Balancing deformation in NW Borneo: Quantifying plate-scale vs. gravitational tectonics in a delta and deepwater fold-thrust belt system

Abstract

a b s t r a c tRecent GPS measurements demonstrate that NW Borneo undergoes 4-6 mm of plate-scale shortening a year, which is not accommodated by plate-scale structures. The only geological structure in NW Borneo described to accommodate on-going shortening is the Baram Delta System located on the outer shelf to basin floor. Delta toe fold-thrust belts are commonly thought to be caused by margin-normal compressional stresses generated by margin-parallel upslope gravitational extension.The Baram Delta System is divided into three neotectonic provinces: 1) an inactive onshore and inner shelf region of inversion superimposed on an older extensional deltaic province, 2) an outer shelf region of present-day deltaic extension, and; 3) a compressional delta toe. However, it is uncertain whether compression in the delta toe area is purely driven by gravity (i.e. shortening is less than upslope extension) or if there is a component of regional shortening involved, which can only be demonstrated if shortening is greater than upslope extension.In order to quantify the balance between the shortening related to the upslope gravitational extension and the shortening related to the regional geodynamic framework in the Baram Delta System, we have used a geomechanical code based on the Finite Element Method and conservation of mass and momentum, Dynel 2D. This enabled us to reconstruct the tectonic evolution of the delta and to demonstrate for the first time that the total shortening observed in the delta toe does not balance against the active extension in the delta top; with the system exhibiting >1.8% shortening overall (or w2.0 km). This additional shortening is therefore attributed to plate-scale shortening across NW Borneo produced by far-field compression. Using a convergence rate of 4 mm y À1 , demonstrated by GPS measurements, the delta toe has accommodated far-field compression only in the last 0.5 My.Crown

Transverse segmentation of the Baram-Balabac Basin, NW Borneo: refining the model of Borneo's tectonic evolution

“…The Belait Formation represents a thick succession of shelf and deep-water sediments shed from the Interior Highland into a foreland basin constructed on attenuated continental crust of the South China Sea. The amount of shortening in the deep-water fold-and-thrust belt is not balanced by the amount of extension on the shelf, leading to the conclusion that shortening is driven by both tectonic and gravitational loading (Hesse et al 2009;King et al 2009b). Transverse segmentation of the Baram-Balabac Basin into distinct domains is expressed by a long strike variations in structural style, depositional systems and the relative amount of tectonic shorting.…”

“…Fig. 1), showing domain boundaries, major onshore faults and earthquake foci (filled white circles from Simmons et al 2007;King et al 2009b). Heavy black line marks edge of composite Asian Plate.…”

Quantifying deformation in North Borneo with GPS

“…This trench may be gradually filled by eroding/gravity sliding sediment material from the Crocker Range that was deposited on the ocean floor (Bol and Hoorn 1980;Pubellier and Morley 2013). The Baram Delta is an active fold-and-thrust belt and situated south of the NorthWest Borneo Trough (Ingram et al 2004;King et al 2010; (Hall 2013). During this period (8-7 Ma) also the Mount Kinabalu granite was intruded into the already existing Crocker Range (Cottam et al 2010(Cottam et al , 2013.…”

“…The region thereby appears to be slowly moving and/or deforming differently from the rigid Sundaland block with a clockwise rotation (Rangin et al 1999;Simons et al 2007;Sapin et al 2013). In addition, a longlived debate about the cause of the present motions of NorthWest Borneo as driven by gravity sliding (King et al 2010;Sapin et al 2013;Hall 2013) or crustal shortening (Simons et al 2007;Franke et al 2008;Hesse et al 2009) is still ongoing. In previous studies, the lack of a dense Global Navigation Satellite System (GNSS) network was a critical constraint to further analyze the behavior of North-West Borneo.…”

The 2015 M w 6.0 Mt. Kinabalu earthquake: an infrequent fault rupture within the Crocker fault system of East Malaysia

“…1b). These two mountain ranges are nearly parallel to an active deep-water fold-and-thrust belt that lies offshore to the northwest and has likely resulted from either gravity-driven mass movements or slow tectonic shortening (Hesse et al 2009;King et al 2010;Sapin et al 2013;Simons et al 2007). Although Sabah's active tectonic setting remains unclear, recent GPS analyses suggest that the region is moving slowly westward relative to the stable Sunda block at a rate of just a few mm/year (e.g., Simons et al 2007;Mustafar et al 2014).…”