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 propaga- tion 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. For permission to copy, contact [email protected] 59 © 2009 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/2/59/3337824/i1553-040X-5-2-59.pdf by guest on 28 September 2021 Morley 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.