Development and use of garnet-based high-resolution P-T-t paths to
constrain the dynamics of Himalayan orogenesis
Abstract
Tectonic models as a universal outcome generate predictions regarding
the travel time paths of rocks as they are displaced due to the
application of particular input parameters and boundary conditions. A
need for most of these models, either as a constraint for realistic
input conditions or to gauge their relevance to a particular natural
system, is pressure‐temperature‐time (P‐T‐t) paths from individual rock
samples that track the conditions they experienced during displacement.
Although arguments can be made that P‐T paths and absolute peak P‐T
conditions may not necessarily be diagnostic of processes involved, this
type of information is clearly a valuable addition to other types of
data, such as timing and microstructural information regarding strain
recorded during rock deformation. Low‐resolution P‐T paths can be
limited in their ability to test ideas regarding lithospheric response
to perturbations, including motion within fault zones. Here we apply
advances in thermodynamic modeling to acquire high‐resolution P‐T paths
that show the conditions responsible for garnet growth within one of the
Himalayas’ major fault systems. The approach we outline can be applied
to any garnet‐bearing assemblage using bulk rock and mineral
compositions and have the potential to significantly increase the
understanding of the dynamics of field areas that contain garnet, from
the mineral’s crystallization to erosion‐driven or tectonically-driven
exhumation. Overall, high-resolution garnet-based P-T paths were
generated for two transects across the Himalayan Main Central Thrust
(MCT) spaced ~850 km apart (along the Bhagirathi and
Marsyangdi drainages) and monazite grains were dated in situ to help
constrain crystallization time. Rocks collected at equivalent structural
positions to the MCT along both transects show similar paths and a shear
zone imbrication model suggest the MCT zone has very high exhumation
rates, up to 12 mm/yr since the Pliocene.