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The deep-water fold-and-thrust belt offshore NW Borneo: Gravity-driven versus basement-driven shortening

S. Hesse, S. Back and D. Franke

Geological Society of America Bulletin 2009;121;939-953 doi: 10.1130/B26411.1

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Notes

The deep-water fold-and-thrust belt offshore NW Borneo: Gravity-driven versus basement-driven shortening

S. Hesse1†, S. Back1, and D. Franke2 1Geological Institute, RWTH Aachen University, Wüllnerstrasse 2, D-52056 Aachen, Germany 2Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, D-30655 Hannover, Germany

ABSTRACT of over 10 km (Fig. 2). The shelf units are com- (Figs. 2B, 3, and 4). Despite signifi cant research monly deformed and exhibit extensional and and exploration efforts in deep-water NW Bor- The deep-water region offshore NW compressional structural features including neo in recent years, there are still questions Borneo is an active fold-and-thrust belt kilometer-scale synsedimentary normal faults concerning the controlling mechanisms of deep- that hosts a signifi cant number of proven (growth faults), shale diapers, and inversion- water folding and thrusting. Based on regional hydrocarbon accumulations. In the past, related anticlines (e.g., James, 1984; Sandal, two-dimensional (2-D) seismic-refl ection data, two mechanisms have been discussed as pri- 1996; Van Rensbergen et al., 1999; Imber et Hinz et al. (1989) interpreted the deep-water mary control for Neogene to Holocene fold- al., 2003; Morley et al., 2003). The continental tectonics offshore NW Borneo as refl ecting a ing and thrusting in this deep-water prov- slope of NW Borneo is underlain by a large, continent-continent collision, and they inferred ince: (1) basement-driven crustal shortening basinward-thinning, middle Miocene to Holo- regional compression to have induced major and (2) gravity-related delta tectonics. In cene deep-water clastic wedge that is deformed thrusting that continues until today. In contrast, this study, new, balanced interpretations of by numerous compressional folds and thrusts for example, Hazebroek and Tan (1993) drew regional, crustal-scale, depth-migrated, two- dimensional (2-D), multichannel seismic- refl ection profi les are presented that provide for the fi rst time quantitative data on tectonic Hainan Philippine shortening throughout the entire deep-water 98 plate fold-and-thrust belt of NW Borneo. We use Eurasian plate our tectonic restorations to compare the 19

Indochina 52 anilaTrench amount of deep-water shortening on the M NW Borneo slope to the amount of extension across the NW Borneo shelf. A key result of Andaman South China Sea this balancing study is the observation that Sea Pliocene to Holocene gravity-driven shorten- 10°N 10 28 ing decreases from south to north, while the Sulu Sea total amount of shortening increases slightly Malaya to the north. Consequently, the amount of Celebes Sea purely basement-driven compression along Sundaland NW Borneo is strongly inferred to increase block toward the north. Because most of the short- Sunda Trench Sumatra Borneo ening is late Pliocene and younger, we inter- 0° pret the tectonic shortening to be ongoing. Sulawesi Banda INTRODUCTION Java Sea Sea Indo-Australian The continental shelf and slope areas of NW plate Java Borneo (Fig. 1) are well known from drilling 60 and seismic-reﬂ ection data (e.g., James, 1984; Indian Ocean Levell, 1987; Hinz et al., 1989; Sandal, 1996; 10°S Java Trench Petronas, 1999; Ingram et al., 2004; Morley, 100°E 110°E 120°E 2007; Gee et al., 2007; Franke et al., 2008). The NW Borneo shelf mainly consists of middle Figure 1. Regional location map for NW Borneo study area (box) in Southeast Miocene to recent prograding shallow-marine Asia. The Sundaland block is a composite region that accreted to the Eurasian clastic sediments that locally attain thicknesses plate during the late Paleozoic and Mesozoic (Metcalfe, 1996; Hall et al., 2008). White arrows indicate the relative convergence across plate boundaries (in mm/yr) †E-mail: [email protected] from Pubellier et al. (2005).

GSA Bulletin; May/June 2009; v. 121; no. 5/6; p. 939–953; doi: 10.1130/B26411.1; 12 ﬁ gures.

Hesse et al.

114°E 115°E 116°E 8°N

-1000 -2000 -1500 -2500

-2500

-1500 BGR86-12 -1500 -1000 BGR86-14 NW -2500Borneo Trough -500 BGR86-18

7°N -velocity body BGR86-20 BGR86-16 High

BGR86-22

BGR86-24

-2000

-2500

6°N

-2500

e n

Kimanis syncli

5°N

Legend:

Thrust fault

Normal fault (deltaic growth fault)

4°N Carbonate platform

BG R86-24 2D seismic section A

slope shelf Belait syncline Depth (km)

Figure 2. (A) Structural map showing key tectonic elements of NW Borneo and the location of the seismic data presented in this paper. Bathymetric contours offshore are drawn at 500 m intervals. Bold black line indicates location of cross-section A–A′. High-velocity body indicated by gray color is discussed in Franke et al. (2008). (B) Regional geological cross section across NW Borneo along line A–A′. The section is based on offshore seismic-reﬂ ection data (Sandal, 1996; Van Rensbergen and Morley, 2000), onshore seismic-reﬂ ection and well data (Back et al., 2008), geological maps (Sandal, 1996; Back et al., 2001, 2005), and models for the tectonic development of northern Borneo (e.g., James, 1984; Morley et al., 2003; Morley and Back, 2008).

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Deep-water Borneo attention to the similarities of structures in their 2000), particularly due to the presence of the (Figs. 2B, 3, and 4). Recent tectonic deforma- “Outboard Belt” with thrusts at the toe of the Telupid ophiolite of central Sabah (see, e.g., tion in the deep-water thrust belt is documented Niger Delta, suggesting that deep-water folding Hutchison, 1996a, his Figure 5.18; Morley and by the prominent seafl oor expression of several and thrusting offshore NW Borneo primarily Back, 2008). thrust hanging-wall fault bend folds, where the developed in response to gravitational delta tec- During Oligocene–early Miocene times, youngest structures form at the thin, distal part tonics. Between these two end-member interpre- the South China Sea was opened by seafl oor of the sediment wedge near the NW Borneo tations (basement-driven versus gravity-driven), spreading. Seafl oor-spreading anomalies in Trough (Figs. 3 and 4; Ingram et al., 2004; Mor- Ingram et al. (2004), for example, proposed the South China Sea oceanic basin range from ley, 2007; Gee et al., 2007). a compressional regime for deep-water NW magnetic anomaly 11 (ca. 32 Ma) to anomaly 5c Borneo dominated by crustal shortening in the (ca. 16 Ma; Taylor and Hayes, 1980; Briais et al., SEISMIC DATA range of 4 cm/yr until recent times (sensu Hinz 1993), with a southward ridge jump at anomaly et al., 1989) and a small contribution by gravity- 7/6b (ca. 27 Ma; Briais et al., 1993). The end of Acquisition and Processing related deltaic toe thrusting (sensu Hazebroek the seafl oor spreading occurred at 20.5 Ma (mag- and Tan, 1993). netic anomaly 6A1; Barckhausen and Roeser, The data used in this study are regional 2-D This paper presents new, balanced interpreta- 2004). With the opening of the South China Sea, multichannel seismic-refl ection profi les acquired tions of regional, crustal-scale, depth-migrated the thinned continental crust of the Dangerous by the Federal Institute for Geosciences and Nat- 2-D multichannel seismic-refl ection profi les Grounds region was rifted away from the south- ural Resources (BGR) in 1986 (survey details in (Fig. 2) covering large parts of the NW Borneo ern margin of China (e.g., Holloway, 1981; Hinz Hinz et al., 1989) and various published seismic deep-water fold-and-thrust belt between Brunei and Schlüter, 1985; Taylor and Hayes, 1983; Bri- profi les. The BGR86 multichannel seismic- Darussalam to the south and the Malaysian- ais et al., 1993). On the southeastern side of the refl ection data set has a total length of 3129 km Philippines border to the north, locally sup- Dangerous Grounds, Hinz and Schlüter (1985) along 27 lines. Seven representative multichan- ported by published 2-D and 3-D industry seis- interpreted an older region of oceanic crust, the nel seismic-refl ection profi les (Fig. 2A) with mics and well data. A stepwise retrodeformation proto–South China Sea. a total length of 1762 km (lines BGR86–12 to of thrusts and folds provides incremental mea- During the Paleogene, this proto–South China BGR86–24) were selected for reprocessing and surements of tectonic shortening for the time Sea closed, most likely due to a SE-directed sub- depth migration, including trace editing, band- between the Miocene and present-day in deep- duction beneath NW Borneo. Following com- pass fi ltering, prestack predictive deconvolu- water NW Borneo. We use the tectonic restora- plete subduction of the proto–South China Sea tion, and amplitude recovery. In a second step, tion results primarily for a comparison between oceanic crust, continental crust of the Dangerous detailed stacking velocity analyses were carried deep-water compression and shelfal extension, Grounds region was partially subducted beneath out on average every 1.5 km. The prestack pro- which enables us to separate the contribution the Crocker Formation basin of NW Borneo in cedure was fi nalized by a multiple suppression, of shallow, gravity-driven shortening from the the latest early Miocene before its buoyancy in which a radon velocity fi lter combined with amount of total shortening of the deep-water locked the system (James, 1984; Levell, 1987; an inner trace mute provided suffi cient results. fold-and-thrust belt. Hazebroek and Tan, 1993; Hutchison, 1996a, After normal-move-out (NMO)-correction, the 1996b; Sandal, 1996; Hall, 1996; Milsom et al., seismic data were stacked. The velocity fi eld for GEOLOGICAL FRAMEWORK 1997). Subsequently, northern Borneo experi- the depth migration was estimated by a step by enced signifi cant compressional deformation, step approach based on the stacking velocities The principal features of the geological evolu- as documented onshore in folded sedimentary determined by semblance analysis. This velocity tion of northern Borneo are summarized in Hinz units of late early Miocene to middle Miocene information was converted into interval veloci- et al. (1989), Hutchison (1996a, 1996b), Mil- ages (e.g., Sandal, 1996; Morley et al., 2003; ties using a smoothed-gradient algorithm. The som et al. (1997), Petronas (1999), Hutchison et Back et al., 2001, 2005, 2008), as well as off- fi nal depth-migration algorithm used was an al. (2000), Hall and Wilson (2000), Morley et al. shore in folded and thrusted middle Miocene to implicit fi nite-difference (FD) migration code. (2003), Morley and Back (2008), and Hall et al. present-day shelf and slope sequences (e.g., Lev- FD time migration was run for quality control of (2008). Northern and Central Borneo were built ell, 1987; Hinz et al., 1989; Hazebroek and Tan, the poststack depth migration. Figure 3 shows from the Mesozoic to Holocene, and they record 1993; Morley et al., 2003; Ingram et al., 2004). a representative depth-migrated section across a complex plate tectonic history involving oce- On the shallow NW Borneo shelf, late Neo- the northern part of the NW Borneo continental anic and continental crust. Today, NW Borneo gene compression coincided with the develop- margin (line BGR86–12), extending from the is a mountainous region exposing Cretaceous- ment of major synsedimentary normal faults Sabah shelf into the NW Borneo Trough. Fig- Eocene deep-water clastic deposits that are in the up to 10-km-thick late Neogene deltaic ure 4 illustrates the typical seismic-refl ection thrusted, folded, and locally metamorphosed to overburden (Fig. 2B). This “thin-skinned” signature of the central to southern portion of phyllites (Crocker and Rajang Groups exposed extensional deformation was superimposed on offshore NW Borneo (line BGR86–20). in the Crocker Mountain Range; for detailed deep-seated compressional structures and gen- description, see Hutchison, 1996a, 1996b), and erated, particularly in the southern part of the Seismic Interpretation Eocene to Lower Miocene sand-rich turbidites NW Borneo shelf, a multitude of complex tec- (Crocker Formation; for a detailed description, tonic features that resulted in the reactivation of The study area is characterized by a wide see Van Hattum et al., 2006). These Crocker major thrusts as normal faults and the inversion variety of slope geometries and structural styles sediments are thought to represent deep-water of synsedimentary normal faults (Morley et al., (Figs. 3 and 4). The north portion (lines BGR86– units in or adjacent to an accretionary prism. 2003). The complex interference of extensional 12, BGR86–14) exhibits a multitude of steep, Several authors have proposed that NW Bor- and compressional features ceased in the vicin- southeast-dipping thrust-related fold anticlines, neo is underlain largely by Mesozoic ophiolitic ity of the shelf break, beyond which a purely of which the youngest, most westward-located rocks (e.g., Hutchison, 1996a; Hall and Wilson, compressional fold-and-thrust belt developed anticlines exhibit a clear seafl oor expression. The

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Hesse et al.

Depth (km) Depth (km) 4 6 0 8 2 10 12 0 4 6 8 2 12 10 SE 140 130 140 130 ? High-velocity body 120 the main tectonic features of the northern the main tectonic features 110 120 of the underlying strata. Different grayscales of the underlying strata. Different izon 1 to horizon 5. Horizon 1, interpreted as izon 1 to horizon 5. Horizon 1, interpreted 11 0 To the SE of the NW Borneo Trough, up to 15 NE- Trough, Borneo the SE of NW To Upper slope 100 100 SeiS-E SeiS-D SeiS-C SeiS-B SeiS-A 80 90 Thrust-related folds 70 80 90 70 Distance (km) Distance (km) Horizon (1) Horizon (5) 60 60 BGR86-12 - Northern Zone Horizon (2) front Horizon (4) Deformation 50 Detachment level Horizon (3) 40 50 40 SeiS-E NW Borneo Trough NW Borneo SeiS-D 30 30 SeiS-C 20 20 SeiS-B 10 10 SeiS-A Seismic units: NW 2 4 6 0 8 0 4 2 6 8 12 10 a detachment, dips between 2° below the shelf to 7° below the thrust front (average dip 3.8°). a detachment, dips between 2° below the shelf to 7° thrust front indicate seismic units SeiS-A, SeiS-B, SeiS-C, SeiS-D, and SeiS-E. Marker horizons separating the seismic units are labeled hor horizons separating the seismic units are indicate seismic units SeiS-A, SeiS-B, SeiS-C, SeiS-D, and SeiS-E. Marker Figure 3. Depth-migrated seismic section BGR86–12 (locations of all seismic lines are shown on Fig. 2) and line drawing showing 3. Depth-migrated seismic section BGR86–12 (locations of all lines are Figure Trough. Borneo the shelf, slope, and NW The seismic section crosses (3× vertical exaggeration). zone of the study area At the landward edge of section, a high-velocity body masks seismic signals imaged. folds are fault-related SW–trending 10

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Deep-water Borneo

Depth (km) Depth (km) 4 6 12 0 8 2 4 8 10 6 2 0 12 10 SE 130 140 130 140 120 120 rn zone of the study area (3× vertical exag- rn zone of the study area lt consists of eight major NE-SW–trending NE-SW–trending lt consists of eight major 6° below the thrust front (average dip 3.5°); 6° below the thrust front 11 0 11 0 Upper slope SeiS-E 100 100 SeiS-D 90 90 SeiS-C 80 80 SeiS-B Distance (km) Distance (km) Thrust-related folds SeiS-A BGR86-20 - Central/Southern Zone 50 60 70 50 60 70 front Detachment level Deformation Horizon (1) Horizon (5) 40 40 SeiS-E Horizon (2) Horizon (4) SeiS-D Horizon (3) NW Borneo Trough NW Borneo SeiS-C 20 30 20 30 SeiS-B 10 10 SeiS-A Seismic units: NW 4 8 0 6 2 6 0 8 4 2 12 10 10 Figure 4. Depth-migrated seismic section BGR86–20 and line drawing showing the main tectonic features of the central and southe 4. Depth-migrated seismic section BGR86–20 and line drawing showing the main tectonic features Figure fold-and-thrust be Borneo deep-water The NW Trough. Borneo the shelf, slope, and NW The seismic line crosses geration). 3. Horizon 1, the detachment, dips between 2° below shelf and as in Figure folds. Grayscales and horizons are fault-related northern Brunei (Morley 2007). offshore for than those proposed only slightly steeper these values are 12

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Hesse et al. anticlines are closely spaced (average distance which are also atypical for diapirs. Therefore, lel to divergent refl ections of moderate to high 2–5 km) and asymmetric. Individual anticlines we believe that the narrow zones of chaotic amplitudes, moderate to high frequency, and show an oversteepened forelimb that developed refl ectivity more likely represent steep faults. high lateral refl ection continuity (Figs. 3 and above the upper tip of an associated thrust fault We have divided the sedimentary column into 4). Internally, unit SeiS-D exhibits alternating (Fig. 3). The observed thrusts range in dip from fi ve seismic units (SeiS-A to SeiS-E, from old intervals of moderate amplitudes and lower fre- 30° near the NW Borneo Trough to 60° a few to young) that are separated by fi ve horizons quency and high-amplitude zones with dense tens of kilometers landward. All thrust faults (1—the basal detachment to 5—the present- refl ector spacing (Fig. 5A). sole into a gently dipping detachment (average day seafl oor) of high refl ectivity and lateral The present-day seafl oor (horizon 5) var- dip 3.8°, maximum dip 7°) that is imaged at continuity (e.g., Figs. 3 and 4). Following the ies in depth between 2.8 km in the NW Bor- depths between 5 and 8 km, i.e., near the defor- interpretation of Hinz et al. (1989), we assume a neo Trough and <100 m in the shelf regions. mation front. High-amplitude refl ector patches late Pliocene to recent age for stratigraphic unit Horizon 5 defi nes the top of unit SeiS-E, a unit further east were interpreted as a continuation SeiS-E, an early Pliocene age for stratigraphic characterized by parallel, subparallel, chaotic of the detachment to depths of ~12 km below unit SeiS-D, a possible latest Miocene age for and divergent refl ections of moderate to high the NW Borneo shelf. Signifi cant parts of the stratigraphic unit SeiS-C, and an early Miocene amplitude with variable frequencies and high upper slope of the northern study area contain age for stratigraphic unit SeiS-B. Marker hori- refl ection continuity (Figs. 3 and 4). Figure 5A the “Major Thrust Sheet” previously defi ned by zon 1 is the acoustic basement forming the basal shows the typical facies pattern of this seismic Hinz et al. (1989). New refraction seismic data detachment surface and is mapped at depths of unit: the back limbs of the thrust anticlines document a high-velocity body at the basinward 5 (NW) to 13 km (SE). The associated refl ec- preserve growth strata with parallel, subparal- edge of this feature (Franke et al., 2008; Fig. 3), tion is positive, of high continuity, and shows lel, and divergent refl ectors of medium to high possibly indicating the presence of a large car- prominent amplitudes, particularly in depths amplitude and refl ector spacing. These stratifi ed bonate body encased in siliciclastics, or a thrust <7 km (Figs. 3 and 4). The acoustic substratum units are commonly interbedded with chaotic block of Paleogene Crocker sediments. (unit SeiS-A) below horizon 1 is characterized seismic facies that may represent slump mass In the central part of the study area (lines by a complex system of horsts, tilted blocks, accumulations (Fig. 5B). The latter may have BGR86–16, BGR86–18, BGR86–20), up to grabens, and half grabens within and westward been derived from the unstable, oversteepened eight successive NE-SW–trending thrust- of the NW Borneo Trough. The grabens either forelimb of the landward-located thrust anti- related fold anticlines were observed (Fig. 4), show onlap fi ll with laterally continuous, paral- cline (see also McGilvery and Cook, 2003; Gee of which only fi ve continue into the south- lel to subparallel refl ections of moderate to high et al., 2007). Another prominent feature within ern part of the study area (lines BGR86–22, amplitudes, or a complex, nearly transparent unit SeiS-E is the widespread occurrence of a BGR86–24). South of the study area, Morley fi ll pattern (Figs. 3 and 4). The horst structures strong bottom-simulating refl ector (BSR) with (2007) documented a further decrease in anti- are commonly overlain by laterally continu- a clear amplitude-reversal in comparison to the cline frequency by mapping four deep-water ous, parallel to subparallel refl ectors with high seafl oor refl ection (Figs. 5A and 5B). The BSR anticlines in the immediate vicinity of the off- amplitudes. The seismic response of horizon 1 is most distinct at the tops of the thrust anti- shore Sabah-Brunei border and three offshore and the underlying unit SeiS-A deteriorates with clines at 250–300 m below seafl oor. the Baram Delta (Fig. 2). As in the northern part increasing depth toward the eastern part of the of the study area, the anticlines are asymmetric study area (Figs. 3 and 4), and only segments of Structural Restoration and exhibit steep forelimbs above the upper tip it exist below 9 km depth. of a basal thrust fault. However, the southern Horizon 2 is a positive refl ector, of high con- In order to evaluate the validity of the seis- anticlines are more widely spaced (distance tinuity, mapped at depths of 3 km to 8 km. This mic-based structural and stratigraphic interpre- 4–15 km) and exhibit less surface expression refl ection forms the top of unit SeiS-B, which tation, reconstruction of the tectonic evolution (Fig. 4). The associated thrust faults dip toward is characterized by laterally continuous, mainly of the NW Borneo deep-water fold-and-thrust the SE and range in dip from 15° near the subparallel to parallel refl ectors of low to mod- belt between the deposition of units SeiS-A and deformation front to 40° below the present-day erate amplitudes (Figs. 3 and 4). In the western SeiS-E, and measure deformation associated shelf edge. The thrusts probably ramp down part of the study area, package SeiS-B onlaps with deep-water folding and thrusting, a series into a southeastward-dipping basal detachment onto and terminates against horizon 1. of 2-D tectonic restorations (unit per unit, from (average dip 3.5°, maximum dip 6°). Horizon 3 is a continuous refl ector that young to old) was carried out on seismic lines South of the study area offshore Brunei Dar- shows a medium- to high-amplitude peak BGR86–12, BGR86–14, BGR86–18, BGR86– ussalam, the proximal portion of the NW Bor- located at depths of 2–5 km. Horizon 3 is the 20, BGR86–22, and BGR86–24 (for locations, neo slope contains shale diapirs (Fig. 2B). On top of seismic unit SeiS-C, a unit characterized see Fig. 2A). Line BGR86–16 was excluded the seismic data presented, there is little indica- by laterally continuous, subparallel to paral- from structural restoration due to limited data tion that shale diapirism of this magnitude infl u- lel refl ections of low to medium amplitudes in quality. On all other lines, the retrodeformation ences the slope region offshore Sabah. Between the NW Borneo Trough, and by higher ampli- work included (1) the fault-by-fault restoration 2 and 6 km depth and 110–150 km along line tudes and a wider refl ector spacing on the NW of thrust displacement, (2) fold-by-fold unfold- BGR86–20 (Fig. 4), dome-shaped chaotic Borneo slope (Fig. 4). Throughout the survey ing, (3) unit-wide decompaction, and (4) unit- refl ections could be possibly interpreted as shale area, unit SeiS-C shows an upward increase in wide isostatic correction. It was assumed that diapirs. However, the velocity analyses carried amplitude and frequency. there was no movement of material into or out out in this study did not show any characteristic Horizon 4 is a positive refl ector, of high of the individual section planes. velocity push-down at the base of suspected dia- continuity, located in depths of 4 km near the For fault restoration, we used a trishear algo- pirs (sensu Van Rensbergen and Morley, 2003). NW Borneo Trough and 1.8 km near the shelf rithm sensu Erslev (1991), in which fault dis- Furthermore, partly continuous refl ections occur edge. Horizon 4 defi nes the top of the seismic placement was accommodated by heterogeneous in the immediate vicinity of the fault zones, sequence SeiS-D, a unit characterized by paral- shear in a triangular zone radiating from the fault

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Deep-water Borneo

A BGR86-20 Central/southern part B BGR86-24 Central/southern part

NW Distance (km) SE NW Distance (km) SE 0 2 4 6 8 10 12 0 2 4 6 8 10 12 14 16 0 0 1.5 1.5

2.5 2 Depth (km) 2 Depth (km) 0.5 Depth (km) 2.5 2.5 3 1 3 3

3.5 3.5 3.5 1.5

2 4

0 0 1.5 1.5 BSR 2 2 2.5 BSR Slump 0.5

Depth (km) 2.5 2.5 3 1 3 3

3.5 3.5 3.5 1.5

0 2 4 6 8 10 12 14 16 2 Distance (km) 4

024681012 Distance (km)

SeiS-C SeiS-D SeiS-E Figure 5. Detailed images of the seaﬂ oor and shallow-subsurface expression of fault-related folds in the central and southern parts of the study area. (A) Part of seismic section BGR86–20 showing a representative fold system near the present-day deformation front, providing stratigraphic detail of seismic units SeiS-C, SeiS-D, and SeiS-E (for facies description, see text). Note strong bottom simulating reﬂ ector (BSR) cutting across the anticline. (B) Part of seismic section BGR86–24 near the present-day deformation front imaging a fault-related fold and a large hanging-wall slump.

tip line (Fig. 6A). After each step of restoring a Figure 7 is a representative example of the maximum of 13 km at line BGR86–22. Short- hanging-wall horizon cutoff to its position before unit-per-unit restoration of the NW Borneo ening decreases 20–40 km northward to 12 km faulting, remnant folding of the target horizon deep-water fold-and-thrust belt based on the ret- (lines BGR86–20 and BGR86–18). At line was balanced using fl exural slip sensu Griffi ths rodeformation of seismic line BGR86–24. Fig- BGR86–14, total shortening was determined to et al. (2002; see Fig. 6B). This two-step fault- ure 8 summarizes the restoration results of all be 10.5 km, before reaching a minimum of 8 km restoration and unfolding approach was carried balanced sections of this study and documents at line BGR86–12. If broken into incremental out systematically within each stratigraphic unit total shortening of each section as well as incre- steps, the spatial distribution of shortening starting at the deep-water deformation front and mental shortening during times of development remains nonuniform, but shortening increases progressively moving landward. Fault displace- of stratigraphic units SeiS-B, SeiS-C, and SeiS- through time (Fig. 8). ment, throw, heave, fold-related shortening, and D. The total shortening values measured across total shortening were measured incrementally the ~120-km-wide study area range from 8 to Compression versus Extension along each section. Complete fault restoration 13 km per section (Fig. 8). Low total shortening and unfolding within each stratigraphic unit is observed in the very north (8 km; line BGR86– The results of the fault restoration and were followed by a sectionwide decompaction 12) and very south of the study area (10 km; line unfolding of six shelf-to-basin transects across sensu Sclater and Christie (1980) and an isostatic BGR86–24). From south to north, total shorten- the NW Borneo deep-water fold-and-thrust belt correction sensu Burov and Diament (1992). ing increases from 10 km (line BGR86–24) to a offshore Sabah indicate a signifi cant regional

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Hesse et al.

A Trishear zone seismic-based interpreted section

orginal bed thickness

Step 1 - removing fault displacement Figure 6. Schematic illustration (trishear - downdip movement) of the structural restoration approach of this study. (A) Rep- resentative interpretation of B a fault-related fold. Dashed circles mark the fault-horizon intersections on footwall and movement landward hanging-wall sides. (B) Section after fault restoration balanc- orginal bed thickness ing displacement of top hori- total shortening amount zon. (C) Flexural-slip unfolding Step 2 - unfolding (flexural slip) of top horizon. Underlying sur- C faces are carried with the top surface along the same movement vectors as it is deformed with the ﬂ exural-slip algorithm. movement landward (D) Target fold after removal of restored sedimentary unit fol- orginal bed thickness lowed by decompaction and total shortening amount isostatic correction.

Step 3 - removal of the complete restored unit / decompaction / isostatic correction D

decompacted bed thickness

Figure 7. Representative example of a tectonic restoration (deep-water section BGR86–24 and interpretation). (A) The present-day geological situation with an 86 km distance between the deep-water deformation front (pin 1 at the upper tip of the youngest thrust fault) and the present-day shelf edge (pin 2). (B) Interpreted section BGR86–24 after complete fault restoration and unfolding of the top of unit SeiS-D, including decompaction and isostatic correction. Fault restoration and unfolding have restored the distance between pins 1 and 2 (91.5 km; 5.5 km of shortening). (C) Section BGR86–24 after retrodeformation, decompaction, and isostatic correction of the top of unit SeiS-C; restored cross-section length between pins 1 and 2 is 93.5 km. (D) Interpreted section BGR86–24 after tectonic balancing, decompaction, and isostatic correction of the top of unit SeiS-B, ﬁ nally restoring the cross section to a length of around 96 km between pins 1 and 2 (2.5 km of shortening, for a total of 10 km of shortening). The change in basement dip (top unit SeiS-A) between A and D is a result of the sequential isostatic compensation after each restoration step. The location of seismic line BGR86–24 is indicated on Figure 2.

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Deep-water Borneo

BGR86-24 - Southern Zone NW Shelf break SE Deformation front

5 km

20 km SeiS-E Pin 2 SeiS-D A Pin 1

SeiS-A Cross section SeiS-B SeiS-C 5 km length 86 km (Pin 1 to Pin 2) 20 km SeiS-D Pin 2 B Pin 1

SeiS-A SeiS-B Cross section SeiS-C 5 km length 91.5 km (Pin 1 to Pin 2) 20 km Pin 2 C Pin 1

SeiS-A SeiS-B Cross section SeiS-C 5 km length 93.5 km (Pin 1 to Pin 2) 20 km Pin 2 D Pin 1

SeiS-A Cross section SeiS-B 5 km length 96 km (Pin 1 to Pin 2) 20 km

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variation in both total crustal shortening as well as incremental crustal shortening through Total shortening (SeiS-B, SeiS-C, SeiS-D) time. We consider the observed shortening SeiS-B SeiS-C SeiS-D variations between the individual transects to 120 BGR86-12 be valid, although absolute shortening values might carry an error of up to 15%, taking into account seismic-processing uncertainties (i.e., 100 BGR86-14 depth migration), horizon-interpretation errors (i.e., across-fault correlation), or uncertainties during restoration (included in Fig. 8). A com- 80 parison of our interpretations with published data of deep-water Sabah (Hinz et al., 1989; BGR86-18 Petronas, 1999; Ingram et al., 2004) and adja- 60 cent areas of Brunei Darussalam (Sandal, 1996; Van Rensbergen and Morley, 2000; Saller and BGR86-20 Blake, 2003; Morley, 2007; Morley and Back, 40 2008; Gee et al., 2007) documents that our Survey distance (km) fault and horizon interpretations fi t well into a margin-scale geological framework. Based 20 BGR86-22 on our stratigraphic subdivision, we calculated a rather uniform shortening of ±3 km in late Miocene and early Pliocene times. Since the 0 BGR86-24 late Pliocene, the study area has undergone a major shortening of 4–8 km, where the maximum shortening is located in the central part of the study area between lines BGR86–22 02468101214 and BGR86–18. Despite a large Pliocene to Shortening (km) Holocene sediment infl ux from the shelf to Figure 8. Shortening values calculated during tectonic restoration for three the continental slope (Sandal, 1996; Petronas, incremental time steps: shortening archived in unit SeiS-D (late Pliocene to 1999; Van Rensbergen and Morley, 2000), recent) is between 5 and 7 km, shortening within unit SeiS-C (early Pliocene the stepped topography of the fold belt is not to late Pliocene) is between 2 and 4 km, and shortening in unit SeiS-B (latest leveled-out on the modern seafl oor (Figs. 3 Miocene to early Pliocene) is between 1 and 3 km. Total shortening calculated and 4; also see detailed 3-D seafl oor images of for latest Miocene to recent times ranges between 8 and 13 km. Morley [2007]; Gee et al. [2007]), indicating that many folds and faults in the study area are active today. The increase in crustal shortening from the Borneo shows an opposite trend: the maximum The interpreted decrease in basement-driven Pliocene until today might be interpreted as of fold-related shortening is found in nearshore compression between lines BGR86–18 and gravity-driven shortening related to major Plio- Sabah north of Gordon-Klias (Fig. 9B), whereas BGR86–12 possibly refl ects the lack of neotec- cene shelfal growth. To test this hypothesis, we Pliocene shortening on the Brunei shelf is lim- tonic data from nearshore and onshore Sabah compiled published seismic data from the Sabah ited to a few inversion features of limited extent around Kota Kinabalu (Fig. 9), or incomplete shelf (e.g., Bol and Van Hoorn, 1980; Levell, (Morley et al., 2003). fault balancing below the high-velocity body 1987; Hazebroek and Tan, 1993; Petronas, Figure 10 is a synoptic plot of Pliocene to of seismic line BGR86–12 (Fig. 3). 1999; Ingram et al., 2004), Brunei shelf (e.g., present-day deep-water, outer shelf, and near- Sandal, 1996; Watters et al., 1999; Van Rens- shore shortening (Fig. 10A) and extension DISCUSSION bergen and Morley, 2000; Imber et al., 2003; estimates (Fig. 10B) of this study between off- Morley et al., 2003; Saller and Blake, 2003), shore Brunei Darussalam and the Gaya Hitam The structural interpretation and restora- and deep-water Brunei (Morley, 2007; Morley area offshore Sabah (Fig. 9). In the very south tion approach in this paper tries to account for and Back, 2008; Gee et al., 2007). This data set of the study area (south of line BGR86–24), and separate the processes that have infl uenced documents large-scale compressional and exten- there is an overall balance between deep-water the geometry and evolution of the deep-water sional features affecting the Pliocene to present- compression and shelfal extension, indicating fold-and-thrust belt offshore NW Borneo. The day basin fi ll offshore NW Borneo (Fig. 9). The that gravity-driven processes could be the pri- following arguments support an interpretation measurements of post-Miocene fault heaves (20 mary control for deep-water shortening. How- of the fold-and-thrust system as being at least sections) and fold-related shortening (six sec- ever, shelfal extension signifi cantly decreases partly controlled by active basement-driven tions) in nearshore northern Sabah and onshore north of line BGR86–24, causing an imbalance shortening. (1) The fault-related folds of the Brunei limit NW-SE–directed extension on between shallow-water extension and deep- present-day deep-water thrust front show a sea- the NW Borneo shelf to a maximum of 6 km water compression. We interpret the shortening bed expression, documenting that deformation offshore Brunei Bay, decreasing northward to surplus in this part of the study area as an indica- is still ongoing today. (2) Our restoration work <1 km offshore Kota Kinabalu. In contrast, the tion of northward-increasing Pliocene to pres- resolves a twofold trend of shortening along the post-Miocene shelfal compression along NW ent basement-driven compression (Fig. 10C). >250-km-long deep-water-margin segment stud-

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Deep-water Borneo

m k 2 ~ Extension Shortening Kota Kinabalu (Back et al., 2008) (Back et al., 1980) Hoorn, (Bol and van 1980) Hoorn, (Bol and van 1987) (Levell, 1999) (Petronas, 1993) Tan, and (Hazebroek m k

3 m ~ k

3 ~ 0.5 km ~ 0.5 km ~

body

H igh-velocity m k

~ Brunei Bay ~ 1 km

m k

k ion values and dominant compression. ~ 2 5 . 3 km 3

~ Comparison of late Pliocene to recent B) m k

5 m k m k 4 Brunei Bay shows an approximate a offshore measur- published data) used in this study for 6 ~ 0.6 km m k m 6 k ~ 1 . m 7 k m 6 k ~ 2 . 7 m k 6 m ~ k

7 m k m 6 . k 5 m 5 k ~ 6 ~ m k 6 ~ km ~ 4 km 5 ~ N S 114°N 115° 116° m 4 k ~ WE (Van Rensbergen and Morley, 2000) (Van and Morley, Rensbergen 2003) (Saller and Blake, 2000) (Van and Morley, Rensbergen 1999) (Petronas, 1999) (Petronas, (this study) (this study) 1999) (Petronas, 1999) (Petronas, 1999) (Petronas, 7°E 6° 5° 23 Kota Kinabalu 20 25 km 19 24

25 18

body High-velocity 22 26 Brunei Bay 17 15 21 16

BGR86-12 3 2 14 BGR86-14 13 4 12 (Sandal, 1996; Watters et al., 1999) et al., Watters 1996; (Sandal, 2008) (Back et al., 2003) (Imber et al., 1996) (Sandal, 1996) (Sandal, 2003) (Morley et al., 1996) (Sandal, 1996) (Sandal, 2003) (Saller and Blake, 1996) (Sandal, 7

R86-16

BG 8 1

- 1

BGR86 8

BGR86-20 6 11 5

BGR86-22 86-24 10 9 N S 114°N 115° 116° BGR WE AB balance between shelfal extension and deep-water compression, whereas the northern study area is characterized by waning extens the northern study area whereas compression, balance between shelfal extension and deep-water deep-water shortening values (calculated for unit SeiS-D) of this study with published data. The southern part of the study are unit SeiS-D) of this study with published data. shortening values (calculated for deep-water fault. GF—growth Figure 9. (A) Location map showing all deep-water, outer-shelf, nearshore, and onshore seismic sections (dashed lines indicate and onshore nearshore, outer-shelf, 9. (A) Location map showing all deep-water, Figure ( below. provided to locations and data sources shortening and extension. Numbers on map refer ing late Pliocene to present-day 7°E 6° 5°

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Hesse et al.

ABC 250 Figure 10. Late Pliocene to Holocene deep-water shortening values and shelfal exten- BGR86-12 sion estimates: (A) Deep-water 200 shortening values offshore BGR86-14 Sabah (this study) and from Shortening published data of deep-water Sabah BGR86-18 surplus Brunei Darussalam (Sandal, 1996; Van Rensbergen and BGR86-20 150 Morley, 2000; Morley, 2007; Gee et al., 2007; Morley and BGR86-22 Back, 2008). (B) Shelfal exten-

Distance (km) sion measured on 20 published BGR86-24 seismic sections (section loca- 100 Shelfal tions and references are shown extension on Fig. 9). (C) Shortening surplus calculated from balancing deep-water shortening and Deep-water shelf extension. The appar- 50 shortening ent decrease of the shortening Brunei surplus in the very north of the study area possibly results from insufﬁ cient information on Pliocene to Holocene folding 0 and thrusting in nearshore and 01234567 01234567 01234567 onshore Sabah. Deep-water shortening Shelf extension Basement-driven (km) (km) deep-water compression (km)

ied for Pliocene to recent times: in the very south increase in the frequency of folds and faults (see of this study, it is diffi cult to judge whether of the study area offshore Brunei, deep-water Figs. 3, 4, and 11). In the northernmost part of the measured basement-driven compression shortening equals estimates of shelfal exten- the study area, the intense deformation observed gradually increases northward, or if there is an sion (Fig. 10). This suggests that shelfal growth may be related to the impact of the high-velocity abrupt increase north of line BGR86–24. might act as an important control for gravity- body interpreted by Franke et al. (2008) as a rigid Figure 1 shows that Borneo is located in driven, shallow-seated, overburden-restricted obstacle that restricted deformation to the distal the tectonically active South China Sea region shortening of the deep-water fold-and-thrust slope portion of the NW Borneo fold-and-thrust close to major plate boundaries. However, the belt. In contrast, further north offshore Sabah, belt. (4) Previous studies have documented Plio- role of Borneo in the large-scale plate-tectonic deep-water shortening cannot be balanced by cene to recent inversion of older fault systems as context of SE Asia both in the past and in the shelfal extension. The observed tectonic imbal- well as folding both onshore Borneo and on the present is still under discussion (e.g., Replu- ance between the shallow-water and deep-water inner NW Borneo shelf; this deformation has maz et al., 2004; Pubellier et al., 2004; Hall provinces indicates the presence of an excess been attributed to plate-margin stress, consistent et al., 2008). A range of modern studies based shortening that is best explained by basement with a tectonically active margin (Morley et al., on global positioning system (GPS) measure- involvement. (3) Figure 11 provides an isopach 2003; Morley, 2007; Back et al. 2008). ments (e.g., Chamot-Rooke and Le Pichon, map of the Pliocene to recent sediment fi ll of We feel that these observations support 1999; Rangin et al., 1999; Simons et al., 2007; the greater study area between offshore Brunei our interpretation of active basement involve- see Fig. 12) proposes recent westward move- (Morley and Back, 2008) and deep-water Sabah ment for crustal shortening of the NW Borneo ment of NW Borneo with respect to the Sunda- (this study). The distribution of modern sedi- deep-water fold-and-thrust belt in recent times. land block (Fig. 1). The GPS vectors of these ment shows a maximum of over 6000 m in the However, in the southern part of the greater studies converge on similar values for different very south of the study area immediately beyond study area, gravity-related delta tectonics may stations in North Borneo (Figs. 12A and 12B) the shelf break offshore Brunei, while the Sabah have played a signifi cant (if not dominant) role but represent only 10 yr worth of observations slope is characterized by Pliocene to recent in the balance and can explain the deep-water in comparison to the restoration of 3 m.y. of sediment fi ll not exceeding 4000 m. This sedi- folds and thrusts. This is not the case in the basement-driven contraction presented in this ment distribution suggests that gravity-driven, northern part of the study area, where shelfal paper (Fig. 12C). The results of our tectonic overburden-controlled processes should decline extension strongly declines, causing a signifi - reconstruction document that pure basement- toward the north of the study area. However, our cant increase in calculated basement-driven related shortening (Fig. 10) is concentrated structural interpretation documents a northward compression (Fig. 10). Given the uncertainties in the northern part of our study area at rates

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Deep-water Borneo

Sabah Shelf

0 1000 2000 Sediment 3000 thickness N Shelf 4000 (m) Brunei 5000 6000

Figure 11. Isopach map of the Pliocene to Holocene sediment ﬁ ll between offshore Brunei (Morley and Back, 2008) and deep-water Sabah (this study). Offshore Brunei exhibits a maximum thickness of 6 km. The thickness of this sedimentary unit decreases to the north, with a maximum of 4 km offshore Sabah. This northward decrease in thickness coincides with a northward increase of fold and fault frequency between offshore Brunei and offshore northern Sabah. BSB—Bandar Seri Begawan.

Sunda plate Sunda land Sunda land A block B block C

3 km NW BorneoTrough NW BorneoTrench 5 km 4 km 6 mm/yr 3 km North Borneo Trench 6 mm/yr 1 km 10 mm/yr 4 mm/yr Borneo Borneo Borneo 3 mm/yr

Figure 12. Global positioning system (GPS)–based movement vectors of NW Borneo with respect to the Sundaland block (Fig. 1) and measurements of basement-driven deep-water-shortening (this study). (A) Movement vector after Rangin et al. (1999) suggesting a WNW-directed motion of northern Borneo of ~10 mm/yr with respect to a “Sunda plate” that is interpreted to be accommodated by crustal shortening in a “North Borneo Trench.” (B) Movement vectors of NW Borneo after Simons et al. (2007), proposing a general westward motion of NW Borneo between 3 and 6 mm/yr with a signiﬁ cant amount of shortening that is interpreted to be accommodated in an active “NW Borneo Trench.” (C) Long-term deep-water-shortening values calculated in this study suggesting a general northwestward-directed compression between 1 and 5 km in 3 m.y. (0.3–1.6 mm/yr).

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between 1 and 5 km per 3 m.y., equivalent to ACKNOWLEDGMENTS Asia: Geological Society of London Special Publica- a long-term shortening rate of 0.3–1.6 mm/yr. tion 106, p. 153–184. Schlumberger is gratefully acknowledged for Hall, R., and Wilson, M.E.J., 2000, Neogene sutures in east- This rate is less than that predicted by modern ern Indonesia: Journal of Asian Earth Sciences, v. 18, GPS monitoring (Fig. 12). Our study indicates providing “Petrel” under an Academic User License p. 781–814, doi: 10.1016/S1367-9120(00)00040-7. Agreement. Midland Valley is acknowledged for a SE-NW–directed compression perpendicular Hall, R., Van Hattum, M.W.A., and Spakman, W., 2008, providing “2DMove” for kinematic structural res- Impact of India-Asia collision on SE Asia: The record to the main fold-and-thrust trend, whereas the toration. Seismic Micro-Technology is thanked for in Borneo: Tectonophysics, v. 451, p. 366–389. land-based short-term GPS vectors are either providing “The Kingdom Suite” for seismic inter- Hazebroek, H.P., and Tan, D.N.K., 1993, Tertiary tectonic pretation. Michael Schnabel and Axel Ehrhardt are evolution of the NW Sabah continental margin: Geo- ESE-WNW (Rangin et al., 1999) or E-W logical Society of Malaysia Bulletin, v. 33, p. 195–210. (Simons et al., 2007). Although the different thanked for their help with the data processing. Rob- ert Hall, an anonymous reviewer, and GSA Associate Hinz, K., and Schlüter, H.U., 1985, Geology of the Dan- motion predictions (Fig. 12) are diffi cult to gerous Grounds, South China Sea and the continental Editor Stephen T. Johnston provided valuable critics margin off Southwest Palawan: Results of SONNE compare with respect to scale and methodol- on the early version of this paper, and their contri- cruises SO-23 and SO-27: Energy, v. 10, p. 297–315, ogy, all models predict present-day tectonic bution to the fi nal paper is gratefully acknowledged. doi: 10.1016/0360-5442(85)90048-9. activity in NW Borneo. This paper is a contribution to projects BA2136/2-3 Hinz, K., Fritsch, J., Kempter, E.H.K., Manaaf Mohammad, and FR2119/1-1 supported by the Deutsche Forsc- A., Meyer, H., Mohamed, D., Vosberg, H., Weber, J., hungsgemeinschaft (DFG). and Benavides, J.J., 1989, Thrust tectonics along the CONCLUSIONS continental margin of Sabah, northwest Borneo: Geol- ogische Rundschau, v. 78, p. 705–730, doi: 10.1007/ REFERENCES CITED BF01829317. 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952 Geological Society of America Bulletin, May/June 2009 Downloaded from gsabulletin.gsapubs.org on 30 September 2009

Deep-water Borneo

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