Sean P. Long

Temperature and strain gradients through Lesser Himalayan rocks and across the Main Central thrust, south central Bhutan: Implications for transport‐parallel stretching and inverted metamorphism

In order to understand mass and heat transfer processes that operated during Himalayan orogenesis, we collected temperature, finite and incremental strain, and kinematic vorticity data through a 5u2009km thickness of Lesser and Greater Himalayan rocks in southern Bhutan. This transect crosses two major shear zones, the Main Central thrust (MCT) and Shumar thrust (ST). Raman spectroscopy on carbonaceous material and garnet-biotite thermometry are integrated with deformation temperatures from quartz petrofabrics. These data define inverted field gradients that correspond in structural position with the MCT and ST, which are separated by sections in which temperatures remain essentially constant. Temperatures increase from ~400-500u2009°C to ~700-750u2009°C between 675u2009m below and 200u2009m above the MCT. This defines a 269u2009±u200944u2009°C/km inverted gradient, interpreted to have formed via high-magnitude (~100-250u2009km) shearing on a discrete MCT zone delineated by the limits of inverted metamorphism. Temperatures increase from ~300-400u2009°C to ~400-530u2009°C across the ST, which is attributed to differences in maximum burial depth of hanging wall and footwall rocks. n nStrain and vorticity data indicate that Lesser and Greater Himalayan rocks were deformed by layer-normal flattening. Transport-parallel lengthening and foliation-normal shortening increase from 38-71% and 36-49%, respectively, between 2.3-1.0u2009km below the MCT. The MCT acted as a ‘stretching fault’, with translation on the order of 100s of km accompanied by transport-parallel stretching of footwall and hanging wall rocks on the order of 10s of km. This demonstrates that stretching accommodated between major shear zones can make a significant contribution to cumulative mass transfer.

Distributed north-vergent shear and flattening through Greater and Tethyan Himalayan rocks: Insights from metamorphic and strain data from the Dang Chu region, central Bhutan

In several places in the Himalaya, there are debates over the location of and defining criteria for the South Tibetan detachment (STD) system. Here, we attempt to resolve this debate in central Bhutan by interpreting temperature, pressure, finite strain, and shear-sense data from an 11-km-thick structural transect through the Dang Chu region. Raman spectroscopy on carbonaceous material and garnet-biotite thermometry define a gradual, structurally upward decrease from 600–700 °C to 400–500 °C, and structural data indicate pure shear-dominant (Wm ≤0.4), layer-normal flattening strain and north-vergent shearing distributed through most of the section. Our data, when combined with published data from central Bhutan, define gradual, structurally upward cooling and an upright pressure gradient that is 1.2–2.4 times lithostatic distributed between 0 and 11 km above the Main Central thrust (MCT). Transport-parallel lengthening varies between ~20%–110% at 2–5 km above the MCT and between ~5%–55% at 5–11 km above the MCT, and north-vergent shearing is distributed between 2 and 11 km above the MCT. These data rule out the presence of a discrete, normal-sense shear zone and instead illustrate distributed structural thinning accommodated by north-vergent shearing. The strain data allow for ~85 km of distributed north-vergent displacement, which may be related to differential southward transport during MCT emplacement. Alternatively, distributed shear may have been translated northward into the STD system in northern Bhutan. Timing constraints for shearing on the MCT and STD allow for both possibilities. Central Bhutan provides a case study for largescale, distributed structural thinning, and highlights the diverse range of processes that accommodate tectonic denudation during orogenesis. LITHOSPHERE; v. 9; no. 5; p. 774–795; GSA Data Repository Item 2017271 | Published online 14 July 2017 https://doi.org/10.1130/L655.1

Geometry and kinematics of the Grant Range brittle detachment system, eastern Nevada, U.S.A.: An end‐member style of upper crustal extension

Documenting the range of styles of normal faulting is fundamental to understanding crustal extension. Here geologic mapping, field relationships, and deformed and restored cross sections illustrate the geometry and kinematic development of a system of west-vergent detachment faults in the Grant Range in eastern Nevada. Faults exhibit brecciation and stratigraphic cutoff angles of 5–15° at all structural levels and deform a 10u2009km thick section of Paleozoic and Paleogene rocks. The fault system is folded across an anticlinal culmination, which grew during extension, as indicated by progressively increasing interlimb angles and incision in the axial zone. The eastern limb consists of an imbricate stack of faults that were emplaced from bottom to top. In the western limb, several faults exhibit apparent thrust relationships. The oldest faults are cut by a ~29u2009Ma dike, and the highest preserved fault cuts ~32u2009Ma volcanic rocks that restore to paleodepths of ~1u2009km. Retrodeformation of folding and minimal structural relief and angularity across a Paleogene unconformity indicate the faults were active at 5–15° angles. Retrodeformation of offset indicates ≥49u2009km (98%) extension. We propose a model of stationary, sustained isostatic uplift and incision at the culmination axis (a “fixed hinge”), with updip excision producing bottom-to-top growth of the imbricate stack and downdip excision producing apparent thrust relationships. The fault system exhibits similarities to core complex detachment systems, though it is confined to upper crustal levels, and there are no preserved high-angle or listric normal faults, indicating a unique extension style dominated by low-angle excision.