Contributions to geodynamic research in the Himalaya

Parts of Tethyan and the Higher Himalaya in Ladakh, Kumaun, Garhwal, Arunachal Pradesh and adjoining areas have been investigated by the Wadia Institute of Himalayan Geology. A summary of the geological work carried out in the last five years is presented.     

The Punjab foreland basin of Pakistan: a reinterpretation of zircon fission‐track data in the light of Miocene hinterland dynamics

Sedimentary basins represent an archive of tectonic events of the hinterland source regions. By determining the variation in sediment lagtime over time, events can be distinguished which may no longer be available as the source has been eroded. In regions characterized by rapid exhumation this is most often the case but the erosion products form a record of these events. Detrital zircon fission‐track ages from sediments of the Siwalik basin, Pakistan, originally presented by Cerveny et al. (New Perspectives in Basin Analysis, Springer‐Verlag, New York, 1988, p. 43), have been reinvestigated and reinterpreted using a revised methodological approach. Detrital age populations were determined from different stratigraphic levels and were correlated through time in order to assess the change in lag time over the stratigraphic section. This information was combined with the many new ages from the hinterland to further interpret events in the source region. The new investigation suggests that steady‐state evolution has not always existed. An overall trend of exhumation increasing by 0.1 mm Myr^?1 (from 0.9 to 2.65 mm yr^?1) from 18 Ma to the present is evident with a major exception of a net pulse between 11.7 and 10.9 Ma associated with an increase in sedimentation increasingly rich in hornblende. Earlier studies suggested that at this time the source of the sediments was the presently outcropping Kohistan Arc. We are able to demonstrate that this cannot be so but was rather the rapidly exhuming Nanga‐Parbat Haramosh syntaxis (> 2 mm yr^?1) coevally with transpressional displacement along the Main Karakorum Thrust, whereby the overlying Kohistan Arc sequences were removed. Furthermore, comparison of our detrital thermochronological data set with another one from the same basin and one from another foreland basin to the east, in NW India suggest that the Himalayan orogenesis was probably not synchronous for the late Early–Middle Miocene. Overall, regions that undergoes today's rapid uplift may be useless to reconstruct earlier phases of exhumation as the levels that may have yielded such info were eroded and deposited into the adjacent basin(s). Such scenario is reproducible in most orogens as in the Himalaya in NW Pakistan stressing the high potential of detrital thermochronological studies to trace hinterland dynamics. Terra Nova, 18, 248–256, 2006     

The pressure–temperature–time path of migmatites from the Sikkim Himalaya

A combined metamorphic and isotopic study of lit‐par‐lit migmatites exposed in the hanging wall of the Main Central Thrust (MCT) from Sikkim has provided a unique insight into the pressure–temperature–time path of the High Himalayan Crystalline Series of the eastern Himalaya. The petrology and geochemistry of one such migmatite indicates that the leucosome comprises a crystallized peraluminous granite coexisting with sillimanite and alkali feldspar. Large garnet crystals (2–3 mm across) are strongly zoned and grew initially within the kyanite stability field. The melanosome is a biotite–garnet pelitic gneiss, with fibrolitic sillimanite resulting from polymorphic inversion of kyanite. By combining garnet zoning profiles with the NaCaMnKFMASHTO pseudosection appropriate to the bulk composition of a migmatite retrieved from c. 1 km above the thrust zone, it has been established that early garnet formed at pressures of 10–12 kbar, and that subsequent decompression caused the rock to enter the melt field at c. 8 kbar and c. 750 °C, generating peritectic sillimanite and alkali feldspar by the incongruent melting of muscovite. Continuing exhumation resulted in resorption of garnet. Sm–Nd growth ages of garnet cores and rim, indicate pre‐decompression garnet growth at 23 ± 3 Ma and near‐peak temperatures during melting at 16 ± 2 Ma. This provides a decompression rate of 2 ± 1 mm yr^?1 that is consistent with exhumation rates inferred from mineral cooling ages from the eastern Himalaya. Simple 1D thermal modelling confirms that exhumation at this rate would result in a near‐isothermal decompression path, a result that is supported by the phase relations in both the melanosome and leucosome components of the migmatite. Results from this study suggest that anatexis of Miocene granite protoliths from the Himalaya was a consequence of rapid decompression, probably in response to movement on the MCT and on the South Tibetan detachment to the north.     

Normal faults in Panjal Thrust Zone in Lesser Himalaya and between the Higher Himalaya Crystallines and Chamba sequence in Kashmir Himalaya,India

Normal faults on mesoscopic scale are observed in the Panjal Thrust Zone in the Dalhousie area of western Htmachal. The boundary between the southern margin of the Higher Himalaya Crystalline (HHC) of Zanskar and the Chamba syncline sequence is also described as a normal fault, referred to as Bhadarwah Normal Fault in the Bhadarwah area of Doda district on the basis of field mapping and shear sense criteria using S-C fabric and porphyroblast rotation. The occurrence of these normal faults suggests that the extensional tectonic regime was not restricted only to the Zanskar shear zone area but that it also occurs south of the Higher Himalayan range. This suggests NE-directed subhorizontal extension and exhumation of deeper level rocks of Higher Himalaya Crystallines.     

Multifractal analysis of earthquakes in Kumaun Himalaya and its surrounding region

Himalayan seismicity is related to continuing northward convergence of Indian plate against Eurasian plate. Earthquakes in this region are mainly caused due to release of elastic strain energy. The Himalayan region can be attributed to highly complex geodynamic process and therefore is best suited for multifractal seismicity analysis. Fractal analysis of earthquakes (mb ?? 3.5) occurred during 1973?C2008 led to the detection of a clustering pattern in the narrow time span. This clustering was identified in three windows of 50 events each having low spatial correlation fractal dimension (D _C ) value 0.836, 0.946 and 0.285 which were mainly during the span of 1998 to 2005. This clustering may be considered as an indication of a highly stressed region. The Guttenberg Richter b-value was determined for the same subsets considered for the D _C estimation. Based on the fractal clustering pattern of events, we conclude that the clustered events are indicative of a highly stressed region of weak zone from where the rupture propagation eventually may nucleate as a strong earthquake. Multifractal analysis gave some understanding of the heterogeneity of fractal structure of the seismicity and existence of complex interconnected structure of the Himalayan thrust systems. The present analysis indicates an impending strong earthquake, which might help in better hazard mitigation for the Kumaun Himalaya and its surrounding region.     

Petrography,geochemistry and regional significance of crystalline klippen in the Garhwal Lesser Himalaya,India

Uphalda gneisses (UG) is a crystalline klippe located near Srinagar in Garhwal Himalaya. These gneisses are compared with Debguru porphyroids (DP) (≈Ramgarh group) of Garhwal–Kumaun Himalaya and Baragaon mylonitic gneisses (BMG) of Himachal Himalaya. Petrographic study reveals that the deformation of UG was initiated at higher temperature (above 350°C) and continued till lowering of temperature and deformation led to the mylonitization.     

Coda wave attenuation characteristics for Kumaon and Garhwal Himalaya,India

Natural Hazards - Uttarakhand Himalayas are among one of the most seismically active continental regions of the world. The Himalayan belt in this region is divided into Kumaon and Garhwal Himalaya....     

Latest Triassic to Early Jurassic Thrusting and Exhumation in the Southern Ordos Basin,North China: Evidence from LA‐ICP‐MS‐based Apatite Fission Track Thermochronology

The contractional structures in the southern Ordos Basin recorded critical evidence for the interaction between Ordos Basin and Qinling Orogenic Collage. In this study, we performed apatite fission track(AFT) thermochronology to unravel the timing of thrusting and exhumation for the Laolongshan-Shengrenqiao Fault(LSF) in the southern Ordos Basin. The AFT ages from opposite sides of the LSF reveal a significant latest Triassic to Early Jurassic time-temperature discontinuity across this structure. Thermal modeling reveals at the latest Triassic to Early Jurassic, a ~50°C difference in temperature between opposite sides of the LSF currently exposed at the surface. This discontinuity is best interpreted by an episode of thrusting and exhumation of the LSF with ~1.7 km of net vertical displacement during the latest Triassic to Early Jurassic. These results, when combined with earlier thermochronological studies, stratigraphic contact relationship and tectono-sedimentary evolution, suggest that the southern Ordos Basin experienced coeval intense tectonic contraction and developed a north-vergent fold-and-thrust belt. Moreover, the southern Ordos Basin experienced a multi-stage differential exhumation during Mesozoic, including the latest Triassic to Early Jurassic and Late Jurassic to earliest Cretaceous thrust-driven exhumation as well as the Late Cretaceous overall exhumation. Specifically, the two thrust-driven exhumation events were related to tectonic stress propagation derived from the latest Triassic to Early Jurassic continued compression from Qinling Orogenic Collage and the Late Jurassic to earliest Cretaceous intracontinental orogeny of Qinling Orogenic Collage, respectively. By contrast, the Late Cretaceous overall exhumation event was related to the collision of an exotic terrain with the eastern margin of continental China at ~100 Ma.     

Neotectonic fault analysis by 2D finite element modeling for studying the Himalayan fold-and-thrust belt in Nepal

This paper examines the neotectonic stress field and faulting in the fold-and-thrust belt of the Nepal Himalaya using the 2D finite element technique, incorporating elastic material behavior under plane strain conditions. Three structural cross-sections (eastern, central and western Nepal), where the Main Himalayan Thrust (MHT) has different geometries, are used for the simulation, because each profile is characterized by different seismicity and neotectonic deformation. A series of numerical models are presented in order to understand the influence of a mid-crustal ramp on the stress field and on neotectonic faulting. Results show that compressive and tensional stress fields are induced to the north and south of the mid-crustal ramp, and consequently normal faults are developed in the thrust sheets moving on the mid-crustal ramp. Since the shear stress accumulation along the northern flat of the MHT is entirely caused by the mid-crustal ramp, this suggests that, as in the past, the MHT will be reactivated in a future large (M_w > 8) earthquake. The simulated fault pattern explains the occurrence of several active faults in the Nepal Himalaya. In all models, the distribution of the horizontal σ₁ (maximum principal stress) is consistent with the sequence of thrusting observed in the fold-and-thrust belt of the Himalaya. Failure elements around the flat–ramp–flat coincide with the microseismic events in the area, which are believed to release elastic stress partly during interseismic periods.     

青藏高原典型地区的地貌量化分析——兼对高原“夷平面”的讨论

尽管过去150年以来,人们对于喜马拉雅造山带有很长的一段研究历史,但是对其几何特征、运动方式、动力学演化仍然理解不深。这种情况的出现,主要是因为人们持续关注的是喜马拉雅造山带的二维构造空间特性,并将某些研究程度较高地区的地质关系向外推广到造山带其他地区。就地理、地层及构造划分而言,概念的混淆和误解在有关喜马拉雅的文章中也大量存在。为了阐明这些问题,并为那些有兴趣探究喜马拉雅造山带地质演化过程的人们提供一个新的平台,文中系统地综述了以前的基本观察。我的综述主要是强调沿走向变化的喜马拉雅地质格架在喜马拉雅剥露、变质和前陆沉积方面所起的作用。文章的主要目的是阐明占据造山带核部的大喜马拉雅结晶岩带(GHC)的侵位历史。因为喜马拉雅大部分地区是由主中央冲断层(MCT)和藏南拆离系(STD)之间的GHC所组成,所以在地图和剖面观察上确定这些一级喜马拉雅构造之间的关系是非常关键的。中喜马拉雅出露的平面模式表明,MCT具有断坪-断坡的逆断层的几何特征。南部的逆冲断坪携带了一个GHC的板片(Slab)叠置在小喜马拉雅层序之上(LHS),并形成了一个在MCT逆冲断层带之南延续100km的巨大上盘断弯褶皱。在西喜马拉雅造山带地区,东经约77°处,MCT呈现为横向逆冲断坡(Mandi倾向逆冲断坡)。在其西边,MCT将低级变质的特提斯喜马拉雅层序(THS)叠置到低级变质的小喜马拉雅之上;而在其东边,MCT将高级GHC叠置到低级LHS之上。这种沿走向变化的地层叠置和横穿MCT的变质等级表明,逆冲断层的断距向西减小,可能是由于地壳短缩总量沿着喜马拉雅造山带向西减小所致。在所有出露的地方,STD大致都沿着THS底部的同一地层面,呈现出一个长度>100km的上盘断坪。这种关系说明:STD可能沿着一个先期存在的岩石接触面,或者沿中部地壳近水平的脆性—韧性转换带而发生。虽然喜马拉雅造山带藏南拆离系的上盘都有THS发育,但是至今没有找到THS切断STD下盘的证据。这样使得估算STD的滑动距离非常困难。STD最南端地层或与MCT(即,Zanskar)相交,或者位于MCT前端1～2km的范围内(不丹),这两种可能都暗示MCT与STD在它们向南的上倾(up-dip)方向有可能结合。虽然这种几何特征在现有的模型中几乎被忽略,但对于整个喜马拉雅造山带的变形和剥露历史具有重要的指示作用。     

Metamorphic P–T profile and P–T path discontinuity across the far‐eastern Nepal Himalaya: investigation of channel flow models

Thermobarometric data and compositional zoning of garnet show the discontinuities of both metamorphic pressure conditions at peak‐T and P–T paths across the Main Central Thrust (MCT), which juxtaposes the high‐grade Higher Himalayan Crystalline Sequences (HHCS) over the low‐grade Lesser Himalaya Sequences (LHS) in far‐eastern Nepal. Maximum recorded pressure conditions occur just above the MCT (～11 kbar), and decrease southward to ～6 kbar in the garnet zone and northward to ～7 kbar in the kyanite ± staurolite zone. The inferred nearly isothermal loading path for the LHS in the staurolite zone may have resulted from the underthrusting of the LHS beneath the HHCS. In contrast, the increasing temperature path during both loading and decompression (i.e. clockwise path) from the lowermost HHCS in the staurolite to kyanite ± staurolite transitional zone indicates that the rocks were fairly rapidly buried and exhumed. Exhumation of the lowermost HHCS from deeper crustal depths than the flanking regions, recording a high field pressure gradient (～1.2–1.6 kbar km^?1) near the MCT, is perhaps caused by ductile extrusion along the MCT, not the emplacement along a single thrust, resulting in the P–T path discontinuities. These observations are consistent with the overall scheme of the model of channel flow, in which the outward flowing ‘HHCS’ and inward flowing ‘LHS’ are juxtaposed against each other and are rapidly extruded together along the ‘MCT’. A rapid exhumation by channel flow in this area is also suggested by a nearly isothermal decompression path inferred from cordierite corona surrounding garnet in gneiss of the upper HHCS. However, peak metamorphic temperatures show a progressive increase of temperature structurally upward (～570–740 °C) near the MCT and roughly isothermal conditions (～710–810 °C) in the upper structural levels of the HHCS. The observed field temperature gradient is much lower than those predicted in channel flow models. However, the discrepancy could be resolved by taking into account heat advection by melt and/or fluid migration, as these can produce low or nearly no field temperature gradient in the exhumed midcrust, as observed in nature.     

Estimation of strain distribution using GPS measurements in the Kumaun region of Lesser Himalaya

The continuous process of continent–continent collision between the Indian and the Eurasian plates has led to the formation of the Himalayan range and continuously caused earthquakes in the region. Large earthquakes with magnitudes of 8 and above occur in this region infrequently, releasing the elastic strain accumulated over years around the plate boundary. Geodetic measurements can help estimate the strain distribution along the fault system. These measurements provide information on active deformations and associated potential seismic hazards along the Himalayan arc. In order to understand the present deformation around the plate boundary, we collected GPS data during three campaigns in the years of 2005–2007 at 16 sites in the Kumaun region of the Lesser Himalaya. Horizontal velocity vectors estimated in ITRF2000 are found to be in the range of 41–50 mm/yr with an uncertainty level of the order of 1 mm/yr. The velocity field indicates that the present convergence of around 15 mm/yr takes place in the Kumaun Himalaya. Further, we estimate the strain components in the study area for understanding the currently active tectonic process in the region. The estimated dilatational strain indicates that the northern part near the Main Central Thrust (MCT) is more compressional than the southern part. Maximum shear strain is mostly accommodated in the northern part too. The maximum shear and dilatational strain rates are about 1.0 and 0.5 μstrain/yr. It is seen that the distribution of high shear strain spatially correlates with seismicity. The maximum of extensional and compressional strains due to the force acting along the Main Central Thrust (MCT) in the NW–SE direction are found to be 0.4 and 0.1 μstrain/yr, respectively. The maximum shear strain in the northern part of the Himalaya appears to be associated with the convergence of the region proposed by other geophysical studies.     

On the possibility of reservoir-induced seismicity in the Garhwal Himalaya

Chander, R., 1991. On the possibility of reservoir-induced seismicity in the Garhwal Himalaya. Eng. Geol., 30: 393–399.

It is argued from a brief review of available evidence that the possibility of reservoir-induced seismicity (RIS) in the Himalaya as a whole cannot be ruled out at the present time. On the other hand, a review of recent local investigations of small earthquakes ( m_b less than 5) and teleseismic investigations of moderate earthquakes (m_b between 5 and 6, mainly) occurring in the Garhwal segment of the Alpide-Himalayan seismic belt provides evidence that RIS in the region can be anticipated. While their epicentral belts coincide geographically, the estimated focal depths of small and moderate earthquakes of the Garhwal Himalaya are in the ranges of 0–14 and 10–20 km, respectively. Small earthquakes occur by reactivation of strike-slip and reverse faults and moderate earthquakes occur on thrust faults. Elsewhere in the world, RIS has been observed most often in the crust at the depths where small earthquakes have been observed in the Garhwal Himalaya. In addition, RIS has been experienced during the impoundment of reservoirs in strike-slip and reverse fault environments, while theoretical analyses indicate that, if suitably located in relation to the reservoir, even a thrust fault may be destabilised by impoundment.     

Identification of active seismicity by fractal analysis for understanding the recent geodynamics of Central Himalaya

Seismicity along the Himalayan front is mostly attributed to the processes of collision between the Indian and the Eurasian plates resulting in the under-thrusting of the Indian Peninsula underneath the Himalaya. The dynamics of the region bears very complex components which require in-depth understanding. Here the overall rate of crustal shortening since ∼ 11 Ma is ∼ 21mm/yr, which is comparable to modern rate of under-thrusting of the northern Indian plate beneath the Himalaya. The region experienced a large number of great earthquakes for the last 100–120 years causing massive destruction. Here an attempt has been made to understand the seismicity pattern of the region using fractal correlation dimension and hence used for the detection of active seismicity. Some clusters of seismicity were found to be indicative of seismically very active zones. Such clusters may enlighten the understanding of recent complex dynamics of Himalayan zone.     

从宽角地震数据得出的特提斯喜马拉雅南部的速度结构

作为ＩＮＤＥＰＴＨ计划的第一阶段，完成了一条跨过特提斯喜马拉雅南缘的深地震共中点（ＣＭＰ）剖面，它绘制出俯冲到喜马拉雅之下的印度大陆地壳的顶部（主喜马拉雅道冲或ＭＨＴ）和底部（莫霍层）轮廓。我们用移动式地震仪记录了ＣＭＰ剖面的爆炸，偏移距最大达１５５ｋｍ。短偏移距数据证实了ＣＭＰ剖面的数据，而我们的大偏移距数据则以强反射带为主。我们将这一反射带的强的初始相位解释为藏南滑脱系（ＳＴＤ），而其最后一个相位则为ＭＨＴ的反映。我们用ＣＭＰ剖面的初动数据去详细地模拟最上部２ｋｍ的结构。亚东裂谷系中年青的伸展盆地的深度约束在２ｋｍ，给出了裂谷东侧的断距为４．６ｋｍ，在特提斯喜马拉雅内的正断层，Ｅ－Ｗ向伸展１．５％。宽角数据用于建立地表到ＭＨＴ的地震波速度模型。ＳＴＤ反射体北倾１３°，从约６ｋｍ深（在ＣＭＰ剖面南端之下）到２２ｋｍ深，然后变平，倾角减至５°。这样，我们的观测提出ＳＴＤ是一个深的基底断裂，对ＭＨＴ，我们观测到倾角为７５°，ＮＮＥ倾，从高喜马拉雅山脊下的－２０ｋｍ海拔到雅鲁藏布江缝合带南约７０ｋｍ处的－３６ｋｍ海拔（地表下４０ｋｍ）。我们提出印度地壳可能俯冲到缝合带地表之下，却不可能是整体俯冲。     

Back-thrusting in Lesser Himalaya: Evidences from magnetic fabric studies in parts of Almora crystalline zone,Kumaun Lesser Himalaya

The present study aims to understand evolution of the Lesser Himalaya, which consists of (meta) sedimentary and crystalline rocks. Field studies, microscopic and rock magnetic investigations have been carried out on the rocks near the South Almora Thrust (SAT) and the North Almora Thrust (NAT), which separates the Almora Crystalline Zone (ACZ) from the Lesser Himalayan sequences (LHS). The results show that along the South Almora Thrust, the deformation is persistent; however, near the NAT deformation pattern is complex and implies overprinting of original shear sense by a younger deformational event. We attribute this overprinting to late stage back-thrusting along NAT, active after the emplacement of ACZ. During this late stage back-thrusting, rocks of the ACZ and LHS were coupled. Back-thrusts originated below the Lesser Himalayan rocks, probably from the Main Boundary Thrust, and propagated across the sedimentary and crystalline rocks. This study provides new results from multiple investigations, and enhances our understanding of the evolution of the ACZ.     

Leucogranite intruding the South Tibetan Detachment in western Nepal: implications for exhumation models in the Himalayas

The most popular models regarding the exhumation of the Greater Himalayan Sequence (GHS), such as extrusion, channel flow, critical taper and wedge extrusion, require prolonged activity of the two bounding shear zones and faults, the Main Central Thrust (MCT) and the South Tibetan Detachment (STD). We present the crystallization age of an undeformed leucogranite that intrudes both the GHS and the Tethyan Himalaya Sequence (THS). Zircon and monazite U‐Pb ages in the leucogranite give ages between 23 and 25 Ma constraining, at that time, the end of shearing along the STD. Our results limit the contemporaneous activity of the MCT and STD to a short period of time (~1–2 Ma) and thus argue against exhumation models requiring prolonged contemporaneous activity of the MCT and STD.     

Exhumation patterns along shallow low‐angle normal faults: an example from the Altotiberina active fault system (Northern Apennines,Italy)

A multi‐method approach (palaeothermal and thermochronological analyses; thermal modelling) is applied to reconstruct the exhumation history of the Altotiberina Fault (ATF), a representative example of crustal‐scale active low‐angle normal faulting in the Northern Apennines (Italy). Thermal maturity and thermochronological data yield similar burial histories but different exhumation patterns for the sedimentary successions in the hangingwall and the footwall of the ATF. Since 3.8 Ma, the ATF footwall has exhumed at rates of 0.90 mm a^?1. Exhumation led to bending and deactivation of the ATF uppermost portion as a result of tectonic unloading and isostatic adjustment, followed by migration of extension and the development of a set of domino‐like, east‐dipping normal faults, rooting on the buried portion of the ATF. ATF activity and isostatic rebound exhumed Triassic rock units from depths of about 4 km. We suggest that isostatic instability is accommodated at shallow crustal levels, in a similar way to what is observed on larger structures at mid‐low crustal levels.     

Late Quaternary–Holocene evolution of Dun structure and the Himalayan Frontal Fault zone of the Garhwal Sub-Himalaya,NW India

Dun structures are common in the Sub-Himalayan zone of the Himalaya bounded by the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT). They are broad synclinal longitudinal valleys formed as a consequence of the exhumation of the range front of the Himalaya. In the Garhwal Sub-Himalaya, these structures have grown since 0.5 Ma, with the peak activity postdating ∼100 ka. A series of out-of-sequence deformation structures have been identified within the MBT-HFT-bounded Dun structures. They are identified on the basis of geomorphic, post-100 ka stratigraphic, and structural expressions, with activity as young as the early Holocene. To the south of the range front of the Himalaya, uplift has been observed in the Piedmont Zone, with peculiar active tectonic geomorphic expressions. Piedmont sediments of 15–5 ka, determined by Optically Simulated Luminescence (OSL), have been affected by the above uplift. The complete tectonic scenario has been analyzed and an attempt has been made to delineate the sequential evolution of these structures during the post-100 ka period (Late Quaternary–Holocene) in the Himalayan range front.     

Timing and mechanisms of Central Himalayan exhumation: discriminating between tectonic and erosion processes

The ability to deduce exhumation mechanisms from thermochronological data is hampered by the fact that assumptions on the thermal state of the lithosphere have to be made. Additional argumentation is generally required to discriminate between erosion-controlled and tectonically induced exhumation. This problem can be overcome by studying the spatial distribution of zircon and apatite (U-Th)/He and fission track data. In this work the variation of four different low temperature isotopic systems generating age trends along a sampling line is used to infer mechanisms of Quaternary exhumation in the Central High Himalayan Metamorphic Belt. Observed zircon age trends with southwards increasing cooling ages (from 0.5 to 1.7 Ma) are attributed to tectonically induced exhumation. The uniform apatite cooling ages clustered c. 0.5 Ma are attributed to erosion.