The Himalayan system is the result of a continental collision that occurred approximately 55 million years ago between the Indian and Asian tectonic plates1,2. The collision resulted in the underthrust of the Indian crust beneath Asia, and a deformed crustal wedge3,4. The Indian plate records a complex pre-Himalayan geological history that includes several basement faults that reach depths of 70 kilometres and are oriented at a high angle to the Himalayan front5. These basement faults bound topographic basement highs that are interpreted to limit the rupture of Himalayan earthquakes6. Recent research suggests the basement faults control the distribution and magnitude of seismicity along the Himalaya7.
The Himalayan system serves as a prototype for novel three-dimensional numerical modelling to understand if and how inherited basement structures influence seismicity in orogenic settings. Models are generated using the Coreform Cubit meshing software and are run on the supercomputer platform hosted at the University of Toronto. The initial model contains three crustal blocks: the Indian crust, a deformed orogenic wedge, and the Asian crust, all cut at a high angle by a lithospheric scale basement fault. The 2015 7.9 MW Gorkha earthquake is simulated, and slip is generated along the Himalayan basal detachment. Seismograms, shear wave potential, and compressional wave potential movies are created to understand if and how the inherited basement faults influence seismicity within orogenic systems. Understanding how these regionally significant basement faults influence seismicity in the densely populated regions of northern India and Nepal is of utmost societal importance.
References
1. Najman, Y., et al., (2010). Timing of India‐Asia collision: Geological, biostratigraphic, and palaeomagnetic constraints. Journal of Geophysical Research: Solid Earth, 115.
2. Hodges, K. V. (2000). Tectonics of the Himalaya and southern Tibet from two perspectives. Geological Society of America Bulletin, 112(3), 324-350.
3. Godin, L., et al., (2019). Influence of inherited Indian basement faults on the evolution of the Himalayan Orogen, in Crustal Architecture and Evolution of the Himalayan-Karakoram-Tibet Orogen, R. Sharma, I. M. Villa, and S. Kumar, Editors, Geological Society of London Special Publication, 481, 251-276.
4. Chung, S.-L. et al. (2005). Tibetan tectonic evolution inferred from spatial and temporal variations in post-collisional magmatism. Earth-Science Reviews 68, 173–196.
5. Godin, L., & Harris, L. B. (2014). Tracking basement cross-strike discontinuities in the Indian crust beneath the Himalayan orogen using gravity data – relationship to upper crustal faults. Geophysical Journal International, 198, 198-215.
6. Gahalaut, V. K., & Kundu, B. (2012). Possible influence of subducting ridges on the Himalayan arc and on the ruptures of great and major Himalayan earthquakes. Gondwana Research, 21(4), 1080-1088.
7. Gahalaut, V, K., & Arora, B. R. (2012). Segmentation of seismicity along the Himalayan Arc due to structural heterogeneities in the under-thrusting Indian plate and overriding Himalayan wedge. Episodes Journal of International Geoscience, 35(4), 493-500.
Determining the geometry and evolution of a basal detachment and its influence on orogenesis is a challenging, but important, aspect to understanding orogenic evolution. The basal detachment of the Himalayan orogen in far west Nepal is presently segmented by a documented tear fault. New pressure-temperature-time-deformation paths from the Himalayan metamorphic core along the Seti Khola river transect were integrated to compare the tectonometamorphic evolution on either side of the basal detachment tear fault to outline its history. Peak metamorphic conditions of 645−745 °C and 0.85−1.1 GPa were reached in the Seti Khola Himalayan metamorphic core rocks during the Oligocene to earliest Miocene, 10−14 m.y. prior to equivalent along-strike rocks in the adjacent Karnali valley, which indicates segmentation of the Himalayan metamorphic core across the tear fault. We interpret the segmentation of the orogen to have been caused by the development of the tear fault in the basal detachment of the Himalayan orogen and differing ramp-flat geometries on either side. The segmentation and change in basal detachment geometry is consistent with the reactivation of an underthrusted Indian plate inherited basement structure, the Great Boundary Fault, during the Oligocene to earliest Miocene. The comparison of tectonometamorphic histories along-strike in far west Nepal highlights the basal detachment geometry through time and the need to consider the pre-orogenic structural features of the plates involved in orogenesis. This study reinforces the importance of combining tectonometamorphic studies with geophysical and geomorphological data to fully understand the causes of along-strike segmentation of orogenic systems through time.
Abstract The channel flow model aims to explain features common to metamorphic hinterlands of some collisional orogens, notably along the Himalaya-Tibet system. Channel flow describes a protracted flow of a weak, viscous crustal layer between relatively rigid yet deformable bounding crustal slabs. Once a critical low viscosity is attained (due to partial melting), the weak layer flows laterally due to a horizontal gradient in lithostatic pressure. In the Himalaya-Tibet system, this lithostatic pressure gradient is created by the high crustal thicknesses beneath the Tibetan Plateau and ‘normal’ crustal thickness in the foreland. Focused denudation can result in exhumation of the channel material within a narrow, nearly symmetric zone. If channel flow is operating at the same time as focused denudation, this can result in extrusion of the mid-crust between an upper normal-sense boundary and a lower thrust-sense boundary. The bounding shear zones of the extruding channel may have opposite shear sense; the sole shear zone is always a thrust, while the roof shear zone may display normal or thrust sense, depending on the relative velocity between the upper crust and the underlying extruding material. This introductory chapter addresses the historical, theoretical, geological and modelling aspects of channel flow, emphasizing its applicability to the Himalaya-Tibet orogen. Critical tests for channel flow in the Himalaya, and possible applications to other orogenic belts, are also presented.
The age and degree of diachroneity of India-Asia collision is critical to construction of models of orogenesis and to understanding the causes of spatial variations in Himalayan evolution along strike. The age of collision is quoted between 65-34 Ma (Jaeger et al 1989; Aitchison et al 2007) and the degree of dichroneity is considered negligible (Searle et al 1997) to substantial (Rowley 1998). We studied the youngest Tethyan succession in the east (Tingri, Tibet) and west (Ladakh, India) of the orogen and used two approaches to date collision: 1) timing of closure of Tethys, by dating the youngest marine strata and 2) first evidence of Asian detritus deposited on the Indian plate, using U-Pb ages of detrital zircon to assess provenance. Both these approaches provide a minimum age to collision. In Ladakh, Indian plate passive margin limestones of the Paleocene Dibling Fm are overlain by the youngest marine facies of the region, the marine Kong Fm and fluvio-deltaic Chulung La Fm (Garzanti et al 1987). The age of the Kong and Chulung La Formations is disputed, from P5/6 (Fuchs & Willems 1990) to P8 (Garzanti et al 1987) the discrepancy possibly the result of research at different locations. Provenance is considered to be either ophiolitic from the Indian plate (Fuchs & Willems 1990) or containing detritus from the Trans-Himalayan arc of the Asian plate (Garzanti et al 1987; Critelli & Garzanti 1994). Our samples from the Kong Fm contained planktic foraminifera indicating a Middle to Early P6 age (54-56 Ma) and larger benthic foraminifera indicating Middle SBZ8 age (53-54 Ma). U-Pb dating of detrital zircons allows discrimination between Asian provenance (dominated by Mesozoic grains from the Trans-Himalayan arc) and Indian provenance (characterized by Precambrian grains and an absence of Mesozoic grains). Our data from the Kong and Chulung La Fms shows a primary provenance from the Asian plate. Thus collision is constrained by arrival of Asian detritus on the Indian plate by 54 Ma. In Tingri, Tibet, Indian plate passive margin limestones of the Zephure Shan Fm extend to the early Eocene, overlain by marine facies of the Pengqu Fm. The youngest marine facies have been dated at 34 Ma (Wang et al. 2002), but this age is disputed by other workers who assign an age of 50 Ma (Zhu et al. 2005). Our new biostratigraphic data from the Pengqu Fm show that calcareous nannofossil species are compatible with an age corresponding to Zones NP11-12 (50.6-53.5 Ma). The dominant population of detrital zircons have Cretaceous-Paleocene ages, derived from the Asian plate, thus indicating that contact between India and Asia had occurred by this time. We therefore conclude that India-Asia collision occurred by 54 Ma in the west, with only extremely limited, if any diachroneity eastward.
Abstract New phase equilibrium modelling, combined with U–Th/Pb petrochronology on monazite and xenotime, and 40 Ar/ 39 Ar geochronology on white mica, reveal the style of deformation and metamorphism near the southern tip of the extruded Himalayan metamorphic core (HMC). In the Jajarkot klippe, west Nepal foreland, greenschist to lower amphibolite facies metamorphism is entirely constrained to the Cenozoic Himalayan orogeny, in contrast with findings from other foreland klippen in the central Himalaya. HMC rocks exposed in the Jajarkot klippe yield short‐lived, hairpin pressure–temperature–time–deformation paths that peaked at 550–600°C and 750–1,200 MPa at 25 Ma. The Main Central thrust (MCT) and the South Tibetan detachment (STD) bound the base and the top of the HMC, respectively, and were active simultaneously for at least part of their deformation history. The STD was active at c . 27–26 Ma and possibly as late as c . 19 Ma, while the MCT may have been active as early as 27 Ma and was still active at c . 22 Ma. The tectonometamorphic conditions in the Jajarkot klippe are characteristic of crustal thickening and footwall accretion of new material at the tip of the extruding metamorphic orogenic core. Our new results reveal that collisional processes active in the middle to late Miocene at the base of the HMC now exposed in the hinterland were also active earlier, during the Oligocene, at the tip of the southward‐extruding middle crust.
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