Showing papers on "Craton published in 1982"

Evolution of continental crust and mantle heterogeneity: Evidence from Hf isotopes

Abstract: We present initial 176Hf/177 Hf ratios for many samples of continental crust 3.7-0.3 Gy old. Results are based chiefly on zircons (1% Hf) and whole rocks: zircons are shown to be reliable carriers of essentially the initial Hf itself when properly chosen on the basis of U-Pb studies. Pre-3.0 Gy gneisses were apparently derived from an unfractionated mantle, but both depleted and undepleted mantle are evident as magma sources from 2.9 Gy to present. This mantle was sampled mainly from major crustal growth episodes 2.8, 1.8 and 0.7 Gy ago, all of which show gross heterogeneity of 176Hf/177Hf in magma sources from eHf=0 to +14, or about 60% of the variability of the present mantle. The approximate eHf=2eNd relationship in ancient and modern igneous rocks shows that 176Lu/177Hf fractionates in general twice as much as 147Sm/144Nd in mantle melting processes. This allows an estimation of the relative value of the unknown bulk solid/liquid distribution coefficient for Hf. DLu/DHf=∼ 2.3 holds for most mantle source regions. For garnet to be an important residual mantle phase, it must hold Hf strongly in order to preserve Hf-Nd isotopic relationships. The ancient Hf initials are consistent with only a small proportion of recycled older cratons in new continental crust, and with quasi-continuous, episodic growth of the continental crust with time. However, recycling of crust less than 150 My old cannot realistically be detected using Hf initials. The mantle shows clearly the general positive eHf resulting from a residual geochemical state at least back to 2.9 Gy ago, and seems to have repeatedly possessed a similar degree of heterogeneity, rather than a continuously-developing depletion. This is consistent with a complex dynamic disequilibrium model for the creation, maintenance and destruction of heterogeneity in the mantle.

Rift Basins in Western Margin of India and Their Hydrocarbon Prospects with Special Reference to Kutch Basin

Abstract: The western continental margin of India can be classed as a divergent or passive margin. The western continental shelf is an extensive carbonate bank (Bombay offshore basin) passing into clastic sediments on the north and south. Three craton-margin embayed basins--Kutch, Cambay, and Narmada--in the northern part of the shelf, are filled predominantly with clastic sediments. These basins occupy grabens bounded by faults diverging seaward. The grabens were formed by three rift systems along major Precambrian tectonic trends. The rifting developed sequentially from north to south around the Saurashtra horst. Kutch basin was formed in the Early Jurassic, followed by Cambay basin in Early Cretaceous time, and the Narmada in the Late Cretaceous. It appears that these rifting ev nts occurred at successive stages during the northward migration of the Indian plate after its break from Gondwanaland in Late Triassic or Early Jurassic. It is inferred that these rift basins opened up successively as a result of the counterclockwise drift of the Indian craton. Bombay offshore and Cambay are two major oil-producing basins in the western margin. These basins are characterized by high geothermal gradients attributed to the shallowness of the mantle in this region. Oil has not been found in Kutch basin, which is mainly an onshore Mesozoic basin. The basin depocenter shifted offshore at the northwestern part of the continental shelf where the shelf is wide.

The Western Canada sedimentary basin

Abstract: The Western Canada Sedimentary Basin, a simple northeasterly tapering wedge of sedimentary rocks more than 6 km thick, extends southwest from the Canadian Shield into the Cordilleran foreland thrust belt. Its internal structure and the lateral variations in its shape reflect a long and complex history of development involving a foreland basin that was superimposed on a cratonic platform and continental terrace wedge. This history, which is inextricably linked to the evolution of the Canadian Cordillera, can be outlined succinctly with reference to the unconformity-bounded transgressive-regressive stratigraphic sequences established by Sloss (Bull. geol. Soc. Am. 74, 93 (1963)), each of which has a distinctive character in Western Canada. The continental terrace wedge was established with the deposition of the Proterozoic Purcell (1500-1350 Ma) and Windermere (850-600 Ma) sequences, but the first record of the platformal phase is the early Palaeozoic transgressive onlap of the early Proterozoic (> 1750 Ma) crystalline basement by the Sauk sequence. Early Palaeozoic subsidence of the margin of the craton may have been due to cooling of the lithosphere after renewed stretching at the ancient rifted western margin of the Precambrian craton, and to isostatic flexure of the lithosphere under the weight of the sediment that had accumulated at the margin in the oceanward prograding continental terrace wedge. During a subsequent Middle Ordovician to Middle Jurassic phase, the cratonic platform became differentiated into an intersecting network of epeirogenic arches with intervening basins. Development of the basins was as much a result of erosion and uplift of the arches between transgressive-regressive cycles as it was a result of differential subsidence of the basins during the cycles. The cause of the long (> 300 Ma) episode of intermittent epeirogenic movements that produced the basins and arches is a major unsolved problem. The foreland basin developed in two stages, in Middle Jurassic to early Cretaceous and late Cretaceous to Palaeocene time, as a result of collisions between North America and two pieces of a tectonic collage of oceanic terranes that were accreted to its western margin. During these two collisions, the continental terrace wedge, which had accumulated outboard from the rifted margin of the continental craton, was compressed and displaced over the western margin of the craton. Part of the supracrustal cover was scraped off the craton and accreted to the overriding mass to form a wedge of imbricate thrust fault slices that was tectonically prograded over the margin of the continental craton. Isostatic flexure of the continental lithosphere in response to the tectonic loading imposed on it by the displaced continental terrace wedge and the accretionary wedge of thrust slices produced the migrating moat in which the outwash of clastic detritus from the evolving thrust belt was trapped to form the foreland basin. The Palaeohelikian flat-lying unmetamorphosed basin remnants high on the Craton show that the Craton was established long before the commencement of the Phanerozoic cycle of basin development examined in this paper. The first significant event in this history was the development, over attenuated crust, of an Atlantic type down-to-ocean faulted margin in the Neohelikian, upon which was deposited a great wedge of clastic sediments across the structural grain of the Archaean platform, followed after a gap of 500 Ma by renewed faulting and subsidence and further loading in Hadrynian time. The subsidence of the continental margin as a result of this imposed load and thermal contraction extended far into the craton and set the stage for the first cycle of deposition represented by the Sauk sequence. Sauk sequence. The Sauk sequence records progressive onlap with the Upper Cambrian strata extending high on the western side of the craton and the depositional strike running northwesterly, parallel to the probable old continental margin. There is a notable lack of arches or isolated basins. This initial sequence is dominated by clastic sediments from the shield. Tippecanoe sequence. Arches interrupted the linear depositional pattern. Marked sedimentary attenuation over highs created a relative thickening in the area of the Williston Basin. This sequence shows thin seams of fine sand and silt suggesting episodic uplift of the burgeoning arches, but the dominance of carbonate rock indicates that they were for the most part covered. The lack of an onlapping relation, and widespread inundation of the craton indicate a very rapid transgression. Like the Cambrian seas, those of the Ordovician and Silurian transgressed from the western margin. Kaskaskia sequence. The strongly developed pre-Devonian arches surrounding and segmenting the Devonian depositional area resulted in a transgression of over 3000 km from the northwest, rather than the west, over a tectonically disturbed and eroded surface, resulting in complex facies patterns. The Mississippian seas covered almost as large an area as those of the Ordovician, onlapping completely the Western Alberta and Peace River arches. Arches in the southern and western part of the Williston Basin continued to grow into late Kaskaskia time, creating sills with salt deposits in the regressive part of the sequence that culminated with a clastic assemblage derived from the flanking arches. Absaroka sequence. The Sweetgrass Arch isolated the Alberta Basin from the Williston Basin for the first time. These opened to the southwest and west respectively. The Williston Basin had a dominant red bed-evaporite assemblage, the Alberta Basin a marine clastic assemblage. This interval is characterized by periods of non-deposition and erosion, with depositional limits of each system indicating much less encroachment on to the craton, which appears to have progressively increased its rate of dip to the west. The progressively increased rate of westward thickening from the Sauk to Kaskaskia to Absaroka in the undisturbed part of the basin also suggests an increasing westward tilt or subsidence of the basin margin. Zuni sequence. The Zuni reflects the late Jurassic orogenic activity in the west. The convergence of allochthonous terranes with the continental margin resulted in the thrusting of the marginal deposits eastward, tectonically loading the lithosphere and providing the provenance for the continental clastic sediment that filled the foredeep which formed in response to the loading. The continental sediment spread across the basin with marine seaways to the north and south that gradually advanced to join together to form a seaway extending from the Arctic to the Gulf of Mexico in mid-Cretaceous time. A second plate convergence resulted in further tectonic thickening with thrusting extending further east, cannibalizing previously formed deposits and generating a second sequence of terrestrial sediments that covered the entire basin again in the late Cretaceous and Palaeocene. Notably absent during the Zuni was the influence of arches that had previously segmented the basin. Also absent are carbonates and evaporites. Instead the basin fill was entirely of clastic sediments. The present structural configuration of the basin was established during this period as the foredeep encroached onto the craton.

The geologic evolution of south america during the archaean and early proterozoic

Abstract: ln the South American continent, Archaean and Lower Proterozoic terranes can be found in all tectonic domains, but especially within the Amazonian and Sao Francisco cratons. They can also be found within the Sao Luis, Luis Alves and Rio de La Plata cratonic fragments, as well as within all mobile belts of the Brasiliano orogenic cycle, as reworked basement. The Arequipa massif is lhe only identified Lower Proterozoic unit in the Andean belt. Within the Amazonian craton, the Central Amazonian province is made up of Archaean rocks, exhibiting cratonic conditions since at least the Lower Proterozoic. It is bounded to the north by the Maroni-Itacaiunas mobile belt, in which the major tectonomagmatic episode occurred in the early Proterozoic, the Transamazonico orogenic cycle. It comprises large portions of supracrustal rocks, identified as greenstone belts, associated to gneisses and migmatites, as well as fragments of high-grade polymetamorphic terranes in which Archaean ages were obtained. Within the Sao Francisco craton, three main types of ancient geologic terranes occur: 1) Archaean granite-greenstone terranes, such as the Brumado-Anajo area in Bahia, and the Quadrilatero Ferrifero area in Minas Gerais; 2) Lower Protcrozoic supracrustal belts, such as the Jacobina, Serrinha and Contendas-Mirante sequences in Bahia, and the Minas Group in Minas Gerais; and 3) Medium to high-grade metamorphic terranes, subjected to extensive granitization during the Transamazonic orogeny (Salvador-Juazeiro mobile belt), and including Archaean "cratonic fragments", of grunulitic composition, such as the Jequie-Mututpe complex in Bahia. Ancient terranes were also found within the mobile belts of the Brasiliano cycle, where they make up their ensialic basement. Lower Proterozoic type radiometric ages are predominant, especially within the Borborema province and the Ribeira belt. Taking into account the general distribution of the ancient terranes, it seems that a major part of the South American continental crust was already existent as such just alter the Transamazonico cycle. Moreover, since the known Transamazonico belts are ensialic, it is clear that large portions of continental material were formed earlier, in Archaean times.

Analogous tectonic evolution of the Ordovician foredeeps, southern and central Appalachians

Abstract: New stratigraphic data suggest that the diachronous evolution of the Ordovician foredeeps in the southern and central Appalachians was remarkably similar. Stratigraphic features that characterize the Middle Ordovician Sevier basin in Tennessee and the Middle and Late Ordovician Martinsbuirg basin in Pennsylvania are in identical ascending order: (1) disconformity on the Knox Group–Beekmantown Group, (2) shelf carbonates, (3) slope deposits, (4) submarine fan turbidites, and (5) contourites and muddy turbidites. We propose that diachronous attempted subduction of the North American craton beneath southeastern microplates and/or volcanic arcs resulted in uplift and erosion of the western shelf followed by its rapid subsidence. Basinward migration of eastern and northeastern terrigenous source areas and associated submarine fan turbidites resulted from continued convergence and filled the basins. Finally, tectonic stabilization and lowering of the source area is recorded by contourites and muddy turbidites. The evolutionary model proposed for the Sevier and Martinsburg basins closely resembles present-day tectonics of the Timor foredeep and the adjoining Sahul shelf north of Australia. Similar comparisons have also been made for Ordovician basins in the northern Appalachians. Analogous tectonic mechanisms, therefore, appear to have operated diachronously along the eastern margin of North America from Tennessee to New England and possibly to Newfoundland during Ordovician time.

MAGSAT scalar anomaly distribution: The global perspective

Abstract: It is established that geographic coincidences exist between high-altitude Magsat scalar anomalies and major geologic and tectonic structures, with oceanic abyssal plains overlain by negative anomalies agreeing well in spatial extent and position and submarine platforms lying beneath positive scalar anomalies. In addition, geographic coincidence is found in the continents between many high-latitude positive anomalies and shields and cratons in North America, Eurasia and Australia. While these correlations are qualitative, they serve to identify regions for detailed study. The global distribution of anomalies provides a basis for comparative study which will be enhanced when reduced-to-pole versions of the Magsat data become available.

The Limpopo Mobile Belt: A result of continental collision

Abstract: The 600-km-long Limpopo Mobile Belt is discussed within the frame of a Proterozoic supercontinent model [Piper, 1976]. Evidence is presented that the Rhodesian and Kaapvaal cratons may have been separated by distances of more than 1000 km of oceanic crust. From about 3350 Ma ago the Kaapvaal Craton appears to have been driven intermittently N, NW, and then WNW against the Rhodesian Craton forming the NE-SW trending collision zone, the Limpopo Mobile Belt, and all the major fold and fracture patterns found. This movement would be similar to the oblique movement of the Pacific plate into the Aleutian trench. When collision ceased around 2500 Ma ago, it is likely that the Great Dyke and other complexes intruded along release fractures formed at right angles to the compression.

Structural Evolution of AN A-Type Subduction Zone, Lofoten-Rombak Area, Northern Scandinavian Caledonides

Abstract: The climactic phase of the Caledonian orogeny in Scandinavia involved collision of the Baltic and Greenland cratons in late Ordovician (?) to Silurian time. We infer that the collision resulted in large-scale subduction of the leading edge of the Baltic craton beneath an ‘Andean-type’ Greenland continental margin. Deep erosional levels in the Lofoten-Rombak area, Norway-Sweden (˜69°N) permit direct examination and evaluation of the structural evolution of a continental or A-type subduction zone. The early stages of collision involved en bloc underthrusting of the continental margin to depths of at least 30 km. During this process, deformational effects in the lower plate were restricted to its upper few hundred meters. Ultimately, en bloc A-type subduction should be limted by buoyancy effects. As convergence continued, the lower plate became involved in a series of imbricate thrusts with the same vergence as the intial subduction zone. The temporal and spatial relationships between these basement-involved thrusts are similar to those observed in foreland fold and thrust terrains. Field and geophysical data bearing on lower plate behavior elsewhere in the Caledonldes and in other collisional orogens suggest that the structural features observed in the Lofoten-Rombak area may be generally characteristic of A-type subduction in continent-continent collisional settings.

Average lead isotope growth curves for shale-hosted zinc-lead deposits, Canadian Cordillera

Abstract: Average lead isotope growth curves for shale-hosted zinc-lead deposits have been constructed empirically for a large but well-defined tectonic segment of the Canadian Cordillera. The model defined is applicable to the dating of galena-lead from shale-hosted deposits in the Selwyn basin and in the Ogilvie, Richardson, Wernecke, and Mackenzie Mountains, in the Eastern fold belt, and in much of the Omineca crystalline belt. This area of the cordillera generally had a coherent geochemical evolution. Lead for deposits was extracted from sediments which had been deposited along the western margin of the North American craton and which were derived largely from upper crustal crystalline rocks that formed the western part of the Canadian Shield.The average shale growth curves involve three stages of evolution. The third (most recent) stage of the three-stage model, starts at 1.89 m.y. and evolves to 0 m.y. with mu of 12.16 and omega of 49.09. The first two stages follow the two-stage model of Stacey and Kramers (1975). The model allows precise model age estimates, with an error generally less than 0.05 m.y., to be made for deposits of known age. This is particularly useful in property evaluation during exploration because in many cases the certainty of a syngenetic or epigenetic origin can control future philosophy about exploration and/or evolution of a deposit or an area. Model ages, in concert with stratigraphic information, commonly indicate the likelihood of epigenetic or syngenetic origins.

Faulting and graben formation in western and central Europe

Abstract: Rifts and rift-related basins play a pre-eminent role among the sedimentary basins of western and central Europe. Through time, grabens developed in a number of different megatectonic settings whereby the principal mechanisms governing their subsidence was crustal stretching and ‘subcrustal erosion’. The level of volcanic activity associated with rifting is highly variable and can change significantly during the development history of a rift. Some rifts are totally non-volcanic. The development of rift domes is generally associated with volcanic activity. Uplifting of a rift dome can induce a reversal in the subsidence pattern of a rift. Intracontinental rifts, with their thinned crust, are prone to inversion when the respective craton is subjected to tangential stresses. In the process of inversion the crust of rifts is mechanically thickened again.

Exotic terranes of western California

Abstract: Numerous distinct geological terranes compose the North American Cordillera1; there may be as many as 50 terranes in California alone2. Critical to deciphering the history of Cordilleran tectonic assembly is an understanding of the displacement history of individual terranes. It is therefore important to know: (1) whether a terrane has undergone significant motion with respect to the stable craton (that is, whether it is allochthonous or exotic); (2) if so, when relative motion started and stopped; (3) from where an individual terrane originated; and (4) the nature of interterrane movements. We consider here the problem of determining whether the now-juxtaposed Salinian and Stanley Mountain terranes of California became amalgamated at or near their present position with respect to cratonic North America, or if they collided at a considerable distance from their present positions and were later accreted to North America as a composite package. The palaeomagnetic data that we present indicate that the latter was the case.

COCORP profiling across the Rocky Mountain Front in southern Wyoming, Part 2: Precambrian basement structure and its influence on Laramide deformation

Abstract: The influence of Precambrian basement structures on subsequent deformation is of considerable relevance to studies of continental evolution COCORP deep seismic-reflection profiles were recently recorded in southeastern Wyoming, where several major, but temporally separated, tectonic elements of the western United States are superimposed Of these, a fundamental boundary between Archean and Proterozoic basements and the eastern front of Laramide deformation were the principal targets of the reflection survey The former may represent an ancient Proterozoic plate boundary; the latter is a prominent physiographic feature that signifies crustal deformation far within the North American craton, more than 1,500 km from the nearest coeval plate margin The major crustal feature controlling a lateral, north-south variation in Laramide tectonic style appears to be the Archean-Proterozoic crustal boundary, known in the nearby Medicine Bow Mountains as the Mullen Creek-Nash Fork shear zone COCORP data in the Laramie Mountains and the Laramie Basin suggest that this shear zone dips ∼ 55° to the southeast Northwest of the shear zone, the seismic basement is also characterized by southeast-dipping events, suggesting that the early Proterozoic tectonics that produced the shear zone were distributed over a wide region Complex reflections down to 15-km depth or more under the Laramie Basin may correspond to structures or erosional truncations in metasediments overlying the Archean basement complex Deep crustal events (between 35 and 40 km) north of the shear zone are short and discontinuous, in contrast with flat, laterally continuous reflections south of the shear zone at about 48-km depth which are interpreted as the crust-mantle transition Thus, COCORP data and published results of regional refraction and gravity surveys suggest that the crust is significantly thinner in the Archean basement terrane northwest of the shear zone than it is in the Proterozoic province to the southeast Differences in crustal thickness may be partly responsible for the difference between Laramide structures in Wyoming and Colorado, and a thin crust may also have facilitated Laramide deformation farther east in the Black Hills, located north of the shear zone

Reconstruction of the early Proterozoic stratigraphy of the Gawler Craton, South Australia

Abstract: Detailed structural mapping on NE Eyre Peninsula, South Australia, has led to a revised stratigraphy and model of sedimentation for Early Proterozoic metasediments of the Gawler Craton. Four stages of deformation have been recognised; three stages are associated with the Kimban Orogeny (c. 1820–1580 Ma) and a fourth stage is known as the Wartakan Event (c. 1500–1450 Ma). The recognition of major D2 folds has shown the previously used stratigraphy to be incorrect and has necessitated its revision. At the base of the sequence, unconformably overlying a 2300 Ma or older basement, is the Warrow Quartzite. A transgressive cycle of schist, dolomite (Katunga Dolomite) and iron formation (Lower Middleback Jaspilite) overlies the quartzite, and this is overlain in turn by a regressive semipelitic unit containing local amphibolites (Cook Gap Schist), and another transgressive iron‐formation bearing cycle (Upper Middleback Jaspilite). At the top of the sequence is the Yadnarie Schist. All units overlying th...

Carpathian Foreland Fold and Thrust Belt and Its Relation to Pannonian and Other Basins

Abstract: The foreland fold and thrust belt of the Carpathians can be traced from Austria through the western and eastern Carpathians to the south Carpathian bend in Romania where most of its structural units plunge beneath younger Pliocene-Pleistocene cover. Folds in the youngest rocks at the south Carpathian bend continue westward, until all surface expression disappears before reaching the Danube River. The fold and thrust belt is flanked by elements of the European, Russian, and Moesian cratonal areas which are overlain by a variable width and thickness of foredeep deposits that are partly involved within the external fold and thrust belt. Internal to the fold and thrust belt are older parts of the Carpathian orogene formed mainly on continental crust. These older structures have significant differences in evolution between the western and eastern Carpathians. The foreland fold and thrust belt consists predominantly of flysch in its inner parts and molasse in its outer parts. These sedimentary rocks are cut into a series of thrust sheets that have moved relatively toward the craton. Oldest flysch units are Late Jurassic and the youngest units are Miocene. Some of the flysch units of the inner part of the belt may have been underlain by oceanic crust, whereas the younger, more external molasse units were underlain by continental crust. Timing of thrusting is only well-constrained locally, but suggests that deformation in the western Carpathians developed during Oligocene to Miocene time. In the eastern Carpathians, structural activity continued into the Pliocene, and deep earthquake activity at Vrancea suggests subduction. Deformation is sti l locally active. A volcanic arc of Miocene to Holocene age lies internal to the fold and thrust belt. Magmatic activity appears to have migrated externally and eastward in time. Contemporaneous with thrusting was the development of Neogene basins in the intra-Carpathian region. These basins exhibit a two-phase subsidence history (except for the Pannonian and Transylvanian basins)--fast initial subsidence, followed by a period of slower linear subsidence. The Pannonian basin shows only a reasonably fast linear subsidence. Structural and thermal models indicate the basins were formed by about 100% stretching. The convex east loop of the Carpathian foreland fold and thrust belt is a result of the collision of continental fragments against a recess in the combined European-Moesian plate. In this region, subduction or downbending of the subducted slab results in an extensional stress field, suggesting that thrusting and apparent compression along the mountain belt are only thin-skinned, superficial effects due to the inability to subduct light upper crustal material and detachment of the crust from the underlying lithosphere. The dominant forces seem to be those acting on the downgoing slab, which produce shortening along the plate boundary, and plate geometry that inhibits rigid plate convergence.

COCORP profiling across the Rocky Mountain Front in southern Wyoming, Part 1: Laramide structure

Abstract: COCORP deep seismic-reflection profiles have been recorded showing significant crustal deformation of the North American craton; profiles were made across the Laramie Mountains, the Wyoming section of the Front separating the southern Rocky Mountains from the Great Plains, and eastern margin, in Wyoming and Colorado. These mountains are underlain by a series of westerly dipping seismic reflectors traceable as deep as 10 to 12 km, arranged en echelon with various dips (20° to 50°), which in some cases can be traced to the surface position of faults flanking the Laramie Mountains. Although other interpretations are possible, the reflectors are thought to be thrust faults, whose distribution and orientation suggest that the mountains were uplifted by horizontal crustal shortening during the Laramide orogeny. These inferred thrusts apparently do not generate such pronounced fault-zone reflections as those seen on COCORP lines recorded across the Wind River Mountains in Wyoming, probably because fault displacements were much smaller. Differential movements along the thrusts may explain variations in the character of the Rocky Mountain Front in the region of the COCORP lines. On the basis of morphological continuity, the Front in Colorado may also have been uplifted by crustal shortening with variations of structural style caused by adjustments to Precambrian and Ancestral Rockies structures. COCORP data have now been recorded across two mountain ranges lying on the eastern and western margins of the region of Laramide basement uplifts in Wyoming. Both ranges (Laramie and Wind River) apparently are underlain by moderately dipping thrust faults. Probably, crustal shortening by lateral compression was the dominant cause of the Laramide basement uplifts. These mountain-building compressional stresses were transmitted to the interior of the continent, 1,000 to 1,500 km from an active plate margin.

Kimberlites in western Liberia: An overview of the geological setting in a plate tectonic framework

Abstract: The Liberian kimberlite field lies in the southern part of the West African craton. Closely associated kimberlites are present in Sierra Leone, Ivory Coast, Guinea, and Mali and, as a group, are structurally controlled within the Archean basement complex of the Leo uplift. Three major kimberlite dike lineaments are recognized in the northwest sector of Liberia all with prominently developed N-NE trends. These lineaments are termed the Kumgbo-Wuese trend, the Border trend, and the Lofa trend. The areas of dike intrusion are covered by the Bopolu geological quadrangle map of Liberia: the Border trend is close to Sierra Leone, the Kumgbo-Wuese trend is to the east, and the Lofa trend is coincident with the Lofa River. Only one kimberlite pipe has been recognized in Liberia (Mano Godua), but a second may be present, and both lie along the Kumgbo-Wuese trend. The kimberlite bodies intrude the ∼2700-m.y.-old Liberian age province, which is comprised of granitic and granodioritic gneiss complexes. Two prominently developed features dominate the province: a general NE-SW basement fabric and an en echelon dolerite dike system that parallels the coast, dated at ∼180 m.y. and considered to represent intrusion at the onset of continental fragmentation. Transecting dikes and related faults suggest a relative age for the kimberlites of < 180 m.y., an estimate that corresponds to the 92- to 140-m.y. age for the Sierra Leone field. There is indirect evidence that kimberlites in Guinea and Mali are of a similar age based on the fact these are on strike with kimberlites in Liberia and Sierra Leone, respectively. Trans-Atlantic correlations between West Africa and northeast South America reveal a cyclic eruption of magma types that were generated in response to plate and continental movements. It is postulated that Liberian kimberlite emplacement was related to postplate motion crustal flexuring that reactivated Precambrian basement sutures. These sutures are parallel to, and correlate with, recognizable paleorift and paleocompressional zones within, and adjacent to, the West African craton. Long sustained periods of tectonic activity can be demonstrated, and it is proposed that, if brittle fractures are continuously reactivated, these will serve as conduits to the upper mantle, tapping kimberlite source regions at depths in excess of 100 km.

Proterozoic tectonic evolution and late Svecokarelian plate deformation of the Central Baltic Shield

Abstract: The continental crust of the Central Baltic Shield evolved by accretion towards the west during the Svecokarelian orogeny 1700–2200 Ma ago. The following features are consistent with a plate tectonic mechanism involving subduction of oceanic crust below an Archean craton in the east: flysch-sediments with serpentinite masses and pillow lavas, linear high-grade metamorphic zones, island-arc type volcanic belts and late tectonic batholiths with porphyry type Cu-Mo deposits. Semi-consolidated new crust was affected by late Svecokarelian deformation (Dn) after 1850 Ma; NNE-trending folds with crenulation cleavage were overprinted on older structures together with associated NW trending ductile transcurrent shear zones that curve the Fn folds into gentle S and Z shapes. The late tectonic batholiths intruded partly at the same time as and partly after the Dn deformation.

Reworking of sialic crust as represented in late Archean–age gneisses, northern Labrador

Abstract: Regionally reworked sialic crust has been recognized in the form of ∼ 2,800-m.y.-old nebulitic rocks (Kiyuktok gneisses) with high initial 87Sr/86Sr ratios (∼ 0.7081) and Pb isotope ratios that suggest a history of the source back to at least 3,500 m.y. B.P. The field, geochemical, and isotopic data are consistent with these rocks having formed by in situ mobilization of granitic components in the ∼ 3,600-m.y.-old Uivak gneisses by the introduction of aqueous fluids some 2,800 m.y. ago. Contemporary but subordinate tonalitic-granodioritic orthogneisses derived from discrete intrusive bodies have contrasting Sr isotope compositions. Some display low initial 87Sr/86Sr ratios (∼ 0.7030) that suggest derivation from a source with mantlelike Sr-isotope composition. However, most of the late Archean–age intrusive rocks display higher initial 87Sr/86Sr ratios (∼ 0.7056, 0.7059). Recognition of these contrasting isotopic patterns suggests that processes of late Archean crust formation and stabilization in the North Atlantic craton were more complex than previously realized. Crustal reworking due to interaction of old crust with aqueous fluids must have been accompanied by the introduction of granitic melts derived by melting of ancient deep crustal rocks or contamination of juvenile granitic melts by ancient deep crustal material.

Seismic crustal structure of the Proterozoic North Australian Craton between Tennant Creek and Mount Isa

Abstract: Seismic recording along a 600-km traverse across the southern part of the North Australian craton indicates that lateral velocity variations exist in the middle/lower crust. Taken together with other data from the Craton, there is evidence for midcrustal velocities increasing from east to west and from south to north. These trends could result from a greater proportion of high-velocity granulites being emplaced at higher levels in the crust. Between Tennant Creek and Mount Isa the velocity structures near the surface are compatible with an assemblage of younger and older metamorphic rocks intruded by granites. Near-surface (0–5 km) P wave velocities on the Tennant Creek Inlier increase from 5.47 km/s at the surface to about 6.2 km/s at 6-km depth and correspond with low-grade metamorphics of the Warramunga Group interspersed with weathered granites overlying amphibolite facies rocks of older metamorphic domains. In the Mount Isa Geosyncline, velocities of 6.03–6.15 km/s correspond to the Leichhart Metamorphics interspersed with granites. In the middle crust, P wave velocities of 6.85 km/s occur at a depth of 26 km near Tennant Creek, whereas such velocities are not evident until depths of about 37 km near Mount Isa. There is therefore a trend for midcrust velocities to increase from east to west. The lower crust is characterized by velocities of 7.3–7.5 km/s, and the upper mantle velocities of 8.16–8.20 km/s are reached at depths of 51–54 km. Exposed granulite facies rocks in central Australia are low in heat-producing elements and are compatible with the low surface heat flow generally measured in Precambrian Australia. These data taken together with laboratory velocity measurements on worldwide samples reported elsewhere indicate that middle to lower crustal rocks could be an assemblage of granulite facies rocks varying from pyroxene granulite in the middle crust, through hornblende granulite, to garnet granulite in the lower crust.

Plate Tectonic Evolution of the Fort Worth Basin

Abstract: The Fort Worth basin, one of several Late Paleozoic foreland basins formed along the front of the growing and advancing Ouachita fold and overthrust belt, has long been considered a preserved remnant of the Ouachita geosyncline. In terms of plate tectonic concepts, however, its geologic evolution represents the Wilson cycle, with opening and subsequent closing of an ocean. A complete accounting of the entire geodynamic evolution from passive plate margin to collisional orogen requires detailed knowledge of the inseparably linked processes of sedimentation, volcanism, metamorphism, and mechanics of deformation. Each of these processes interacted as the area passed through the various stages of its evolution. The reconstruction of the tectono-sedimentary history of the Fort Worth basin begins with part of a rifted and retreating plate margin over which Early Paleozoic seas advanced to deposit the shelf carbonates of Cambro-Ordovician age in Texas. Thicker sequences of this same facies accumulated in West Texas and Oklahoma in the more rapidly subsiding Delaware and Wichita aulacogens. During the Middle Paleozoic, a period of tectonic unrest resulted in erosion or non-deposition of sediments in this time interval in the Fort Worth basin area. The Ouachita facies sediments were deposited synchronously with the shelf carbonate facies. With the closure of a marginal sea, the Ouachita fold-thrust belt evolved as the subduction complex composed of the Ouachita facies strata scraped from the subducting oceanic crust. By Mississippian time, the continental margin was nearing the subduction zone. The subduction complex grew and was thrust over the continental margin and thus became the major source not only for the early synorogenic flysch sequence of Mississippian-Pennsylvanian age but also the later molasse sequence represented by the Atoka and Strawn deposits. As plate convergence continued, the outer arch or rise flexed the subducting craton. The resultant basinal hingeline controlled sediment facies distribution as it retreated cratonward in response to continued cratonic flexing, as the continental margin entered the subduction zone and Pangaea formed. With the rifting and breakup of Pangaea in the Triassic, and the formation of the Gulf of Mexico, the southern margin of North America once again became a trailing and subsiding plate margin over which Early Cretaceous seas advanced to deposit the Comanche Series that today overlie the tilted and eroded Paleozoic sequence of the Fort Worth basin.

Incipient Rift Zone, Western Sverdrup Basin, Arctic Canada

Abstract: Normal faults, linear magnetic anomalies, gabbro dikes, aligned evaporite domes, and modern earthquake epicentres define a broad tectonic belt in upper Paleozoic and Mesozoic rocks of western Sverdrup Basin. From Melville Island, at the southern margin of the basin, the belt strikes northeastward, toward the continental margin, at northern Ellef Ringnes Island. The age and geometry of the structural elements indicate that the belt represents a long-lasting domain of crustal dilation, as an incipient rift in the northern margin of the craton. Fracture systems, developed from dilation along the belt, could have served as conduits for migrating hydrocarbons, contributing to large natural gas accumulations at Sabine Peninsula, King Christian Island, and western Ellef Ringnes Island, and oil and gas under the offshore regions between the islands.

Limits to northward drift of the Paleocene Cantwell Formation, central Alaska

Abstract: Volcanic rocks of the Paleocene Cantwell Formation in central Alaska apparently originated at a paleolatitude of 83°N (α 95 = 9.7°), as indicated by paleomagnetic results. When compared with the Paleocene pole for the North American craton, the 95% confidence limits of the results suggest that terranes north of the Denali fault have moved no more than 550 km northward relative to the North American craton since Paleocene time.

Paleogeographic relationship of a Mississippian barrier-island and shelf-bar system (Hartselle Sandstone) in Alabama to the Appalachian-Ouachita erogenic belt

Abstract: A Mississippian clastic facies in the southern United States is within one of several regionally extensive clastic wedges that record the evolution of the Appalachian-Ouachita orogenic belt along the eastern and southern margin of the North American craton. In the eastern fringe of the wedge in Alabama, the Hartselle Sandstone is a northwest-trending linear sandstone along the regional facies boundary between clastic-wedge rocks (Floyd-Parkwood) and carbonate rocks (Monteagle-Bangor) typical of the shallow-marine shelf of the craton. Internal facies distribution in the Hartselle Sandstone defines a depositional system consisting of a northwest-trending barrier-island complex on the southwest and migrating sand bars on a shelf on the northeast. Barrier-island facies include horizontally laminated foreshore sandstone that contains limestone interbeds, as well as crossbedded longshore bar sandstone that contains fossil logs. The shallow shelf was dominated by migration of bars, wave reworking, and deposition of fine sediment from suspension in interbar areas. These processes are indicated by a complex vertical succession of crossbedded sandstones, rippled sandstones, and mudstones. Shelf processes reworked the upper part of the barrier-island sediment during regional southwestward marine transgression that is confirmed by southwestward extent of the Bangor Limestone over the Hartselle Sandstone. The northwest-trending barrier island of the Hartselle indicates sediment supply from the southwest, and that sediment dispersal system suggests uplift at a tectonically active converging continental margin along the Appalachian-Ouachita orogen during the Mississippian.