High-resolution P-T paths from garnet-bearing rocks across the Himalayan
Main Central Thrust: Implications for understanding the crustal response
to orogenic processes
Abstract
Barrovian-grade pelites in the Greater Himalayan Crystallines and Lesser
Himalayan Formations exposed in the Himalayan core are separated by the
Main Central Thrust (MCT). This fault system accommodated a significant
amount of India-Asia convergence and is the focus of several models that
explore ideas about the development of the range and collisional belts
in general. Units separated by the MCT provide critical information
regarding the mechanisms of heat transfer within collisional belts.
Garnets collected across the MCT record their growth history through
changes in chemistry. These chemical changes can be extracted and
modeled using a variety of thermodynamic approaches. Here we describe
and apply particular thermobarometric techniques to decipher the
metamorphic history of several garnet-bearing rocks collected across the
MCT in central Nepal, the Sikkim region, and NW India. Comparisons are
made between the results of previously-reported conventional rim P-T
conditions and P-T paths extracted using the Gibb’s method to isopleth
thermobarometry and high-resolution P-T path modeling using the same
data and assemblages. Regardless of calibrations used, the P-T
conditions and paths, along with previously-reported timing constraints,
are consistent with an imbrication model that suggest the MCT shear zone
developed as rock packages within the LHF were progressively
transferred. In this model, samples within the LHF travel along the MCT
at a 5 km/Ma speed rate from 25 to 18 Ma. The hanging wall speed rate is
10 km/Ma, and topography progressively accumulates until a maximum
height of 3.5 km. Once the topography is achieved at 18 Ma, a period of
cessation is applied to the MCT between 18 and 15 Ma, and topography is
reduced at a rate of 1.5 km/Ma. The model returns to activity within the
MCT shear zone with the activation of the MCT footwall slivers from 8 to
2 Ma. P‐T changes recorded by the footwall garnets result from thermal
advection combined with alterations in topography. For most MCT footwall
samples, the P-T paths match the model predictions remarkably well. The
P-T paths for some samples in central Nepal are also consistent high
exhumation rates (>12mm/year) within the MCT shear zone
since the Pliocene, a scenario predicted by this imbrication model.