Geometry, kinematics, and AUTHORS Nathan P. Benesh Department of Earth and displacement characteristics of Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138; present tear-fault systems: An example address: ExxonMobil Upstream Research Compa- ny, Houston, Texas; [email protected] from the deep-water Niger Delta Nathan P. Benesh is a research geologist in the Struc- ture and Geomechanics Group at the ExxonMobil Upstream Research Company. He previously re- Nathan P. Benesh, Andreas Plesch, and John H. Shaw ceived his Ph.D. in earth and planetary sciences at Harvard University. His research interests focus on geomechanical modeling and the quantitative evaluation of structures at the trap to regional scale. ABSTRACT Andreas Plesch Department of Earth and We use three-dimensional seismic reflection data and new map- Planetary Sciences, Harvard University, 20 Oxford based structural restoration methods to define the displacement Street, Cambridge, Massachusetts; history and characteristics of a series of tear faults in the deep- [email protected] water Niger Delta. Deformation in the deep-water Niger Delta Andreas Plesch is a senior research associate in the is focused mostly within two fold-and-thrust belts that accom- Structural Geology and Earth Resources Group in modate downdip shortening produced by updip extension on the Earth and Planetary Science Department, Harvard University. He received his Ph.D. from the continental shelf. This shortening is accommodated by a Free University, Berlin, Germany. His research in- series of thrust sheets that are locally cut by strike-slip faults. terests revolve around three-dimensional model- Through seismic mapping and interpretation, we resolve these ing and the analysis of structures on the reservoir strike-slip faults to be tear faults that share a common detach- to mountain belt scale with a focus on quantitative aspects. ment level with the thrust faults. Acting in conjunction, these structures have accommodated a north–south gradient in John H. Shaw Department of Earth and westward-directed shortening. We apply a map-based resto- Planetary Sciences, Harvard University, 20 Oxford ration technique implemented in Gocad to restore an upper Street, Cambridge, Massachusetts; [email protected] stratigraphic horizon of the late Oligocene and use this analysis John H. Shaw is the Harry C. Dudley professor of to calculate slip profiles along the strike-slip faults. The slip structural and economic geology and chair of the magnitudes and directions change abruptly along the lengths Department of Earth and Planetary Sciences, of the tear faults as they interact with numerous thrust sheets. Harvard University. Prior to joining the Harvard The discontinuous nature of these slip profiles reflects the man- faculty, Shaw worked as an exploration and pro- ner in which they have accommodated differential movement duction geologist. His research interests include the structural characterization of complex traps between the footwall and hanging-wall blocks of the thrust and reservoirs in both conventional and uncon- sheets. In cases for which the relationship between a strike- ventional petroleum systems. slip fault and multiple thrust faults is unclear, the recognition of this type of slip profile may distinguish thin-skinned tear faults from more conventional deep-seated, throughgoing ACKNOWLEDGEMENTS strike-slip faults. We thank CGGVeritas for providing the seismic data used in this study and Landmark Graphics Corporation for donating the software through its University Grant Program. We also thank Exxon- Mobil for its support of this work and Chevron Corporation for providing additional data sets. The AAPG Editor thanks the following reviewers for Copyright ©2014. The American Association of Petroleum Geologists. All rights reserved. their work on this paper: Christopher F. Elders, Manuscript received January 25, 2011; provisional acceptance March 30, 2011; revised manuscript Stephen J. Naruk, and Sandro Serra. received May 5, 2013; final acceptance June 25, 2013. DOI:10.1306/06251311013 AAPG Bulletin, v. 98, no. 3 (March 2014), pp. 465–482 465 INTRODUCTION deltaic deposits on the continental shelf (Wu and Bally, 2000). This updip extension is fed at depth The Niger Delta, situated in the Gulf of Guinea at onto several detachment surfaces within thick over- the southern end of the Cretaceous Benue trough pressured shales of the early to middle Paleogene rift basin, represents one of the largest modern that underlie the postrift deltaic deposits (Bilotti deltas and most productive regions for petroleum et al., 2005; Corredor et al., 2005). Shortening in exploration in the world (Doust and Omatsola, the deep water began in the late Miocene to the 1990; Figure 1). The delta serves as a prime ex- early Pliocene to accommodate the slip that ex- ample of gravity-driven tectonic deformation, a tends downdip along these detachments. The characteristic of many deep-water passive margins thrust faults produced by this shortening occur in (Evamy et al., 1978; Doust and Omatsola, 1990). two distinct provinces, termed the “inner fold-and- The delta began to form as sediments that shed thrust belt” and the “outer fold-and-thrust belt” from the Niger River gradually filled the Benue (Connors et al., 1998; Corredor et al., 2005). trough during the opening of the equatorial At- With the advent of petroleum exploration in the lantic, and, by the late Eocene, the delta had be- deep-water Niger Delta in the 1990s, high-quality gun to prograde on top of the continental margin. two-dimensional (2-D) and three-dimensional (3-D) Two distinct fold-and-thrust belts within the delta seismic reflection data have been acquired, which accommodate the shortening and downdip de- have allowed for the investigation of the nature of formation produced by gravity-driven extension these gravity-driven contractional structures. These on the continental shelf. This gravity-driven ex- studies have mainly focused on the analysis of the tension is caused by rapid sediment deposition deep-water thrust faults and fault-related folds, in- that leads to differential loading and the subse- cluding their structural geometries and kinematics quent formation of large normal faults within the (e.g., Connors et al., 1998; Shaw et al., 2004; Suppe Figure 1. A generalized map of the main structural provinces of the Niger Delta and a schematic diagram of the stratigraphy in the area of interest. Also shown are the locations of the three-dimensional (3-D) seismic reflection data volume and two-dimensional (2-D) seismic survey used in this study. Qua. = Quaternary; Plio. = Pliocene; Oligo. = Oligocene; Paleo. = Paleocene; Cret. = Cretaceous. 466 Geometry, Kinematics, and Displacement Characteristics of a Tear-Fault System et al., 2004; Corredor et al., 2005), and the effects of thrust-fault systems using 3-D seismic reflection high basal fluid pressures (e.g., Bilotti et al., 2005) data that image such structures in the outer fold- and multiple detachment surfaces (e.g., Corredor and-thrust belt of the Niger Delta. The system we et al., 2005; Briggs et al., 2006) on their structural analyze contains multiple strike-slip faults that styles. It is generally accepted that these thrust- work in tandem with the thrust structures to parti- related structures provide the dominant means by tion strain and accommodate a gradient in shorten- which shortening is accommodated in the com- ing across the fold-and-thrust belt. In addition to pressional toe of the delta; however, in this study, analyzing the geometric and genetic relationships we show that transport-parallel strike-slip faults among the faults, we quantify shortening across the are also an important factor. We use both high- region, and, by means of a map-view surface resto- resolution 2-D and 3-D seismic data (Figure 1) to ration, we determine diagnostic slip profiles for the analyze a system of tear faults in the outer fold- strike-slip tear faults. and-thrust belt that partition distal contractional deformation. The term “tear fault” has long been applied to GEOLOGIC SETTING strike-slip faults that abruptly terminate thrust sheets alongstrike (e.g., Twiss and Moores, 1992). The focus of this study is a series of tear faults that The Jacksboro and Russell Fork faults, which bracket occur near the northern termination of the outer thePineMountainthrustsheetinthesouthernAp- fold-and-thrust belt of the Niger Delta. The outer palachian Valley and Ridge Province in the eastern fold-and-thrust belt represents the more distal of United States, provide a well-known classic exam- the two fold-and-thrust belts that accommodate ple of a tear-fault system (Mitra, 1988). Beyond outboard shortening driven by gravitational col- the Appalachians, thin-skinned tear faults that only lapse and extension of the deltaic sequence on the involve the shallow sedimentary section have also continental shelf. For most of the outer fold-and- been noted in the Canadian Rocky Mountains thrust belt, the primary detachment lies within the (Benvenuto and Price, 1979), the Western Foothills Akata Formation (Corredor et al., 2005), a time- of Taiwan (Mouthereau et al., 1999), the Santa transgressive, thick marine shale that generally sits Barbara Channel (Shaw and Suppe, 1994), and the atop an Upper Cretaceous sedimentary sequence, Maracaibo Basin in Venezuela (Escalona and Mann, which itself overlies an Early Cretaceous oceanic 2006), among other localities. These tear faults