Chris J. Dart

Geological evolution of the southeastern Red Sea Rift margin, Republic of Yemen

The tectonic evolution of the southeastern margin of the Red Sea Rift in western Yemen has been investigated using a multi-disciplinary field study of an east-west transect between Al Hudaydah and Sana9a. Slow subsidence of up to 1 km occurred over the area during a 100 m.y. period before rifting. There was a major episode of flood volcanism between ca. 30 and 20 Ma, and important extensional faulting began after the eruption of the volcanic rocks and ceased before middle to late Miocene sediments and volcanic rocks were deposited unconformably on top of rotated fault blocks on the coastal Tihama Plain. Surface uplift has produced the Yemen highlands, whose highest peak reaches an elevation of 3660 m. This is attributed to plume heating and eruption of >3000 m of volcanic rocks. Apatite fission-track ages indicate early to middle Miocene exhumational cooling ages, postdating the major volcanic phase and contemporaneous with rifting. Volcanism was accompanied by emplacement of subvertical dike swarms, which generally strike north-northwest to northwest, broadly parallel to the Red Sea coastline. Major faults indicate northeast-southwest-directed extension. Large granitic sheets and plutons (up to 25 km wide) intruded the volcanic rocks. Approximately 30 km of extension has taken place across a 75-km-wide zone (β = 1.7) in 6-8 m.y. The relative timing of volcanism followed by extension and uplift does not fit conventional models of passive or active rifting. We suggest that the proto-Red Sea Rift was caused by regional plate stresses that exploited lithospheric weakening caused by the Afar plume. Appreciable doming only occurred after the main episode of volcanism, which suggests that magmas extruded before maximum thermal expansion of the lithosphere took place.

Sequence stratigraphy of (?)Pliocene-Quaternary synrift, Gilbert-type fan deltas, northern Peloponnesos, Greece

Abstract Seismic-scale cliff section exposures of exhumed (?)Pliocene-Quaternary, footwall-derived, Gilbert-type fan deltas are located along the faulted southern margin of the Gulf of Corinth rift. The location of these Gilbert-type fan deltas is controlled by antecedent drainage systems and relay zones. Each Gilbert-type fan delta body is made of depositional sequences and systems tracts that can be interpreted within a sequence stratigraphic framework. Sequence architectures were developed against a backdrop of rapid basin deepening due to tectonic subsidence. Sequences are bounded by composite downlap/truncation and toplap surfaces and are dominated by highstand and shelf margin systems tracts. The architecture of systems tracts and key stratal surfaces are interpreted in terms of hangingwall subsidence, sea/lake level changes, hinterland uplift and climatically induced variations in surface runoff. Gross Gilbert-type fan delta architecture is interpreted as the response to a background relative base level rise which was presumably dominated by rates of hangingwall subsidence that generally exceeded other base level control mechanisms.

The geometry and evolution of a transpressional strike-slip system: the Carboneras fault, SE Spain

The Carboneras fault system is a 40 km long, 1 km wide Neogene NE-SW-trending left-lateral transpressional strike-slip fault system which is part of the Trans-Alboran shear zone in SE Spain. The Carboneras fault system is an anastomosing array of sub-vertical, individual fault planes or fault zones which surround pods or lenses of less intensely strained rocks. Displacements along the individual fault planes exhibit reverse components of slips and form positive flower structures. Faults have either sharp boundaries or wider bands of gouge, typically a few metres thick and are hundreds of metres in length. Second-order fault splays are well developed and usually exhibit P-shear rather than Riedel-shear orientations. These are interpreted to be related to the transpressional displacement and may also characterize other oblique convergent zones elsewhere. The second-order faults are interpreted to have formed as shear strain increased along the first-order fault and was transferred laterally to branch segments. This process produced pods, or shear lenses, bounded by the fault segments. The length to width aspect ratios of the shear lenses were found to be scale-independent across five orders of magnitude with the most common values between 3:l and 6:l. A model is proposed for the development of the fault zone by incremental finite displacements along the segmented fault surfaces. This model is based on field evidence that displacement switched with time from one fault strand to another.

Stratigraphy and structure of the Maltese graben system

The Maltese graben system is a series of Miocene–Quaternary extensional basins located in the foreland of the Sicilian Apennine–Maghrebian fold and thrust belt. The tectono-sedimentary development of the graben was determined from the first fully integrated analysis of the area to include onshore field and hydrological borehole data, together with offshore reflection seismic data and exploration wells. The NW–SE- and ENE–WSW-trending rifts have a coeval, four phase tectono-sedimentary evolution revealed by a succession of platform and pelagic carbonates. The pre-rift phase (>21 Ma) is followed by a early syn-rift phase (21–6 Ma) characterized by evidence for relatively minor extensional faulting including the development of neptunian dykes. Major fault activity occurred during the succeeding late syn-rift phase (<5 Ma) which is characterized by growth faults and fault control on facies and the areas of deposition/non-deposition. This was followed by passive graben infilling during the post-rift phase (probably < 1.5 Ma). Despite low measured extension factors (β = 1.03–1.17) across the rift structures, major basin bounding extensional faults have throws of up to 2.2 km. Fault slip data indicate that both NW–SE and ENE–WSW rift trends were generated in response to N–S stretching. These results are in accord with Argnanis (1990) description of a major N–S orientated transfer fault located 50 km west of the present study area.

The timing of magmatism, uplift and crustal extension: preliminary observations from Yemen

Abstract The Red Sea and the Gulf of Aden form young, oceanic rift basins, situated between the diverging African and Arabian plates and bordered by highly elevated, volcanic margins. Yemen in the southeastern Red Sea, was once centred over the Afar plume/triple-junction (c. 30 Ma) forming part of the Arabian ‘passive’ margin. The present high elevation of the Afro-Arabian rift-flanks (up to 3.6 km as in Yemen), is the combined result of a number of endogenic rift processes which served to generate both the initial crustal uplift and also preserve the elevated topography. A further isostatic response generating uplift is likely to have been driven by differential erosion of the rift-flanks. However, the sedimentary record of the pre-Jurassic to early Tertiary period provides little evidence for major changes in relief or elevation. Furthermore, structural and volcanological observations indicate that most of the crustal extension occurred during mid-late Tertiary. The voluminous Oligo-Miocene basalt-rhyolite magmatism of Yemen was not apparently associated with pre-volcanic (> 30 Ma) uplift despite the commonly held belief that the Afar plume existed beneath the region 30 Ma ago. Geological data point to an episode of uplift that occurred after the initiation of magmatism. Fission track data indicate that uplift related exhumation postdates magmatism by some 10–15 Ma, perhaps the amount of time needed to change the thermal character of the Pan-African lithosphere above the Afar plume. A sequence of magmatism followed by synchronous crustal extension and uplift for Yemen does not fit with the traditional categories of active (uplift-magmatism-rifting) and passive (rifting-uplift-magmatism) rifting. Clearly such end-member models do not simply apply to the Red Sea or the Great Basin of the western USA where a period of tectonic quiescence, followed by post-volcanic extension and uplift (1 km), post-dated the Oligo-Miocene ignimbrite flare-up.

3D analysis of inverted extensional fault systems, southern Bristol Channel basin, UK

Abstract East-West trending inverted extensional fault systems offset Triassic to Lower Jurassic strata close to the southern margin of the Bristol Channel basin along the north Somerset coast. Field mapping, using exceptionally detailed aerial photographs, has revealed a three phase tectonic evolution. (i) North South orientated stretching, resulting in a well developed extensional fault system. The faults are segmented, linked by relay ramps and horsetail toward their tips. (ii) North-South oriented compression, resulting in partial inversion of the extensional fault system, with the development of hangingwall buttress anticlines and zones of intense folding. (iii) North-South orientated compression, resulting in NW-SE trending dextral and NE-SW trending sinistral strike-slip faults. Comparison of hangingwall buttress anticlines exposed in North Somerset with similar larger scale structures observed on seismic profiles from the Bristol Channel shows that both are directly analogous. Correlation with regional data on the tectonic evolution southwest England, and the Bristol Channel, indicates that extension occurred during the Jurassic-Lower Cretaceous, and contractional inversion and strike-slip deformation during the Tertiary.

Basinward migration of rift-border faults: Implications for facies distributions and preservation potential

Basinward migration of half-graben footwall margins occurs throughout the active Aegean extensional province and has significant implications for facies distributions and preservation potential in synrift fills worldwide. Recognition of this phenomenon is particularly important for predicting the preservation and distribution of hydrocarbon reservoirs in rift basins. Using an example from the Gediz graben of western Turkey, it is shown that basinward migration of footwall margins exhumes proximal facies of lateral sedimentary systems, severely limiting their preservation potential and providing a ready source of easily erodible material. This in turn leads to the generation of extensive footwall-derived depositional systems, which frequently displace axial drainage systems toward the hanging-wall dip slope.

Computer modelling of the influence of tectonics on sequence architecture of coarse-grained fan deltas

Abstract Fan delta deposition and extensional faulting are combined in a computer model using a general tectonosedimentary forward modelling equation. Fan delta forms and architectures resulting from tectonic activity are investigated using this model. Fan deltas formed during periods of tectonic quiescence are shown to have a sheet-like form and grow mainly as a result of progradation. Topset development is limited, resulting in the development of oblique to sigmoidal clinoforms. Fan deltas formed during periods of tectonic activity have a more wedge-shaped form and are restricted to a region adjacent to the controlling fault. These fan deltas grow as a result of both progradation and aggradation, resulting in the development of sigmoidal clinoforms. Under conditions of constant tectonic subsidence and sediment supply, fan deltas are shown to undergo a brief period of progradation followed by aggradation and a long period of retrogradation. This is a consequence of an increasing amount of sediment being deposited as topset material as the delta progrades and the position of the offlap break being displaced basinward during hangingwall subsidence. Other controls on fan delta architecture, such as variations in sediment supply and sea/lake level, are also considered. Pleistocene-Recent, Gilbert-type fan deltas from the southern margin of the Gulf of Corinth rift, Greece contain many of the features described above and modelling is used to investigate whether it is possible to identify the controls on their sequence architectures.