The Paleozoic tectonothermal evolution of the Bachu Uplift of the Tarim Basin, NW China: Constraints from (U–Th)/He ages, apatite fission track and vitrinite reflectance data

Abstract

This study uses a new application of the apatite and zircon (U–Th)/He thermochronometry technique to investigate the thermal history and tectonic uplift of sedimentary basins. Based on measured apatite and zircon (U–Th)/He ages, apatite fission track data and equivalent vitrinite reflectance measurements of borehole core samples, the thermal history of the Bachu Uplift of the Tarim Basin was modeled. Our analysis indicates a complicated thermal history in the Bachu Uplift region during the Paleozoic. The thermal gradient initially increased from 28–30 °C/km in the Cambrian period to 30–33 °C/km in the Ordovician period and increased up to 33–34 °C/km in the Silurian period. The thermal gradient began to decrease during the Devonian period and was estimated to be approximately 30 °C/km during the Late Devonian and the Early Carboniferous periods. The low thermal gradient phase was followed by an additional high gradient phase from the Late Carboniferous to the Early Permian when the gradient was approximately 32 °C/km. Thermal gradients in the Paleozoic were substantially different than the present-day gradient (21 °C/km). Tectonothermal modeling results also revealed uplift events in the Bachu Uplift region that shifted from south to north (e.g., ∼150 Ma to ∼100 Ma) and from west to east (e.g., ∼250 Ma to ∼150 Ma). This thermal evolution of the Bachu Uplift is consistent with its tectonic progression. The thermal spike in the Late Carboniferous and Early Permian periods could be related to magmatic activities in the Early Permian period. The Paleozoic thermal history of the Bachu Uplift region of the Tarim Basin could not be reconstructed properly due to a lack of reliable thermal indicators in the Lower Paleozoic successions, and a poor understanding of the thermal evolution of Lower Paleozoic source rocks during the Paleozoic has hindered petroleum exploration in this area. This study provides a new Paleozoic thermal history and shows that the Paleozoic thermal history plays an important role in the maturation process of the Lower Paleozoic source rocks.