Major Strike-slip Fault System Research Articles

Regional 2D seismic interpretation and exploration potential of offshore deepwater Sierra Leone and Liberia, West Africa

The offshore region of Sierra Leone and Liberia has been underexplored on the continental shelf and unexplored in deepwater. However, a reconnaissance interpretation of new 2D seismic data, in conjunction with a review of the reservoir and geochemical information available to date, is positive about the hydrocarbon potential of the region. Three distinct prospective hydrocarbon basins are present—Sierra Leone Basin, Liberia Basin, and Harper Basin (Figure 1). Figure 1. Location of Sierra Leone, Liberia, and Harper basins. Fault lines (blue) indicate major strike-slip fault systems. Nine exploratory wells have been drilled and abandoned on the shelf, at the limits of the study area, considerably shoreward of the potential deepwater basins. In terms of exploration maturity, considering a well density of one well per 6700 km2, the area should be classified as “frontier.” However, the regional potential is far from unknown. The nine wells provide an abundance of key data, which contribute to the identification of several hydrocarbon systems. The primary petroleum system contains at least three oil-prone, marine, and lacustrine source rock zones deposited during Lower Cretaceous time (Early Albian to Early Ceno-manian). A secondary petroleum system may have a Late Cenomanian to Turonian shale source rock. Trap types are numerous and relatively widespread, in particular those associated with Cretaceous syn-rift and transform related faulting and unconformities. In addition, traps on the slope and in the basin are ideally located for access to oil migration. The basins developed in two phases, a syn-rift phase and a passive margin phase, both significantly overprinted by wrenching associated with Atlantic transform fault systems. The main rift phase, accompanied by continental to marginally marine sedimentation, took place from Lower Cretaceous Aptian to Middle Albian time. The passive margin and wrenching phase, initiated with seafloor spreading, began in Late Albian time and continues to the …

SEISMIC DELINEATION OF STRUCTURE-CONTROLLED CARBONATE CEMENT AND POTENTIAL ECONOMIC IMPLICATIONS, ANGEL FIELD, NORTH WEST SHELF

Major carbonate-cemented zones occur in Late Jurassic Angel Formation sandstones of marine mass flow origin that contain large hydrocarbon reserves in the Angel Field, Dampier Sub-basin. Preliminary results suggest that poikilotopic dolomite cement is dominant. The carbonate-cemented zones are identifiable from wireline log response and 3D seismic data, and occur in discrete intervals with a cumulative thickness of approximately 165m at Angel-2. These intervals produce a zone of high amplitude reflections of about 100 ms two-way time. Field-wide seismic mapping indicates that these carbonate-cemented zones sharply abut the northern margin of a major east-west trending strike-slip fault system that traverses this field. The carbonate-cemented zones extend in a wedge-like shape towards the northeast and concentrate along the crest of the main structural trend.The results underscore the importance of 3D seismic data for a better estimation of reservoir risk and reserves in variably carbonate-cemented sandstones.The carbonate-cemented zones may represent a 'plume' related to migration of petroleum and/or carbon dioxide. Therefore delineation of major carbonate-cemented zones using seismic data may aid in the identification of petroleum migration pathways and pools in the North West Shelf. Alternatively, carbonate cements dissolved south of the major fault zone and possibly in downdip locations in which case dissolution pores may exist in these areas. Further research is required to evaluate these hypotheses.

Aspects of sedimentary basin evolution assessed through tectonic subsidence analysis. Example: northern Gulf of Thailand

Tectonic subsidence and subsidence rate analyses were conducted using a forward burial technique for the Cenozoic sediments of the northern Gulf of Thailand, a region presently bounded and intersected by major strike-slip fault systems. Basins represented by the seven wells studied are the Thon Buri, Hua Hin, Chumphon, Kra, and Pattani basins. The total observed subsidence was stratigraphically calibrated using well biostratigraphy and/or regional seismic stratigraphy. Tectonic subsidence was subsequently determined assuming local Airy isostasy by correcting decompacted sediments for sediment loading and variations in paleowater depths. Statistical comparison of the observed tectonic subsidence profile versus the theoretical thermal subsidence profile reveals zero-intercept times of incipient thermal-rifting and furthermore helps differentiate times of thermal subsidence from episodes of fault-controlled mechanical subsidence. Differences in tectonic subsidence, tectonic subsidence rates, and in the zero-intercept times of thermal rifting imply the Paleogene thermal associated rifting of the northern Gulf of Thailand was neither restricted spatially nor universally synchronous among the basins, but instead both spatially and time transgressive. Although coupled thermal-mechanical subsidence played a major role in the evolution for most of these basins, in some basins, e.g. the Thon Buri and northern Kra basins, subsidence was not thermally initiated. Instead, these basins experienced relatively slow-paced sediment loaded subsidence until a sudden fault-associated acceleration commenced in the Pliocene. Additional evidence for temporal and spatial changes in local strain is demonstrated by observed asynchronous episodes of “see-saw” subsidence-uplift of the basin floors. For example, while the northern Hua Hin Basin experienced Miocene-Pliocene alternations of subsidence and uplift, portions of the Pattani Basin to the southeast underwent periods of subsidence and uplift which differed in magnitude and phase. In comparison to the tectonic subsidence rates compiled for basin classes worldwide, these Gulf of Thailand basins exhibit time-transient characteristics of cratonic, rift, and wrench style basins. The opening of the northern Gulf of Thailand, basin formation, and subsequent basin evolution have varied both in time and space. Therefore, in terms of hydrocarbon potential, it would be both inappropriate and inaccurate to suggest the thermal histories and subsequent hydrocarbon maturities of these wrench-associated basins to be equivalent. Though tectonically related, each basin has had its own geohistory.

Neogene lateral extension during convergence in the Central Alps: Evidence from interrelated faulting and backfolding around the Simplonpass (Switzerland)

The Simplon Fault Zone occurs at the western end of the Lepontine metamorphic dome in the Central Alps and represents a major, low-angle (25° dip) normal fault associated with WSW-ENE-directed lateral extension. Movement began in the early Miocene and continued until at least 3 Ma, accumulating a total vertical throw of around 10 km, corresponding to 24 km displacement parallel to the fault. This displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic “detachment fault” (the Simplon Line). In the central region (Simplon village to Zwischbergen), some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone. Due to the higher exposed structural level, the importance of the cataclastic component increases towards the northwest. In the northern “steep belt” around the Simplonpass, retrograde greenschist-facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with SSE-vergent, large-scale backfolding, representing NNW-SSE shortening and crustal thickening. The combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen, allowing lateral escape of continental material. Normal faulting and the resultant lateral escape was not just superficial. It extended to depths of at least 15 km, and probably to depths exceeding 25 km. The overall geometry and kinematics of the Simplon Fault Zone, and in particular the interplay between folding and faulting, is very similar to that described from the metamorphic core complexes and detachment faults of the Cordillera of the southwestern USA. Within the Central Alps, extension was associated with rapid exhumation of the Lepontine metamorphic dome, as evidenced by very rapid cooling rates determined from mineral ages in the footwall. Similar to the Cenozoic tectonics of the Himalayas and Tibet, orogen-parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction. More limited extension of the same age and orientation is known from a large area of the European foreland. Greater extension in the mountain belt compared with the foreland was accommodated by major strike-slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern Alps of Haute Provence (France). Coeval folding and orogen-parallel extension, such as at the Simplonpass, were developed in a dextral transpressive regime due to oblique WNW-ESE convergence across the Alps during the Neogene.

CONTINENTAL SHELF BASINS ON THE WEST TASMANIA MARGIN

The continental margin of western Tasmania is underlain by the southern Otway Basin and the Sorell Basin. The latter lies mainly under the continental slope, but it includes four sub-basins (the King Island, Sandy Cape, Strahan and Port Davey sub-basins) underlying the continental shelf. In general, these depocentres are interpreted to have formed at the 'relieving bends' of a major left-lateral strike-slip fault system, associated with 'southern margin' extension and breakup (seafloor spreading). The sedimentary fill could have commenced in the Jurassic; however, the southernmost sub-basins (Strahan and Port Davey) may be Late Cretaceous and Paleocene, respectively.Maximum sediment thickness is about 4300 m in the southern Otway Basin, 3600 m in the King Island Sub-basin, 5100 m in the Sandy Cape Basin, 6500 m in the Strahan Sub-basin, and 3000 m in the Port Davey Sub-basin. Megasequences in the shelf basins are similar to those in the Otway Basin, and are generally separated by unconformities. There are Lower Cretaceous non-marine conglomerates, sandstones and mudstones, which probably include the undated red beds recovered in two wells, and Upper Cretaceous shallow marine to non-marine conglomerates, sandstones and mudstones. The Cainozoic sequence often commences with a basal conglomerate, and includes Paleocene to Lower Eocene shallow marine sandstones, mudstones and marl, Eocene shallow marine limestones, marls and sandstones, and Oligocene and younger shallow marine marls and limestones.The presence of active source rocks has been demonstrated by the occurrence of free oil near TD in the Cape Sorell-1 well (Strahan Sub-basin), and thermogenic gas from surficial sediments recovered from the upper continental slope and the Sandy Cape Sub-basin. Geohistory maturation modelling of wells and source rock 'kitchens' has shown that the best locations for liquid hydrocarbon entrapment in the southern Otway Basin are in structural positions marginward of the Prawn-1 well location. In such positions, basal Lower Cretaceous source rocks could charge overlying Pretty Hill Sandstone reservoirs. In the King Island Sub-Basin, the sediments encountered by the Clam-1 well are thermally immature, though hydrocarbons generated from within mature Lower Cretaceous rocks in adjacent depocentres could charge traps, providing that suitable migration pathways are present. Whilst no wells have been drilled in the Sandy Cape Sub-basin, basal Cretaceous potential source rocks are considered to have entered the oil window in the early Late Cretaceous, and are now capable of generating gas/condensate. Upper Cretaceous rocks appear to have entered the oil window in the Paleocene. In the Strahan Sub-Basin, mature Cretaceous sediments in the depocentres are available to traps, though considerable migration distances would be required.It is concluded that the west Tasmania margin, which has five strike-slip related depocentres and the potential to have generated and entrapped hydrocarbons, is worthy of further consideration by the exploration industry. The more prospective areas are the southern Otway Basin, and the Sandy Cape and Strahan sub-basins of the Sorell Basin.

North Caribbean neotectonic events: The Trans-Haitian fault system. Tertiary record of an oblique transcurrent shear zone uplifted in Hispaniola

The left-lateral relative motion between the Caribbean and the North American plates has previously been inferred as occurring along a fault zone located north of Hispaniola. East of the northern Dominican Republic, a relatively linear fracture zone (the Septentrional Fault Zone, SFZ) extends into the Puerto Rico subduction zone. Similarly, south of Haiti, the trace of the inactive Enriquillo-Plantain Garden Fault Zone (EPGFZ) extends into the Muertos Trench and may represent a major fault along the plate boundary. Between these two major strike-slip fault systems, the central part of Haiti shows a diffuse fracture zone that trends N130. The chronology of deformation involves an initial Paleocene to Eocene suturing of allochthonous terranes, a Late Miocene development of a strong spaced cleavage within the late Paleocene to Late Miocene strata that overlap the terrane suture, and diffuse Pliocene to Pleistocene strike-slip faulting along traces that reactivate the older spaced cleavage planes. During the Pleistocene, basalts ranging in age from 0.4 to 1.3 Ma were extruded in pull-apart basins associated with N130-trending faults (antithetic features of the SFZ) and N30 normal faults (antecedent synthetic features of the SFZ). This event is coeval with the compression recorded at the Muertos-Beata collision front in the southwestern Dominican Republic. The most recent phase of tectonism involves strong uplifts and broad, open folding along NW-striking axes, which is consistent with the regional maximum deformation pattern predicted for E-W left-lateral shear along the North America—Caribbean plate boundary.

Archaean greenstone belt tectonism and basin development: some insights from the Barberton and Pietersburg greenstone belts, Kaapvaal Craton, South Africa

The sediments in two of South Africa's major Archaean greenstone belts, the Barberton and Pietersburg greenstone belts, span an age range of some 800 million years. Both greenstone belts represent remnants of extensive fold and thrust belts with complex, but different polyphase tectonic histories. The oldest sediments were deposited between circa 3470 and 3490 M.a. on oceanic like crust preserved in the Barberton belt, possibly at the same time as sedimentation on similar oceanic crust preserved in the Pietersburg belt. Thereafter, the geologic evolution of these two belts diverged considerably. In the Barberton belt, there is clear evidence that the oceanic crust and sediments were obducted onto an intra-arc basin environment within 50 million years of its formation. The sequence was later further imbricated by northwest directed thrust stacking between 3300-3200 M.a. Basin development during both periods of thrusting took place in close proximity to active “calc-alkaline” arc systems. Deformation of the sediments within these basins took place while the same sediments were being deposited. Sedimentation took place predominantly in subaqueous environments, ranging from submarine mid-fans below the photic zone to tidal flats and deltaic plains. The sediments represent a polyhistory successor-type basin: early basins developed along a complex subduction related plate boundary; these basins later evolved into foreland depositories along and within collisional environments of an accretionary orogen. Late in the history of the Barberton greenstone belt (circa 3100 M.a.), the rocks were in places thermally reactivated and probably subjected to extensional processes; these processes overlapped in time with the main episodes of economic gold mineralization, and are of “early Witwatersrand-basin” age. The oceanic-like crust (including associated sediments) preserved in the Pietersburg belt was not significantly deformed until at least 500 million years after its formation: tectonic imbrication of this crust occurred between about 2900 and 2700 M.a., during north-directed thrusting. Sedimentary basins formed in response to this thrusting and associated bulk-flattening, and the sediments in these basins were deformed during sedimentation. Sedimentation took place predominantly in subaerial environments, in alluvial fans of an active foreland basin. It is likely therefore that the oceanic-like crust had previously been tectonically emplaced onto a continental environment as an allochthonous sheet, but the timing of this is not clear. The foreland basin of the Pietersburg belt which developed on top of this simatic basement is almost certainly part of the greater upper Witwatersrand basinal systems; it was most likely formed in an intermontane tectonic setting of the Cordillera-type such as that along the Mesozoic-Cenozoic margins of western North America. Late in the history of the Pietersburg belt ( circa 2.7 G.a.), the rocks were tectonically deformed in a major strike-slip fault system with both transpressional and transtensional kinematics. Sediments which were deposited and deformed in local contemporaneous basins, form an integral part of the greenstone belt's lithotectonic sequence, and are probably of Ventersdorp age. Unravelling the kinematic details of the basin evolution in both greenstone belts is now a major challenge for further understanding of the tectonic evolution of these belts, and almost the entire 800 million years of Archaean history of the Kaapvaal craton as recorded in these belts. A better understanding of the intimate relationship of tectonism to basin formation from greenstone belt studies, is of fundamental importance to understanding the famous Witwatersrand gold deposits and the source of this gold. This major task can only be achieved through better integrated and focused field and laboratory studies, in which precise chronostratigraphy must play a central role.

Sedimentology, structural evolution and tectonic setting of the late Precambrian Longmyndian Supergroup of the Welsh Borderland, UK

Abstract The Longmyndian Supergroup is an approximately 6500 m thick sedimentary sequence of late Precambrian age. Major unconformities within this sequence are lacking. The Longmyndian Supergroup can be divided into four facies associations: (1) turbidite, (2) subaqueous delta, (3) alluvial floodplain, and (4) braided alluvial. These are organized into a single, broadly upwards-coarsening, progradational sequence, with the turbidite association at the base and the braided alluvial association towards the top. The sediments are interpreted to have been deposited in a forearc, or possibly interarc, basin which lay to the north of a contemporaneous magmatic arc, of which the (now adjacent) Uriconian Volcanic Complex is considered to have been a part. The Longmyndian Supergroup has been deformed into a tight NNE-SSW trending syncline. Fault patterns and strain markers within the Longmyndian indicate that it was deformed during sinistral transpression. The adjacent Pontesford-Linley and Church Stretton fault systems are considered to be major strike-slip fault systems which were initiated at about the same time as the deformation of the Longmyndian Supergroup. Strike-slip movements along these fault systems resulted in the juxtaposition of the Uriconian Volcanic Complex with the Longmyndian Supergroup. Late Precambrian/early Cambrian deformation is indicated by some stratigraphic and radiometric age-dating evidence. Since there is evidence that the fault systems have suffered strike-slip displacements during the Palaeozoic it is difficult to be certain of the magnitude and style of the late Precambrian/early Cambrian deformation. However, the recognition of late Precambrian/early Cambrian sinistral transpressional deformation in northwest Wales suggests that the Precambrian of the Welsh Borderland was similarly deformed during the late Precambrian/early Cambrian.

Plate tectonic evolution of the Mediterranean-Middle East region

An interpretive model for the Mesozoic-Cenozoic plate tectonic evolution of the Mediterranean and adjacent areas is illustrated by a series of paleoposition maps at selected intervals between the Late Triassic and Recent. This interval witnessed an important period of tensional development during the Triassic and Jurassic that fragmented Pangea after its Late Paleozoic consolidation. A number of oceanic areas evolved through Jurassic time, all of which have since been consumed during the Alpine orogeny. During the Cretaceous and Tertiary, sea-floor spreading geometry in the North and South Atlantic resulted in convergence between Africa and Eurasia that controlled the evolution of the Mediterranean and adjacent Middle East areas from the Late Mesozoic to Recent. The interpretation departs from several previous ones in the following respects: 1. (1) The Eastern Mediterranean and adjacent Middle East area are interpreted to have developed as two seaways with an intervening continental sliver derived from Africa that now comprises Central Turkey and West Iran. These seaways persisted from the Late Triassic to Late Cretaceous, by which time they were largely consumed by northward subduction. 2. (2) Spreading that produced the present Eastern Mediterranean Sea developed during the Late Cretaceous, driving the Tripolitza-Eratosthenes-Iskenderun continental fragments northward from Africa to collision with Central Turkey as the preceding southern arm of the Mesozoic Tethys sea-floor was entirely consumed. Initial block faulting in this zone occurred during the Jurassic and Early Cretaceous but without subsidence reaching clearly bathyal depths until the Cenomanian-Turonian. This differs from the timing of events in the “Ionian” sequence of the Pindic Nappe of Crete, which shows a change from shallow carbonate shelf in the Triassic to deep basin in the Liassic to the north of equivalent platform facies of the Tripolitza Nappe. ∗ ∗ Sestini (1984). 3. (3) Moesia and Rhodope are concluded to have been a single plate until a rifting event that started during the Late Jurassic. The Balkan Mountains are interpreted to represent an Alpine inversion of this Mesozoic rift system that extended west from the Black Sea Basin. 4. (4) The Eastern Carpathian microcontinental sliver is concluded as continuing southward as the Serbo-Macedonian Massif that separates Rhodope from the Vardar Zone rather than as Rhodope itself. 5. (5) The Scutari-Pec Lineament is considered as representing a major discontinuity between the Apulia-Pannonian plate and adjacent Greece and Turkey. Ophiolites to the northwest of this lineament originated in a single oceanic area, the Vardar Zone. Ophiolites to the southeast of this lineament evolved from two oceanic areas, those of interior Greece and northern Anatolia and the associated Ankara melange relating to the Vardar Zone, the former north arm of Tethys. and the Pindos-southern Anatolia-Zagros-Oman Ophiolites relating to a former south arm of Mesozoic Tethys. 6. (6) Based on the history of development of various elements of the Alpine system, there is a strong suggestion that important dextral movement took place along a major strike-slip fault system reaching from the Middle East to the Central Alps between Late Cretaceous and Middle Eocene times.

Seismotectonics of western New Guinea.

We study seismic activity and focal mechanisms in the vicinity of Indonesian New Guinea (Irian Jaya) in order to clarify the tectonics of this region. Epicentral distribution and vertical profiles of earthquake foci across New Guinea indicate that seismic activity is not associated with the two principal geologic features, the New Guinea Trench and the Medial Mountains, but rather is concentrated in the region between them. We find no seismological evidence of subduction along the New Guinea Trench west of 140°E; moreover, one well-constrained focal mechanism at the New Guinea Trench yields a strike-slip focal mechanism.The region between the New Guinea Trench and the Medial Mountains is characterized by strike-slip faulting and reverse faulting. The focal mechanism of the 1979 Ms =7.6 Yapen Island earthquake off the northwestern coast of New Guinea suggests that active strike-slip motion is occurring along this portion of the Sorong fault system. Thrust-type mechanisms with one nodal plane dipping steeply to the northeast, including that of the 1971 Ms = 8.0 earthquake, are most common in the Meervlakte Basin area between the northern coastal range and the Medial Mountains. Additional strike-slip faulting and thrust faulting are seen in the Tarera and Wandamen Fault Zones in Bird's Neck, and south of the Medial Mountain Suture Zone.We suggest that the oblique convergence between the Caroline and Australian plates in western New Guinea is divided into the strike-slip component represented by the left-lateral shear motion along the major strike-slip fault systems and the dip-slip component represented by the intraplate thrusting such as the reverse faults in the Meervlakte Basin. East of 140°E along the New Guinea Trench, the Bismarck plates are subducting, producing the intermediate-depth seismicity beneath central New Guinea.