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

Abstract

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.

Thermometry and Microstructural Analysis Imply Protracted Extensional Exhumation of the Tso Morari UHP Nappe, Northwestern Himalaya: Implications for Models of UHP Exhumation

“…Parameters that quantify the strength (i.e., nonrandomness) of quartz fabric development have been used as proxies for relative finite strain magnitude in several recent studies to investigate strain patterns across major Himalayan shear zones (Hunter et al, 2015(Hunter et al, , 2018Larson, 2018;Larson et al, 2017;Long et al, 2019;Starnes et al, 2020). We calculated the cylindricity index (B) from our seven CPO samples, which is a unitless value between 0, which represents a completely random fabric, and 1, which represents a completely nonrandom fabric (Vollmer, 1990).…”

“…Several thin sections of metasedimentary rocks from the TMN and most thin sections from the Tetraogal and Mata nappes contain elongated quartz grains that are isolated within a matrix of mica or calcite (Figures 12a-12c). These are interpreted as stretched detrital quartz clasts that represent deformed markers from which finite strain can be measured (e.g., Larson & Godin, 2009;Law et al, 2004;Long et al, 2017Long et al, , 2019. In order to quantify the magnitude and orientation of 3D finite strain, we performed the Rf-ϕ method (e.g., Dunnet, 1969;Ramsay, 1967) on quartz clasts from 34 total samples of metasedimentary rocks that span all three transects (15 samples from the TMN, five from the Tetraogal nappe, and 14 from the Mata nappe).…”

Jurassic–Cenozoic tectonics of the Pequop Mountains, NE Nevada, in the North American Cordillera hinterland

“…is commonly observed in Himalayan shear zones (Long et al, , 2019 and Sevier thrust structures (Yonkee, 2005). Similarly, extensional shear zones can involve local contractional folding (e.g., Miller et al, 1983;Snoke et al, 1997).…”