Structural Interpretation of extensional Deformation along the Dali Fault System, Southeastern Margin of the Tibetan Plateau

Abstract

The Dali fault system is a prominent active tectonic feature along the southeastern margin of the Tibetan plateau, developed between the Three River fold belt and the South China block. It includes three main strands bending eastward, from east to west—the Chenghai, Heqing, and Heihuijiang faults. Quaternary deformation along these strands involves both left-lateral strike slip and normal dip slip. Left-lateral strike slip occurs primarily along southern fault segments, oriented Ne-SW. To the north, these faults curve northward, whereas to the south they terminate against the NW-trending Diancang Shan massif. Our studies of the interaction between these structures and faults along the boundaries of the Diancang Shan massif reveal numerous indications of recent fault slip, such as fault scarps, fault gouge or breccia, syn-tectonic sedimentation, tilting, landslides, and fault-related folds. Kinematic studies indicate that Ne-SW—directed normal faulting along middle segments of the Heqing and Heihuijiang faults resulted in relative uplift of the Yulong Shan massif and concomitant subsidence of the Heqing and Lijiang basins. Similarly, Ne-SW—directed normal faulting along the northern and southern boundaries of the Diancang Shan massif have resulted in uplift of the massif and subsidence of the Qiaohou and eryuan basins, as well as erhai Lake. The basins preserve a Pliocene and early Pleistocene sedimentary record, suggesting initiation of the fault system at this time. Ne-SW—directed normal faulting along the middle segment of the Chenghai fault has resulted in subsidence of the Binchuan Basin, which preserves an early to Middle Pleistocene sedimentation record. Finally, Ne-SW—directed normal faulting along the southern end of the Chenghai fault and the northern boundary of the Diancang Shan massif resulted in crustal shortening along the Nancun thrust-fold belt. All these geological arguments indicate that left-lateral movement along the southern segments of the Chenghai, Heqing, and Heihuijiang faults have been largely transferred into both transtensional and transpressional motions, resulting in a characteristic landscape dominated by basins alternating with mountains. Reconstruction of the basin strata indicate that extension across the Heqing and Binchuan basins amounts to 2 and 3.7 km, respectively. Based on both the shear sense and irregular geometric features of these strands of the Dali fault system, we interpret fault slip to have accommodated clockwise rotation of the intervening blocks, including the Diancang Shan massif, around a vertical axis. Sedimentary and isotopic data show that rotational movement of the Dali block likely was initiated in the south during Pliocene time, and subsequently migrated to the north by early to Middle Pleistocene time. We attribute rotation to N-S right-lateral shear along the southeastern margin of the plateau, generated by the northward motion of India relative to eurasia.