WHAT IS THE EVIDENCE FOR SUBDUCTION IN

XI Simposio Bolivariano: PETROLEUM EXPLORATION IN SUBANDEAN BASINS “Knowledge Integration - The key to success” (Cartagena de Indias, Colombia, August 29th to September 1st ,2012)

WHAT IS THE EVIDENCE FOR SUBDUCTION IN THE CARIBBEAN MARGIN OF COLOMBIA? ROSSELLO, EDUARDO A., CONICET – FCEN - Universidad de Buenos Aires. 1428 Buenos Aires, Argentina; and COSSEY, STEPHEN P.J., Cossey & Associates Inc, P.O. Box 1510, Durango, CO 81302, USA.

Abstract In recent years, there has been a growing interest in the origin and evolution of the Colombian Caribbean Margin (CMC) by both academia and the petroleum industry. Conflicting hypotheses have emerged of the regional tectonic context and nature of the boundary of the Caribbean and South American plates. Many authors describe the subduction of the Caribbean plate under the South American plate with sutures and other related tectonic phenomena, such as the subsequent development of an accretionary prism. However, there is no clear surface morphological evidence such as a trench or onshore mountains to support this hypothesis. Nor is there any subsurface evidence such as a magmatic arc or dipping seismicity that is typically seen in other known subduction margins (e.g. Chile-Peru, Indonesia, Aleutians, etc). It is proposed that the CMC is a result of the dynamic balance of an early extensional margin related to a Mesozoic to Cenozoic passive margin style where the continental platform progrades onto the oceanic plate and a late transpressional inversion is associated with oblique convergence. The controversy over the tectonic setting of the CMC is extremely important because it impacts the petroleum potential of the region. It could be resolved by a incorporating tectono-sedimentary and 4D deformational analyses with recent surface and subsurface data. KEYWORDS: Caribbean margin, tectonics, tectonosedimentation, Colombia.

INTRODUCTION In recent years, there has been a growing interest in the origin and evolution of the Colombian Caribbean Margin (CMC) by both academia and the petroleum industry. Conflicting hypotheses have emerged of the regional tectonic context and nature of the boundary of the Caribbean and South American plates. The CMC develops between the Gulf of Urabá and the Guajira Peninsula -at the margins of the oceanic Caribbean and continental South American plates (Fig. 1). Because of this location, the tectonosedimentary evolution is very controversial (Duque Caro, 1984; Flinch, 2003; Kerr & Tarney, 2005; Rossello, 2006; Cobbold et al., 2007; Ceron et al., 2007; Higgs, 2009; Moreno et al., 2009; Pindell et al., 2010; Vinnels et al., 2010; James, 2010, etc.).


Figure 1. Schematic distribution of the Neogene stress field on the different segments of the CMC.

In this paper, the evidence (or lack of) for subduction in the Caribbean margin of Colombia is discussed and compared to other proven subduction margins based on the tectonics, petrology, geomorphology, and geophysical data available. The controversy over the tectonic setting of the CMC is extremely important because it impacts the petroleum potential of the region. It could be resolved by a incorporating tectono-sedimentary and 4D deformational analyses with outcrop and subsurface data. THE GEOLOGICAL SETTING The onshore part of the CMC is represented by the Sinu and San Jacinto fold belts, located on the north-western margin of Colombia where few outcrops are available. This area is characterized by a very low topographic slope and abundant mud volcanoes. It is composed of Upper Cretaceous pelagic rocks, a thick lower Tertiary turbiditic sequence follow by Oligocene-Miocene pelites and extensive fine-grained upper Miocene and Pliocene turbidites overlain by shallow-water Quaternary fluvial, carbonatic and lacustrine sediments (Duque-Caro, 1979; Case and Holcombe, 1980; Vinnels et al., 2010). Northwest-verging folds and faults trend approximately N20ºE. Narrow mud-cored anticlines with steeply dipping limbs are separated by broad, gentle synclines associated with domes and argillaceous diapirs, sometimes reaching the surface as active mud volcanoes. In the offshore region, seismic profiles across the margin suggest active folding of the youngest sediments, with undeformed reflectors beneath the fold belt dipping gently to the southeast. These characteristics are comparable with other well studied passive margins


where processes of gravitational gliding are interpreted (Atlantic margins of Brazil, Angola, etc.). TYPICAL SUBDUCTION MARGINS VS. THE CARIBBEAN MARGIN OF COLOMBIA. Many authors describe the contact between the Caribbean plate and the South American plate as a typical subduction margin with sutures and other related tectonic phenomena, such as the subsequent development of an accretionary prism (Duque Caro, 1984; Flinch, 2003; Kerr & Tarney, 2005; Ceron et al., 2007; Higgs, 2009; Pindell et al., 2010). A weakly defined southeast-dipping Wadati-Benioff zone has been interpreted as the product of slow subduction of Caribbean crust beneath South American plate (Dewey, 1972; Pennington, 1981; Kellogg and Bonini, 1982). GPS geodetic measurements show convergence between the Caribbean and South American plates of about 1 to 2 cm/yr (Vega et al., 1991, Audemark et al., 2005). Toto and Kellogg (1992) consider the Sinu-San Jacinto fold belt as an accretionary wedge related to an aseismic subduction with very low angle surface topographic slope (≥ 2º), forming a wedge of sediments up to 12 km thick which has been accreted to the South American margin throughout the Cenozoic (Duque-Caro, 1979). However, there is no clear surface morphological evidence, such as a trench or onshore mountains, to support this hypothesis (Fig. 2). Nor is there any subsurface evidence such as a magmatic arc or dipping seismicity that is typically seen in other known subduction margins: e.g. Chile-Peru, Indonesia, Aleutians, etc. (Uyeda, 1982).

Figure 2. The main characteristics of a typical subduction zone..

Evidence which does not support a typical subduction zone is as follows: i) The convergence rates are very low (1 to 2 cm/yr) and the convergence orientation very oblique in relation of other typical subduction zones where the rates are over 6 to 7 cm/yr and the convergence direction is orthogonal. ii) There is no typical related seismicity and the dip of the top of the oceanic crust is horizontal to very gentle. Thus, the gravimetric and magnetometric surveys do not show compatible patterns related to subduction. iii) There is no trench in the contact zone and the typically adjacent cordillera is absent.


vi) There are no middle to high metamorphic grades in the rocks associated with the oceanic crust slabs forming the accretionary prism, as well as no contemporaneous calc-alkaline magmatism. DISCUSSIONS AND CONCLUSIONS The present deformation of the CMC is the result of the oblique convergence between the Caribbean plate and the northwest margin of South America. The irregular morphology of the CMC and the different rates of sedimentation, facies distribution and sequences deposited between the Cretaceous and the present can be explained by a passive margin system (Fig. 3).

Figure 3. Schematic evolution for the CMC. Upper: passive margin during the Cretaceous? to Lower Tertiary times. Lower: Present context showing the Sinú-San Jacinto belt uplifted by a concentration of the deformation on the weaker transitional crust.

Through the preliminary analysis of surface and subsurface available information, it is possible to recognize a partitioning of the tectonosedimentary evolution (Fig. 4). From the Mesozoic, the sedimentation of the CMC evolved in an extensional tectonic regime associated with the opening of the Caribbean. Volcano-sedimentary depositional centers developed at the active edges of the rift, where northward vergence occurred. This extensional margin was progressively superimposed by sedimentary sequences of a passive margin fed by the Magdalena and Cauca paleo-rivers. These thick turbiditic sequences are then successively overlain by coastal or platform sequences prograding to the north. However, from the Miocene, the Sinú-San Jacinto fold belt was uplifted and produced a partial restriction, forcing the Cauca river to merge with the Magdalena. This caused a decrease in the sedimentation rate in the south of the CMC and an increase in the north, creating the huge Magdalena delta (Fig. 5).


Figure 4: Left) Sedimentological history showing the influence of the Sinú-San Jacinto belt in the evolution of the Cauca river. Right) Deformational history showing compressional and dextral wrenching compound due to the oblique convergence of the Caribbean and South American plates (modified from Rossello, 2006).

Concomitant with the sedimentary evolution, the tectonic setting was also changing from typically extensional towards more oblique convergence between the South America and Caribbean plates with the main axis of compression being WNW – ESE. This change would have started in the Miocene, although it wasn't strong enough to exhibit the typical features of subduction such as volcanism and associated seismicity. In the sections where the convergence is more orthogonal, compression increases (eg. Sinú-San Jacinto and La Guajira fronts, Fig. 1) and when the compression direction decreases, there is more wrenching with a dextral compound (eg. Austral front and Oca faulting, Fig. 1).

Figure 5. Schematic evolution of the Cauca and Magdalena rivers due to the uplift of the Sinú-San Jacinto belt (taken from Rossello et al., 2011).

There are two principal factors which make the CMC margin different from other passive margins. i) Sedimentological changes (fluvial input) which develops through time and eventually focusses in the northern part of the CMC, and ii) Deformational influence (oblique convergence) on the level base change and/or the basin subsidence and exhumations along the CMC. This high time-space variability in the types of factors and their relative intensities produce the different tectonosedimentary scenarios in the surface and subsurface. The evidence presented in this paper does not support the existence of a subduction zone in the CMC. It is therefore proposed that the CMC is a result of the dynamic balance of an early extensional margin related to a Mesozoic to Cenozoic passive margin style where the


continental platform progrades onto the oceanic plate and a late transpressional inversion is associated with oblique convergence. The in-situ model for the Caribbean plate proposed by James (2009; 2010) or the alternative interpretations of Moreno et al. (2009) conform better with the available data. The tectosedimentary evolution of the CMC looks like very similar in time and space to the important oil and gas producing northern margin of Borneo (Malaysia and Brunei) (Morley et al., 2011). The controversy over the tectonic setting of the CMC is extremely important because it impacts the petroleum potential of the region. It could be resolved by a incorporating tectono-sedimentary and 4D deformational analyses with outcrop and subsurface data.

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