Narrow Rift Research Articles

Magmatic emplacement of northwestern Geece ophiolites

New field observations reinforce the interpretation that was admitted from 1956 to 1970 according to which the ophiolites of Northwestern Greece were generated and emplaced as a massive submarine flow of mantelle magma which had risen through the opening of an orogenic rift. As it opened, the still narrow rift left out submarin tuffs and lavas (pillows) which have been found in different places under the basal peridotites of ophiolitic suits.

Dynamics of magma emplacement in centrifuge models of continental extension with implications for flank volcanism

A set of experimental small‐scale models of continental extension investigated the emplacement of orthogonal and oblique magma chambers, initially underplated at the base of the crust. The models are driven by a centrifugal body force which simulates the role of gravity in nature. The models represent crustal conditions analogous to relatively mature continental rifts and consider both symmetric and asymmetric extension. The experimental results suggest that magma emplacement is controlled by the interactions between tectonics and rheology of the crustal layers. In particular, extension is mainly accommodated by lateral flow and ductile doming in the viscous layer simulating the lower crust and by listric normal faults, in the overlying brittle layer, associated with the ductile domal uplift. Stretching of the continental crust induces a “reactive” migration of viscous layers into the footwall of major normal faults, where magma accumulation at the core of the domes takes place. This behavior suggests that widespread magmatism is expected to localize in the footwall of major normal faults, a situation that is often observed in core complex structures. Although the models presented here are intended to simulate magma emplacement during extension of a two‐layer brittle‐ductile system, lateral flow and migration of magma initially underplated at the base of the crust beneath narrow rifts (e.g., the Ethiopian rift) may also provide a similar mechanism and a possible explanation for the occurrence of important volcanoes on the plateaus flanking the rift zones. In the suggested model, such volcanoes are related to large basaltic magma reservoirs, located at the base of the crust, to accommodate the space vacated by lateral flow and thinning of the ductile crust. The similarity in structure of the models with natural examples of continental extension modes (such as rift systems and core complexes) may suggest a close similarity of dynamic processes.

The Laptev Sea Rift

In north-eastern Siberia the active mid-ocean Gakkel Ridge interacts with the continental shelf of the Laptev Sea. Extension has affected the shelf since at least the Early Tertiary and has resulted in the formation of a complex horst and graben system. We present new seismic data from the Laptev Sea including deep seismic soundings.The most prominent rift basin is the Ust' Lena Rift with a minimum E–W width of 300km at latitude 75°N and a Cenozoic infill up to 13km in thickness. The asymmetric shape of the basin and conclusive evidence for a detachment imply a simple-shear geometry. The suggested rift model combines a ramp and flat geometry for the detachment with ductile stretching beneath the detachment. A major west-dipping, hingeline, listric fault separates the Ust' Lena Rift from the Laptev Horst.The 100–150km wide Laptev Horst is subdivided into three units by narrow rift grabens. Another prominent rift graben is the Anisin Basin, which is located in the northern shelf area.Though the Laptev Sea Rift formed in interaction with an active mid-oceanic ridge, there are indications that the Laptev Sea rift is of the ‘passive rift’ type. The rift was developed east of a SW–NE trending transfer zone which links the Gakkel Ridge to the Laptev Sea Rift.

Rifted(?) crust at the East Antarctic Craton margin: gravity and magnetic interpretation along a traverse across the Wilkes Subglacial Basin region

Early geophysical studies hypothesized a continental rift structure beneath the Wilkes Subglacial Basin. Recent models favour a flexural origin for the basin linked to Transantarctic Mountains uplift and to East Antarctic Craton lithospheric rigidity. Flexural modelling predicts crustal thickening beneath the basin. Gravity modelling along the International Trans-Antarctic Scientific Expedition traverse (1998/99), however, reveals crustal thinning beneath the basin. At 75°S the crust thins from 37 km beneath the Transantarctic Mountains to 31±2 km beneath the Wilkes Basin. The western flank of the basin features a sharp magnetic break. This signature may arise from a fault separating highly magnetic Precambrian craton crust from weakly magnetic Neoproterozoic(?) crust. Much later crustal extension may have focussed along the craton margin. The eastern flank of the Wilkes Basin exhibits a prominent aeromagnetic signature. Potential field modelling predicts 1–4 km thick sedimentary infill within the Wilkes extended terrane, interpreted mainly as Beacon Supergroup intruded by Jurassic Ferrar tholeiites. The adjacent Adventure Subglacial Trench is a narrow rift basin with 25±5 km thick crust and a 10±4 km sedimentary infill.

Crustal structure of the northern Baltic Sea palaeorift

Integrated analysis of the gravity data measured in the northern Baltic Sea in 1997 and the near-vertical BABEL data shot in 1989 provides important constraints for the Subjotnian (1600–1500) rift-related evolution of the Svecofennian orogenic crust.The Subjotnian is characterised by voluminous bimodal rapakivi granite magmatism that is manifested as large granite batholiths associated with ovoid crustal thickness minima and large Bouguer gravity minima (−20 to −40mgal). In the seismic sections, the batholiths are transparent whereas the lower crust beneath is highly reflective and bears signs of intra- and underplating.The Bouguer anomaly map together with the seismic results imply that the crust of the Southern Gulf of Bothnia is mostly composed of Subjotnian intrusions with a large rheologically stronger, mafic Svecofennian block in the middle. The largest batholiths Åland, Bothnian and Rödön are mostly covered by water, but even the minor plutons at Strömsbro, Reposaari and Siipyy are interpreted to be small satellites to larger marine batholiths. A 3–4km deep Jotnian sandstone basin, the Strömmingsbodan basin, was developed on top of the Bothnian rapakivi granite. We suggest that it was initiated by cauldron subsidence. In the Postjotnian, the granites and sandstones were intruded by diabase sills, the contacts of which are imaged by high-amplitude reflections. The rapakivi granites are separated by a large transparent and dense body in the lower to middle crust. It is interpreted as an older Svecofennian mafic intrusion.The northern Baltic Sea has many characteristic features of paleorifts: topographic low (now under water), thin crust with large crustal thickness gradients and the products of voluminous bimodal magmatism. The Gulf of Bothnia aborted rift is a part of a honeycomb-like wide rift area that extends from Lake Ladoga to the Caledonides (800km×800km). It has seeds of many localised narrow rifts (100km×100km) not connected at the lower crustal level.

Deep structure of the Namibia continental margin as derived from integrated geophysical studies

During the Geophysical Measurements Across the Continental Margin of Namibia (MAMBA) experiments, offshore and onshore refraction and reflection seismic as well as magnetic data were collected. Together with the existing free‐air gravity data, these were used to derive two crustal sections across the ocean‐continent transition. The results show that the Early Cretaceous continental breakup and the separation of South Africa and South America were accompanied by excessive igneous activity offshore. Off Namibia we found a 150–200 km wide zone of igneous crust up to 25 km thick. The upper part of this zone consists of an extrusive section comprising three units of basaltic composition: two distinct wedges of seaward dipping reflectors (SDRs) separated by flat‐lying volcanic flows. The inner wedge of SDRs can be modeled as the source of a long‐wavelength magnetic anomaly that borders long parts of both South Atlantic margins (anomaly G). The crust underneath these extrusives is characterized by high‐velocity and high‐density material (average values 7 km s−1, 3×103 kg m−3). Free‐air gravity anomalies along both sides of the high‐density crust are interpreted as edge effects resulting from juxtaposition with normal oceanic and continental crust on either side. We define the abrupt landward termination of this zone as the continent‐ocean boundary, and consequently, the crust seaward is interpreted as exclusively igneous material and not intruded continental crust. Extrapolation of the interpreted geophysical features along the southwest African margin suggests a fast prograding narrow rift zone and sharp lithospheric rupture leading to the formation of a margin‐parallel magmatic belt south of the Walvis Ridge. The influence of the Tristan da Cunha mantle plume may explain the widening of this thick igneous crust near the Walvis Ridge.

Seismic volcanostratigraphy of large‐volume basaltic extrusive complexes on rifted margins

Large‐volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments. The presence and amount of water is of main importance, while local rift basin configuration, erosion, and resedimentation determine the overall geometry of the volcanic constructions. We have developed the concept of seismic volcanostratigraphy, a subset of seismic stratigraphy, to analyze volcanic deposits imaged on seismic reflection data. The method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units. Interpretation of seismic reflection data along the Atlantic and Western Australia rifted margins reveals six characteristic volcanic seismic facies units named (1) Landward Flows, (2) Lava Delta, (3) Inner Flows, (4) Inner Seaward Dipping Reflectors (Inner SDR), (5) Outer High, and (6) Outer SDR. These units are interpreted in terms of a five‐stage tectonomagmatic volcanic margin evolution model comprising (1) explosive volcanism in a wet sediment, broad basin setting, (2) subaerial effusive volcanism forming Gilbert‐type lava deltas along paleoshorelines, (3) subaerial effusive volcanism infilling a fairly narrow rift basin, (4) shallow marine explosive volcanism as the injection axis is submerged below sea level, and finally (5) deep marine sheet flow or pillow‐basalt volcanism. Further, erosion and resedimentation processes are particularly important during the shallow marine stages. Seismic volcanostratigraphy provides important constraints on rifted‐margin development, in particular, on the prevolcanic basin configuration, relative timing of tectonomagmatic events, total amount of volcanic rocks, location of paleoshorelines, and margin subsidence history. These parameters give key boundary conditions for understanding the processes forming volcanic margins and other large‐volume basaltic provinces.

Rheological controls on extensional styles and the structural evolution of the Northern Carnarvon Basin, North West Shelf, Australia

The extensional architecture of the Northern Carnarvon Basin can be explained in terms of changes in lithospheric rheology during multiphase extension and lower crustal flow. Low‐angle detachments, while playing a minor role, are not considered to have been the primary mechanism for extension as suggested in previous models. Early extension (Cambrian‐Ordovician) in the Northern Carnarvon Basin is characterised by low‐angle detachment structures of limited regional extent. These structures have a spatial association with a Proterozoic mobile belt on the margin of the Pilbara Craton. Thermo‐mechanical conditions in the mobile belt may have predisposed the highly deformed crust to thin‐skinned extension and detachment development. Permo‐Carboniferous extension generated an extensive wide rift basin, suggesting ductile rheologies associated with intermediate lithospheric temperatures and crustal thickness. Thick Upper Permian to Upper Triassic post‐rift sequences and marked thinning of the lower crust occurred in association with only a small amount of extension in the upper crust. This observation can be reconciled by considering outward lower crustal flow, from beneath the basin towards the basin margin, following extension. Strong mid‐crustal reflectors, which occur over large areas of the Northern Carnarvon Basin, probably represent a boundary between flow and non‐flow regimes rather than detachment fault surfaces as in previous models. Crustal thinning and thermal decay following Permo‐Carboniferous extension contributed to the increased strength and brittle behaviour of the lithosphere. Consequently, Late Triassic to Early Cretaceous extension resulted in the development of far more localised narrow rift systems on the margins of the preceding wide rift basin. Diapiric intrusions are associated with the narrow rift basin development, resulting from either remobilisation of ductile lower crustal rock or the initial formation of sea‐floor spreading centres.

Seismic stratigraphy of the Miocene–Pleistocene sedimentary basins of the Northern Tyrrhenian Sea and western Tuscany (Italy)

Seismic and stratigraphic data of the inland Volterra Basin and of the Tuscan Shelf (Northern Tyrrhenian Sea) have been analysed to determine the tectono‐sedimentary evolution of this part of the Northern Apennines from the early Miocene (about 20 Ma) to the present. The area is a good example for better understanding the evolution of postcollisional related basins.The study area is characterized by a series of sedimentary basins separated by tectonic ridges. Similar environmental conditions existed both onshore and offshore as indicated by the occurrence of similar seismic units. The units are separated by major unconformities. The cross‐sectional geometries of the deposits of these basins, as defined through seismic reflection profiles, change in a quasi‐regular manner through time and space.Early stages (late Burdigalian to early Tortonian) of evolution of the basins are marked by either flat‐lying deposits, quasi‐uniform in thickness, probably remnants of originally wider and shallow settings, or, in places, by relatively small bowl‐shaped basins. The latter may have been strongly affected by the pre‐existing topography and tectonics, as they developed at or near the leading edges of pre‐Neogene substrate thrusts. These early deposits represent sedimentation during a transitional period from the end of compressional tectonics to the start of an extensional phase and represent a pre‐narrow rift stage of evolution of the region. The subsequent stage of tectonic evolution (late Tortonian to early Messinian), where preserved, is recorded by fault‐bounded triangular‐shaped basins interpreted as half‐grabens. This is one of the periods of major development of narrow rifts in the area. The following stage (late Messinian to Early Pliocene) is marked by variable types of basins, showing wide and deep bowl‐shaped geometries persistent in the offshore, whereas inshore (Volterra Basin) they alternate with half‐graben, synrift deposits. This period thus represents a transitional stage where part of the system is still affected by synrift sedimentation and part is developing into incipient post‐rift conditions. This stage was followed in early to middle Pliocene times by wide bowl‐shaped to blanket‐type deposits both in offshore and in inshore areas, indicating regional post‐rifting conditions. The pre‐, syn‐ and post‐rift stages have followed each other through time and space, starting first in the westernmost offshore area and shifting later toward the east, inshore.

Sand‐box modelling of basement‐controlled transfer zones in extensional domains

The interaction modalities of transfer zones connecting rift segments may be influenced by several factors. Amongst these, the location and architecture of transfer zones in narrow rifts has been repeatedly associated with the presence of inherited basement anisotropies. Sand‐box models were made to investigate how the orientation, geometry and kinematics of transfer zones depend upon pre‐existing basement anisotropies. Analogue models reproduced offset rift segments linked by transfer zones bordered by arcuate normal faults. Strike‐slip faults are present inside the transfer zone, provided that angle α (between the extension direction and the axis of the transfer zone) is less than 50°. Narrower transfer zones, striking oblique to the extension direction, occur for angle φ (between the direction perpendicular to the extension direction and the direction of the basement anisotropy) less than 90°; wider transfer zones, subparallel to the extensional direction, occur for φ > 90°. Increasing the overstep induces narrower transfer zones striking subparallel to the extension direction. Similar geometrical and kinematical patterns have been found at transfer zones in narrow rifts. The comparison between experimental and natural data shows how the geometry, kinematics and orientation of natural transfer zones depends upon the trend of inherited anisotropies: transfer zones along inherited basement structures set at lower angles to the extension direction, and display lower aspect ratios, than transfer zones where inherited anisotropies are absent.

Signs of Plate Tectonics on an Infant Mars

PLANETOLOGYPivotal to the discovery almost 40 years ago of plate tectonics were rank upon rank of magnetic stripes that march across the sea floor, each marking the orientation of Earth's flip-flopping magnetic field at the moment the crust was born. Now on pages [790][1] and [794][2] of this issue, researchers report magnetic stripes on Mars. The data, gathered by the Mars Global Surveyor spacecraft, suggest that in its early days, Earth's diminutive cousin resurfaced itself the way Earth does today, spreading freshly made crust away from long, narrow volcanic rifts. Yet the shape and pattern of the martian stripes are so different from Earth's that geophysicists are reserving judgment for the moment. [1]: http://www.sciencemag.org/cgi/content/short/284/5415/790 [2]: http://www.sciencemag.org/cgi/content/short/284/5415/794

Narrow rifts versus wide rifts: inferences for the mechanics of rifting from laboratory experiments

Laboratory experiments on analogue models of the lithosphere are useful tools to study tectonic processes and, in particular, to test physical hypotheses. They complement numerical modelling becaus...

Lithospheric structure of the Pannonian basin derived from seismic, gravity and geothermal data

Abstract The structure of the Pannonian basin is the result of distinct modes of Mid-Late Miocene extension exerting a profound effect on the lithospheric configuration, which continues even today. As the first manifestation of extensional collapse, large magnitude, metamorphic core complex style extension took place at the beginning of the Mid-Miocene in certain parts of the basin. Extrapolation of the present-day high heat flow in the basin, corrected for the blanketing effect of the basin fill, indicates a hot and thin lithosphere at the onset of extension. This initial condition, combined with the relatively thick crust inherited from earlier Alpine compressional episodes, appears to be responsible for the core complex type extension at the beginning of the syn-rift period. This type of extension is well documented in the northwestern Pannonian basin. Newly obtained deep reflection seismic and fission-track data integrated with well data from the southeastern part of the basin suggests that it developed in a similar fashion. Shortly after the initial period, the style of syn-rift extension changed to a wide-rift style, covering an area of much larger geographic extent. The associated normal faults revealed by industry reflection seismic data tend to dominate within the upper crust, obscuring pre-existing structures. However, several deep seismic profiles, constrained by gravity and geothermal modeling, image the entire lithosphere beneath the basin. It is the Mid-Miocene synrift extension which is still reflected in the structure of the Pannonian lithosphere, on the scale of the whole basin system. The gradually diminishing extension during the Late Miocene/Pliocene could not advance to the localization of extension into narrow rift zones in the Pannonian region, except some deep subbasins such as the Makó/Békés and Danube basins. These basins are underlain coincidently by anomalously thin crust (22–25 km) and lithosphere (45–60 km). Significant departures (up to 130 mW m −2 ) from the average present-day surface heat flow ( c. 90 mW m −2 ) and intensive Pliocene alkaline magmatism are also regarded as evidence for the initiation of two newly defined narrow rift zones (Tisza and Duna) in the Pannonian basin system. However, both of these narrow rifts failed since the final docking of the Eastern Carpathians onto the European foreland excluded any further extension of the back-arc region.

The continental margin off east Tasmania and Gippsland: structure and development using new multibeam sonar data

The continental margin facing the southern Tasman Sea off southeastern Australia is of high petroleum and geological significance because it contains Australia’s richest oil province, the Gippsland Basin. Exploration activity in the region has concentrated on the shelfal part of this basin. Off Tasmania the upper margin is host to important deep sea trawl fisheries, and regional geophysical data suggest some long-term petroleum potential.In early 1997, 20,000 km2 of the margin were surveyed by the Australian Geological Survey Organisation, in cooperation with Scripps Institution of Oceanography and the University of Sydney. The SeaBeam 2000 multibeam sonar system of the US RV Melville was used to map the topography and character of the seafloor in unprecedented detail. The SeaBeam 2000 is a 12 kHz, 121-beam echo-sounder that collects both bathymetry and backscatter (sidescan) data across a swath of ~3.4 times the water depth. The survey also collected 3500 km of magnetic and gravity profile data.The margin off Gippsland is dominated by a large embayment 100 km across and floored by a ESE-trending chasm, the Bass Canyon, which is 60 km long and 10-15 km wide. The canyon has cut down about 2 km into the margin and is bounded to the north and south by very steep inner canyon walls 1000 m high. Sediments transported down the canyon debouch at its mouth, at~4000 m depth, and spread onto the abyssal plain via a network of distributary channels and levees. Sediment from the shelf is channelled into the entrance of Bass Canyon by three major, deeply-incised tributary canyons and a number of smaller ones. Bass Canyon has probably acted as a conduit for clastic sediment to the deep sea floor since breakup at ~80 Ma. Seismic profiles over the Bass Canyon complex show that canyon erosion has exposed sections of almost the entire Gippsland Basin sequence down to the Early Cretaceous and underlying basement. Numerous suitable dredge and core sampling sites in the canyons have been identified for a geological sampling program.Off east Tasmania, the survey showed the margin to be generally steep and rugged, with relatively little sediment having been deposited on the continental slope, except in narrow rifts, since breakup. The adjacent abyssal plain is commonly underlain by 2 km of Late Cretaceous-Cainozoic sedimentary section. The continental slope is cut by an extensive system of canyons, some more than 30 km long and 500 m deep. Large fault blocks on the lower slope show structural trends consistent with a NW-NNW rift direction and NE-ENE transfer direction. The upper slope appears to be underlain by a subsided Miocene carbonate shelf. Volcanics, of probable late Cainozoic age, are fairly widespread on the seafloor off northeast Tasmania. Such volcanic terrain includes a number of scattered cones, the largest 450 m high.

The crustal and upper-mantle geophysical signature of narrow continental rifts in the Pannonian basin

The Pannonian basin provides a very good opportunity to study extensional tectonics. The image of narrow continental rifts, which represent one of the modes of continental extensional tectonics, was studied in detail in the Békés basin. In order to constrain the lithosphere structure beneath the narrow rifts in the Pannonian basin, density models and interpretation of magnetotelluric measurements along the Pannonian Geotraverse are presented. The results obtained agree with the most recent deep seismic data. Most of the narrow rifts in the Pannonian basin are characterized not only by thinner crust but also by thinner lithosphere. A typical phenomenon of the gravity field over the central part of the narrow rifts in the Pannonian basin is the existence of a relative local gravity high. These gravity highs are probably due to intrusions of high-density masses into the lower crust and lower part of the upper crust beneath the narrow rifts, which are related to the extension of the basins (subbasins).

Rift processes at the Valles Marineris, Mars: Constraints from gravity on necking and rate‐dependent strength evolution

Recent spherical harmonic representations of the gravity field of Mars have sufficient resolution to examine the regional crustal and lithospheric structure of the Valles Marineris and are used as constraints for rift modeling. While gravity signatures of individual troughs are not evident, a broad 190–260 mgal low indicates that the central chasmata as a whole are compensated at a depth of 30–80 km, representing the thickness of crust surrounding the troughs. Furthermore, at the time large‐scale relief was established, the effective thickness of the elastic lithosphere was <30 km, corresponding to heat flow >20 m W m−2. The calculated range of crustal thickness and heat flow can be used to test two semianalytic models of rift formation. In the first model, the distinction between single (“narrow rift”) and multiple (“wide rift”) troughs is determined by the wavelength of necking instabilities. In the second model, narrow‐versus‐wide morphology is controlled by the evolution of lithospheric strength during rifting. The large‐scale, parallel, multiple troughs of the central Valles Marineris are morphologically similar to terrestrial wide rifts, but a lack of distinct faulting in some of the chasmata could imply that the faulted troughs may have formed as isolated narrow rifts. Our results are only marginally consistent with necking leading to a wide rift; allowable combinations of crustal thickness and heat flow lead to decoupling within the lithosphere resulting in a second principal necking wavelength that is smaller than the main trough spacing, for which evidence is equivocal. At high heat flow (>40 m W m−2), however, necking of the strong upper crust alone can yield a shorter wavelength characteristic of a single trough, allowing narrow‐rift origins of faulted chasmata. In contrast, the inferred range of crustal thickness and heat flow poorly match the narrow‐rift regime of the strength‐evolution model but are in good agreement with its predictions for wide rifting. Furthermore, the distinction between wide rift and core complex in this model places an upper bound on heat flow of 70 m W m−2.

Detrital zircon constraints on the tectonic evolution of the Gravina belt, southeastern Alaska

Upper Jurassic - Lower Cretaceous marine clastic strata and mafic to intermediate volcanic rocks of the Gravina belt are part of a complex suture zone separating the Alexander and Wrangellia terranes on the west from the Yukon-Tanana and Stikine terranes to the east. U-Pb ages have been determined on 118 single detrital zircon grains from Gravina strata in an effort to determine the tectonic setting of the Gravina belt and the paleoposition of outboard terranes prior to their Late Cretaceous juxtaposition against inboard terranes. Samples from five stratigraphic units yield ages of 105-120 (n = 5), 140-165 (n = 56), 310-380 (n = 17), 400-450 (n = 19), 520-560 (n = 5), 920-1310 (n = 5), and 1755-1955 Ma (n = 5). The 105-120 and 140-165 Ma grains were shed primarily from arc-related plutons that lie outboard of the Gravina belt. The lack of 120-140 Ma ages coincides with a lull in magmatism in the outboard arc and in the western United States, which suggests that Gravina strata accumulated during major changes in plate motion along the Cordilleran margin. The 400-560 Ma zircons were derived from rocks of the Alexander terrane which also lie to the west. In contrast, the 310-380 and >900 Ma grains were apparently shed from inboard regions. Likely sources include the Yukon-Tanana and Stikine terranes in the northern Cordillera and assemblages in the northern California region which contain igneous rocks and detrital zircons of the appropriate ages. Our data accordingly support models in which the Gravina basin formed in narrow rift or transtensional basins, whereas the outboard Alexander and Wrangellia terranes were located along the California - Oregon - Washington - British Columbia - Alaska margin. Our data are less supportive of models in which the Gravina strata and underlying Alexander and Wrangellia terranes were separated from western North America by a large ocean basin, or were located along the coast of Mexico.

Detrital zircon constraints on the tectonic evolution of the Gravina belt, southeastern Alaska

Upper Jurassic - Lower Cretaceous marine clastic strata and mafic to intermediate volcanic rocks of the Gravina belt are part of a complex suture zone separating the Alexander and Wrangellia terranes on the west from the Yukon-Tanana and Stikine terranes to the east. U-Pb ages have been determined on 118 single detrital zircon grains from Gravina strata in an effort to determine the tectonic setting of the Gravina belt and the paleoposition of outboard terranes prior to their Late Cretaceous juxtaposition against inboard terranes. Samples from five stratigraphic units yield ages of 105-120 (n = 5), 140-165 (n = 56), 310-380 (n = 17), 400-450 (n = 19), 520-560 (n = 5), 920-1310 (n = 5), and 1755-1955 Ma (n = 5). The 105-120 and 140-165 Ma grains were shed primarily from arc-related plutons that lie outboard of the Gravina belt. The lack of 120-140 Ma ages coincides with a lull in magmatism in the outboard arc and in the western United States, which suggests that Gravina strata accumulated during major changes in plate motion along the Cordilleran margin. The 400-560 Ma zircons were derived from rocks of the Alexander terrane which also lie to the west. In contrast, the 310-380 and >900 Ma grains were apparently shed from inboard regions. Likely sources include the Yukon-Tanana and Stikine terranes in the northern Cordillera and assemblages in the northern California region which contain igneous rocks and detrital zircons of the appropriate ages. Our data accordingly support models in which the Gravina basin formed in narrow rift or transtensional basins, whereas the outboard Alexander and Wrangellia terranes were located along the California - Oregon - Washington - British Columbia - Alaska margin. Our data are less supportive of models in which the Gravina strata and underlying Alexander and Wrangellia terranes were separated from western North America by a large ocean basin, or were located along the coast of Mexico.

Isotopic Composition of Carbonates and Sulphates, Potash Mineralisation and Basin Architecture of the Nagaur-Ganganagar Evaporite Basin (North-Western India) and Their Implications on the Neoproterozoic Exogenic Cycle

The Nagaur-Ganganagar evaporite basin covers a large part of the north western India in Western Rajasthan. The evaporite bearing Hanseran Evaporite Group (HEG) is the northern subsurface facies variant of the exposed Bilara Group of carbonates. While the Bilara Group is underlain by Jodhpur Sandstone (disposed on top of the Malani acid effusives, dated -745 Ma), HEG is also underlain by the same sandstones as documented in numerous deep bore holes. South of Bilara outcrops, near Birmania the facies changes to phosphatic carbonates. As documented by Ilyin (1990), this period of the earth history experienced development of narrow rift basins, many of these are repositories of evaporite deposits. The Nagaur-Ganganagar basin located west of the main NE-SW trending Aravalli Range, came into existence due to rejuvenation of Precambrian lineaments, producing horsts and grabens and extensive acid volcanism. This led to the formation of an extensive, shallow marine basin bounded by Jodhpur-Pokaran-Malani ridge in the south, Aravalli range in the east and Delhi-Sargodha-Lohore subsurface ridge to the north and north east. Salt Range in Pakistan and Hormuz basin in Iran are located in the northern extension of this basin (Dasgupta et al., 1988). The HEG exhibits lateral facies variations and deposition of atleast seven halite cycles alternating with maroon clays, marl, limestone, stromatolitic and foetid smelling OM-rich dolomite and magnesite. The halite (mainly the 2nd halite cycle) is associated with several potash minerals like sylvite, polyhalite, langbeinite sylvinite and carnallite. Analysis of subsurface strata and structural interpretation suggests deepening of the basin towards the central part and in WNW direction due to steep, step-like NNE-SSW gravity faults with their down thrown side deepening progressively towards west. Thickness of the HEG and the number of halite cycle increases from the margin of the basin towards the centre. Two bore holes (P/47 and P/12) near Lakhasar in Bikaner district with continuous and well preserved cores were logged and studied. The dolomite and the interbedded anhydrite were analysed for carbon and sulphur isotopes. Petrographic studies show widespread diagenetic replacement features in the carbonates, sulphates and gangue minerals. Oxygen isotopic data for the dolomite also show evidences of meteoric water alteration which is documented by the co-variance of several C-O isotopic data sets. On the other hand, C and S isotopic curves show an inverse correlation (Fig. 1), peculiar for this time range and conforms to the similar observations made by the earlier workers. Pronounced increase of ~34S (mean = 31.6%o) is accompanied by a change in the 6~3Cc~b values from 2.1 to --8%0. Allowing some compensation for the diagenetic modification of the isotopic values the magnitude of the isotopic shift for both the isotopes is about 7%o. Such an inverse correlation of C and S isotopic curves imply that Neoproterozoic and Phanerozoic exogenous cycles operated on similar modes (Strauss,1993).

Magnetotelluric Phase Anisotropy above Extensional Structures of the Neogene Pannonian Basin

Neogene extensional forces generated narrow rift zones (deep subbasins) in the Pannonian Basin, among others one of the deepest, the 7 km deep Békés graben. Above this structure strong magnetotelluric (MT) phase anisotropy (phase-deviation in two orthogonal directions) has been observed indicating the upwelling of the partially molten asthenosphere. This upwelling is certainly the first to prove the validity of the deep mantle structure of Buck's extensional narrow rift model by an electromagnetic method. A geotectonic relation will also be pointed out between the causes of MT phase anisotropy in cratonic and orogenic belts.