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Commonly, intra-continental wrenching is associated with a high degree of crustal faulting and fragmentation. The resulting continental blocks can undergo vertical-axis rotations, which in turn can lead to the generation of intricate fault networks within and along their boundary regions. Investigations into these structural complexities can support understanding of when and how these continental blocks rotate, and what their position was prior to transform margin formation. In the case of the Falkland Islands Microplate (part of the Falkland Plateau transform margin), its position between South Africa, South America, and East Antarctica prior to the break-up of Gondwana is still debatable. This uncertainty affects the reliability of plate models for this region. Here we integrate gravity and 2D and 3D seismic reflection data from the eastern margin of the microplate (west side of the Falkland Plateau Basin) to provide insights into the tectono-stratigraphic architecture of this area from Jurassic onwards, and into the evolution of the Falkland Islands Microplate. Our findings show that the western part of the Falkland Plateau Basin is an integral part of the microplate, and it underwent deformation in a relatively fast-changing stress regime. Stress field configuration estimates across the Falkland Islands Microplate support an alternation between a NE-SW and NW-SE/WNW-ESE orientation of σ3 during the Jurassic and an ENE-WSW oriented σ3 during the Lower Cretaceous. Correlations of this local stress configuration with the regional support a Middle to Upper Jurassic rotation of the microplate in a predominantly extensional setting facilitated by the early fragmentation of south-western Gondwana.

The early stages of transform margin formation are associated with crustal fragmentation and block rotation. The restricted size of the resultant microcontinental blocks precludes palaeogeographical reconstructions and reliable estimations of the amount of rotation they can undergo. An example considered here is the Falkland Plateau. This is located adjacent to the Agulhas–Falkland Fracture Zone and its westernmost province is the Falkland Islands microcontinent. The position of the plateau and the islands prior to Gondwana break-up remains contentious. This study integrates seismic reflection and gravity data to propose a revised position of the Falkland Islands microcontinent constrained by (1) the presence of a mega-decollement, controlling the Gondwanide Orogen, described north of the Falkland Islands and underneath South Africa and the Outeniqua Basin, and (2) the similar architecture of fault networks mapped north of the islands and in the northernmost Outeniqua Basin. This revised position requires a re-evaluation of the timing and rate of rotation of the Falkland Islands microcontinent and affects the expected crustal architecture adjacent to the islands. Our model yields rotation rates between 5.5° and 8° Ma−1 and two potential times for rotation, and predicts more unstretched crust beneath the basin east of the Falkland Islands than previous models.

The palaeogeographic reconstruction of the Falkland Plateau transform margin in a Gondwana pre-break-up configuration has been the subject of debate for years. This is mainly due to the uncertainty in the position of the Falkland Islands microplate. The islands were an extension of the south-east coast of South Africa, being either i) part of a rigid Falkland Plateau fixed to the South American plate or ii) undergoing a vertical-axis clockwise rotation of between 80° to 120° along the transform faults generated during the initial stages of fragmentation of south-western Gondwana. The absence of documented evidence of this rotation within the sedimentary infill of the basins surrounding the Falkland Islands represents an ongoing issue. Furthermore, a structural framework of the eastern continental shelf of the islands that takes into account the most recent seismic reflection surveys has not been published yet.This study presents an updated description of the structural configuration of the Falkland Plateau Basin, focusing on the Volunteer and Fitzroy sub-basins. This structural framework, based on extensive 2D and 3D seismic reflection data and aided by seismic attribute mapping, provides new insights into the evolution of the Falkland Islands microplate and the Falkland Plateau Basin.Three main structural trends were identified across this section of the Falkland Plateau. WNW-ESE trending half-grabens were mapped north-west of the Volunteer sub-basin; these correlate laterally with linear gravity anomalies following the same trend north of the Falkland Islands. NNE-SSW to N-S normal faults are predominant west of the Volunteer sub-basin and are believed to control the western margin of the Falkland Plateau Basin. Locally, the NNE-SSW trend is subdued by NNW-SSE striking en-échelon normal faults suggestive of left-lateral movement along a NNE-SSW direction. A similar trend is interpreted in the southern part of the Fitzroy sub-basin, supporting sinistral wrenching along the western margin of the Falkland Plateau Basin.These results suggest intra-plate deformation that is consistent with a clockwise rotation of the Falkland Islands microplate along the transform faults that accommodated the initial fragmentation of Gondwana. The interpreted fault network allows us to understand the temporal variation in the orientation of the minimum horizontal stress across the Falkland Islands microplate. By comparing this variation with the regional stress regime in south-western Gondwana, the timing and mechanism of the rotation of the islands can be better constrained.

The architecture of magma-poor continental margins, from break-up to the establishment of an oceanic spreading centre has received significant recent interest. A general consensus as to their characteristics exists and although it has been applied to margins globally is still dominated by the Iberian-Newfoundland “type” margin. Additionally, many studies rely upon sparsely distributed 2D seismic reflection profiles, or single seismic sections. Questions remain as to the validity of the model across other magma-poor margins, in particular the role of magmatic additions during the rifting phase and their relationship to the creation of true oceanic crust at the onset of sea-floor spreading. Our data comprise a 2D seismic reflection survey from the Obbia Basin, northern Somalia passive margin. The data are extremely well imaged to over 13 s two-way time, providing insights into the crustal structure. Here, we present 4 representative sections from this larger survey, which combined with gravity modelling, describe the character and crustal architecture along the margin from the proximal rifted continental crust to the oceanic domain. We find that the Somali margin is a poorly understood example of a magma-poor margin exhibiting Iberian-type architecture. The margin comprises a stretched proximal domain that passes seaward into a strongly thinned necking zone across which the lower crust is completely thinned. We identify a high amplitude reflection (HAR) that corresponds to the decoupling level, and into which crustal faults sole out. Seaward, the HAR merges with the top of the exhumed mantle directly underlying post-rift sediments. In contrast to other magma-poor margins, the Somali margin has a narrow necking zone and a wide zone of exhumed mantle (in excess of 100 km). On most sections true oceanic crust is never encountered, with only the distal portion of one section exhibiting oceanic crustal character. We place the continent-ocean transition zone in excess of 200 km east of the shoreline. In the north, the basin is affected by transform faults; where this occurs we note a presence of volcanics within the crust and post-rift sediments. Gravity modelling supports the addition of magmatic material in these regions. This addition of volcanic material, in contrast to that proposed at transform faults in other passive margins, is spatially restricted to the transform faults, emplaced post-kinematically, and is not associated with oceanic crust formation. The results of this study not only provide new insights into the architecture on non-typical non-magmatic margins but also reveal the importance of incorporating these findings into plate-reconstructions.

T he crustal structure along the passive margin of offshore Somalia has remained largely unknown, due to limited geological and geophysical data collection during the long period of political instability and civil war. The extent of geological information in the area has for many years been restricted to regional-scale features defined from poor quality legacy seismic data (Bunce and Molnar, 1977) and potential field data (Coffin and Rabinowitz, 1987; Rapolla et al., 1995). These observations allowed the development of early plate reconstruction models for Africa and Madagascar. However, since 2012 the region has experienced a period of relative political stability following the inauguration of the Federal Government of Somalia. Two 2D seismic acquisition programmes were undertaken in 2014 and 2016. The newly acquired seismic data have facilitated the development of new theories and understanding of the evolution of the Somalian passive margin (Kearns et al., 2016). In this study, we integrate newly acquired long offset seismic data with wells and regional satellite gravity and magnetic data, to critically assess the structural configuration, and map the continental to oceanic crust transition. We use gravity modelling techniques to support our observations. We also discuss the possible implications of our findings on source rock thermal maturity.