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The offshore East African Rift System: Structural framework at the toe of a juvenile rift

2015; Wiley; Volume: 34; Issue: 10 Linguagem: Inglês

10.1002/2015tc003922

1944-9194

Dieter Franke, Wilfried Jokat, S. Ladage, Harald Stollhofen, Jennifer Klimke, Rüdiger Lutz, Estevão Stefane Mahanjane, A. Ehrhardt, Bernd Schreckenberger,

earthquake and tectonic studies

TectonicsVolume 34, Issue 10 p. 2086-2104 Research ArticleFree Access The offshore East African Rift System: Structural framework at the toe of a juvenile rift Dieter Franke, Corresponding Author Dieter Franke Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, Germany Correspondence to: D. Franke, Dieter.Franke@bgr.deSearch for more papers by this authorWilfried Jokat, Wilfried Jokat Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, GermanySearch for more papers by this authorStefan Ladage, Stefan Ladage Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, GermanySearch for more papers by this authorHarald Stollhofen, Harald Stollhofen Geozentrum Nordbayern, Friedrich-Alexander Universität, Erlangen, GermanySearch for more papers by this authorJennifer Klimke, Jennifer Klimke Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, GermanySearch for more papers by this authorRuediger Lutz, Ruediger Lutz Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, GermanySearch for more papers by this authorEstevão Stefane Mahanjane, Estevão Stefane Mahanjane Instituto Nacional de Petróleo, Maputo, MozambiqueSearch for more papers by this authorAxel Ehrhardt, Axel Ehrhardt Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, GermanySearch for more papers by this authorBernd Schreckenberger, Bernd Schreckenberger Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, GermanySearch for more papers by this author Dieter Franke, Corresponding Author Dieter Franke Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, Germany Correspondence to: D. Franke, Dieter.Franke@bgr.deSearch for more papers by this authorWilfried Jokat, Wilfried Jokat Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, GermanySearch for more papers by this authorStefan Ladage, Stefan Ladage Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, GermanySearch for more papers by this authorHarald Stollhofen, Harald Stollhofen Geozentrum Nordbayern, Friedrich-Alexander Universität, Erlangen, GermanySearch for more papers by this authorJennifer Klimke, Jennifer Klimke Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, GermanySearch for more papers by this authorRuediger Lutz, Ruediger Lutz Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, GermanySearch for more papers by this authorEstevão Stefane Mahanjane, Estevão Stefane Mahanjane Instituto Nacional de Petróleo, Maputo, MozambiqueSearch for more papers by this authorAxel Ehrhardt, Axel Ehrhardt Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, GermanySearch for more papers by this authorBernd Schreckenberger, Bernd Schreckenberger Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, GermanySearch for more papers by this author First published: 19 September 2015 https://doi.org/10.1002/2015TC003922Citations: 60AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract The Cenozoic East African Rift System (EARS) extends from the Red Sea to Mozambique. Here we use seismic reflection and bathymetric data to investigate the tectonic evolution of the offshore branch of the EARS. The data indicate multiple and time transgressive neotectonic deformations along ~800 km of the continental margin of northern Mozambique. We observe a transition from a mature rift basin in the north to a juvenile fault zone in the south. The respective timing of deformation is derived from detailed seismic stratigraphy. In the north, a ~30 km wide and more than 150 km long, N-S striking symmetric graben initiated as half-graben in the late Miocene. Extension accelerated in the Pliocene, causing a continuous conjugate border fault and symmetric rift graben. Coevally, the rift started to propagate southward, which resulted in a present-day ~30 km wide half-graben, approximately 200 km farther south. Since the Pleistocene, the rift has continued to propagate another ~300 km, where the incipient rift is reflected by subrecent small-scale normal faulting. Estimates of the overall brittle extension of the matured rift range between 5 and 12 km, with an along-strike southward decrease of the extension rate. The offshore portion of the EARS evolves magma poor, similar to the onshore western branch. The structural evolution of the offshore EARS is suggested to be related to and controlled by differing inherited lithospheric fabrics. Preexisting fabrics may not only guide and focus extension but also control rift architecture. Key Points Structural images of the offshore East African Rift Extension estimates and development of the offshore rift Influence of inheritance on rift localization and architecture 1 Introduction Onshore, the Cenozoic East African Rift System (EARS) extends for thousands of kilometers in the eastern part of Africa, consisting of two major branches (western and eastern; cf. Figure 1) [e.g., Chorowicz, 2005; Ebinger, 2012; Macgregor, 2015]. Little is known about the geological fabric of the southward offshore continuation of the EARS along the coastline of Mozambique. This continuation has been established by GPS measurements, earthquake slip vector data, the spatial distribution of earthquake focal mechanisms, and transform fault azimuth data [Calais et al., 2006; Saria et al., 2014; Stamps et al., 2014]. Previous to this study, however, only Mougenot et al. [1986] published offshore seismic reflection data, which provided first insights into the structural framework of the offshore extension of the southeastern branch of the rift. Figure 1Open in figure viewerPowerPoint General geological overview of the study area. Dark grey lines indicate the position of geophysical profiles acquired during R/V Sonne cruise SO-231 in 2014. Earthquake locations and magnitudes (1973–2014; mb > 4.0) are shown as magenta circles according to the National Earthquake Information Center catalog and earthquake moment tensors from the Global Centroid Moment Tensor catalog [Ekström et al., 2012]. The Lurio Belt separates the northern from the southern high-grade metamorphic basement of northern Mozambique [Emmel et al., 2011]. The inlay shows the main faults of the western and eastern branches of the East African Rift System (from Chorowicz [2005] and Macgregor [2015]). Locations of the reflection seismic lines A–E (Figures 3, 5, 7, 8, and 9) are indicated. The dashed black box shows the position of Figure 4. Our recently acquired marine multichannel reflection seismic and bathymetry data off Mozambique (Figure 1) allow a more detailed view on the EARS offshore, or southeastern branch. The southeastern toe of the EARS provides critical constraints on the initial development, the propagation and unzipping of rift structures, and the timing of deformation. We discuss the influence of old/inherited Pan-African lithospheric structure and the crustal configuration resulting from the Jurassic opening of the Western Somali Basin on the rift location, structure, and architecture. 2 Geological Setting and Stratigraphy 2.1 Geological Setting The offshore EARS partly follows structures, which are inherited from the formation and early dispersal of Gondwana. Northern Mozambique lies in the core of the Pan-African orogeny at ~620–530 million years (Myrs) that amalgamated the Gondwana supercontinent [e.g., Emmel et al., 2011]. The Mozambique mobile belt runs from the Red Sea in the north to northern Mozambique and Malawi in the south and is polyorogenic in nature. It results from a large-scale continent-continent collision zone, developed between eastern and western Gondwana, with incorporated ophiolites, regional-scale westward vergent nappes, and thrusts [Fritz et al., 2013]. The Precambrian metamorphic basement of northern Mozambique is subdivided into a northern and a southern province, separated by the ENE trending Lurio Belt (Figure 1) that approaches the shoreline at about 13°S [Emmel et al., 2011; Fritz et al., 2013]. Both basement provinces consist of a collage of Mesoproterozoic to Neoproterozoic gneiss complexes, which are structurally overlain by granulite facies nappes and klippen. Following the Pan-African orogeny, the onset of Karoo rifting during latest Carboniferous to Early Permian represents the first significant regional tectonic event in southern and eastern Africa, finally leading to the breakup of Gondwana and the opening of the West Somali and Mozambique Basins [Catuneanu et al., 2005]. Prior to the onset of continental breakup, Madagascar was located in central Gondwana, adjacent to the present-day Tanzania, Kenya, and Somalia margins. The first oceanic crust in the West Somali Basin formed in the late Middle Jurassic, at ∼165 Myr [Coffin and Rabinowitz, 1987; Gaina et al., 2013; Jokat et al., 2003; Reeves, 2014]. Contemporaneously to the southward drift of Madagascar until the Aptian, both the Western Somali and the Mozambique basins opened [Coffin and Rabinowitz, 1988; Gaina et al., 2013; Jokat et al., 2003; Leinweber and Jokat, 2012]. This configuration results in a fairly simple structural setting: northeastern Kenya/Somalia and northern Madagascar resemble conjugate passive rift margins, whereas southeastern Kenya/Tanzania/northern Mozambique and western Madagascar are conjugate transform margins [Coffin and Rabinowitz, 1992]. As a consequence of the Mesozoic southward drift of Madagascar, a pronounced bathymetric elevation (the Davie Ridge or Davie Fracture Zone) (Figure 1) formed. Its strike direction is subparallel to the Mozambique suture [Fritz et al., 2013], a remnant of the ~550 Myr Pan-African collision. Dredging and coring revealed that the Davie Ridge is, at least locally, comprised of continental crust. Crystalline rocks were recovered at 14–15°S, from the base of the western ridge flank and included tectonized gneisses and altered semipelites, subsequently covered by sediments [Bassias and Leclaire, 1990; Bassias, 1992; Leclaire et al., 1989]. Continental basement is likely also present farther north and south along Davie Ridge, in an area where mainly basalts have been dredged and cored. Alkaline basalts, recovered from the southern parts of the ridge, indicate geochemical affinities with Cretaceous volcanics in southeast Africa and Madagascar [Bassias and Leclaire, 1990]. The presence of these intraplate basalts and the general absence of ultramafic rocks corroborate, according to Bassias [1992], the continental origin of Davie Ridge. Recent offshore extensional deformation is discernible from seismicity scattering along the Davie Ridge between 10°S and 18°S over a distance of ~800 km along the NE Mozambique continental margin (Figure 1). Earthquake slip vectors along the Davie Ridge consistently indicate E-W extension [Foster and Jackson, 1998; Saria et al., 2014]. Fault plane solutions of five earthquakes in the Kerimbas Graben and four at the southern Davie Ridge all reveal roughly E-W extensional failure (Figure 1) with focal depths clustering at ~19 km and ~40 km in the Kerimbas Graben and at ~19 km at 18°S, offshore Madagascar [Yang and Chen, 2010]. 2.2 Stratigraphic Framework A detailed seismic stratigraphy allows conclusions about the respective timing of extensional deformation. Thus, a summary of the lithostratigraphy, the main regional events, unconformities, and seismic marker horizons is given in Figure 2. Sedimentary deposits, ranging in age from the Middle Jurassic to Cenozoic, have been drilled in southern Tanzania and northern Madagascar [Nicholas et al., 2007; Salman and Abdula, 1995]. The presence of Early Permian to Middle Jurassic sediments in northern Mozambique, as proposed by Salman and Abdula [1995], may be inferred from the paleo-position of Madagascar and the similarity of predrift sediments in the Karoo basins of Tanzania and Madagascar [Catuneanu et al., 2005; Geiger et al., 2004; Hankel, 1994]. However, extensive uplift and erosion probably caused a widespread absence of Karoo sediments, resulting in a regional Early to Middle Triassic hiatus [Hankel, 1994]. Figure 2Open in figure viewerPowerPoint Summary of the lithostratigraphy and the main regional events, unconformities, and horizons, interpreted in the offshore multichannel seismic data. J indicates an unconformity of Middle to Late Jurassic age, BC the Base Cenozoic, O the top Oligocene, and LM a late Miocene horizon. The Jurassic-Cretaceous successions offshore northern Mozambique are related to the progressive breakup of southeastern Gondwana [Gaina et al., 2013; Jokat et al., 2003; Rabinowitz et al., 1983]. During the Early to Middle Jurassic rifting phase, vast regional depressions developed [Salman and Abdula, 1995]. This rift phase is well documented offshore west Madagascar, where late Early Jurassic marine strata are unconformably overlying a Carboniferous-Triassic sequence and wedge-shaped Toarcian to Aalenian half-graben fills [Geiger et al., 2004]. In the northern Mozambique Basin, the upper parts of the postbreakup successions were drilled, providing stratigraphic control on the seismic marker horizons: the Neocomian Unconformity, the Top Cretaceous/Base Tertiary, and the Pliocene Unconformity [Mahanjane et al., 2014]. Results of commercial exploration of the Rovuma Basin (Figure 1) provide a relatively well-defined Late Mesozoic to recent stratigraphy in northern Mozambique [e.g., Key et al., 2008; Nicholas et al., 2007; Salman and Abdula, 1995]. In the offshore Rovuma Basin of northern Mozambique (Figure 1) [Salman and Abdula, 1995] and in northwestern Madagascar [Razafindrazaka et al., 1999] the Lower Cretaceous strata are essentially marine shalestone, siltstone, and sandstone. Seafloor spreading ceased in the West Somali Basin in the Early Cretaceous [Coffin and Rabinowitz, 1992]. Consistently, a passive margin sedimentary system has been reported since the beginning of the Late Cretaceous in the West Somali Basin [Key et al., 2008; Nicholas et al., 2007] in conjunction with enhanced sediment supply [Walford et al., 2005]. The establishment of deepwater bottom currents in offshore Mozambique caused the formation of contourites, including shingled clinoforms [Coffin and Rabinowitz, 1992]. The Upper Cretaceous succession offshore Mozambique and Madagascar is made up of several thousands of meters of mainly marine sediments [Key et al., 2008; Razafindrazaka et al., 1999]. A major marine transgression resulted in a condensed section at the top of the Upper Cretaceous deposits, potentially including a paraconformity. This transgression reached its maximum in the Eocene, with the onset of regression during the Late Eocene [Key et al., 2008]. The offshore Rovuma deltaic complex offshore northern Mozambique (Figure 1) consists of a thick, eastward prograding wedge of Cenozoic fluvio-deltaic deposits and shallow-water marine to deep-marine strata that unconformably overlie the interface of Palaeocene-Eocene successions [Key et al., 2008; Salman and Abdula, 1995]. Paleocene-middle Eocene aged clays were proposed by Mahanjane and Franke [2014] to form the detachment plane of the offshore Rovuma Basin deep-water fold-and-thrust belt. A Miocene transgression led to shallow water marine sedimentation during progradation of the Rovuma delta, contemporaneous with rift-related sedimentation in the onshore East African Rift System [Key et al., 2008]. Following the late Miocene, significant uplift and erosion is reported from southern Tanzania and Mozambique [Nicholas et al., 2007; Walford et al., 2005]. This resulted in a regional peak in sediment supply to adjacent offshore areas. On the eastern flank of Davie Ridge, at about 15°S, DSDP Leg 25, Site 242 (Figure 1) penetrated 676 m of Quaternary to upper Eocene sediments [Simpson et al., 1974], providing well control for the youngest part of the seismic sequence analyzed in this study. 3 Data and Methodology Geophysical data (Figure 1) were acquired during a cruise of the German research vessel Sonne in early 2014. Bathymetric mapping of the seafloor was conducted continuously during the cruise using the ship's SIMRAD EM120 multibeam echo sounder. In total, 27 multichannel reflection seismic profiles with a total length of 4300 km were acquired. The reflection seismic source was a G gun array with a total volume of 3100 in3 (50.8 L; working pressure 14,5 Mpa) and a towing depth of 6 m throughout the survey. The G gun array was fired at regular 50 m intervals. Seismic signals were recorded with a 324 channel (4050 m active length) digital streamer, towed at a depth of 12 m. The record length was 14 s with a sampling rate of 2 ms. Prestack seismic data processing included trace editing, filtering, and a signal deconvolution, followed by surface-related multiple suppression. Stacking velocities were determined at regular 3 km intervals. The lines were either poststack time migrated or prestack migrated. The prestack Kirchhoff time migration was performed on common-offset gathers, in some cases followed by a FK demultiple process or a mute in the radon domain. Residual moveout velocities were determined before stacking. Magnetic data were acquired with a towed arrangement consisting of a SeaSPY gradient magnetometer array with two scalar Overhauser sensors and a vector magnetometer sensor. The gravity field was recorded with a KSS32M sea gravimeter system. Extension across the rift was roughly estimated by determining the fault heaves from the hanging wall and footwall cutoffs of well-imaged marker horizons on seismic lines, following the procedures described by, e.g., Bell et al. [2009] and Lamarche et al. [2006]. Multibeam bathymetry was used to ensure measurement perpendicular to the structural grain of the rift. The calculated extension is given as the sum of major fault heaves across the basin (Table 1). Table 1. Estimates of Approximate Extension on the Basis of Fault Geometriesa Time Area Cumulative Extension (m) Miocene 10–12°S - Kerimbas Graben 300–600 ~Pliocene 10–12°S - Kerimbas Graben 400–1500 12–13.5°S - Zone of diffuse extension and Lacerda Graben 200–800 ~Pleistocene–Recent 10–12°S - Kerimbas Graben 4000–10,000 12–13.5°S - Zone of diffuse extension and Lacerda Graben 1000–5000 13.5°–15°S - Transition to the southern tip 2000–3000 15°–18°S - Southern tip of the rift >1000 a The onset of extension occurs in post-Oligocene times but before the latest Miocene. The ~Pliocene extension is not stratigraphically constrained but estimated on the basis of sedimentation rates. Similarly, the ~Pleistocene to Recent extension has been derived. Flattening of seismic data is a common seismic interpretation tool to remove structural overprint by shifting the data to a reference horizon. This enables the interpretation of significant features present at a particular time. 4 Interpretation 4.1 Seismic Stratigraphy and Key Horizons The general stratigraphic framework offshore Mozambique and Madagascar is summarized in Figure 2. Our approach is to tie our reflection seismic data to the relatively well-known stratigraphic framework of the deepwater areas offshore northern Mozambique, as described in section 2.2 and to correlate the post-Paleogene strata with the drilling results of DSDP Site 242. In between the West Somali and the Mozambique basins our stratigraphy relies on previous correlations with the well-known stratigraphy of the Zambezi Delta and a comparison to the onshore and offshore stratigraphy of western and northwestern Madagascar (see section 2.2). In addition, we have tied our stratigraphic concept to previous offshore investigations of Mougenot et al. [1986] and Coffin and Rabinowitz [1988, 1992] (Figure 2). Occasionally, there are deep reflections in the seismic data that may be interpreted as Karoo-aged sediments. Identification of these successions remains, however, arbitrary. Therefore, Karoo-aged strata are not interpreted separately, but are rather incorporated into the acoustic basement. Wedge-shaped strata occurring in deeply buried half-grabens are consistently capped by an unconformity (e.g., Figure 3). This unconformity is interpreted to reflect the change from rifting to transform-controlled passive margin development as Madagascar moved southward relative to mainland Africa. From correlative unconformities (Figure 2), we identify a regional unconformity—that we label "J unconformity" and that we tentatively assign an early Late Jurassic age. Figure 3Open in figure viewerPowerPoint (bottom) Reflection seismic line A and (top) interpretation across the Kerimbas Graben offshore northern Mozambique, at 11.5°S (for a margin-wide structural image, see Mahanjane and Franke [2014]). The symmetric rift graben is ~30 km wide. Major faults from both earlier extensional deformation dissecting the basement and recent deformation dissecting the seafloor are indicated. The interpreted acoustic basement is shown in brown color. Interpreted horizons in the sedimentary successions are J - Late Jurassic; BC - Base Cenozoic; O - Top Oligocene; LM - late Miocene. Vertical exaggeration is ~2. Line location is indicated in Figure 1. The overlying successions are made up of alternating packages of relatively low and high reflectivity. We correlate this succession with the drift phase in the West Somali Basin, ranging in age from the Late Jurassic to the Early Cretaceous. A condensed section at the top of the Late Cretaceous deposits [Key et al., 2008] correlates with a distinct seismic reflector as previously identified as the top of the Cretaceous strata [Mascle et al., 1987; Mougenot et al., 1986; Rogers et al., 2000] that we label as base Cenozoic reflector BC (Figures 2 and 3). In our reflection seismic data, the majority of Cenozoic sediments is characterized by well-stratified successions with thicknesses of up to ~3 s (two-way time, TWT). The stratigraphic interpretation of Cenozoic deposits in the study area is based on DSDP Leg 25, Site 242 (Figure 1) [Simpson et al., 1974]. We have traced the top Oligocene reflector throughout our survey (indicated as O; Figure 2), which is well defined in both the Rovuma Basin and at the DSDP Site 242. The horizon is equivalent to horizon A1 of Mougenot et al. [1986]. Finally, we interpreted a late Miocene reflector (LM; Figures 2 and 3) that has been tied to the well-known stratigraphy of the Rovuma Basin and DSDP Site 242. It also corresponds to the earlier interpreted reflector A3 of Mougenot et al. [1986]. Additionally, the presence of canyons that are known to cut into the middle Miocene strata [Walford et al., 2005] also gave hints on interpreting this horizon as late Miocene reflector LM. 4.2 Structure and Architecture of the Offshore East African Rift System 4.2.1 Kerimbas Graben—Line A (11.5°S) Our northernmost reflection seismic line runs in a NE direction across the Kerimbas Graben (Figure 3). Sediment thicknesses along this line reach at least 4 s (TWT). A top acoustic basement reflection is distinct at the western and eastern portions of the profile; it is less clear below the graben, where it is partly masked by seafloor multiples. Where distinct, the basement is affected by extensional deformation, resulting in the formation of deeply buried half-grabens. The half-graben fills are typically wedge shaped and capped by an unconformity (e.g., km 8–18 and km 60–70; Figure 3). The syn-extensional sedimentary infill probably is of Jurassic age, and we interpret its top boundary as the J unconformity. Most of the basement faults terminate before reaching the J unconformity. Based on the reflection characteristics and patterns, we have traced the relatively well-known stratigraphy from the Rovuma Basin (Figure 1; cf. section 2.2) across the Kerimbas Basin to the east (Figure 3). A distinct low-frequency horizon (horizon BC; cf. Figures 2 and 3) is interpreted as the base of the Cenozoic succession. The high reflective pattern underlying this reflector is distinct along the entire line. On the western graben flank, the Mocimboa Thrust Belt (Figure 3) developed above an Eocene shale detachment [Mahanjane and Franke, 2014]. The top Eocene is a distinct reflector (km 0–15: at ~5 s (TWT); Figure 3) overlying an about 500 ms (TWT) thick seismically more transparent succession that is traced eastward with confidence. Similarly, the top Oligocene horizon (O) (km 0–15: at ~4 s (TWT); Figure 3) is characterized by a distinct reflector, overlying a relative transparent succession. The late Miocene reflector (LM) is sealing the deformation structures observed in the Mocimboa Thrust Belt and continues as the top of a moderately reflective sediment package across the Kerimbas Graben. A small sediment-filled canyon at the eastern margin of the Kerimbas Basin (km 60–62; Figure 3) here confirms the approximate vertical position of the late Miocene reflector that we traced on the basis of the reflection characteristics and the thickness of the Miocene succession. Neogene to recent extensional deformation is distinct throughout the graben. Most of the normal faults dissect the seafloor, indicating recent tectonic activity. Slope failures at the graben shoulders are observed (km 12–15 and 49–54; Figure 3). Two major basin-bounding listric faults are developed on either side of the graben. Recent extensional deformation concentrates on the conjugate border faults but also affects the center of the graben structure. Outside the Kerimbas Graben there is merely no indication of any recent deformation (Figure 3). By combining the reflection seismic data with the bathymetry, we construct a seafloor structural map (Figure 4). The Kerimbas Basin is a more than 150 km long and about 30 km wide symmetric graben that is about 1 km deep on average. The graben is bounded by prominent, tens of kilometers long, N-S trending faults on either side (Figure 4). The bathymetric data reveal distinct SSE trending transfer faults dissecting the N-S trending graben. The most prominent of these is a right-lateral fault, which also affects the St. Lazare Seamount (dashed line; Figure 4, left). Figure 4Open in figure viewerPowerPoint Zoom into the area where a clear seafloor expression of the offshore EARS is present. (left) Major normal faults interpreted on multichannel seismic and bathymetry data. Line thickness correlates with the throw of the individual faults. Underlain is the EMAG2 2 arc min resolution Earth magnetic anomaly grid [Maus et al., 2009]. The onshore structural trend has been modified from Vauchez et al. [2005]. The location of reflection seismic lines A–C is indicated. (middle) The corresponding seafloor bathymetry. (right) Additional earthquake epicenters and fault plane solutions are presented. The lithology of dredge sites from Davie Ridge is shown according to Leclaire et al. [1989]. 4.2.2 Zone of Diffuse Extension—Line B (13°S) The E-W trending line B (Figure 5) is located at a position without a noticeable seafloor expression of Davie Ridge (Figure 4). Acoustic basement reflections are distinct along the line, except in its central part. Along this line very weak magnetic anomalies have been recorded (Figure 5, top). At the western end of the line (km 0–25; Figure 5), deeply buried seaward dipping consecutive normal faults dissect the basement. The resulting narrow half-grabens are sealed by the J unconformity (Figures 2 and 5), marking the transition from rifting to transform margin development. Strike-slip faulting is interpreted in the center of the line, affecting the basement and the overlying sedimentary successions (km 50–60; Figure 5). The two faults may represent reactivated and transpressionally deformed normal faults. In any case, this fault activity phase postdates the extensional deformation, because it resulted in deformation of much younger overlying sediments. Similar to line A farther north, we interpret a distinct high reflective pattern as representing the Cretaceous succession and mark its top reflection as BC. This horizon has been affected by the strike-slip deformation in the center of the line. We assume that strike-slip movements are related to the southward displacement of Madagascar with respect to Africa during the Middle Jurassic (~165 Myr) and Early Cretaceous (~120 Myr) [Mascle et al., 1987; Rabinowitz et al., 1983] and possibly a subsequent reactivation in the Late Cretaceous [Mascle et al., 1987]. The Cretaceous sequence often develops an unconformable contact to underlying strata showing downlap truncations onto horizon J. The interpretation of the Paleogene and Neogene successions has been cross correlated to the Kerimbas Basin in the north and to DSDP Site 242 in the south [Simpson et al., 1974]. The Eocene and Oligocene successions generally grade from a nearshore transparent reflection pattern to a well-stratified and clearly reflective pattern farther away from the shoreline. The top of this succession is marked as horizon O. The interpretation of the Miocene succession, including the top reflector LM is further constrained by the presence of a sediment-filled paleo-channel (km 60–70; Figure 5), overlain by a recent canyon. Figure 5Open in figure viewerPowerPoint The E-W trending (bottom) reflection seismic line B at 13°S and (top) the interpreted line are shown. In Figure 5 (top) the ship track gravity (green line) and magnetic (red line) data are pres