CALCITE STRAIN ANALYSIS OF LIMESTONES ACROSS THE CAMBRIAN-ORDOVICIAN UNCONFORMITY IN THE SPITI AND ZANSKAR REGIONS, INDIAN HIMALAYA

One of the most enigmatic tectonic episodes in the Himalaya is represented by a Cambrian-Ordovoician unconformity that occurs along a length of the NW Himalaya. The unconformity is overlain by coarse conglomerate and is associated with local faults and folds. Current debate about the Cambrian-Ordovician unconformity revolves around whether it marks uplift and erosion associated with the development of a fold-thrust belt or rift system along the northern Indian continental margin. We collected oriented block samples of low-grade Cambrian and Carboniferous limestone above and below the unconformity in order to compare strain patterns in rocks that bracket this tectonic episode. Twinning strains calculated for the Cambrian and Carboniferous rocks using all of the data from each sample are similar, the shortening strains range from 3% to 10%, X/Z ratios range from 1.05 to 1.24, and intermediate axial lengths are roughly 1.00 indicating plane strain. Our results emphasize that there is no Early Paleozoic twinning strain in the Cambrian samples that is distinct from twinning strains in the Carboniferous samples above the unconformity. The negative expected values (NEV), which may reflect inhomogenous strain or an additional non-coaxial strain, are generally higher in Cambrian rocks (20%-43%) than in Carboniferous rocks (9%-24%). However, removal of 20% of the largest magnitude deviations to eliminate the grains with the largest inhomogeneous strains and possibly measurement errors, significantly decreases the NEV for both Cambrian and Carboniferous samples (7%-19% NEV). In addition, separate analysis of positive expected value (PEV) and NEV data shows similar principal shortening (e1) strain patterns for both Cambrian and Carboniferous samples, which would not be expected if Cambrian rocks contained a separate noncoaxial Early Paleozoic strain that is absent in the Carboniferous samples. The similarity of twinning strains in the Cambrian and Carboniferous rocks indicates that early Paleozoic deformation was either roughly coaxial (<45 degrees) to post-Carboniferous (Cenozoic?) deformation or, alternatively, related to rifting rather than thrusting, which would not have necessarily caused a widespread twinning strain.