Using Quartz Petrofabric Intensity Parameters to Delineate Shear Zones: A Case Study From the Main Central Thrust in Western Bhutan

Starnes, Jesslyn K.; Long, Sean P.; Zhang, Jingyi; Gordon, Stacia M.

doi:10.1130/abs/2017am-297707

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“…The fabric patterns from these seven samples are indicative of deformation within a 3-D strain field that approximates plane strain (e.g., Schmid and Casey, 1986). Statistical parameters can be used to quantify the intensity (i.e., nonrandomness) of fabric development, and have recently been applied as strain magnitude proxies to characterize strain patterns across Himalayan shear zones (Larson et al, 2017;Starnes et al, 2017). Here, we utilize the cylindricity index (B) of Vollmer (1990), which is based on eigenvector analysis of c-axis directions (e.g., Woodcock, 1977;Mainprice et al, 2014).…”

Section: Quartz Crystallographic Fabricsmentioning

confidence: 99%

A structural model for the South Tibetan detachment system in northwestern Bhutan from integration of temperature, fabric, strain, and kinematic data

et al. 2019

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Despite playing a fundamental role in all models of Himalayan tectonics, minimal data constraining the structural evolution, metamorphic history, and offset magnitude of the South Tibetan detachment system (STDS) are available. Here, we integrate petrofabric, finite strain, and kinematic data with metamorphic and deformation temperatures to generate a structural model for the STDS in northwestern Bhutan. We divide the STDS into an ∼2-km-thick lower level that accommodated ∼6–13 km of thinning via ≥30–76 km of simple shear-dominant displacement within Greater Himalayan rocks, and an ∼3-km-thick upper level that accommodated ≥21 km of displacement via an upward decrease (from 44% to 2%) in transport-parallel lengthening within Tethyan Himalayan rocks. Peak metamorphic temperatures in the lower level are ∼650–750 °C, and two distinct intervals of telescoped isotherms in the upper level define a cumulative upward decrease from ∼700 to ∼325 °C. These intervals are separated by an abrupt upward increase from ∼450 to ∼620 °C, which we interpret as the result of post-STDS thrust repetition. Above the upper telescoped interval, temperatures gradually decrease upward from ∼325 to ∼250 °C through a 7-km-thick section of overlying Tethyan Himalayan rocks. Telescoped isotherms lie entirely above the high-strain lower level of the STDS zone, which we attribute to progressive elevation of isotherms during protracted intrusion of granite sills. This study demonstrates the utility of using gradients in fabric intensity and thin section-scale finite strain to delineate shear zone boundaries when field criteria for delineating strain gradients are not apparent.

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