Geological Society of America Bulletin - UMass Amherst et al_GSABull_2009.pdf · INTRODUCTION The continental ... South China Sea Indo-Australian Java Sea plate Indian Ocean Sulu

Geological Society of America Bulletin

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

S. Hesse, S. Back and D. Franke versus basement-driven shorteningThe deep-water fold-and-thrust belt offshore NW Borneo: Gravity-driven

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ABSTRACT

The deep-water region offshore NW Borneo is an active fold-and-thrust belt that hosts a signifi cant number of proven hydrocarbon accumulations. In the past, two mechanisms have been discussed as pri-mary control for Neogene to Holocene fold-ing and thrusting in this deep-water prov-ince: (1) basement-driven crustal shortening and (2) gravity-related delta tectonics. In this study, new, balanced interpretations of 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 shortening throughout the entire deep-water fold-and-thrust belt of NW Borneo. We use our tectonic restorations to compare the amount of deep-water shortening on the NW Borneo slope to the amount of extension across the NW Borneo shelf. A key result of this balancing study is the observation that Pliocene to Holocene gravity-driven shorten-ing decreases from south to north, while the total amount of shortening increases slightly to the north. Consequently, the amount of purely basement-driven compression along NW Borneo is strongly inferred to increase toward the north. Because most of the short-ening is late Pliocene and younger, we inter-pret the tectonic shortening to be ongoing.

INTRODUCTION

The continental shelf and slope areas of NW Borneo (Fig. 1) are well known from drilling and seismic-refl ection data (e.g., James, 1984; Levell, 1987; Hinz et al., 1989; Sandal, 1996; Petronas, 1999; Ingram et al., 2004; Morley, 2007; Gee et al., 2007; Franke et al., 2008). The NW Borneo shelf mainly consists of middle Miocene to recent prograding shallow-marine clastic sediments that locally attain thicknesses

of over 10 km (Fig. 2). The shelf units are com-monly deformed and exhibit extensional and compressional structural features including kilometer-scale synsedimentary normal faults (growth faults), shale diapers, and inversion-related anticlines (e.g., James, 1984; Sandal, 1996; Van Rensbergen et al., 1999; Imber et al., 2003; Morley et al., 2003). The continental slope of NW Borneo is underlain by a large, basinward-thinning, middle Miocene to Holo-cene deep-water clastic wedge that is deformed by numerous compressional folds and thrusts

(Figs. 2B, 3, and 4). Despite signifi cant research and exploration efforts in deep-water NW Bor-neo in recent years, there are still questions concerning the controlling mechanisms of deep-water folding and thrusting. Based on regional two-dimensional (2-D) seismic-refl ection data, Hinz et al. (1989) interpreted the deep-water tectonics offshore NW Borneo as refl ecting a continent-continent collision, and they inferred regional compression to have induced major thrusting that continues until today. In contrast, for example, Hazebroek and Tan (1993) drew

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939

†E-mail: [email protected]

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

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, Germany2Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, D-30655 Hannover, Germany

Sundaland

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Eurasian plate

Philippine

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South China Sea

Java SeaIndo-Australian

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Indian Ocean

Sulu Sea

Celebes Sea

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100°E 110°E 120°E

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0°

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98

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Figure 1. Regional location map for NW Borneo study area (box) in Southeast Asia. The Sundaland block is a composite region that accreted to the Eurasian 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) from Pubellier et al. (2005).



Hesse et al.

940 Geological Society of America Bulletin, May/June 2009

NW

Born

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BGR86-24

BGR86-22

BGR86-20

BGR86-18

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Carbonate platform

2D seismic section

Legend:

A

B

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shelfslope Belait syncline

Dep

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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).



Deep-water Borneo

Geological Society of America Bulletin, May/June 2009 941

attention to the similarities of structures in their “Outboard Belt” with thrusts at the toe of the Niger Delta, suggesting that deep-water folding and thrusting offshore NW Borneo primarily developed in response to gravitational delta tec-tonics. Between these two end-member interpre-tations (basement-driven versus gravity-driven), Ingram et al. (2004), for example, proposed a compressional regime for deep-water NW Borneo dominated by crustal shortening in the range of 4 cm/yr until recent times (sensu Hinz et al., 1989) and a small contribution by gravity-related deltaic toe thrusting (sensu Hazebroek and Tan, 1993).

This paper presents new, balanced interpreta-tions of regional, crustal-scale, depth-migrated 2-D multichannel seismic-refl ection profi les (Fig. 2) covering large parts of the NW Borneo deep-water fold-and-thrust belt between Brunei Darussalam to the south and the Malaysian-Philippines border to the north, locally sup-ported by published 2-D and 3-D industry seis-mics and well data. A stepwise retrodeformation of thrusts and folds provides incremental mea-surements of tectonic shortening for the time between the Miocene and present-day in deep-water NW Borneo. We use the tectonic restora-tion results primarily for a comparison between deep-water compression and shelfal extension, which enables us to separate the contribution of shallow, gravity-driven shortening from the amount of total shortening of the deep-water fold-and-thrust belt.

GEOLOGICAL FRAMEWORK

The principal features of the geological evolu-tion of northern Borneo are summarized in Hinz et al. (1989), Hutchison (1996a, 1996b), Mil-som et al. (1997), Petronas (1999), Hutchison et al. (2000), Hall and Wilson (2000), Morley et al. (2003), Morley and Back (2008), and Hall et al. (2008). Northern and Central Borneo were built from the Mesozoic to Holocene, and they record a complex plate tectonic history involving oce-anic and continental crust. Today, NW Borneo is a mountainous region exposing Cretaceous-Eocene deep-water clastic deposits that are thrusted, folded, and locally metamorphosed to phyllites (Crocker and Rajang Groups exposed in the Crocker Mountain Range; for detailed description, see Hutchison, 1996a, 1996b), and Eocene to Lower Miocene sand-rich turbidites (Crocker Formation; for a detailed description, see Van Hattum et al., 2006). These Crocker sediments are thought to represent deep-water units in or adjacent to an accretionary prism. Several authors have proposed that NW Bor-neo is underlain largely by Mesozoic ophiolitic rocks (e.g., Hutchison, 1996a; Hall and Wilson,

2000), particularly due to the presence of the Telupid ophiolite of central Sabah (see, e.g., Hutchison, 1996a, his Figure 5.18; Morley and Back, 2008).

During Oligocene–early Miocene times, the South China Sea was opened by seafl oor spreading. Seafl oor-spreading anomalies in the South China Sea oceanic basin range from magnetic anomaly 11 (ca. 32 Ma) to anomaly 5c (ca. 16 Ma; Taylor and Hayes, 1980; Briais et al., 1993), with a southward ridge jump at anomaly 7/6b (ca. 27 Ma; Briais et al., 1993). The end of the seafl oor spreading occurred at 20.5 Ma (mag-netic anomaly 6A1; Barckhausen and Roeser, 2004). With the opening of the South China Sea, the thinned continental crust of the Dangerous Grounds region was rifted away from the south-ern margin of China (e.g., Holloway, 1981; Hinz and Schlüter, 1985; Taylor and Hayes, 1983; Bri-ais et al., 1993). On the southeastern side of the Dangerous Grounds, Hinz and Schlüter (1985) interpreted an older region of oceanic crust, the proto–South China Sea.

During the Paleogene, this proto–South China Sea closed, most likely due to a SE-directed sub-duction beneath NW Borneo. Following com-plete subduction of the proto–South China Sea oceanic crust, continental crust of the Dangerous Grounds region was partially subducted beneath the Crocker Formation basin of NW Borneo in the latest early Miocene before its buoyancy locked the system (James, 1984; Levell, 1987; Hazebroek and Tan, 1993; Hutchison, 1996a, 1996b; Sandal, 1996; Hall, 1996; Milsom et al., 1997). Subsequently, northern Borneo experi-enced signifi cant compressional deformation, as documented onshore in folded sedimentary units of late early Miocene to middle Miocene ages (e.g., Sandal, 1996; Morley et al., 2003; Back et al., 2001, 2005, 2008), as well as off-shore in folded and thrusted middle Miocene to present-day shelf and slope sequences (e.g., Lev-ell, 1987; Hinz et al., 1989; Hazebroek and Tan, 1993; Morley et al., 2003; Ingram et al., 2004).

On the shallow NW Borneo shelf, late Neo-gene compression coincided with the develop-ment of major synsedimentary normal faults in the up to 10-km-thick late Neogene deltaic overburden (Fig. 2B). This “thin-skinned” extensional deformation was superimposed on deep-seated compressional structures and gen-erated, particularly in the southern part of the NW Borneo shelf, a multitude of complex tec-tonic features that resulted in the reactivation of major thrusts as normal faults and the inversion of synsedimentary normal faults (Morley et al., 2003). The complex interference of extensional and compressional features ceased in the vicin-ity of the shelf break, beyond which a purely compressional fold-and-thrust belt developed

(Figs. 2B, 3, and 4). Recent tectonic deforma-tion in the deep-water thrust belt is documented by the prominent seafl oor expression of several thrust hanging-wall fault bend folds, where the youngest structures form at the thin, distal part of the sediment wedge near the NW Borneo Trough (Figs. 3 and 4; Ingram et al., 2004; Mor-ley, 2007; Gee et al., 2007).

SEISMIC DATA

Acquisition and Processing

The data used in this study are regional 2-D multichannel seismic-refl ection profi les acquired by the Federal Institute for Geosciences and Nat-ural Resources (BGR) in 1986 (survey details in Hinz et al., 1989) and various published seismic profi les. The BGR86 multichannel seismic-refl ection data set has a total length of 3129 km along 27 lines. Seven representative multichan-nel seismic-refl ection profi les (Fig. 2A) with a total length of 1762 km (lines BGR86–12 to BGR86–24) were selected for reprocessing and depth migration, including trace editing, band-pass fi ltering, prestack predictive deconvolu-tion, and amplitude recovery. In a second step, detailed stacking velocity analyses were carried out on average every 1.5 km. The prestack pro-cedure was fi nalized by a multiple suppression, in which a radon velocity fi lter combined with an inner trace mute provided suffi cient results. After normal-move-out (NMO)-correction, the seismic data were stacked. The velocity fi eld for the depth migration was estimated by a step by step approach based on the stacking velocities determined by semblance analysis. This velocity information was converted into interval veloci-ties using a smoothed-gradient algorithm. The fi nal depth-migration algorithm used was an implicit fi nite-difference (FD) migration code. FD time migration was run for quality control of the poststack depth migration. Figure 3 shows a representative depth-migrated section across the northern part of the NW Borneo continental margin (line BGR86–12), extending from the Sabah shelf into the NW Borneo Trough. Fig-ure 4 illustrates the typical seismic-refl ection signature of the central to southern portion of offshore NW Borneo (line BGR86–20).

Seismic Interpretation

The study area is characterized by a wide variety of slope geometries and structural styles (Figs. 3 and 4). The north portion (lines BGR86–12, BGR86–14) exhibits a multitude of steep, southeast-dipping thrust-related fold anticlines, of which the youngest, most westward-located anticlines exhibit a clear seafl oor expression. The



Hesse et al.


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Hesse et al.


anticlines are closely spaced (average distance 2–5 km) and asymmetric. Individual anticlines show an oversteepened forelimb that developed above the upper tip of an associated thrust fault (Fig. 3). The observed thrusts range in dip from 30° near the NW Borneo Trough to 60° a few tens of kilometers landward. All thrust faults sole into a gently dipping detachment (average dip 3.8°, maximum dip 7°) that is imaged at depths between 5 and 8 km, i.e., near the defor-mation front. High-amplitude refl ector patches further east were interpreted as a continuation of the detachment to depths of ~12 km below the NW Borneo shelf. Signifi cant parts of the upper slope of the northern study area contain the “Major Thrust Sheet” previously defi ned by Hinz et al. (1989). New refraction seismic data document a high-velocity body at the basinward edge of this feature (Franke et al., 2008; Fig. 3), possibly indicating the presence of a large car-bonate body encased in siliciclastics, or a thrust block of Paleogene Crocker sediments.

In the central part of the study area (lines BGR86–16, BGR86–18, BGR86–20), up to eight successive NE-SW–trending thrust-related fold anticlines were observed (Fig. 4), of which only fi ve continue into the south-ern part of the study area (lines BGR86–22, BGR86–24). South of the study area, Morley (2007) documented a further decrease in anti-cline frequency by mapping four deep-water anticlines in the immediate vicinity of the off-shore Sabah-Brunei border and three offshore the Baram Delta (Fig. 2). As in the northern part of the study area, the anticlines are asymmetric and exhibit steep forelimbs above the upper tip of a basal thrust fault. However, the southern anticlines are more widely spaced (distance 4–15 km) and exhibit less surface expression (Fig. 4). The associated thrust faults dip toward the SE and range in dip from 15° near the deformation front to 40° below the present-day shelf edge. The thrusts probably ramp down into a southeastward-dipping basal detachment (average dip 3.5°, maximum dip 6°).

South of the study area offshore Brunei Dar-ussalam, the proximal portion of the NW Bor-neo slope contains shale diapirs (Fig. 2B). On the seismic data presented, there is little indica-tion that shale diapirism of this magnitude infl u-ences the slope region offshore Sabah. Between 2 and 6 km depth and 110–150 km along line BGR86–20 (Fig. 4), dome-shaped chaotic refl ections could be possibly interpreted as shale diapirs. However, the velocity analyses carried out in this study did not show any characteristic velocity push-down at the base of suspected dia-pirs (sensu Van Rensbergen and Morley, 2003). Furthermore, partly continuous refl ections occur in the immediate vicinity of the fault zones,

which are also atypical for diapirs. Therefore, we believe that the narrow zones of chaotic refl ectivity more likely represent steep faults.

We have divided the sedimentary column into fi ve seismic units (SeiS-A to SeiS-E, from old to young) that are separated by fi ve horizons (1—the basal detachment to 5—the present-day seafl oor) of high refl ectivity and lateral continuity (e.g., Figs. 3 and 4). Following the interpretation of Hinz et al. (1989), we assume a late Pliocene to recent age for stratigraphic unit SeiS-E, an early Pliocene age for stratigraphic unit SeiS-D, a possible latest Miocene age for stratigraphic unit SeiS-C, and an early Miocene age for stratigraphic unit SeiS-B. Marker hori-zon 1 is the acoustic basement forming the basal detachment surface and is mapped at depths of 5 (NW) to 13 km (SE). The associated refl ec-tion is positive, of high continuity, and shows prominent amplitudes, particularly in depths <7 km (Figs. 3 and 4). The acoustic substratum (unit SeiS-A) below horizon 1 is characterized by a complex system of horsts, tilted blocks, grabens, and half grabens within and westward of the NW Borneo Trough. The grabens either show onlap fi ll with laterally continuous, paral-lel to subparallel refl ections of moderate to high amplitudes, or a complex, nearly transparent fi ll pattern (Figs. 3 and 4). The horst structures are commonly overlain by laterally continu-ous, parallel to subparallel refl ectors with high amplitudes. The seismic response of horizon 1 and the underlying unit SeiS-A deteriorates with increasing depth toward the eastern part of the study area (Figs. 3 and 4), and only segments of it exist below 9 km depth.

Horizon 2 is a positive refl ector, of high con-tinuity, mapped at depths of 3 km to 8 km. This refl ection forms the top of unit SeiS-B, which is characterized by laterally continuous, mainly subparallel to parallel refl ectors of low to mod-erate amplitudes (Figs. 3 and 4). In the western part of the study area, package SeiS-B onlaps onto and terminates against horizon 1.

Horizon 3 is a continuous refl ector that shows a medium- to high-amplitude peak located at depths of 2–5 km. Horizon 3 is the top of seismic unit SeiS-C, a unit characterized by laterally continuous, subparallel to paral-lel refl ections of low to medium amplitudes in the NW Borneo Trough, and by higher ampli-tudes and a wider refl ector spacing on the NW Borneo slope (Fig. 4). Throughout the survey area, unit SeiS-C shows an upward increase in amplitude and frequency.

Horizon 4 is a positive refl ector, of high continuity, located in depths of 4 km near the NW Borneo Trough and 1.8 km near the shelf edge. Horizon 4 defi nes the top of the seismic sequence SeiS-D, a unit characterized by paral-

lel to divergent refl ections of moderate to high amplitudes, moderate to high frequency, and high lateral refl ection continuity (Figs. 3 and 4). Internally, unit SeiS-D exhibits alternating intervals of moderate amplitudes and lower fre-quency and high-amplitude zones with dense refl ector spacing (Fig. 5A).

The present-day seafl oor (horizon 5) var-ies in depth between 2.8 km in the NW Bor-neo Trough and <100 m in the shelf regions. Horizon 5 defi nes the top of unit SeiS-E, a unit characterized by parallel, subparallel, chaotic and divergent refl ections of moderate to high amplitude with variable frequencies and high refl ection continuity (Figs. 3 and 4). Figure 5A shows the typical facies pattern of this seismic unit: the back limbs of the thrust anticlines preserve growth strata with parallel, subparal-lel, and divergent refl ectors of medium to high amplitude and refl ector spacing. These stratifi ed units are commonly interbedded with chaotic seismic facies that may represent slump mass accumulations (Fig. 5B). The latter may have been derived from the unstable, oversteepened forelimb of the landward-located thrust anti-cline (see also McGilvery and Cook, 2003; Gee et al., 2007). Another prominent feature within unit SeiS-E is the widespread occurrence of a strong bottom-simulating refl ector (BSR) with a clear amplitude-reversal in comparison to the seafl oor refl ection (Figs. 5A and 5B). The BSR is most distinct at the tops of the thrust anti-clines at 250–300 m below seafl oor.

Structural Restoration

In order to evaluate the validity of the seis-mic-based structural and stratigraphic interpre-tation, reconstruction of the tectonic evolution of the NW Borneo deep-water fold-and-thrust belt between the deposition of units SeiS-A and SeiS-E, and measure deformation associated with deep-water folding and thrusting, a series of 2-D tectonic restorations (unit per unit, from young to old) was carried out on seismic lines BGR86–12, BGR86–14, BGR86–18, BGR86–20, BGR86–22, and BGR86–24 (for locations, see Fig. 2A). Line BGR86–16 was excluded from structural restoration due to limited data quality. On all other lines, the retrodeformation work included (1) the fault-by-fault restoration of thrust displacement, (2) fold-by-fold unfold-ing, (3) unit-wide decompaction, and (4) unit-wide isostatic correction. It was assumed that there was no movement of material into or out of the individual section planes.

For fault restoration, we used a trishear algo-rithm sensu Erslev (1991), in which fault dis-placement was accommodated by hetero geneous shear in a triangular zone radiating from the fault



Deep-water Borneo


tip line (Fig. 6A). After each step of restoring a hanging-wall horizon cutoff to its position before faulting, remnant folding of the target horizon was balanced using fl exural slip sensu Griffi ths et al. (2002; see Fig. 6B). This two-step fault-restoration and unfolding approach was carried out systematically within each stratigraphic unit starting at the deep-water deformation front and progressively moving landward. Fault displace-ment, throw, heave, fold-related shortening, and total shortening were measured incrementally along each section. Complete fault restoration and unfolding within each stratigraphic unit were followed by a sectionwide decompaction sensu Sclater and Christie (1980) and an isostatic correction sensu Burov and Diament (1992).

Figure 7 is a representative example of the unit-per-unit restoration of the NW Borneo deep-water fold-and-thrust belt based on the ret-rodeformation of seismic line BGR86–24. Fig-ure 8 summarizes the restoration results of all balanced sections of this study and documents total shortening of each section as well as incre-mental shortening during times of development of stratigraphic units SeiS-B, SeiS-C, and SeiS-D. The total shortening values measured across the ~120-km-wide study area range from 8 to 13 km per section (Fig. 8). Low total shortening is observed in the very north (8 km; line BGR86–12) and very south of the study area (10 km; line BGR86–24). From south to north, total shorten-ing increases from 10 km (line BGR86–24) to a

maximum of 13 km at line BGR86–22. Short-ening decreases 20–40 km northward to 12 km (lines BGR86–20 and BGR86–18). At line BGR86–14, total shortening was determined to be 10.5 km, before reaching a minimum of 8 km at line BGR86–12. If broken into incremental steps, the spatial distribution of shortening remains nonuniform, but shortening increases through time (Fig. 8).

Compression versus Extension

The results of the fault restoration and unfolding of six shelf-to-basin transects across the NW Borneo deep-water fold-and-thrust belt offshore Sabah indicate a signifi cant regional

0

3

4

2.5

3.5

0 2 4 6 8 10 12

0

1

2

0.5

1.5

SeiS-DSeiS-C

Distance (km)

SeiS-E

BSR

0 2 4 6 8 10 12

3

4

2.5

3.5

0

1

2

0.5

1.5

BGR86-20 Central/southern part

SENW

2 4 6 8 10 12 14 16

SENW

0

2

3

2.5

3.5

1.5

2

3

2.5

3.5

1.5

BGR86-24 Central/southern part

2 4 6 8 10 12 14 160

2

3

2.5

3.5

1.5

2

3

2.5

3.5

1.5

Slump

BSR

Depth

(km

)

Distance (km)

Distance (km)Distance (km)

A B

0

Depth

(km

)

Depth

(km

)D

epth

(km

)

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.



Hesse et al.


Step 2 - unfolding (flexural slip)

orginal bed thickness


Trishear

zone

seismic-based

interpreted section

Step 1 - removing fault displacement (trishear - downdip movement)

movement landward

movement landward


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

total shortening amount

B

A

C

D

total shortening amount

decompacted

bed thickness

Figure 7. Representative example of a tectonic restoration (deep-water section BGR86–24 and interpretation). (A) The present-day geologi-cal 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.

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



Deep-water Borneo


BGR86-24 - Southern Zone

20 km

5 km

Shelf break SENW

Cross section

length 91.5 km

(Pin 1 to Pin 2)

Cross section

length 93.5 km

(Pin 1 to Pin 2)

Cross section

length 96 km

(Pin 1 to Pin 2)

Deformation front

Cross section

length 86 km

(Pin 1 to Pin 2)

B

C

D

Pin 1

Pin 2

Pin 1

Pin 1

Pin 1

Pin 2

Pin 2

Pin 2

20 km

5 km

20 km

5 km

20 km

5 km

20 km

5 km

SeiS-C

SeiS-A

SeiS-ESeiS-D

SeiS-B

SeiS-C

SeiS-ASeiS-B

SeiS-D

SeiS-C

SeiS-ASeiS-B

SeiS-ASeiS-B

A



Hesse et al.


variation in both total crustal shortening as well as incremental crustal shortening through time. We consider the observed shortening variations between the individual transects to be valid, although absolute shortening values might carry an error of up to 15%, taking into account seismic-processing uncertainties (i.e., depth migration), horizon-interpretation errors (i.e., across-fault correlation), or uncertainties during restoration (included in Fig. 8). A com-parison of our interpretations with published data of deep-water Sabah (Hinz et al., 1989; Petronas, 1999; Ingram et al., 2004) and adja-cent areas of Brunei Darussalam (Sandal, 1996; Van Rensbergen and Morley, 2000; Saller and Blake, 2003; Morley, 2007; Morley and Back, 2008; Gee et al., 2007) documents that our fault and horizon interpretations fi t well into a margin-scale geological framework. Based on our stratigraphic subdivision, we calculated a rather uniform shortening of ±3 km in late Miocene and early Pliocene times. Since the late Pliocene, the study area has undergone a major shortening of 4–8 km, where the maxi-mum shortening is located in the central part of the study area between lines BGR86–22 and BGR86–18. Despite a large Pliocene to Holocene sediment infl ux from the shelf to the continental slope (Sandal, 1996; Petronas, 1999; Van Rensbergen and Morley, 2000), the stepped topography of the fold belt is not leveled-out on the modern seafl oor (Figs. 3 and 4; also see detailed 3-D seafl oor images of 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 Pliocene until today might be interpreted as gravity-driven shortening related to major Plio-cene shelfal growth. To test this hypothesis, we compiled published seismic data from the Sabah shelf (e.g., Bol and Van Hoorn, 1980; Levell, 1987; Hazebroek and Tan, 1993; Petronas, 1999; Ingram et al., 2004), Brunei shelf (e.g., Sandal, 1996; Watters et al., 1999; Van Rens-bergen and Morley, 2000; Imber et al., 2003; Morley et al., 2003; Saller and Blake, 2003), and deep-water Brunei (Morley, 2007; Morley and Back, 2008; Gee et al., 2007). This data set documents large-scale compressional and exten-sional features affecting the Pliocene to present-day basin fi ll offshore NW Borneo (Fig. 9). The measurements of post-Miocene fault heaves (20 sections) and fold-related shortening (six sec-tions) in nearshore northern Sabah and onshore Brunei limit NW-SE–directed extension on the NW Borneo shelf to a maximum of 6 km offshore Brunei Bay, decreasing northward to <1 km offshore Kota Kinabalu. In contrast, the post-Miocene shelfal compression along NW

Borneo shows an opposite trend: the maximum of fold-related shortening is found in nearshore Sabah north of Gordon-Klias (Fig. 9B), whereas Pliocene shortening on the Brunei shelf is lim-ited to a few inversion features of limited extent (Morley et al., 2003).

Figure 10 is a synoptic plot of Pliocene to present-day deep-water, outer shelf, and near-shore shortening (Fig. 10A) and extension estimates (Fig. 10B) of this study between off-shore Brunei Darussalam and the Gaya Hitam area offshore Sabah (Fig. 9). In the very south of the study area (south of line BGR86–24), there is an overall balance between deep-water compression and shelfal extension, indicating that gravity-driven processes could be the pri-mary control for deep-water shortening. How-ever, shelfal extension signifi cantly decreases north of line BGR86–24, causing an imbalance between shallow-water extension and deep-water compression. We interpret the shortening surplus in this part of the study area as an indica-tion of northward-increasing Pliocene to pres-ent basement-driven compression (Fig. 10C).

The interpreted decrease in basement-driven compression between lines BGR86–18 and BGR86–12 possibly refl ects the lack of neotec-tonic data from nearshore and onshore Sabah around Kota Kinabalu (Fig. 9), or incomplete fault balancing below the high-velocity body of seismic line BGR86–12 (Fig. 3).

DISCUSSION

The structural interpretation and restora-tion approach in this paper tries to account for and separate the processes that have infl uenced the geometry and evolution of the deep-water fold-and-thrust belt offshore NW Borneo. The following arguments support an interpretation of the fold-and-thrust system as being at least partly controlled by active basement-driven shortening. (1) The fault-related folds of the present-day deep-water thrust front show a sea-bed expression, documenting that deformation is still ongoing today. (2) Our restoration work resolves a twofold trend of shortening along the >250-km-long deep-water-margin segment stud-

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14

Total shortening

(SeiS-B, SeiS-C, SeiS-D)SeiS-C SeiS-DSeiS-B

Shortening (km)

Su

rve

y d

ista

nce

(km

)

BGR86-12

BGR86-14

BGR86-18

BGR86-20

BGR86-22

BGR86-24

Figure 8. Shortening values calculated during tectonic restoration for three incremental time steps: shortening archived in unit SeiS-D (late Pliocene to recent) is between 5 and 7 km, shortening within unit SeiS-C (early Pliocene to late Pliocene) is between 2 and 4 km, and shortening in unit SeiS-B (latest Miocene to early Pliocene) is between 1 and 3 km. Total shortening calculated for latest Miocene to recent times ranges between 8 and 13 km.



Deep-water Borneo


114

°N11

5°

116

°7°E

6° 5°

19

20

18

1

3 2

4

8

5

62

1

22

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9

11

10

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14

7

15

16

17

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High-velocity

bodyB

run

ei

Ba

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N S

WE

11

4°N

11

5°

11

6°

7°E

6° 5°

mk 4 ~

mk 5 ~

mk 6 ~

mk 6 ~

mk

6 ~

mk

4 ~

~ 2

km

~ 1

km

~ 0

.5 k

m

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km

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-24

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R86-1

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, 200

0)

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(San

dal

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6)

(San

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(San

dal

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6)

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(San

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99)

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is s

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003)

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ron

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, 199

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Hesse et al.


ied for Pliocene to recent times: in the very south of the study area offshore Brunei, deep-water shortening equals estimates of shelfal exten-sion (Fig. 10). This suggests that shelfal growth might act as an important control for gravity-driven, shallow-seated, overburden-restricted shortening of the deep-water fold-and-thrust belt. In contrast, further north offshore Sabah, deep-water shortening cannot be balanced by shelfal extension. The observed tectonic imbal-ance between the shallow-water and deep-water provinces indicates the presence of an excess shortening that is best explained by basement involvement. (3) Figure 11 provides an isopach map of the Pliocene to recent sediment fi ll of the greater study area between offshore Brunei (Morley and Back, 2008) and deep-water Sabah (this study). The distribution of modern sedi-ment shows a maximum of over 6000 m in the very south of the study area immediately beyond the shelf break offshore Brunei, while the Sabah slope is characterized by Pliocene to recent sediment fi ll not exceeding 4000 m. This sedi-ment distribution suggests that gravity-driven, overburden-controlled processes should decline toward the north of the study area. However, our structural interpretation documents a northward

increase in the frequency of folds and faults (see Figs. 3, 4, and 11). In the northernmost part of the study area, the intense deformation observed may be related to the impact of the high-velocity body interpreted by Franke et al. (2008) as a rigid obstacle that restricted deformation to the distal slope portion of the NW Borneo fold-and-thrust belt. (4) Previous studies have documented Plio-cene to recent inversion of older fault systems as well as folding both onshore Borneo and on the inner NW Borneo shelf; this deformation has been attributed to plate-margin stress, consistent with a tectonically active margin (Morley et al., 2003; Morley, 2007; Back et al. 2008).

We feel that these observations support our interpretation of active basement involve-ment for crustal shortening of the NW Borneo deep-water fold-and-thrust belt in recent times. However, in the southern part of the greater study area, gravity-related delta tectonics may have played a signifi cant (if not dominant) role in the balance and can explain the deep-water folds and thrusts. This is not the case in the northern part of the study area, where shelfal extension strongly declines, causing a signifi -cant increase in calculated basement-driven compression (Fig. 10). Given the uncertainties

of this study, it is diffi cult to judge whether the measured basement-driven compression gradually increases northward, or if there is an abrupt increase north of line BGR86–24.

Figure 1 shows that Borneo is located in the tectonically active South China Sea region close to major plate boundaries. However, the role of Borneo in the large-scale plate-tectonic context of SE Asia both in the past and in the present is still under discussion (e.g., Replu-maz et al., 2004; Pubellier et al., 2004; Hall et al., 2008). A range of modern studies based on global positioning system (GPS) measure-ments (e.g., Chamot-Rooke and Le Pichon, 1999; Rangin et al., 1999; Simons et al., 2007; see Fig. 12) proposes recent westward move-ment of NW Borneo with respect to the Sunda-land block (Fig. 1). The GPS vectors of these studies converge on similar values for different stations in North Borneo (Figs. 12A and 12B) but represent only 10 yr worth of observations in comparison to the restoration of 3 m.y. of basement-driven contraction presented in this paper (Fig. 12C). The results of our tectonic reconstruction document that pure basement-related shortening (Fig. 10) is concentrated in the northern part of our study area at rates

0

50

100

150

200

250

0 1 2 3 4 5 6 7

Dis

tan

ce

(km

)

Deep-water shortening

(km)

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Bru

ne

iS

ab

ah

BGR86-12

BGR86-14

BGR86-18

BGR86-22

BGR86-24

BGR86-20

A B C

Deep-water

shortening

Shelfal

extension

Shortening

surplus

Basement-driven

deep-water compression

(km)

Shelf extension

(km)

Figure 10. Late Pliocene to Holocene deep-water shorten-ing values and shelfal exten-sion estimates: (A) Deep-water shortening values offshore Sabah (this study) and from published data of deep-water Brunei Darussalam (Sandal, 1996; Van Rensbergen and Morley, 2000; Morley, 2007; Gee et al., 2007; Morley and Back, 2008). (B) Shelfal exten-sion measured on 20 published seismic sections (section loca-tions and references are shown on Fig. 9). (C) Shortening sur-plus calculated from balanc-ing deep-water shortening and shelf extension. The appar-ent decrease of the shortening surplus in the very north of the study area possibly results from insuffi cient information on Pliocene to Holocene folding and thrusting in nearshore and onshore Sabah.



Deep-water Borneo


Sabah Shelf

Brunei S

helf

1000

2000

3000

4000

5000

6000

0

N

Sediment

thickness

(m)

6 mm/yr

6 mm/yr

4 mm/yr

3 mm/yr

B

NW

Bor

neo

Tren

ch

Sunda land

block

Borneo

10 mm/yr

A

Nor

th B

orne

o Tr

ench

Sunda plate

Borneo

3 km

5 km4 km

3 km1 km

C

NW

Bor

neo

Trou

gh

Sunda land

block

Borneo

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).

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.



Hesse et al.


between 1 and 5 km per 3 m.y., equivalent to a long-term shortening rate of 0.3–1.6 mm/yr. This rate is less than that predicted by modern GPS monitoring (Fig. 12). Our study indicates a SE-NW–directed compression perpendicular to the main fold-and-thrust trend, whereas the land-based short-term GPS vectors are either ESE-WNW (Rangin et al., 1999) or E-W (Simons et al., 2007). Although the different motion predictions (Fig. 12) are diffi cult to compare with respect to scale and methodol-ogy, all models predict present-day tectonic activity in NW Borneo.

CONCLUSIONS

The deep-water portion of NW Borneo has been structurally balanced and restored in order to investigate the controlling mechanisms of deep-water folding and thrusting. Key results of this tectonic reconstruction are:

(1) Total crustal shortening measured along six shelf-to-basin transects across the NW Bor-neo deep-water fold-and-thrust belt ranges from 8 to 13 km. These values represent the sum of shortening calculated for three incremental time steps, late Pliocene to recent (5–7 km per sec-tion), early Pliocene to late Pliocene (2–4 km per section), and latest Miocene to early Plio-cene (1–3 km per section). Our data indicate that total shortening generally increases through time. The seafl oor expression of active folds indicates signifi cant recent tectonic activity of deep-water faults and folds.

(2) When analyzed against shelfal extension, the NW Borneo deep-water fold-and-thrust belt exhibits a twofold trend of shortening along the >250 km portion studied: in the south (offshore Brunei Darussalam and southern Sabah), Plio-cene to recent deep-water shortening equals shelfal extension. This observation suggests that modern shelfal growth probably acts as a pri-mary control for gravity-driven, shallow-seated, overburden-restricted shortening of the fold-and-thrust belt. In contrast, offshore northern Sabah, deep-water shortening exceeds shelfal extension. In this region, the observed domi-nance of deep-water shortening is interpreted to refl ect Pliocene to recent deep-rooted, collision-related basement tectonics.

(3) The subtraction of shelfal extension from total shortening provides information on colli-sion-related basement compression throughout the study area. If viewed from south to north along the deep-water fold-and-thrust belt, base-ment-driven shortening increases northward. We interpret this trend to document a generally northward increase of basement-driven com-pression that signifi cantly contributes to present-day deep-water folding and thrusting.

ACKNOWLEDGMENTS

Schlumberger is gratefully acknowledged for providing “Petrel” under an Academic User License Agreement. Midland Valley is acknowledged for providing “2DMove” for kinematic structural res-toration. Seismic Micro-Technology is thanked for providing “The Kingdom Suite” for seismic inter-pretation. Michael Schnabel and Axel Ehrhardt are thanked for their help with the data processing. Rob-ert Hall, an anonymous reviewer, and GSA Associate Editor Stephen T. Johnston provided valuable critics on the early version of this paper, and their contri-bution to the fi nal paper is gratefully acknowledged. This paper is a contribution to projects BA2136/2-3 and FR2119/1-1 supported by the Deutsche Forsc-hungsgemeinschaft (DFG).

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