Paleostrain stratigraphic analysis of calcite twins across the Cambrian–Ordovician unconformity in the Tethyan Himalaya, Spiti and Zanskar valley regions, India

Abstract

Calcite strain analyses were conducted on low-grade Cambrian and Carboniferous limestone samples collected above and below the Cambrian–Ordovician unconformity in the Spiti and Zanskar valley regions of the NW Himalaya in order to compare strain patterns in rocks that bracket an enigmatic early Paleozoic tectonic episode. All samples record a layer-parallel shortening strain at a high angle to folds and faults in the Tethyan Himalayan fold-thrust belt. In the Carboniferous samples, we relate these layer-parallel strains to the onset of Cenozoic deformation within the Tethyan Himalayan fold-thrust belt. The Cambrian sample from the Spiti area contains a layer-parallel shortening strain even though the Cambrian–Ordovician unconformity is angular. This suggests that the twinning strains in the Cambrian sample may have formed at the onset of early Paleozoic folding and subsequent erosion, and that early phases of Cenozoic shortening were coaxial to early Paleozoic shortening. The maximum shortening axis in the Carboniferous samples, which is probably parallel to the early thrust transport direction in the Tethyan Himalayan fold-thrust belt, is parallel to the NE movement of India with respect to Eurasia in the Middle Eocene, suggesting that it might closely correspond to the India/Eurasian slip direction during this time period.