Tectonic Evolution, Sequence Stratigraphic Filling Response and Genetic Mechanism of Asymmetric Graben in Continental Rifted Lacustrine Basin

Abstract

The tectono‐sedimentary evolution of continental grabens is fundamentally governed by the differential kinematics of boundary faults. While classic models traditionally conceptualize grabens as symmetric structures, their actual evolution is profoundly asymmetric. Using the Sujiatun sag (Songliao Basin) as a natural laboratory, this study integrates 3D seismic, wireline log, and core data to reconstruct the high‐resolution sequence‐stratigraphic and tectono‐sedimentary dynamics of an asymmetric graben. We propose a comprehensive evolutionary model and elucidate the deep‐shallow coupled geomechanical drivers behind this asymmetry. The syn‐rift succession is partitioned into six third‐order sequences (SQ1–SQ6). During the initial rift phase (SQ1–SQ2), the dominant eastern key fault (F2) pinned the depocenter to the east, generating an aggradation‐dominated sequence filled by steep‐slope fan‐deltas. During the intensive rift phase (SQ3–SQ4), the conjugate fault system underwent a profound kinematic ‘see‐saw’ reversal. This tectonic switching drove a cross‐basin, westward migration of the depocenter, plunging the basin into a highly underfilled state dominated by deep‐lacustrine source rocks. In the recession phase (SQ5–SQ6), tectonic relaxation and geomorphological planation facilitated the basinward progradation of widespread, overfilled braid‐delta systems across a symmetrized basin profile. Geomechanical analysis reveals that initial fault differentiation stemmed from the selective reactivation of inherited basement fabrics. The subsequent kinematic reversal was dictated by the stress feedback of surface mass redistribution: massive sedimentary loading ‘clamped’ the eastern fault, whereas erosional unloading ‘unclamped’ the western fault. Macroscopically, asymmetric asthenospheric upwelling and magmatic underplating dictated the deep rheological heterogeneities driving the initial rupture. This ‘dynamic asymmetric graben’ model challenges static exploration paradigms, offering critical mechanistic insights into the heterogeneous distribution of source rocks and subtle lithostratigraphic traps, at the same time providing a robust predictive framework for hydrocarbon exploration in similar continental rifts worldwide.