Ian Davison

Damage zone geometry around fault tips

Damage zones are described around small scale normal, strike-slip, and reverse faults cutting horizontally-bedded carbonates, shales and siltstones in the Bristol Channel basin, U.K. Two different types of brittle damage zone have been recognized: (a) fractures branching directly from the fault tip; and (b) fractures forming an en echelon array, which are disconnected from the fault tip. Similar damage zones are repeated at regular intervals along the faults and they are interpreted to represent paleo-tip lines or sticking points along fault planes. Between these zones there is little visible damage outside the fault planes, which are typically 25–1000 μm thick along small displacement (<0.1 m) normal faults. Hence, faults propagated within their own plane at the scale of observation (25 μm), but dilational out-of-plane fractures are preserved at their arrested tips. Strike-slip and thrust faults were observed to produce more variable damage zone geometries compared to those at normal fault tips. Damage zone geometries around lateral tips of normal and strike-slip faults indicate that they can form by a different mechanism than up- and down-dip tip zones. Subsidiary fracture patterns can be used to recognize the direction of fault propagation.

Basement control and transfer tectonics in the Recôncavo-Tucano- Jatobá rift, Northeast Brazil

Abstract The Reconcavo-Tucano-Jatoba rift consists of a series of asymmetric grabens which are separated by basement highs and transfer faults. Opening of the rift took place in a NW direction oblique to the N-S rift trend. Well defined transfer faults parallel the opening direction. They were responsible both for offsetting en-echelon depocenters in the Tucano and Reconcavo basins and for switching of the asymmetry of half-grabens across the Vaza-Barris fault zone. The transfer faults show characteristic features such as change of movement sense along strike and with time, and “cactus-shaped” fault structures as well as flower structures in cross section. The sigmoidal plan-form of the rift is probably due to faulting following pre-existing weaknesses through a complex mosaic of basement blocks. Basement anisotropy is thought to have controlled the asymmetry of the half-grabens and the localisation of the Vaza-Barris transfer. A 2° anticlockwise rigid rotation of the East Brazilian Microplate relative to the Sao Francisco Craton around a pole located near the eastern termination of the Jatoba Graben describes the calculated 20% extension in the South Tucano and Reconcavo grabens, the oblique northwestward opening, and the eastward shallowing of the Jatoba Graben. Gravity modelling suggests important crustal thinning, locally up to 45%, below the rift. Mantle upwarping is localized near the faulted margin of the rift. With localized thinning in a 100 km wide basin during a 20 Ma rifting event, lateral, as well as vertical heat conduction would be important and may account for the absence of a post-rift thermal subsidence phase.

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.

Overburden deformation patterns and mechanisms of salt diapir penetration in the Central Graben, North Sea

Abstract Active and passive diapirism control the deformation and geometry of hydrocarbon traps in the overburden, and a more detailed understanding of this process will help reservoir prediction and hydrocarbon recovery. Cores studies of seven Central Graben diapirs indicate Zechstein salt penetrated Late Cretaceous chalk by extreme tectonic thinning with high-angle (>70° to bedding) normal faulting, tensile fracturing and pressure solution. Attenuation of the chalk significantly weakens the overburden, allowing buoyancy forces to dome up the overburden. Doming created enough topography for downslope sliding of chalk slabs on slip planes parallel to bedding or, in the case of the Kyle diapir, for chaotic debris flows of lithified chalk. Significant extensional bedding-parallel faults and slump folds are developed within Palaeocene shale on the diapiric flanks. Inter-granular slip in unconsolidated clastic material was probably the dominant deformation mechanism. Diapirs have penetrated the Palaeocene clastic sediments by maintaining topographic relief, so that unlithified sediment continually slid off the crest, producing translated intact rafts up to several tens of metres in thickness.

Tectonic evolution of the Sergipano Fold Belt, NE Brazil, during the Brasiliano orogeny

Abstract The Sergipano Fold Belt (SFB) represents the southern segment of a Brasiliano-age (∼600 Ma) continental collision zone which extends over NE Brazil and continues into Central Africa as the North Equatorial Fold Belt. Major sinistral transcurrent fault/shear zones and thrusts separate domains with individual lithological, tectonometamorphic and magmatic characteristics. The principal steeply-dipping strike slip zones trend east-west to WNW-ESE. Lithological correlations across the shears have not proved possible, which suggests that displacements were probably large, but as yet, are unquantified. Major folding and thrusting affected all the domains. Folds have a southerly vergence with NW-SE to WNW-ESE, trends, consistent with a NE-SW to NNE-SSW shortening direction; although small differences in shortening trends exist between individual domains. The southernmost allochthonous domain of carbonate to clastic shelf-slope sediments was subject to a single phase of folding and thrusting onto the Sao Francisco Craton, accompanied by greenschist-facies metamorphism. North of this domain lies the Macurure Domain, which is separated from the southernmost allochthonous domain by the major Sao Miguel do Aleixo transpressive shear zone. Polyphase deformation, amphibolite facies metamorphism and massive granitic intrusion characterize this domain. North of the Macurure Domain lie the migmatites and granites of the Poco Redondo Domain, which represent a deeper crustal level. They are separated from the Macurure Domain by the Belo Monte Shear. A late tectonic leucogranite in the Poco Redondo Domain yields a 600 Ma Rb/Sr isochron. Finally, the Poco Redondo Domain was overthrust from the north by the Caninde Gabbroic Complex, associated with carbonates, pelites and amphibolites metamorphosed to greenschist and upper amphibolite facies; this has previously been interpreted as an ophiolite sequence. Although the relative movements or absolute ages of the domains are unknown, their distinct geological differences suggest that they are separate crustal blocks which underwent large lateral movement during the Brasiliano collision.

Geology and tectonics of the South Atlantic Brazilian salt basins

Abstract This paper first reviews the salt basins and depositional ages in the South Atlantic salt province. This comprises a series of salt basins separated by basement highs, deep graben (that never dried up), later volcanic highs and subaerial ocean spreading ridges. Initial halite and anhydrite deposition occurred first in the Sergipe-Alagoas Basin of NE Brazil at c. 124.8 Ma, and was closely followed by deposition in the Kwanza Basin, Angola between 124.5 and 121 Ma. The later potassium-magnesium-rich salts were deposited in the Sergipe-Alagoas and Gabon-Congo basins before 114.5 Ma. The age of the main Santos-Campos salt is not known precisely, but the latest anhydrites deposited on the southern margin of the Santos Basin post-date volcanic rocks dated at 113.2 Ma. The paper then compares the salt tectonics of the wide Campos-Santos Basin segment with the narrow South Bahia basins segment. Sediment loading in the Santos Basin produced a landward-dipping base salt, which led to the development of counter-regional faults, and inhibited downslope sliding, and enhanced later contractional effects caused by either gravity spreading or regional tectonic compression. Folding occurred in simultaneous pulses across the Santos Basin, suggesting that regional tectonic compression occurred. The narrow salt basins of South Bahia have a steeply dipping base salt horizon (4°) and pronounced folding, which initiates at the oceanward pinch-out of the salt and propagates back up the slope. The topographic highs, above fold anticlines, are rapidly eroded on narrow margin slopes, which allows the folds to grow more easily to large amplitudes at the top salt horizon.

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.

Wide and narrow margins of the Brazilian South Atlantic

Two contrasting end-members of passive continental margins, here referred to as wide and narrow margins, are present in the South Atlantic, although some margin segments are intermediate between these two styles. Narrow margins are characterized by a large bounding fault (>4 km throw) near the shelf edge; a sharp transition zone (10–20 km wide) from normal thickness to substantially thinned continental crust or oceanic crust; where the total width of extended continental crust is usually less than 100 km. Wide continental margins are characterized by a broad continental shelf and a wide zone (50–600 km) of thinned crust with an even distribution of faults. Wide margins usually have a more gentle continental shelf and slope. Narrow–narrow, wide–wide, and wide–narrow conjugate margin pairs exist in the South Atlantic and are suggested to be controlled mainly by the opening kinematics and the thermal structure of the lithosphere during rifting. Basement structure and strain rate appear to have exerted little control on the style of margin extension. Although basement architecture appears to control the trend of the initial rifts, the positions of fracture zones which separate narrow and wide segments, and the rate of lateral rift propagation.

Tectonics and hydrocarbon distribution along the Brazilian South Atlantic margin

Abstract Break-up of the Brazilian margin produced a wide variety of structural styles depending on opening kinematics and thermal configuration of the lithosphere. Wide margins developed in Santos and Campos, where stretched continental crust extends for up to 600 km from the shoreline; whereas narrow margins may be only 40 km wide. The wide margins host the largest oil fields, such as those discovered in the Campos Basin. Wide margins have good hydrocarbon potential in the post-rift sequence, but narrow margins probably have the better unknown syn-rift plays which have still to be drilled in deepwater. Little is known of the narrow margins, such as the Jacuípe and Pernambuco-Cabo Basin area, as most of the prospective hydrocarbon structures lie in water depths > 1500 m. Hydrocarbons are hosted by reservoirs ranging from fractured Neocomian volcanics to Miocene turbidites, with over 80% of the reserves trapped in the Campos Basin turbidite sandstones. These sandstones occur as constrained channel-fills on the upper platform and slope, and as unconstrained lobes up to 30 km in long dimension on the mid to lower continental slope which are situated in 500+ m water depths at the present day. The wide margins will continue to be the most productive in Brazil, but deepwater production technology has advanced enough to make the deeper parts of the narrower margins attractive exploration areas.

Thrust Tectonics and the Role of Evaporites in the Ionian Zone of the Albanides

This study focuses on the Alpine fold and thrust belt in the Ionian basin of Albania, where there are giant onshore oil fields. The evaporites have acted as a detachment layer but also have been cut by thrusts that carry evaporites to the surface in their hanging walls. Where the detachment is located at the top of the evaporites, they are dragged up in the footwall along thrusts, and faults develop at the base and the top of the evaporite. Simple shear deformation along the thrusts causes greater than 80% thinning of the evaporite, so that sheets of only 40-200 m thickness are preserved, whereas the original evaporite thickness probably was well in excess of 2000 m. Where ramps cut through the evaporite and cause repetition of the complete evaporite layer, the salt is remobilized into salt-filled compressional anticlines or escapes at the surface to produce smeared-out surface extrusions. The most spectacular example of this is the Dumrea structure, where the evaporites have developed into a complex shape from an overthrusted allochthonous sheet that has been extruded at the surface. The Dumrea structure is up to 5.6 km thick proved by drilling and has been displaced laterally by up to 25 km, and it has the effective geometry of a giant sheath fold.