Precambrian Blocks Research Articles

Geology and genesis of peraluminous granites in East Tianshan Upper Paleozoic island arc belt

A peraluminous granite belt occurs along the connecting zone between the Turpan-Hami Precambrian block and the Upper Paleozoic island arc belt. Muscovite granite and twomica granite are the essential lithological components of that belt. All the potassium feldspars in these granites are microcline. Heavy minerals are dominated by magnetite. SiO2 contents of these granites are greater than 73% with most of the A/NKC values greater than 1.1, normative corundum values greater than 1. Plots of CIPW norms in the (Al-K-Na)-Ca-(Fe2++ Mg) diagram are mostly situated in the plagioclase-cordierite-muscovite region. The rocks are characterized by very low contents of minor elements and ΣREE with strong Eu depletion, δ18O values between 6.6‰ and 7.0‰, Rb-Sr isochron age of 260.2 ± 6.2 Ma and an initial87Sr/86 Sr ratio of 0.7052. These granites might have been produced by partial melting of moderately acidic volcanites and low-maturity sediments in the basement sequences and could be genetically connected with the southward A-type subduction of the Turpan-Hami block following the closure of the Middle Carboniferous back-arc basin.

The East Siberia Transect

The new East Siberia transect, constructed by synthesizing recently analyzed geological and geophysical data, runs in a broken line through the vicinities of the towns of Nizhneangarsk, Chita, and Borzya, traversing the Siberian Platform margin and the Baikal and Mongolia-Okhotsk fold areas. The transected region encompasses a number of terranes, most of which once were arc-trench systems with fore-arc and back-arc basins. The island arcs involved Early Precambrian blocks that were sialic cores of some of their islands. The terranes have complex spatial relations in the present tectonic framework, with most of the rocks belonging to magmatic arcs, whereas the associated basins comprise allochthons thrust over continental margins. Accretion of the terranes to the Siberian continent occurred in two stages, the Early Proterozoic and the Early Paleozoic. Middle Paleozoic and Late Paleozoic–Early Mesozoic subduction on the periphery of the Mongolia-Okhotsk ocean, which actually was an enormous guK of the Pacif...

Precambrian Tasmania: a Thin-skinned Devil

An interpretation of geological and geophysical data is presented to suggest that the Precambrian rocks of Tasmania comprise a series of Palaeozoic thrust slices embedded in the western part of the Lachlan Fold Belt and the eastern part of the Kanmantoo Fold Belt. This interpretation contrasts with previous interpretations which regarded the Precambrian rocks as the basement on which the Palaeozoic rocks formed. In western Tasmania, east- or west-facing thrusts and involving detached Precambrian blocks occur in discrete zones. West-facing thrusts only occur northwest of an axis from Elliott Bay in southwest Tasmania to Penguin in northern Tasmania. This thrust system overlaps an older east-facing system which becomes more intense eastward but which may be recognised across the entire island.

Early Middle Paleozoic Intraplate Orogeny in the Ogcheon Belt (South Korea): A new insight on the Paleozoic buildup of east Asia

The polycyclic Ogcheon belt of South Korea represents the boundary between two Precambrian blocks. Investigation on the timing and kinematics of the block agglomeration is of considerable bearing on the understanding of the constitution of the Asian continent. We report here structural evidence showing that the earlier tectonic event dates back to early middle Paleozoic (the “Ogcheon tectonism” or “orogeny”). The ductile piling up of nappes of the Ogcheon belt corresponds to an intracontinental orogeny involving the opening and the subsequent closure of an aborted rift and not of a wide oceanic area. The nappes contain unequivocal evidence for ductile shearing with a northwest over southeast sense of movement. Field evidence includes the presence of S1 and S1–2 foliations bearing an extensive stretching lineation perpendicular to the belt, associated with a strongly noncoaxial deformation regime. The F1 isoclinal folds are commonly “A type” and sheath folds whose axes are parallel to the stretching lineation, whereas the southeast facing F2 recumbent folds have subhorizontal axes parallel to the belt. All lines of evidence (unconformity, superposed deformations, age of the metamorphism, etc…) imply that the D1–2 “Ogcheon tectonism” is pre‐middle Carboniferous (Late Silurian‐Early Devonian). The subsequent Indosinian (Middle Triassic) F3 upright synfolial folds that trend NNE‐SSW and later structures clearly overprint the middle Paleozoic ductile thrusts and isoclinal folds. The early Paleozoic Ogcheon Supergroup is formed of a thick volcanosedimentary pile deposited above an early platform sequence in a rift basin. On the basis of the geochemical characteristics of the metavolcanics, it appears that oceanization did not occur and Ogcheon rift aborted soon after it was formed. We suggest that middle Paleozoic ductile nappe structures have been formed by the structural inversion of the rift extensional features. During the ductile stacking episode, the early formed foliation and isoclinal folds have been folded in a continuum during the same tectonic event. Middle Carboniferous terrigenous formations have been deposited unconformably upon folded and thrust older rocks. A tentative correlation with early Paleozoic Imjingang, Qinling and Cathaysian belts within the Asian continent reveals that Ogcheon rift was emplaced within the South China plate and that Imjingang belt should represent the limit between North and South China blocks in the Korean peninsula.

Geotectonic evolution of the Siberian platform during the Precambrian and comparison with the lower Precambrian complexes of eastern Asia

The main stages of the geotectonic evolution during the Precambrian of the Siberian platform are older than those in the other blocks of eastern Asia. Upper Proterozoic orogenic stages took place in north China-Korean blocks near the collision boundary with the south China block. Such Upper Proterozoic stages are characteristic for the south China, Bureya and Omolon massifs. The distribution of the various Precambrian blocks in eastern Asia can be explained as follows. 1. (1) Precambrian blocks of north China-Korean, south China, Bureya and Omolon massifs were affected by the activization and destruction (with the separation from the Siberian platform) in the Upper Proterozoic. 2. (2) These blocks (especially Omolon, Bureya, south China) are Gondwana continental fragments which joined the northern blocks during the Paleozoic and Triassic.

Thickness Variation of Simpson Group in South-Central Oklahoma and Its Tectonic Significance: ABSTRACT

The Middle Ordovician Simpson Group in the Southern Oklahoma aulacogen is composed of interbedded sandstone, limestone, and shale. Several pulses of subsidence controlled the deposition of these sediments. Simpson Group thickness variations, based on an isopach map and corresponding regional cross sections, define the presence of two distinct depositional basins flanked on their northern sides by a stable cratonic shelf. The anomalous thickness of Simpson sediments within these basins is related to syndepositional subsidence along zones of weakness initiated during the rifting stage of aulacogen development. The larger basin covers the western part of south-central Oklahoma. The northern flank of this basin illustrates a zone of rapid thickening of sediments. The updip portion of the northern flank is the northernmost limit of the Southern Oklahoma aulacogen. The depocenter of the larger basin is positioned in the Ardmore basin. Paleostress studies using calcite twin lamellae along the southeastern portion of the Sulfur fault within the smaller basin reveal an east-west compression followed by north-south compression. Surface folds formed by the east-west compression are highly faulted and overturned, whereas the folds formed by the north-south compression are open, slightly asymmetric, with rounded hinges and limbs. Similar east-west-trending structures in the subsurface couldmore » be a favorable target for hydrocarbon exploration. In general, these basins are genetically related but are separated by a large Precambrian basement block (Tishomingo Granite).« less

Early Precambrian Basement uplift and block faulting along the western margin of the Bushveld Complex, southeastern Botswana (Eugene J.L. Dietvorst)

Early Precambrian Basement uplift and block faulting along the western margin of the Bushveld Complex, southeastern Botswana (Eugene J.L. Dietvorst)

Geodynamic regimes in the precambrian of the urals

Of all Precambrian rocks in the Urals the late Proterozoic is best represented. The Archaean and lower Proterozoic are reconstructed from separate blocks (massifs) which are roughly correlated with the crystalline basement of the adjacent platforms. Radiometric dates for the majority of early Precambrian blocks exceed 2 Ga. Features of very old (2.9 Ga) protocrustal material have been identified with grey gneiss complexes. Archaean blocks represent remnants of charnockite-granulite and migmatite-amphibolite complexes, and the geodynamic regime of their generation is the most primitive and does not occur in later periods of geological history. The Karelian age blocks appear to be relics of nongeosynclinal mobile belts. Their petrochemical characteristics and REE patterns suggest rift-related conditions of their formation. Late Proterozoic deposits (1650-590 Ma) comprise thick terranes, both at the western slope and in the axial zone of the Urals. Revision of rock associations disclosed that these terranes were previously wrongly interpreted as geosynclinal formations. They consist of platform sediments and include magmatic rocks which are typical of zones of diaschysis (tectonomagmatic activation) and rifting. Recurrent rifting of the Elsonian, Grenville and Baikal (Assynthian) cycles of diastrophism was responsible for crustal extension and thinning, followed by complete continental breakup and development of oceanic basins in the early Palaeozoic only. Extension in the lower crust during rifting was responsible for dynamometamorphism under different temperature conditions and dropping lithostatic pressure. Blastomylonites and schists produced during this process were previously mistaken for products of orogenic metamorphism. They were considered as evidence of geosynclinal cycles occurring in the late Proterozoic and similar to those of the Palaeozoic. Diaschysis and rifting were probably predominant geodynamic regimes in the late Precambrian of the Urals. They resulted in destruction or extensive reworking of the original Archaean and early Proterozoic crust to variable degrees and in the accumulation of volcano-sedimentary and terrigenous rifted-depressional successions. There is no reliable evidence for the existence of an oceanic basin in the late Proterozoic of the Urals.

Cathaysia, Gondwanaland, and the Paleotethys in the evolution of continental Southeast Asia

Continental Southeast A ia is dominated by Precambri an continenral blocks overlain by Late Proterozoic to Paleozoic platform successions, representing Atl an ti c-type rifted miogeocl in al margin s. All th e blocks appear to have rifted and drifted from the Australian part of Gondwanaland. The timing and ex tent of their eparati on is ana lysed by th e distribution of Penni an Cathays ian Gigamop­ leris and Gondwana Glossop1eris floras, assisted by dated tectono-stru ctura l units, pa leoclimate ind icators, and good quality paleomagnetic data. Between th e blocks lie narrow intensely folded Ph anerozoic mobile belts, which developed on the oceanic crust of th e Paleoteth ys ocean, c haracteri zed by pela gic-turbidite flysch equences which shallowed as th e oceans narrowed. The narrowing was effected by subduction resultin g in island arcs within the oceans, and cordill eran volcano-plutonic arcs along the bl ock marg in . Extinction of th e bas in s resulted in colli s ion zones containin g S-type granites and utu re zones containin g dismembered ophi olites. Post-consolid ation pl ate readju tments res ulted in wrench and rift fa ulting in several places while convergence conti nu ed elsewhere. The tectoni c analysis has been carried out by recognizing tectonic elements (s tructural-formati onal unit~) for selected Phanerozoic time frame . We also pre ent a Phanerozoic sequence of palinspatic reconstructiors fo r the ri fti ng and drift in g of the blocks from northern Australia . These are consistent with the known ages and distribution s of th e tectono-structural units a nd th e good quality paleomagnetic data. We also present an ana lys is of the first-order re lat ionship between the di tributi on of mineral deposits and the igneous events which resulted from tectonic reactions between the Precambrian blocks and th e mobile be lt s.

Determination of the three-dimensional seismic structure of the crust and upper mantle in the Central Midlands of England

Summary. Teleseismic P-wave residuals relative to CWF, a permanent shortperiod seismic station on Charnwood Forest in the Central Midlands of England, have been determined for two small aperture arrays deployed over the Precambrian block of Charnwood and its surrounding Phanerozoic sediments. The data have been inverted to produce a block model of the P-wave velocity variations in the crust and upper mantle beneath the study region. The results are consistent with significant variations penetrating to a depth of at least 50 km. Low velocities are associated with two upper crustal intrusive bodies, the Caledonian Mountsorrel granodiorite and the South Leicestershire diorites. A longer-wavelength variation at lower crustal/upper mantle depths could arise from the Moho dipping to the south-west beneath the study region, and whose strike sub-parallels the dominant Charnian trend of the major basement structures in this part of Central England.

Periglacial structures and palaeoclimatic significance of a late Precambrian block field in the Cattle Grid copper mine, Mount Gunson, South Australia

A tabular body of quartzite breccia at least 5 km2 in area and up to 20 m thick is preserved beneath the aeolian Whyalla Sandstone of late Precambrian age at the Cattle Grid copper mine on the Stuart Shelf W of the Adelaide Geosyncline. The breccia (Cattle Grid Breccia) exhibits numerous periglacial structures and is interpreted as a fossil block field formed through in situ frost shattering of silicified sandstone (Pandurra Formation) during the Marinoan Glaciation ca. 680 Ma ago. Periglacial structures displayed by the breccia include: primary sand‐wedge polygons; anticlines and tepee‐like structures, some cut by reverse faults, ascribed to frost heaving and frost thrusting; and truncated earth mounds. Sand wedges, drop involutions, periglacial injections, and frost‐heaved blocks also occur within the basal few metres of the Whyalla Sandstone, indicating that periglacial activity continued after burial of the block field. During formation of the breccia the mean annual air temperature probably was no hi...

Offshore extensions of the Avalon Zone of Newfoundland

Gravity and magnetic data from the nearshore region of the Avalon Zone of Newfoundland provide evidence of the nature of a sedimentary basin immediately east of the Avalon Peninsula. The data also suggest a strong correlation between the arcuate magnetic patterns of the offshore portion of the Avalon Zone and the Precambrian mafic volcanics mapped onshore. The offshore basin is interpreted to have been formed by Precambrian block faulting and subsequently filled by sediments derived from surrounding paleotopographic highs. Two-dimensional gravity and magnetic modelling results suggest the presence of mafic material beneath the modelled 9+ km of sediment in the basin.

Tectonic and magmatic evolution of the Andes of northern Argentina and Chile

Abstract Two orogenic cycles, both with different evolution, are developed in the western margin of the South American continent in northern Argentina and Chile: the Paleozoic “Hercynic” cycle and the Meso-Cenozoic “Andean” cycle. The Hercynic cycle. A wide marine basin extending westward of the Cordillera Oriental which developed in Cambrian-Ordovician times marks the beginning of this cycle. In contrast to Peru and Bolivia where this basin developed between two Precambrian blocks, the western margin of this basin in northern Argentina and Chile is still unknown. The Ordovician sedimentation and accompanied volcanism ends with the Ocloyic deformation phase and its synkinematic granitic plutonism. Two basins were developed in the Silurian-Devonian separated by the Puna arc, uplifted during this ocloyic phase. The shallow-water marine terrigeneous sediments which were deposited in them were deformed by a new tectono-magmatic associated phase (Chanic phase). Carboniferous to Lower Permian marine carbonates were deposited west of the Puna arc and red beds east of it. Later on, during the Permian to Triassic, a magmatic belt was developed along the Cordillera Occidental. The rhyolites, ignimbrites and the granitic to granodioritic related intrusives are well represented in Chile. Although the overall geologic history of this period is known, many problems concerning its origin and relations to plate tectonics are still unsolved. The Andean cycle. During this cycle, a series of magmatic-arc systems related to the subduction of the Pacific crust was built up along the western margin of South America. Huge volumes of calc-alkaline lavas and related plutons were emplaced since the Jurassic, along belts parallel to the present coastline, showing a general eastward migration trend. Up to the Lower Cretaceous, an ensialic back-arc basin was formed east of this magmatic arc. Thousands of meters of marine and continental sediments were deposited in it. This basin disappeared during the Middle Cretaceous, probably as a result of the final opening of the Atlantic and the active westward movement of the South American Plate. Since Middle Cretaceous times, the magmatic-arc has been the fundamental paleogeographic element. The arc progressively migrated stepwise eastward, each step marked by a tectonic phase. Subduction-related crustal erosion could explain the lack of fore-arc petrotectonic assemblages. The eastward magmatic polarity which characterizes this section of the Andes, could also be explained by such a mechanism.

Geometry of Tertiary Coal Seams as a Response to Changes in Structural Framework: ABSTRACT

Diminished subsidence of the Williston basin at the end of the Cretaceous was followed by accumulation of thick lignite seams in the Ravenscrag Formation (Paleocene). The formation of seven coal fields in southern Saskatchewan was also influenced by contemporaneous subsidence structures that originated as collapse of overlying strata above areas of salt leaching from the buried Devonian Elk Point evaporite basin. Lignite seams, up to 10 m thick, accumulated in successively younger coal fields that prograded to the southeast across southern Saskatchewan toward the cratonic depocenter in western North Dakota. The shape of the older coal basins conformed to the areas of salt solution activity and both the coal fields and component beds accumulated as circular forms. Salt-sol tion collapse effects diminished upsection with more pronounced cratonic influence that originated with intermittent movements between Precambrian crustal blocks forming the Williston basin. The thicker areas of coal seams in the middle of the 300-m coal-bearing section accumulated parallel with the cratonic lineaments but oblique to the coal-basin forms that retained a geometric relation to the salt solution. The uppermost coal basins were almost completely dominated by cratonic influence with thick coal bed areas parallel with cratonic lineaments and dispersed in a reticulate pattern. Younger coal fields distant from the cratonic depocenter were elongated parallel with lineaments oriented toward the cratonic depocenter, whereas coal fields next to the center of the Williston basin accu ulated as arcuate forms along the strike of the basin. The activity of salt-solution subsidence structures during the formation of the uppermost coal fields was sufficiently weak to favor greater regional bed thickness but not affect lateral basin form. End_of_Article - Last_Page 553------------

Preliminary model of regional Mesozoic groundwater flow and uranium deposition in the Colorado Plateau

Qualitative and numerical simulation of regional groundwater flow in the Colorado Plateau during Late Jurassic and Early Cretaceous time has led to a model of uranium deposition in areas of upwelling solutions. Paleogeographic reconstruction shows that surface drainage and therefore groundwater flow was generally toward the northeast and east. Croundwater flowing in these directions, principally through Triassic-Jurassic and Permian sandstones, encountered sediments of variable thickness due to buried uplifted or downdropped Precambrian blocks. The buried uplifted blocks caused upward movement of groundwater around them. These inferred zones of upwelling are closely associated with concentrations of Jurassic- and Cretaceous-age uranium deposits. The results are consistent with hypotheses of an upwelling brine mixing and reacting with descending meteoric water and causing uranium precipitation at the fluid interface. Whether the uranium came from above or below the interface is an unsolved problem, but this model helps to identify better the sources of the waters and also suggests target areas for exploration.

Etude comparee des structures profondes et des anomalies magnetiques allongees reconnues en manche occidentale et en baie d'audierne: Existence possible d'une suture cryptique au nord-ouest du massif armoricain (France)

The magnetic anomaly which spreads off the Audierne Bay is caused by an ophiolitic complex which can be studied onshore. This complex, which is a remnant of oceanic crust, separates a monometamorphic Paleozoic unit from a polymetamorphic Precambrian block. Magnetic susceptibilities have been measured on rocks of the complex. Calculated models show that the main magnetic anomaly of the English Channel can be explained by such a kind of rocks. This information completes the study of the deep structures (the main basic intrusion of the English Channel separates a Paleozoic basin from a polymetamorphic basement) and leads us to believe that another oceanic suture exists in the northwestern part of the Armoricain area. The relations between those two sutures are discussed.

Deep-seated inclusions, kimberlites, and the problem of continental drift

Composition of the deep-seated Inclusions of ultrabasic and basic rocks in kimberlites and basalts is correlated with the type of construction of the crust. We suggest that these inclusions reflect the composition of the lower horizons of the crust and the top of the mantle, and that ktmberlite was originally a gassaturated tectonite, developing on the boundary between mantle harzburgite and. the lower horizons of the crust (the Mohorovicic geophysical surface). This surface is probably one of the principal tectonic planes during the displacement of continents. Geological analysis suggests that the deep-seated inclusions in kimneriites are rocks from the base of the crust of the Precambrian metamorphic blocks. This once again makes it possible to the raise the question of the interpretation of the diamond occurrences in the rocks of the deep Precambrian. —.Authors.

Structural deformation of the Idaho-Wyoming overthrust belt (U.S.A.), as determined by Triassic paleomagnetism

A paleomagnetic study of Triassic redbeds from fourteen sites within the Idaho-Wyoming overthrust belt and three sites from the adjacent stable foreland was undertaken in an attempt to decipher the origin of the arcuate shape of the thrust-fold belt in north-western Wyoming. This belt rises from beneath the Snake River lava plain in eastern Idaho and trends east-southeast into Wyoming. South of Jackson, Wyoming, the belt swings 45 to 70 degrees to trend almost due south into west-central Wyoming. A total of 193 samples were paleomagnetically analyzed using spinner and cryogenic magnetometers. Thermal demagnetization up to 681° C reveals that the natural remanent magnetization consists characteristically of two directional components. A small secondary (chemical) magnetization is revealed at temperatures below 500° C, whereas a proportionally large characteristic magnetization appears to be primary and conforms to the expected Triassic latitudes of the North American craton. The site-mean directions have Fisher precision k ranging from 14 to 149, with confidence cones of 95%-probability ranging between 4.8° and 16.2°. The characteristic (primary) directions of magnetization indicate that the structural bend in the overthrust belt originated through tectonic rotations of the thrust sheets in the horizontal plane, as the paleomagnetic declinations show a systematic change corresponding to the structural configuration mentioned above. We propose that as overthrusting progressed from west to east in the Late Mesozoic and Early Tertiary, the thrust sheets experienced an increasing resistance to movement due to the proximity of the ancient arcuate foreland margin. The buttressing effect of the foreland hinge on the overthrust plates caused the frontal edges of the thrusts to assume the configuration of the foreland margin resulting in horizontal rotations of the thrust sheets. The easternmost Prospect—Cliff Creek—Jackson thrust sheet came into actual contact with the ancestral Teton—Gros Ventre Precambrian block on the north and the Game Hill reverse fault on the east; as a result the northern sections of the thrust sheet rotated by an amount of up to 60 degrees in a counter-clockwise sense and the southern and eastern sections rotated in a clockwise direction by an amount of up to 30 degrees.

Earthquakes and Block Tectonics in the Illinois Basin

Historic earthquakes in Illinois occur on faults that have been intermittently active since Precambrian time. Vertical deflection of the lithosphere continues, but at an abbreviated pace. Evidence for this statement follows: (1) blocks of undeformed and coherent Precambrian lithosphere are outlined by gravity closures; (2) Paleozoic strata are folded and, in most cases, unfaulted over Precambrian blocks that have undergone vertical, differential displacements; (3) gravity closures, which contain negative residuals, overlie crustal blocks that have moved upward relative to blocks that are overlain by gravity closures, which contain positive residuals; and (4) historic earthquakes are not located within closures but are in high gravity gradient areas on the edges of closures or in the lower gradient areas lying between closures.

GEOLOGICAL APPRAISAL OF THE MOOTWINGEE AREA, NEW SOUTH WALES

The results of various photogeological, geological and geophysical programmes in the Mootwingee Area of northwestern New South Wales are discussed.In a structurally complex area, a central horst of outcropping Precambrian rocks trends north-northwestwards. This is flanked both to the east and west by basins containing considerable thicknesses of Palaeozoic sediments.The problem of whether the western structural basin, now represented by the Topar syncline, is coincident with the Cambro-Ordovician basin of sedimentation (?Bancannia Trough) is also discussed. The presence of infolded and down-faulted remnants of Cambro-Ordovician sediments within the Precanibrian basement, to the north of the area discussed, suggests that Cambro-Ordovician sedimentation may have extended further eastward than is now revealed in outcrop.The main source-potential of the area appears to be confined to the Cambro-Ordovician sequence with perhaps very limited potential in the thick Devonian sediments. The drilling of a stratigraphic well in each of the basinal areas respectively to the west and east of the centrally-outcropping Precambrian block, appears justified. Such drilling should resolve some of the above-mentioned problems of depositional limits, as well as assist in evaluating the petroleum potential of the region.