Karakoram fault. Strain partitioning in the Himalaya is similar to that observed in the Sumatran arc, where the right-lateral Sumatran fault is situated 300 km inboard of the Java Trench (McCaffrey, 1991).
MECHANICS OF THRUSTING The Sub-Himalaya of northwest India and Pakistan provides good examples of changes in mechanical response due to rocks of differing strengths (Davis and Lillie, 1994). Although the Kangra reentrant is very wide and has a narrow cross-sectional wedge taper (-4°), this taper is not likely due to salt at the dcollement level, as in Pakistan (Davis and Lillie, 1994). The Precambrian Shah Group, which includes minor evaporites, crops out at the northern margin of the reentrant (Srikantia and Sharma, 1976); Yeats and Lihhie (1991) suggest that these strata may contribute to the broad width of the Sub-Himalaya there. Variation in the strike of the Main Boundary thrust, - EURASIA 50mm INDIA withinHimalayan(1990);forelandFigure the 16. the Himalayathrusting (A) southernFrontal Diagram is offault vectoroblique northwest showing(HFF) is to from Indianand India velocity BUrgmannthe (boxplate Karakoram vectors inmotion. A). et al. ofAt fault, The(1996). thethe Indiannorthernsouthernthrust (B) plate.vergence Diagram vector,edge Note of isIndia(not the inperpendicular theHimalayanto relative scale) area under illustratingto block,toEurasia, thrustinvestigation between strainistraces from partitioning (large the (box)DeMets arrowthat et al. (HIM)northwestKarakoramarrows)extending is leaves from India,from fault. aAvouac Karakoram componentwith MBT=Main Tibet and Tapponnierasfaultof Boundary aoblique single to HFF). rigidconvergence (1993),thrust. North-northeast block. (C)and VelocitiesSimplified thatthe Himalayanis likelydirected of velocity-space the accommodated blockconvergenceTibetan relative block diagram bybetween to(TIB) right-lateral India of therelative India(IN) Himalayan is movementand to based the Eurasia Himalayan collisionon onthis (small the study. in block gray Velocity of India relative to the Tibetan block is speculated. C 41 however, controls the shape of the Kangra reentrant. Likewise, where the Himalayan Frontal fault steps southward into the foreland relative to the Suruin-Mastgarh anticline, deformation is restricted to the Tertiary section, which does not include evaporites. The broad width of the Sub-Himalaya thus results largely from a preexisting structural feature rather than a weak d&ollement; whether the surface geometry of the Main Boundary thrust is affected by salt at the Main Himalayan detachment beneath the Lesser Himalaya is unclear. The Kangra reentrant bears both south- and north-vergent thrusts, with a frequency that likens it to the eastern Pot-war Plateau (Pennock et al., 1989). Although the mechanical response to shortening in these two regions is similar, the greater rock strength at the décollement in the Kangra reentrant results in a greater taper angle of the thrust wedge (4° vs 1.6° for the eastern Potwar Plateau). To the southwest, in the Nahan salient, the internal strength of the rocks (Bilaspur Limestone) involved in thrusting increases. This increased strength is reflected in an increased décollement dip and topographic slope, such that the taper angle is 6°-7°.
HYDROCARBON PROSPECTS Although the Oil and Natural Gas Corporation has had an ongoing program of exploration in the Sub-Himalaya, no economically viable oil or gas discoveries have been made. Unlike Pakistan, there are no hydrocarbon-bearing Cambrian through Mesozoic platform strata. On the basis of this and other studies, there are two potential source rock groups. The first is the Subathu and Lower Dharmsala, which have been identified as having high organic carbon contents and which are mature (Agarwal et al., 1994; Biswas, 1994). Numerous gas shows along the Suruin-Mastgarh anticline and Jawalamukhi thrust led the Oil and Natural Gas Corporation to study these structures in detail, but deep wells did not encounter commercial hydrocarbons. Existing surface shows likely result from southward migration of hydrocarbons beneath the Kishandpur- Mandili (north of the Suruin Mastgarh anticline in the vicinity of Jammu) and Jawalamukhi thrusts, as suggested by Acharyya and Ray (1982). The wedge of Siwalik strata imaged beneath the Nahan salient (Appendix D) lends further support for this interpretation. Gravity models of the foreland also suggest that some low density Tertiary strata extend beneath the Lesser Himalaya (Lyon-Caen and Molnar, 1985) The second potential source rock may be found in the Vindhyan Group. In places, Vindhyan rocks are weakly metamorphosed (as beneath the Mohand anticline) but not throughout most of the Vindhyan basin to the east. Srivastava et al. (1983) 42 identify shale sequences within the Vindhyan group containing high organic carbon. These shales, however, are restricted to the eastern part of the Vindhyan basin. On the other hand, the results of this study clearly show that Vindhyan strata lie beneath the Sub-Himalaya. One cannot, therefore, rule out the possibility that subthrust reflectors on seismic profiles mark strata capable of generating and storing hydrocarbons.
EARTHQUAKE HAZARDS On April 4, 1905, the décollement beneath the Sub-Himalaya of northwest India ruptured in the great (M=8) Kangra earthquake (Ni and Barazangi, 1984). Isoseismals, drawn after the event (Middlemiss, 1910), indicate two zones of strong ground motion: one (max intensityX) focused around Kangra in the northwest part of the reentrant, and a second (max intensity Vil-Vill) centered around Dehra Dun. Molnar (1987) suggested that this apparent shaking pattern could be attributed either to rupture on two fault segments with relatively aseismic creep between, or to rupture on one fault segment with less strain release around Dehra Dun. Assuming an average Indo-Himalayan convergence rate of 14 mm/yr and 5 of slip during a great earthquake such as the Kangra event, the recurrence interval for such an earthquake is -350 years. Although this number is a general estimate, given the uncertainty of both rate and slip amount, it shows that the potential for catastrophic loss along the Himalayan front is very high. Interestingly, the Nurpur, Jawalamukhi-B, and Balh wells are increasingly overpressured with depth, whereas the Janauri well is not (Fig. 17). Although overpressuring is to be expected based on the Coulomb wedge theory of Davis et al. (1983), where the wedge is always close to critical failure, one would expect similar overpressures throughout the thrust wedge. Assuming that the increased overpressures are of tectonic origin, as in the Lilla well south of the Salt Range thrust (Jaum and Lillie, 1988), then perhaps strain is accumulating along this zone of tight structures within the Kangra reentrant. This situation is plausible, as interseismic strain accumulation is documented across the Dehra Dun reentrant (Yeats and Lillie, 1991; Gahalaut and Chander, 1991). Although the sources of great Himalayan thrust earthquakes are located on the Main Himalayan thrust beneath the Lesser Himalaya (Ni and Barazangi, 1984; Molnar, 1990), smaller events may originate closer to the Main Boundary thrust. The evaporite-bearing Shah Group crops out on the north edge of the Kangra reentrant, suggesting that the Main Himalayan thrust may bear a flat within a layer of ductile strata north of the reentrant. Yeats and Lilhie (1991) point out that this 43
0
2000-
4000-
6000-
8000 20 40 60 80 100 Pressure (MPa)
Figure 17. Plot of well pressures vs. depth (relative to well tops) from reported mudweights. Note that the Mohand, JMI-B, and Janauri wells are close to the hydrostatic gradient, whereas the Balh and Nurpur wells become more overpressured with depth. The increase in overpressurization of JMI-B occurs in the footwall of the Jawalamukhi thrust (Fig. 5). 44 may effectively weaken the d&ollement, such that moderate earthquakes may be more common than in adjacent reaches of the Himalaya where the basal thrust is locked. 45 CONCLUSIONS
Seismic-reflection profiles, surface geology, and well data integrated in this study place important constraints on the structural development of the Sub-Himalaya of northwest India. The important conclusions are as follows: The Sub-Himalaya of northwest India is a foreland fold-and-thrust belt that is detached from pre-Tertiary basement along a shallow dipping décollement. in the Kangra structural reentrant, the d&ollement dips 2.5°, and in the Debra Dun reentrant, 6°. This steepening may reflect a lateral ramp on the Main Himalayan thrust between the Kangra and Dehra Dun reentrants, and beneath the Lesser Himalaya; only balanced cross sections of the Lesser and High Himalaya, geodetic surveys, or deep seismic- reflection profiles, however, will document this. Structural style in the reentrants changes toward the south. In the north, where the detachment is deep and rock strength great, the structures are fault- propagation folds with steep to overturned limbs. In the south, where the detachment is shallower and rock strength weaker, foreland strata are gently deformed. The reentrants contrast with narrow parts of the Sub-Himalaya where imbricate thrusting is common. A basement warp or offset promotes ramping of the Sub-Himalayan thrust sheet beneath the Janauri anticline. Likewise, perturbations on the basement surface are documented in seismic data beneath the Indo-Gangetic plains. On the basis of similarities between the Sub-Himalaya of India and Pakistan, there may be other warps or offsets that have controlled the location of thrust structures. The southern margin of the Vindhyan basin parallels the southern edge of the Sub-Himalaya. In order to model the India-Eurasia collision from the Indo- Gangetic plains to the Indus-Tsangpo suture, knowledge of the paleogeometry of the downgoing Indian slab is critical. These data suggest strata of India's northern passive margin are much thinner beneath the décollement of northwest India than in the overlying Lesser Himalaya. Palinspastic restoration of cross sections shows that 23 km of shortening has occurred in the Kangra reentrant since 1.5-1.9 Ma. The Indo-Himalayan convergence rate is thus 14±2 mm/yr in northwest India. These data agree with a comparison of shortening rates from other locations and by different methods, which suggests that the average Indo-Himalayan convergence rate is 14±4 mm/yr. These data also exclude the possibility of significant activity on the Main Boundary thrust during the past 2 Myr; otherwise shortening rates would be too high. 46 Strain is likely accumulating in the central part of the Kangra reentrant and along the Suruin-Mastgarh anticline. These data, pius regional geologic observations, suggest that these regions may experience a high frequency of moderate magnitude earthquakes, rather than one great event. Siwalik and Dharmsala strata extend beneath the Nahan salient andmay extend beneath the Lesser Himalaya. Surface geology north of the Suruin-Mastgarh anticline is similar to that of the Nahan salient; undeformed foreland stratamay thus lie beneath the thrust wedge here. If so, the undeformed subthrust strata area likely source of gas shows along the Suruin-Mastgarh anticline and south of the Jawalamukhi thrust. 47 REFERENCES CITED
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Rao, D.P., 1986, An approach to the quantitative estimates of rates of recent movements: The Sub-Himalayan example: Royal Society of New Zealand Bulletin, v. 24,p. 507-517. Raynolds, R.G., and Johnson, G.D., 1985, Rates of Neogene depositional and deformational processes, northwest Himalayan foredeep margin, Pakistan, in Snelling, N.J., ed., The chronology of the geologic record: Geological Society of London Memoir 10,p. 297-311. Sah, M.P., and Srivastava, R.A.K., 1992, Morphology and facies of the alluvial-fan sedimentation in the Kangra Valley, Himachal Himalaya: Sedimentary Geology, v. 76,p.233-242. Schelling, D., and Arita, K., 1991, Thrust tectonics, crustal shortening, and the structure of the far-eastern Nepal Himalaya: Tectonics, v. 10,p.85 1-862. Seeber, L., Armbruster, J.G., and Quittmeyer, R.C., 1981, Seismicity and continental subduction in the Himalayan arc, in Gupta, H.K., and Delany, F.M., eds., Zagros, Hindu Kush, Himalaya, Geodynamic Evolution: American Geophysical Union Geodynamic Series, v. 3, 2 15-242. Srikantia, S.V., and Sharma, R.P., 1976, Geology of the Shah belt and the adjoining areas: Geological Survey of India Memoirs, v. 106, p. 3 1-166. Srivastava, B.N., Rana, M.5., and Narendra, K.V., 1983, Geology and hydrocarbon prospects of the Vindhyan Basin: Petroleum Asia Journal, v. 6, p. 179-189. Srivastava, P., and Mitra, G., 1994, Thrust geometries and deep structure of the outer and lesser Himalaya, Kumaon and Garhwal (India): Implications for evolution of the Himalayan fold-and-thrust belt: Tectonics, v. 13,p. 89-109. Stein, R.S., and Yeats, R.S., 1989, Hidden earthquakes: Scientific American, v. 260,p. 48-57. 51
Suppe, J., 1983, Geometry and kinematics of fault-bend folding: American Journal of Science, v. 283,P. 684-72 1. Suppe, J., and Medwedeff, D.A., 1990, Geometry and kinematics of fault-propagation folding: Eclogae Geologicae Helveticae, v. 83,p.409-454. Thakur, V.C., 1992, Geology of Western Himalaya: New York, Pergamon Press, 366p. Valdiya, K.S., 1992, The Main Boundary Thrust zone of the Himalaya, India: Annales Tectonicae, v. 6 (supplement),p.54-84. Wiltschko, D.V., and Eastman, D., 1983, Role of basement warps and faults in localizing thrust fault ramps, in Hatcher, R.D., Williams, H., and Zeitz, I., eds., Contributions to the tectonics and geophysics of mountain chains: Geological Society of America Memoir 158,p.177-190. Yeats, R.S., and Lillie, R.J., 1991, Contemporary tectonics of the Himalayan frontal fault system: folds, blind thrusts and the 1905 Kangra earthquake: Journal of Structural Geology, v. 13,p. 215-225. Yeats, R.S., Nakata, T., Farah, A., Fort, M., Mirza, M.A., Pandey, M.R., and Stein, R.S., 1992, The Himalayan Frontal Fault System: Annales Tectonicae, v. 6 (supplement),p. 85-98. Zhao, W., Nelson, K.D., and project INDEPTH team, 1993, Deep seismic reflection evidence for continental underthrusting beneath southern Tibet: Nature, v. 366, P. 557-559. 52
APPENDICES 53 APPENDIX A
VELOCITY ANALYSIS USED FOR TIME-TO-DEPTH CONVERSION OF SEISMIC PROFILES
Velocities of the Tertiary rocks of the Sub-Himalaya were determined from a velocity survey conducted by the India Oil and Natural Gas Corporation in the Janauri- 1 well and through correlation of wells with seismic-reflection data. The Oil and Natural Gas Corporation determined interval velocities from a velocity vs. depth curve assuming linearity of average velocity with depth. These values were then used to determine average formation velocities for Lower (4000 mIs), Middle (3500 mIs), and Upper (2500 m/s) Siwalik strata. A value of -4900 m/s was determined for the Dharmsala, but was considered too high when compared with values for rocks of similar age and lithology in Pakistan. Given a known velocity of 4000 rn/s for the Lower Siwalik and a known depth of the Jawalamukhi thrust on the seismic-reflection profile Kangra-2 (Fig. 7), a velocity of 4100 rn/sec was determined for Upper Dharrnsala in the hanging wall of the thrust. The velocities established thusly were then used to plot the JMI- 1, JMI-2, JMI-B, and Balh wells on seismic-reflection profiles. In as much as the strong reflectors at 3.0 to 3.3 s on Kangra-2 likely represent pre-Tertiary basement, and as the JMI-B well does not penetrate pre-Tertiary rocks even at a depth of 5895 m below sea level, Lower Dharmsala velocity must increase. A minimum value of 4300 rn/s was determined, which keeps the base of the well above the pre-Tertiary horizon. For the purpose of constructing cross section C-C' across the Nahan salient (Appendix D), Eocene Subathu strata and Precambrian Bilaspur Limestone were assigned velocities of 4300 rn/s and 4400 m/s, respectively. Although calculated velocities all fall within reasonable limits for the various lithologies, it is important to note the increase in formation velocity from Pakistan to India. Within the Siwalik group, this apparent increase is offset by the low velocity determined for the Upper Siwalik. For the purpose of time-to-depth conversion, the Upper Siwalik is taken here to include Quaternary alluvium (Neogal, Fig. 5). The average velocity for Siwalik strata as whole, therefore, is roughly 3000 m/s. Velocities of the Dharmsala Group are 700-1000 rn/s higher than those of the equivalent Rawalapindi Group in Pakistan, which could be attributed to greater burial depth and thus greater induration of the formation in India. 54 APPENDIX B
ADDITIONAL NOTES AND SEISMIC PROFILESKANGRA REENTRANT (CROSS SECTION A-A')
Three unmigrated seismic-reflection profiles (Fig. 18), Kangra- 1 (Fig. 19), Kangra-2 (Fig. 9), and Kangra-3 (Fig. 20)were interpreted to clarify the complicated structure south of the Jawalamukhi thrust and to constrain the depth of basement beneath the middle part of the Kangra reentrant. On all three, high-amplitude reflectors between 3.0 and 3.5 s are interpretedas the top of pre-Tertiary strata. Although there are not many continuous reflectors, surface geology and well data constrain subsurface formation boundaries. Likewise, Siwalik and Dharmsala strataare distinguished by a general increase in reflection amplitude. In as much as the three reflection profiles are unmigrated, there is some interference between normal-incidence reflections and hyperbolic point-source reflections, which become broader with depth. Such interference ispresent beneath the BaIh anticline on Kangra-1 and Kangra-2 and within the zone between 1.7 and 3.3s on Kangra-3. On Kangra-2, hyperbolic reflections in the footwall of the Jhor fault likely result from tight, perhaps kink, folding in thecore of the Balh anticline. Beneath these events, between 2.1 and 2.7 s, north-dipping hyperbolic reflections are interpreted to result from folding of Dharmsala strata ina small duplex. The crests of such hyperbolas constrain hinge lines, facilitating construction of balanced cross sections. The wells JMI-1 and JMI-2 lie near the line Kangra-2, but JMI-Bwas projected 4 km to the southeast onto the line. Both above and below the Jawalamukhi thrust, the JMI-B well encountered anomalously thick intervals of Lower Siwalik strata (Fig. 21). In addition, it is difficult to reconcile the well and seismic data in the footwall of the J awalamukhi thrust, as there is no seismic evidence suggesting a moderately south dipping, Upper/Lower Dharmsala contact. Along strike stratigraphic variationsare thus likely in the subsurface beneath the JMI wells. In the Jawalamukhi thrust footwall beneath the JMI-B well, Lower Siwalik strata in placesare uplifted or overturned, as commonly occurs with growth of fault-propagation folds. Other variations in subsurface stratigraphy may result from small-scale faulting within the horse between the J awalamukhi thrust and Jhor fault, as suggested by offset groups of similar reflectors. Such apparent offsets, however,may also be the result of noise that pervades the central part of Kangra-2. Steep, northeast and southwest dips at the base of the JMI-B well are attributed to complex deformation at the base of the thrust sheet. 55
32
A Hoshiarp& Adampur
3V- + Sutiej A / Rupar Q '7
Siwalik (Lower,Middle,Upper) C
Dharmsala
Subathu
Lesser Himalaya metasediment I Bilaspur Ls.
30 High (crystalline) & Tethys (metasedimentary) Himalaya+ Saharanp Seismic reflection profile 9 2 10 60 80 B100 4> Exploratory well KILOMETERS 76 77. 78
Figure 18. Geologic map of the Sub-Himalaya of northwest India showing location of seismic lines and structure sections discussed in text and appendices. Refer to Fig. 4 for names of Sub-Himalayan structures. Structures: HFF=Himalayan Frontal fault, MBT=Main Boundary thrust MCT=Main Central thrust. Seismic-reflection profiles: DN=Doon-N, DS=Doon-S, K1=Kangra- 1, K2=Kangra-2, K3=Kangra-3, K4=Kangra-4, N1=Nahan- 1. 56
B Topogrhy (approx. k is = 1550 rrt 0 4 Both Dstunr= Jawotmjthi Ivust SLOMETERS Jor thrust(TD4474m) 40 500m Bal syn 12 ,oASL +d--70 55 60
-MS.voxhk - LSrwathr
UDharmsaia -
I. DhrnsotSabathu - -3
4
Figure 19. Unmigrated seismic-reflection profile Kangra-1 (line Ki, Fig.18; A: uninterpreted, B: interpreted). Linecrosses the Jawalamukhi thrust, southeast of Kangra- 2. Although the overall structure is similarto that on Kangra-2 (Fig. 7), notice that slip on the Jhor fault is greater, such that its surface expression is a south-vergent thrust. 57
Dtum Topgrhy(pox. Im s=1500) 50Cm Ps I1 Jw6hj thnmt 20 20
a
0 1 2 3 4 5 Oatum= T00ogpby pprox. Im 1= 1600 n) I I I I KILOMETERS soon, Pnn,a tit Jawn,h 00001 15 13 20 2& :TO M SwaJ.k MSwe6k I Swnhk
L Snnal.k
U DCmmno U Dhnomnnle
L00m& L Dhn,nno&Sub -. Sub00hu
PR 5-TERTIA fly - Vndhynn Décotlement
Figure 20. Unmigrated seismic-reflection profile Kangra-3 (line K3, Fig. 18; A: uninterpreted, B: interpreted). Line crosses the Jawalamukhi thrust northwest of Kangra- 2. Note the change in structural style of the entire Tertiary section, from tight folds to imbricate thrusting. The top of basement (Vindhyan strata) appears with correct northeast dip, because average velocities down to 3.2 seconds are consistent across the length of the section. 58
SW NE JM!-B JMI-2
1 1!
LS SPA LEVEL
MS
L
LS
UD UD
.tD LD
/ LD
Al ScaIel'=lkm,VE=l:l
Figure 21. (A) Diagram illustrating the complications of projecting exploratory wells in zones of complex structure. Solid, horizontal, dashed lines mark formation contacts within the wells. Gray, northeast-dipping dashed lines are inferred formation contacts used to construct cross section A-A'. JMI-1 and JMI-2 can be reconciled, but logs from JMI-B, which was projected 4 km onto the line of section A-A', do not correlate with the other two wells. (B) Diagram illustrating possible evidence for small-scale deformation of Dharmsala strata in the footwall of the Jawalamukhi thrust. Note offset of moderate-amplitude reflectors between the marked faults. 59 Well-log interpretations are alsosuspect with respect to formation boundaries within the Dharmsala Group. 'Whereas the abundance of sandstone decreases within the Lower Dharmsala (Raiverman, 1983; Karunakaran and Ranga Rao, 1976), descriptions of the Upper and Lower Dharmsala from the JMI-B well are similar (Upper Dharmsala: greenish-gray sandstone, purple siltstone and claystone; Lower Dharmsala: alternating gray and purple sandstone and claystone, unpublished Oil and Natural Gas Corporation well report). Such structural complications are difficult to see at the resolution provided by seismic-reflection data and at the scale of the entire Sub-Himalaya. Final interpretations of Kangra-1, -2, and -3 are close approximations of the true structure and were made such that the sections would balance when retrodeformed. The unmigrated seismic-reflection profile Kangra.-4 (Fig. 8) was interpreted to constrain the depth to pre-Tertiary basement beneath the northern part of the Kangra reentrant. As with the Kangra-1, -2, and -3 profiles, Siwalik strata are characterized by moderate-amplitude reflectors, and Dharmsala strata by high-amplitude reflectors. In as much as Kangra-4 clearly illustrates the backlimb of the Paror anticline and relatively horizontal strata beneath the Palampur thrust, subsurface formation boundaries were traced from surface contacts after hand-migration of steep backlimb reflectors. The result is that formation boundaries seem to crosscut reflectors. 60 APPENDIX C
ADDITIONAL NOTESDEHRA DUN REENTRANT (CROSS SECTION B-B') The Doon-S (Fig. 10) and Doon-N (Fig. 11) seismic-reflection profiles provide tight constraints on Siwalik structure south of and onto the Mohand anticline, and also within the Doon Valley. The reflectors that mark formation boundaries within Siwalik strata were determined by projecting the Mohand well 11 km west-northwest along the strike of the anticline. The Mohand well (ground elevation= 507 m) penetrates a repeated section of Middle Siwalik strata (1750 m each), yieldinga fault depth of 1250 m below sea level. However, a north dipping fault is identified at 0.33 s on Doon-S, which yields a fault depth of 577 m below sea level given anaverage velocity of 3500 rn/s for the Middle Siwalik. This deepening of the fault towards the southeast may be related to the 30 change in strike of the anticline, southeast of the Mohand well. Transfer of slip from the Frontal fault to the Bhimgoda backthrustmay also contribute to the observed deepening. Although the hanging-wall geometry of the Frontal fault changes, the footwall geometry is assumed to remain roughly constant beneath the central part of the anticline. In as much as the maximum thickness of the Middle Siwalik is known (1800 m, Kumar and Nanda, 1989), the formation top is at a depth of 1200 rn (0.83 s) on Doon-S. The depth of the fault is 577 m, which yields a 623-rn-thick section of Upper Siwalik in the footwall of the fault. These corrected depths were then converted to time for projection onto Doon-S. On seismic profile Doon-S, the Upper Siwalik is characterized byan upper layer of high-amplitude reflectors, an underlying seismically transparentzone, and another layer of high-amplitude but somewhat discontinuous reflectors at its base. The Middle and Lower Siwalik are characterized by moderate- to high-amplitude reflectors thatcan be traced the entire length of Doon-S. Rocks of the Dharmsala Group provide moderate-amplitude reflectors, which are complicated by an increasing amount of "noise" with depth. In addition, migration of Doon-Smay have disrupted deep reflectors. The last strong reflector (at 2.4 s) beneath the projected Mohand well top corresponds with the Dharmsala-Vindhyan contact. In the middle part of Doon-S, the base of Dharmsala strata is located at 2.9s depth which corresponds to a unconformity between horizontal reflectors and underlying north- and south-dipping reflectors, which are interpreted to represent Vindhyan strata. The moderate-amplitude Vindhyan 61 reflectors, however, are markedly different than the high-amplitude reflectors observed at 2.85 s depth over the southern 3 km of Doon-S. This observation likely reflects a change in lithology, such that pre-Tertiary basement here may be Aravalli or Delhi crystalline or metamorphic rock. Formation boundaries on Doon-N were established on the basis of similar Upper Siwalik reflectors in the two profiles. The unconformity at 2.75 s at the southern end of Doon-N likely represents the detachment beneath the Doon valley and the northern extension of the Mohand thrust. Offset of Lower Siwalik and Dharmsala reflectors resulted from slip on two blind thrusts that also root in this detachment. Disrupted reflectors at the north end of Doon-S are due to interference from steep to overturned Siwalik strata in the Santaurgarh anticline. 62 APPENDIX D
ADDITIONAL NOTES AND SEISMIC PROFILESNAHAN SALIENT (CROSS SECTION C-C')
The unmigrated seismic-reflection profile, Nahan-1 (Fig. 22) shows high- amplitude reflections within the deforming wedge (down to 2.0 s), north of the Bilaspur thrust, which are interpreted as thrust blocks cored by Bilaspur Limestone. Interpretation of Nahan-1 shows that these thrusts do not root in a décollement atop high-amplitude pre-Tertiary reflectors. Rather, they root in a décollement within the Middle Siwalik. Disruption of Siwalik reflectors to the south may be a result of slip on a lower d&ollement that rests atop pre-Tertiary basement. Raiverman et al. (1993) show a profile south of Nahan-1 that also shows disruption of Siwalik reflectors down to 3.0 s which corresponds to the pre-Tertiary Siwalik contact at the south end of Nahan-1. This disruption of Siwalik reflectors (beneath the detachment and south of the Bilaspur thrust), however, may be due to raypath-bending effects, in which case the detachment within the Middle Siwalik extends to the Himalayan Frontal fault. Nahan- 1 also shows that the pre-Tertiary basement is not everywhere subhorizontal. Rather, the basement surface is warped such that the plate-boundary décollement does not always lie directly above it. The apparent south dip of the Lower to Middle Siwalik contact is attributed to the effects of velocity pullup and raypath bending, produced by the overlying high-velocity Bilaspur Limestone. In addition, the geologic section C-C' (Fig. 23) is generalized, as there is abundant small-scale deformation within Subathu and Dharmsala strata that cannot be resolved at the scale of the seismic profiles and geologic maps. A minimum velocity of 4400 m/s was applied to the Bilaspur Limestone, but the true value may be as high as 5000 m/s. If that is the case, the detachments and contacts at the northern end of the section may be as much as 500 m deeper. 63
Figure 22. Unmigrated seismic-reflection profile Nahan- 1 (line Ni, Fig.18; A: uninterpreted, B: interpreted). Line crosses the central part of theNahan salient. Note the high-amplitude reflectors at 2.0 s, marking the base of an upperthrust stack of Eocene Subathu strata and Precambrian Bilaspur Limestone.Siwalik strata southwest of the line of section are deformed in broad anticlines; the anticlines mayhave resulted from slip on the same detachment at the base of the Bilaspur thruststack or a lower detachment above high-amplitude reflectors at 2.5 to 3.0 s. Moderateamplitude reflectors down to 3.0 s at the southwest end of the profile are characteristicof Siwalik reflectors observed elsewhere. Southwest-dipping reflectors areinterpreted as reverse faults that cut Lower and Middle Siwalik strata, and a lowerdetachment is favored. Velocity-pullup effects are pronounced in the northeastern part ofthe section. A depth conversion of the profile (cross section c-c', Fig. 23), with a minimumvelocity of 4400 rn/s for the Bilaspur Limestone, suggests that the Lower toMiddle Siwalik contact is at least horizontal, if not gently north-dipping. 64
cN c) zw ki4 !,tLt II 2 2
0
C £ z 0 w
0-
a 0 I 1 2 I 3 I KroI thrust Datum=900 mASL B Topography (approx. for is = 1 500 m( Majhauli thrust 40 Biaspur thrust . Surajpur-*44- thrust 5 Saraulith rust - KILOMTSS 65 Ranon thrust +. 0- -: 50 LStwalik20 LSiwaljk °(D .5 850T Subathu Subathu o 8O(OT 40 1 USaIuk -. I-Ui BIaspur LimestonePASTEE1TIAFIY Mulik L SrwaIik. - - PR-TERTIARY Vixdhyan Figure 22b cSw Saharanpur (projected) INDO-GANGETIC PLAINS NE DISTANCE (KM) Alluvium Precambrian Bilaspur Limestone Cenozoic_molasse LowerMiddleUpper SiwalikSiwalik (VindhyanPrecambrian(DelhiPrecambrian and Group) andAravalli crystalline Cambrian Groups) basement metasedimentary rocks Figure 23. Structural cross section c-c' (Fig. 18) across the Nahan salient. This section was used to constrain the depth SubathuLowerUpper Dharmsala A SeismicExploratory reflection well profile fromappliednorthfold;of the therift-related enddécollement to Himalayan ofthe the Bilaspur profile,normal beneath Frontal Limestone. marksfaulting, the fault Nahanwhere but(HFF) The athesalient reverse-faultdowndropped is d&ollement a blind(Fig. 14).thrust. interpretation blockwould The The frontalof be dashedVindhyan if a cannotSiwalik velocity line strata beat anticline ofruled5 km5000at theout.depth, was m/s 40 kmmodeled (ratherwithin mark thanVindhyan asperhaps a 4400 fault-propagation resultedm/s)strata were at the