Growth of Folds in a Deep-Water Setting

Home , Anticline, Fault block, Thrust fault

Growth of folds in a deep-water setting

Chris K. Morley* Baan Yosawaadi, 7 Pahonyohothin Road, 10400, Bangkok, Thailand

ABSTRACT relative changes in sediment supply to indi- about the detailed structure of deep-water folds, vidual piggyback basins are defi ned by rela- and how fold growth has interacted with sedi- Regional 3D seismic data of the deep- tive changes in sediment supply (generally mentation and gravity (e.g., Hooper et al., 2002; water area offshore NW Borneo provide a increasing with time) with respect to growth McGilvery and Cook, 2003; Ingram et al., 2004; detailed picture of the interaction between in anticline amplitude. Such changes may Corredor et al., 2005; Heinio and Davies, 2006; sedimentary processes on a continental slope be due to sediment infi lling of depressions Morley, 2007a, 2007b). Deep-water fold belts and the growth of major folds over a time farther updip and prograding downslope form very large provinces, equal in complexity period of ca. 3.5–5 Ma. In the deep-water with time or to changes in rate of deforma- to the external zones of orogenic belts, such as area, the estimated rates of fold propagation with time. Initial sediment pathways the Western Alps and Zagros Mountains, but are tion of tens of mm/yr−1, shortening rates of were predominantly subparallel to growing much less completely described. mm/yr−1, and fold segment lengths of tens of faults. As folds matured, canyons and chan- Models for the interaction of growing folds kilometers indicate the studied folds are simi- nels exploited low points (e.g., fold linkage or with synkinematic sedimentation have been lar in scale and deformation rate to folds in intersection points) or weak points (e.g., mud determined from a range of studies including orogenic belts such as the Zagros Mountains pipes) on folds to initiate transverse channel theoretical stratal geometries determined from and Himalayas. Feedbacks between sedi- systems. At a late stage, transverse channels trigonometry-based, fold-growth simulations, ment dispersal patterns, anticline growth, carved up the once long and linear surface to the detailed interaction of sedimentation and and structural style are manifested in many ridges into isolated segments. fold growth, usually based on well-exposed ways, and are enhanced by the presence of outcrops or 2D seismic data (e.g., Mutti et weak, poorly lithifi ed, synkinematic sedi- 1. INTRODUCTION al., 1988, 2000; Suppe et al., 1992; Shaw and ments at fold crests that undergo mass wast- Suppe, 1994; Hardy and Poblet, 1995; Hardy ing as the fold grows. As folds tighten they Deep-water fold belts occur in three main et al., 1996; Delcaillau et al., 1998; Evans and range in geometry through simple folds— settings: (1) accretionary prisms, (2) toe thrust Elliott, 1999; Lopez-Blanco, 2002). Geometric folds affected by crestal normal faults, folds and fold regions associated with gravitational approaches describing fault-propagation folds with crestal normal faults and rotational deformation of oceanward-dipping, passive have assumed a simple buildup of synkinematic slides, and folds with forelimb degrada- margin sediments underlain by mobile salt, sediments onto a growing fold high that can tion complexes and pronounced erosional and (3) passive or active margins where rapid generate a range of patterns of thinned, rotated, unconformities. The unconformity surfaces sedimentation and development of overpres- and onlapping growth sequences (e.g., Suppe are either elongate parallel to the fold axes sured mobile shales are associated with input et al., 1992; Hardy and Poblet, 1995; Hardy et (related to local mass wasting) or perpendic- from one or more major deltas. These regions al., 1996). These models appear to work well ular (related to fl ows crossing the anticlines). are home to fold-thrust belts equivalent in size for folds growing in continental and shallow Mass wasting processes in the study area to the external regions of major orogenic belts marine environments. However, the deep-water that are large in scale compared with folds such as the Jura Mountains of the Alps and the environment has different erosional and sedi- (i.e., giant landslides) have modifi ed anticline Zagros Mountains of Iran and Iraq (e.g., Morley mentary characteristics than folds in other set- shape by erosion, and are little affected by and Guerin, 1996; Rowan, 1997; Trudgill et al., tings. Only in deep-water settings can poorly anticline topography. More commonly, grav- 1999; Ajakaiye and Bally, 2002; Hooper et al., lithifi ed syntectonic sediments accumulate on ity fl ows are relatively small compared with 2002; Jackson et al., 2008). While deep-water top of growing anticlines that have a strong the anticlines, and transport pathways are folds have been known for a long time from topographic expression at the seafl oor. The infl uenced by anticline surface topography. 2D seismic-refl ection data, the detailed geom- strong topography causes interplay between The factors infl uencing sediment pathway etry and evolution of deep-water folds have not structure and gravity fl ows moving down the changes during anticline growth include: been documented until recently. The advent of continental slope. The patterns of synkinematic proximal to distal propagation of folds, lat- deep-water drilling technology has led to the sediment deposition, sediment removal, and eral propagation of folds, and changing loca- commissioning of numerous 3D seismic sur- sediment redistribution signifi cantly impact fold tions of fold growth with time. Assuming an veys in the search for hydrocarbons. The 3D shape (e.g., McGilvery and Cook, 2003; Nigro overall consistency in sediment supply, local seismic data provide a wealth of information and Renda, 2004). Using a 3D seismic data set

*Former address: Department of Petroleum Geoscience, Universiti Brunei Darussalam, Tunku Link, Bandar Seri Begawan, Brunei Darussalam; E-mail: chrissmorley@ gmail.com.

Geosphere; April 2009; v. 5; no. 2; p. 59–89; doi: 10.1130/GES00186.1; 26 ﬁ gures.

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gathered across young (Pliocene-Recent) folds faults, shale diapirs, and inversion-related anti- strate that the NW Borneo trough is the site of offshore Brunei as an example, this paper inves- clines (Morley et al., 2003). inactive subduction where the oceanic crust of tigates the ways in which folds develop through The NW Borneo fold-thrust belt is developed the proto–South China Sea was subducted dur- time in a deep-water setting and interact with on the slope in the deep-water equivalents of ing the Paleogene but became jammed in the sediment transport down a continental slope. It the extensively drilled middle Miocene–Recent latest Early Miocene when thinned continental is believed that the 3D data set provides a level shelf sequences (Fig. 1). The slope dips between crust of the Dangerous Grounds block entered of detail about fold-sediment interaction that has one and three degrees into the deepest region of the subduction zone (Levell, 1987; Hall, 1996). not been so comprehensively demonstrated pre- the South China Sea—the Northwest Borneo The Miocene uplift and erosion history of NW viously, although the fundamentals of the pro- trough. Until recently the fold-thrust belt was Borneo has at least in part been attributed to cesses are well established. only known through sparse industrial and aca- oceanic slab breakoff during the middle–late demic 2D seismic lines (James, 1984; Hinz and Miocene and buoyant rise of the leading end of 2. PREVIOUS WORK Schluter, 1985; Hinz et al., 1989; Sandal, 1996; the Dangerous Grounds continental crust (e.g., Schluter et al., 1996; Ping and Hailing, 2004). Hutchison et al., 2000; Morley and Back, 2008). The NW margin of Borneo has been exten- The 2D seismic data show an extensive train of Despite the termination of subduction, global sively explored for hydrocarbons, and as a offshore-verging folds, spaced between 5 and positioning system (GPS) data indicate differ- result, the geology of the shelf and upper slope 15 km apart, affects the slope. They appear to be ential motion continues to occur between NW areas of Brunei, Sabah, and Sarawak are well mostly fault propagation folds related to imbri- Borneo and Sundaland driven by collisional known from drilling and seismic-reﬂ ection data cate faults at depth that sole out into one or more events to the south and east of Borneo (Simons (James, 1984; Sandal, 1996; Petronas, 1999). detachments. The thrust belt has a classic taper et al., 2007). The shelf is dominated by prograding shallow geometry, with the basal décollement dipping The present-day fold-thrust belt is largely marine sequences of middle Miocene–Recent shelfward; at the shelf-slope break the detach- of latest Miocene–Holocene age (Sandal, age that attain thicknesses in excess of 10 km in ment probably lies at a depth of ~10 km (Fig. 1; 1996; Ingram et al. 2004). Hence the deforma- places. The sequences are deformed by growth Morley, 2007a). The 2D seismic data demon- tion is unrelated to the earlier period of active

113° 114° 115° 116° 117°

South China Seas 7° 200 m Bathymetry Syncline Anticline Offshore 0 40 km Thrust Normal Brunei fault fault Darussalam 2000 m NW Borneo2500 m Trough 1000 m Palawan Sabah 110° 1500 m + + + South China Seas 6° ++ Fig. 2 Malaysia Spratley Rise- Dangerous Grounds Brunei Sabah

Sarawak 5° Borneo 0° Sabah Sunda Shelf Kalimantan Brunei + Sumatra + + + ++ N Baram Delta N + Province 4° Sarawak Java Sarawak 500 km

Meligan Sandstone Paleogene Rajang Undifferentiated Paleogene Miri Zone, Formation (Oligocene- Group flysch (on oceanic crust?) (on continent basement?) Early Miocene) Upper Cretaceous-Eocene Belait, Liang and Setap tectonized ophiolite with ++ Neogene igneous rocks Shale Formations remobilized serpentinite (Miocene-Holocene)

Figure 1. Regional map of NW Borneo showing location of the NW Borneo trough and study area, offshore Brunei Darussalam.

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subduction of oceanic crust. The active folds GPS data (Simons et al., 2007) show that north- maximum horizontal stress direction (NW-SE) trend NE-SW, parallel to the old trench (Fig. 2), ern Borneo is moving west at ~4–6 mm/yr with determined for the shelf immediately adjacent indicating the suture zone between the Danger- respect to Sundaland (i.e., South Borneo, Pen- to the GPS points (Tingay et al., 2005). While ous Grounds block, and NW Borneo is a major insula Malaysia, and Indochina). This motion the stress tensor for the deep-water area may zone of weakness that reactivated from the lat- is oblique to the NE-SW–striking folds of the have a component related to gravity-driven est Miocene–Recent (Hall and Morley, 2004). continental slope and oblique to the modern deformation, another signiﬁ cant component

Figure 2. Edge map of the water-bottom refl ection mapped from 3D seismic-refl ection data, for the outer shelf and slope region of offshore Brunei Darussalam (the seafl oor morphology shown here has also been illustrated by Demyttenacre et al. [2000] and McGilvery and Cook [2003]). See Figure 1 for location. The darker gray shading indicates greater slope dips on the external anticlines compared with the upper slope. Black arrows mark some examples only of the terminations of headless channels along the upper slope. I, II, III, IV, V, VI, VII, VIII, IX, and X denote anticlines. Letters A to M denote locations along important sediment pathways from the upper slope to the lower slope. Incised canyons through anticlines occur at locations A, E, F, I, and M. Inset shows a perspective view (looking SE) of the edge map.

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of the stress tensor is regional stress related to package characteristics due to changes in sedi- intervals between horizons C and D, B and C, A Australia-Sundaland collision. The gravity- ment type, structural location, and timing of and B, and seafl oor and A; (3) edge maps of the driven component of stress may help explain the deformation. All the horizons were picked fi ve mapped horizons; and (4) RMS amplitude difference between the modern stress orienta- within refl ection packages that in places thin maps for the fi ve horizons. tion and GPS-defi ned block motions. Hence the onto anticlines and thicken into synclines, and The vertical cross sections show the detailed deep-water folds probably arose due to the com- hence are synkinematic with respect to folding. interaction between refl ections and fold growth, bined effects of gravity-driven deformation and For high-resolution interpretation across the as shown by thinning-thickening patterns, from regional stresses (Morley et al., 2008). area, horizons B, C, and D were picked every refl ector termination patterns (e.g., onlap, ero- Recently the ability to explore for hydrocar- 25 lines and traces, and horizon A was picked sional truncation, and downlap), sedimentation bons in deep water has opened up the margin every 50 lines and traces. For 3D seismic data, style (e.g., presence of degradation complexes, to extensive 3D seismic surveys (Demyttenacre the location of vertical sections through the data slumps, and landslides), and unconformities. To et al., 2000; Ingram et al., 2004). Publications that lie parallel to the acquisition direction are understand how folds and sedimentation patterns using the 3D data have described the petroleum called lines or inlines, those orthogonal to the have evolved during fold growth in map view, system of the fold-thrust belt in Sabah and the acquisition direction are called traces, while sec- the isochron maps are used in conjunction with modern sedimentary pathways from the shelf to tions selected in any other orientation through the RMS amplitude, edge, and difference maps. deep-water offshore Brunei (Demyttenacre et the data are called arbitrary lines. In some areas The isochron maps show thickness changes for al., 2000; McGilvery and Cook, 2003; Ferguson of high-density faulting, inlines and traces a particular interval, which tend to be related to et al., 2004; Gee et al., 2007; Morley, 2007a). were picked every 2–10 lines. In areas where fold growth, erosion, and deposition in fans and The 3D data enable detailed mapping of the there was high refl ection continuity and struc- channels. The geometry of thickness changes seafl oor, which provides highly detailed images tural disruption was minimal, horizons were cross-checked with refl ection characteristics of modern depositional systems (Fig. 3). picked every 50 inlines and traces. Line spac- on vertical cross sections makes for a clear ing is every 12.5 m, hence the spacing of the interpretation of the isochron maps. Typically, 3. DATA 25 × 25 grid is 312.5 m. The main mapped area sedimentary patterns, such as sinuous channel, is ~50 km long in a strike direction (NE-SW) or fan-shaped features visible on the isochron Ten thousand square kilometers of 3D seismic and up to 60 km long in a dip direction (NW- maps, also show on the RMS amplitude maps. data were acquired by Petroleum Geo-Services SE). The fi nal infi ll of the picked grid to produce The RMS amplitude, difference, and edge maps (PGS) in 2000 and 2001 across the deep-water a continuous horizon was conducted with Land- are derived for a particular horizon, or narrow area of Brunei. The data were originally pro- mark’s zoned auto-picker (ZAP). Automatic time window around the horizon, where the cessed by PGS in 2001. This paper is part of a infi ll of the picked grid uses seismic waveform best resolution may be ~15–30 m. The isochron study at the University of Brunei Darussalam characteristics constrained within a user-defi ned maps represent a thicker interval of investiga- utilizing the deep-water 3D survey provided by interpretation window, and the picked seismic tion with the isochron interval ranging from zero an agreement between the Petroleum Unit, Total, data as seed points, to produce a continuous thickness up to around 500 ms (~450–500 m). and Brunei Shell. The data set used in this study horizon within 3D seismic data. This means the isochron map is less detailed was clipped at one second below the seafl oor; Derivative images of time-structure map dip- in resolving specifi c sedimentary features than the deeper seismic data were not available for and-strike characteristics based on changes in the other maps. However, in general terms there study. Calibration of refl ections from the 3D curvature can be generated by a number of dif- is a good correspondence between the features survey with the well and seismic data shown in ferent algorithms (e.g., Brown, 1996). In this imaged on the different map types. Ingram et al. (2004) and Sandal (1996) from the study, edge maps were found to give the best shelf and adjacent deep-water areas indicates results for detecting subtle structural and strati- 5. MODERN SETTING the upper one second of data (~0.9–1 km thick- graphic signatures of comparatively minor fea- ness of section) is of predominantly Pliocene- tures such as channels, scarps, craters, and small Figures 2, 3, and 4 show the regional context Holocene age. The upper one second of data faults, as well as showing large-scale folds and of the area in map view and in cross section. provides a wealth of detailed information about fans (e.g., Fig. 2). Figure 2 is an edge map of the water-bottom how sedimentation and folding have interacted To determine the sedimentation patterns and time-depth across the outer shelf, slope, and within the synkinematic section. fault patterns in map view from contrasts in seis- Borneo trench area. The outer shelf is relatively mic amplitude, two methods were used: (1) the smooth, and is cut by a few active growth faults. 4. METHODOLOGY amplitudes along the auto-picked horizon, and The continental slope is strikingly different. It is (2) the root mean square (RMS) of amplitude val- dominated by an array of terraces (underlain by Seismic interpretation was conducted using ues for a 10 ms window of data sampled above or synclines) that step down the slope via a series of Landmark software. Five horizons were picked below a smoothed version of the picked horizon prominent, relatively steeply dipping, NE-SW– for the study: the seafl oor (Fig. 4); a shallow hori- using Landmark StratAmp software. Contrasts in trending ridges related to the crests and fore- zon (horizon A) that coincides with an unconfor- amplitude values in map view can show distinc- limbs of growing anticlines. The synclines form mity on the crests of several folds; the top (hori- tive sedimentary forms such as channel or fan- sedimentary depocenters with a subhorizontal zon B) and bottom (horizon C) of a distinctive shaped bodies of lower or higher amplitude rela- seafl oor (Fig. 4). Anticlines tend to have a strong low refl ectivity package; and the deepest pos- tive to the adjacent areas (e.g., Fig. 3). surface expression and locally steeper slopes on sible horizon (D) that could be mapped around To understand how folds have interacted with the fold limbs than the regional dip of the con- most of the study area (Figs. 4 and 5). It is not deep-water sediments with time, the following tinental slope, but this expression changes sig- possible to generalize too much about the char- aspects of the seismic interpretation were used nifi cantly passing down the continental slope. In acteristics of the different horizons because there most extensively (see section 8): (1) interpreta- Figure 2 the steeper slopes are associated with are important lateral changes within refl ection- tion of vertical sections; (2) isochron maps of the more distal 4–5 anticlines. Anticlines more

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Fig. 4b

IX Fig. 4c

Fig. 7a M Fig. 7b Fig. 7c VI Fig. 10 I VIII

J IV Fig. 11 H V F B K IV Fig. 23b V I Fig. 23a Fig. 21b III Fig. 4a E Fig. 9c Figure 3. (A) Root mean square Fig. 9b I D Fig. 20 U2 (RMS) amplitude map of the Fig. 21a Fig. 4b seaﬂ oor, illustrating the main Fig. 9a VII II structural and sedimentary V features active today (also see C U1 McGilvery and Cook [2003] for a similar water-bottom Fig. 4c map). See Figure 1 for location. N (B) Interpretation of the main

A structural and sedimentary features seen on the amplitude A 10 km map in Figure 3A. Black arrows mark some examples only of the NE edge of giant landslide (see Gee et al. 2007) terminations of headless chan- Region of flow-aligned Region of high amplitudes ridges within giant associated with downslope nels along the upper slope. I, landslide (glacier-like creep?) transport of sediments (channels and fans) II, III, IV, V, VI, VII, VIII, IX, Crestal area of anticline with relief higher than surrounding seafloor and X denote anticlines. Letters Mud pipe/mud volcano A to M denote locations along IX Trace of normal fault scarp important sediment pathways Trace of mound crest from the upper slope to the Channel crossing (gravity-driven fold?) M saddle region Scarps along anticline crest, lower slope. Incised canyons in anticline typically due to landslides through anticlines occur at Erosional striations where flows have traversed locations A, E, F, I, and M. Inset L low-relief portion of anticline shows the location of the ﬁ gure Ridges in late landslide Segments of within giant landslide with respect to the edge map in Channel crossing turbidite Cliff at I Figure 2. Red—high amplitude, VIII anticline crest channel edges trailing edge of break-away black—low amplitude. region J IV H V F B K IV I

III

E I D U2 Channel crossing Coalesced channels saddle region VII in anticline

V C U1

Upslope region of numerous channel imput areas

A B

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NW SE I II A IV III

2..0 VI

2.5 VIII

IX 3.0

3.5

4.0

0 10 km 4.5

II III B IV 2..0 V

2.5 VI VIII

3.0

IX Mapped horizons 3.5 A

TWT (s.) TWT B

4.0 C D 0 10 km 4.5

NW II SE 1.5 C 2000 II (U1)

2500 Mud pipe III HA and crater 2..0 D HB 3000 IV 0 5 km

HC 3500 V (F) 2.5 HA, B, C = Horizons A, B & C 4000

V1 (I) Knickpoint canyon on forelimb VIII 3.0 M 0 5 km

Folded low-angle unconformity 3.5

Unconformity

4.0 Arbitrary line

Figure 4. Regional seismic lines showing the mapped horizons; two (A and B) are inlines, while (C) is an arbitrary line. The arbitrary line follows the modern cross-fold sediment transport path down the slope (see Fig. 3 for location). TWT—Two-way time.

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I II proximal to the shelf are strongly dissected by III A IV NW-SE–trending linear to anastamosing channels and are lower relief features than the dis- VI tal anticlines (Figs. 2 and 4). In Figure 2, the smooth area of the outer shelf is not cut by any channels; all the channels arise along the upper slope or at anticlines and do not connect with any channel system on the shelf. The southwest- ern part of the shelf in Figure 2 is cut by some active growth faults, and they are not disturbed by any channels. Hence it is clear the channels crossing the slope are headless. A region lacking clear ridges and channels, with a speckled, blocky appearance, dominates V the western half of Figure 2. As described in VII detail by Gee et al. (2007), this region is covered I II by a giant, young landslide, called the Brunei III B IV Slide. Figure 3 shows an amplitude map with an interpretation of the main structural and sedi- VI mentary features of the study area. Two fans (K and J; Fig. 3), fed by channel systems oriented transverse to the fold axes, are prominent fea- Figure 5. 3D visualization tures of piggyback basin development (McGil- of structure maps for four very and Cook, 2003). The amplitude map mapped horizons for this study shows that the areas of higher-energy sedimen- (A) horizon A, (B) horizon B, tation (channels, landslides, and fans) tend to be V (C) horizon C, and (D) horizon regions of relatively high amplitude (yellow-red D. View toward the NE. Black VII colors); the areas of more aggradational depo- areas are regions where the sition (anticlinal crests and more isolated parts horizon could not be mapped of synclinal basins) are lower-amplitude areas I due to either unconformities, II (shades of gray). The more landward folds are III C the horizon being deeper than IV commonly affected by shallow mass movement one second from the seaﬂ oor, VI processes and mud volcanoes (Fig. 3), while the or unmappable within degra- western part of the area covers the eastern mar- dation complexes or mud-pipe VVIII gin of the Brunei Slide.

and mud-volcano provinces. I, VIX II, III, IV, V, VI, VII, VIII, IX, 6. VERTICAL SEISMIC PROFILES and X denote anticlines. 1.5 In many fi gures, vertical seismic lines are 2.0 presented; their vertical scale is given in two- V 2.5 way time (TWT), in units of seconds (s) or mil- VII liseconds (ms). An approximate depth conver- VX 3.0 Two way time (s.) Two sion can be made assuming an average seismic 3.5 velocity of 1900–2000 m/sec for the fi rst second 5 km 4.0 (TWT) of data below the seafl oor, which means I 100 ms are equal to ~95–100 m. The sections II III D below describe changes in fold geometry pass- IV ing upslope from the most external or distal to VI the more proximal folds.

VVIII VIX 6.1. Simple Folds

In the most distal anticline, the internal folded refl ections lie subparallel to the seafl oor. The seafl oor is folded and has a convex-up geometry. Much of the section maintains its thickness

V around the fold, and only the uppermost reﬂ ectors show thinning toward the fold crest. The VII VX fold is unaffected by normal faulting of the crest (Fig. 4B, anticline IX).

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6.2. Simple Fold with Normal Faults and folds and thrusts at the downdip termination disturbed by post-depositional gravity processes. of the slide. Figure 8A shows an edge map of the Conversely, mass wasting or erosion by currents In many folds, including the most external fold seafl oor where slump scars break up a forelimb or fl ows to produce degradation complexes epi- in some places, planar to slightly curved faults, that is also affected by planar normal faults. An sodically affects the fold crests and more steeply dipping ~40°–65° affect the crest (Figs. 6 and amplitude map of horizon B (Fig. 8B) shows the dipping anticline forelimbs (Morley, 2007b). 7). These faults tend to penetrate deepest (500– deeper fault pattern within the anticlines. The In the relatively steeply dipping forelimbs of 700 ms, ~460–650 m) at the fault crest, and die result of the interaction between slumps and some folds the continuity of refl ections is com- out at shallower levels passing toward the fold normal faults produces an angular discordance pletely lost. These areas are characterized by limbs. In map view the faults can be strongly between the seafl oor refl ection and the inter- chaotic or seismically transparent zones with curved (Fig. 8). Faulting is most intense at the nal refl ections of the fold, here called forelimb disorganized wedge and lens-shaped packets of fold crest and dies out laterally (down plunge) unconformities (Fig. 7). Multiple unconformity high-amplitude refl ections (labeled degradation as fold amplitude decreases. In cross section the surfaces are frequently present, indicating rep- complexes in Fig. 9 and at location a in Fig. 10). planar nature of the faults and different depths etition of the faulting and slumping process Some weak internal refl ections tend to show at which they die out downward indicate that during fold growth (Fig. 9). Landslides produce lower dips than the overall dip of the forelimb despite their curved map patterns, which sug- depressions, benches, and bulges in the slope (Fig. 10). In places the wedge-shaped packets gest rotational detachment type faults, the nor- that modify the overall convex-up fold geometry (Fig. 10, location a) terminate against refl ective mal faults actually do not link up with a detach- (Fig. 8A, locations a and c). units within the forelimb syncline. These very ment. Hence they cannot be viewed as classic distinctive forelimb packages are interpreted to rotational slides. The faults are most intense on 6.4. Mature Folds with Angular represent degradation complexes, where mate- the steeper anticline slopes, and fault orientation Unconformities and Degradation rial removed by mass movement from the anti- changes as the strike of the slope changes. These Complexes cline crest and upper forelimb is redeposited observations suggest the faults are gravity-driven at the foot of the forelimb (Heinio and Davis, features, related to slope instability created by In the folds described above it is generally 2006; Morley, 2007b). growing anticlines (Morley, 2007b). possible to trace horizons around from the back- On dip lines, the presence of a crestal uncon- limb to the forelimb, particularly deeper in the formity can be diffi cult to defi ne due to com- 6.3. Simple Fold with Normal Faults and fold, except where slumps have locally removed plications with gas effects causing strong ampli- Mass Movement material or have disorganized layering. In tudes below a bottom simulating refl ection mature folds it is not possible to correlate hori- (BSR) associated with gas hydrates. However, Overall the convex-up fold geometry is still zons directly across the fold on dip lines from the on strike lines, one or more angular unconfor- preserved, and the fold resembles simple folds forelimb to the backlimb. The thickness of the mities tend to stand out clearly (Fig. 11). The with normal faults as described above, but addi- backlimb section is much greater than the fore- deepest unconformity tends to cut through units tionally the fold forelimb is affected by rotational limb due to the fold asymmetry and its effects tilted and rotated by normal faults, and dip in the slides, which tend to detach parallel to the fore- on where sediment accumulates (Figs. 10 and same direction as the forelimb. Some folds are limb slope at depths of ~100 m or less (Figs. 7B, 11). The more gently dipping backlimb areas, at this stage today (Figs. 7B and 7C). Usually 8A [locations a, b, c, d], and 9C). These slides while displaying thinning onto the fold crest, are sediments are deposited above the unconformity are linked with listric extensional faults updip generally areas of sediment accumulation, little slope. In the simplest case, these units are little

NW Crestal normal faults SE

HA 3.5 TWT (s.)

HB HB

HC HC 4.0

1 km

Figure 6. Seismic line across the most external anticline (IX), which shows the seaﬂ oor is folded quasi-conformably with deeper reﬂ ections. HA, HB, and HC—seismic horizons A, B, and C, respectively. See Figure 3 for location. TWT—Two-way time.

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deformed, and show low dips. In more complex cases, the overlying units are cut by new or reac- NW SE tivated normal faults, and may contain internal A angular unconformities and show considerable HA changes in dip (Figs. 4C and 9). HB 3.0

TWT (s.) 6.5. Mature Folds with Mud Pipes and Gas Chimneys HC Folds at the mature stage of development display angular unconformities and degrada- 3.5 tion complexes; they are also commonly associated with mud pipes and gas chimneys. Pipes and chimneys are readily identifi able in cross HB section as narrow, vertical zones of transpar- Gas chimney ent to chaotic zones of seismic character. Mud HC pipes typically terminate at a local depression in the seafl oor. In map view the depressions are 1 km circular to oval features a few hundred meters across. For example, the seafl oor amplitude B Erosional unconformity map (Fig. 3A) shows numerous circular fea- truncating tilted fault blocks tures, interpreted to be mud volcanoes that are HA aligned along the topographic ridges (Fig. 3B). HB 3.0 In general the external folds are little affected by Slumped region any large pipes, and only one small gas chimney HC affects external fold IX. The large mud pipes are the only type of fl uid escape feature that, based on geochemical analysis of water-bottom grab samples, is associated with thermogenic hydro- 3.5 carbons (Warren et al., 2009). This suggests a deep origin (probably >3 km) for the associated HB mud and fl uids (Fig. 10D). Smaller pipe-like features appear to be gas-rich fl uid escape features. HC The seismic refl ections of multiple horizons are locally disturbed (pulled-up or pushed-down) in a narrow vertical band (e.g., Gay et al., 2003). These changes are attributed to a lateral change C Erosional unconformities in velocity (velocity pull-up or push-down) truncating tilted fault blocks and are known as velocity-amplitude anomaly structures (VAMPS). In the case of dewatering HA features, the pull-downs may in part be a geo- HB 3.0 logical feature, not a velocity feature, which arose due to volume loss from the fl uid source HC layer. Figure 10E shows two examples of vertical pipe-shaped anomalies. The velocity pull-up anomaly suggests the presence of gas hydrates; the pull-up effect dies out downward. The chim- 3.5 ney with pull-down geometry (Fig. 10E) indi- HB cates the presence of free gas in pore spaces and/ or mechanical collapse of layers due to dewater- HC ing. The vertical pipe anomalies do not extend deeper than 600–700 ms (~550–650 m); hence they are shallow features and appear to rise off a seismically transparent (white) layer that, from drilling upslope, is known to be very shale Figure 7. Seismic lines a, b, and c across anticline VIII showing rotational rich. Clusters of these pipes are found only on slide affecting forelimb of anticline, also affected by deeper penetrating and the backlimb side of anticlines (Fig. 10D). The more planar normal faults. The sections are dip lines that are higher toward pipes are interpreted to represent lateral migra- the fold crest passing from a to c; HA, HB, and HC—seismic horizons A, B, tion of fl uids out of the synclinal depocenter, and C, respectively. See Figure 3 for location. TWT—Two-way time. updip to a level where fl uid pressure is large

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enough to exceed the minimum principal horizontal stress, and permit hydraulic fracturing up A to the surface. Fold height and maturity appear to control mud-pipe and gas-chimney spatial density, suggesting a strong link between shortening, pore space reduction, and overpressuring. While shortening is clearly manifest at a large scale as folding and thrusting, what is less well known is the degree to which bulk shortening of the young, and porous, deep-water section is achieved by tectonic compaction. It has been IX observed in fold-thrust belts that bulk shortening by pressure-solution cleavage precedes folding (e.g., Morley, 1986a, 1986b; Whitaker and Bartholomew, 1999; Tavani et al., 2006), and bulk shortening by loss of pore space in the shallower, less compacted deep-water sediments is c M likely to occur as well. This would result in a d component of the overpressure being related a to shortening, not just disequilibrium compac- N tion associated with rapid burial of fi ne-grained sediments (e.g., Osborne and Swarbrick, 1997). Uplift of overpressured units in the cores of anti- b clines raises them higher relative to the same units on the anticline fl anks and synclines and VIII thus brings them closer to a stress state where natural hydraulic fracturing will occur. The B effects of mass movement and erosion removing some of the overburden at the crest will further enhance the propensity of anticlinal fold cores to preferentially spawn mud pipes. In some folds, mud pipes are associated with forelimb degradation complexes (Fig. 10), and hence in addition to erosion and slumping of material, the degradation complexes also contain mud- fl ows resulting from eruptions of overpressured mud from the pipes.

Figure 8. (A) Edge map of the seaﬂ oor reﬂ ection showing normal faults and slumped area associated with the external anticlines (IX and VIII); see Figure 3 for location. Location a—upper ridge associated with a extensional rotation in the upper part of a major rotational slide; location b—most intense region of mass movement cutting back high into the fold crest at the apex of the fold; location c—ridge associated with toe thrust and fold region of slide; d—anticlinal ridge at tip of imbricate thrust fault (deep penetrating). M—incised canyon on anticline crest. (B) Root mean square (RMS) amplitude map of horizon B. The rotational slide region of location a disturbs the reg- 0 5 km ular pattern of deep-penetrating normal faults seen beyond location a.

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NW SE

V-shaped lateral terminations to Transport path unconformity Minor erosional bench disorganized reflection packages merging with forelimb wedge-shaped packages = mass unconformity movement complexes on forelimb 2.5 HB Latest folding HA episode

Folding HC Quiescence 3.0 TWT (s.) HB HD Early Folding

No pronounced thinning 3.5 onto fold crest A Thinning of upper non-kinematic layers in forelimb by slumping during early stages of fold growth? 1 km NW SE Unconformity

HB Degradation 2.5 complex BSR TWT (s.)

HA HC 3.0 HD

HB Decreasing fold interlimb angle

3.5 B

Slump/landslide scarp NW SE Successive forelimb unconformities HA

Degradation 2.5 HB

complex TWT (s.)

HA Figure 9. Examples of changes in uncon- HC formity geometry passing along the strike 3.0 of anticline V. (A) Anticline morphology in HD saddle area, where ﬂ ows can traverse an HB anticline, passing toward the anticline; crest sections (B) and (C) show increasing erosion, the effects of normal faults, and mass wasting. HA, HB, HC, and HD—mapped 3.5 seismic horizons A to D, respectively. See C Figure 3 for location. BSR—bottom simulating reﬂ ection; TWT—Two-way time.

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NW Low-angled internal SE A reflections to Mud pipe degradation complex 0 Degradation Normal faults Mini basin complex Forelimb Backlimb BSR TWT (s.)

BSR a 1.0

Approximately V.E. = 2:1

Unconformity 1 km

B Anticline VIII Anticline VI

Approximately V.E. = 1:1 C 0 HA

HB TWT (s.)

HA HC a HB HB 1.0 HC HC Approximately V.E. = 2:1

Velocity pull-ups associated with gas chimneys Velocity pull-down associated D E 100 m with fluid/gas chimney Mixed aggradational sediments, locally slumped 0 sediments and mud volcano products Fluid pipes and gas chimneys BSR Thermogenic system Biogenic system 0

E 1 Mud Pipe Degradation complex 2 TWT (s.) Overpressured fluid migration path? 3

4 km

Top of disequilibrium compaction overpressure near lithostatic pressure Older, compacted units, not Overpressured detachment zone mobile or strongly overpressured 0.7

Figure 10. Example of two anticlines (VIII and V), the geometry of which at the seafl oor is strongly modifi ed by mass-wasting and mass-movement processes. The northwestern fold (VIII) is a simple fold with normal faults; the southeastern fold (V) is affected by erosion, forelimb degradation complexes (labeled “a”), and a mud pipe. HA, HB, and HC—mapped seismic horizons A to C, respectively. BSR— bottom simulating refl ection interpreted to indicate the presence of gas hydrates. (A) Sketch illustrating the geological features of the seismic line; (B) seismic line at approximately vertical = horizontal scale; (C) seismic line at more typical display scale of 2× vertical exaggera- tion (better for seeing details of seismic character); (D) geological cross section based on (A) above and deeper 2D seismic-refl ection data illustrating likely overpressured fl uid-distribution and fl uid-migration pathways; (E) examples of shallow fl uid and gas pipes developed on the backlimb of the anticline, from lines along strike from the section shown in (B) and (C) above. See Figure 3 for location. TWT—Two- way time.

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7. TIME-STRUCTURE MAPS tinuous seismically transparent, internally cha- during deposition of units represented by hori- otic to partially organized refl ections frequently zons A and B. The time-structure maps (Figs. 5A The regional horizons A, B, C, and D were occur in the piggyback basins. These transpar- and 5B) also illustrate the erosional degradation picked within refl ection packages that in places ent units represent landslides, debris fl ows, tur- of the anticline crests and folds. However, the thin onto anticlines and thicken into synclines, bidites, and other products of mass movement details of this erosional degradation are much and hence are synkinematic with respect to fold- and mass wasting (e.g., McGilvery and Cook, better imaged on isochron, amplitude, and edge ing (Fig. 4). The deepest horizons (D and C) gen- 2003; Gee et al., 2007). maps, and are discussed below. erally lie within lower-amplitude, more continu- Time-structure maps of horizons A, B, C, and ous refl ection packages, and represent the early D are shown in Figure 5. The deeper horizons (c 8. LARGE-SCALE SEDIMENTATION stages of fold development. In general, refl ection and d in Fig. 5) defi ne well-developed, simple PATTERNS ASSOCIATED WITH FOLD continuity is good, and refl ectors tend to be sub- fold shapes. The upper slope anticlines (I, II, and GROWTH parallel; however, in places, some highly discon- III) are relatively low amplitude compared with tinuous to chaotic units are present. anticlines IV, V, VI, and VIII, which have deep The large-scale patterns of fold evolution Much of the synclinal basin fi ll is character- synclinal depocenters. The fold hinges are curvi- were discerned from the isochron maps derived ized by a relatively high-amplitude package of linear with prominent saddles and crests devel- from time-structure maps, coupled with ampli- refl ections with an irregular internal geometry oped along their length. The anticline trend VI tude extractions of the mapped horizons. For that overlies horizon C; they also thin and onlap and VII has three distinct crests and saddles that this study there were no wells within the area onto anticlines. Horizon B is located at the base probably refl ect the connection of three isolated of 3D seismic data with which to calibrate rock of the high-amplitude refl ection package in the folds. Horizons A and B (Figs. 5A and 5B) defi ne data with amplitudes; however, Ingram et al. central slope area, and horizon A is a shallow very subdued fold shapes compared with horizons (2004) provide some indication about litholo- horizon within the higher amplitude basin fi ll. D and C. Synclines form fl at-bottomed lows, and gies and stratigraphy based on well and seismic The synclinal basin fi ll contains a wide range the anticline ridges are mounded features dis- data from adjacent areas of Sabah. The ampli- of refl ection-pattern types, ranging from par- sected by NE-SW–trending low linear features. tude map of the seafl oor (Fig. 3) shows that high allel, continuous refl ections, to discontinuous It is apparent from vertical seismic sections that amplitudes are associated with clear channel, mounded or wavy forms. Erosional truncations horizons A–D infi lled fold-created bathymetry fan, and lobe complexes (also see McGilvery are frequent. Extensive tabular to highly discon- and that the fold geometry was smoothed out and Cook, 2003). These higher-amplitude areas

SW NE

Unconformity

HA HA HB HB 2.5

HC TWTT (s.)

3.0

1 km

Figure 11. Example of seismic trace along crest of anticline V, showing development of angular unconformities during fold growth. HA, HB, and HC—mapped seismic horizons A to C, respectively. See Figure 3 for location. TWTT—Two-way traveltime.

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may be associated with increased sand content the anticlines VII and VIII on the isochron map (area H) there was a high in the synclinal trough within a predominantly shale-prone section. (Fig. 12C) suggests that the maximum height (Fig. 12C). The isochron map for horizons C–B Similarly on amplitude maps of deeper hori- of the anticlines above the synclinal depocenter shows that the syncline adjacent to anticline V zons, high amplitudes tend to be associated with was ~350 m. continued to deepen at a relatively high rate, the fi ll of synclines, and also commonly defi ne The isochron map for horizon B–A (Fig. 12B) while adjacent to anticline IV subsidence in channel and fan geometries. shows a signifi cant change in geometry from the syncline slowed. This resulted in an along- Isochron maps of the early synkinematic the deeper isopachs. Instead of the dominant strike ramp dip of the synclinal fl oor toward stage (between horizons C–D and B–C) show NE-SW trends, the contours display a more rec- the anticline V synclinal area (Fig. 5B). Dur- that the anticlines tend to be long (25–30 km), tilinear pattern; the anticlinal ridges are no longer ing its early development the piggyback basin continuous features that are infrequently so continuous. This pattern refl ects the breach- showed patchy development of high-amplitude breached by channels and canyons (Fig. 12). ing of anticlines by NW-trending channels and areas (Fig. 13). There is a possible feeder sys- Most sediment transport was axial, along the canyons (Fig. 15A). The amplitude map of hori- tem into the piggyback basin through area H, synclinal piggyback basins, as indicated by zon B (Fig. 16) shows some differences with and some poorly developed potential channels higher amplitude areas in the synclines com- the deeper horizons. The channel systems from transported sediment out of the basin around pared with the anticlines (Figs. 13, 14, and 15). area B fed sediment into the piggyback basins area J (perhaps refl ecting an episode when the Areas H and B (Figs. 13 and 14) are the main between folds II, III, and IV to the west. Small basin around area Q was completely fi lled; see regions of early transport across folds. Area H transverse canyons are also highlighted crossing Fig. 13). Horizon B shows the highest- amplitude occurs at the down-plunge termination of fold V folds III and VI. Area E to K marks an area of concentration occurs in the area between K and against fold IV (Figs. 13 and 14), while area B high amplitudes in a low area between folds VII E (Fig. 16). This is the main isopach thick of is a persistent area of SE- to NW-directed trans- and VI; it does not show the organization and the basin (Fig. 12B). Late in the basin fi ll during port on all amplitude maps except horizon D well-developed transverse feeder systems and deposition of horizon A, extensive deposits of (Fig. 15). The persistence of area H to cross-fold fans that characterize the subsequent infi ll of the gravity-driven sediments affected the area, nota- transport is due to the relatively low wavelength piggyback basin (Figs. 3, 15a, and 17). bly fans J and K (Fig. 17). The WSW-plunging and amplitude of the folds (Fig. 4C), whose The amplitude map of horizon A clearly piggyback basin between anticlines VI and IV growth appears to have kept pace with sedi- shows that between horizons B and A well- displays basin oblique transport paths that link mentation; hence the folds created little surface developed transverse canyons and channels with a high-amplitude channel system that runs topography. Passing to the SW the same folds started to dissect folds I to VI (Figs. 15A and parallel to anticline VI. The linked system of (II and III) increase in amplitude and interrupt 17). Elongate, sinuous, high-amplitude pack- channels then cuts across the anticline at loca- NW-SE transport during horizon C and D times ages both cut across anticlines and run parallel tion I (Figs. 15A and 17). This feeder system is (Figs. 13 and 14). For horizon C, at location C to synclines. The amplitude geometries are inter- not seen today (Fig. 3). (Fig. 14), the high-amplitude refl ections curve preted to represent channel and fan patterns very The isopach and amplitude maps described around the plunging nose of fold VI, indicating similar to those seen on the seafl oor amplitude above show that as the folds evolve and grow the fold was a signifi cant feature at the seafl oor. map (Fig. 3). The horizon A–seafl oor isochron the initial ridge and synclinal piggyback basin The situation for horizon D is different—high- map also shows further development toward a morphology becomes modifi ed, and anticlinal amplitude refl ections cover the region of the strongly rectilinear contour pattern where trans- ridges become breached. In assessing the impact plunging nose, and SW of location D (Fig. 13) verse (i.e., NW-SE trends) features are almost as of folds on sedimentation patterns, it is impor- a channel-like feature trends perpendicular to strong as the fold trends (Fig. 12A). tant to consider both the lateral propagation the anticline axis, suggesting there was no fold While many areas have evolved considerably and evolution of folds, as well as the internal topography during horizon D times. The iso- with time, area B on amplitude maps of horizons to external (seaward) propagation. For a mature chron maps for horizons D and C also support C, B, A, and the seafl oor (Figs. 3, 14, 15, 16, and fold, the changes in fold geometry through time post–horizon D propagation of anticline IV into 17) has remained an area of persistent SE- to show similar patterns to the external to inter- the area SW of location C (Figs. 12C and 12D), NW-directed sediment transport. Only for hori- nal changes in fold geometry discussed for the and establishment of an anticlinal ridge during zon D (Fig. 15) is the pattern not well developed. modern setting. The next sections examine how the interval between horizons C and B. However, the isopach maps show that for the anticline geometry changed through time, and The amplitude map of horizon C (Fig. 14) horizon D to C interval the area was a low, and how the anticline breaching channels and can- shows that a straight, NW-trending, narrow, hence a likely trap for sediments. Subsequent lat- yons developed in response to fold geometry. linear feature runs across anticline VIII west of eral propagation of anticlines I and II into area area M, unaffected by the fold, indicating the B eliminated the piggyback basin, but anticlinal 9. ORIGIN OF VARIATIONS IN FOLD fold was not present at that time. The piggyback uplift was not strong enough to be a barrier to GEOMETRY basin between anticlines VIII and VI contains sedimentation, and deposition largely kept pace small, patchy, high-amplitude areas (Fig. 14 with, or exceeded the anticline growth rate. Thus Fold growth in the most external anticlines locations O and N) that may be poorly devel- the SE to NW sediment transport pathway was rotates the seafl oor parallel to the dip of shal- oped, or partially eroded fans fed from the east kept open from horizon C to the present. low refl ections (Figs. 4 and 7). As folds become (area Q) through channels in the vicinity of J. The piggyback basin between anticlines VI more tightly folded and higher amplitude, their Conversely, area P (Fig. 14) appears to be sup- and IV changes geometry with time. For hori- geometry changes, and the seafl oor slope angle plied from the west. In the cases of fans O, N, zon C the basin is deepest adjacent to anticline can diverge from that of the shallow refl ection and P, the fans were fed by fl ows that followed IV (Fig. 5); this is also shown in the isochron (Figs. 4, 18, and 19). To try and quantify these synclinal axes, and the sediment source areas map (Fig. 12C, location Q). The deep syncline changes, a number of basic fold measurements lay outside of the study area (Fig. 15C). The is bounded by two anticlines (V and IV), and at were compared with slope angle (half wave- thickness difference between the syncline and the oblique intersection of these two anticlines length from the anticline crest to the footwall

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A B

M VI VI Q I I

VIII H IV J H IV J V V P F K IV B K IV B V I V I III III E E VII II I II D VII D I N

C 0 C 0 50 100 100 150 200 A 200 300 250 300 A 400 500 Thickness Thickness in milliseconds in milliseconds C D

M M VI VIII L VI VIII L I Q

J H IV IV V H P V K IV B V V I K IV B X X V I III F E III E VII II I E D II I K VII D N

N 0 0 C 100 C 50 200 100 300 Rapid thinning onto 150 400 backlimb of anticline 200 500 A 250 A 600 300 350 5 km Thickness Thickness in milliseconds in milliseconds

Figure 12. Isochron map for the intervals between: (A) horizons A–seaﬂ oor, (B) B–A, (C) C–B, and (D) C–D. Black areas are regions where the horizons could not be mapped due to either unconformities, the horizon being deeper than one second from the seaﬂ oor, or unmappable within degradation complexes or mud-pipe and mud-volcano provinces. I, II, III, IV, V, VI, VII, VIII, IX, and X denote anticlines. Letters A to M denote locations along important sediment pathways from the upper slope to the lower slope. Incised canyons through anticlines occur at locations A, E, F, I, and M.

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VI Q

VIII IV J H V

F IV V B K I

III

E VII II I D

C -2000 0

10000 N

10 km 22000

Figure 13. Amplitude map for horizon D. Black areas are regions where the horizons could not be mapped due to either unconformities, the horizon being deeper than one second from the seaﬂ oor, or unmappable within degradation complexes or mud-pipe and mud-volcano provinces. I, II, III, IV, V, VI, VII, VIII, IX, and X denote anticlines. Letters B to M denote locations along important sediment pathways from the upper slope to the lower slope. Incised canyons through anticlines occur at locations A, E, F, I, and M. Inset shows location of map with respect to edge map in Figure 2. Scale is relative amplitude in arbitrary units. High-amplitude areas—red; low amplitudes—black.

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2 Shear grooves

Proximal IX channels

N VI Q

VIII O IV J H V

F IV V B K I Faint shear III grooves P 1 E VII II I D

VII N

10 km

Figure 14. Amplitude map for horizon C. Black areas are regions where the horizons could not be mapped due to either unconformities, the horizon being deeper than one second from the seaﬂ oor, or unmappable within degradation complexes or mud-pipe and mud-volcano provinces. I, II, III, IV, V, VI, VII, VIII, IX, and X denote anticlines. Letters A to M denote locations along important sediment pathways from the upper slope to the lower slope. Incised canyons through anticlines occur at locations A, E, F, I, and M. Fan geometries are labeled N, O, and P. A high-amplitude area associated with axial transport along a syncline is labeled Q. Inset 1—Detailed amplitude map of fan P; inset 2—detailed amplitude map of fan O showing radial shear grooves and channels. Inset 3 shows location of map with respect to edge map in Figure 2. Scale is relative amplitude in arbitrary units. High-amplitude areas—red; low amplitudes—black.

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AB Mini-basin deep, with plunge direction IX IX Anticline with VIII VIII plunge direction

VII I Sediment transport MM M L direction L N VI

I L I VI K K I VII J K J VII J

H E F F H V F V H E 10 km V IV V E IV IV D IV C D D III C III

II B U2 II I B I U1 B I A I A A

C D IX IX VIII VIII M M L

VI VI VII K I K I J VII J

F V F V H H V E IV V E IV IV IV D III D III C C

II II B I B I I I A A

Figure 15. Summary maps of amplitudes and fold evolution for the following horizons: (A) horizon A, (B) horizon B, (C) horizon C, and (D) horizon D, with interpretation of sediment transport paths, to show how the geometry and location of active structures and sedimentation patterns has changed with time. I, II, III, IV, V, VI, VII, VIII, IX, and X denote anticlines. Letters A to M denote locations along important sediment pathways from the upper slope to the lower slope. Incised canyons through anticlines occur at locations A, E, F, I, and M. High-amplitude areas—red; low amplitudes—black.

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VIII

IV J H V

F IV B K I

III E II VII D I -3474 0

10000

C 20000

30000 N

10 km

Figure 16. Amplitude map for horizon B. Black areas are regions where the horizons could not be mapped due to either unconformities, the horizon being deeper than one second from the seaﬂ oor, or unmappable within degradation complexes or mud-pipe and mud-volcano provinces. Inset shows location of map with respect to edge map in Figure 2. I, II, III, IV, V, VI, VII, VIII, IX, and X denote anticlines. Letters B to M denote locations along important sediment pathways from the upper slope to the lower slope. Incised canyons through anticlines occur at locations A, E, F, I, and M. Scale is relative amplitude in arbitrary units. High-amplitude areas—red; low amplitudes—black.

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L VI

VIII IV J H L V F B K IV V I

III E II VII D I U2

C U1

A N

10 km

Figure 17. Amplitude map for horizon A. Black areas are regions where the horizons could not be mapped due to either unconformities, the horizon being deeper than one second from the seaﬂ oor, or unmappable within degradation complexes or mud-pipe and mud-volcano provinces. I, II, III, IV, V, VI, VII, VIII, IX, and X denote anticlines. Letters A to M denote locations along important sediment pathways from the upper slope to the lower slope. Incised canyons through anticlines occur at locations A, E, F, I, and M. U1 and U2 denote large transport-path unconformities. Inset shows location of map with respect to edge map in Figure 2. Scale is relative amplitude in arbitrary units. High-amplitude areas—red; low amplitudes—black.

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20 20

18 AB18

16 16

14 14

12 12

10 10

Slope dip (°) 8

6 6

4 4

2 2

0 0 0 500 1000 1500 2000 0 1000 2000 3000 4000 5000 6000 7000 Amplitude (m) Fold half wavelength (m)

Figure 18. Graphs showing how slope dips associated with the crests and forelimbs of anticlines vary with (A) fold half wavelength (anticline crest–forelimb–syncline trough) and (B) fold amplitude (from forelimb syncline trough to fold crest for horizon C). The graphs show some correlation between slope dips and fold wavelength and amplitude. However, the data are very scattered.

syncline, amplitude, interlimb angle, Figs. 18 above the anticline, with points coded to traveltime to depth. The graph shows that in and 19). Of these different measures the clearest represent the different fold styles described general the transition to more intense faulting association of changes in anticline slope angle above. Slope dip was calculated from depth- and mass movement processes is accompanied was with the interlimb angle (Fig. 19). converted seafl oor profi les. Depth conversion by an increase in interlimb angle and in surface Figure 19 shows a graph of fold interlimb used 1500 m/s as the velocity of seismic waves slope dip. For the category of simple folds, the angle plotted against average surface slope dip traveling through water to convert two-way interlimb angle values are quite widely scattered between 180° and 150°. This is because included in the category are the small upper slope folds in 90 the eastern corner of the area (Fig. 4A) that have relatively small interlimb angles (165°–150°) 100 but appear to have been almost continuously buried by sediment. Also in the category are 110 the youngest folds in the province, with high interlimb angles (180°–170°). Hence crest uplift 120 rate relative to sedimentation rate, as well as interlimb angle, is an important (but diffi cult to quantify) factor. The deep normal faults tend to 130 develop in folds with interlimb angles between 170° and 140°, and surface slopes of 4°–12°. 140 Large rotational slump scars plus normal fault-

Interlimb angle (°) ing tend to occur where interlimb angles are 150 smaller than 165° and slope angles are greater

160

170 Figure 19. Fold interlimb angle versus slope dip of seaﬂ oor above anticlines; symbols 180 indicate the type of associated fold geometry. 0 5 10 15 20 Note: variation within the simple fold group is affected by fold amplitude and wave- Surface slope dip (°) length, not just interlimb angle, because Simple fold buried simple fold where sedimentation has kept pace with the

Simple fold with normal faults growth of relatively short-wavelength, low- amplitude folds, the fold crests of relatively Simple fold with normal faults/slope affected by mass movements tight folds can be buried and consequently Fold with degredation complex and internal are not exposed to mass-movement and angular unconformities mass-wasting processes.

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than 6°. For folds affected by degradation complexes, surface slope dip does not increase beyond ~12°–13°, while the interlimb angle increases. Once a critical slope angle is reached, NW SE mass movement processes remove the crest of A the anticline as it continues to tighten.

The data discussed above show that grav- 2.0 ity exerts a strong modifying inﬂ uence on fold geometry in deep-water environments. Slope 2.25 dip is very important both as a pathway for sedi-

TWT (s.) 2.50 ment transport and dispersal and for the initiation of sediment movement due to slope insta- 2.75 1 km bility (also see McGilvery and Cook, 2003). The shallow fold geometry strongly refl ects the NW SE infl uence of gravity on fold shape and internal B structure. The typical changes in fold geometry described above vary according to either: 2.0 Sea floor (1) the maturity of the fold (i.e., passing from the youngest, deepest water folds to the older 2.25 folds located toward the continental shelf), or TWT (s.) 2.50 Minor growth (2) lateral changes in fold style along an anticli- to no growth synkinematic section nal ridge. In both cases it is the vertical rate of 2.75 1 km Pre-kinematic section? fold growth (i.e., the effects of fold geometry and Forelimb unconformity Transport path unconformity (TPUC) rate of shortening), coupled with sedimentation Late synkinematic sediments preserved around fold crest and backlimb, missing from the forelimb Backlimb unconformity rate, that dictates whether a particular segment of an anticline will act as a barrier to sediment, C or be breached. Sedimentation rate is important because if sedimentation can keep up with fold growth, the fold will never develop a topographic ridge, but will be continuously covered with sed- Sea floor iment. Hence in Figure 4A both the most external fold (IX), and the small, buried internal folds (I, II, and III) have a simple fold shape. A planar slope profi le is associated with the arbitrary line that follows a well-developed sediment transport pathway down the slope (Fig. 4C), showing that D locally a combination of sedimentation infi lling lows, and erosion smoothing of highs can elimi- or nate structural topography. Sea flo

10. LOCAL UNCONFORMITY DEVELOPMENT AROUND FOLDS

As folds grow, the initial pattern of parallel refl ectors, or refl ectors that thin toward the E fold crest, become modifi ed. The development

of local erosion surfaces or unconformities is Sea floor widespread, and their location and geometry varies greatly around folds. The early stages of unconformity development occur where slumps and mass movement affect the forelimb (Figs. 7, 8, and 9; forelimb unconformities) and give rise to truncation of tilted fault blocks in the forelimb of the anticline. Later deposition parallel to or onlapping the slope may then be incorporated Figure 20. Example of episodic fold growth (fold IV) based on stratal geometries. (A) Unin- into the fault blocks and preserved. Alterna- terpreted seismic line. (B) Interpreted seismic line showing evidence for episodic growth tively, renewed mass movement may repeatedly of the anticline. (C–E) Earlier stages of fold growth illustrated in line drawings. (C) Final thin the forelimb and erode deeper into the fore- stage of fold growth prior to inﬁ ll of the topography; (D) early fold topography smoothed limb, crest, and even the upper backlimb of the out, period of deposition and no folding; (E) early stage folding. See Figure 3 for location. anticline (Fig. 9C). At the foot of the forelimb, TWT—Two-way time.

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sediments in the adjacent piggyback basin thin Fold growth is not necessarily smooth and processes discussed in this paper. The giant Bru- onto or onlap the forelimb. Forelimbs may dis- progressive; there is commonly evidence for nei slide is related to sediment input from the play a steeply dipping unconformity surface in several cycles of growth and unconformity Baram delta and forms the uppermost deposi- the forelimb that is onlapped by sediments in the development. Particularly the forelimb and tional unit in the western part of the area (Fig. 2); adjacent syncline. The unconformity in places crests can show outward and upward propaga- it has been described in Gee et al. (2007), and bounds a degradation complex and passes into a tion of unconformities (Figs. 4 and 21). Where so is just briefl y mentioned here. This landslide high- or low-angle unconformity at the fold crest low-angle unconformities have developed, the is over 150 km long, 50 km wide, and tens of (Fig. 9). As the forelimb grows, the synkinematic overlying sediment is gently inclined above a meters thick. It represents a catastrophic slope strata evolve from progressive thinning onto the fold at depth. In cases where the fold starts to failure event that resulted in a widespread ero- crest to nondeposition on the forelimb (Figs. 7 grow again, folding of the sediment above the sion surface at the base of the landslide that and 9). The nondeposition stage results in the unconformity produces a broader wavelength truncates the crests of several anticlines. Hence late synkinematic section resembling a large fold with much gentler limb dips than the core it is important to note that although anticlines extensional tilted fault block, where the rotated of the fold (Fig. 4C, fold II; Figs. 20 and 21). generally control mass wasting location and dis- fold backlimb is equivalent to the footwall block, West of the study area is a different type of tribution, large mass wasting events can elimi- and the forelimb is similar to the degraded nor- erosion surface and unconformity related to nate the anticline topography and override the mal fault scarps described from the North Sea mass movement on a much larger scale than the structural control. (e.g., Stewart and Reeds, 2003; Fig. 20). On more steeply rotated back limbs, sediments may also onlap a local (backlimb) unconformity. NW SE Passing along strike from the crest of an anticline to a lower saddle area, steeply dipping A forelimb unconformities tend to pass laterally into conformable section. Then in the lowest part 2.0 of the saddle, a different geometry unconformity is developed that is elongate parallel to the sediment transport direction. The transport path 2.25 unconformity initially forms at a low angle (1°– 3°; Figs. 4C and 21), and is elongate subparallel to the surface slope (Figs. 17 and 22). The differ- TWT (s.) 2.50 ence in geometry between the two unconformities refl ects the changes in processes that created them. Forelimb and backlimb unconformities 2.75 result from local mass movements that degrade 1 km the crest and steep forelimbs and/or backlimbs. Such unconformities are elongate parallel to Forelimb unconformity Transport path unconformity the anticline. In the saddle areas fold growth is slower, or has ceased, and the region has become NW SE a pathway for sediment movement transverse to the anticlinal ridges. Sediment transport causes B erosion of the saddle area; hence the transport path unconformities display dips similar to the present-day slope. In some folds the transport path unconformities cut into folds with older forelimb and backlimb unconformities resulting in the higher angle unconformities meeting the transport path unconformities (Figs. 9A and 23D). As seen in accretionary prisms and salt mini-basins, knickpoint systems can arise along deep-water channels that incise because the updip basin has fi lled to spill point (see review in Mitchell, 2006). Figure 4C shows an arbitrary seismic line following a major channel system down the NW Borneo slope; the gradient Late synkinematic sediments preserved around fold crest is remarkably constant, steepening only in one and backlimb, missing from the forelimb place where a knickpoint affects anticline VIII. Hence while transport path unconformities may Figure 21. Example of low-angle transport path unconformity affecting the low-relief sad- partially have arisen as knickpoints, as shown in dle area of a fold. (A) The main transport area with very fl at-lying unconformity. (B) Mov- Figures 4C and 21, they are more generally tracts ing east of the main transport area, the top of the fold begins to develop a little topography. of constant gradient that maintain their position For more complete topographic development still on the margins of the transport path, see by erosion keeping pace with anticline uplift. Figure 20. See Figure 3 for location. TWT—Two-way time.

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11. CONTROLS ON LOCATION OF pathway from one piggyback basin to another overlies a tightly folded anticline. In contrast, TRANSVERSE CHANNELS AND (Figs. 13, 14, location H). (3) Mud pipes may to the east, the less tightly folded part of anti- CANYONS weaken and facilitate erosion of a fold crest so cline V (with no transport path unconformity; that late-stage transverse channels develop path- Figs. 23A and 23C) is eroded and onlapped by In the data set there are a number of impor- ways across the former fold crest (Fig. 22C). For the post-folding sediment, and lies at the edge tant pathways for sediment transport transverse example, a large canyon system crosses the crest of the pathway. to anticlinal ridges (Figs. 15, locations B, C, D, of the anticline V at location F (Figs. 3 and 17). Slightly emergent fold crests, and low-relief E, H, I, J, and M). The structural causes for the (4) Regions where fold growth is slow relative portions of plunging folds have been traversed pathways include (Fig. 22): (1) fold saddles, to sediment supply can also become pathways. by gravity fl ows. Evidence for this is a series of where two en-echelon or inline anticlines have Such areas may be due to the initial stages of fold striations (shear grooves) caused by shear at the propagated toward each other and linked to growth or to shortening being distributed across base of one or more subaqueous landslides, pos- form a region of relatively low relief (Fig. 22B). an unusually large number of folds, resulting sibly enlarged by turbidite fl ows formed along a Continued fold growth failed to eliminate the in below-average uplift on the individual folds large proportion of the fold (Figs. 3 and 24). relatively low relief area of the linkage zone. involved (Figs. 2 and 15, location B). This is a structurally low region, not just a Figure 23 shows two seismic lines where a 12. ESTIMATES OF RATES OF region where erosional incision has kept pace transverse channel cuts a fold crest with mud DEFORMATION with anticline growth (Figs. 3 and 15, locations pipes (3 above). The gently inclined section E and M). (2) Overlapping folds whose axes above horizon B (Figs. 23B and 23D) lies uncon- It is diffi cult to obtain sedimentation and are oblique result in one fold meeting partway formably on the eroded anticline V, and was uplift rates in the absence of detailed dating of along the length of the other fold (Fig. 22A). For deposited after the transverse channel cut across the section. Ages can, however, be estimated by folds IV and V, this geometry sets up an early the anticline. This important sediment pathway carrying seismic horizons dated by wells from the shelf and upper slope of Brunei in San- dal (1996) into the deep-water area, and from Anticline with plunge directions indicated dates provided in Ingram et al. (2004) for an A adjacent deep-water area in Sabah. These data show the upper one second of seismic data is largely of upper Pliocene to Recent age, with Fold-parallel flow finds oblique pathway through to next basin lower Pliocene to possibly late Miocene age strata present in the cores of the most uplifted anticlines. The character of the refl ection packages indicates horizon D overlies the Lingan Fan II of Ingram et al. (2004), suggesting an age of ca. 3.7–3.5 Ma for horizon D. The thick- Flow reflects back down mini-basin at nesses of piggyback basin fi ll between horizon the narrowing/rising end of the mini-basin D and the seafl oor typically range from 550 to 900 m, yielding average sedimentation rates of B Pathway at saddle between ~0.16–0.25 mm yr−1. For a longer time inter- Pathway at plunging fold termination two joined folds val (9 m.y.), the age data provided by Ingram et al. (2004) yield similar deposition rates of 0.28 mm yr−1in a syncline and 0.13 mm yr−1on an anticline crest. Flows deflected away Flows deflected away from anticlinal from anticlinal For the most external anticline (IX, Fig. 6) crest find another crest find another the seafl oor at the fold crest lies 350 ms above pathway across pathway across the seafl oor in the adjacent piggyback basin C Eventually a pathway is forced through (262 m), while the fold amplitude for horizon C Mud Pipes/volcanoes (at sea floor) is 500 ms (450 m). The depth conversion from two-way time assumes a velocity of 1500 m/s for water and 1800 m/s for the poorly compacted uppermost part of the sedimentary section. The Boundary of unconformity 188 m difference in height from anticline crest Transport path to syncline trough between fold amplitude and Successive flows arrive at the unconformity anticlinal crest and start to the seafl oor depth refl ect both thinning of sec- wear away at the crest Fold parallel tion onto the anticline, and thinning by normal unconformity faulting. For anticline IX, fold growth appears to Backlimb (onlap) unconformity have begun just below horizon B; strata thickness to the seafl oor in the piggyback basin is ~450 m. Figure 22. Schematic diagrams of maps illustrating the ways that different fold geometries Assuming average sedimentation rates between can infl uence where gravity fl ows can traverse growing folds at the seafl oor. (A) Transport 0.16 and 0.28 mm yr−1 for the piggyback basin path between en-echelon folds. (B) Transport path across saddle region in fold. (C) Trans- fi ll, then the age of onset of folding lies between port path across fold crest. 2.8 Ma and 1.6 Ma. The absolute ages are based

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NW SE NW SE AB 2.5 2.5

3.0 3.0

3.5 TWT (s.) TWT 3.5

C Anticline IV Anticline V

Forelimb unconformity

HC 1 km

D Anticline IV Anticline V HB Transport path Forelimb unconformity unconformity Mud pipe

1 km

FLUC

Figure 23. Example of sediment transport pathway across the crest of a fold, affected by mud pipes, anticline V. (A) and (C) are uninterpreted and interpreted versions of the same line, and (B) and (D) are uninterpreted and interpreted versions of the same line. See Figure 3 for location. HA, HB, and HC—mapped seismic horizons A to C, respectively. TWT—Two-way time.

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on the ages of seismic horizons discussed at up the continental slope, growth started earlier, similar sedimentation rates to those above, aver- the start of this section. For 450 m uplift, the and uplift has been greater (Fig. 13, anticlines age uplift rates for anticlines V and VI range average uplift rate of the anticline crest above II, III, and IV). For anticline V, the fold ampli- between 0.24 and 0.42 mm yr−1. Hence there is the level of the seafl oor in adjacent synclinal tude of horizon C is ~1190 m (Fig. 9), and at the at least a hint from the uplift estimates that anti- areas is between 0.16 and 0.28 mm yr−1, similar crest, horizon C has been eroded off. Anticline clines affected by unconformities and degrada- to the sedimentation rates of the adjacent pig- VI (Fig. 5) shows a very similar situation with a tion complexes are associated with both greater gyback basin. For the anticlines located higher fold amplitude for horizon C of 1200 m. Using total uplift, and somewhat more rapid uplift rates than simple anticlines. The amount of shortening across the fold belt, based on line-length measurements of horizon C is small; 6.5 km across a region that today is 55 km long, or ~10%. Assuming a 3 Ma age for part of giant landslide horizon C based on correlation of seismic horizons to wells on the shelf and the deep-water wells in Ingram et al. (2004), the approximate shortening rate is 2 mm yr−1. This shortening rate is close to the 3–4 mm yr–1 difference in westerly motion between south (Sundaland) Borneo and NW Borneo derived from GPS data (Simons et al., 2007). The differential motion is occurring at ~45° to the orientation of the folds; hence if there is strain partitioning, then the amount of shortening in the fold-thrust belt Anticline VII should be considerably less than the westerly motion. Horizon C lies within the synkinematic section, so the shortening estimates are not the maximum amounts. The maximum percentage shortening across the belt based on line-length N measurements of deeper, pre-kinematic section from 2D seismic data is ~17%. The rates estimated above are the same order of magnitude as external folds from the subduction-related 2 km setting of Taiwan, where a study by Delcaillau A et al. (1998) estimated uplift rates for the Pak- uashan anticline of ~0.3–10 mm yr−1 over the past 500 ka, with an E-W shortening rate of B ~4–8 mm yr−1. Zagros folds have also been estimated as shortening at similar rates (3–4 mm/yr−1; Oveisi et al., 2007). From the amplitude maps, it is possible to see the distal propagation of folds evolving between the mapped horizons. Between horizons D and B the most external fold moved from fold VI to fold IX (Fig. 15), a distance of 20 km over a time period of ca. 1.2–2.0 my, based on sedimentation rates discussed above. Within this period of time, fold VIII propagated to its full length (50+ km). Indicating lateral propagation UP Ridge-parallel lineations Linear ridges perpendicular to of a fold is faster than propagation of folds in the due to faults anticline due to turbidites transport direction by at least 2.5 times. Chen et and debris flows traversing al. (2007) report lateral fold propagation up to N the ridge W 1 km 40 km/m.y. based on magnetostratigraphic timing of outcrop sections, for the Kashi anticline. Figure 24. (A) Detail of root mean square (RMS) amplitude (red—high ampli- This is a very similar rate to lateral propagation tude, gray—low amplitude), water-bottom map (Fig. 3) showing NW-SE– estimated here. trending striations crossing the crest of anticline VII, due to gravity fl ows that Figure 25 shows two stages of fold devel- have traversed and scoured the low-relief anticline crest. (B) 3D perspective of opment interpreted as fault propagation folds, the same anticline in (A); amplitude draped over time-structure map (green— based on 3D seismic data for the upper 1 km low amplitude, red—high amplitude). View to the SE showing the monoclinal discussed in this paper and 2D seismic for the geometry of the anticline at the seafl oor. deeper section in the adjacent area of Sabah

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(Ping and Hailing, 2004). The sections show tion complex and erosional unconformities are resulting in the anticline crest being spared from that near the fault tip the more external anti- associated with a thrust that was emergent or major erosional truncation (e.g., Apennines cline (VIII) dips at a lower angle (20°) than the very close to the surface. foredeep basin, Bally, 1989). These have been more internal anticline (35°). In a simple kink- referred to as fi ll-to-the-top models (see review band, fault-propagation fold model, this would 13. DISCUSSION in Castelltort et al., 2004; Fig. 26B). require that the initial footwall ramp dips were Examples of syntectonic fold growth used different in the two cases. However, regional How growing folds at the surface interact to support kink-band–style, fold-growth mod- 2D seismic data show the imbricate faults have with their environment varies signifi cantly pass- els (e.g., Suppe et al., 1992; Shaw and Suppe, a listric shape, matched by the fold backlimbs ing from continental, to shallow marine, to deep 1994) typically come from fi ll-to-the-top set- having a smoothly curving shape (James, 1984; marine settings (Fig. 26A). Continental and tings in shallow marine and continental envi- Ping and Hailing, 2004). As the fault continued shallow marine environments have two modes ronments. The pre-kinematic sedimentary to propagate, it steepened; consequently, for a of fold-environment interaction as follows: strata in these settings are typically well lith- constant strain rate, uplift associated with a fold (1) Nondeposition or erosion of the anticline ifi ed, and the erosional products of the folds would increase due to the increase in fault dip. crest due to its emergence either at the land give rise to coarse-grained clastic sediments. Hence the higher uplift rates discussed above surface or above wave base. Any sedimenta- As discussed by Nigro and Renda (2004) and for the more internal anticlines (e.g., V and VI) tion associated with the anticline onlaps the Morley (2007b) deep-water folds (Fig. 26C) compared with the external anticlines can be fl anks and is deposited in the synclinal basins are different, for three main reasons: (1) folds explained by changes in fault dip (e.g., IX and (Fig. 26A). The evolution of drainage and are protected from subaerial and wave-related VIII). The listric shape of the imbricate faults uplifted terraces can be used to infer fault devel- erosion; hence thicker, and more widely dis- and the different depths at which the blind imbri- opment and propagation in a continental setting tributed synkinematic sedimentation can occur cate fault terminates with respect to fold geom- (e.g., Delcaillau et al., 1998). (2) The anticline is across anticlines, as well as within synclinal etry supports this interpretation. The simple fold covered by sediment. If a fold develops within a piggyback basins; (2) the synkinematic sedi- geometry with normal faults developed while subsiding basin (usually a foredeep basin or a ment deposited across anticlines during growth the thrust fault was blind, the fault tip lying large thrust-sheet–top basin), then it is possible is poorly lithifi ed, predominantly fi ne grained ~0.4 km or deeper below the seafl oor. Degrada- for sedimentation to keep up with fold growth and weak, and hence liable to be very sensitive

NW SE

1 SE dip to basal detachment A 0V.E. = 1:1 5 km

NWUnconformity SE NW SE Degradation complex Sea floor

Horizon C

Mud Pipe 1.2 km heave

Top pre-kinematic section/ base synkinematic section

Overpressured fluid migration path? B C Figure 25. Relationship of shallow structure determined from 3D seismic data (Fig. 9) and the deeper fold geometry (based on regional 2D seismic lines; Ping and Hailing, 2004). The folds are modiﬁ ed fault-propagation folds with listric imbricate fault geometry. (A) Simple fold with crestal normal faults; (B) fold affected by degradation complexes and overpressured mud and ﬂ uid pipe; (C) illustration of fold morphology in (B) without the effects of erosion and mass wasting of the crest and forelimb.

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to the effects of gravitational force during fold able surface expression. Gravity-driven sedi- (Fig. 12C) that developed between horizons B growth; and (3) the main erosional processes are ment transported through syntectonic topogra- and C and then is much less active subsequently a variety of mass-movement and mass-wasting phy related to different structural styles (folds (Fig. 12B), while areas N and P along the same phenomena, including rotational slides, fast and and rifts) exploits variations in fold or normal fold trend remained active between horizons B slow creep mass movement, debris fl ows, and fault geometry to move down the regional slope and A. It is also possible for saddle areas along turbidity fl ows, and currents (e.g., McGilvery via structural lows such as fold saddles and fault anticlines to remain as relative lows because and Cook, 2003; Heinio and Davies, 2006; Gee displacement lows and the plunging termina- they represent the areas of latest fold develop- et al., 2007). Sheet-type fl ows are considerably tions of en-echelon folds or the equivalent relay ment, and once fold segments are joined, the more extensive in the deep-water environment ramps in normal faults. saddle areas subsequently grow in amplitude at than they are in shallow marine or continental The folds investigated here show mature link- similar rates to the crestal areas. However, this settings, and the effects of their erosive power age geometries after only ~17% shortening, commonly does not appear to be the case. At on the poorly lithifi ed sediments forming anti- indicating, that like normal faults, in many cases the crests of more mature folds (e.g., anticline clines (e.g., Fig. 24, Gee et al., 2007) will be linkage of folds and their underlying thrust faults III) there is evidence for continuous growth as much greater, and create different transport occurs at a relatively early stage in their strain shown by progressive rotation and thinning of paths in deep water than in continental and shal- history (e.g., Morley, 1999). Another aspect of section onto the anticline. In the saddle areas for low marine settings. normal fault growth is how displacement pat- the same age sequences, there are periods where Propagation and linkage geometries in nor- terns become reorganized during linkage: does uniform thicknesses of sediment were depos- mal faults have been extensively documented, the area of maximum displacement relocate to ited across the anticline, followed by renewed and the less well documented lateral propaga- the center of the linked fault system, or do the folding (Fig. 20). In the case of area K between tion of folds in response to linkage of underly- linkage points remain persistent regions of low folds VII and VI, early fold activity (Fig. 12C) ing thrust faults has been treated in a similar displacement during the life of a long fault (e.g., was followed by inactivity with no reactivation way (e.g., Dahlstrom, 1969; Nicol et al., 2002; Cartwright et al., 1995; Morley, 1999)? In the (Figs. 12A and 12B). Thus anticline growth in Delcaillau et al., 2006). In both normal faults examples of folds described here, some of the the saddle areas is commonly more episodic and folds, propagation and linkage patterns have changes in syncline depocenter locations sug- than in the crests, suggesting incomplete kine- been determined from synkinematic sedimenta- gest shifting patterns of thrust displacement at matic linkage of the underlying fault systems. tion patterns and geomorphology associated depth; the best example is NW of anticlines V With some caveats, the different ways fl ows with active faults and folds that have consider- and IV where area Q is a prominent depocenter exploit fold transfer zones and linkage points described here are applicable in terms of fold evolution and rate of processes to a wide vari- Mass wasting and ety of tectonic settings (see section 11). For Landslidesfluvial erosion of crest example, geomorphic criteria to help identify Land surface subaerial lateral fold propagation have been documented by Delcaillau et al. (2006, 2007) from the Siwalik ranges of the Himalaya. These workers have been able to distinguish between anticlines propagating in a single direction ver- A sus linkage of several inline folds. The Zagros Mountains show paleodrainage patterns that Land surface or sea floor are incompatible with present-day fold geometry, but which can be interpreted in terms of lateral propagation of fault and fold segments and linkage fold segments that are typically 20–40 km long (Ramsey et al., 2008). The data suggest that for Himalayan, Zagros, and deep- B water folds, shortening rates are in the order of mm/yr−1, while lateral and transport direction Mass wasting, current, and gravity flow erosion of crest fold-propagation rates are at least an order of Small normal faults magnitude faster. Water Degradation complex Landslides Sedimentary fl ows have originated from a number of sources: (1) locally from growing anticlines, (2) from high up the continental slope, sourced by headless channel systems, which are active today, and (3) from the shelf. C Shelf-sourced (more sand-rich) sediments appear as a source of fl ows primarily during Figure 26. Schematic sections illustrating how variations in the type of ero- times of sea-level lowstands and tectonic uplift sional process and protection from erosion due to subsidence and burial of the shelf (Ingram et al. 2004). The present-day of growing folds by sediment or water affect fold morphology (A) on land, time of sea-level highstand is refl ected by the (B) in a rapidly subsiding continental or shallow marine basin, and (C) in absence of channels on the shelf connecting to deep water. the deep water, the presence of headless channel

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systems along the slope and the dominance of matic strata, which is lacking in many orogenic and appear to be a deep-seated, mass-movement mud and silt, and virtual absence of sand in belts, offers the potential for using deep-water phenomenon. The normal faults die out down- numerous grab samples taken across all envi- fold belts to better quantify the growth and ward at different levels and do not detach into ronments in the deep-water area. behavior of fold-thrust belts in general. Despite rotational slides. Shallow rotational slides are The way in which deep-water fold-belt struc- the many similarities between different kinds of common features affecting the forelimbs of ture infl uences sediment pathways on a slope major fold-thrust belts, fold growth in a deep- many folds with interlimb angles between 165° described here is not just applicable to over- water setting occurs in an environment that pro- and 140°. Rotational slides tend to occur where pressured shale-prone parts of large deltas and tects synkinematic and pre-kinematic sediments the seafl oor dips above the forelimb attain angles accretionary prisms (e.g., McAdoo et al., 1997). from wave and subaerial erosional processes of 6°–11°. Where folds have tightened to inter- There are many parallels to be found between that affect folds in continental and shelfal envi- limb angles less than ~145°, the crests of folds the development of deep-water, salt-related ronments. Consequently deep-water folds are become strongly modifi ed by erosional uncon- mini-basins and the thrust-related piggyback formed in weak rocks that differ in lithology, formities and forelimb degradation complexes. basins discussed here. One common feature degree of lithifi cation, and mass movement pro- Generally the seafl oor slope above folds does of tectonically active continental slopes may cesses compared with orogenic belts. There are not attain dips greater than 13°, indicating that be the triggering of landslides at higher slope strong feedbacks between sedimentation, fold mass-movement and mass-wasting processes angles than on tectonically quiescent margins. growth, and fl uid migration in the deep-water keep pace with fold growth at such dips. McAdoo and Watts (2003) note that the Califor- environment. The key points arising from this (5) Large (hundreds of millimeters in nia, Texas and Louisiana, and New Jersey and study are as follows: diameter), mud-pipe intrusions and gas chim- Maryland continental margins undergo land- (1) In the deep-water environment, the neys are mostly located in anticline fold cores, slides on slopes less than 4°, while for the Cas- structural evolution of folds is strongly linked particularly the tightest, highest-amplitude cadia accretionary complex, offshore Oregon, with sedimentary processes and their modifying folds. The presence of hydrothermally generated most landslides occur on slopes over 15°. In the infl uence on fold morphology. hydrocarbons suggests a deep (>3 km) source anticlines described here, sediment accumula- (2) Shortening in the toe fold-thrust belt for the fl uids. The fl uids probably migrate up tion and episodic mass wasting is observed to may result from a stress fi eld that combines the zones of weakness at depth (thrust planes) until increasingly modify fold geometry as surface effects of gravity-driven and far-fi eld stress. they are able to propagate subvertical hydraulic slope increases, up to a maximum of ~13° dip of However, estimated shortening rates (~10% fractures to the surface. The presence of shal- the seafl oor on anticline forelimbs. shortening in 3 m.y., at a rate of ~2 mm yr−1) low overpressured fl uids in the crests of anti- Salt-related depocenters, deltaic deep-water seem appropriate for purely regional-driven clines is also likely to aid slumping of shallow fold belts, and accretionary prisms create dep- strains as determined by GPS measurements crestal and forelimb sediments. Migration of ocenters between actively uplifting ridges or (Simons et al., 2007). fl uids arising from compaction of sediments in mounds and force gravity-driven sediments to (3) Deep-water sedimentation rates time- synclinal depocenters occurs in clusters of small defl ect around them, or carve a path through averaged over millions of years for the area (<100-m-diameter) pipes in the backlimbs of them, commonly exploiting structural lows (e.g., range up to ~0.28 mm yr−1. Such rates are suf- anticlines. The pipes rise from source depths of McAdoo et al., 1997; Sinclair and Tomasso, fi ciently slow that for most growing anticlines, ~500–700 m. 2002). In both deformation styles, the mini- the uplift of the fold crest above the seafl oor (6) Folds exert a strong infl uence on sedi- basins farthest upslope fi ll fi rst; then sediments in the areas of the synclines exceeds the sediment pathways. During the early synkinematic spill over in basins farther downslope. In a sepa- mentation rate by ~0.16–0.42 mm yr−1. Hence stages of fold development, most sediment rate paper focusing on the evolution of the mini- topography associated with growing anticlines transport is along synclinal piggyback basins basin between anticlines IV and VIII described exerts a strong infl uence on sediment pathways parallel to the fold trends. Consequently early here, Morley and Leong (2008) discuss basin along the continental slope, particularly on sedi- sediment pathways from one piggyback basin to development in terms of “fi ll and spill” cycles ments transported in systems linked to headless another lie along low-angle entry points in par- developed for salt-related basins of the Gulf slope channels. Conversely much larger sub- ticular at stepovers between en-echelon folds. of Mexico (e.g., Prather et al., 1998; Sinclair aqueous landslides, such as the giant slide in the As folds grow, pathways transverse to the folds and Tomasso, 2002; Booth et al., 2003). While western part of the area, are little infl uenced by begin to be exploited. Drainage transverse to the there are potentially signifi cant differences in structural topography, and can actually decapi- fold direction develops at structurally low areas, strain rate, range of structural styles, sequence tate the crests of anticlines. such as: (a) saddles in a fold trend, where two of deformation, ridge geometry, mini-basin area (4) As anticlines tighten and increase in folds have joined along-strike, (b) regions where and shape, between the main deep-water defor- amplitude (either along-strike, or with time as two oblique fold trends meet, (c) areas where mation styles, which may lead to some signifi - folds grow), folds show the following tenden- fold growth has been slow, perhaps due to short- cant differences in structure-sediment interac- cies: (a) at high interlimb angles (180°–165°) ening being distributed across more folds than is tion, in general, the similarities are compelling. the fold shape is simple, and the seafl oor lies typically the case. However, transverse channels subparallel to the upper internal refl ections of can also develop at structural highs weakened 14. CONCLUSIONS the fold. The excess seafl oor dip overlying the by mud pipes where rapid erosion can occur. forelimb ranges up to 4°; (b) interlimb angles (7) An overall internal to external young- The folds described in this study display between 170° and 140° are associated with ing of fold initiation is discerned for the deep- dimensions, growth rates, propagation and link- a range of simple fold geometry that is either water offshore area; however, across a 40- to age geometries that are very similar to large- weakly or strongly modifi ed by swarms of 50-km-wide belt (in the transport direction), it scale folds seen in orogenic belts. Consequently normal faults that affect the fold crest. Gener- is apparent that fi ve to seven folds can be active the ability to obtain high-quality, 3D seismic ally as interlimb angle decreases, fault density at the same time, and that could involve any images of deep-water folds and their synkine- increases. The normal faults are gravity driven, fold in the belt (Fig. 15). For the duration of the

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Jackson, M.P.A., Hudec, M.R., Jennette, D.C., and Kilby, also arise where the rate of fold growth changes Demyttenacre, R., Tromp, J.P., Ibrahim, A., Allman-Ward, R.E., 2008, Evolution of the Cretaceous Astrid thrust P., and Meckel, T., 2000, Brunei deep water exploration belt in the ultradeep-water Lower Congo Basin: Amer- from one part of a fold trend to another with from seafl oor images and shallow seismic analogues to ican Association of Petroleum Geologists Bulletin, time. This is particularly evident for folds V and depositional models in a slope turbidite setting: Gulf v. 92, p. 487–511. Coast Section of the Society of Economic Paleontolo- James, D.M.D., 1984, The geology and hydrocarbon IV, and the way the location of the thickest sedi- gists and Sedimentologists Foundation, 20th Bob F. resources of Negara Brunei Darussalam: Berhad, Bru- mentary section in the adjacent piggyback basin Perkins Annual Research Conference, p. 304–317. nei Darussalam, Special Publication, Muzium Brunei changes with time (Figs. 12B and 12C). Evans, M.J., and Elliott, T., 1999, Evolution of a thrust- and Brunei Shell Petroleum Company, 164 p. sheet-top basin: The Tertiary Barreme basin, Alpes- Levell, B.K., 1987, The nature and signifi cance of regional de-Haute-Provence, France: Geological Society of unconformities in the hydrocarbon-bearing Neogene ACKNOWLEDGMENTS America Bulletin, v. 111, p. 1617–1643, doi: 10.1130/ sequences offshore West Sabah: Geological Society of 0016-7606(1999)111<1617:EOATST>2.3.CO;2. Malaysia Bulletin, v. 21, p. 55–90. 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nas, Kuala Lumpur, 665 p. the Añisclo anticline, southern Pyrenees: Journal of MANUSCRIPT RECEIVED 27 APRIL 2007 Ping Yan, and Hailing Liu, 2004, Tectonic-stratigraphic divi- Structural Geology, v. 28, p. 695–712, doi: 10.1016/j REVISED MANUSCRIPT RECEIVED 20 NOVEMBER 2008 sion and blind fold structures in Nansha Waters, South .jsg.2006.01.009. MANUSCRIPT ACCEPTED 28 NOVEMBER 2008

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