Rift Faults Research Articles

Melting of metasomatized subcontinental lithosphere: undersaturated mafic lavas from Rungwe, Tanzania

This paper uses the geochemistry of primitive mafic lavas from the Rungwe volcanic province (southwestern Tanzania) to infer the source mineralogy and melting history. Post-Miocene mafic lavas from Rungwe include alkali basalts, basanites, nephelinites and picrites with up to 18.9 wt% MgO; nephelinites (>13.5% normative nepheline) are restricted to Kiejo volcano in the southern portion of the province. Rungwe lavas differ from most Western Rift volcanics in that they are not unusually potassic (K2O/Na2O ca. 0.40). Sparsely phyric mafic lavas contain phenocrysts and xenocrysts of plagioclase (An82–90), clinopyroxene (4.5–9.5 wt% Al2O3), and olivine (Fo79–88); one basanite contains a 1 mm xenocryst of apatite included in magnesian clinopyroxene. All samples have high abundances of incompatible elements (e.g., 0.7–2.2 wt% P2O5) and are enriched in REE relative to HFSE (Hf, Zr, Ti, Y), Cs, Ba, and K. Some incompatible element ratios are constant throughout the Rungwe suite (e.g., Zr/Nb, Sr/Ce, K/Rb), but other ratios are extremely variable and exceed the range measured in global Ocean Island Basalts (OIB) (e.g., Ba/Nb, Sm/Zr, La/Nb, Pb/Ce, Nb/U). The range in degree of silica saturation, and its excellent correlation with P2O5/Al2O3, indicate that the Rungwe suite records variable degrees of melting. Variations of individual incompatible trace element abundances in nephelinite and basanite samples suggest that the source contains metasomatic amphibole, ilmenite, apatite, and zircon. The Rungwe suite is interpreted as a series of low-percentage melts of CO2-rich peridotite at pressures that span the garnet-spinel transition. A geochemical comparison of Rungwe samples to lavas from other Western Rift volcanic centers requires that the source mineralogy varies along the rift axis, although each province is underlain by metasomatized peridotite. The incompatible trace element signatures of Western Rift lavas indicate that the source area is typically homogeneous on the scale of individual volcanoes, although lavas from each volcano reflect a range in degree of melting. Significantly, volcanoes with distinct geochemistry are always separated by major rift faults, suggesting that volcanic and tectonic surface features may correspond to metasomatic provinces within the subcontinental lithospheric mantle.

Rejuvenation of the Tethyan passive continental margin of northern Sinai: deformation style and age (Gebel Yelleq area)

Detailed field mapping and structural analysis of the northeastern and southeastern parts of Gebel Yelleq (the largest fold among the northern Sinai folds) indicate the presence of two ENE-oriented dextral wrench zones. The northern (Meneidret El Etheili) wrench zone is represented by right-stepped, en échelon doubly plunging folds whereas throughgoing wrench faults characterize the southern (Minsherah) wrench zone. The steep (southeastern) flank of Gebel Yelleq asymmetric fold makes an acute angle with the wrench zones and is orthogonal to the NW-SE-directed shortening direction indicated by mesostructures. Similar structural relations are evident in the other major folds of northern Sinai, Gebels El Maghara and Halal. A structural model is proposed for the deformation of northern Sinai. Late Cretaceous-Tertiary rejuvenation of the early Mesozoic passive continental margin of the Neotethys proceeded via right-lateral wrenching in northern Sinai and is related to the oblique convergence between Africa-Arabia and Eurasia during the closure of the Neotethys. As a result, wrench zones developed above the early rift faults, e.g., Meneidret El Etheili and Minsherah wrench zones. Also, NW-SE-directed shortening related to and parallel to the convergence of Africa-Arabia and Eurasia deformed the blocks lying between the wrench zones through the development of NE-SW-oriented thrust faults, at acute angles to the wrench zones. These thrusts ruptured the Precambrian basement and propagated upward through the overlying sedimentary cover as listric thrust faults forming large fault-propagation folds in Gebels Yelleq, El Maghara and Halal. These thrusts underlie the steep southeastern flanks of these folds and are exposed in Gebels El Maghara and Hamayir. The depth to detachment of these thrusts progressively increases southward to a value, underneath the Gebel Yelleq area, close to the brittle-ductile transition of the crust. The deformation reached its zenith in the early late Senonian and probably continued into the Early Miocene. The northern Sinai fold belt characterizes an unstable crustal block separated from a tectonically stable crustal block to the south, in central and southern Sinai, by a belt of ENE-oriented closely spaced faults forming what is called the central Sinai “hinge belt”.

A mechanical theory of normal fault orientation

Some regional fracturing can be difficult to explain in terms of Anderson's shear-based theory of faulting. An alternative strain-based theory leads to definite predictions for orientations of extensional faulting, including regional normal faulting, rift faulting, sub-vertical jointing and cleating, and some forms of shrinkage cracking. The examples we present are quite typical, and qualitatively bear out the theory predictions.

The North Tanganyika hydrothermal fields, East African Rift system: Their tectonic control and relationship to volcanism and rift segmentation

The two branches of the East African Rift system include numerous hydrothermal fields, which are closely related to the present fault motion and to volcanic and seismic activity. In this study structural data from Pemba and Cape Banza hydrothermal fields (western branch, North Tanganyika, Zaire) are discussed in terms of neotectonic phenomena. Different types of records, such as fieldwork (onshore and underwater) and landsat and spot imagery, are used to explain structural controls on active and fossil hydrothermal systems and their significance. The Pemba site is located at the intersection of 000–020°-trending normal faults belonging to the Uvira Border Fault System and a 120–130°-trending transtensional fault zone and is an area of high seismicity, with events of relatively large magnitude ( M s < 6.5). The Cape Banza site occurs at the northern end of the Ubawari Peninsula horst. It is bounded by two fault systems trending 015° and is characterized seismically by events of small magnitude ( M s < 4). The hydrothermal area itself is tectonically controlled by structures striking 170–180° and 080°. The analysis of both hydrothermal areas demonstrates the rejuvenation of older Proterozoic structures during Recent rift faulting and the location of the hydrothermal activity at the junctions of submeridian and transverse faults. The fault motion is compatible with a regional direction of extension of 090–110°. The Cape Banza and Pemba hydrothermal fields may testify to magma chambers existing below the junctions of the faults. They appear to form at structural nodes and may represent a future volcanic province. Together with the four surface volcanic provinces existing along the western branch, they possibly indicate an incipient rift segmentation related to ‘valley-valley’ or ‘transverse fault-valley’ junctions, contrasting with the spacing of the volcanoes measured in the eastern branch. These spacings appear to express the different elastic thicknesses between the eastern and western branches of the East African Rift system, perhaps related to a difference in stage of evolution of the two branches.

The Palaeoproterozoic Ubendian shear belt in Tanzania: geochronology and structure

The Ubendian belt is a linear, NW-SE trending orogenic belt in western Tanzania. It is part of a larger Palaeoproterozoic orogen, developed around the west and south-western margin of the Archaean Tanzanian craton. The Ubendian Belt has experienced several periods of reactivation since the Palaeoproterozoic, acting as a zone of displacement during successive orogenic and rift-forming events. The Ubendian Belt is characterized by an early deformation and granulite-facies metamorphism, isotopically dated at 2100–2025 Ma, and marked by an E-W to ESE-WNW trending foliation. This phase also affected the adjacent Usagaran (and Bangweulu?) Belt and is interpreted as a product of collisional orogeny along the SW margin of the Tanzanian, and possibly Congo, cratons. A second phase of deformation, apparently restricted to the Ubendian Belt, is characterized by large, NW-SE trending, dextral shear zones. This phase is responsible for the creation of the eight crustal blocks developed throughout the belt and overprints much of the earlier deformation fabric. This second deformation phase is terminated by late- to post-kinematic calc-alkaline granitic batholiths ( ca 1860 Ma). A phase of tectonic reactivation occurred locally at ca 1725 Ma. Notable by its absence, is any evidence of Kibaran isotopic ages in the Ubendian Belt. Major Kibaran orogenic belts occur to the north (Burundi) and south (Zambia) of the Ubendian Belt, although they are apparently not isotopically recorded within it. A third phase of deformation is characterized by Neoproterozoic ( ca 750 Ma) reactivations of the Ubendian shear zones. This deformation is marked by the development of sinistral, brittle-ductile shear zones, displaying retrograde metamorphic mineral assemblages, and intruded by alkaline plutons. These late shear zones were the preferential locus for the brittle rift faults of the western branch of the East African Rift.

An earthquake study in the Lake Baringo basin of the central Kenya Rift

A 100 × 80 km 2 earthquake recording network was operated for three months (January–March 1990) in the Lake Baringo region of the Kenya Rift Valley. Twenty-nine seismic sites were occupied by short-period stations over a region including the Elgeyo escarpment, the Kerio Valley, the Tugen Hills and Lake Baringo itself. Eighty local events of M L < 2.0 have been located within 50 km of the network. These events are situated within the central part of the rift, showing some association with the main rift faults but mainly clustering beneath Lake Baringo at a depth of about 5 km and occurring as swarm activity. Ninety percent of the events occurred at depths shallower than 12 km. The brittle-ductile transition zone in this area is determined at a depth of 12–16 km, similar to that in the Lake Bogoria area immediately to the south. A preliminary study of focal mechanisms of suitable events indicates WNW-ESE extension across the Lake Baringo basin and suggests the presence of sub-vertical rupture surfaces beneath the lake. These may be caused by the insertion of basic dykes into the upper crust, and the rupture process assisted by high temperature and pressure geothermal fluids. The inversion of local P-wave arrival time data for the upper crustal velocity structure beneath Lake Baringo identifies a low-velocity zone beneath surface geothermal activity, a relationship previously demonstrated for the Lake Bogoria region.

The geothermal fields of the Kenya rift

From the standpoint of geothermal energy, Kenya's resources are due to the presence of the Kenya rift which is part of the East African rift system. Geological, geophysical and geothermal studies indicate that Neogene volcanic activity has led to the presence of near surface heat generating sources. Geothermal fields of the Kenya rift occur in two types of environments. The main geothermal fields are associated with Quaternary volcanoes. The second type is associated with fissures that are related to active fault zones. In either case, these fields are dissected by numerous rift faults that give rise to a number of geothermal springs and fumaroles.

Potential field studies of continental rifts—the Great Lakes region: Introduction

The ongoing tectonic activity of modern continental rifts commonly results in striking physiographic forms, intense volcanism, and seismicity, all of which are of special interest to geoscientists. As a result, they have been investigated since the very inception of scientific geological studies. Continental and oceanic rifts are fundamental elements of modern plate tectonic theory. Plate tectonics, apparently, has operated in a similar fashion for at least 2500 Ma and ancient rifts date back to Early Proterozoic time. These paleorifts, or more specifically the remnants thereof, are generally no longer active under an extensional stress field. An outstanding example is the roughly 2000 km long North American Midcontintent Rift (MCR) System, dated at about 1100 Ma. For most of its length it remains buried beneath Phanerozoic sedimentary cover of the continental interior. The rift surfaces only in the Great Lakes region, mainly around the shores of Lake Superior, and developmental theories relating to the rift have been established primarily on the basis of geological and geophysical data obtained in this area. The series of papers presented in this volume focuses on the Lake Superior segment of the MCR. Paleorifts are of interest for both scientific and practical reasons. Ancient rift faults are potential zones of weakness that may be reactivated under ambient stress fields totally different from the original rift-causing extensional forces. As a result, paleorifts may be a seismic hazard in an otherwise aseismic craton. A corollary of this is that related structural deformation may be propagated upward, modifying the surface morphology, which in turn may control the lithofacies of sediments being deposited in overlying basins. These structures and lithologic variations may have an important role in the hydrologic regime of sedimentary basins and the entrapment of hydrocarbons. These basins are often the host for important petroleum deposits, as a significant proportion of the world's known petroleum reservoirs occurs in paleorifts. Furthermore, numerous mineral resources of sedimentary and hydrothermal origin occur within rift-fill assemblages, as well as within overlying, late-stage sedimentary basins. The study of modern rifts is important because of the oppor-

The influence of preexisting structure on the evolution of the Cenozoic Malawi rift (East African rift system)

This paper analyzes the importance of preexisting structure for the evolution of the Cenozoic Malawi rift, which constitutes the southernmost part of the western branch of the East African rift system. Kinematic analyses demonstrate that the regional extension direction rotated clockwise from ENE to SE during rifting. Cenozoic rift faults (of dip‐, oblique‐, and strike‐slip character) rejuvenated crustal structures whenever those were properly oriented with respect to the extension directions. Depending on these boundary conditions the rift attempts to find the easiest pathway in order to develop in a mechanically modest way. Proterozoic basement structures represent the basic anisotropy which already influenced pre‐Cenozoic rifts and also the Cenozoic Malawi rift. Amongst these basement structures, Pan‐African retrograde to cataclastic, strike‐slip shear zones have the strongest influence on later rift structures. Rift‐related transfer faults appear as differently striking passive structures, and their orientation seems to be controlled by a variety of steep basement structures (retrograde shear zones, foliation planes, and fracture arrays).

Extension tectonics offshore Namibia and evidence for passive rifting in the South Atlantic

The Namibian offshore region has been studied using more than 14 300 km of new multifold seismic data to provide insights into the history of lithospheric extension in the South Atlantic. Initial subsidence related to crustal stretching began at least as early as Permo-Triassic times. Continued stretching caused multiphase rifting which migrated northwards with time, as broadly recognized in other South Atlantic basins north of the Agulhas fracture line. Rifting was followed by thermal subsidence in the Late Cretaceous and Tertiary. Geophysical data indicate that south of the Walvis Ridge, major rift faults dip landwards. A structural model based on reflection seismic, gravity and magnetic data suggests that rifting was asymmetric and accommodated by the development of a major easterly dipping crustal shear and mantle wedge. The model suggests that thinning of the crust does not correspond vertically with synrift sedimentation represented in the Central Half Graben. Seaward dipping rift faults north of the Walvis Ridge suggest that the major crustal shear dips westwards. This rift geometry may have been produced by simple shear analogous to that of the Basin and Range model of extension. The analogy is corroborated by major onshore structural features and the distribution of silicic volcanics on both sides of the South Atlantic. Our model suggests that initial Gondwanaland break-up was accomplished by simple shear related to passive stretching of the crust. As there was an association between rifting in the South Atlantic and subduction processes active on the Pacific margin of West Gondwanaland, the stretching may be related to changes in plate boundary stresses. Close to the convergent margin in South America, rifting was influenced by the north-west trending Late Permian accretionary fabric, but the main break was parallel to the Late Proterozoic-Cambrian Pan African-Braziliano sutures of the Lavalleja-Dom Feliciano and Ribeirio Belts.

Crustal structure associated with Gondwana graben across the Narmada-Son lineament in India: An inference from aeromagnetics

Aeromagnetic data over an 80-km-wide belt along the ENE-trending Narmada-Son lineament (NSL), starting from Baroda in the west and continuing to the south of Jabalpur in the east, has been studied to understand the structural and tectonic framework of the region. The area is covered by generally E-W-trending steeply dipping and folded Archean phyllites and quartzites as basement, with Bijawars (Upper Precambrian), upper Vindhyans (Upper Proterozoic), and Gondwanas (Upper Carboniferous) overlying them. Overlapping them all are the Deccan trap (Cretaceous-Eocene) flows. Aeromagnetic linements and their disposition and pattern in this region suggest major dislocations in the crust. The region around Hoshangabad, which is the intersection point of the NSL and the northwestern extension of the Godavari lineament, appears to have been intensely disturbed. Spectral analysis of aeromagnetic profiles across the NSL belt brought out a deep magnetic interface within crust at depths varying from 4 km to about 20 km below the surface, perhaps corresponding to the discontinuity characterized by the interface of granitic and basaltic rocks. There is a significant downwarping of this interface under the Hoshangabad region, suggesting that this is perhaps related to the evolution of the Gondwana basin structure in this area. This warping of the magnetic interface may be a reflection of the crustal flexuring and rift faulting. Elsewhere in the world, concentrations of carbonatite complexes and dike swarms are known to occur in areas of crustal flexuring and rift faulting. The occurrence of carbonatite complexes in this region (e.g. at Amba Dongar and Barwaha, and dike swarms in the Dadiapada region) gives credence to the present inferences from the aeromagnetic study.

Geodetic network for study of crustal movements across the Jordan-dead sea rift

Since estimates of ongoing slip along the Dead Sea Rift transform differ widely, a geodetic monitoring network, 2.5 × 4 km area, was established at Kefar Hanassi, north of Lake Tiberias. It includes 12 deep-anchored micropile benchmarks, six on each side of the main rift fault. Network geometry is such that repeated measurements will detect displacements not only on the main fault but also on the oblique subsidiary faults. The accuracies attained in the first set of surveys (Tellurometer MA100 and Geodimeter AGA114, triangulation and precise levelling) suggest that a consistent tectonic slip at the rate of a few mm/a should emerge from the range of operational noise within a decade. To improve the resolution, and to extend the network to the rift margins, higher quality EDM and GPS measurement systems are now deployed.

The Malawi rift: Geology, tectonics and seismicity

The Malawi (Nyasa) rift is a southern extension of the western branch of the Cenozoic East African rift system. It extends for SOCl km, from the Rungwe volcanics in southern Tanzania to the Middle Shire River. The rift structures extend for a further 600 km to the south by the Urema graben and Dombe trough in Mozambique (Fig. 1) (Crow and Crossley, 1980). The Malawi rift, which is largely occupied by Lake Malawi, consists of a series of half grabens. The geology of the lake bed has been revealed by seismic reflection data obtained under Project CEGAL (Cenozoic Environments and Geodynamits of African Lakes) in 1981 (Rosendahl and Livingstone, 1983; Ebinger et al., 1984) and Project Probe (1985-87), (Scholz et al., 1989). The seismic reflection data has shown that the lake consists of several asymmetric basins with complex fault geometries, infilled with thick sedimentary sequences with dyke-like bodies (Scholz et al., 1989). The rift is seismically active with most of the activity being associated with the rift faults. The major part of the Malawi rift is underlain by Precambrian gneisses and granulites of sedimentary or igneous origin. Karroo sequences of conglomerates, sandstones and coal measures are found onshore in Livingstone Province, Bandawe-Metangula-Nkhotakota Province and Chikwawa/Nsanje in the Shire Province. In the

Back‐arc rifting in the Izu‐Bonin Island Arc: Structural evolution of Hachijo and Aoga Shima Rifts

Abstract Multi‐ and single‐channel seismic profiles are used to investigate the structural evolution of back‐arc rifting in the intra‐oceanic Izu‐Bonin Arc. Hachijo and Aoga Shima Rifts, located west of the Izu‐Bonin frontal arc, are bounded along‐strike by structural and volcanic highs west of Kurose Hole, North Aoga Shima Caldera and Myojin Sho arc volcanoes. Zig‐zag and curvilinear faults subdivide the rifts longitudinally into an arc margin (AM), inner rift, outer rift and proto‐remnant arc margin (PRA). Hachijo Rift is 65 km long and 20–40 km wide. Aoga Shima Rift is 70 km long and up to 45 km wide. Large‐offset border fault zones, with convex and concave dip slopes and uplifted rift flanks, occur along the east (AM) side of the Hachijo Rift and along the west (PRA) side of the Aoga Shima Rift. No cross‐rift structures are observed at the transfer zone between these two regions; differential strain may be accommodated by interdigitating rift‐parallel faults rather than by strike‐ or oblique‐slip faults. In the Aoga Shima Rift, a 12 km long flank uplift, facing the flank uplift of the PRA, extends northeast from beneath the Myojin Knoll Caldera. Fore‐arc sedimentary sequences onlap this uplift creating an unconformity that constrains rift onset to ∼1‐2Ma. Estimates of extension (∼3km) and inferred age suggest that these rifts are in the early syn‐rift stage of back‐arc formation. A two‐stage evolution of early back‐arc structural evolution is proposed: initially, half‐graben form with synthetically faulted, structural rollovers (ramping side of the half‐graben) dipping towards zig‐zagging large‐offset border fault zones. The half‐graben asymmetry alternates sides along‐strike. The present ‘full‐graben’ stage is dominated by rift‐parallel hanging wall collapse and by antithetic faulting that concentrates subsidence in an inner rift. Structurally controlled back‐arc magmatism occurs within the rift and PRA during both stages. Significant complications to this simple model occur in the Aoga Shima Rift where the east‐dipping half‐graben dips away from the flank uplift along the PRA. A linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting. Rifts are better developed between the arc edifices; intrusions may be accommodating extensional strain adjacent to the arc volcanoes. Pre‐existing structures have little influence on rift evolution; the rifts cut across large structural and volcanic highs west of the North Aoga Shima Caldera and Aoga Shima. Large, rift‐elongate volcanic ridges, usually extruded within the most extended inner rift between arc volcanoes, may be the precursors of sea floor spreading. As extension continues, the fissure ridges may become spreading cells and propagate toward the ends of the rifts (adjacent to the arc volcanoes), eventually coalescing with those in adjacent rift basins to form a continuous spreading centre. Analysis of the rift fault patterns suggests an extension direction of N80°E ± 10° that is orthogonal to the trend of the active volcanic arc (N10°W). The zig‐zag pattern of border faults may indicate orthorhombic fault formation in response to this extension. Elongation of arc volcanic constructs may also be developed along one set of the possible orthorhombic orientations. Border fault formation may modify the regional stress field locally within the rift basin resulting in the formation of rift‐parallel faults and emplacement of rift‐parallel volcanic ridges. The border faults dip 45–55° near the surface and the majority of the basin subsidence is accommodated by only a few of these faults. Distinct border fault reflections decreases dips to only 30° at 2.5 km below the sea floor (possibly flattening to near horizontal at 2.8 km although the overlying rollover geometry shows a deeper detachment) suggesting that these rifting structures may be detached at extremely shallow crustal levels.

Contemporary, Holocene, and Quaternary deformation of the Asal Rift, Djibouti: Implications for the mechanics of slow spreading ridges

Because the frequency and character of rifting events along mid‐ocean ridges are largely unknown, how the repetition of such events gives rise to rift structures is unexplored. The Asal rift in the Afar depression of Djibouti, Africa, provides the world's best subaerial analogue for young slow spreading mid‐ocean ridges. Seismic, geodetic, and field observations of a seismovolcanic event in 1978 at Asal yield estimates of the fault and dike locations, geometry, displacement, and volume of basalt extruded in a rifting event. A 6–9 kyr‐old lake shore highstand at Asal has been warped downward by 70 m, providing a Holocene measure of the vertical deformation across the rift. The rift topography furnishes an older datum, which we infer to be 34±6 kyr old using the Holocene deformation rate. We find that faults throughout the rift valley are active; Holocene slip rates diminish beyond 4 km from the rift axis; late Quaternary rates decrease beyond 6–7 km. The Holocene slip rates are used to estimate repeat times by taking the displacement on the faults which slipped in 1978 as characteristic; we find tectonic events on individual faults recur every 200– 300 years. Half the rift faults slipped together in the 1978 event. If this is typical, then groups of faults are activated every 100–150 years. We suggest that half the events take place in the rift axis accompanied by volcanic extrusion; the remainder occur peripheral to the neovolcanic zone and involve fault slip only, both events having a repeat time of 200–300 years. Given the 10 km width of the rift and its 16 mm yr−1 spreading rate, the mean age of the material in the rift should be ∼350 kyr, an order of magnitude older than the inferred age of the formation of the rift topography. The subsidence rate of the rift axis during the past 35 kyr is 8–9 mm yr−1, with the rate of infilling by volcanic extrusion &lt;1 mm yr−1. The resulting net subsidence rate, about equal to the half‐spreading rate of the rift, could not be sustained for 300 kyr without significant infilling by lavas. Thus both observations suggest that the long‐term vertical deformation in the rift has not been steady state. Instead, we suggest that there is a rifting/filling cycle at Asal, with the most recent filling episode ending ∼35 kyr.

The Late Palaeozoic Rifting on Hainan Island, China1

Abstract There occurred rifting on Hainan Island in the Late Palaeozoic. Bimodal volcanic rocks composed of basalt and rhyolite developed in the Carboniferous. Widespread in the Late Palaeozoic formations are several layers of fluvial intermontane conglomerates whose distribution is controlled by rift faults. The Late Palaeozoic deposits dominated by clastic rocks are, for a major part, of marine facies and of continental facies in the lower and upper parts. Lithological and lithochemical studies indicate that the detrital rocks were formed in a tectonic setting of continental rifting. The evolution of the rifting terminated at the stage of transition form an intra continental rift to an intercontinental one and the rift basin was a bay opening westward to the sea.

The Spi Group: a post-Archean, pre-2.1 Ga rift succession, Trans-Hudson hinterland

A succession of mafic flows intercalated with sandstone and overlain by conglomerate, herein named the Spi Group, lies unconformably on Archean basement and is preserved in a syncline near Spi Lake, N.W.T. An alluvial fan setting near active rift fault escarpments is the interpreted environment of deposition. The nearby Montgomery Lake Group, Mackenzie Lake metasediments, and Padlei Formation at the base of the Hurwitz Group are possible correlatives of the Spi Group; diabase dykes of the Kaminak swarm, common in the Archean basement, are possibly related to the mafic flows. Correlation with the upper, coarse clastic part of the Hurwitz Group (Tavani Formation) is discounted because the Spi conglomerate does not contain clasts of Hurwitz lithology. The Montgomery Lake, Padlei and Spi successions are interpreted as being rift-related, and are older than the 2.1 Ga gabbro that intrudes the Hurwitz Group. Recognition of widespread rifting in this part of the Hearne Province constrains tectonic modelling of the preorogenic history of the Trans-Hudson hinterland.

Vulcan Graben, Timor Sea; Regional Structure From a Magnetic Survey

In late 1989 the Australian Bureau of Mineral Resources (BMR) carried out a regional aeromagnetic survey by contract over the Vulcan Graben region. Elongate short-wavelength anomalies, with wavelengths of 3 to 12 km, amplitudes of 0.2 to 2.0 nT, and with generally northeast strikes, can be correlated for distances of up to 100 km on the basis of their shape and amplitude. Their source appears to be principally at depths of between 0.7 and 3.0 km. Some of the relatively larger amplitude and wavelength anomalies overlie major northwest-dipping normal faults which have been mapped by seismic. All of the elongate anomalies are thought to be due to structures or magnetic deposits associated either directly with NE-trending normal rift faults, or with faults in the post-rift sediments which are related to rift fault reactivation. ENE fault trends, which developed in the Neogene as a result of the collision with Timor, do not appear on the aeromagnetic data. We have mapped numerous northwest-trending faults. These have a spacing of approximately 10 to 25 km. They are thought to be strike-slip because some offset the major northeast-trending normal faults. The northwest trending faults may be due to the reactivation of Palaeozoic trends, the reactivation of Mesozoic transfer faults, or possibly some may be younger and independent of the earlier faults. Some of these faults have previously been mapped on seismic and correspond to seismic 'bad data' areas. These faults may have a major role in the entrapment of hydrocarbon accumulations in the Timor Sea, with most of the significant fields being close to a northwest-trending fault or its extension. In the Timor Sea, high resolution aeromagnetics have proven to be a relatively cheap and cost-effective tool which both complements and supplements the existing seismic data. It should prove equally useful in other areas with extensive seismic coverage. In frontier areas, it should rapidly define the geometry of the major tectonic elements of the basin, allowing for better positioning of seismic programs.

Jurassic Faults of the Southwest Alabama and Offshore Areas

Four fault groups affecting Jurassic strata occur in the southwest and offshore Alabama areas. They include the regional basement rift trend, the regional peripheral fault trend, the Mobile graben fault system, and the Lower Mobile Bay fault system. The regional basement system rift and regional peripheral fault trends are distinct and rim the inner margin of the eastern Gulf Coastal Plain. The regional basement rift trend is genetically related to the breakup of Pangea and the opening of the Gulf of Mexico in the Late Triassic-Early Jurassic. This fault trend is thought to have formed contemporaneously with deposition of Late Triassic-Early Jurassic Eagle Mills Formation and to displace pre-Mesozoic rocks. The regional peripheral fault trend consists of a group of en echelon extensional faults that are parallel or subparallel to regional strike of Gulf Coastal Plain strata and correspond to the approximate updip limit of thick Louann Salt. Nondiapiric salt features are associated with the trend and maximum structural development is exhibited in the Haynesville-Smackover section. No hydrocarbon accumulations have been documented in the pre-Jurassic strata of southwest and offshore Alabama. Productive hydrocarbon reservoirs occur in Jurassic strata along the trends of the fault groups, suggesting a significant relationship betweenmore » structural development in the Jurassic and hydrocarbon accumulation. Hydrocarbon traps are generally structural or contain a major structural component and include salt anticlines, faulted salt anticlines, and extensional fault traps. All of the major hydrocarbon accumulations are associated with movement of the Louann Salt along the regional peripheral fault trend, the Mobile graben fault system, or the Lower Mobile Bay fault system.« less

High-Resolution Aeromagnetics as an Exploration Tool in the Timor Sea, Northwestern Australia

The Mesozoic Vulcan Graben in the Timor Sea, northwestern Australia, is one of Australia's most active oil exploration areas, with a number of discoveries made within the last five years. To further assist in understanding the structural setting of the region, the Australian Bureau of Mineral Resources (BMR) carried out a regional high-resolution aeromagnetic survey over the Vulcan Graben in late 1989. Three to twelve km wavelength, 0.2-2.0 nT amplitude magnetic anomalies have northeast strikes and correlate well with the position and orientation of the rift faults, as defined by seismic reflection data. Significantly, the magnetic trends closely match the fault trends at the prospective Valanginian unconformity horizon, where all of the major producing reservoirs are located. Numerous northwest-trending (strike-slip ) faults have also been mapped as zones of low anomaly, some of which correspond with offsets in the major northeast-trending anomalies. These faults, which are probably due to the reactivation of either Mesozoic transfer faults or Paleozoic normal faults, may have a major role in the entrapment of hydrocarbons in the Timor Sea, as most producing fields are located close to a northwest-trending fault or its extension. The northwest-trending faults have not previously been defined using seismic data. Thismore » survey shows that even in an area with extensive seismic coverage such as the Timor Sea, high-resolution aeromagnetics can provide important information that both complements and supplements the seismic data.« less