Thermochronological dating of brittle structures in basement rocks: A case study from the onshore passive margin of SW Sri Lanka

摘要

The brittle structural inventory of southern and southwestern Sri Lanka has been studied by kinematic, mineralogical and thermochronological techniques. Thermochronological analyses of faults comprise apatite and zircon fission track (FT) data and apatite (U-Th-Sm) /He ages from fault planes and undisturbed host rocks, and range between ～65 and ～230 Ma. The ages of both settings are undistinguishable for topographic altitudes below ～100 m, while fault planes from higher elevations are significantly younger than the corresponding host rocks. Thermal history modeling and qualitative interpretation of the thermochronological data identify at least five episodes of thermal overprint associated with faulting activity occurring at 159 ± 18, 144 ± 14, 120 ± 10, 94 ± 8, and 70 ± 10 Ma. The kinematic, mineralogical and thermochronological data collectively show that Sri Lanka was subjected to major N-S oriented extension subsequent to the Gondwana breakup. The resulting brittle structures exert primary control on the island's geomorphology, especially on the Southern Escarpment of the Sri Lankan Highlands.