Formation mechanism of salt piercement structures in a compressive environment: An example from the Kuqa depression, western China

Abstract

The formation mechanism of piercement structures is more complex than that of concealed piercement structures, and little research has been conducted on this topic. In this work, based on surveys of field geological outcrops and detailed interpretation of seismic data, combined with the evolution of the tectonic-sedimentary environment and stratum growth characteristics, the geometry and evolutionary history of salt piercement structures in the Kuqa Depression are investigated. Through discrete element numerical simulation experiments, the influences of various factors on the formation and evolution of the piercement structure are discussed. The results show that the piercement structures in the Kuqa Depression are mainly developed in the northern margin of the Wensu paleohigh, the Quele passive salt diapir, and at the top of the Kelasu basement fault. The salt can directly pierce the overlying strata or contact the surrounding rock through faulting. The characteristics of the growth strata reveal that the Kelasu piercement structure formed first, followed by the Wenshu piercement structure, and the Quele piercement structure formed later. The evolution of the piercement structure can be divided into three stages: quiet, weak compression and strong compression. Relying solely on tectonic compressive stress, it is difficult for salt layer to form piercement structures. The most advantageous location for the development of the piercement structure in a compressive environment is in the margin of the paleohighs and low bulges and at the top of early passive salt diapirs and preexisting basement faults. The front of the sedimentary wedges is also a preferential location for the development of piercement structures. The barrier of the paleo-high, reactivation of the preexisting basement fault, priority activation of the early passive salt dome, and progradation of the sedimentary wedge from the orogenic belt to the basin interior are favorable factors inducing piercement structure formation. An important mechanism for controlling salt piercement is to promote strong thrusting in suprasalt strata, which provides a channel for salt upwelling.