OSCILLATORY- AND SECTOR-ZONED GARNETS RECORD CYCLIC (?) RAPID THRUSTING IN THE CENTRAL NEPAL HIMALAYA

Garnets with oscillatory-zoning (repeated compositional reversals) and sector-zoning (crystallographically-controlled compositional differences) were discovered in Lesser Himalayan rocks from near the Main Central Thrust, in the Langtang region, central Nepal. The oscillations occur as 0.005 to 0.02 mole% spikes in Ca and Y, superimposed on general rimward depletions. The sector zoning occurs as subtle, 0.005 mole% higher Ca contents in zones corresponding to former {112} faces compared to adjacent zones corresponding to the common {011} faces. Both the oscillatory and sector zoning result from crystal growth kinetic effects, including repeated depletion/enrichment of the matrix in compatible/incompatible elements (oscillations), and preservation of chemical differences on different crystal faces (sectors).

Both types of zoning are interpreted to result from extraordinarily rapid, possibly cyclic thrusting. If cyclic thrusting occurred, then time-scales of thrust rate inhomogeneities are ~25 Kyr. This timescale implies that (1) satellite determination of modern fault movement rates may not accurately reflect slip rates or strain partitioning in the Himalaya for times previous to the late Pleistocene, and (2) strain in the Indo-Asian collision must be cyclically partitioned on different major structures over tens of Kyr. This latter observation is consistent with variations in thrust rates within the MCT zone from ~2 to >4 cm/yr on Myr timescales (Kohn et al., 2004). Sector zoning in Ca but not Y, Mn, or Mg implies maximum intra-crystalline diffusion rates for Ca of ~10^-21 cm²/sec at 550 °C (consistent with experimental determinations), but faster rates for other elements.