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Himalayan Geology

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Tectonic aneurysm and climate forcing of orogenic wedge dynamics in
Sikkim-Darjeeling Himalaya

S. SINHA ROY Birla Institute of Scientific Research, Statue Circle, Jaipur 302001, India Email: ssinharoy@yahoo.com

It has long been recognized that a cause-and-effect relation exists between erosional unloading and passive isostaticresponse. However, in recent years a new focus on the role of surface processes in active tectonics has emerged (Zeitler et
al 2001). Influence of erosion on tectonic evolution has been examined at various spatial scales (Pavlis et al. 1997). The issue is complex because record of unroofing that has traditionally been viewed as evidence for tectonic activity,could in fact document erosion events driven by climate (Molnar & England 1990). It can also be argued that tectonics can force a climate response and vice versa. Many studies have shown that the concentration of erosion energy leads to a similar concentration of mechanical energy (Koons 1990), and that erosion can exert a strong control on particle paths through an orogen, thus forcing crustal exhumation (Willet et al. 1993). As a result, erosion influences the deformation pattern within orogenic belts. This precinct is supported by critical taper theory and its application to orogenic wedges that link topography and tectonics in a convergent wedge, and predicts that deformation should be very sensitive to mass redistribution by surface as well as internal processes (Dahlen 1990).

The Himalaya being a classic example of collision mountain belt having an orogenic wedge geometry it has offered a study ground to understand many aspects of climatic and tectonic forcing of its geodynamics and growth. This paper explores this aspect on the basis of geologic, tectonic and geomorphologic features of the Tista valley in Sikkim- Darjeeling Himalaya and its foreland piedmont region. The Tista valley exposes the Higher Himalayan crystallines (HHC) and leucogranites in its upper reaches. MCT-I separates the HHC and its substrate consisting of the Darjeeling Gneisses and metamorphics of the Lesser Himalaya, showing inverted metamorphism. At a lower structural level another thrust zone (MCT-II) separates greenschist-facies Daling Group of the footwall and the inverted metamorphic sequence of the hangingwall. MCT-II zone and its footwall contain slivers of deformed Lingtse Gneiss (1.7 Ga), considered reworked basement of the Mesoproterozoic Daling Group. Gondwana rocks are exposed in the Lesser Himalayan belt within tectonic windows (Rangit and Pachekhani) beneath MCT-III that transported the Daling nappe to the foothill foreland, encroaching, and at places, overstepping the MBT.

The latter thrust juxtaposes the Gondwana rocks against the Siwaliks. The Quaternary sediments of the piedmont belt are thrust below the Siwaliks along the MFT. The Tista river is antecedent to and follows the axial trace of a regional north-plunging anticline which is the youngest structure in deformation chronology. The longitudinal profile of the Tista River from Makha, located near MCT-II at its footwall, up to the mountain front (MFT), shows three prominent knickpoints at altitudes of 502 m (N. Makha), 438 m  (Sirwani) and 350 m (S. Singtam), and these are associated with river valley deflection of 35, 68 and 56, respectively.

These knickpoints are river response to fault-controlled uplift and tilting of MCT-III hangingwall block The Tista valley within the study stretch (50 km) contains a 4-tier strath terrace system, best developed and preservedat Makha, Sirwani, S. Singtam, Rangpo and Kalijhora (Sinha- Roy, 1980). The elevation of the strath surface from the present thalweg is variable along the valley (oldest T1 = 110 - 54 m, T2 = 53 - 9 m, T3 = 15 - 4 m, youngest T4 = 2 -1 m). The downstream slope (1.54%) of reconstructed T1 strath surface is higher than the gradient (0.66%) of the present river. The younger strath surfaces are generally in grade with the present river profile. T1 and T2 are generally paired up to knickpoint-II at Sirwani, but are unpaired downstream. This suggests almost straight (sinuosity = 1.5) channel in the upper reaches during T1-T2 stage, and meandering (sinuosity = 2.3) channel during
T2-T3 stage. This feature points to differential block tilting, causing lower and higher valley slopes, respectively in the upstream and downstream of the knickpoints. Knickpoint-III at Singtam is the ‘boulder-out’ point for T1 terrace sediments, but this point shifts downstream for successive younger T2 and T3 terraces, meaning an increase of stream power at T2-T3 aggradation stage. Channel base shear velocity deduced from bedload granulometry is high(22.99 cm/sec) for T1 up to ‘boulder-out’ point at Singtam, but it reduces to 6.35 cm/sec downstream. For T2 the mean shear velocity increases to 12.79 cm/sec all along the valley while for T3 it reduces to 4.50 cm/sec. However, T4 shows an increase of basal shear velocity to 9.82 cm/sec. These data would suggest an increase in stream power and bedload abrasion during T2 and T4 stages, signifying high river discharge. T2 stage corresponds to the on-set of monsoon, its intensity fluctuating between T2 and T4 stages with intensification during T4 stage

The ages of the terrace sets have been extrapolated from  the mean ages of terrace systems of similar disposition in Nepal (Avouac 2003), Sikkim (Mukul et al. 2007), and Arunachal Pradesh (Srivastava & Mishra 2008). The ages assigned are T1 (12 ka), T2 (8 ka), T3 (6 ka) and T4 (3 ka).Using these ages and the strath elevations the mean incision rates have been estimated between T1-T2 stage at 8.5 mm/yr, between T2-T3 stage at 10.8 mm/yr, and between T3-T4 stage at 2.4 mm/yr. Notably, the incision rates are variable at different locationsalong the valley, a feature that seems to be due to alongvalley spatial variation of bed-rock erodibility, channel morphology and channel roughness at various incision stages. The maximum incision rate obtained between T2-T3 stages corresponds to the highest stream power related possibly to high intensity of monsoon during 8 - 6 ka. A positive feedback exists between tectonics and erosion in the Lesser Himalaya. The Tista anticline folds MCT-I that  serves as the tectonic floor of the extruded hot HHC, and hence, the feedback mechanism causing Higher Himalayan aneurysm must have been transmitted to the Lesser Himalayan domain. An incision rate of 10–8 mm/yr in the Tista river valley that is higher than the regional denudation rate in adjacent Nepal (0.6 - 0.4 mm/yr, cf. Montgomery & Stolar 2006; 0.8–0.2 mm/yr, cf. Wobus et al. 2005), and tectonic emplacement of reworked basement gneisses (Lingtse Gneisss) within the Daling nappe of the Tista anticline would argue in favour of crustal aneurysm for the Tista anticlinal uplift.

 The concentrated uplift is caused by high river incision and consequent crustal weakening that led to crustal flow and mass accretion in the valley.The crustal flow and accretion are key factors of orogenic wedge dynamics. The Quaternary tectonic-erosion couplingand tectonic aneurysm, depicted in the Tista valley, is a legacyof orogenic wedge evolution during at least the last 25 Ma. Post-collision convergence and hinterland-ward material flow during 55–25 Ma Period produced to the south of the suturebackstop a thickened, internally deformed and self-similarly grown supercritical orogenic wedge (HHC) of high surface slope, floored by high-friction MHT basal decollement. This supercritical wedge collapsed at MCT-I ramp during 25–15 Ma, and caused the pro-wedge to return to critical taper state. During 15–8 Ma active slip along low-friction MHT decollement transported MCT-I footwall toward the foreland,  a feature that shed molassic sediments in the Siwalik foreland
basin in response to high wedge erosion, forced by monsoon on-set at ca. 10–8 Ma (Gurjic et al. 2004). Underthrust foreland sediments along MHT accreted material to the wedge, and at ca. 8 Ma MBT ramp broke the surface. The pro-wedge attained
supercriticality because of continued mass accretion under  condition of high influx (Fa) to efflux (Fe) ratio (Fa>Fe) and high basal decollement friction (μ>0.5) at relatively higher crustal level.

Hinterland crustal thickening and internal stress accumulation resulted in out-of-sequence thrusting, producing MCT-II at ca. 6 Ma. Further accretion and internal wedge deformation formed MCT-III between 6–2 Ma. Frontal accretion in the external part of the pro-wedge led to the development of foreland fold-thrust belt involving MCT-III and MBT zones while underplating along MHT decollement in the internal part led to the formation of antiformal duplex stack with basement slivers within MCT-II hanging wall. Foreland propagation of out-of-sequence thrusts indicated by the encroachment and overstepping of MBT by MCT-III, and high erosion (Fe>Fa) brought the wedge back to critical taper state. High erosion efflux from the wedge filled the foreland flexural basin. The critical taper wedge failed along MFT at post-2 Ma time, making the taper subcritical. The rateof southward wedge propagation between MCT-I and MFT is 4 mm/yr which is comparable to the rate of uplift of the orogen. This indicates that wedge growth rate gauged by crosssectional area change is controlled by a positive feedback  where tectonic forcing outpaced climatic forcing (Fa>Fe) The foreland Quaternary basin was filled by coalesced fan deposits that formed the piedmont zone (Sinha-Roy 1981). A number of erosion surfaces on tilted fans and the incision of the fans by the Tista and its tributaries indicate active uplift of the piedmont. Orogen-parallel fault scarps and longitudinal warps within the piedmont fans suggest that the tip line of the orogenic wedge has migrated to the south of the mountain front (MFT) into the piedmont zone. The expanded wedge is likely to have reached the early stage of super-criticality in consequence of which a blind thrust ramp (Main Piedmont Thrust = MPT) has developed in the hanging wall of MHT decollement. The surface-breaking normal-sense fault system and the warps in the piedmont fan deposits are interpreted as trishear response of upward propagating MPT ramp.

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Gurjic, D., Courtand, I., Bookhagen, B., Blythe, A. 2004. Dissimilar denudation histories along the Himalaya: climatic causes – tectonic and landscape responses. In: Proc. 2nd Swiss Geoscience Meeting, Lousanne, 123-124.

Koons, P.O. 1990. The two-sided wedge in orogeny, erosion and collision from the sandbox to the Southern Alps, New Zealand. Geology, 18, 679-682.

Molnar, P., England, P.C. 1990. Late Cenozoic uplift of mountain ranges and global climate change, chicken or egg? Nature, 346, 29-34.

Montgomery, D.R., Stolar, D.B., 2006. Reconsidering Himalayan river anticlines. Geomorphology, 82, 4-15. Mukul, M., Jaiswal, M., Singhvi, A.K. 2007. Timing of recent out-ofsequence actiove deformation in the frontal Himalayan wedge: insight from the Darjiling sub-Himalaya, India. Geology, 35 (11), 999-1002

Pavlis, T.L., Hamburger, W., Pavlis, G.L. 1997. Erosional processes as a control on the structural evolution of an actively deforming fold and thrust belt: an example from the Pamir–Tien Shan region, Central Asia. Tectonics, 16, 810-822.

Sinha-Roy, S. 1981. Alluvial fan model for the Himalayan piedmont deposits. Journal of Geological Society of India, 22(4), 164-174. Srivastava, P., Mishra, D. K., 2008. Morpho-sedimentary records of active tectonics at the Kameng River exit, NE Himalaya. Geomorphology, 96, 187-198. Willet, D., Beaumont, C., Fullsack, P. 1993. Mechanical model for the tectonics of doubly vergent compressional orogens. Geology, 21, 371-374.

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