Late Miocene Topographic Inversion in Southwest Tibet Based on Integrated Paleoelevation Reconstructions and Structural History

Earth and Planetary Science Letters 282 (2009) 1–9

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Earth and Planetary Science Letters

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Late Miocene topographic inversion in southwest Tibet based on integrated paleoelevation reconstructions and structural history

Michael A. Murphy a,⁎, Joel E. Saylor b, Lin Ding c a University of Houston, 312 Science & Research Bldg. 1, Houston, TX 772004-5007, United States b University of Arizona, Tucson, Arizona 85721, United States c Institute of Tibetan Plateau Research and Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, People's Republic of China article info abstract

Article history: Investigations of the deformation history of the Himalayan orogen support interpretations of rapid and Received 4 June 2008 striking changes in the landscape of the Tibetan Himalaya and High Himalaya. We examine this issue by Received in revised form 15 October 2008 integrating oxygen isotope-based paleoelevation reconstructions of the Zada basin in southwestern Tibet Accepted 2 January 2009 with information on the structural evolution of the High and Tibetan Himalaya between 79°E and 84°30′E. Available online 5 April 2009 18 18 δ Opsw values were calculated from δ Occ values from pristine ﬂuvial Miocene gastropod shells. Analyses 18 18 18 Editor: C.P. Jaupart comparing the most negative δ Osw and reconstructed δ Opsw values to Δδ Osw versus elevation relationships based on both thermodynamic models and an empirical data set suggest a decrease in the Keywords: mean watershed elevation of 1 to 1.5 km since the Late Miocene. Geologic mapping and structural data from Himalaya crustal scale fault systems in the Zada region and regions to its east indicate a phase of arc-normal shortening syncollisional extension and vertical thickening since the Middle Miocene, followed by ongoing arc-parallel extension and vertical oxygen isotopes thinning. These results suggest that regions in this part of the orogen transitioned from undergoing arc- paleoelevation normal shortening to arc-parallel extension in the Late Miocene, and that arc-parallel extensional structures root deeply within the Himalayan thrust wedge. When combined with data on the distribution, age, and provenance of sedimentary basins, our geologic mapping, structural data, and paleoelevation results suggest that this transition from shortening to extension was accompanied by a topographic inversion from mountains to basins in b4 m.y. These observations can be explained by a foreland propagating fault system that accommodates outward radial expansion of the Himalayan orogen. © 2009 Elsevier B.V. All rights reserved.

1. Introduction conditions (Yin, 1993), oroclinal bending (Klootwijk et al., 1985; Ratschbacher et al.,1994), oblique convergence (McCaffrey and Nabelek, Convergent orogens worldwide share some common characteristics 1998), outward radial expansion of the Himalayan thrust front (Seeber in their evolution: all result in horizontal crustal shortening, leading to and Armbruster, 1984; Molnar and Lyon-Caen,1988; Seeber and Pecher, thickened crust and surface uplift. Following crustal thickening, crests of 1998; Murphy and Copeland, 2005), and mass accumulation and the mountain ranges often undergo crustal extension, while crustal dissipation processes (Hodges et al., 2001). shortening continues on the ﬂanks of the mountain range (e.g., Molnar Southwestern Tibet and northwest Nepal in the western Himalaya and Lyon-Caen,1988)(Fig. 1). It has been unclear whether the transition preserve geologic features that archive the transition from shortening to extension marks attainment of maximum elevations or if crustal to extension in the Himalayan fold-thrust belt (Fig. 2). Extension is thickening through shortening outpaces crustal thinning through expressed in part in the largest topographic depression (250 km×90 km) extension (e.g., Molnar and Lyon-Caen, 1988). Although this transition in the Himalayan range. This depression includes the Zada and Pulan is recognized in nearly all convergent orogens (e.g., Dewey, 1988), our basins. The Zada basin ﬁll is broadly constrained to be Late Miocene to understanding of its temporal and spatial relationship with respect to Pleistocene in age (Saylor et al., 2009; Wang et al., 2008)andis ongoing processes within orogens is poor. composed, in part, of sediments derived from north of the basin. The The transition from shortening to extension is important to under- basin currently lies in the hinterland of the Himalayan fold-thrust belt stand because it is a record of how forces evolve within and outside an and developed on a regionally extensive thrust system known to be active orogen. Tectonic models explaining the change from shortening to in the Middle Miocene. This suggests that this part of the orogen extension in the Himalayan orogen have invoked collapse of a growing transitioned from a topographic high to a topographic low in only a few thrust wedge (Davis et al., 1983), changing thrust wedge boundary million years. Below, we present a study which integrates oxygen isotope-based ⁎ Corresponding author. paleoelevation reconstructions of the Zada basin in southwestern E-mail address: [email protected] (M.A. Murphy). Tibet with information on the structural evolution of the High and

Fig. 1. Tectonic map of the Himalayan convergent margin showing Tertiary fault systems that accommodate arc normal shortening and arc-parallel extension. Open box symbol indicates regional faults that accommodate arc-parallel extension. Abbreviations are: ADD, Ama Drime Detachment; CD, Chame detachment; CFS, Chomolhari fault system; DF, Dangardzong fault; GCT, Great Counter thrust; GMD, Gurla Mandhata detachment system; GMH, Gurla Mandhata-Humla fault system, KFS, Karakoram fault system; LPSZ, Leo Parghil shear zone; MCT, Main Central thrust zone; MBT, Main Boundary thrust; MFT, Main Frontal thrust; QNF, Qusum normal fault; STD, South Tibet detachment.

Tibetan Himalaya between 79°E and 84°30′E. A detailed documenta- trace is onlapped by the youngest strata of the Zada basin in the tion and synthesis of the oxygen and carbon isotope analyses and northeast corner of the basin. magnetostratigraphy is presented elsewhere (Saylor et al., 2009). The The GCT is mappable through the Mt. Kailas area (Yin et al., 1999) purpose of this contribution is to present our data and interpretations and eastward to the Lopukangri area (84°30′E) (letter K on Fig. 2). pertaining to the Miocene to present structural evolution and Rocks immediately east of Lopukangri (7024 m) were mapped at a paleogeography of rocks exposed in southwestern Tibet and north- scale of 1:100,000. The GCT extends WNW–ESE across the mapped west Nepal and their broader tectonic implications. area and consists of 5 subparallel thrust faults distributed within a 4 km-wide zone. Striations on the fault surfaces yield a mean slip 2. Distribution and kinematics of shortening structures direction of N05E (letter K on Fig. 2). This system of faults juxtaposes Paleozoic through Mesozoic phyllites, slates, sandstones, and lime- Early to Middle Miocene horizontal shortening is manifested along stones of probable Paleozoic through Mesozoic age (TSS) in its south-directed thrust-sense shear zones (Main Central thrust zone — hanging wall against boulder-cobble conglomerate in its footwall. MCT) and north-directed thrust faults (Great Counter thrust — GCT) The slip direction of the GCT estimated in the Zada and Lopukangri (Figs.1 and 2). The mapped position of the GCT in the Mt. Kailas area is regions demonstrate that it is approximately arc-normal (Fig. 2). based on mapping by Yin et al. (1999). Its position to the northwest Structural reconstructions of deformation in the Mt. Kailas area indi- and east of Mt. Kailas as well as the fault-slip data shown on Fig. 2 is cate 64 km of north-south shortening across the GCT (Murphy and based on ﬁeldwork conducted during this study. The age of rock units Yin, 2003). Similarities between the rocks juxtaposed along the GCT are based on those cited by Cheng and Xu (1987). The GCT is a regional suggest that this shortening estimate may be applicable to the Zada north-directed thrust system that has been identiﬁed across the entire and Lopukangri areas. On the basis of modeled K-feldspar 40Ar/39Ar length of the Himalaya from northwest India to Eastern Tibet (this data from clasts in the Kailas conglomerate in the Mt. Kailas area, Yin study, Heim and Ganser, 1939; Ganser, 1964; Harrison et al., 1992; et al. (1999) suggested that the thrust was active between 19–13 Ma. Ratschbacher et al., 1994,Yin et al., 1999). In general, the fault system These ages overlap those determined by Quidelleur et al. (Quidelleur juxtaposes the Tethyan sedimentary sequence (TSS) in its hanging et al., 1997) for slip on the GCT (locally termed the Renbu Zedong wall against a N2.5-km thick Late Oligocene-Middle Miocene thrust) in eastern Tibet. Movement on the GCT created as much as nonmarine clastic sedimentary sequence, referred to as the Kailas 10 km of crustal thickening (Yin et al., 1999) and associated (or Gangrinboche) Conglomerate— in Tibet and the Indus Molasse in topography in the hanging wall. India (Fig. 2, Ganser, 1964; Harrison et al., 1993; Aitchison et al., 2002). Where observed in the Zada basin, near Mt. Kailas and in the The eastern margin of the Zada basin was mapped at a scale of Lopukangri region the GCT juxtaposes low-grade sedimentary and 1:100,000. Our mapping focused on the geologic relationships meta-sedimentary rocks of the Tethyan sedimentary sequence (TSS) exposed along and adjacent to the trace of the GCT. The trace of the in the hanging wall against a cobble-boulder conglomerate in the GCT extends northwestward across the mapped area. It juxtaposes footwall. This conglomerate lies unconformably on top of porphyritic phyllites and slates of probable Ordovician age (TSS) in its hanging granite (Fig. 2). It consists of two mappable units based on clast wall against boulder-cobble conglomerate in its footwall. The GCT composition. Clasts in the youngest conglomerate unit are dominated along the eastern margin of the Zada basin is a 1–3 m-thick zone of by sedimentary and volcanic rocks of unknown age, while the older brecciated rock and throughgoing fault surfaces lined with a dark unit almost exclusively consists of clasts of porphyritic granite. We brown to red clay gouge. Striations in the clay gouge yield a mean slip interpret this conglomerate sequence as the Kailas conglomerate and direction of N55E along the northeastern region of the basin margin correlated it to its type location exposed in the Mt. Kailas area based (letter G on Fig. 2) and N39E along the east-central margin of the basin on the similarity in their structural position and composition. As in the (letter H on Fig. 2). The hanging wall of the GCT as well as its surface Zada and Mt. Kailas regions, the conglomerate in the footwall of the M.A. Murphy et al. / Earth and Planetary Science Letters 282 (2009) 1–9 3

Fig. 2. Early to Middle Miocene Faults shown in yellow. Late Miocene to Recent faults shown in red. Stereoplots are equal area lower hemisphere. Letters correspond to data collection sites. Site A: Qusum normal fault, site B: Karakoram fault, site C: Karakoram fault, site D: Gurla Mandhata-Humla fault system, site E Gurla Mandhata-Humla fault system, site F: Gurla Mandhata-Humla fault system, site G: Great Counter thrust, site H: Great Counter thrust, site I: Main Central thrust zone, site J: Lesser Himalayan imbricate thrust faults, site K: Great Counter thrust. Base map is a shaded relief map derived from Shuttle Radar Topography Mission data (SRTM 90 m). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

GCT near Lopukangri lies unconformably on top of porphyritic granite on imbricate thrusts in the Lesser Himalaya, immediately south of the and consists of two mappable units distinguished, as above, based on MCT in northwestern Nepal is S19W±3° (Fig. 2). clast composition which can be correlated to those recognized in the Mt. Kailas and Zada areas. 3. Distribution and kinematics of extensional structures The trace of the MCT zone extends across the southern flanks of the Western and Central Himalaya between 2200 and 3800 m elevation Late Miocene to Recent arc-parallel extensional deformation is and dips moderately to the north. The average slip direction on faults expressed in the Western and Central Himalaya by three regional within the MCT zone in the Central Himalaya is approximately arc- structures; the Karakoram fault, Leo Parghil horst (Qusum normal normal (S10W±4°) (Fig. 2). The timing of slip on the MCT in the fault), and Gurla Mandhata-Humla fault system (Fig. 2). Between 79°E Central Himalaya is estimated by 40Ar/39Ar thermochronology and 81°30′E deformation along the Karakoram fault occurs within a and provenance data in foreland basin deposits as Early Miocene narrow zone (2–20 km wide) consisting of right-lateral faults, normal (~21–22 Ma) (DeCelles et al., 2001). Other major thrusts to the south faults, and right-lateral ductile shear zones that strike N40W north of of the MCT broke in a generally southward younging sequence, 31°30′N and N55W south of 31°30′N(Fig. 2)(Armijo et al., 1986; beginning in middle Miocene time with the Ramgarh thrust and Ratschbacher et al., 1994; Murphy et al., 2000). Fault-slip data show following sequentially with the development of a large duplex in that the direction of displacement along the fault rotates to more Lesser Himalayan Sequence rocks, the Main Boundary thrust, and the easterly orientations from north to south between 79°E and 82.5°E Main Frontal thrust (DeCelles et al., 2001). The average slip direction (Fig. 2). This trend broadly parallels the trace of the Himalayan thrust 4 M.A. Murphy et al. / Earth and Planetary Science Letters 282 (2009) 1–9 front (Figs. 1 and 2). The Karakoram fault cuts, and therefore must be central part of the basin, a N800-m-thick succession of upper Miocene younger than, the GCT in the Mt. Kailas area (Fig. 2, Murphy et al., to Pleistocene deposits records an upward transition from fluvial to 2000). Thermal histories derived from rocks exhumed along the lacustrine sedimentation and finally alluvial fan deposits. The Karakoram fault south of 32.5°N indicate it was active at ca. 10 Ma (Yin formation rests unconformably on the TSS which is deformed by et al., 1999). South of the Mt. Kailas area a system of top-to-west low- shortening structures. Zada basin strata were deposited on an angle normal faults, referred to as the Gurla Mandhata–Humla fault erosional surface with significant relief; where observed, basin fill is system (GMH) is interpreted to be kinematically linked to the subhorizontal and undeformed. In the northeast corner of the basin, Karakoram fault. Displacement along the GMH facilitated exhumation the basinfill onlaps the hanging wall and trace of the GCT (Fig. 2). of mid-crustal rocks (Murphy et al., 2002; Murphy and Copeland, Paleocurrent direction indicators show that porphyritic granite clasts 2005). Motion on the GMH is ~E–W(Fig. 2). 40Ar/39Ar data from the in the uppermost Zada formation whose composition matches footwall rocks indicate that they cooled below 400 °C ca. 9 Ma. Pressure granites in the footwall of the GCT are indeed derived from the and temperature estimates for the structurally deepest footwall rocks footwall of the GCT (Poage and Chamberlain, 2001). Along the indicate that they equilibrated at 5 to 7.6 kbars and 575 to 690 °C northwestern margin of the basin, the basin fill abuts the Qusum (Murphy et al., 2002). Consideration of the original depth and dip normal fault. Paleocurrent indicators show that mylonitic clasts in the angle of the detachment fault prior to exhumation of the footwall uppermost Zada basin fill are derived from rocks of similar composi- yields minimum slip estimates between 66 and 35 km across the GMH. tion in the footwall of the Qusum normal fault (Saylor et al., in The Leo Parghil horst is bounded on the north by the NW-dipping review). We interpret the presence of a late Miocene basin in the Leo Parghil shear zone (Thiede et al., 2006)andonitssouthbythe hanging wall of the GCT to signify an inversion of the topography SE-dipping Qusum normal fault (Fig. 2). Fault slip data on the Leo between the latest movement on the GCT and deposition within the Parghil shear zone indicates top-to-NW displacement (Thiede et al., Zada basin (Fig. 2)(Yin et al., 1999; Aitchison et al., 2002). 2006). White mica and biotite from the flank of the Leo Parghil dome indicate an onset of exhumation at 15 Ma (Thiede et al., 2006). The 5. Oxygen isotope paleoaltimetry Qusum normal fault dips moderately to the southeast and juxtaposes mylonitic garnet-bearing schists and gneisses along with variably de- In the following section we summarize the results of an oxygen formed leucogranite bodies in its footwall against Triassic-Jurassic strata isotope paleoelevation study the full details of which are presented by and Zada basin deposits in its hanging wall. Slickenlines on the Qusum Saylor et al. (2009). The principle underlying oxygen isotope normal fault and antithetic faults within the fault zone indicates that the paleoelevation reconstructions is that the oxygen isotopic composi- 18 mean slip direction is S30E (Fig. 2). Leucogranite bodies in the footwall tion of meteoric water (expressed as δ Omw, in units ‰) decreases by locally contain a shear fabric similar to the attitude of the Qusum normal ~2.8‰/km (Siegenthaler and Oeschger, 1980; Chamberlain and Poage, fault, one of which yielded a K-Ar age of 16–15 Ma (Zhang et al., 2000). 2000; Garzione et al., 2000a,b; Poage and Chamberlain, 2001; Currie et al., 2005; Cyr et al., 2005; Blisniuk and Stern, 2005; Rowley and 4. Basins associated with extensional structures Currie, 2006; Rowley and Garzione, 2007). In the ideal case, the δ18O 18 18 value of surface water (δ Osw)reflects the average δ Omw value in The Zada basin (N7,500 km2) is the largest Late Miocene to the catchment. Departures from the ideal case tend to increase the 18 16 Pleistocene basin in the Himalaya (Cheng and Xu, 1987)(Fig. 1). δ Osw values via preferential evaporation of H2 O, which results in a Research in the Zada basin has produced an array of conclusions lowering of calculated paleoelevation. Carbonates ultimately derive 18 18 regarding basin formation and evolution (Cheng and Xu, 1987; their oxygen from surface water and so carbonate δ O values (δ Occ) 18 18 Chamberlain and Poage, 2000; Garzione et al., 2000a,b; Currie et al., provide a record of paleo-surface water δ Ovalues(δ Opsw), 2005; Cyr et al., 2005; Saylor et al., in review). Below we summarize dependent on the temperature of carbonate formation. After correct- conclusions by Saylor et al. (in review). Currently the hypsometric ing for other climatic factors, paleoelevation can be reconstructed. mean elevation of the Zada basin watershed is 4855 m Fig. 3. In the Local departures from the modern global average δ18Oversus elevation lapse rate can potentially be redressed by direct measure- ment and modeling of local lapse rates as has been done for southern Tibet (Garzione et al., 2000b; Rowley et al., 2001). Modern calibration is based on 28 water samples collected at 20 locations ranging from the Sutlej River mainstream to small seep springs throughout Zada Basin over multiple years. Virtually no local rain fell during the sampling campaigns, meaning we were sampling higher elevation run-off. We calculated the elevation at which precipitation fell by obtaining hypsometric mean and maximum 18 watershed elevations for each sample. δ Osw values range from −17.9 to −11.9‰, and δDsw values from −137 to −86‰ (VSMOW). Modern water from the Zada Basin plots on the global meteoric water line meaning that it is free of extensive evaporation. Late Miocene-Pliocene water data is based on fossil gastropods collected from a variety of depositional settings in two measured sections spanning the lower ~550 m (8–2.5 Ma) of the Zada Formation. We are confident that gastropod samples are unaffected 18 by diagenesis and retain their original δ Occ values for the following four reasons: (1) all samples which returned usable X-ray diffraction results (11 of 12) were aragonite, and none showed evidence of Fig. 3. Δδ18O (VSMOW ‰) of modern water from the Zada Basin plotted against the recrystallization; (2) the samples are visually pristine; and (3) 18 mean catchment elevation for water samples. Δδ O is calculated assuming a low- samples which we microdrilled showed seasonal variation which is elevation δ18O value for New Delhi of -5.8‰. Also shown are thermodynamically based unexpected if the samples were reset to a regional δ18O value during lapse rate models (based on work by Rowley et al., 2001 and Rowley and Garzione, sw 2007) with low-elevation temperature of 300 K (black line) and 295 K (dark grey line) diagenesis. (4) The results from this representative sampling can be and an empirical lapse rate (from Garzione et al., 2000b, light grey line). applied across the basin because the sediments were never buried M.A. Murphy et al. / Earth and Planetary Science Letters 282 (2009) 1–9 5

derive from a ±0.5‰ uncertainty due to variation in the most 18 negative values of δ Occ in Mio-Pliocene low-elevation paleosol carbonates between western Nepal and Pakistan (Quade et al., 1995), and ±1.5‰ uncertainty due to seasonal changes in the temperature of aragonite precipitation.

We applied the Rayleigh fractionation models with Ti =295 K and Ti =300 K (both with initial relative humidity of 0.8) and the empirically based model to our data in order to compare the 18 18 elevations predicted by Δδ Osw and Δδ Opsw values. In light of the observed 3‰ increase in δ18O values from Himalayan and Tibetan glaciers in the 20th century (Thompson et al., 1997, 2000; Kang et al., 18 2001), we added 3‰ to our Δδ Opsw value. Even with this correction the models still yield a difference in predicted elevations between the 18 18 Δδ Opsw and Δδ Osw values that is greater than the uncertainty associated with those data points (Fig. 4). The addition of 3‰ to the 18 Δδ Opsw value may not be warranted, as interpretation of the glacial 18 record remains unclear and the inferred increase in δ Omw values may be due to a short-term, transitory change. However, without the δ18 δ18 δ18 correction for the changes in O values observed in Tibetan glaciers, Fig. 4. Opsw (paleo-surface water) values were calculated from Occ values from – pristine ﬂuvial Miocene gastropod shells and the most negative values compared with our estimates of paleoelevation would increase by ~0.5 0.7 km. 18 the most negative values of modern water from the Zada Basin. Uncertainty comes from Possible alternative explanations for extremely negative δ Occ a±1.5‰ uncertainty due to seasonal changes in temperature of gastropod shell values from Zada basin include; gastropods precipitated their shells in precipitation and ±0.5‰ due to variability in the Miocene low-elevation paleosol warm water (41 °C), the lapse rate has changed between the Miocene record between India and Pakistan. Three models are used in the comparison (Empirical and the present or the low-elevation temperature was 4.5 °C cooler in ﬁt to Nepal data from Garzione et al., 2000b, Thermodynamic Models based on work by Rowley et al., 2001 and Rowley and Garzione, 2007) and in each of the models the the Miocene. However, all of these require extreme and unobserved decrease in elevation between the Miocene and present is greater than the uncertainty scenarios. associated with the data points. This is the case regardless of whether a +3‰ correction for 20th century climate change is incorporated into the calculation (Corrected Miocene 6. Discussion Zada Water).

6.1. Paleogeography below ~800 m and so are not subject to regional metamorphism or hydrothermal alteration. The spatial and temporal relationship between the, Zada basin, and We assumed that aragonite precipitated at temperatures similar to shortening and extensional structures suggests a rapid and striking the modern in light of the broad similarities in climate between the change in the landscape between the Tibetan plateau and Himalaya. Late Miocene and the present. Conservatively assuming that gastro- The age of the GCT and estimates of crustal thickening indicated by pods grow dominantly during the warmer months, we calculated the 40Ar/39Ar modeling (Yin et al., 1999) indicate that the hanging wall of average temperature for the months of April–October (months when the GCT, which currently underlies the Zada basin ﬁll was elevated and

Taverage N0 °C) and set T=7 °C. Observed intrashell variation of ±1.5‰ may have supplied detritus northward into the Kailas basin from the corresponds to a seasonal variation of ±7 °C and that uncertainty is Oligocene–Miocene (e.g., Yin et al., 1999; Aitchison et al., 2002). Thus, applied to all samples. based on the aerial extent of the GCT and Kailas conglomerate, the We apply current water source, moisture pathway, and climate region immediately south of the GCT between 79°E and 84°30′Ewas conditions to reconstruct the δ18O versus elevation lapse rate because arguably topographically higher than the region to the north (Figs. 2 18 there is no significant change in MAT or δ Occ values evident from the and 6). Based on timing estimates on the GCT, we interpret this first- low-elevation Late Miocene records. Modern mean annual tempera- order characteristic of the landscape to have existed from the Early to ture in the Gangetic foreland is close to Miocene MAT estimates Middle Miocene. (Awashi and Prasad, 1989; Sarkar, 1989; Quade et al., 1995). Development of the Zada basin and the Pulan basin during the Late 18 Additionally, average paleosol δ Occ values from Neogene deposits Miocene to Pleistocene occurred within the region occupied by the at low elevation in the northern Indian sub-continent show no change hanging wall of the GCT, a region of previously elevated relative post-8 Ma (Quade et al.,1995). Being dominated by the same monsoon topography. The aerial extent of the Zada basin fill and the Pulan basin climate system, paleosols from northern India would have experienced fill in the Mt. Kailas area (Fig. 2) implies that a semi-discontinuous the same climate changes and changes in source water δ18O values as 300 km long topographic depression formed on the south side of the the Zada samples (Araguas-Araguas et al., 1998; Dettman et al., 2001; GCT trace between the last movement on the GCT and initiation of Tian et al., 2001). As the monsoon was established by at least 10.7 Ma sedimentation in the Zada basin (i.e.: between the Middle and Late (Dettman et al., 2001), the same source and pathway applies for Miocene. Coeval with the development of the Zada and Pulan basins was Miocene Zada water as for modern Zada water. The balance of evidence initiation of local slip along the Karakoram fault, GMH, Leo Parghil shear suggests that we can use the modern δ18O versus elevation relation- zone, and Qusum normal fault. This implies a genetic link between arc- ships, as measured by Garzione et al. (2000b) and modeled by Rowley parallel extension and basin development in the Tibetan Himalaya and et al. (2001), to understand the ancient record. High Himalaya. These geologic observations indicate that the region 18 18 We compared the most negative δ Osw and reconstructed δ Opsw occupied by the Zada basin evolved from being a topographic high to 18 values to Δδ Osw versus elevation relationships based on both a topographic low during the transition from arc-normal shortening to simple Rayleigh fractionation model (Rowley et al., 2001; Rowley and arc-parallel extension in this part of the orogen. Our paleoelevation Garzione, 2007) and an empirical data set (Garzione et al., 2000b). In results suggest that, in addition to the decrease in elevation between the 18 18 calculating Δδ O we used the modern, low-elevation δ Omw value Middle to Late Miocene, continued elevation loss between the Late for New Delhi of −5.8‰ (VSMOW, Rozanski et al., 1993) and a Mio- Miocene to present may have been as much as 1.5 km. In support of 18 Pliocene low-elevation δ Occ value of −6.0‰ (VSMOW, Quade et al., a topographic inversion in the Zada region is the presence of wind gaps 18 1995; Dettman et al., 2001). Uncertainties in our Δδ Opsw values in the mountain ranges to the east and west of the basin (Fig. 2). We 6 M.A. Murphy et al. / Earth and Planetary Science Letters 282 (2009) 1–9

extension Fig. 5). These are (1) the transition and the timing of the transition from arc-normal shortening to arc-parallel extension, (2) the topographic inversion, (3) the large topographic depression which the Zada basin occupies and its uniqueness in the Himalayan range and (4) the magnitude and depth of exhumation accommodated by extensional structures. Corollary observations to these include crustal thinning implied by a loss of elevation of 1–1.5 km and the distribution of extensional structures associated with the Zada topographic depression. Table 1 compares the predictions of the tectonic models mentioned in the introduction to the observations listed above. This comparison shows that two of the models that best explain arc- parallel extension of the Himalayan orogen are outward expansion of the arc-shaped Himalayan thrust front (Seeber and Armbruster, 1984; Klootwijk et al., 1985; Molnar and Lyon-Caen, 1988; Ratschbacher et al., 1994; Seeber and Pecher, 1998; Murphy and Copeland, 2005), and oblique convergence along an arc-shaped boundary (McCaffrey and Nabelek, 1998). McCaffrey and Nabelek (1998) show that coeval arc-normal Fig. 5. A kinematic model for outward radial expansion of the Himalayan arc. The shortening and arc-parallel extension can result from basal shear magnitude of extension is determined by calculating the difference in arc-length as a caused by the Indian plate sliding obliquely beneath the Himalayas result of foreland propagation. Thrust faults initiate successively towards the foreland and Tibetan plateau. The model predicts, due to greater obliquity which results in a pattern of extension that shows an increase in magnitude towards the hinterland. See text for details. between the convergence direction and the orogen front, the rate of arc-parallel extension is greater in the western Himalaya compared to that in the central Himalaya. This may provide an explanation for the interpret the inception of entrenchment of rivers on the eastern and presence of large Late Miocene basins in the Tibetan Himalaya of the western margins of the Zada region to have occurred during the Early to western Himalaya and their apparent absence in the central Himalaya. Middle Miocene phase of arc-normal shortening. We envision that rivers Oblique convergence links the timing of the transition from arc- on the eastern margin ﬂowed to the northeast and those on the western normal shortening to arc-parallel extension systematically to events margin ﬂowed to the southwest. We interpret the wind gaps to have within the orogenic wedge by linking it to a change in the force formed during the transition from shortening to extension as the Zada imposed by basal shear stress. This change could be explained by basin developed (Fig. 6). initiation of a forward propagating thrust system which increases the area of the orogenic wedge in contact with the basal detachment, 6.2. Explanations for arc-parallel extension thereby increasing the force imposed by the obliquely subducting plate to the point where it overcomes the internal strength of the The discussion above highlights several foundational observations rocks to resist translation and they are translated towards the that must be incorporated into models explaining arc-parallel syntaxes. This predicts that prior to breaking the forward propagating

Table 1 Comparison of predications of proposed models for arc-parallel extension with observations from southwest Tibet.

Observations from Collapse of Changing thrust Oroclinal bending Oblique convergence Radial Mass accumulation/ southwest Tibet growing thrust boundary conditions expansion dissipation processes wedge (critical taper) Arc-normal Yes Yes Yes Yes Yes Yes shortening Arc-parallel Not explicit Not explicit but possible Yes (limited to zone of Yes Yes Yes (if lateral variations extension but possible arc-normal shortening) in potential energy are present) Transition from arc- Yes Yes No Yes Yes Yes normal shortening to arc-parallel extension Timing of transition Adjusting from Increase in pore-pressure Not predicted. Arc-normal Not explicit but possible. Passage of No (possibly synchronous, super-critical or initiation of a forward shortening and arc-parallel Caused by change in force shortening possibly due to an abrupt state propagating thrust system. extension are synchronous. imposed by basal shear. zone increase in potential Linked to change indirection energy within the orogen) or rate of convergence. Topographic Yes Yes Yes Yes Yes Yes inversion Uniqueness and Not predicted Not predicted Not Predicted Yes Not explicit Not explicit but possible location of Zada but possible Crustal thinning Yes (orogen- Yes (orogen-wide) not Yes (orogen-wide) not Yes (locally) Yes (orogen- Yes (orogen-wide) due wide) not localized localized wide) not to minimization of excess localized localized potential energy Arc-parallel Yes (possible) Yes (possible) Yes. But limited to zone Yes Yes Yes (if lateral variations extensional of active arc-normal in potential energy are structures shortening. present) Strike slip structures Not predicted Not predicted Not predicted Yes Yes Not explicit Depth of Not predicted Not predicted but possible Possible Yes-to basal detachment Yes-to basal Limited by southward exhumation but possible detachment extruding MCT zone M.A. Murphy et al. / Earth and Planetary Science Letters 282 (2009) 1–9 7 thrust system only the zones of greatest obliquity (i.e. the zones The mechanism for extension favored in this study is only closest to the syntaxes) should have experienced translation. applicable to the Himalayan thrust wedge from the GCT in the north Although not formally discussed, the oblique convergence model of to the Main Frontal thrust in the south where structural trends define McCaffrey and Nabelek (McCaffrey and Nabelek, 1998) also predicts an arcuate pattern and slip directions trend outward radially. Although that the strike-slip faults on which the blocks are translated towards extension is prevalent throughout the Tibetan plateau to the north the syntaxes will propagate towards the center of the orogen in (Lhasa and Qiangtang blocks) much of the region is characterized by discrete steps related to forward propagation of the thrust front Mesozoic and early Cenozoic north-south shortening and therefore (Murphy et al., 2000). unlikely to be a component of the Neogene Himalayan thrust wedge. Models calling upon outward radial growth of the Himalaya also Nonetheless, if extension within the Tibetan plateau were driven by offer an explanation for how an orogen may transition from outward radial growth of the Himalayan arc, then the geometry of shortening to extension (e.g., Murphy and Copeland, 2005). The extensional structures is predicted to be arc-normal resulting in geologic observations presented above demonstrate that arc-parallel northeast-trending rifts in western Tibet and northwest-trending rifts extension shortly followed arc-normal shortening within the High in eastern Tibet (Seeber and Armbruster, 1984). Kapp and Guynn and Tibet Himalaya; is contemporaneous with crustal shortening in (2004) showed that the first-order geometry of Tibetan rifts is exactly the Lesser and Subhimalaya; and that the slip direction on extensional opposite to this prediction, with northwest-trending rifts in western faults parallels the trace of the Himalayan arc. The fault slip data on Tibet and northeast-trending rifts in eastern Tibet. The kinematics of the GCT and MCT between 79°E and 84°30′E are a manifestation of an Tibetan rifts, although poorly known, do not show slip parallel to the Early to Middle Miocene phase of deformation dominated by arc- trace of the Himalayan arc. In western Tibet, the Lunggar rift displays a normal shortening (Fig. 2). Assuming that these structures root into a shear sense towards the east-northeast (Pullen et al., 2008), while the regional south-directed thrust as indicated by seismological, geodetic, shear sense of the Nyainqentanghla extensional system in eastern and structural data (Pandey et al., 1999; DeCelles et al., 2001; Jade Tibet is top-to-southeast (Harrison et al., 1992). Thus, extension of the et al., 2004), this motion requires the Himalayan arc to lengthen Tibetan plateau north of the India-Asia suture zone is likely a result of assuming no deformation of the downgoing Indian plate. In the some other mechanism, such as eastward extrusion of central Tibet simplest scenario, whereby the Himalayan arc grows radially outward relative to southern Tibet (Taylor et al., 2003) or gravitational collapse along a single thrust, the magnitude of extension is highest at the of thickened lithosphere (Molnar et al., 1993). Therefore, the age, thrust front since the radius of curvature of the arc is greatest at this lifespan, and kinematics of normal faulting within the Tibetan position, and decrease towards the hinterland. However, in the lithosphere may not correlate to that within the Himalaya. Himalaya whereby multiple thrust faults have initiated successively Estimates of the along-arc extension rate as derived by geodetic from north to south (i.e. hinterland to foreland), the magnitude of arc- methods range from 4.5±3.5 mm/yr (Jouanne et al., 1999)to14± parallel extension should increase toward the hinterland, as the 2 mm/yr (Chen et al., 2004). The higher rate estimated by Chen et al. rearmost hanging wall has been translated furthest towards the (2004) is consistent with the rate of east-west extension between Leh foreland. As the magnitude of arc-parallel extension is directly and Lhasa calculated by Jade et al. (2004). The onset of extension in proportional to that of arc-normal translation, the rearmost hanging the Himalayas is estimated to be 8 Ma (Harrison et al., 1995), 11 Ma walls will undergo greater extension than the hanging walls of (Murphy et al., 2002), and 14 Ma (Coleman and Hodges, 1995). Taking younger thrusts at the thrust front (Fig. 5). The magnitude of the slip rate estimated by Chen et al. (2004) with these timing extension can be expressed as estimates yields a magnitude of arc-parallel extension of 112 km, 154 km and 196 km, respectively. This range overlaps the estimate ½− απ = ð Þ e = r2 r1 180 1 calculated in the kinematic model presented here as well as the model presented by McCaffrey and Nabelek (1998). The extension rate where r2 and r1 are the post expansion and pre-expansion radii of estimated by Jouanne et al. (1999) yields magnitudes of extension that curvature, and α is the central angle which describes the sector length are significantly lower than that estimated from the geologic data and considered. The pattern of extension calculated for a thrust belt in therefore may not be representative of the long-term rate. which thrusts initiate successively towards the foreland is consistent with the widespread exposure of mylonitic rocks in the core of the 6.3. Integrated paleogeography and tectonic model Gurla Mandhata metamorphic core complex which signify a large magnitude of extension within the high Himalaya (Murphy et al., The outward radial expansion model described above explains the 2002; Murphy and Copeland, 2005). Structural reconstructions across structural relationships and basin development observed in south- the Himalayan fold-thrust belt suggest 228 km of horizontal short- west Tibet (Fig. 6). Arc-normal shortening during the Early Miocene to ening have occurred in the region between the Main Central Thrust Middle Miocene resulted in vertical thickening, surface uplift, and and the Main Frontal Thrust in northwest India (Srivastava and Mitra, denudation of rocks between the MCT zone and Indus–Yalu suture 1994) and in western Nepal (DeCelles et al., 1998). Assuming a simple zone (Yin et al., 1999; Aitchison et al., 2002). Onset of arc-parallel scenario whereby this shortening is fed into a single top-to-south extension in the Late Miocene resulted in vertical thinning and detachment implies that the rocks in the high Himalaya have been subsidence of the region previously thickened. It is envisioned that translated toward the foreland for a distance of 228 km. The kinematic Zada basin fill accumulated in such a locality. During this transition we model presented here predicts 150 km of arc-parallel extension within suggest that wind gaps, discussed earlier, formed on the eastern and the high Himalaya (hanging wall of the MCT) between 77.5° E and western margins of the Zada basin. We interpret the transition from 90°E which represents a 45° sector of the Himalayan arc. Moreover, shortening to extension to be a consequence of foreland propagation this model provides an explanation for how the arcuate geometry of of arc-normal shortening structures. As the zone of arc-normal the Himalayan orogen may have maintained steady-state equilibrium shortening propagated towards the foreland, the region thickened is since the Late Miocene. brought into a zone of arc-parallel extension. This creates a transition Arc-parallel extension as described above requires extension of the zone between the foreland dominated by crustal shortening and the entire thrust wedge. This is consistent with field observations as well hinterland dominated by arc-parallel extension. as isotopic data that show that the GMH assists in exhumation of Whether or not this model is applicable to the entire Himalayan arc Greater and Lesser Himalayan rocks, cuts the South Tibetan detach- remains an open question. Structures equivalent to the GCT span the ment system as well as the MCT zone, and extends into the Lesser length of the Himalayan arc (Heim and Ganser, 1939; Ganser, 1964; Himalaya (Ratschbacher et al., 1994; Murphy, 2007). Harrison et al.,1992; Ratschbacher et al.,1994; Quidelleur et al.,1997; Yin 8 M.A. Murphy et al. / Earth and Planetary Science Letters 282 (2009) 1–9

Fig. 6. Structural model (block diagram and cross-section) showing evolution of Himalayan convergent margin between 79°E and 84°E. (A) Early to Middle Miocene geologic setting characterized by crustal thickening in region between MCT and GCT. Deposition of the Kailas Conglomerate occurs north of this region. (B) Middle Miocene to Recent geologic setting characterized by movement along faults accommodating arc-parallel extension, inversion of topography in the Zada region, and foreland propagation of the Himalayan fold-thrust belt. Isotopic data from the Gurla Mandhata footwall rocks correlate to Lesser Himalayan sequence, implying that the Gurla Mandhata-Humla fault system (GMH) roots deeply. Field mapping presented by Murphy and Copeland (Murphy and Copeland, 2005) shows that the GMH cuts the Main Central thrust in northwestern Nepal. et al.,1999; Wang et al., 2000; Aitchison et al., 2002), indicating that the information on the distribution, age, and kinematics of faulting TSS in the hanging wall of the GCT may have been regionally elevated, as between 79°E and 84°30′E yields the following results: it is today, since the Early Miocene. An intriguing recent observation in δ18 δ18 the eastern Himalaya by Li and Yin (2008) is the presence of a ~100-km 1. The most negative Osw and reconstructed Opsw values were Δδ18 wide zone of left-slip faults that strike parallel to the trace of the compared to Osw versus elevation relationships based on both Himalayan arc. The slip rate on this fault system is estimated to be thermodynamic models and an empirical data set and suggest a 4–8 mm/yr which is broadly comparable to slip rate estimates along the decrease in the mean watershed elevation of 1 to 1.5 km since the Karakoram fault system suggesting a common mechanism for the Late Miocene. development of arc-parallel strike-slip faults. Nonetheless, our current 2. A phase of arc-normal shortening and vertical thickening, followed understanding of the geology of the Himalaya indicates that the Zada by arc-parallel extension and vertical thinning that is ongoing. basin is unique as no known equivalent has been recognized in the 3. The spatial relationship between shortening and extension implies ﬁ central Himalaya. This indicates that arc-normal crustal thickening and rst order changes in the landscape that is supported by our oxygen uplift spanned the length of the Himalayan arc but arc-parallel extension isotope-based paleoelevation reconstructions. and elevation loss is focused in the western portion of the arc. While the 4. Regions in this part of the orogen transitioned from undergoing model presented above does not provide an explanation for the arc-normal shortening to arc-parallel extension in the Late apparent absence a possible explanation may involve varying rates of Miocene. convergence between the western and central Himalaya. If the rate of 5. These observations can be explained by foreland propagating fault convergence in the western Himalaya is less than the rate of arc-parallel systems that work together to accommodate outward radial extension then the crust may be thinning faster than crustal thickening expansion and maintain the arcuate shape of the Himalayan orogen. via underthrusting/underplating leading to a net decrease in elevation. By contrast to the east, if the rate of convergence is faster or equal to the Acknowledgements rate of extension then crustal thinning in response to arc-parallel extension is equal to crustal thickening such that no major holes/basins This research was supported by National Science Foundation grant are produced and elevation is in a semi-steady state. EAR-0106808 and EAR 0438826 to Mike Murphy and National Natural Science Foundation of China grant 40625008 to Ding Lin. We thank 7. Conclusions Rasmus Thiede, Alexander Webb, and anonymous reviewer for their helpful comments. Logistical assistance with work in the Nepal Our integrated study of oxygen isotope-based paleoelevation Himalaya was provided by Bhim Chand with Earth's Paradise Treks reconstructions of the Zada basin in southwestern Tibet with and Expeditions Ltd. M.A. Murphy et al. / Earth and Planetary Science Letters 282 (2009) 1–9 9

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