DISTRIBUTED STRETCHING GRADIENTS BETWEEN MAJOR HIMALAYAN SHEAR ZONES, AND THEIR ROLE IN MASS TRANSFER (Invited Presentation)

Accurate assessment of mass transfer processes is critical for understanding how orogenic systems evolve. However, in the ductile portions of orogens, strain can be complexly partitioned between structures of various scales and styles, ranging from regional shear zones to distributed grain-scale fabrics. Here, I discuss three case studies of gradients in transport-parallel stretching (TPS), obtained from measurements of 3D crystal-plastic strain (Rf-phi method) of relict quartz porphyroclasts in rock samples (n=84) distributed above and below the Main Central thrust (MCT) and South Tibetan detachment system (STDS) in Bhutan. These data are used to quantify the relative contributions of offset on shear zones versus strain accommodated within rocks between them.

In central Bhutan, rocks 2.5-4.5 km below the MCT experienced 14% average TPS. Between 2.5-1.0 km below the MCT, TPS increases upward from 38% to 71%, which defines a ≥45 km upward increase in southward transport distance. The MCT is estimated to have accommodated 100-250 km of top-to-S displacement within a ~1 km thick interval. TPS is ~65% average between 2-5 km above the MCT and ~30% average between 5-11 km above the MCT. These data allow for as much as 85 km of distributed top-to-N displacement, which may have been fed northward into the STDS. In northwestern Bhutan, the basal ~2 km of the STDS zone accommodated ≥30-75 km of top-to-N offset. Above this, TPS decreases upward from 44% to 2% through a 4.5 km thickness of Tethyan Himalayan rocks, which accounts for ≥20 km of additional top-to-N displacement.

These examples show that displacement on major shear zones was accompanied by distributed TPS of hanging wall and footwall rocks on the order of 10’s of km, demonstrating that TPS gradients between shear zones can make a significant contribution to cumulative mass transfer. As a condition of strain compatibility, stretching gradients enhance displacement magnitude in the transport direction, and are thus an important component of the deformation field that must be considered for accurate mass balance assessment. This process explains the development of ubiquitous shear zone-subparallel foliations and transport-parallel stretching lineations within exhumed metamorphic terranes, and is predicted to become more efficient at higher deformation temperatures.