Marine and Petroleum Geology 25 (2008) 606–624 Seismic images of a collision zone offshore NW Sabah/Borneo Dieter Franke a,Ã , Udo Barckhausen a , Ingo Heyde a , Mark Tingay b , Nordin Ramli c a Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655 Hannover, Germany b School of Earth & Environmental Sciences, University of Adelaide, Geology and Geophysics, SA 5005, Australia c Petroliam Nasional Berhad (PETRONAS) Kuala Lumpur City Centre, 50088 Kuala Lumpur, Malaysia Received 17 September 2004; received in revised form 8 November 2007; accepted 20 November 2007 Abstract Multichannel reflection seismic data from the southern South China Sea, refraction and gravity modelling were used to investigate the compressional sedimentary structures of the collision-prone continental margin off NW Borneo. An elongated imbricate deepwater fan, the toe Thrust Zone bounds the Northwest Borneo Trough to the southeast. The faults separating the individual imbricates cut through post-Early Miocene sediments and curve down to a carbonate platform at the top of the subsiding continental Dangerous Grounds platform that forms the major detachment surface. The age of deformation migrates outward toward the front of the wedge. We propose crustal shortening mechanisms as the main reason for the formation of the imbricate fan. At the location of the in the past defined Lower Tertiary Thrust Sheet tectonostratigraphic province a high velocity body was found but with a much smaller extend than the previously defined structure. The high velocity structure may be interpreted either as carbonates that limit the transfer of seismic energy into the sedimentary layers beneath or as Paleogene Crocker sediments dissected by remnants of a proto-South China Sea oceanic crust that were overthrust onto a southward migrating attenuated continental block of the Dangerous Grounds during plate convergence. r 2007 Elsevier Ltd. All rights reserved. Keywords: Northwest Borneo margin; Sedimentary succession; Seismic data 1. Introduction The NW Borneo continental margin lies within a broad zone of lithospheric deformation at the boundary between Borneo to the south, the Sulu Sea and Celebes Sea regions to the east, and the South China Sea to the northwest (Fig. 1). A striking onshore structural element of this wide and highly complex deformation belt is the Crocker- Rajang mountain belt which extends along the central part of Borneo, from Sabah to central-south Sarawak (Fig. 1; Hamilton, 1979; Benard et al., 1990; Hutchison et al., 2000). It is generally assumed that this mountain belt formed as an accretionary complex during south-directed subduction of a proto-South China Sea (Hamilton, 1979; Hall, 1996; Hutchinson, 1996; Meng, 1999; Pubellier et al., 2003). The closure of the proto-South China Sea began at 44 Ma (e.g. Hall, 1996, 2002). Collision of the proto- South China Sea crust is believed to have first occurred in the SW (Luconia Shoals) during the Late Eocene and commenced progressively later towards the NE until the Early Miocene (Hutchinson, 1996). A large deepwater foreland fold and thrust belt, 100 km wide is present adjacent to the NW Borneo Trough (Figs. 1 and 2). The origin of the 5–8 km thick imbricate fan in this Thrust Zone remains uncertain. Hinz et al. (1989) and Ingram et al. (2004) attribute the development of the Thrust Zone to crustal shortening while Tan and Lamy (1990), Hazebroek and Tan (1993) and Hutchinson (2004) interpret the thrusts as gravitational induced compressive deformation at the toe of the Tertiary delta system from NW Borneo. An important argument favouring the latter interpretation is that oceanic spreading in the South China Sea basin, which is commonly interpreted to be coincident with the plate convergence at the NW Borneo continental margin, ceased in the Early Miocene or early Middle Miocene (Barckhau- sen and Roeser, 2004; Taylor and Hayes, 1980; Briais et al., 1993). However, the crustal shortening hypothesis is strengthened by the observation of Miocene and Pliocene (post oceanic spreading) fault reactivation and inversion ARTICLE IN PRESS www.elsevier.com/locate/marpetgeo 0264-8172/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2007.11.004 Ã Corresponding author. Tel.: +49 511 643 3235; fax: +49 511 643 3663. E-mail address: Dieter.Franke@bgr.de (D. Franke).