METAMORPHIC AND CHRONOLOGIC CONSTRAINTS ON HIMALAYAN THERMAL-MECHANICAL MODELS

Himalayan thermal mechanical models differ in two basic ways: channel flow models presume large-scale (100s of km) of flow of the Greater Himalayan Sequence (GHS) between lower and upper ductile bounding shears [the Main Central Thrust (MCT) and the South Tibetan Detachment System (STDS), respectively]. In contrast, critical taper models presume large-scale transport on the MCT, but little or no flow of the GHS, and minimal movement on the STDS. Structural studies document a ductile flow style of deformation for the GHS, but do not quantify the scale of flow. Metamorphic and chronologic analysis instead offers insight into the scale of heat transfer, which correlates with mass transfer. These data distinguish between large-scale vs. small-scale flow, and hence between contrasting thermal-mechanical models. Metamorphic P-T conditions, P-T paths, and cooling histories around the MCT are inconsistent with large-scale flow of the GHS. For example, channel flow predicts hotter P-T conditions, P-T paths characteristic of much greater heat advection, and much later cooling than indicated by rocks in Central Nepal. Critical taper models instead match these data more closely. Chronologic studies around the STDS indicate that ductile flow lasted for less than 5 Myr, and likely ≤3 Myr: ductile extension on the STDS did not occur until 24 Ma at the earliest, and more likely 22 Ma, whereas undeformed plutons that cross-cut the ductile STDS in NW India are 19 Ma. Metamorphic monazite chronology in Bhutan similarly indicates a duration of ductile movement of only ~2 Myr (Chambers et al., this meeting). A duration of 2-3 Myr does not accommodate more than a few 10s of km ductile shear on the STDS, consistent with recent regional structural analysis in Bhutan (Long and McQuarrie, this meeting). Thus, metamorphic and chronologic data support critical taper models and not channel flow, although a brief period of wedge flattening and ductile extension on the STDS is indicated.