The gneisses of the Nanga Parbat–Haramosh Massif (NPHM), Pakistan, experienced peak metamorphic temperatures in the interval from 25 to 30 Ma, as revealed by 40 Ar/ 39 Ar cooling ages of hornblende and the ages of the youngest intrusions of the Kohistan batholith located immediately adjacent to the NPHM. 40 Ar/ 39 Ar and fission-track mineral ages indicate that the postmetamorphic cooling history of the NPHM has been controlled over the past 5 to 10 m.y. by active tectonism associated with the Raikhot Fault, although passive uplift and erosion in response to overthrusting of the NPHM by the Kohistan Arc has been underway as well. Net cooling rates for NPHM gneisses exposed today along the Indus River at low elevations have accelerated, from 20°C/m.y. at ∼ 20 Ma to 300°C/m.y. at 0 to 0.4 Ma. Following emplacement of aplite dikes at about 30 to 35 Ma, portions of the Kohistan Batholith adjacent to the NPHM experienced cooling rates similar to the NPHM of about 20°C/m.y. over the period 25 to 10 Ma, but the net cooling rates for the batholith of ∼30°C/m.y. over the past 10 m.y. have been much lower than those experienced within the NPHM. Ion microprobe and conventional U/Pb analyses of zircon show that the protoliths for the Iskere Gneiss and the structurally lower Shengus Gneiss of the NPHM are, respectively, ∼1850 Ma and 400 to 500 Ma in age. Zircons from the Iskere Gneiss have thin, relatively high U rims that yield ages from 2.3 to 11 Ma. These rims indicate that metamorphism of the NPHM gneisses is Tertiary, not Precambrian, in age. The ages and Concordia systematics of analyses of Shengus Gneiss zircons suggest that this gneiss may be a metamorphosed equivalent of the Mansehra Granite and other Paleozoic S-type granites found throughout the Himalaya.

Pressure-temperature (P-T) paths observed in pelitic schists on either side of the Main Mantle Thrust in northern Pakistan record the dynamics of the collision between the Kohistan Island-Arc and Indian plate. Geothermometry studies, mineral reaction textures, and thermodynamic modeling of zoned garnets suggest that the rocks in the Kohistan Arc and the Nanga Parbat–Haramosh Massif experienced different pressure-temperature histories as a result of imbrication of these two terranes during thrusting. Rocks in the Kohistan Arc followed decreasing pressure-temperature paths, with early garnet growth occurring at high pressures (9.5 kbar) and later garnet growth at lower pressures (8.5 kbar). Conversely, rocks in the Nanga Parbat–Haramosh Massif record an increasing P-T path history. The early P-T history within the massif was at low pressures (4.0 kbar) and low temperatures (450°C). Later, both pressure and temperature increased to a maximum of 7.5 kbar and 580°C. The contrasting P-T paths observed within these two terranes provide evidence for overthrusting of the Kohistan Arc over the Nanga Parbat–Haramosh Massif along the Main Mantle Thrust.

Two distinct granite plutons occur above the Main Central Thrust north of Uttarkashi in the upper valley of the Bhagirathi River and its source, the Gangotri Glacier, in Garhwal, India. The structurally lower pluton is a biotite granite with mineralogic and major and trace element characteristics similar to late Precambrian to early Paleozoic plutons of the Northern and Lesser Himalayan belts of Indian shield granites. The structurally higher pluton, intruded into the Martoli Formation and Vaikrita Group of the Tethyan sedimentary rocks, is an aluminous S-type muscovite-tourmaline leucogranite similar to other Cenozoic High Himalayan leucogranites with respect to its mineralogy and major element chemistry, as well as its high concentrations of Rb, Cs, and U, combined with low concentrations of Sr, Zr, Th, and rare-earth elements. Whole-rock Rb-Sr data for five samples from the main leucogranite define two clusters on an isochron of 64 ± 11 Ma. This isochron may reflect (1) a significant age, (2) variations of initial 87 Sr/ 86 Sr in the magma at the time of crystallization, or (3) multiple pulses of magma with different isotopic compositions. An Rb-Sr mineral isochron for one of the leucogranite samples yields an age of 21.1 ± 0.9 Ma, whereas a K-Ar age on a muscovite separate from the same rock yields an age of 18.9 ± 1.3 Ma. Whatever the actual age of the leucogranite magma within the range of these different determinations, it clearly had a high initial 87 Sr/ 86 Sr, greater than 0.746. The initial 143 Nd/ 144 Nd is correspondingly low, less than 0.51190. These data suggest that the granitic magma developed by anatectic melting of older continental crust. Although the timing of anatectic melting is equivocal, constraints on the conditions required to produce the melts imply that postcollisional, intracontinental subduction along the Main Central Thrust (MCT) could not produce the leucogranites without some preheating of the Indian continental crust. This heating may have occurred when delaminated Indian subcontinental lithosphere was replaced by hot asthenosphere in the initial stages of the continental collision. Crustal anatexis to produce the leucogranites might have already occurred at this early stage of collision. Alternatively, once the crust was so heated, crustal anatexis may have occurred within the hot crystalline Higher Himalayan slab due to influx of volatiles from the Lesser Himalayan slab as the former was emplaced over the latter by intracontinental subduction. Continued continental convergence led to uplift of the Higher Himalaya due to displacement along a major crustal footwall ramp of the Main Central Thrust. The mineral ages are interpreted as dating cooling related to this uplift, which may have coincided with termination of active motion of the MCT and the initiation of motion along the Main Boundary Thrust.

The central Karakoram can be divided into three main tectonic units from north to south: a northern Karakoram terrane, the Karakoram batholith, and the Karakoram metamorphic complex. In the Baltoro Glacier region the Karakoram magmatism includes intrusive suites that predate and postdate the India-Eurasia collision. The oldest subduction-related phases include Jurassic hornblendite to biotite monzogranite of the Hushe complex, and Cretaceous (ca. 82 to 75 Ma) hornblende-biotite metagranitoids of the Muztagh Tower unit, all of which were deformed during the India-Kohistan-Karakoram collision. Volumetrically dominant is a postcollisional granite, the Baltoro Plutonic Unit (BPU), which consists of biotite monzogranites to two-mica ± garnet leucogranites and pegmatite-aplites of mildly peraluminous affinity. The BPU represents the youngest magmatic phase of the composite Karakoram batholith with a U-Pb age of 21 ± 0.5 Ma and K-Ar mica cooling ages ranging from 11.7 to 5.25 Ma. The Masherbrum migmatite complex (MMC), one of a series of such complexes along the southern margin of the batholith, immediately predates the BPU. Leucocratic dikes that cross-cut the MMC yield an Rb-Sr age of 14.1 ± 2.1 Ma and K-Ar ages of 17 to 10 Ma. The BPU is interpreted as a crustal melt ultimately derived from deep crustal levels and may not be related to leucogranite generation associated with the migmatite terrain to the south. Petrogenesis of the BPU is fundamentally different from that of the High Himalayan granites and may involve a degree of selective mantle contamination. Four major metamorphic-deformation phases can be distinguished in the central Karakoram. The earliest, M 1 , is represented by low-pressure andalusite-staurolite–bearing assemblages that are spatially associated with igneous components of the Hushe complex of Jurassic age. The dominant thermal event was a widespread Barrovian-type metamorphism (M 2 ), which was syntectonic to the main deformation and overprinted M 1 assemblages. M 2 -related structures are cut by the 37.0 ± 0.8 Ma Mango Gusar two-mica granite pluton. M 2 kyanite-garnet-plagioclase-quartz-muscovite-biotite-staurolite assemblages indicate minimum pressure temperature (P-T) conditions of 550°C and 5.5 kbar (550 MPa). Thermal effects related to intrusion of the BPU constitute M 3 . Along its northern margin at Mitre Peak, the assemblage andalusite-cordierite-chlorite-biotite-muscovite-quartz-plagioclase indicates a maximum pressure of 3.75 kbar (375 MPa, ≃ 12.5-km depth). Along its southern margin at Paiyu, the presence of granitic melt pods with sillimanite, muscovite, plagioclase, and quartz indicates a minimum pressure of ca. 3.5 kbar (350 MPa) and a temperature 75° higher than local M 2 assemblages. The replacement of kyanite by sillimanite and the appearance of granitic melt pods approaching the BPU along the Baltoro Glacier transect, may be an M 3 overprinting of M 2 . M 4 (<5 Ma) is a syntectonic retrogressive metamorphism along the hanging wall of the Main Karakoram Thrust—a breakback thrust responsible for the recent uplift of the Karakoram. Structural culminations of midcrustal rocks occur in the K2 and Broad Peak areas within Carboniferous-Lower Cretaceous sediments of the Gasherbrum Range. Metamorphism of the K2 gneiss (dominantly biotite-hornblende-K-feldspar orthogneiss) occurred during middle to Late Cretaceous time. Pegmatite dikes dated as 70 to 58 Ma (K-Ar-mica) cut the gneisses.

The Chilas Complex is a large mafic-ultramafic body closely associated with the Kohistan Arc sequence in the western Himalaya of northern Pakistan. The arc and the Chilas Complex occupy an area of 36,000 km 2 , bounded on the north and south by major sutures. The arc formed close to the margin of Eurasia in response to the northward subduction of neo-Tethyan ocean lithosphere in Late Jurassic to middle Cretaceous time, and consists of intra-arc sediments, calc-alkaline volcanics, and diorite-tonalite-granite plutons. At its base is the Chilas Complex, which extends for more than 300 km and which has a maximum width of 40 km. Most of the complex consists of massive (although locally layered) gabbro-norites, which comprise variable amounts of plagioclase (An 64-40 ), orthopyroxene (En 76-48 ), clinopyroxene (mg = 75-55), magnetite, ilmenite, ±quartz, ±K-feldspar, ±hornblende, ±biotite, ±rare scapolite. In the central part of the complex, near the base, there are minor discordant dikes and intrusive bodies as large as 5 km 2 of a dunite-peridotite-troctolite-gabbronorite-pyroxenite-anorthosite association that displays excellent layering, graded bedding, slump breccias, and syndepositional faults. These rocks contain olivine (Fo 94-71 ), relatively Mg-rich orthopyroxene (En 91-65 ), clinopyroxene (mg = 85-67), and calcic plagioclase (An 98-83 ), ±hornblende, ±chrome spinel, and ±pleonaste, and represent a more primitive magma batch emplaced into the base of the gabbro-norite magma chamber. The mafic complex is not an ophiolite. Rocks of the complex have more petrographic and compositional similarities with plutonic blocks from island arcs and with other major mafic complexes such as the Border Ranges Complex of Alaska and those from the Ivrea Zone in the Alps. Trace-element patterns of the gabbro-norites have marked negative Nb anomalies, positive Sr, Ba, and P anomalies, and high K/Rb ratios, features consistent with melting of a hornblende-bearing sub-arc mantle source. The Chilas Complex either represents the root zone magma chamber of the Kohistan island arc, or magma generated by diapirism in the early stages of intra-arc rifting during formation of a back-arc basin.

The Salt Range and its Trans-Indus extension bridges the reentrant between the outer ranges of the northwestern Himalaya and the Sulaiman Mountain arc. Upper Proterozoic to Recent successions occur in the range, which makes up the southern thrust front of the orogen. There are two regional features of particular interest. The first is the occurrence of thick saliferous deposits of Eocambrian age, overlying Precambrian basement in the Potwar Plateau and thrust southward in the Salt Range over the alluvial Cenozoic. Thick, saliferous deposits also occur within the Eocene sequence of Kohat. These incompetent formations played a significant role in determining structure. The second feature is the presence of four major unconformities: between the marine Eocambrian to Cambrian sequence and the glacial, Lower Permian conglomerates, and below the Paleocene, the Miocene, and the late Pliocene–Pleistocene formations. Metamorphic rocks, linking with the Precambrian crystalline basement of northwestern India, crop out only in the Kirana Hills some 80 km south of the Salt Range. Within the Salt Range and related areas, unmetamorphosed sedimentary rocks compose the exposed succession, mainly shallow-water marine, until mid-Tertiary time, and lacustrine and fluvial from Miocene time onward. Prior to Quaternary time, only epeirogenic forces affected the region, accompanied occasionally by local warping. In contrast, during Quaternary time, the effects of the Himalayan orogeny extended southward. Accentuated by movement within the Eocambrian saliferous formation, the Salt Range developed as a complex anticlinorium, emplaced southward along a major thrust, which has recently been determined by seismic reflection measurements to involve a décollement of at least 20 km. Complex fold and fault structures resulted elsewhere within the region.

Recent chronologic and stratigraphic studies in the northwestern Himalayan foreland basin have led to better constrained deformational and depositional histories. In order to test the hypothesis that considerable pre-Pleistocene uplift occurred in the Salt Range of northern Pakistan, the stratigraphic record adjacent to the central and eastern Salt Range has been examined. Unconformities, paleomagnetically documented tectonic rotations across these unconformities, and changes in the paleocurrent directions, provenance, and rates of sediment accumulation serve to delineate an interval of early Pliocene uplift of the Salt Range, as well as several late Pliocene–Pleistocene uplift events in this range and adjacent structures. Stratigraphic, reflection seismic, and structural data indicate that these uplift events resulted from thrusting related to the salt-lubricated Potwar detachment. When considered in conjunction with the chronology of deformation in other parts of the foreland, these data clearly indicate that out-of-sequence thrusting has occurred on a large scale (>100 km) during the past 6 m.y. This pattern of deformation supports the concept that an irregular spatial and temporal distribution of shortening should be expected to occur within an advancing thrust wedge.

Heavy mineral and paleocurrent direction data suggest that the ancestral Indus is an analog of the modern Indus River (Pakistan). During the past 18 m.y., the Indus has maintained a relatively stationary outlet position along the Himalayan Front. Sandstones from three sections of Siwalik strata have been sampled and their heavy mineral suites analyzed. These stratigraphic sections cover some 200 km in an east-west direction in the Potwar Plateau area. In the western and central Potwar Plateau sections, blue-green hornblende makes a conspicuous first appearance at 11 Ma, the boundary between the Chinji and Nagri Formations. Above this boundary, blue-green hornblende dominates the heavy mineral assemblage of Siwalik sand. In the Upper Siwalik beds the heavy mineral suite contains as much as 75 percent blue-green hornblende. Unroofing of the Kohistan Arc terrane is the most likely explanation for this detrital hornblende. In marked contrast, the eastern Potwar region does not show the same abundance of blue-green hornblende. Significantly, the modern rivers of the eastern Potwar do not carry abundant blue-green hornblende either; only the Indus River does. Paleocurrent measurements taken in the Chinji Village section near the center of the Potwar Plateau indicate a northwest-to-southeast flow direction, whereas those in the Trans-Indus 120 km west of Chinji Village indicate a northeast-to-southwest flow direction. These data indicate that the Siwalik sequence of northern Pakistan is configured as a large-scale alluvial fan with the ancestral Indus shifting course back and forth across the Potwar Plateau region with a frequency of 10 4 to 10 5 yr/cycle. River sinuosity varied systematically from side to side of this fan, with minimum sinuosity attained along a north-south axis. As indicated by the absence of blue-green hornblende, the ancestral Indus did not reach the eastern Potwar Plateau (Kotal Kund area, 100 km east of Chinji) during the past 11 m.y.

India collided with the northern Kohistan/Asian plate at about 55 Ma. Subsequently, Asia has overridden India, developing a wide range of thrust slices at the top of the Indian plate. Balanced sections in the imbricated sedimentary cover of the Indian plate indicate a minimum displacement of more than 470 km since collision. This requires the Kohistan region to the north to be underlain by underthrusted middle to lower Indian crust, the internal ductile deformation and thickening of which accounts for the main overall crustal thickening beneath Kohistan. In the Besham area of north Pakistan, a stratigraphy can be documented for the northern part of the Indian plate that includes basement sequences of quartzo-feldspathic gneisses of the Besham Group, and of Precambrian schists of the Tanawal Formation intruded by the Swat-Mansehra granite. The basement rocks are unconformably overlain by carbonate-rich Paleozoic sedimentary rocks. Sedimentary rocks of both the basement and cover sequences were metamorphosed at an early stage of the Himalayan deformation during tectonic burial associated with crustal thickening. Structures just south of the suture related to this crustal thickening include a sequence of ductile mylonites thickened by thrust-related folding, a folded thrust stack involving basement rocks imbricated with cover strata, and late cross-folds. Much of the thickening of the Indian plate in the footwall of the Main Mantle Thrust can be related to the necessary changes in thrust wedge shape as it climbs through the crust.

Recent recognition of the existence of a deep fault of regional dimension along the northern boundary of the Great Himalayan (Himadri) lithotectonic subprovince is a repudiation of the time-honored notion of the transition from high-grade metamorphics of the basement complex forming the Great Himalaya into the Phanerozoic fossiliferous sedimentary succession of the Tethyan domain. This plane of dislocation has attenuated and truncated basal Tethyan units (e.g., by the Malari Fault in Kumaun); it has disharmonically deformed or backfolded the lower Paleozoic formations (as discernible north of Nanda Devi in Kumaun and north of Kanjiroba-Annapurna in western Nepal); it has split the lithologic succession into a schuppen zone (as done by the Chomolungma/Main North Himalayan Thrust in the Sagarmatha [Everest] region in northeastern Nepal and by the Trans-Axial Fault in northwestern Sikkim); and it has caused considerable deformation, including mylonitization of the basement metamorphics, migmatites, and mid-Tertiary (28 to 18 Ma) granites that occur as concordant bodies and cross-cutting dikes. The Trans-Himadri Fault (T-HF) may represent the southernmost of the old normal faults in the continental-margin basin in the frontal part of the northward advancing Indian plate. This fault (which becomes a low-angle thrust in northeastern Nepal) was reactivated following the blocking of movements along the Indus-Tsangpo Suture Zone (ITSZ) and slowing down of sliding along the Main Central Thrust (MCT). The T-HF may therefore be accommodating a part of the convergence of the Indian and Asian plates. The domal structural architecture north of Annapurna, the pronounced upwarps of the metamorphic basement immediately to the south of the ITSZ in the Mansarovar region, and the faulted crystalline basement blocks and slices thrust northward in the Tso Morari area in Ladakh bear testimony to the blocking of movements and the consequent upwarping of the Indian crustal plate at its leading edge. The fall and then rise of the Moho in the Kashmir-Nanga Parbat-Pamir section and its abrupt deepening from 55 km under the Sagarmatha to 70 km a few tens of kilometers north, through to the Tibetan country, must be viewed in the context of postcollision, perhaps neotectonic, movements along the deep T-HF.

The Nanga Parbat–Haramosh (NPHM) massif is a unique structural and topographic high in the northwestern corner of the Himalayan convergence zone. Previously, the NPHM was thought to be bounded by the Main Mantle Thrust (MMT), a fault along which the Kohistan-Ladakh island arc was obducted onto the northern margin of India. This study presents field evidence that the recently active dextral reverse Raikot fault truncates the MMT and forms the western boundary of the NPHM. The Raikot fault separates medium-grade, Mesozoic to middle Cenozoic mafic metasedimentary and intrusive rocks of the Kohistan island arc (Kohistan Sequence) from high-grade Proterozoic metasedimentary rocks (Nanga Parbat Group) and orthogneisses of the Indian craton. The Kohistan Sequence rocks have experienced one tight to isoclinal folding event, probably associated with obduction of the island arc, and a second folding event associated with movement on the Raikot fault. The Nanga Parbat Group rocks were transposed by an early (possibly Proterozoic) isoclinal folding event and have subsequently been folded around east-trending axes in the early Cenozoic by the obduction of Kohistan, then around north-trending axes in late Cenozoic time in association with the uplift of the NPHM and initiation of the Raikot fault. The Raikot fault consists of both mylonite zones and numerous major and minor faults. Slickensides and mylonitic lineations both indicate dextral reverse slip. The Raikot fault and associated folds appear to have accommodated as much as 15 to 25 km of uplift during late Cenozoic time. The localization of the uplift and the involvement of the Moho suggest that the Raikot fault follows a major crustal structure, possibly a pre-collision Indian plate boundary. If this is the case, rotational underthrusting of greater India along the MMT would require dextral slip along the Raikot fault. It is proposed that the Raikot fault is a terminal tear fault on the MCT.

Skardu Basin is a northwest-trending intermontane basin along the Indus River in the Karakoram Himalaya Mountains of Pakistan. Seismotectonic domain boundaries in the Karakoram Himalaya commonly cross lithologic and some older structural boundaries. Four major structural-seismotectonic domains exist in the Skardu area: the Himalayan seismic zone, characterized by thrust tectonics; the complex Hindu Kush–Pamir seismic zone; the Skardu quiet zone, characterized by strike-slip, extensional, and rotational tectonics with relatively little seismicity; and the southern edge of Eurasian lithosphere (Tarim–Kun Lun–Tibet) northeast of the Karakoram fault. The Skardu quiet zone is interpreted to be within the Himalayan thrust prism, above an aseismic detachment along which stable sliding or ductile faulting accommodates displacement. Stresses transmitted into the Skardu quiet zone laterally from the Himalayan seismic zone toward Eurasia and perhaps upward from the inferred basal detachment result in gross clockwise rotation, translation to the north-northwest, and a right-lateral sense of shear in the Skardu region. Landsat lineaments defined by major drainages suggest an array of fractures and faults in the Skardu quiet zone. Field data suggest that the lineaments generally reflect distributed shear along myriad small faults rather than displacement exclusively localized on major, discrete fault surfaces. Extensive glacial and fluvial erosion have accentuated trends characterized by relatively dense fracturing and faulting. At its confluence with the Indus at Skardu, the Shigar River flows through a breach that may have originated as a pull-apart structure similar to the pull-apart basin along the upper Sutlej River. The preserved vestiges of the upper Cenozoic Bunthang sedimentary sequence reflect Skardu’s early basin phase. Uplift along the Nanga Parbat–Haramosh syntaxis and along the northeastern margin of the Himalayan seismic zone may have contributed to the ponding of the Indus River in the Skardu Basin during Bunthang time. These axes of uplift may be related to movement of the Himalayan thrust wedge from a region of easy basal slip (Skardu quiet zone) to a region of increased resistance to basal slip (Himalayan seismic zone, or, in the case of the NP-H syntaxis, the Hindu Kush-Pamir seismic areas). Regional uplift within the Skardu quiet zone may reflect thickening of the thrust prism in response to variations in shear resistance along the detachment. Quaternary glacial lake beds located on the floor of Skardu Basin are generally undeformed in the western half of the basin. Local deformation within the lake beds in the eastern half of the basin is probably due to interaction with glaciers.

The Shigar Valley is crossed by a large, southwest-verging reverse fault containing pods of serpentinized ultramafic rock; the fault is correlated with the Northern Suture, which separates Paleozoic shelf-type sedimentary rocks of the Asian plate from Cretaceous volcanic rocks of the Ladakh-Kohistan arc. In the Shigar valley, the Asian plate is represented by a series of folded schists and marbles (the Daltumbore Formation) that is faulted southward over metasedimentary rocks and volcaniclastics (the Bauma-Harel Formation) belonging to the volcanic arc. Cretaceous turritellid gastropod fossils were found in the Bauma-Harel Formation. Metamorphism on both sides of the suture occurred in a regime of high temperature but only low to moderate pressure. Metamorphic isograds are cut by the suture, so metamorphism must have occurred before faulting along the suture. Two main phases of igneous intrusion are exposed in the arc terrane: a pre-tectonic, possibly tholeiitic phase about 100 m.y. old, and a post-tectonic, calc-alkaline to subalkaline phase 40 to 60 m.y. old. The Northern Suture does not have the appearance of a major suture, but the Ladakh-Kohistan Arc seems to have been a separate plate from the Asian continent. The suture probably marks the closure of a small ocean basin in late Cretaceous to early Eocene time.

Gravity data along a north-south profile from Kohistan to the Punjab plain of Pakistan have been incorporated into recent interpretations of the gross structure of the foreland fold-and-thrust belt of the Himalaya. In northern Pakistan, large deviations from Airy Isostatic equilibrium are observed. An excess of mass characterizes the northern Kohistan arc, and a deficit of mass underlies a broad area extending from southern Kohistan to the Salt Range, while to the south a slight excess of mass seems to prevail in the region of the Sargodha high. This anomalous distribution of mass can be understood if the Indian elastic plate, which is assumed to overlie a buoyant “fluid,” is flexed down under the weight of both the overthrust mountains and the sediments eroded off the mountains and deposited in the foredeep basin. In many respects the intracontinental subduction of India beneath the Himalaya is similar to island arc formation, including the seismically active Sargodha high, a basement ridge analogous to the flexural bulge encountered seaward of oceanic trenches. Analysis of Bouguer gravity anomalies along a profile extending from the Sargodha high to the Main Mantle Thrust (MMT) shows that most of the negative-northward gravity gradient can be attributed to crustal thickening. In the Sargodha high area, an additional contribution of about 25 mgal appears to be due to excess of mass at lower crustal or upper mantle levels. The Moho discontinuity is interpreted to bulge up beneath the Sargodha high, then gradually increase in dip from 1° to 3° beneath the Salt Range and Potwar plateau (approximately equal to the change in dip of the basement surface). The Moho is interpreted to change from upwardly convex to upwardly concave beneath southern Kohistan. Finally, north of the Main Mantle Thrust it appears to bend down again, but at a steeper angle of about 15°. Shorter wavelength anomalies, superimposed on the regional Bouguer gradient, are modeled in terms of upper crustal density changes, including those due to: (1) offsets of the basement surface; (2) variable thickness of the Eocambrian evaporite sequence that forms the basal décollement; (3) thrusting and folding of relatively high-density, older parts of the stratigraphic section to higher structural levels, particularly in the Salt Range and northern Potwar plateau; and (4) thickening of the low-density Neogene molasse sequence into the axis of the Soan Syncline, a structural depression between the Salt Range and northern Potwar plateau. Subsurface densities of the overthrust wedge, as well as the definition of the shape of the top surface of the Indian plate interpreted from seismic reflection and drilling data, place bounds on the flexural rigidity of such a plate and the forces that deform it. In northern Pakistan, a steeper Bouguer gravity gradient suggests that the flexural rigidity of the elastic plate (D = 4.0 [± 2.0] × 10 23 Nm) is a factor of 10 smaller than the current values interpreted for the central and eastern Himalaya. Moreover, the maximum flexural stresses are probably concentrated within the crust, which may account for the seismic activity of the Sargodha high and southern Kohistan. At the end of the Indian elastic plate (arbitrarily chosen at the MMT), a large positive vertical shear stress, 9.2 × 10 12 N/m < S 0 < 1.6 × 10 13 N/m, is applied to account for the topographic load north of the MMT. In addition, to fit the gravity constraints it was necessary to apply a large negative bending moment, −1.4 × 10 18 N < M 0 < −0.85 × 10 18 N, at the end of the plate. The negative bending moment can be explained by the combined effect of the northward migration of the Indian plate and the southward differential compressional force generated by the crustal rocks stacked at mid-upper crustal levels beneath the northern Kohistan arc. In addition, buoyancy of the crustal rocks at deeper levels beneath the Kohistan arc may contribute to the negative bending moment. Consequently, in southern Kohistan the surface of the Indian plate is concave up; compressional stresses in the upper part of the plate are probably the primary source of the Hazara seismic zone, where incipient reverse faulting seems to take place. In contrast, the pronounced upward convexity developed along the flexural bulge can account for (1) tensional stress in the upper part of the Indian plate, which is large enough to produce basement normal faults interpreted beneath the Salt Range and Sargodha high; and (2) compressional stress in the lower portion of the crust, which causes the excess of mass and seismicity beneath the Sargodha high.

The main collision between the Indian and Asian lithospheric plates occurred during late Eocene time (40 to 50 m.y. ago). Continued northward migration of the Indian plate since that time at the rate of 5 cm/yr has resulted in approximately 2,000 km of closure between the two plates. In northern India it has been suggested that, while perhaps 500 to 1,000 km of the closure can be accounted for by crustal shortening, underthrusting, and thickening, the balance (1,000 + km) may be accounted for by eastward motion along strike-slip faults in China. However, in northern Pakistan, the problem is complicated because there is no Tibetan plateau analog and no evidence of strike-slip structures that could have removed significant amounts of crustal material. In order to place tighter constraints on tectonic models for the Indian-Asian collision in the western Himalaya, it is important to be able to estimate the amount of crustal shortening that has occurred. Current estimates of 500 to 700 km of crustal shortening in northwestern Pakistan are calculated from balanced cross-section methods. An important step in estimating the amount of shortening that has occurred is to determine the volume of crust that remains in the orogen. The crustal models based on observed gravity profiles presented in this chapter suggest that there may be enough crustal volume in the western Himalaya to account for between 570 and 1,140 km of shortening. This is still significantly less than the 2,000 km of closure that has possibly occurred. The balance of the closure might be accounted for by erosion and/or diffuse deformation, or it might suggest that less than 2,000 km of closure has occurred in the northwestern Himalaya. The crustal models also suggest that the style of underthrusting in the northwestern Himalaya may be significantly different than that proposed for the Himalaya of northern India. Here the underthrusting seems to be occurring at a very steep angle when compared to the shallow underthrusting proposed for northern India.