Showing papers on "Continental margin published in 1985"

Carbonate Platform Facies Models

Abstract: Various types of carbonate platforms are characterized by distinctive profiles, facies, and evolutionary sequences. Ramps may be homoclinal or distally steepened, and may have fringing or barrier shoal-water complexes of ooid-pellet sands or skeletal banks. Homoclinal ramps pass seaward into deeper water without major break in slope, and lack deep-water breccias. Distally steepened ramps may be low energy, and characterized by widespread, shallow, subwave-base mud blankets, or high energy with coastal beach/dune complexes and widespread skeletal sand blankets. Slope facies may contain abundant breccias of slope-derived clasts. Rimmed shelves have relatively flat tops, and marked break in slopes at the high energy, shallow-shelf edge where they pass into deep water. Such shelves may be aggraded with peritidal facies extending over much of the shelf, or incipiently drowned, depending on magnitude of sea level fluctuations. They may be accretionary, or bypass types that include gullied slope, escarpment, and high-relief erosional forms. Intrashelf basins occur on some shelves, controlling distribution of reservoir and source beds. Isolated platforms are surrounded by deeper water and may be located on rifted continental margins, or on submarine volcanoes. Most have high-relief rimmed margins. Platforms that have been subjected to rapid sea level rise may be incipiently drowned, and characterized by raised rims, elevated patch or pinnacle reefs, and widespread subwave-base carbonate or fine clastic blankets. Completely drowned shelves develop where the shelf is submerged to subphotic depths, terminating shallow water deposition, and commonly resulting in blanketing of the shelf by deeper water facies. Some margins show extensive down-to-basin faulting that is contemporaneous with carbonate deposition, or associated with thick prograding clastic sequences. The various types of platforms change in response to variations in sedimentation, subsidence or sea level rise, and may form distinctive evolutionary sequences. The relatively few models presented appear to accommodate most geological examples, some of which contain major reservoir facies.

Punctiform initiation of seafloor spreading in the Red Sea during transition from a continental to an oceanic rift

Abstract: Transition from a continental to an oceanic rift occurs in the Red Sea by initial emplacement of oceanic crust in regularly spaced ‘hot points’, which serve as nuclei for axial propagation into segments of oceanic crust accretion and for initiation of seafloor spreading. Hot points develop above upwelling mantle diapirs caused by density / viscosity inversion beneath continental and oceanic rifts.

Late Quaternary Shelf-Margin Deltas, Northwest Gulf of Mexico

Abstract: Interpretations of 35,000 km (21,900 mi) of single-channel, high-resolution, seismic profiles traversing the continental shelf and upper continental slope of the northwest Gulf of Mexico indicate the existence of five late Wisconsinan shelf margin deltas, including the Rio Grande and Mississippi deltas. The deltas were recognized by geomorphic pattern, high-angle clinoform seismic reflections, and association with buried river systems. Isopach patterns show that the deltas range in size up to 5,000 km2 (1,900 mi2) and reach thicknesses of over 180 m (590 ft). The deposits are elongate parallel with depositional strike, indicating subsidence of the shelf margin as a whole. Internal reflection patterns show the deltas to be fluvially dominated. Multilo ate structure resulted from both short-term eustatic sea level fluctuations and delta switching. The late Quaternary shelf-margin deltas provide models for analogous deposits in the ancient record. They are primary indicators of the position of ancient shelf margins, and are important for predicting sand occurrence in that environment as well as farther downslope. As exploration moves to the shelf edge and beyond, instability hazards posed by late Wisconsinan deltas, as well as older deposits, must be understood and dealt with.

Active rifting in southwestern Mexico: Manifestations of an incipient eastward spreading-ridge jump

Abstract: A major, ongoing episode of continental rifting has formed a system of three intersecting grabens in the western Mexican volcanic belt. These structures may mark the initial stage of an eastward spreading-ridge jump beneath continental crust. Eastward ridge-segment jumps have been propagating northward along this part of the East Pacific Rise for the past 12 m.y., but this is the first plate-boundary reorganization event to involve continental crust since the separation of Baja California in the Miocene. Since Pliocene time, small volumes of unusual alkaline magmas have erupted in the graben areas in close association with the more abundant, subduction-related calc-alkaline magmas erupted from andesitic stratovolcanoes. The alkaline magmas are varieties found in zones of active rifting elsewhere in the world.

Biological communities at vent sites along the subduction zone off Oregon

Abstract: An abundant deep-sea benthic community, dominated by the vestimentiferan tube worm, Lamellibrachia barhami, a giant white clam, Calyptogena sp., and a second clam, Solemya sp., was discovered along the northeast Pacific continental margin during dives of DSRV Alvin off Oregon. The localities are tectonically controlled and occur along submarine ridges that result from sediment deformation and accretion at the leading edge of plate subduction. Ancient sea-water buried with the sediment, dissolved methane and ammonia, and methane-derived carbonate are discharged to the ocean floor along this tectonically active margin. We hypothesize that the unusual benthic community is sustained by a chemosynthetic-energy metabolism based on the oxidation of methane by free living and symbiotic microbial populations. These organisms represent another significant adaptation of deep-sea biota to a site where the abundanc of food (in this case not directly related to particulate input from the sea surface but to tectonically generated methane), allows higher than normal levels of productivity and metabolism, despite the high pressures and low temperature of the deep-sea habitat. It is tempting to relate the global distribution of such faunas to chemosynthetic food sources generated by tectonic forces.

Geological manifestations of ridge collision: Evidence from the Golfo de Penas-Taitao Basin, southern Chile

Abstract: Recent geological and geophysical studies in the southern Andes adjacent to the intersection of the Chile Rise with the Peru-Chile Trench (ANT-NAZ-SAM triple junction) have revealed a number of features and a Neogene geologic history that are unique along the Pacific margin of South America. This history includes (1) development of a Tertiary-Quaternary marine basin with up to 3 km of sediment infill (Golfo de Penas-Taitao basin, GTB), (2) disruption of the region by a series of faults with both normal and strike slip movements, and (3) localization of silicic, near-trench volcanism and epizonal plutonism and related hydrothermal activity. The northern portion of the GTB began to subside in the Late Miocene (possibly earlier), and has subsequently been deformed, uplifted, and exposed. Gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today (Golfo de Penas). Subsidence and uplift have thus occurred diachronously in the region, although it is unclear when subsidence began in the Golfo de Penas. Tectonic disruption of the region is likely related to the Liquine-Ofqui fault (LOF), a major, NS-trending, crustal shear zone that curves westward and terminates in the Golfo de Penas. The LOF has both down-to-the-west and right lateral offset and separates the main Andean Cordillera on the east from a large crustal block (the Chiloe block) on the west. We hypothesize that the GTB has developed as a pull-apart basin in response to northward movement of the Chiloe block along the LOF. We propose a dynamic model whereby a stress gradient that decreases longitudinally away from the Chile Rise/Peru-Chile Trench intersection is set up because the youngest, most buoyant, oceanic lithosphere is being subducted at the triple junction. The Chile Rise is viewed as a type of indenter which is acting to drive the Chiloe block northward in front of the northward-migrating triple junction. This model explains the unique set of geologic features found in the region, and suggests that ridge-trench interactions may be an important factor in orogenesis at active continental margins.

A transform margin in the Mesozoic Tethys: evidence from the Swiss Alps

Abstract: Remnants of the Liguria-Piemont Ocean with its Jurassic ophiolitic basement are preserved in the South Pennine thrust nappes of eastern Switzerland. Analysis of South Pennine stratigraphy and comparison with sequences from the adjacent continental margin units suggest that South Pennine nappes are relics of a transform fault system. This interpretation is based on three arguments: (1) In the highly dismembered ophiolite suite preserved, Middle to Late Jurassic pelagic sediments are found in stratigraphic contact not only with pillow basalts but also with serpentinites indicating the occurrence of serpentinite protrusions along fracture zones. (2) Ophiolite breccias (»ophicalcites«) occurring along distinct zones within peridotite-serpentinite host rocks are comparable with breccias from present-day oceanic fracture zones. They originated from a combination of tectonic and sedimentary processes: i.e. the fragmentation of oceanic basement on the seafloor and the filling of a network of neptunian dikes by pelagic sediment with locally superimposed hydrothermal activity and gravitational collapse. (3) The overlying Middle to Late Jurassic radiolarian chert contains repeated intercalations of massflow conglomerates mainly comprising components of oceanic basement but clasts of acidic basement rocks and oolitic limestone also exist. This indicates a close proximity between continental and oceanic basement. The rugged morphology manifested in the mass-flow deposits intercalated with the radiolarites is draped by pelagic sediments of Early Cretaceous age.

Sedimentation and tectonics in the Scottish Dalradian

Abstract: Synopsis Sedimentological research on the Scottish Dalradian has progressed from the recognition of sedimentary structures in the 1930s, via the identification of sedimentary facies from the 1950s onwards, to the integration, in the 1970s, of sedimentological data with that from studies of stratigraphy, tectonics and volcanism. This has now led to an understanding of the pre-orogenic evolution of the Dalradian terrane in terms of progressive lithospheric stretching associated with the break-up of the Proterozoic Supercontinent. The Appin and Argyll Groups were deposited on the NW side of a late Precambrian marine gulf which developed over a complex zone of crustal thinning between the Laurentian and Baltic parts of the Super-continent. As extension accelerated, subsidence rates increased and the Dalradian area of the gulf evolved from a relatively shallow shelf into a series of turbidite basins. Thinning of the lithosphere gave rise, in Argyll Group times, to locally intense igneous activity. Subsequently, complete continental rupture along the gulf axis led to the birth of the Iapetus Ocean. By Southern Highland Group times the Dalradian terrane had become part of the new, thermally-subsiding, Laurentian continental margin. One can envisage the geometry and facies variations of many horizons within the Dalradian in terms of a pattern of numerous fault blocks defined by listric normal faults, dipping SE towards the site of continental rupture, and NW–SE trending transfer faults, which divided the gulf and subsequent margin into a series of compartments. It was movements on these faults that largely controlled Dalradian stratigraphic evolution. For example, pulses of rapid stretching, and consequent fault activity, produced basin-deepening sequences which mark the base of the Easdale and Crinan Subgroups.

Rhone Deep-Sea Fan: Morphostructure and Growth Pattern

Abstract: A detailed bathymetric survey of the Rhone deep-sea fan and its feeder canyon using Sea-Beam, reveals morphologic features such as very tight meanders of the canyon and channel courses, cutoff meanders, and downslope narrowing of the inner channel floor. Striking similarities exist between these deep-sea features and some continental landforms, especially in alluvial plain areas or desert environments. Sea-Beam also reveals evidence of huge slump scars affecting the slope and fan. The superficial structure of the Rhone Fan results from the stacking of numerous lenticular acoustic units displaying specific seismic characters in which we recognized channel and levee facies. Except in the upper fan area, these units have not been constant; they have generally migrated, owing to shifting of the channel throughout fan evolution. Construction of the fan probably began as early as the early Pliocene and continued to the close of the Wurmian (late Wisconsinian). In our opinion, the fan's growth pattern could be associated with climatic fluctuations. The principal sedimentary mechanism responsible for the growth of the fan appears to be turbidity currents, but mass gravity flows have also been an important factor in building the fan by occasionally blocking the main channel and forcing it to migrate.

Seismic structure across the active subduction zone of western Canada

Abstract: The Vancouver Island Seismic Project was conducted in 1980 to study the structure of the subducting oceanic Juan de Fuca plate and the overriding continental America plate. The principal seismic refraction line (line I) was a 350-km onshore-offshore profile perpendicular to the continental margin. An array of 32 receivers was located on the America plate on the mainland and across Vancouver Island and extended offshore with three ocean bottom seismometers (OBS's). Two shots were fired at the eastern end of the line, and 17 shots were located along the westernmost 100 km of the profile. Control for the interpretation of the onshore-offshore profile was provided by a reversed refraction profile along the length of Vancouver Island and by a marine refraction profile recorded on the OBS'S. Modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two-dimensional media. The major features of the refraction structural model are that (1) the oceanic lithosphere dips at 3° or less beneath the continental slope, so the bend in the subducting slab occurs landward of the foot of the slope. (2) the oceanic lithosphere dips at 14°–16° beneath the continental shelf until it passes beneath the continental Moho at 37 km depth below western Vancouver Island, (3) an upper mantle reflector may correspond to the base of the subducting lithosphere, and (4) a segment of high-velocity material above the downgoing crust, with velocity 7.7 km/s and depth range 20–25 km, may represent a remnant of subducted lithosphere, perhaps detached when the subduction zone jumped westward to its present position.

Mid America: Tectonic Setting for the Pacific Margin from Southern Mexico to Northwestern Colombia

Abstract: This chapter presents a synthesis of the most relevant geological and tectonic features of the land and continental margin that borders the Pacific Ocean from southern Mexico to northwestern Colombia (Fig. 1). The objectives are to emphasize the variations in geological constitution of different segments of the region and to review briefly the major problems in understanding the geological history and the tectonic processes that resulted in its present configuration.

Evolution of the Archean Continental Crust

Abstract: Some 20 years of plate tectonic theory, combined with new insights into the fine structure of the lithosphere, the application of multielement geo­ chemical and isotopic studies, paleomagnetism, and geophysical modeling of mantle processes, have profoundly influenced present thinking on the origin and evolution of the Earth's early continental crust; previously, our knowledge of the continental crust was based almost exclusively on field geological observations. Although there is now general agreement on how the Earth worked for the last 200 m.y. because of observable evidence in the oceans and continents (e.g. Bird 1980, Condie 1982), it has proved difficult to extend this history into more ancient times in view of the lost oceanic record and the ambiguity and complexity of the pre-Mesozoic rock relationships in the continents (Dewey 1982). However, preserved characteristic rock as­ semblages uniquely identifying modern-type Wilson-cycle processes (i.e. opening and closure of oceans underlain by oceanic crust) have now been recognized in continental terranes as old as �900 m.y., and these assemblages provide strong evidence for the conclusion that the present global tectonic regime has governed the evolution of the lithosphere at least since the late Precambrian (Kroner 1977, 1981a,b; Goodwin 1981). Profound disagreement on the older crustal history, however, prevails to the present day, since typical features characterizing Phanerozoic accretionary terranes (such as obducted ophiolites, blueschists, and Franciscan-type melanges) have not been found in more ancient regions. Thus, two types of evolutionary models have been developed. One type postulates uniformitarian development back to the earliest Archean (Burke

"Paleomagnetism and geology of Eocene volcanic rocks of southwest Washington, Implications for mechanisms of tectonic rotation""

Abstract: Paleomagnetic and geologic investigations in Eocene volcanic rocks of the southwest Washington Coast Range demonstrate a close relationship between tectonic rotations and the local structural geology. The allochthonous middle Eocene submarine basalt basement of the Crescent Formation consists of several fault-bounded structural domains up to 30 km across that are characterized by different amounts of clockwise rotation (20° to perhaps as much as 65°) when compared to the Eocene reference pole for North America. Structural analysis shows the differential rotations postdate middle Eocene folding of the Crescent Formation against the continental margin and predate the unconformably overlying upper Eocene Goble Volcanics, which are rotated about 23° and do not show the same domains of rotation as the underlying Crescent Formation. Post-Goble rotations may be accommodated by a fault pattern very similar to that expected for areas caught in a simple dextral shear couple along transcurrent faults. Major north-northwest trending faults with several kilometers of dextral displacement form the boundaries of cross-faulted shear domains in which the clockwise rotation of elongate crustal slices is accommodated by west-northwest trending sinistral R′ Riedel shears. Thirty-five to 100% of the observed post-late Eocene rotations could have occurred by this shear rotation mechanism. Other paleomagnetic study areas in the Coast Range and western Cascades have a similar fault geometry and may also have undergone significant shear rotations. Long-term northward oblique subduction of the Farallon plate beneath the Coast Range throughout most of the Tertiary could have been the driving force for the shear rotations and could explain the rapid eastward decrease in rotation away from the continental margin. Shear rotations could eliminate many of the structural and stratigraphic difficulties associated with models involving rotation of large, rigid plates.

Mesozoic and Cenozoic Geology of the U.S. Atlantic Continental Slope and Rise

Abstract: We have studied single·channel and multichannel seismic-reflection profiles to identify and trace key reflecting horizons and seismic sequences from . the North American Basin into the U.S. Atlantic conti· nental rise and slope. Using seismic·facies analyses and isopach maps of seismic sequences together with availa· ble borehole data, we interpret the geologic history of the continental margin seaward of the shelf break. This area has a complex geologic record because deposition was controlled by the interaction of a variety of sedimen· tary processes. These processes had their origins in both shallow-water (e.g., sea-level) and deep-ocean (e.g., abyssal· current) impulses. Unfortunately, our interpretations are constrained by borehole data only along the continental slope and the lowermost continental rise. Until the neces· sary samples are recovered in intervening areas, this report serves as a preliminary analysis. from the time of initial sea·floor spreading in the Middle Jurassic until the end of the Eocene, the sedimen· tary record was shaped principally by cross·slope processes such as turbidity currents and debris flows. The sedimen· tary prism that formed along the margin in the Middle Jurassic probably was carbonate-rich, and it had a rela· tively uniform thickness with respect to the strike of the adjacent continental hinge zone. It is not clear whether the margin was a carbonate ramp or had a fringing reef. In contrast, by Late Jurassic time a shelf·edge reef was present, and it controlled seaward dispersal of sediment. Abyssal fans formed at the foot of the continental slope below gaps in the reef trend!..notably seaward of the Baltimore Canyon trough and northeast of the Blake Plateau basin. Extensive construction of fans occurred throughout the Early Cretaceous as segments of the reef gradually died or were overstepped by prograding shelf sediments. Seismic-reflection profiles show cut·and·fill structures and presumably coarse-grained channel fill within the abyssal fans, but boreholes have sampled.only the distal, fine-grained edges of these deposits. The bore· holes do show the dominant effect of deep-ocean processes on the outermost margin, namely a sharp rise in the calcite compensation depth (CCD) at the end of the Neocomian with subsequent deposition of carbon-rich black sl!ales beneath intermittently anoxic bottom water. During the Late Cretaceous the basin again became oxygenated, and high eustatic sea level caused flooding of the shelf. This restricted offshore transport of sediment, and a conformable drape facies was deposited across most of the continental rise. In the Paleocene and espe· cially during the Eocene, seaward transport of sediment in turbidity currents reached its zenith. High productiv· ity in surface waters, shelf erosion during sea-level low stands, and rapid sediment accumulation on the continental slope contributed to mass wasting that carried sediments up to 1,100 km across the broad continental rise and into the deep basin. The dominance of cross-slope sedimentation processes was dramatically curtailed at the beginning of the Oligocene. Global cooling and the opening of the Greenland Sea to the Arctic Ocean allowed cool. strongly circulating bottom water to enter the North Atlantic at this time. The abyssal western boundary current that was formed along the U.S. margin deeply eroded the conti· nental rise and oversteepened the continental slope. Current speeds probably decreased during the Oligocene. The flow, however, remained swift enough that the only sediment retained on the continental rise accumulated from large mass movements off the slope during a period of accelerated shelf erosion caused by a major sea-level low stand in the mid·Oligocene (29 Mal. In the early to middle Miocene bottom-current speeds decreased further, the Gulf Stream swept sediment off the Blake Plateau. and a large clastic wedge prograded across the shelf of the Baltimore Canyon trough. These factors led to rapid deposition that formed the cores of the Blake Outer Ridge and another large sediment drift ( Chesapeake Drift) beneath the upper continental rise off New Jersey. These drifts continued to develop under the influence of gradually decreasing current speeds during the late Miocene and Pliocene. At the same time, the growth of two secondary sediment drifts, the Hat teras and Bahama outer ridges, was accelerated, and very-well-developed sediment waves were formed there. The trend of gener· ally decreasing bottom-current speeds was interrupted by short pulses of intense deep circulation in the late middle Miocene (12 Ma) and the late Pliocene (3 Ma). These erosional pulses locally truncated beds on the flariks of sediment drifts, and they formed recognizable seismic boundaries (reflectors Merlin and Blue, respectively) between seismic sequences. The pulses are primarily responsible for modem sea-floor outcrops of Miocene and Pliocene sediments in ridge· flank locations. When Northern Hemisphere glaciation began in the late Pliocene (3 Ma), downslope sedimentary processes again assumed an important role, particularly by incising sub· marine canyons and channels into the continental slope and rise. The Quaternary sedimentary record has been shaped under the combined influence of these downslope processes and of contour-following bottom currents.

Input of river-derived sediment to the New Zealand continental shelf: I. Mass

Abstract: The input of river-borne sediments to the New Zealand continental shelf has been calculated for all the major rivers and basins in New Zealand. South Island yields 284 ± 40 × 106 tonnes per year of sediment from a land area of 152 977 km2 and North Island yields 105 ± 9·4 × 106 tonnes per year from a land area of 114 621 km2. Particularly high discharges are noted off the west coast of South Island and east coast of North Island and result in higher offshore sedimentation there. The data are compatible with measured sedimentation rates on the New Zealand continental shelf. The specific sediment yield from South Island is amongst the highest previously recorded.

Disintegration of the continental margin of northwestern Gondwana: Late Devonian of the eastern Anti-Atlas (Morocco)

Abstract: The Devonian–early Carboniferous sequence in the eastern Anti-Atlas represents a complete record of the last stage of the depositional and tectonic evolution along the northwestern margin of Gondwana. As a consequence of early Variscan block faulting, a platform and basin topography was established during the Middle and Late Devonian. Platforms were covered by condensed cephalopod limestones; sedimentation in the basins was mainly argillaceous with calcareous and turbiditic intercalations. In the latest Famennian/early Tournaisian the whole area was covered by delta deposits in the south passing into turbidites and olistostromes toward a continental slope farther north. This sedimentary and structural evolution reflects the gradual foundering and disintegration of the northwestern continental margin of Gondwana prior to the collisional stage in the late Visean/late Carboniferous.

Microearthquake seismicity and fault plane solutions related to arc-continent collision in the eastern sunda arc, indonesia

Abstract: The collision of Australia with the eastern Sunda and Banda arcs of Indonesia represents a modern example of the early stages of arc-continent collision. In order to obtain a more detailed view of the tectonics of the collision zone, a microearthquake survey was conducted around the Savu Sea, the region that encompasses the transition from subduction of Indian Ocean lithosphere on the west to collision or subduction of Australian continental litho- sphere on the east. Intermediate depth earth- quakes were concentrated near Pantar Island, the easternmost active volcano of the Sunda arc, and outline a northwest dipping zone that strikes N65oE from 70 to 150 km depth. The seismic zone probably marks the western edge of the collision zone and its more northerly strike, and the orientations of nodal planes from fault plane solutions indicate a 25 o bend in the subducted slab to a depth of at least 150 km. Because the convergence rate and the timing of collision from geologic observations on Timor suggest that the oart of the slab now at 150 km depth was at the trench when Australia first collided with Timor, we infer that continental crust (or at least rifted continental margin crust) caused the bend in the convergent margin and has been subducted to 150 km depth. Fault plane solutions of several events show nearly vertical nodal planes that trend parallel to the strike of the seismic zone, along which a northwest-side-down sense of dis- placement is indicated: these events are concentrated at the southern (top) side of the seismic zone and suggest that the subducted lithosphere is presently detaching in the 50-100 km depth range beneath the eastern Savu Sea. Fault plane solutions for earthquakes within the upper part of the detaching slab (70-100 km depth) have P and T axes whose trends lie parallel to the strike of the seismic zone and horizontal, southeast trending B axes. These earthquakes are interpreted as due to stresses caused by the bend in the subducting plate.

U-Pb Isotopic evidence for the accretion of a continental microplate in the Zalm region of the Saudi Arabian Shield

Abstract: The area covered by this work includes three of the main tectonic units of the Arabian Shield: the Afif continental terrain, the Nabitah suture with its associated mobile belt, and the Asir ensimatic arc terrain. The geology of the Zalm area is well understood, and this geochronologic and isotopic study confirms that the southern Afif terrain was a continental microplate in the late Proterozoic. The study also provides a time frame for the crustal evolution of this part of the Shield. The Kabid formation is the oldest in the region and comprises pelitic, arkosic and felsic high-grade paragneisses. U-Pb zircon data from a pelitic garnet-sillimanite gneiss show that this part of the continental basement in the southern Afif terrain may be as old as 1770 Ma. Isotopic nalyses indicate that lead from the Kabid gneiss resided in the upper continental crust for a long period before 1770 Ma, and consequently Archaean source rocks may be present within the southern Afif terrain. Pb and Rb-Sr isotopic data in the Zalm region reveal a change in the nature of the underlying crust, from continental basement in the northeast, to less radiogenic marginal arc rocks in the southwest. This change is coincident with both aeromagnetic data, and a facies change within a pre-collision marginal basin. Miogeosynclinal continental shelf facies of the Siham group lie unconformably over the Kabid formation, and are in the area of continental lead signatures. Eugeosynclinal deep water sediments and volcanics, in association with ultramafic rocks, occur in the area of marginal arc signatures. U-Pb zircon age determinations show that this 9Andean9 continental margin developed before about 720 Ma, and emplacement of calc-alkaline plutonic rocks continued until about 690 Ma. During the period 685-640 Ma, the continental Afif microplate collided with the Asir terrain as part of the Nabitah orogeny. At approximately 640 Ma ago, the Najd strike-slip faulting commenced with a dextral phase that controlled the emplacement of granitic plutons as well as the development of a series of large pull-apart grabens. Some of these grabens were floored by new oceanic crust and were filled with volcanosedimentary rocks of the Bani Ghayy group. Subsequently, the Najd fault system changed to sinistral strike-slip motion at about 620 Ma ago.

Total tectonic subsidence: A parameter for distinguishing crust type at the U.S. Atlantic Continental Margin

Abstract: Total tectonic subsidence (TTS) is the difference between the prerifting continental crust elevation and the present, sediment-unloaded, basement depth in a sedimentary basin. TTS is calculated using observations of bathymetry and basement depth and assumptions about sediment density and the loading response of the lithosphere. I use the extensional model for sedimentary basin formation to interpret the TTS of a passive continental margin. That model predicts that the TTS on an old margin should define the major crust-type boundaries and indicate the lateral distribution of extension within the extended continental crust. TTS analysis of the U.S. Atlantic continental margin indicates that the boundary between extended continental and oceanic crusts lies along the East Coast Magnetic Anomaly (ECMA) to the north of the Blake Spur Fracture Zone (BSFZ), and along the Blake Escarpment to the south. There is no TTS anomaly associated with the Blake Spur Magnetic Anomaly which seems to be underlain by normal oceanic crust. The ECMA appears to be associated with a TTS minimum, but I attribute that to artificially shallow estimates of depth to magnetic basement under the ECMA. TTS estimates of extended continental crust thickness agree well with gravity models except where the problem with magnetic basement depth estimates interferes. The amount of crust extension prior to seafloor spreading varies along the strike of the margin. The most abrupt change is at the BSFZ. Across the BSFZ the amount of extension varies by 160 km, but the volume of continental material does not vary. The extension variation may be caused by lateral changes in the gross rheology of the continental lithosphere. In the case of the BSFZ it would be the suture of more “ductile” pre-late Paleozoic African craton with more “brittle” North American craton that could cause the abrupt change.

The Puncoviscana trough — a large basin of Late Precambrian to Early Cambrian age on the pacific edge of the Brazilian shield

Abstract: During the Late Precambrian to Early Cambrian a large basin with a homogeneous psammitic-pelitic sediment fill existed in the area of the NW-Argentine Andes. It is now exposed in different tectonic levels. This basin of meridional elongation was situated on a stable continental margin at the western edge of the Brazilian shield. It was underlain by segmented older continental crust. According to the modal and chemical composition of the greywackes and subgreywackes a predominant metasedimentary source may be supposed. Four clastic facies types indicate a transport by gravity currents within a submarine fan system, while massive red pelites represent cut-off periods, and rare carbonates deposition on rises. The gravity currents derived from easterly directions. The sedimentation age is proved by a rich ichnofauna and some medusoid impressions.