Tectonic evolution analysis constrained jointly by in-situ calcite U-Pb dating and apatite fission track for southwestern Ordos Basin

The lower Paleozoic strata of Ordos Basin is rich in oil and gas resources. The Lower Paleozoic marine carbonate in the southwestern Ordos Basin is characterized by large thickness and high abundance of organic matter. The source rocks are in dry gas window.However, the tectonic evolution has not been well evaluated. The timing of the earlier tectonic events is a key scientific problem and needs to be addressed timely.In this study, under the constraints of detailed field investigations, microstructure characteristics, in-situ trace element analysis and regional deposition-burial evolution history, the secondary calcite in-situ U-Pb dating is combined with apatite fission-track time-temperature simulation curves of the Lower Paleozoic carbonate strata and its fault facture zone to reveal four tectonic events including the Late Ordovician, Late Triassic, Late Jurassic and end of the Early Cretaceous-Cenozoic.A hydrothermal event occurred at the Late Ordovician 452±13 Ma, which might be linked to the subduction and closure of the Proto-Tethys Ocean during the Caledonian period.A limited Indosinian tectono-thermal event occurred at 214±22 Ma, which might be caused by the regional fault movement.The intense tectonic activities during the Yanshanian and Himalayan periods induced a strong uplift and denudation.The southwestern Ordos Basin experienced multi-stage complex tectonic evolution and the caprock was seriously damaged, leaving rather unfavorable preservation conditions for hydrocarbon.This study conducts a novel tectonic evolution analysis by combining low-temperature thermochronology with in-situ U-Pb dating of fault-related calcite, providing a new way to analyze tectonic evolution and later transformation process at the margin of complex superimposed basins.