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Transfer zones in the East African Rift System and their relevance to hydrocarbon exploration in rifts
Abstract
Transfer zones in extensional regions display a wide range of geometries from discrete fault zones to zones of broad warping. The classification of extensional fault displacement transfer zones developed in this paper includes three main criteria: (1) primary subdivision of transfer zones by relative attitude and direction of throw of the major faults (synthetic and conjugate); (2) secondary subdivision of conjugate transfer zones into transfer zones where the normal faults dip toward each other (convergent) and where the transfer zones occur between faults that dip away from each other (divergent); (3) the tertiary subdivision of conjugate relationships of transfer zones and secondary subdivision of synthetic transfer zones are by the fault terminations in plan view; fault tips approach, or they overlap, completely overlap (termed collateral), or are in line (termed collinear). A high abundance of overlapping transfer zones occur in the East African rift where extension is low and large-scale cross faults (photo-transform faults?) are uncommon. Commonly, transfer zones in this region are relatively high areas with complex internal fault geometries flanked by deeps. Transfer zones in rifts contain complex but somewhat predictable structural geometries that make them optimum locations for structural hydrocarbon traps. The classification presented can help define and delineate those zones and to some degree predict their internal structural geometry.
Understanding the long‐term tectono‐stratigraphic evolution of active extensional faulting is crucial for unraveling the mode through which continental rifting propagates in space and time. The Pliocene‐Quaternary L’Aquila Intermontane Basin (AIB) in central Apennines offers a natural laboratory for studying a propagating continental rift. Seismicity is related to NW‐SE‐striking normal faults that have been accommodating crustal stretching since the Late Pliocene. Through a multidisciplinary approach integrating field, mineralogical, geochemical (C‐O stable and clumped isotopes) and geochronological (⁴⁰Ar/³⁹Ar, U‐Th) analyses, this study focuses on the structural connection between the Mount Pettino Fault (MPF) and the Paganica Fault, two active, left‐stepped basin boundary faults of the AIB. A two‐stage tectono‐stratigraphic evolution is proposed during transition from localized to delocalized deformation and fault linkage. Stage‐1 (pre‐Middle Pleistocene) corresponds to nucleation and growth of the MPF, characterized by a ∼5 m thick exhumed fault core, consisting of an isotopically closed cataclasite (T (∆47) ∼33–50°C). Stage‐2 corresponds to the development of a distributed zone of NW‐SE and E‐W extensional faulting in the overlay zone with the Paganica Fault, which is interpreted as a transfer zone linking the basin boundary faults, with maximum long‐term slip rates comparable to those of the connected faults. Structurally controlled circulation of meteoric fluids promoted carbonate veining and travertine formation (T (∆47) ∼8°C). U‐Th carbonate dating of Stage‐2 mineralizations constrains the tectonic activity in the transfer zone at least at ∼182–331 ka. Implications on the tectono‐stratigraphic evolution and on the seismotectonic scenario of the AIB are discussed, providing geodynamic inference at regional scale.
Extensive seismic reflection interpretation of this high allowed better understanding of its proximal and distal structure. Seismic lines helped map the salt base and recognize the primary geological interfaces, unconformities, faults, and volcanic intrusions. The proximal Cabo Frio High is a continental basement uplift extending mainly into the Campos Basin. The Cabo Frio Outer High, which is of volcanic origin, lies between Santos and Campos Basins. This volcanism is concentrated at the crossing of two significant structures: Cruzeiro do Sul Lineament and Rio de Janeiro Fracture Zone. The Cabo Frio High fits an NW-SE fold axis, which is well correlated with distension along NW-SE direction generating a regional E-W sinistral plate movement, mainly related to the ENE-WNW sinistral Rio de Janeiro transfer zones. The accumulated uplift is approximately 4 km, achieved by at least four uplifting pulses. The uplifting starts in the Albian-Santonian, characterized by the elevation of the northeast portion of Cabo Frio High, followed by erosion. The sill and dikes intruded in the Upper Cretaceous, and the uplift of the southwest portion of the Cabo Frio High characterizes the second uplift pulse. Subaerial Eocene volcanos characterize the third uplifting pulse. The uplifting of the Blue Marker horizon (Oligocene) characterizes the last pulse.
The links between tectonics, surface processes and magmatism govern the evolution of rifted and transform margins. Quantifying the control of surface and deep Earth processes, lithosphere rheology and plate kinematics is challenging because of their non-linear interactions. We designed and conducted systematic 3D magmatic-thermo-mechanical numerical experiments coupled with surface processes modelling to better understand the formation of rifted and transform continental margins. Oceanic transform faults are formed by either the opposite polarity of oceanic detachment faults or their formation is linked to the gradual interaction between two propagating rift and spreading centers. Lower divergence velocities, faster crustal and slower mantle thinning, lower surface processes (i.e. erosion and sedimentation) rates, and lower mantle potential temperature lead to the formation of magma-starved continental margins, mantle exhumation and eventually the formation of a stable transform fault zone with a magma-starved, deep transform valley. Suppressed melting and small-scale mantle instabilities govern the along-ridge variation of magmatic and non-magmatic segments, often leading to V-shaped zero-offset oceanic fracture zones. In contrast, faster divergence, lithospheric mantle inherited weak zones, enhanced erosion and sedi-mentation, result in enhanced mantle melting, and rift magmatism and the formation of a spreading center in the transform zone. Models simulating the temporal increase of divergence velocities show the evolution from an initial magma-poor to a final magma-rich oceanic basin. In models without simulating mantle melting, enhanced surface processes lead to delayed break-up linked to a longer continental hyper-extended stage. However, enhanced surface processes and a more localized and accelerated lithospheric mantle thinning can promote earlier mantle melting and the formation of magma-chambers beneath the crust.
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The distribution and timing of Neogene extensional structures along the offshore Tanzania margin and their influence on submarine sediment dispersal pathways remain poorly constrained. This knowledge gap limits understanding of the propagation of the East African Rift System (EARS) in the western Indian Ocean. In this study, we use 2D and 3D seismic reflection data to explore a portion of the upper slope region offshore the Rufiji River delta which led to the discovery of a new extensional structure. Horizon maps and seismic sections extracted from the 3D volume reveal that the slope was intersected by W‐E‐oriented turbidite channels during the Cenozoic until the early Pliocene (5.3 Ma). Since then, the opening of this graben, whose timing is also constrained by stratigraphic horizon flattening, has led to a southward reorientation of these channels, a pattern that persists today, as evidenced by the flow direction of the channels at the modern seafloor. 2D seismic profiles reaching depths of 10 s two‐way travel time (TWT) indicate that the formation of this graben is not related to the reactivation of Mesozoic structures. In detail, seismic data show that the acoustic basement is intersected by extensional faults, likely related to the Jurassic rift tectonics, which is reactivated during the middle Cretaceous forming a gentle monocline. The lack of deformation in the post‐Cretaceous suggests a period of tectonic quiescence which persists until the establishment of a new extensional regime responsible for the graben's opening, indicating a decoupling between Mesozoic and Neogene tectonics. Considering the similarity in kinematics, orientation and timing between the graben and other structures along the margin, onshore and offshore, we interpret this graben to be generated by a later tectonic phase of the EARS. These new results may indicate that tectonic stresses associated with the EARS migrated from the Tanzania craton, where the oldest rift structures are dated to ca. 25 Ma, to the western Indian Ocean, where the tectonic activity started during the middle‐late Miocene to Pliocene.
Major normal fault systems are composed of segments that link as displacement accumulates, with linkage zone characteristics that reveal fault zone evolution. The steeply west-dipping Sevier fault zone in southwestern Utah, displays a complex fault network that developed between two long (>10 km), en echelon segments near the town of Orderville. Geologic map data and cross-sections of the transfer zone between the Mt. Carmel segment in the south and the Spencer Bench segment in the north reveal more than ten normal faults and four relay ramps displaying a range of geometries, including two relay ramps that display ramp-parallel folds. We suggest that transfer zone deformation was initially dominated by faults subparallel to the primary segments with later cross-faults that hard-linked these faults across most of the transfer zone. When the transfer zone was a soft-linked system, a displacement deficit likely existed relative to fault segments to the north and south. This early fault configuration would have reduced the efficiency of slip propagation associated with major earthquakes (>M7.0). In contrast, the present-day transfer zone, with a complex but hard-linked fault network, shows displacements that transition smoothly from the higher displacement (~800 m) southern segment to the lower displacement (~400 m) northern segment. That transition, combined with extensional strain within the zone, suggests that the Orderville fault network would be unlikely to impede propagation associated with future major earthquakes. The kinematic model of fault evolution presented here has implications for those investigating geothermal energy potential, groundwater flow, natural gas and oil reservoirs, mineral deposit formation, or seismic hazards.
Sequence architectures along the margins of rift basins are still poorly documented compared to passive continental margin settings. The Eocene Shahejie Formation on the rift margin of the Dongying Depression records a complex sequence stratigraphic distribution of conglomerate, sandy conglomerate, sandstone and mudstone. These facies have been mainly attributed to fan delta and marginal subaqueous fan depositional settings that developed during segmented fault activity along the rift margin. We utilize three‐dimensional (3D) seismic data, conventional cores, and wireline log data to dissect the overall wedge‐shaped upper sub‐member strata of the Shahejie Formation's fourth member. The study interval is a third‐order sequence formed between 45.4 and 42.5 Ma and contains a lowstand–transgressive systems tract (LST–TST) and a highstand systems tract–falling‐stage systems tract (HST–FSST). We found that the LST–TST developed several huge amalgamated depocenters along the Chennan border fault; whereas, these depocenters gradually diminished or even disappeared during the HST–FSST period, illustrating less significant control by the border fault. Through calculations of strata growth rates, we confirmed that the segmented activity of the border fault influences the stratigraphic distribution and facies evolution during these two periods. Specifically, deep‐water depositional systems, represented by marginal subaqueous fans were widely developed in the LST–TST period and were influenced by overall strong tectonic activity, including retrogradational (R) and aggradational (A) patterns. However, the aggradational to progradational (AP) deltas and progradational to degradational (PD) deltas dominated the generally muted tectonic activity setting during the HST–FSST. Additionally, influenced by localized segmented fault activity, these systems tracts exhibit incomplete vertical development, resulting in spatial variability in stratigraphic stacking patterns.
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