Contrasted styles of rifting in the eastern Gulf of Aden: A combined wide‐angle, multichannel seismic, and heat flow survey

Abstract

[1] Continental rifts and passive continental margins show fundamental along-axis segmentation patterns that have been attributed to one or a number of different processes: extensional fault geometry, variable stretching along strike, preexisting lithospheric compositional and structural heterogeneities, oblique rifting, and the presence or absence of eruptive volcanic centers. The length and width scales of the rift stage fault-bounded basin systems change during the late evolution of the new plate boundary, and the role of magmatism may increase as rifting progresses to continental rupture. Along obliquely spreading ridges, first-order mid-ocean ridge geometries originate during the synrift stage, indicating an intimate relationship between magma production and transform fault spacing and location. The Gulf of Aden rift is a young ocean basin in which the earliest synrift to breakup structures are well exposed onshore and covered by thin sediment layers offshore. This obliquely spreading rift is considered magma-poor and has several large-offset transforms that originated during late stage rifting and control the first-order axial segmentation of the spreading ridge. Widely spaced geophysical transects of passive margins that produce only isolated 2-D images of crust and uppermost mantle structure are inadequate for evaluation of competing rift evolution models. Using closely spaced new geophysical and geological observations from the Gulf of Aden we show that rift sectors between transforms have a large internal variability over short distances (∼10 km): the ocean-continent transition (OCT) evolves from a narrow magmatic transition to wider zones where continental mantle is probably exhumed. We suggest that this small-scale variability may be explained (1) by the distribution of volcanism and (2) by the along-strike differences in time-averaged extension rate of the oblique rift system. The volcanism may be associated with (1) the long-offset AlulaFartak Fracture Zone, which may enhance magma production on its younger side, or (2) channeled flow from the Afar plume material along the newly formed OCT and the spreading ridge. Oblique extension and/or hot spot interactions may thereby have a significant control on the styles of rifting and continental breakup and on the evolution of many magma-poor margins.

Architecture of rifted continental margins and break-up evolution: insights from the South Atlantic, North Atlantic and Red Sea–Gulf of Aden conjugate margins

“…In addition, some recent studies have proposed the occurrence of exhumed mantle between the rift basin and the first oceanic crust, based on observations of outcropping rocks in the Alpine basins and the results of DSDP boreholes in Iberia and " " Newfoundland (Beslier et al 1996;Withmarsch et al 2001). This possibility has also been suggested for the Gulf of Aden eastern margin (Leroy et al 2004;d'Acremont et al 2005;Leroy et al 2010b;Watremez et al 2010;Lucazeau et al 2010). However, the evidence for a wide belt of exhumed mantle below the Neogene sediments in the Red Sea, as proposed by some authors (Voggenreiter et al 1988;Bohannon 1989;Voggenreiter & Hotzl 1989) is scanty.…”

“…Low velocity anomalies in the upper mantle are imaged below the northern margin of the Gulf of Aden, indicating that partial melting occurs under the rift basins (Basuyau et al 2010), which could be related to the Afar plume influence affecting even the eastern parts of the Gulf (Leroy et al 2010a). However, several studies indicate the possibility of mantle exhumation in the magma-poor segments of the Gulf of Aden, particularly eastwards of the Alula-Fartak Fracture Zone (Leroy et al 2010b;Leroy et al 2012). …”

“…Rift shoulder uplift and faulting are generally reported as being of lower Oligocene in age, whereas another phase of faulting is inferred to occur at the Oligo-Miocene boundary. Rift blocks are observed in seismic profiles offshore the northern Gulf of Aden (Leroy et al 2004;d'Acremont et al 2005;d'Acremont et al 2006;Autin et al 2010;Leroy et al 2010b), indicating that extensional tectonics affected the rifted crust until the Late Miocene (Leroy et al 2012). …”

“…21) indicates that the margins were characterized by inherited rifts onshore and offshore Yemen and Somalia, some of which were formed during pre-Tertiary extensional events (Bott et al 1992;Leroy et al 2012). In the " " Neogene, the separation of the Somalia and Arabian plates by the Sheba Ridge resulted in extensional stresses and the rifted crust developed several basins that were controlled by accommodation zones which became transform faults or fracture zones during the continental margin drift (e.g d'Acremont et al 2010; Leroy et al 2010b).…”

“…23) shows that the Neogene rift troughs in Somalia-Socotra and Yemen-Oman are quite narrow, and the OCT is marked by extensional faulting (d'Acremont et al 2005;Autin et al 2010;Leroy et al 2010b).…”