The NW Borneo deep-water fold-and-thrust belt, offshore Sabah, southern South China Sea, contains a structurally complex region of three to four seafloor ridges outboard of the shelf-slope break. Previous studies have suggested the seafloor ridges formed either above shale diapirs produced by mass movement of overpressured shales (i.e., mobile shale) or above an imbricate fold-and-thrust array. Here, we performed tectonostratigraphic analyses on a petroleum industry three-dimensional (3-D) seismic volume that imaged the full growth stratal record. We show fold growth history, deformation styles, along-strike structural variabilities, and synkinematic sedimentation during triangle zone–style fold growth. Nine seismic horizons within growth strata were mapped and correlated to petroleum industry seismostratigraphy. Synkinematic sedimentation interactions with growing folds and near-surface strains were analyzed from seismic attribute maps. We interpret that the seafloor structures were formed by imbricate thrusts above multiple detachments. We estimate ∼8 km minimum shortening since the late Miocene ca. 10 Ma. The folds show oversteepened fold forelimbs, back-rotated backlimbs, and forward-vergent (NW to NNW) “blind” thrust ramps that terminate within the growth strata. Fold cores show evidence of internal shear. Immature folds show detachment fold geometries, whereas mature folds show forelimb break thrusts, type I triangle zones, and rotated forward-vergent roof thrusts. Thrust linkages spaced ∼10 km apart were exploited as thrust top synkinematic sedimentation pathways; the linkages also partition near-surface strains. Our comprehensive, three-dimensional documentation of triangle zone fold growth and sedimentation in a deep-water fold belt highlights internal shear, multiple detachments, and opposite thrust vergence; mobile shales are not required to explain the deformation.
Triangle zones and wedge thrust systems (herein referred to as “triangle zones”) are thrust-related folds that are formed by linked, oppositely dipping thrusts that sole into one or more detachment levels (Fig. 1; e.g., Price, 1981; McClay, 1992; Couzens and Wiltschko, 1996; Shaw et al., 2005; von Hagke and Malz, 2018). Research on triangle zone architectures and structural evolution has been driven by their importance for hydrocarbon exploration, earthquake risk assessments, and fold-and-thrust structural models (Jones, 1982; Harrison and Bally, 1988; Barnes and Nicol, 2004; Duerto et al., 2006; Kelsey et al., 2008). Triangle zones have been well documented within many subaerial fold-and-thrust belts, particularly near the deformation front (e.g., Price, 1981; Couzens and Wiltschko, 1996; Espurt et al., 2012; Malz et al., 2016; Qui et al., 2019). Submarine (i.e., deep-water) fold belt triangle zones are less documented (Barnes and Nicol, 2004; Corredor et al., 2005; Morley et al., 2017) but significant because they contain substantial growth strata that allow growth kinematics to be more fully understood (cf. Suppe et al., 1992). Furthermore, at offshore fold-and-thrust belts, the growth strata and synkinematic sedimentation depositional patterns can be imaged by high-quality marine seismic reflection surveys, such as in this study.
This paper describes the three-dimensional (3-D) architecture and growth history of triangle zone–style fold-thrusts within the inboard offshore Sabah NW Borneo deep-water fold-and-thrust belt, southern South China Sea (Fig. 2A). The fold-and-thrust structures occur in a structurally complex part of the deep-water fold belt (Fig. 2) and have been variably interpreted to be the surface expression of deeper shale ridges, shale diapirs, or alternatively, an array of forward-vergent (i.e., NW- to NNW-verging) imbricate thrusts (e.g., Hinz et al., 1989; Morley, 2009a; Cullen, 2010). The study presented here used a spatially extensive (1788 km2), high-quality petroleum industry 3-D seismic volume recorded to 7 s two-way traveltime (TWT; Figs. 2A and 2C) that imaged the full growth stratal record. The study area was chosen to encompass a number of lateral fold terminations to permit stratal correlations around fold tip lines from the footwalls to the hanging walls. Sequential fold growth was deduced from structural analysis of two-dimensional (2-D) cross-sectional profiles and 3-D visualizations. Seismic horizon–based 3-D maps of synkinematic sedimentation patterns, regional isochron thickness variations, and near-surface strains were analyzed and compared against inferred fold evolutionary sequences to present a comprehensive 3-D tectonostratigraphic history. The new insights were compared against previous studies from the Brunei sector deep-water fold belt, located <100 km along-strike to the southeast (Fig. 2A; e.g., Morley, 2007b, 2009b; Morley and Leong, 2008), which has provided many influential examples of thrust-related deformation above overpressured shale detachments. The results shown here highlight the role of triangle zone structures, internal shear, and multiple detachment levels within deep-water fold belts formed above overpressured shales, and they reaffirm the fact that mobile shale tectonics are not required to explain the inboard NW Borneo deep-water fold belt structures.
REGIONAL GEOLOGIC SETTING
The geology of the NW Borneo margin is not well constrained at depth but is thought to be floored by Mesozoic ophiolite basement and early Paleogene Rajang Group marine sediments (Fig. 3; e.g., James, 1984). Early to mid-Cenozoic plate-tectonic reconstructions of NW Borneo and the surrounding region are highly debated (e.g., Hall, 2002; Cullen, 2010; Zahirovic et al., 2014; Advokaat et al., 2018; Wu and Suppe, 2018). It is generally agreed that proto–South China Sea subduction and South China Sea seafloor spreading were significant regional tectonic events (e.g., Hall, 2002; Cullen, 2010; Zahirovic et al., 2014; Wu and Suppe, 2018). Borneo rotated up to ∼45° counterclockwise after the late Eocene (Advokaat et al., 2018). From the early Cenozoic to the mid-Miocene, NW Borneo was apparently an active margin due to southward proto–South China Sea subduction under Borneo (Fig. 3; Hutchison, 1996). During this period, the Rajang Group marine strata were uplifted, deformed, and metamorphosed (Hutchison, 1996), and the West Crocker Formation and its shale-dominated offshore equivalents (e.g., Temburong Formation and other strata) were deposited (e.g., Lambiase et al., 2008). At the mid-Miocene, subduction apparently ceased due to a collision between NW Borneo and Dangerous Grounds extended continental crust, the so-called “Sabah orogeny” (e.g., Hutchison et al., 2000). Some studies have linked the Sabah orogeny to a prominent regional unconformity on regional seismic sections named the Deep Regional unconformity (DRU; Fig. 3; Levell, 1987; Hutchison et al., 2000). The chronostratigraphic significance of the Deep Regional unconformity is still debated, but the unconformity seems to mark the end of folding and thrusting in the Crocker Range and the start of Baram basin extension (Sapin et al., 2011; Morley, 2016).
The Miocene to present-day stratigraphic section of NW Borneo is the main stratigraphic sequence of interest for this study (Fig. 3). The Miocene to present-day section is relatively better constrained than older stratigraphy as a result of petroleum exploration studies (e.g., James, 1984; Hazebroek and Tan, 1993; Sandal, 1996). NW Borneo has experienced 4–8 km of uplift since the mid-Miocene (Hutchison et al., 2000). Topographic growth was particularly rapid in the Neogene (Roberts et al., 2018). Up to 12 km of sediments were rapidly deposited along the NW Borneo margin and other circum-Borneo offshore sedimentary basins within a series of prograding deltas after the mid-Miocene (Hall and Nichols, 2002; Morley et al., 2003; Morley and Back, 2008). Two main deltaic depocenters were established near the study area during this period: the older Champion Delta to the northeast and the younger Baram Delta to the southwest near Brunei (Sandal, 1996). Rapid delta progradation apparently produced high overpressures within the thick marine shale sequences of the Setap Formation (Fig. 3; Sandal, 1996).
Regional NW Borneo Structure
Uplift of the NW Borneo margin is evidenced by exposed older ophiolitic basement and deformed Rajang Group strata to the north and east of the NW Borneo margin (Fig. 2A). Toward the southwest, the Brunei sector was the locus of post–mid-Miocene marine deltaic deposition (yellow area in Fig. 2A). The deltaic sediments near the Brunei coastline have been deformed into N-S–, NE-SW–, and E-W–oriented synclines, anticlines, and thrusts by structural inversion during the late mid-Miocene and early Pliocene (Fig. 2A; Morley et al., 2003). Within the present-day NW Borneo shelf, inboard of the 200 m bathymetric contour, extensional growth fault arrays have been superimposed upon the broad, N-S– to NE-SW–trending inversion anticlines (Fig. 2A; Sandal, 1996; Morley et al., 2003). The growth faults began to form in the middle Miocene and progressively stepped outward (i.e., to the NW) as the delta prograded (Morley et al., 2003).
The present-day deep-water fold belt is formed on the slope outboard of the 200 m bathymetric contour (Fig. 2A). Axial traces of the deep-water folds trend approximately NE-SW and are margin-parallel (Fig. 2A; e.g., Grant, 2004). The deep-water fold belt terminates to the SW near the West Baram line, a widely recognized, major crustal-scale boundary that separates NW Borneo from the Luconia block stable platform (e.g., Hutchison, 1996; Cullen, 2010, 2014). Toward the NE, the deep-water fold belt changes strike and terminates laterally near the so-called “major thrust sheet” (Fig. 2A), an enigmatic region characterized by chaotic and high-velocity strata in seismic profiles (e.g., Hinz et al., 1989; Hazebroek and Tan, 1993; Franke et al., 2008). The fold belt deformation front occurs within the NW Borneo Trough, a long and linear bathymetric depression that contains middle Miocene to Pliocene hemipelagic sediments that overlie and progressively onlap the South China Sea Dangerous Grounds attenuated continental crust basement and Oligocene–Miocene synrift fill (e.g., Fig. 2). A more detailed description of the deep-water fold belt is provided in the next section.
The present-day NW Borneo margin is aseismic but exhibits minor (<1 cm/yr) westerly geodetic motions relative to Sundaland (Simons et al., 2007; Mustafar et al., 2017). The present-day small westward motions are not straightforward to explain because they are oblique to the NE-SW strike of the deep-water folds (Fig. 2A) and to the NW-SE maximum horizontal stress directions within the shelf nearest to the global positioning system (GPS) observations (Tingay et al., 2005, 2007; Hesse et al., 2009; King et al., 2009). It has been debated whether deep-water fold belt deformation since the mid-Miocene has been purely gravity-driven or is a result of a combination of far-field tectonic stress and gravity sliding (e.g., Hesse et al., 2009; Morley et al., 2011; Hall, 2013; Sapin et al., 2013). Evidence for both tectonic-driven and gravity-driven stresses includes the inversion of inner-shelf and onshore growth faults (Morley et al., 2003), maximum horizontal stress direction distributions that do not resemble other Tertiary passive-margin delta systems (King et al., 2010b), and an excess of deep-water fold belt shortening relative to inboard growth fault extension (Hesse et al., 2009). It is also debated whether the NW Borneo margin is tectonically active at present. Hall (2013) and Sapin et al. (2013) attributed the present small geodetic motions to gravity-driven failure of the uplifted Borneo margin. It was argued that NW Borneo is presently tectonically quiescent based on the apparent lack of significant seismicity, magmatism, or evidence for a dipping slab (Hall, 2013).
NW Borneo Deep-Water Fold Belt Structure
The NW Borneo deep-water fold-and-thrust belt is an imbricate fan containing up to 10 major forward-vergent (i.e., NW- to NNW-verging) thrusts (Fig. 2A; e.g., Ingram et al., 2004; Hesse et al., 2009; Cullen, 2010). The frontal thrusts sole into a basal detachment above bright seismic reflectors that overlie the upper synrift Oligocene–Miocene carbonate stratigraphy of the Dangerous Grounds block (Fig. 2C; e.g., Hazebroek and Tan, 1993; Hutchison, 2004; Franke et al., 2008). Cullen (2010) recognized that the bright reflectors were a progressive unconformity and named them the South China Sea unconformity, which we will adopt for this study. The age and significance of the South China Sea unconformity are debated; in places, it is an onlap surface overlain by uppermost Miocene to base Pliocene strata, whereas in other areas, it is an unconformity that is overlain by mid- to late Miocene strata (Morley, 2016). Depth-migrated 2-D regional seismic sections reveal a tapered thrust wedge formed by a 1° to 2° seaward-dipping upper bathymetric surface and an ∼2° to 6° landward-dipping basal detachment near the deformation front (Hesse et al., 2009). The fold belt has a wedge thickness of 2–3 km near the deformation front and is ∼13 km thick underneath the shelf (Hesse et al., 2009). Near Brunei, the deep-water fold belt has a comparable 2° to 5° landward-dipping basal detachment and a 1° to 2.5° seaward-dipping slope that fits a critically tapered thrust wedge (Morley, 2007b). The relatively low critical taper angle near Brunei indicates the likelihood of very high (i.e., near-lithostatic) pore-fluid pressures within the thrust wedge (Morley, 2007b; King and Morley, 2017; Berthelon et al., 2018). The existence of highly overpressured fluids and upward fluid migration is supported by studies that have characterized seafloor mud volcanoes or chimneys above highly fractured seep structures within the Brunei sector (Warren et al., 2014). Some studies have also suggested the existence of shale diapirs or ridges within poorly imaged seismic zones, as further reviewed below.
Within the inboard fold belt, which is the main area of interest for this study, imbricate thrust arrays have been interpreted that link at depth into one or more detachment levels (e.g., Ingram et al., 2004; Hesse et al., 2009; Cullen, 2010). Total shortening within the deep-water fold belt since the latest Miocene has been estimated at 8–14 km (Hesse et al., 2009). An imbalance of updip shelf extension (∼1–2 km) relative to downdip fold belt compression (∼7 km) since the Pliocene was estimated by Hesse et al. (2009) near the study area. The imbalance suggests that shortening within the study area has been driven by both gravity-driven and basement-driven (i.e., tectonic) shortening (Hesse et al., 2009; King et al., 2010a). However, thrust belt shortening estimates are dependent on the structural model. Therefore, we will later discuss the Hesse et al. (2009) analyses against shortening estimates from the new structural model presented here.
Individual Fault-Related Fold Structures
The offshore NW Borneo fault-related folds typically have shorter forelimbs and longer backlimbs and are spaced 5–15 km apart (e.g., Fig. 2C). Many thrust faults do not reach the seafloor but terminate upward within the sedimentary section (Fig. 2C). Some thrust faults appear to have blind tip-line splays or frontal trishear zones (e.g., Hesse et al., 2009). The fold geometries are not well described by classic fault-bend fold or fault-propagation models (i.e., Suppe, 1983; Suppe and Medwedeff, 1990) due to fold backlimb stratal dips that are typically less than the thrust ramp angles (e.g., Morley, 2009a; Hesse et al., 2010a). Growth strata above the folds display fanning limb dips that suggest limb rotation during fold growth (e.g., Fig. 2C; see also Results section). Syntectonic strata above the fold forelimbs are typically deformed by crestal normal fault arrays (Morley, 2007a). A general forward-breaking fold-and-thrust sequence toward the NW was interpreted based on observed folding of progressively younger growth strata toward the outboard NW Borneo Trough (Franke et al., 2008).
Cullen (2010) suggested that the offshore NW Borneo fold belt was segmented along-strike into four structural domains; the segmentation was possibly controlled by deep structures within the rifted South China Sea continental crust. Our study area lies in Domain C of Cullen (2010), which was characterized by deep-water fold-thrust spacings of ∼10 km, near-symmetric folds, and, in map view, narrow and elongate thrust-related ridges that showed only minor segmentation (e.g., Fig. 2). Thickening of growth strata within the fold backlimbs indicated a strong pulse of deformation during the latest Miocene to early Pliocene (Cullen, 2010). Analysis of deep structures from 2-D seismic lines indicated triangle zone geometries and multiple detachment levels within the fold belt (Cullen and de Vera, 2012), but specific structural details, along-strike variabilities, and synkinematic sedimentation interactions have not been fully studied. Other previous studies have alternatively interpreted mobile shale-style deformation within the NW Borneo deep-water fold belts. These arguments are briefly reviewed in the next section.
Do Mobile Shale Zones Exist within the Offshore NW Borneo Deep-Water Fold Belt?
It is controversial whether overpressured muds or shale substrates can deform and remobilize within the subsurface to form large-scale masses such as shale diapirs or shale ridges (i.e., mobile shale; e.g., Van Rensbergen et al., 1999; Morley et al., 2011; Wood, 2011; Morley et al., 2017). Studies of the offshore NW Borneo deep-water fold belt have been central to the debate; poorly imaged zones within anticlinal crests offshore Brunei and offshore Sabah were originally interpreted as mobile shale diapirs or shale ridges (Hinz et al., 1989; Hazebroek and Tan, 1993; Sandal, 1996). Later evaluation of Brunei outcrops found evidence for shale injectites (i.e., shale dikes and sills) but did not find salt diapiric–like shale masses; therefore, it was argued that the extent of mobile shale had been overinterpreted (Van Rensbergen et al., 1999; Van Rensbergen and Morley, 2003). Elsewhere, classic mobile shale-style structural interpretations have been further challenged by improved seismic images that revealed nonplastically deformed strata within regions that had formerly shown chaotic “shale diapirs” (Elsley and Tieman, 2014; Morley et al., 2017). On the other hand, mobile shale has been interpreted and modeled within the offshore Niger Delta inner fold belt (Morley and Guerin, 1996; Ings and Beaumont, 2010; Wiener et al., 2014). Shale diapirs have also been interpreted offshore SW Taiwan (Chen et al., 2014; Doo et al., 2015; Deffontaines et al., 2016).
Folds 1–3 from this study are of particular importance to the mobile shale debate because the fold cores are among the least well imaged within the offshore NW Borneo fold belt and show shale diapir–like chaotic seismic zones within the fold cores (Fig. 2C). Indeed, previous studies proposed that folds 1–3 could be shale diapirs or ridges (Hinz et al., 1989). In contrast, more recent regional 2-D seismic studies have interpreted folds 1–3 as deep triangle zone–style imbricate structures above multiple detachment levels (Cullen and de Vera, 2012), or an imbricate fan array over a single basal detachment (Ingram et al., 2004; Franke et al., 2008). Here, we studied folds 1–3 using the advantage of a full 3-D seismic record. This allowed us to further investigate whether the structures are best explained by mobile shales or thrust-related folding, and if the latter, to document specific deformation details within a structurally complex area.
DATA AND METHODS
3-D Seismic Data
The 3-D seismic data set in this study covered a total area of 1787 km2 (37.4 km × 47.8 km; Fig. 2A) and was recorded to 7 s TWT at a 4 ms sample rate (e.g., Figs. 4–6). The seismic volume had trace (i.e., dip-oriented) and line (i.e., strike-oriented) shotpoint spacings of 12.5 m and 6.25 m, respectively. The long axis of the data set was oriented NE-SW, subparallel to the strike of the deep-water folds and shelf-slope break (Fig. 5). The study area included a number of fold terminations and fold-thrust transfer zones that allowed correlation of growth strata across thrust top basins.
Structural filtering was applied to the seismic data using standard workflows to remove high-frequency noise and enhance the most coherent and dominant seismic reflectors for structural analysis (e.g., Fig. 6C; Landmark Graphics Corporation, 2003). In this study, a three-step workflow was performed using Landmark Poststack/PAL software that included Butterworth band-pass filtering, deconvolution, and a dip-scan stack. Coherency filtering was undertaken to reveal seismic discontinuities from features such as crestal normal faults (e.g., Fig. 6D). Coherency filtering was performed using LANDMARK Poststack/PAL’s Structure Cube algorithm (Landmark Graphics Corporation, 2003), which generated a Structure Cube volume by comparing all traces in an analysis window with the average trace value within the window. The analysis window size used in this study was 3 traces by 3 lines by 15 samples. A dip correction was applied that prevented steep-dipping seismic events from creating apparent discontinuities. A more sophisticated “dip-steered coherency filtered” volume was also generated using OpenDTect seismic software and an OpenDTect processing workflow (Brouwer, 2009). The filter was designed to increase the contrast and the resolution and reduce the number of spurious events in the coherency volume. The processing applied the following steps: (1) calculation of a steering cube; (2) median-filtering of the steering cube; and (3) calculation of a similarity attribute in two orientations.
Seismic Attribute Extractions
Seismic attributes were extracted along interpreted horizons to provide amplitude patterns for seismostratigraphic interpretation. Seismic amplitudes were extracted on interpreted horizons using Landmark Seisworks 3D and Landmark Geoprobe software (Landmark Graphics Corporation, 2003). Average absolute amplitude and root mean square (RMS) amplitudes were extracted from a horizon-guided gate of ±10 ms TWT using the Landmark Poststack/PAL software. Horizon-guided dip, azimuth, dip-azimuth, and difference attributes were also routinely extracted for all interpreted horizons using Landmark Seisworks 3D software (Landmark Graphics Corporation, 2003).
Seismic Interpretation Workflow
Nine horizons were picked throughout the study area using the Landmark Openworks suite of seismic interpretation software (Landmark Graphics Corporation, 2003). The picked horizons were named pregrowth 1 (i.e., PG1), and syngrowth 1 to syngrowth 8 (abbreviated SG1 to SG8; Fig. 4). Horizon seed picks were first manually interpreted over a widely spaced 400 × 400 in-line and cross-line grid. The seed picks were correlated around fold-thrust tip lines to improve growth strata correlations between thrust top basins. Picks were then infilled to produce a higher-resolution 50 × 50 in-line and cross-line grid. The horizon was then autotracked using Landmark ZAP! software using stringent autotracking criteria (Landmark Graphics Corporation, 2003). After assessing the results of the first-pass autotracking, further manual infill picking was performed as necessary, and the horizon was autotracked a second time with reduced autotracking correlation criteria. Finally, interpolation and smoothing were used to close any remaining data gaps and create a complete horizon. Landmark Geoprobe software (Landmark Graphics Corporation, 2003) was used to visually inspect the horizon in 3-D, and further manual infill picking was performed as necessary. After horizon picks were finalized, isochron maps between successive horizons were calculated in Landmark Seisworks 3D (Landmark Graphics Corporation, 2003) to analyze 3-D growth stratal thicknesses.
Seismic Stratigraphy and Regional Stratigraphic Correlation
The nine picked seismic horizons from this study were correlated to a proprietary regional sequence stratigraphic framework provided by Shell Malaysia that was based on seismostratigraphic correlations, well data, micropaleontology, and correlation to sea-level curves following Ingram et al. (2004) and Cullen (2010). The Shell stratigraphic framework was also referenced by previous Brunei sector deep-water fold belt studies (e.g., Morley, 2009a), which facilitates our later comparison between the two regions. Figure 4 shows an example of our interpreted seismostratigraphy at a thrust top basin within the study area, including the seismic character, megasequence boundaries, and structural detachment levels relative to the nine picked horizons.
Three main tectonostratigraphic sequences were identified from the 3-D seismic data set based on seismic character, internal stratal geometries, and reflector continuities:
prekinematic megasequence: late Miocene and older (>10 Ma);
growth megasequence I–IV: late Miocene to mid-Pliocene (ca. 10–4 Ma); and
growth megasequence V–VI: mid-Pliocene (ca. 4 Ma) to present.
The lowermost prekinematic megasequence had parallel to subparallel, lower-amplitude reflectors that were onlapped by overlying strata (Fig. 4). Correlation to the Shell sequence stratigraphic framework indicated that the prekinematic sequence within the study area was likely mid- to late Miocene aged (i.e., ca. 10–15 Ma; Fig. 4). The lithology of our prekinematic sequence is unknown but was inferred to be overpressured offshore and prodelta shales by previous studies (e.g., Sandal, 1996; Morley et al., 2003). Several possible detachment levels were interpreted within the uppermost 300 ms to 500 ms TWT (∼315–525 m) of the prekinematic sequence (Fig. 4). Higher-amplitude, discontinuous, and shingled seismic reflection characters were observed within our interpreted detachment levels, in contrast to the characteristically low amplitudes within this megasequence. Our interpreted detachment levels correlated at or slightly above the “3.1 SB” Shell horizon ca. 11.6 Ma (Fig. 4; Cullen, 2010). A basal detachment was projected into the study area at ∼7 s TWT (∼13 km depth) based on published regional studies (Cullen, 2010; Cullen and de Vera, 2012).
The intermediate “growth megasequence I–IV” was correlated to a late Miocene to mid-Pliocene (ca. 10–4 Ma) age from the Shell stratigraphy (Fig. 4). This megasequence was characterized by moderate- to high-amplitude, parallel to subparallel seismic reflectors that displayed internal and basal onlapping and offlapping relationships (Fig. 4). The megasequence was subdivided into five mapped horizons named syngrowth 1 at the base to syngrowth 5 at the top. The maximum thickness of the megasequence in the study area was ∼1300 ms TWT (∼1.35 km). Based on regional studies, the growth megasequence I–IV likely includes offshore to prodeltaic turbidite sandstones interbedded with hemipelagic and pelagic shales (Ingram et al., 2004).
The uppermost “growth megasequence V–VI” was correlated to a mid-Pliocene (ca. 4 Ma) to present age from the Shell stratigraphy. Seismic reflectors in this megasequence were moderate to high amplitude, discontinuous at the megasequence base, and parallel and continuous at the top (Fig. 4). The horizons were generally less tilted than the horizons in the megasequence below. The syngrowth 5 seismic horizon at the base of the megasequence displayed a regional erosional unconformity (Fig. 4). This unconformity was often onlapped or overlapped by high-amplitude, chaotic seismic reflectors that we interpreted as mass transport deposits (Fig. 4). The maximum thickness of the megasequence in the study area was ∼2100 ms TWT (∼2.2 km). Regional studies indicate the megasequence lithologies are offshore to prodelta turbiditic sandstones that are interbedded with hemipelagic and pelagic shales (Ingram et al., 2004).
Line Length Cross-Section Balancing
Line length cross-section balancing was undertaken to provide first-order shortening estimates from the two-way time seismic sections, which do not contain reliable vertical length scales, but which likely have sufficiently preserved horizontal scales within the upper half of the seismic section to conduct line length analyses (e.g., Fig. 6). It is noted that line length balancing will produce lower than actual shortening estimates in regions with contractional pure shear or ductile strain (e.g., Bulnes and McClay, 1999; Suppe et al., 2004). These strain types have been previously shown in the adjacent Brunei sector deep-water fold belt to the southwest (Morley, 2009a). Therefore, it is best to view the values obtained in this study as first-order, minimum shortening estimates.
General Structural Setting of the 3-D Seismic Study Area
The 3-D seismic volume is located within the inner slope of the central NW Borneo deep-water fold-and-thrust belt just outboard of the shelf-slope break (Fig. 2). Water depths in the study area range from 275 m to 1875 m (Fig. 5A). A seafloor depth map revealed three main NE-SW–oriented emergent ridges within a general NW-sloping seafloor (Fig. 5A). The emergent anticlines have up to 460 m of seafloor relief and are at varying stages of erosion and burial (Fig. 5A). Seafloor canyons are developed that transect the seafloor ridges; the canyons are up to 200 m in depth relative to the surrounding seafloor and 500–1000 m in width (Fig. 5A). Slump scarps with up to 120 m of relief degrade the forelimbs of folds 2 and 3 (Fig. 5A). Local seafloor gradients generally show basinward dips that range from 1.5° to 2.0° within the seafloor canyons; 0.6° to 1.7° within the fold backlimbs; and 5.9° to 10.5° within the fold forelimbs. Landward-dipping seafloor gradients are not observed. Regional seafloor gradients measured from dip section across the study area are between 1.3° to 1.6°.
Our map of the lowermost top prekinematic horizon SG1 is presented in Figure 5 to show the main trend of the four main subparallel, NE-SW–trending thrust-related folds within the study area. We will herein refer to these structures as fold 1 to fold 4 (Figs. 5B and 5C). The emergent seafloor ridges in Figure 5A generally follow the deep structural trends (Figs. 5B and 5C), suggesting a relationship between the bathymetry and deep structures. The mapped fold crests within the top prekinematic SG1 are spaced between 9.5 km and 13.9 km apart and exhibit four-way anticlinal closures along strike (Fig. 5). To the north, folds 3 and 4 laterally terminate along a complex oblique linkage zone (Fig. 5). As discussed earlier, the main fold belt basal detachment is likely near 7 s TWT (∼13 km depth) based on published studies (e.g., Cullen, 2010; Cullen and de Vera, 2012). The basal detachment has been projected into Figure 5C from these studies.
Figures 6A and 6B shows a dip-oriented seismic transect from the 3-D seismic volume that characterizes the typical structural styles within the study area. Figures 6C and 6D show structural and coherency filtered displays of the Figure 6A dip line. All seismic profiles shown in this section have been vertically scaled to show the growth strata sequence at a realistic ∼1:1 scale based on an estimated ∼2100 m/s TWT interval velocity from previous studies (e.g., Franke et al., 2008; Morley, 2009a). It should be noted that our choice to show the growth strata at a 1:1 vertical scale will artificially “thin” the deeper section such that the pregrowth strata will appear thinner than reality; thus, the deep (>4.5 s TWT) thrust fault segments will appear with artificially low dip angles.
The study area is generally characterized by emergent folds that have been partially buried by synkinematic sedimentation (Figs. 5 and 6). The main thrusts terminate upward within the growth strata (i.e., blind) and do not reach the seafloor (e.g., Fig. 6). The fold synclines are conspicuously broader than the narrow anticlines (Figs. 6A and 6B). The fold anticlines are rounded and exhibit short (<5 km) forelimbs relative to longer ∼5–10 km backlimbs (Figs. 6A and 6B). Growth strata are generally well imaged and show onlaps and fanning limb dips (Fig. 6B).
Fold crests are generally deformed by forward- or backward-dipping crestal normal faults (Fig. 6). The crestal normal faults were revealed in more detail by the coherency filtered displays (e.g., Fig. 6D), which showed ∼60°-dipping crestal normal fault arrays (Figs. 6B and 6D). Similar crestal normal faults have also been shown in the Brunei sector (Morley, 2007a). Forward-dipping, low-angle (10° to 21° dipping) slumps were also developed over some fold crests (e.g., folds 2 and 3 in Fig. 6B).
The fold cores are not well imaged, but additional thrust fault cutoffs were revealed by structural filtering (Fig. 6C). The folds showed apparent footwall syncline geometries (Figs. 6A and 6B). Although the footwall synclines could be interpreted as seismic velocity pull-up artifacts, well-imaged thrust faults only showed minor seismic velocity pull-ups that were not sufficient to fully explain the footwall synclines (see Supplemental Figure S11). Instead, we will later argue that the footwall synclines are structural remnants from an inferred early phase of detachment folding. Similar conclusions for the Brunei sector deep-water fold belt were reached by Morley (2009a).
In the following section, growth stratal and fault-fold geometries will be analyzed within the best-imaged outboard fold 3 using four seismic dip lines that step sequentially from the fold 3 lateral tip to the crest. A first-order shortening estimate will be assessed for fold 3 using line length balancing, which is an appropriate technique for this study given the limitations of the two-way time (i.e., non-depth-converted) seismic data.
Outboard Fold 3 Structural Styles
Fold 3 Lateral Termination
Figure 7 shows a series of four seismic dip lines that step progressively southwestward across fold 3 from its lateral termination to its crest, based on the relief shown in the SG1 top prekinematic horizon time structure map (Fig. 5B). Seismic line locations are shown in Figure 5B. Interpreted versions of the Figure 7 seismic lines are shown in Figure 8. At its northern lateral termination, fold 3 has near-symmetric detachment fold geometry and is rounded, broad (∼11 km), and low relief (∼500 m; Fig. 7A). An incipient forward-vergent break thrust was imaged at depth within the fold forelimb (Figs. 7A and 8A). The break thrust showed maximum fault displacements at the center that decreased toward the top and bottom tips (Fig. 7A). These fault displacement patterns are consistent with the initiation of a break thrust within the stratigraphic section, followed by upward and downward tip propagation (Morley, 1994; Tavani et al., 2006). Similar break-thrust initiation patterns were also observed at the frontal offshore Brunei deep-water fold belt (Morley, 2009a).
Seismic coherency filtered displays not shown here indicated that the fold crest was degraded by both basinward- and landward-dipping crestal normal faults (Fig. 8A). Thinning of growth strata above the fold within growth megasequence V implies that folding initiated here within the growth V strata (i.e., between the SG5 and SG6 horizons), which was correlated to ca. 4–3 Ma based on the Shell seismostratigraphy (Fig. 8A). Seismic-amplitude extraction maps presented later in the section Subsurface Mapping show deflected synkinematic sedimentation patterns within the growth V strata that apparently indicate active uplift, supporting the ca. 4–3 Ma fold growth timing inferred here. An upward shallowing of limb dips within the growth strata (i.e., fanning limb dips) above both the forelimb and backlimb indicates fold growth by progressive limb rotation (Fig. 8A; Hardy and Poblet, 1994). At depth, the upper pregrowth strata within the detachment fold core show a thickened package of relatively bright and shingled reflectors (Fig. 8A). The thickened pregrowth interval was observed to generally thicken toward the fold core within other structures in the region (Fig. 8; Fig. S1). Thus, we conclude the observed thickened section is unlikely to be stratigraphic in nature (i.e., a turbidite deposit), but, rather, it is a mechanically weak zone that was structurally thickened due to shear or internal imbrication.
Figures 7B and 8B show a dip line that is located ∼2.4 km along strike from Figures 7A and 7B and closer to the fold crest. Here, fold 3 has an asymmetric faulted detachment fold geometry formed by a shorter 1.8-km-long forelimb relative to a 3.5-km-long backlimb (Fig. 7B). The forelimb break thrust does not reach the seafloor (i.e., blind; Fig. 7B). The fold crest is deformed by a set of crestal normal faults that are mostly forward-dipping but includes some backward-dipping normal faults (Fig. 8B). At depth, the fold core exhibits a footwall syncline and shows a thickened section of bright, shingled reflectors (Fig. 8B) similar to Figure 7A. Backlimb strata show a conspicuously lower dip than the underlying thrust ramp (Fig. 8B), which indicates probable internal shear within the fold (Suppe et al., 2004). The thinned growth IV seismic package suggests that folding initiated here ca. 5.3–4 Ma (Fig. 8B). This fold growth timing is slightly earlier than the observed 4–3 Ma timing at the fold 3 termination (Fig. 8A), which suggests that fold 3 grew laterally along-strike in the Pliocene. Limb rotation–style fold growth is suggested by fanning limb dips within the growth strata (Fig. 8B).
Fold 3 Center
Figure 7C shows a seismic transect across a higher-relief section of fold 3 that is located ∼7.5 km along strike from Figure 7B. Here, fold 3 is highly asymmetric and has a very short (∼2.2 km) forelimb relative to a long (∼8.3 km) backlimb (Fig. 7C). Fault cutoffs apparently show a nonplanar, forward-vergent thrust ramp formed by a shallow-dipping upper thrust and a steeper-dipping lower thrust that are vertically offset at 3.5–4.5 s TWT (Fig. 7C). The kinked thrust ramp and deep footwall syncline are interpreted to be the result of displacement along a deeper, linked backthrust (Fig. 8C), which has been observed in other wedge thrust-style triangle zones (e.g., Harrison and Bally, 1988). The deep footwall syncline could also be a seismic velocity artifact, but as argued earlier, well-imaged portions of fold 3 show only minor seismic velocity pull-ups (∼100 ms TWT) that would be insufficient to explain the deep structure (Fig. S1). The fold crest is deformed by an array of forward-dipping crestal normal faults (Fig. 8C). In contrast to the fold 3 lateral tip, backward-dipping crestal normal faults were not observed here. Thinned growth strata within the growth II or growth III megasequences suggest that fold 3 growth began here around 8 Ma, which means that this portion of fold 3 is relatively older than the lateral fold tip (i.e., Figs. 8A and 8B).
Figure 7D is located 23 km to the southwest from Figure 7C and shows the fold 3 crest within the study area, defined by the point of highest relief on the SG1 prekinematic structure map (Fig. 5B). Here, a conspicuous set of flat-lying reflectors that fit a type I triangle zone footwall block geometry (e.g., Fig. 1A) are imaged within the fold 3 core at 4.5–5.5 s TWT (Fig. 7D). The triangular footwall block is bounded by an interpreted deep forethrust and backthrust that forms a triangle zone fold (Fig. 8D). The triangle zone fold has a short (∼2.3 km) forelimb, a long (∼7.4 km) backlimb, and a footwall syncline (Figs. 7D and 8D) that shows similarities to the less-well-imaged triangle zone in Figure 8C. Thinned growth strata indicate initiation of fold growth during the growth II megasequence ca. 8–7.3 Ma (Fig. 8D), similar to fold 3 crest shown in Figure 8C. The fold crest is strongly onlapped; fanning limb dips indicate fold growth by limb rotation (Fig. 8D). Similar to the Figure 8C triangle zone, we interpret the growth stratal geometries as the result of fold amplification due to displacement along a deep backthrust (Fig. 8D). A shallow, low-angle slump has offset the seafloor and formed a prominent scarp (Fig. 8D). The fold forelimb is not well imaged, but the upper thrust tip is apparently “blind” and has been buried by synkinematic sedimentation (Fig. 8D).
Shortening Estimate within Fold 3 Triangle Zone
Shortening at the fold 3 crest was assessed by simple line length balancing using a datum horizon (shown in blue) within the prekinematic sequence (Fig. 9). Line lengths were measured from pins located at either end of the section where interbed slip was less likely (Fig. 9). Because the Figure 9 seismic profile was available in two-way time only and not depth-scaled, only the horizontal dimension was restored; no attempt was made to balance areas or restore vertical dimensions. The line length balancing produced an estimate of 3.7 km of shortening at this location within fold 3 (Fig. 9). Gaps in line lengths above the SG1 top prekinematic marker are likely from crestal erosion, slumping, and/or nondeposition above the fold crest due to progressive shortening. The gaps above SG1 fit with our interpretation that these units are growth strata based on onlap patterns (Fig. 9).
3-D Visualization of Fold 3 Triangle Zone Geometries
Figure 10 shows a 3-D visualization of four serial seismic dip sections spaced 4.4 km apart near the fold 3 triangle zone–style fold analyzed in Figure 10 (see map in Fig. 5B). Here, fold 3 is characterized by a relatively short forelimb, a longer backlimb, a blind, forward-vergent thrust ramp, and forward-dipping crestal normal faults (Figs. 10B–10E). Growth strata show fanning limb dips (Fig. 10) that indicate limb rotation–style fold growth. The seafloor shows a well-developed, scalloped slump scarp (Fig. 10A) that terminates near the Figure 10E transect. Displacements along the deep backthrust have vertically amplified fold 3 (Figs. 10B–10D) and may have contributed to the seafloor slumping above the fold (Fig. 10A).
Figure 10 shows a deep footwall syncline (developed in Figs. 10B–10D), whereas the footwall in Figure 10E is nearly flat-lying. This provides further evidence that the footwall synclines are truly present and not simply imaging artifacts; if significant seismic velocity artifacts were truly present, “seismic pull-up” would be expected under all four structures, given the general similarities in their thrust configurations. Instead, the along-strike changes shown in Figure 10 indicate that the fold 3 triangle zone (Figs. 7D and 8D) is not a consistent structural feature but varies along strike. These along-strike variabilities have been seen in other shale-prone submarine fold belts (Morley et al., 2017) and suggest that any attempt to present a single, idealized structural model for this region is likely unrealistic.
Summary of Fold 3 Triangle Zone Observations
The following general characteristics were observed at fold 3:
Thinning of growth strata indicate that fold 3 initiated near its present-day crest in the southwest part of the study area during growth II megasequence ca. 8–7.3 Ma (Fig. 10). Fold growth at its present-day lateral tip to the northeast began later during the growth V megasequence at ca. 4–3 Ma (Fig. 7).
Fanning of limb dips within the growth strata above both the forelimb and backlimb suggests fold growth by limb rotation (e.g., Fig. 9).
Limited internal shear within the fold core is indicated by fold backlimbs that show lower dips than the underlying thrust ramp (e.g., Suppe et al., 2004). The sheared section may be within a thickened and shingled package of seismic reflectors within the fold core pregrowth strata, similar to observations in the offshore Brunei deep-water fold belt (Morley, 2009a).
Detachment fold and faulted detachment fold structural styles were observed at the youngest part of fold 3 near its lateral tip (Figs. 7 and 8). Triangle zone geometries were observed near the fold 3 crest (Figs. 7D, 8D). In some cases, a “triangular footwall block” geometry was directly imaged within the fold core (Fig. 7D), which showed similarities to idealized triangle zone models (e.g., Fig. 1A).
The triangle zone folds showed focused amplification and uplift around fold crests characterized by short fold forelimbs, long backlimbs, back-rotated growth strata above the fold crest, and fanning limb dips (e.g., Fig. 10).
Minimum shortening within the highest-relief portion of fold 3 was on the order of ∼3.7 km (Fig. 9).
Mobile shale–style deformation (e.g., expulsion rollovers, shale pipes, shale ridges, and shale diapirs) was not observed and is not necessary to explain the deformation features.
The triangle zone folds show along-strike variability (Fig. 10).
Inboard Folds: Fold 1 and 2 Structural Styles
Figure 11 shows a seismic section across the inboard folds 1 and 2 within the central study area. The folds display short forelimbs relative to long backlimbs (Fig. 11A), bearing resemblance to fold 3 (e.g., Fig. 8). The folds are separated by a broad syncline and show tight interlimb angles (87° to 126°; Fig. 11A) that are amongst the tightest within the offshore NW Borneo thrust belt (Hesse et al., 2010b). Although fold cores were not clearly imaged, the thrust ramps clearly terminate within the growth strata and do not reach the seafloor (i.e., blind thrusts; Fig. 11A), similar to fold 3 (Fig. 8). The fold crests are onlapped by growth strata that show fanning limb dip geometries (i.e., limb rotation; Fig. 11). Truncated reflectors within the growth strata above the fold crests show evidence for crestal erosion during the mid-Pliocene (ca. 4 Ma; SG5) and late Pliocene (ca. 3 Ma; SG6; Fig. 11B). Growth stratal thinning within the oldest growth I strata in green indicates that fold growth began ca. 10 Ma (Fig. 11B). The youngest growth VI strata are nearly flat-lying and have apparently buried folds 1 and 2 (Fig. 11B). Therefore, we interpret that this location has not been very structurally active in the recent past.
Growth strata draping the fold-thrust crests have been asymmetrically folded such that forelimb growth strata are generally more steeply dipping and curvilinear, whereas backlimb growth strata are more gently dipping and planar (Fig. 11B). The steeply dipping forelimb growth strata are interpreted to be the result of a shallow “blind” backthrust that has folded and upturned the forelimb strata (Fig. 11B). If the shallow backthrust were connected to the main forward-vergent thrust tip, it would have formed a small wedge thrust (Fig. 11B). A similar small-scale wedge thrust structure may be presently forming within the fold 3 forelimb (e.g., Fig. 8D). The shallower-dipping backlimb growth strata are interpreted to have been carried piggyback-style above an underlying forward-vergent thrust (Fig. 11B). For the most part, the main roof thrust detachment level (i.e., ∼0.8 s TWT or 850 m below the SG1 horizon) is bedding-parallel (Fig. 11). Therefore, the shallow detachment is a thrust “flat” and not a “ramp.” This implies the thrust flats have been rotated by deeper deformation within the poorly imaged fold cores. In the “Discussion,” we schematically interpret a series of deep forward-vergent thrusts (i.e., verging to the NW) that link from the regional basal detachment to the upper detachment level (Figs. 11, 12, and 13) and comment on other possibilities. A minimum shortening of 4.6 km was estimated for the Figure 11 transect based on a line length balance. Figure 12 shows an example of folded growth strata above a tightened fold 2. Here, the roof thrust has been back-rotated and displays a prominent axial surface pinned to the deep wedge thrust tip (Fig. 12). The axial surfaces seem to extend across the entire growth stratal section up to the seafloor, which suggests active or recent folding (Fig. 12).
Figure 13 shows a 3-D visualization of four serial dip sections spaced 6.2 km apart and seafloor bathymetry across folds 1 and 2. The visualization shows how deformation of folds 1 and 2 is similar across the 3-D study area at the first order (Figs. 13 and 14) but is variable along strike. For example, the fold 1 backlimb shows additional gentle folding of the growth I strata in green that could be indicative of deeper structures and/or minor imbrication within the upper prekinematic sequence in blue (Figs. 13B–13D). Complex transfer zones occur along strike (e.g., fold 2 in Fig. 13D), and these are analyzed in further detail in the next section.
Thrust Fault Displacement–Distance Analysis
Fault displacement variations within the main forward-vergent thrusts T1 to T4 (Fig. 5) were analyzed by constructing fault displacement–distance plots (Williams and Chapman, 1983; Totake et al., 2018). This analysis was designed to highlight local fault displacement minima, which we show are probably linkage zones between originally separate thrust segments. To present our analysis in spatial units, we converted our measured fault displacements from two-way time to depth using a uniform velocity of 2500 m/s based on Upper Miocene to Lower Pliocene growth strata interval seismic velocities from previous studies (Franke et al., 2008).
The along-strike fault displacement–distance graphs revealed seven local thrust displacement minima (A to G in Figs. 15 and 16), which showed >250 m fault displacement reversals. The mapped thrust displacement minima corresponded well to “saddle-point” structural lows within the thrust-related fold anticlines (Fig. 5B). Based on the “bow-and-arrow rule” of Elliott (1976), we interpret the identified thrust displacement minima in Figure 15 to be possible remnant lateral thrust linkages between originally separate thrust segments. At the right-hand side of Figure 15, an overlap zone between thrusts T3 and T4 shows steep and opposing displacement gradients near their fault tips (Fig. 15), which suggest that these thrusts could be kinematically linked within a complex linkage zone. The identified thrust linkage zones A to G underlie many of the present-day seafloor canyons between the seafloor ridges (Fig. 17B). Further evidence for our hypothesized thrust linkage zones will be shown by analysis of synkinematic sedimentation patterns in the “Subsurface Mapping” section below.
3-D Fold Growth History
Figure 16A shows a fold growth summary map for the deep-water NW Borneo study area based on growth stratal analysis. The main thrusts associated with folds 1–4 have been color-coded by their initiation time based on growth stratal thinning (Fig. 16A). Thrust-related fold anticlines were shaded yellow to indicate recently active fold-thrusts (Fig. 16) based on observed regions where the youngest strata, including the present-day seafloor, appear to be folded and where fold axial planes extend to the seafloor (e.g., Fig. 12). Figure 16B shows that the recently folded areas from our analysis are generally uplifted relative to surrounding fold crests. The identified thrust linkage zones in this study (shown by A to G in Fig. 16B) are generally located below the seafloor canyons. Therefore, it seems plausible to conclude that the present seafloor bathymetry has been influenced by the deeper fold-and-thrust structures (Fig. 14).
Our growth stratal analysis indicates a general forward-breaking thrust sequence that began around SG1 ca. 10 Ma in the southeast and proceeded toward the north and west to SG2 ca. 8 Ma (Fig. 16A). After 8 Ma, fold growth was limited to minor, outward lateral growth of the tips of folds 3 and 4 (Fig. 16). At the present day, most of fold 3 within the study area appears to be actively folding the youngest strata near the seafloor (Figs. 10 and 16), except the oblique linkage zone between folds 3 and 4 near the northern study area (Fig. 16A). The northeast halves of folds 1 and 2 exhibit folded young strata near the seafloor and are interpreted to be recently active (e.g., Fig. 13A), whereas the southwest area appears to be presently buried and relatively inactive (e.g., Fig. 11). In the next section, we will compare our fold growth model in Figure 16A against seismic attribute observations from the 3-D seismic volume.
Syntectonic Sedimentation Patterns from RMS Amplitude Extractions
Previous RMS amplitude extractions from offshore Sabah and offshore Brunei 3-D seismic surveys have revealed ancient sediment dispersal pathways (e.g., turbidite fans and channel complexes) and gravity mass-wasting features (e.g., mass transport complexes and basal scours) that offer insights on paleo-seafloor bathymetry and areas of active fold growth and uplift (e.g., McGilvery and Cook, 2003; Morley and Leong, 2008; Morley, 2009a; Cullen, 2010). These insights are of potential importance for this study because they can be compared to our fold growth model (Fig. 16), which was based on relatively independent growth stratal analyses. In this study, submarine sedimentation patterns within the syngrowth strata were revealed by nine RMS amplitude extractions, from which four representative maps are shown (Fig. 17). A 3-D visualization of Figure 17C is shown in Figure 18. High RMS amplitudes are shown in red, and lower RMS amplitudes are shown in gray to black. Following previous studies (e.g., Morley and Leong, 2008), sheet-like or channel-like high-RMS-amplitude bodies are interpreted as turbiditic, sand-rich sedimentation, whereas low RMS amplitudes may indicate hemipelagic shale-rich areas. The previously interpreted thrust linkage zones A to G from Figure 15 were overlain on the maps for comparison.
The top of the prekinematic SG1 horizon (ca. 10 Ma) showed sheet-like, high RMS amplitudes that were confined to the fold 1 backlimb, and a lobate, intermediate amplitude at the eastern study area that straddled fold 2 (Fig. 17A). The confined sedimentation patterns suggest that the inboard folds 1 and 2 were active during this stage and had formed adequate seafloor topography to block sedimentation from reaching the outboard area, whereas fold 3 was not yet active. This compares well to our growth stratal thinning–based structural model, which suggests that folds 1 and 2 had formed by the time of the SG1 horizon (Fig. 16).
The early Pliocene–aged SG4 horizon (ca. 5.3 Ma) amplitude extraction showed narrow, chaotic swaths of high RMS amplitudes between folds 1 and 3 and a sheet-like, high-RMS-amplitude body outboard of fold 3 (Fig. 17B). These are interpreted to show high-energy, basinward flow of submarine sedimentation across active fold scarps. In the eastern study area, high-amplitude basal scours showed deflections around the crests of folds 1 and 2 (Fig. 17B), which presumably exhibited significant seafloor topography during this time period. Other sediment pathways apparently traversed the folds across low-relief areas (e.g., A, B, C, D, and F in Fig. 17B), which correspond to the interpreted linkage zones from the fault displacement–distance analyses (Fig. 15).
The early Pliocene–aged SG7 horizon (ca. 1.8 Ma) showed extensive sheet-like, high-amplitude regions that traversed the folds within the western study area (Figs. 17C and 18). In particular, the area above our identified thrust linkage G was an important sediment pathway (Fig. 17B). By contrast, low RMS amplitudes in black indicate sediment bypass areas along the easternmost folds 1 and 2, and fold 3 (Figs. 17B and 18). This suggests that the easternmost folds 1 and 2 and fold 3 had positive seafloor topography during this time period and were potentially active, which compares well with our identified areas of present-day uplift and folding as shown by the yellow areas in Figure 16. Synkinematic sedimentation appears to have been buttressed by the fold 3 crest and forced to flow around its NE lateral termination (Fig. 17C). The fold 3 eastern lateral termination shows a prominent low-amplitude region that suggests it was bypassed by sedimentation (Figs. 17C and 18), in contrast to earlier periods (e.g., Figs. 17B and 18). The full suite of amplitude extraction maps (not shown) indicated an abrupt change in sedimentation patterns at the fold 3 eastern termination between SG4 and SG5 (i.e., between ca. 5.3 and 4 Ma). These changes compare well to the predicted initiation of fold growth at the fold 3 and fold 4 tips based on growth stratal thinning analysis (Fig. 16A).
The present-day seafloor horizon shows mainly low amplitudes above most of folds 1 and 2 and above the fold 3 crest (Fig. 17D). Narrow belts of high amplitudes occur within seafloor canyons that formed above potential thrust linkage zones F, D, and B (Fig. 17D). The low amplitudes above folds 1, 2, and 3 apparently indicate present-day active sediment bypass around the study area. We suggest that this sediment bypass is largely driven by active uplift areas along folds 1, 2, and 3 as identified in Figure 16.
Near-Surface Strain Patterns
Near-surface strain patterns above deep-water folds are partially controlled by seafloor erosional processes (i.e., canyon development, slumps), but they can also indicate seafloor relief changes linked to deeper structural geometries (i.e., linkage zones, fold plunges, or oblique ramps; e.g., Morley, 2007a). Near-surface strain patterns within our study area were revealed by dip-steered coherency filtering of the Quaternary SG8 horizon (ca. 0.5 Ma), which is located at ∼100 m below the seafloor (Fig. 19). Crestal normal faults were detected above all four folds (Fig. 19D). The largest population of crestal normal faults was oriented parallel to the strike of the seafloor ridges (Fig. 19A) and was seaward-dipping (black lines in Fig. 19D). A few ridge-parallel crestal normal faults were landward-dipping (white lines in Fig. 19D) and occurred near identified thrust linkage zones labeled A to G in Figure 19D. Ridge-front faults oriented transverse to the ridge axes (e.g., Fig. 19B) occurred above the forelimbs of folds 1–3 (orange lines in Fig. 19D). In seismic sections, these ridge axis–transverse faults were small-offset, subvertical normal faults. The faults had displacements that died out to the top and bottom similar to crestal normal faults in the Brunei sector deep-water fold belt (Morley, 2007a). The fold-axis transverse faults generally occurred ahead of the highest-relief fold segments (e.g., Fig. 5B), but it was not possible to clearly link these patterns to any mapped deep structure. Fold axis–oblique crestal normal faults that were en echelon and had 20°–35° obliquity were detected at the southwest end of fold 2 (Fig. 19B), where late lateral growth of fold 3 was interpreted from growth strata (Fig. 16A). Crosscutting crestal normal faults were revealed at the lateral termination of fold 3 (Fig. 19C). In general, the strain patterns appeared to be most disrupted by deep thrust linkage zones (Fig. 19D), which indicates that the deep linkages continue to influence strain patterns to the present day.
Vertical Thickness Variations from Isochron Maps
Isochron maps show the vertical thickness variations in two-way time between two seismic horizons. These maps can be useful for deducing fold growth timing from growth stratal thickness variations, or they may reveal regional sedimentation thickness and erosion trends. It should be noted that isochron maps will show distortions in regions with steeply inclined bedding, particularly in the dip direction; therefore, we limited our isochron map analysis to show general trends within the study area, particularly in the along-strike direction. In addition, fold growth timing inferred from isochron thickness variations was confirmed by comparisons with seismic sections.
Figure 20 shows a series of four isochron maps; thicker areas are shown by blue-purple colors, whereas thinner areas are in red-yellow. The late Miocene isochron map computed between the SG1 and SG4 horizons (ca. 10–5.3 Ma) shows a general pattern of thinning above fold crests and thickened thrust top basins (Fig. 20A). A thickened lobe-shaped area can be observed within the northeast study area across folds 1 and 2 (Fig. 20A). The lobate thickened area shows similarities to the sheet-like, high RMS amplitudes at the backlimbs of folds 1 and 2 within the SG1 amplitude map (ca. 10 Ma; Fig. 17A). This suggests that the 3-D study area received sedimentation from a sediment source to the northeast of the study area in the mid- to late Miocene. A similar conclusion was reached by Cullen (2010) based on a regional RMS amplitude extraction and calibrations to exploration well lithofacies. These inferred sediment sources are consistent with mid- to late Miocene reconstructions of the Champion Delta (Sandal, 1996).
The Lower Pliocene isochron map between the SG4 and SG5 horizons (ca. 5.3–4 Ma) revealed significantly thinned or absent growth strata, particularly above the fold 2 crests (Fig. 20B). Comparison to seismic cross sections confirmed the absent strata and showed that the thickness variations were largely from crestal erosion and not nondeposition (e.g., Fig. 13). Lobe-shaped thickened areas ahead of fold 2 were formed by forelimb fold degradation complexes (e.g., fold 2 in Fig. 13A).
The Upper Pliocene isochron map computed between the SG5 and SG7 horizons (ca. 4 Ma to 1.8 Ma) revealed an asymmetric thickening toward the southwest within an overall pattern of thinned crests and thickened thrust top basins (Fig. 20C). The thickened area suggests that sedimentation patterns shifted in the early Pliocene to a southwesterly source, which was likely the Baram Delta.
The Quaternary isochron map computed between the SG7 and seafloor horizons (ca. 1.8 Ma to present day) revealed a thickened area within the southwest study area (Fig. 20D). This overall thickening was also seen in the Upper Pliocene isochron map (Fig. 20C) and supports the notion of a Baram Delta sediment source for the study area from the late Pliocene to present day. Thinned areas were observed above the eastern fold 1 and 2 crests (Fig. 20D). These thinned areas correspond well to the sediment bypass or channel blocking seen within our seismic amplitude extractions (Figs. 17B–17D). This may suggest that the easternmost folds 1 and 2 have been structurally active since the late Miocene (Fig. 16).
Growth of Folds within the 3-D Study Area
Early Fold Growth by Detachment Folding and Internal Shear
The youngest folds in the study area occur at the northeast termination of fold 3 (Fig. 16A). Here, fold 3 showed a broad, near-symmetric, detachment fold geometry that displayed minor internal thickening within the fold core (Fig. 8A). A forelimb break thrust developed within the stratigraphy that had upward and downward thrust tips (Fig. 8A). A remnant footwall syncline was developed near the break thrust (Fig. 8B). The dips of the backlimb strata are shallower than the underlying thrust ramp, which suggests the likelihood of internal shear within the fold (Suppe et al., 2004). Therefore, we infer that early fold growth within the study area was accommodated by detachment-style folding and internal shear (Figs. 21A and 21B). Morley (2009a) noted similar structural styles within the youngest folds at the frontal Brunei sector deep-water fold belt and termed these “shear detachment folds,” since they did not fit classic fault-bend or fault propagation fold styles (Suppe, 1983; Suppe and Medwedeff, 1984). A kinematic model based on internal simple shearing was used to explain their internally thickened fold cores, remnant footwall synclines, and lack of connection to a basal detachment (Morley, 2009a). The shear detachment fold model (Morley, 2009a) appears to satisfactorily explain the early fold growth in this study area (e.g., Figs. 21A and 21B) and further supports our argument that the observed footwall synclines are truly present and not seismic artifacts.
Fold 3 Amplification through Triangle Zone–Style Fold Growth
The higher-relief central fold 3 (Figs. 8C and 8D) showed many typical characteristics of shear fault-bend folds (cf. Suppe et al., 2004), including: (1) shorter forelimbs relative to long backlimbs, (2) growth strata that displayed upward fanning dips, and (3) lower backlimb dip angles relative to the underlying ramp. These observations indicate the important role of shear-related deformation during deep-water fold growth, which has also been recognized at the Cascadia and Nankai accretionary prisms (Suppe et al., 2004) and the offshore Niger Delta (Corredor et al., 2005). At some areas of fold 3, the main forward-vergent thrusts are kinked and vertically segmented (Figs. 6C, 8C, and 12). The kinked fold 3 forethrusts may have been generated by late folding due to displacements along a deep backthrust, as shown in our geometric model in Figure 21. However, this structural model can only be locally applied because structural styles change along strike over relatively short distances within fold 3 (Fig. 10). Indeed, our analyses of fold 3 show that shortening is accommodated by variable combinations of internal shear, forethrusts, and backthrusts along multiple levels, and shallow crestal normal faulting (Figs. 6, 8, 11, and 19). This is not surprising, as recent studies have indicated that a single, universal structural solution is unrealistic for most fold-and-thrust belts (Butler et al., 2018). Previous studies also noted that shale-prone thrust systems show highly variable structural styles that are present over short distances and change rapidly (Morley et al., 2017). Therefore, our fFold 3 structural model (Fig. 21) is best presented as an example of triangle zone–style deformation within a deep-water fold belt, but it should not be singularly applied to the study area.
Growth of the Older Folds 1 and 2
The older folds 1 and 2 showed more complex first-order geometries relative to fold 3 (Figs. 11 and 13) and require an alternative structural model. Folds 1 and 2 are detached on a bedding-parallel detachment ∼0.8 s TWT (∼850 m) below the SG1 horizon, which has been rotated by deeper deformation (i.e., within the poorly imaged fold core; e.g., Fig. 11). Upward tilting of both the fold forelimb and backlimb is supported by fanning limb dips within the growth strata (Figs. 12 and 13). Fold tightening has produced minor wedge thrusts within the growth strata (Figs. 11 and 14) and, in some cases, back-rotated the roof thrust (Fig. 12). Here, we tentatively interpret deeper forward-vergent thrusts that are kinematically linked to the regional basal detachment (red dashed lines in Figs. 11, 12, and 13), but other solutions are possible, which are discussed below. Regardless of the deeper uncertainties, the overall geometries of folds 1 and 2 (Figs. 11 and 12) show the importance of multiple detachment levels within the deep-water NW Borneo fold belt, which confirms previous interpretations (Cullen and de Vera, 2012). Our study also provides new details to show along-strike variabilities and interactions between the thrust linkages and synkinematic sedimentation (Figs. 13, 15, and 17).
The internal geometries of the deep fold cores are not well imaged by the 3-D seismic in this study (e.g., Fig. 13) but are likely formed of shale-dominated strata (Fig. 4). Previous offshore NW Borneo from structural studies (de Vera and McClay, 2008) and triangle zone sandbox models (Couzens-Schultz et al., 2003; Pichot and Nalpas, 2009) suggests the fold cores could contain complex, shallow-dipping (<15° dip) imbricate thrusts or duplex structures. Studies of shale-prone thrust zones (Morley et al., 2017) indicate that duplex structures, if they exist, are likely to be complex and disrupted shale zones, in contrast to classic, well-ordered thrust duplexes. For example, the northern Makassar Straits deep-water fold belt shows deep fold cores that are variably deformed by arrays of minor thrusts (or a single major thrust) that do not (does not) show consistent vergence (Morley et al., 2017). Alternatively, the fold cores here may possibly resemble a “mushwad,” a large-scale, “ductile” duplex of stratigraphically weak layers that deform below an overlying stiffer layer (Morley et al., 2017).
Implications for Triangle Zone Structures
Triangle zones are “structures with a triangular shape in section view accommodating shortening by coeval activity of a basal thrust and an associated backthrust of opposite vergence (e.g., Fig. 1; von Hagke and Malz, 2018). This study provides new details of triangle zone and wedge thrust structures within a submarine fold belt formed across multiple detachments within overpressured shales, especially in the third dimension (i.e., along strike). The development of the multiple detachment levels and oppositely vergent thrusts shown here was likely linked to the very high fluid overpressures, which have been documented within the NW Borneo deep-water fold belt (Morley, 2007b; Berthelon et al., 2018). The offshore Niger Delta, Gulf of Guinea, provides another example of a deep-water fold belt that has developed above multiple detachment levels (Corredor et al., 2005). Similar to the examples presented here (e.g., Figs. 12 and 13), movement along deeper, underlying wedge thrusts at the offshore Niger Delta resulted in a conspicuous pattern of kinked and folded upper thrust sheets (Corredor et al., 2005). Several deep-water fold belts, including the offshore Niger Delta structure and offshore West Sulawesi fold belt, northern Makassar Straits, show relatively rapid (i.e., within 4 to 6 km) changes in fault vergence and detachment levels along strike (Higgins et al., 2007, 2009; Morley et al., 2017). Within this context, the triangle zone styles observed here from 3-D seismic data further illustrate the diversity in structural styles within limited regions of shale-prone deep-water fold-and-thrust belts.
The triangle zone thrusts shown in this study differ in style from typical triangle zones at subaerial fold belts, which usually occur as a single structure near the deformation front and are often buried by syntectonic sediments (e.g., Price, 1981). They also differ from triangle zones that are formed above a single detachment (i.e., the type I triangle zone; Fig. 1A), which are simpler structures that can only accommodate very limited shortening (von Hagke and Malz, 2018). In contrast, the type 1 triangle zone in fold 3 shown here likely overlies deeper structures above the basal detachment (Figs. 7D and 8D). Thus, deformation across multiple detachments should be considered when restoring shortening. Minor wedge thrusts that show similarities to type II triangle zones (Fig. 1B) may also form within the synkinematic strata (e.g., growth strata I to VI in Fig. 11; see also Fig 14).
Regional Tectonic Implications
Comparisons between shortening and extension within the NW Borneo margin have been used to infer the relative amounts of gravity-driven and far-field tectonic convergence in northern Borneo since the late Pliocene (Hesse et al., 2009; King et al., 2010a). Simple line length section balancing using our interpreted triangle zone structural styles indicates 8.3 km of minimum total shortening across the inboard folds 1–3 of the deep-water fold belt (∼3.7 km for fold 3 in Fig. 9; ∼4.6 km for folds 1 and 2 in Fig. 11). The minimum of 8.3 km of shortening within the inboard fold belt obtained by our study exceeds the ∼2–4 km of updip extension within the shelf growth faults (Hesse et al., 2009). Similar conclusions were reached by Hesse et al. (2009) using an alternative structural model that had an imbricate thrust array over a single basal detachment. Our results show that excess downdip fold belt shortening relative to updip shelf extension is a valid conclusion. This strengthens the argument of Hesse et al. (2009) that deformation within the NW Borneo deep-water fold belt likely involved a component of tectonic shortening (i.e., deformation cannot be simply explained by gravity-driven shortening).
Implications for NW Borneo Deep-Water Fold Belt Overpressured Shale Tectonics
The poorly imaged fold cores in the study area have been interpreted as shale ridges or shale diapirs, or alternatively, an imbricate thrust formed above a single or multiple detachment levels (Hinz et al., 1989; Hazebroek and Tan, 1993; Hesse et al., 2009; Cullen and de Vera, 2012). Our structural analysis shows that the poorly imaged seismic zones within the NW Borneo deep-water fold belt (e.g., the inboard folds 1–3 in Fig. 2C) can be reconciled as thrust imbricates above multiple detachments (Fig. 11). Moreover, we did not observe mobile shale–related features such as salt withdrawal–style synclines that would indicate down-building along fold flanks (Wiener et al., 2014). Therefore, we conclude that mobile shales are not required to explain deformation in the study area. On the other hand, there is ample evidence for fluid escape features above fold crests (Figs. 19A and 19B). Fluid escape features have also been seen within the Brunei sector from outcrop (Morley, 2003) and seismic studies (Warren et al., 2014).
A structural analysis was conducted on a 3-D seismic survey that imaged the full growth stratal section of the NW Borneo deep-water fold belt, offshore Sabah, southern South China Sea. The study area shows three to four seafloor ridges outboard of the shelf-slope break that were previously interpreted to be the surface expression of mobile shale tectonics or deeper imbricate thrusts. Our results show that the offshore Sabah deep-water fold belt is best explained by triangle zone–style thrusts that formed across multiple detachment levels in overpressured, shale-prone strata. The most outboard fold 3 initiated as detachment folds that had slightly thickened fold cores from internal shear. The forelimbs were later deformed by a break thrust, leaving a remnant footwall syncline. Shear fault-bend fold-style deformation then produced structures that had short forelimbs relative to long backlimbs, upward fanning limb dips within the growth strata, and backlimb dips that were lower than the underlying thrust ramp. A portion of fold 3 was amplified from triangle zone wedge thrust-style folding that steepened fold forelimbs and back-rotated the backlimb strata. In contrast, the inboard folds 1 and 2 were more structurally complex and showed rotated roof thrusts and minor wedge thrusting in the growth strata above multiple detachments. Our analyses show significant structural variation along strike, including thrust linkage zones spaced ∼10 km apart that were exploited as thrust top synkinematic sedimentation pathways. The variable deformation styles preclude a single structural model and instead highlight structural diversity within shale-prone thrust systems.
Shortening estimates based on our triangle zone–style structural models indicate ∼8 km of minimum shortening within the three most inboard NW Borneo deep-water folds. Our inboard fold belt shortening estimate exceeds updip shelf extension measured from previous studies. This supports the notion that shortening along NW Borneo since the mid-Miocene included tectonic shortening and cannot simply be explained by gravity-driven tectonics. Growth stratal thinning showed a general forward-breaking thrust sequence after the late Miocene ca. 10 Ma. Mobile shale–style tectonics, including shale diapirs and the mass movement of mobile shales, are not required to explain the structural geometries within the study area.
Shell International Exploration and Production provided funding for the research through the “4D Deepwater Fold Belts” project at Royal Holloway, University of London, UK. Bill Wilks, Andrew Cullen, Mike Foley, Stephen Wright, Charlie Lee, Thibault Burckhart, Jonny Guddingsmo, Senira Kattah, Yaw Tzong, Mark Griffiths, Phlemon George, Homerson Uy, and Brent Cooper are thanked for their help and expertise during a visit to the Shell Sarawak office by the authors. Shell and Petronas granted permission to publish the seismic data through the efforts of Charlie Lee, Joe Balan, and Lee-Fah Jong. Josep Anton Munoz and Jake Hossack are thanked for constructive comments on an earlier version of this paper. John Suppe and Luther Strayer are thanked for helpful discussions. Joe Cartwright and Ian Clark are thanked for their input and discussion. We thank reviewers Chris Morley and Cara Burberry and Science Editor Andrea Hampel for helpful criticism and comments that significantly improved the final manuscript.