Frontal expansion of an accretionary wedge under highly oblique plate convergence: Southern Indo-Burman Ranges, Myanmar

Abstract

The formation of accretionary wedges with oversteepened slopes and uplifted axial zones has been demonstrated to be potentially associated with highly oblique plate convergence by numerical and analog studies. The direct role of this mechanism, or other factor(s) in producing the described structural and morphological features in nature, however, has yet to be confirmed. We used seismic reflection sections, detrital zircon U-Pb ages, and detrital apatite fission-track thermochronological data to examine the effects of highly oblique convergence and sediment reworking on accretionary wedge growth in the Indo-Burma Subduction Zone. A detailed subsurface structural analysis of a two-dimensional seismic survey from the outer wedge of the southern Indo-Burman Ranges, Myanmar, yielded three primary characteristics. These are (1) a narrow, steep deformation front (average width 15.6 km) and a vast, low-relief shelf terrace (average width 49 km); (2) a comparatively long-lived growth thrust fault (FT1) with a convex-up geometry at the rear of the deformation front that controlled the vertical stack of the progradational sequences in the shelf terrace; and (3) a group of NE-striking transtensional faults that cut through entire outer-wedge successions and displays as a series of negative flower-like structures. These characteristic features are roughly consistent with the results of laboratory analog modeling of highly oblique plate convergence but significantly differ from those of natural accretionary wedges that formed under highly oblique convergence conditions, such as those in the Sumatra, Hikurangi, Chile, and Cascadia. We further analyzed the sediment provenance of the southern Indo-Burman Ranges and discovered that the outer-wedge rocks are a product of sediment reworking of the hinterland wedge that began to be uplifted and exhumed in the early Miocene (22−12 Ma) due to transpressional motion between the Indian plate and West Burma Terrane. Our analyses indicate that active sedimentation behind the major growth thrust fault (FT1) provided additional basal shear stress that strengthened the coupling of the interface between the wedge base and décollement and promoted the vertical expansion of the outer wedge of the southern Indo-Burman Ranges from the Neogene to the present day. In contrast, the outer wedge of the central Indo-Burman Ranges has experienced stronger forward accretion since the late Miocene, which could be explained by a smaller degree of obliquity and weaker sediment reworking. Our findings demonstrate that both highly oblique plate convergence and sediment reworking were the primary driving forces that triggered vertical development of accretionary wedges. The results of this research have significant implications for understanding the structures and kinematic evolution of wedge systems at other convergent plate margins, in which seamount passage or subduction erosion is often interpreted as the cause of the steeply tapered wedges.