Structure of the Sinu Fold Belt between the Magdalena Fan and the Uramita Fault (offshore northwest Colombia)

Abstract

The Sinu Fold Belt (SFB) represents the western termination of the South Caribbean Deformed Belt. Deformation since Late Miocene times has resulted in an accretionary wedge. A detailed structural map of the SFB has been constructed by means of geological interpretation of seismic data. Different styles of deformation have been identified in the SFB, and changes in several structural parameters have been analysed both, across and along-strike. Keywords: Sinu Fold Belt; Caribbean; Accretionary prism; Tear fault; Shale diapirism Introduction The westernmost part of the South Caribbean Deformed Belt (SCDB) is known as the Sinu-San Jacinto accretionary prism (S-SJAP). The Sinu Fold Belt (SFB) is the westernmost and external part of the S-SJAP and is mostly located offshore Colombia, south of the Magdalena Fan and north of the the Uramita Fault at the Golfo of Uraba. A sedimentary sequence formed by Oligocene to Miocene alternation of siltstones and sandstones, is deformed since Late Miocene time giving place to a series of W to NW vergent thrusts and related folds. Numerous studies present an interpretation of the SFB structure, including cortical or regional cross-sections (e.g., Toto and Kellogg, 1992; Flinch et al., 2003; Bernal-Olaya et al., 2015a; Flinch and Castillo, 2015), although some address more detailed aspects (e.g., Duque-Caro, 1984; Ruiz et al., 2000; Alfaro and Holz, 2014; Bernal-Olaya et al., 2015b). However, despite the large number of seismic profiles, no detailed structural map that includes the trace of the faults, folds and other structural elements is available, except for the Vinnels et al. (2010) map, which reflects the trajectory of the main anticlines in a portion of the SFB. Thus, thanks to the high quality seismic and bathymetric data, a structural map has been constructed, which has allowed us to analyse the structural variations throughout the SFB. Geological Setting The Caribbean Plate is an independent portion of lithosphere originated in the Pacific region whose actual configuration results from a complex story of collision, subduction, accretion and transpression with the surrounding plates (Mann, 1999; Pindell and Kennan, 2009; Escalona and Mann, 2011). The SCDB is an accretionary prism that runs along the convergent margin between the Caribbean and South American plates. Relative movement of the Caribbean Plate since Late Miocene towards ESE has led to different styles of deformation depending on the configuration of the South American coast. In the northwestern Colombian coast movement is manly orthogonal, which produces a compressional setting where the SFB was formed. The most relevant structures that bound the SFB are: (1) SCDB trench to the north and northwest; (2) Uramita Fault to the west; (3) Sinu Fault zone (SFZ) to the east, that separates Sinu and San Jacinto Fold Belts, and (4) Magdalena Fan to the northeast, where high sediment supply since Middle Miocene has broken the continuity of SCDB. Data and Methodology This study involved geological interpretation of a 2D seismic grid of 67 lines. These lines cover an area of 17000 km2 from the SFZ to the abyssal plain. All these lines are time-migrated and five, long lines, are also depth-converted. These five lines are span lines, acquired by GX Ion and proprietary of the ANH of Colombia. After interpretation in section, the tip line of each thrust fault was projected vertically on a bathymetric map in order to construct a structural map (figure 1). Two types of bathymetric data were used: a satellite map from GIS map services (1 km Global Bathymetry) and a high-resolution map taken from Vinnels et al. (2010). Structure The SFB is an active accretionary prism whose main structures follow a general NE-SW trend. In map view, it is a slightly asymmetric curved belt convex to the NW, with a minor recess and salient in the southern area. It is more than 200 km long, and 130 km and 30 km wide in its widest and narrowest parts respectively. Different structural styles can be distinguished in SCDB: Longitudinal structures are mainly thrusts and related folds. These open anticlines and synclines deform the sea bottom from the middle continental slope to the deformation front, while they are buried by a thick sequence of syn- and post-tectonic sediments from middle slope to the SFZ. In map view, thrust and associated anticline traces are slightly curved, passing from N-S direction in the south to NE-SW direction in the north. Thrusts are mainly westward directed and form an imbricate thrust system. They emanate from a detachment that dips gently to the SE, possibly located within Oligocene over-pressured shales. East of the shelf-slope break, some thrusts emanate from shallowest levels, suggesting a shallower detachment. Spacing between thrusts changes both across and along-strike. In different cross sections perpendicular to the main structures, the average spacing between thrusts exceeds, in general, 3 km but does not reach 4 km. The spacing increases progressively from SE to NW reaching maximum values higher than 4 km in the outermost zone. The spacing values are relatively regular in the central zone of the SFB, while in the NE and SW edges of the belt they are much more variable. In the continental shelf, mud walls following a general N015E direction are separated by wide synclines. Shale diapirism is responsible for folding of thrusts-surfaces at depth and the generation of normal extensional faults with NE-SW strikes in the continental shelf. Several features, perpendicular to main structures, interrupt the continuity of highs on the bathymetric map. These features have been also identified in seismic lines and interpreted as tear faults, that strike normal to the thrust front. These structures act as lateral boundaries that segment the thrust sheets as they propagate to the foreland. The width of the blocks in between two tear faults ranges between 10 and 50 km. Number, spacing and geometry of thrust and folds are different on both sides of each tear fault; however, structures keep the same direction. In the northern part of the SFB, two faults have been identified oblique to the main folds and thrusts. The fault located more to the south could correspond to the Rosario Fault (Vernette et al., 1992). This fault is more than 73 km long, its average strike is N076E and exhibits a steep dip towards the north-northwest. The fault located to the north possibly corresponds to the Canoas Fault (Vernette et al., 1992; Ruiz et al., 2000). It is a fault of more than 41 km in length, it has an approximately an E-W strike and a steep dip to the north. Both faults display an apparently right-lateral motion on the map. These faults are developed in a portion of the SFB where a noticeable change in the strike of the main longitudinal structures, i.e., thrusts and related folds, takes place from NNE-SSW south of the faults to NE-SW to the north. The deformation front is characterized by a change in vergence of the frontal thrusts along-strike. In the southern area of the SFB, the deformation front is dominated by seaward vergence of thrust faults, in the central area dominates landward vergence of thrust faults, and in the north area it is characterized by double-vergent structures. The topographic slope angles, estimated from sections perpendicular to the slope, vary from 3.4∞ to 4.7∞ in the outer wedge, and from 2.2∞ to 4.2∞ for the transitional zone (division used by Bernal et al., 2015a) increasing, in both cases, from south to north. The dip of the basal detachment is approximately constant throughout the SFB (from 3.5∞ to 3.8∞) except for the north area where it dips 2.5∞. Just below the deformation front, its depth ranges from 6.0 km to 8.9 km, being the shallowest values in the central area. The taper angle varies along-strike from 3.6∞ to 7.9∞, decreasing progressively from south to north. Summary and Conclusions The general direction of the SFB, between the Magdalena fan and the Uramita Fault, is NNE-SSW, although it shows a slightly curved shape. Changes in the structural style and in some structural parameters occur throughout this area. The normal faults and large shale diapirs interpreted in the inner areas, within the continental shelf, are absent in the outer zones. Ductile deformation related to shale domes is attributed to overpressure. Shale diapirism was induced by a high sediment supply coming from the continent during the Andean Orogeny combined with compressional tectonic activity within the wedge (Duque-Caro, 1979; Vernette et al., 1992, Ruiz et al., 2000). In the outer areas, between the continental shelf and the deformation front, the structure mainly consists of an imbricated thrust system and related folds, and some perpendicular and oblique faults. Two large strike-slip faults occur in the northern portion of the SFB. These structures accommodate the transition from contractional deformation, due to orthogonal convergence, south of the Magdalena fan, and the transpressional regime to the east and north out of the studied area (Ruiz et al., 2000). In the central-north part of the SFB, where the belt width is maximum, parameters such as taper angle, number of thrusts and sediment thickness are maximum with respect to southern and northern areas. The effect of tear faults in the south and central area, and strike-slip faulting in the north could contribute to form a stepped deformation front. Orientation and movement of thrust related folds, tears and strike-slip can only occur in a scenario where transport vector direction of the Caribbean plate ranges from N090E to N120E. This range agrees with the vector direction obtained with the bow and arrow rule (113.) and direction given in other works derived global Global Positioning System data (e.g., Alfaro and Holz, 2014; Egbue and Kellogg, 2010).

AAPG Datapages/Search and Discovery Article #90330©2018 AAPG Hedberg: Geology of Middle America – the Gulf of Mexico, Yucatan, Caribbean, Grenada and Tobago Basins and Their Margins “Integration of the geology of the area from the southern onshore of North America to the northern onshore of South America”, Siguenza, Guadalajara, Spain, July 2-5, 2018