Geometry of an oblique thrust fault zone in a deepwater fold belt from 3D seismic data

Growth of a 12 km long, deepwater anticline during the late Pliocene–Recent is documented from 3D seismic reflection data across NW Borneo. The fold is part of a train of folds formed along the slope at the distal margin of the Baram Delta Province. Growth of the anticline involved fold lateral propagation and linkage of two thrusts formed in the anticline forelimb as either break thrusts or as imbricates ramping up from a master detachment (at ∼3 km depth). For the southwestern anticline, the northern tip of the NE–SW striking, SE-dipping thrust passes into an E–W striking oblique termination, which lies in the linkage zone between the two anticlines. The thrust termination is characterised by the following changes passing east towards the fault tip: 1) the fault zone dips steeply to the south, then 2) passes to a vertical segment (inferred to have oblique motion), and 3) furthest east the fault dips northwards with an extensional component of displacement. The fault zone terminates in a transtensional graben. This graben does not fit with a simple pull-apart geometry or simple oblique ramp geometry. In the future if the thrust faults propagate together and link this oblique fault zone may develop into an oblique ramp that acts as a transfer zone between the faults. However at present the oblique fault zone appears to be a region of 3D strain, where deformation at the fault tip, and the gravity effects of plunging folds have affected the shallow, weak sediments and given rise to a complex thrust termination at a early stage of thrust and fold development. The oblique structure may have developed in response to strains imposed by the encroachment of the fold and thrust belt on an uplifted basement or volcanic high that forms a pronounced topographic feature across a narrow part of the thrust front, 14 km NW of the most external (oceanward) fold.