Rift Origin Research Articles

Crustal structure in the Falcón Basin area, northwestern Venezuela, from seismic and gravimetric evidence

Abstract The Falcon Basin in northwestern Venezuela has a complex geological history driven by the interactions between the South American and Caribbean plates. Igneous intrusive bodies that outcrop along the axis of the basin have been associated with crustal thinning, and gravity modeling has shown evidence for a significantly thinned crust beneath the basin. In this study, crustal scale seismic refraction/wide-angle reflection data derived from onshore/offshore active seismic experiments are interpreted and forward-modeled to generate a P-wave velocity model for a ∼450 km long profile. The final model shows thinning of the crust beneath the Falcon Basin where depth to Moho decreases to 27 km from a value of 40 km about 100 km to the south. A deeper reflected phase on the offshore section is interpreted to be derived from the downgoing Caribbean slab. Velocity values were converted to density and the resulting gravimetric response was shown to be consistent with the regional gravity anomaly. The crustal thinning proposed here supports a rift origin for the Falcon Basin.

A mantle- and a lower crust-derived bimodal suite in the Yusufeli (Artvin) area, NE Turkey: trace element and REE evidence for subduction-related rift origin of Early Jurassic Demirkent intrusive complex

The Yusufeli area, in the Eastern Black Sea Region of Turkey, contains a crystalline complex that intruded into the Carboniferous metamorphic basement and is composed of two intrusive bodies: a gabbro-diorite and a tonalite-trondhjemite. The mafic body (45–57 wt% SiO2) displays a broad lithological spectrum ranging from plagioclase-cumulate to quartz diorite. Primitive varieties of the body have Mg-number, MgO and Cr contents that are close to those expected for partial melts from mantle peridotite. Data are consistent with the magma generation in an underlying mantle wedge that was depleted in Zr, Nb and Ti, and enriched in large ion lithophile elements (K, Rb, Ba, Th). However, high Al2O3, CaO and generally low Ni (<65 ppm) contents are not in agreement with the unfractionated mantle-derived primitive magmas and require some Al2O3- and CaO-poor mafic phases, in particular, olivine and orthopyroxene. Absence of orthopyroxene in crystallization sequence, uralitization, and a common appearance of clinopyroxene surrounded by hornblende imply an anhydrous phase fractionated from highly hydrous (5–6%) parent. Geochemical modelling suggests derivation by 15–20% melting of a depleted-lherzolitic mantle. The tonalite-trondhjemite body (58–76 wt% SiO2) ranges in composition from quartz diorite to granodiorite with a low-K calc-alkaline trend. Although LILE- and LREE- enriched characteristics of the primitive samples imply a metasomatic sub-arc mantle for their source region, low MgO, Ni and Cr concentrations rule out direct derivation from the mantle wedge. Also, lack of negative Eu anomalies suggests an unfractionated magma and precludes a differentiation from the diorites of mafic body, which show negative Eu anomalies. Their Na enrichments relative to Ca and K are similar to those of Archean tonalites, trondhjemites and granodiorites and Cenozoic adakites. However, they exhibit important geochemical differences from them, including low-Al (<15 wt%) contents, unfractionated HREE patterns and evolution towards the higher Y concentrations and lower Sr/Y ratios within the body. All these features are obtained in experimentally produced melts from mafic rocks at low pressures (≤5 kbar) and also widespread in the rocks of arc where old (Upper Cretaceous or older) oceanic crust is being subducted. Major and REE modelling supports formation of the quartz dioritic parent to the felsic intrusive rocks by 70% partial melting of a primitive gabbroic sample (G694). Therefore, once taking into account the extensional conditions prevailing in the Pontian arc crust in Early Jurassic time, former basic products (gabbros) seem to be the most appropriate source for the tonalite-trondhjemite body. Magmatic emplacement of stratigraphically similar lithologies in the Pulur Massif, just southwest of the Yusufeli, was dated to be 184 Ma by the 40Ar/39Ar method on amphibole, and is compatible with the initiation of Early Jurassic rifting in the region.

Geology, geochronology and tectonic setting of late Cenozoic volcanism along the southwestern Gulf of Mexico: The Eastern Alkaline Province revisited

A NNW-trending belt of alkaline mafic volcanic fields parallels the Gulf of Mexico from the U.S. border southward to Veracruz state, in eastern Mexico. Previous studies grouped this volcanism into the so-called “Eastern Alkaline Province” (EAP) and suggested that it resulted from Gulf-parallel extensional faulting migrating from north to south from Oligocene to Present. On the basis of new geologic studies, forty-nine unspiked K–Ar and two 40Ar– 39Ar ages, we propose a new geodynamic model for the volcanism along the southwestern Gulf of Mexico. We studied in detail four of the six recognized fields of mafic alkaline volcanism in Veracruz state: 1) The lavas flows of Tlanchinol area (7.3–5.7 Ma), 2) the Alamo monogenetic field and Sierra de Tantima (7.6–6.6 Ma), 3) the Poza Rica and Metlatoyuca lava flows (1.6–1.3 Ma) and 4) the Chiconquiaco–Palma Sola area (6.9–3.2 Ma). Other two mafic volcanic fields may represent the continuation of alkaline volcanism to the southeast: the Middle Miocene lavas at Anegada High, offshore port of Veracruz, and the Middle to Late Miocene volcanism at the Los Tuxtlas. The existence of major Neogene extensional faults parallel to the Gulf of Mexico (i.e., ∼N–S to NNW–SSE) proposed in previous works was not confirmed by our geological studies. Elongation of volcanic necks, vent alignment, and faults mapped by subsurface data trend dominantly NE to ENE and NW to NNW. These directions are parallel to transform and normal faults that formed during the Late Jurassic opening of the Gulf of Mexico. Ascent of mafic magmas was likely facilitated and controlled by the existence of these pre-existing basement structures. Coupled with previous studies, our data demonstrate the occurrence of three magmatic episodes in Veracruz: 1) A Middle Miocene (∼15–11 Ma) episode in southern Veracruz (Palma Sola, Anegada, and Los Tuxtlas); 2) A Late Miocene to Early Pliocene (∼7.5–3 Ma) pulse of mafic alkaline volcanism throughout the study region; and 3) A Late Pliocene to Quaternary transitional to calc–alkaline volcanism in southern Veracruz (Palma Sola, Los Tuxtlas). Whereas the first and third episodes may be considered part of the subduction-related Trans-Mexican Volcanic Belt, the second pulse of mafic alkaline volcanism has a more complex origin. The absence of significant extensional faulting precludes a rift origin. We favor a model in which a transient thermal anomaly and melting of the mantle was triggered by the tearing and detachment of part of the subducted slab.

Secular geochemical variations of the Lower Cretaceous siliciclastic rocks from central Tibet (China) indicate a tectonic transition from continental collision to back-arc rifting

Fifty-five Lower Cretaceous siliciclastic rocks from central Tibet were analyzed for major and trace elements, in an attempt to evaluate their provenance and the type of tectonic setting in which they were deposited. The spatial–temporal variation of both sediment maturity indices (SiO 2/Al 2O 3, La/V, and Zr/TiO 2), the ratios of immobile trace elements (Cr/Th, Th/Sc, Th/Co, La/Sc, and La/Co), and SiO 2, K 2O/Na 2O, Eu/Eu*, (Gd/Yb) n , and ferromagnesian contents, displays that the Berriasian–Valanginian (BV) sediment could have been derived from northern orogenic source areas, whereas the Hauterivian–Lower Barremian (HB) sediment could have been closely related to the syndepositional volcanism in the northern Lhasa block itself. The Na 2O/Al 2O 3–K 2O/Al 2O 3 relations also place approximately half of HB samples in the field of volcanic rocks of back-arc rifting origin, whereas little volcanic clasts exist in the BV rocks. There are distinct signatures that the mafic/ultramafic materials in the BV rocks were derived from oceanic island basalts (ophiolites) in the Bangong–Nujiang suture. Therefore, the collisional orogen due to the Qiangtang–Lhasa collision should have played a key role in supply of the sediments during BV time. Bimodal sedimentary basaltic and rhyolitic tuffs, with respective distinguished geochemical features, exist in the HB sequence. Therefore, a distinct tectonic transition from continental collision to back-arc rifting in central Tibet is believed to have occurred during Early Cretaceous time. Extreme depletion of Na of most Lower Cretaceous samples compared to the upper crust, and their high average chemical index alteration (CIA) values and Th/U ratios, point out the source rocks formed in tropical climatic regions with little tectonic uplift, although the weathering degree of the source rocks for HB samples could likely have been affected by the syndepositional volcanism. During Middle Cretaceous time, the entire southern Eurasian margin could have been characterized by a series of back-arc rift basins (including those in Kohistan and Ladakh).

Permian of West Yunnan, Southwest China: a biostratigraphic synthesis

Recent progress in the study of Permian stratigraphy of western Yunnan, southwest China, is reviewed with particular references to the Tengchong and Baoshan blocks and the Changning–Menglian Belt. Where confusion or controversy exists in stratigraphical nomenclature and/or dating, we attempt to clarify the situation based on our recent field observations and newly obtained research results. The Permian within the Changning–Menglian Belt embraces different stratigraphic successions, suggesting different tectonic settings, ranging from passive margin and active margin, to oceanic basin and seamounts. Permo-Carboniferous faunas in the carbonate sequences of the Changning–Menglian Belt are of typical Cathaysian affinity, as demonstrated by abundant fusulinaceans and compound rugose corals. The Permian stratigraphy and faunas of the Tenchong and Baoshan blocks are markedly different from those of the Changning–Menglian Belt. The Baoshan Block lacks Upper Carboniferous deposits, and its subsequent Lower Permian sequence consists predominantly of siliciclastic strata yielding cool-water faunas and possibly glaciogene diamictites, overlain by thick basaltic lava and volcaniclastics of probably rift origin. The upper part of the Permian in the Baoshan Block is characterized by carbonates containing mixed Cathaysian and Gondwanan faunas. The Tengchong Block has a similar evolutionary history to the Baoshan Block, but completely lacks volcanic rocks.

Early Proterozoic magmatism in Yukon, Canada: constraints on the evolution of northwestern Laurentia

Northwestern Laurentia, after cratonization at about 1.85 Ga, underwent a series of tectonic and magmatic events during the Proterozoic that were followed by separation of Laurentia from another landmass, probably Australia. The oldest magmatic event produced the Bonnet Plume River Intrusions (BPRI), which intruded the Wernecke Supergroup as short dikes and small stocks. The BPRI are hydrothermally altered tholeiitic diorites, gabbros, and subordinate anorthositic and syenitic rocks, with trace element signatures consistent with a rift origin. Depleted mantle model ages range from 2.29 to 2.57 Ga and εNdvalues range from +0.7 to –1.7. An increasing crustal component is apparent in rocks with more evolved compositions. Four U–Pb zircon ages (1705.9 ± 0.7, 1709.4 ± 1.4, 1711.1 ± 5.1, and 1713.6 ± 12.7 Ma) indicate a Paleoproterozoic age for the BPRI. These dates constrain the age of the Wernecke Supergroup to [Formula: see text] ca. 1710 Ma, making it the oldest supracrustal succession in western Laurentia, e.g., &gt;240 Ma older than the Belt Supergroup of southeastern British Columbia and the northwestern United States. The Wernecke Supergroup was deposited in the first rift basin to open along the western margin of Laurentia, but was later inverted by the pre-1.6 Ga Racklan Orogeny, an event possibly influenced by transmission of compression from the Yavapai and Mazatzal orogenies in southern Laurentia. The Neoproterozoic southwestern United States – east Antarctica (SWEAT) reconstruction, which places Australia next to northwestern Laurentia, is supported by linkages between Paleoproterozoic and Mesoproterozoic geological features in northwestern Canada and Australia.

Mode of Cenozoic east‐west extension in Tibet suggesting a common origin of rifts in Asia during the Indo‐Asian collision

Cenozoic rifts in Tibet were traditionally interpreted as a result of topographic collapse of the Tibetan Plateau, reaching the maximum elevation that can be supported by its mechanical strength. Recent studies have emphasized possible similarities between rifting in Tibet and extension in the Basin and Range of the western United States. However, when examined in detail, one finds that spacing of long (&gt;100 km) rifts in Tibet (∼100–300 km) is significantly greater than that in the Basin and Range (∼20–40 km). From south to north, rift spacing decreases systematically: from 191±67 km in the Himalaya south of the Indus‐Yalu suture to 146±34 km in south Tibet between the Indus‐Yalu and Bangong‐Nujiang sutures and farther north to 101±31 km in central Tibet between the Bangong‐Nujiang and Jinsha sutures. Instability analysis suggests that the mantle lithosphere must have been involved in east‐west Tibetan extension. Specifically, the widely spaced rifts in The Himalaya and Tibet may have been related to the presence of a relatively light crust (density &lt;∼2.90 g cm−3) and a strong mantle lithosphere (∼40 km thick and a factor of 5 stronger than the upper crust). The observed systematic decrease in rift spacing can be explained by the known decrease in the crustal thickness in Tibet, from ∼70–80 km in The Himalaya in the south to ∼50–55 km in central Tibet in the north. A regional comparison of rifts in east Asia suggests that both the involvement of the mantle lithosphere and the age of rift initiation are similar for Tibetan rifts, the Baikal rift, and the Shanxi graben. This implies that topographic collapse or a convective event in the mantle cannot be the sole cause for the development of the Tibetan rifts. A regional boundary condition applied throughout east Asia must be required.

Late Cretaceous and Tertiary structural evolution of the northeastern part of the Vøring Basin, Norwegian Sea

Abstract The Late Cretaceous-Tertiary structural evolution of the northeastern part of the Vøring Basin, mid-Norway, is highly complicated. Although tectonic activity occurred throughout Cretaceous time in much of the Vøring Basin, including the Gjallar Ridge and along the Fles Fault Zone, in the Vema Dome-Nyk High area evidence of such activity is not observed until the latest Maastrichtian time. In the Vema Dome-Nyk High area, several faults with both a NW-SE orientation and a NE-SW orientation have experienced lateral movements. The NE-SW orientation is the old Caledonian trend. The complex structural evolution of the Vema Dome-Nyk High area is probably related to the existence of continental weakness zones, which are the onshore extension of known oceanic fracture zones. The two lineaments that delineate these weakness zones, and that delineate the Vema Dome-Nyk High area, namely the Bivrost and Surt Lineaments, diverge slightly from one another toward the SE. This complex structural framework of NW-SE oriented lineaments and NE-SW oriented deep-seated basement structures of Caledonian compressional and/or Mesozoic rift origin, facilitated minor clockwise rotation of the area between the two lineaments during the extensional regime before the break-up of the North Atlantic, as well as during the compressional regime that has been proposed after the break-up.

Structure and genetic mechanisms of the Precambrian rifts of the East-European Platform in Russia by integrated study of seismic, gravity, and magnetic data

Integrated models of the deep structure and origin of rifts located within the Russian portion of the East-European Platform have been developed from recent DSS results, new gravity and magnetic modelling, and geological and older geophysical data acquired over the last 50 years. The Mezen rift province, the Middle-Russian rift, the Valday rift, the Pachelma rift, and the rifts within the Pre-Caspian depression were studied. All of these rifts were affected by extension and filled with syn-rift sediments at different times through the Riphean (1650–650 Ma). Post-rift sedimentary basins developed from the end of the Neoproterozoic until the Cenozoic. The models indicate that the crustal structure and genesis of the individual rifts are different. The Mezen rift province was formed under a condition of limited extension of the continental crust. The McKenzie pure strain mechanism is acceptable for lithosphere extension in the Middle-Russian rift. The Wernicke model best expresses the Valday and Pachelma rifts. The rift process in the Pre-Caspian area is explained in terms of large-scale sliding apart of lithospheric plates, and approached the stage of development of oceanic crust.

BASEMENT LITHOLOGY AND ITS CONTROL ON SEDIMENTATION, TRAP FORMATION AND HYDROCARBON MIGRATION, WIDURI‐INTAN OILFIELDS, SE SUMATRA

The Widuri‐Intan oilfields produce from late Oligocene sandstones of the Talang Akar Formation, which were deposited in a fluvial‐to‐deltaic setting on the NW side of the Asri Basin, offshore SE Sumatra. The Asri Basin is of rift origin and formed during the early Oligocene, with its axis oriented in a NE‐SW direction. Approximately 310 million brls of oil have been produced from the fields within the 12‐by‐12 mile (20‐by‐20 km) study area. The oil occurs in a series of structural and stratigraphic traps within slightly sinuous to meandering channel sandstone bodies. The reservoir sequence (sandstone interbedded with minor mudstone and coal) overlies basement rocks, which are predominantly Cretaceous in age. Forty‐nine well penetrations have shown that the basement is composed of one of four lithologies: (1) hornblende granodiorite; (2) metamorphic rocks, mainly mica schist; (3) plugs of metabasalt and related volcanic rocks; or (4) dolomitic limestone.A combination of drill cuttings, sidewall and conventional cores and FMS/FMI images has been used to identify and map the distribution of basement rock type. The basement was subjected to exposure and deep weathering prior to the formation of the Asri Basin, as evidenced by the zones of surface alteration encountered during drilling. The basement palaeotopography had a strong influence on the later distribution of major fluvial channels and sand pinch‐outs. Several major faults appear to be controlled by basement lithology, especially at the boundaries of granodiorite and metabasalt intrusives. An important shear zone, oriented NW‐SE, appears to have offset the basement between the main Widuri and Intan fields, and was subsequently the site of silicification of the mica schists in the basement. The Lidya field is situated where the reservoir pinches out onto eroded areas of basement silicification along this shear zone. Palaeocurrents in the upper 34–2 and 34–1 channel sandstones in the Widuri field were controlled by the orientation of this basement feature. Drape and compaction of Oligocene Talang Akar Formation sediments over eroded volcanic plugs have defined or enhanced a number of structural/stratigraphic plays, including the Widuri and Chesy fields. From seismic and well evidence, the reservoir sequence at the Indri field is underlain by dolomitic limestone and exhibits a series of unusual karst‐related sinkhole and collapse structures. These are circular to slightly elliptical in shape, and extend from basement level to over 900 ft vertically into the overlying Talang Akar Formation.

A mantle-derived bimodal suite in the Hercynian Belt: Nd isotope and trace element evidence for a subduction-related rift origin of the Late Devonian Br�venne metavolcanics, Massif Central (France)

In the Brevenne Series (NE Massif Central), a low-grade bimodal association of metabasalts and metarhyolites is exposed, together with intrusive trondhjemite bodies. Zircon U-Pb dating constrains their magmatic emplacement at 366 ± 5 Ma and 358 ± 1 Ma, respectively. The metabasalts are characterized by a distinct enrichment in incompatible elements (e.g. Th and LREE) and positive ɛNdi (from +5 to +8). Combined isotope and trace element systematics rule out crustal contamination of mafic melts as a suitable cause of the LILE (large ion lithophile element)-enrichment. Rather, a mixing process between a component similar to mid ocean ridge basalts and an enriched end-member with ɛNdi > +5 is suggested. An enriched-mantle source of ocean island basalt affinity is precluded by the relative depletion of high field strength elements, especially Nb which shows negative anomalies in chondrite-normalized patterns. On the contrary, a subduction-related origin for the LILE enrichment would be more consistent. It may be inferred that arc-like melts [enriched in Th and LREE (light rare earth elements) and depleted in Nb, with ɛNdi > +5] were produced through partial melting of a depleted-mantle source, to which a small amount of crustally derived component had been added. The metarhyolites are enriched in LILE, and have a close genetic relationship with the metabasalts, as evidenced by their high ɛNdi (from +4.7 to +6.8). Although the chemical evidence remains ambiguous, it is suggested that fractional crystallization, accompanied by subordinate assimilation, is the petrogenetic process most consistent with the data. The trondhjemites are isotopically distinct from the metarhyolites. Their ɛNdi values (from −1.0 to +2.2) reflect an important contribution of continental crust to their genesis, and disprove their inferred cogenetism with the felsic volcanics. A review of modern environments in which such bimodal suites are exposed, shows that settings involving incipient rifting of a volcanic arc fringing a continental margin, or built upon young, thin continental crust might provide suitable analogues. Geodynamic reconstructions are complicated by subsequent tectonic events which disrupted the initial patterns, and by Mesozoic-Cenozoic sedimentary cover. However, this subduction-related magmatism enlarges the growing body of evidence for southward subduction processes until the Late Devonian during the evolution of the northern flank of the European Variscides. As a general implication, it is suggested that the combined use of the Sm-Nd system with incompatible elements relatively resistant during alteration and low-grade metamorphism (REE, Th, Zr, Nb) may provide diagnostic criteria for recognizing the tectonic setting of bimodal metaigneous suites in ancient orogenic belts.

Late Silurian‐Early Devonian transpressional rift origin of the Quebec Reentrant, northern Appalachians: Constraints from geochemistry of volcanic rocks

Silurian‐Devonian rocks in the northwestern mainland Appalachians form part of an overstep sequence deposited across the vestiges of Iapetus after the collision of Laurentia and Gondwana. They occur in three synclinoria (Gaspé‐Connecticut Valley, Merrimack‐Aroostook, and Fredericton synclinoria), separated by two anticlinoria (Munsungun‐Pennington‐Boundary Mountain‐Bronson Hill and Miramichi). Silurian rocks in Gaspé‐Connecticut Valley and Merrimack‐Aroostook synclinoria consist of shelf and trough sediments derived mainly from Laurentia to the northwest, with some local contributions from an ephemeral landmass (Appalachia) and from the Miramichi Highlands, hi contrast, Early Devonian rocks in all three synclinoria are mainly flysch derived from the southeast in Avalonia. Interbedded volcanic rocks are predominantly Late Silurian‐Early Devonian in age. They are generally bimodal with the mafic rocks exhibiting a change from transitional alkalic‐tholeiitic to tholeiitic around the Siluro‐Devonian boundary, broadly coincident with the Salinic disturbance in the Gaspe Peninsula. The geochemical characteristics of the basalts indicate that they were erupted in a continental intraplate environment with melting migrating upward through time across the garnet to spinel phase boundary as stretching increased. Thermal uplift during rifting is inferred to have led to erosion associated with the Salinic disturbance. The start of the volcanism appears to coincide with a switch from sinistral to dextral transpression along the orogen that may be related to the change from clockwise to anticlockwise rotation of Laurentia relative to Gondwana. The effects of dextral transpression vary with the trend of the orogen; intense deformation and metamorphism occurred in the Central Mobile belt opposite the New York and St. Lawrence promontories, whereas rifting developed in the Quebec Reentrant, leading to thinning of the crust, up‐welling of the asthenosphere, melting, and magmatism.

Structural Development of McKelligon Canyon, Franklin Mountains, Texas

McKelligon Canyon is located in the south-central Franklin Mountains of west Texas. The Franklin Mountains are a north-south-trending, tilted fault block mountain range. Most workers who have studied the Franklins agree that range uplift occurred after the Cenomanian. Rio Grande rift extension and Laramide age compression are the most commonly cited mechanisms for range uplift. Recently, McKelligon Canyon was suggested to contain Laramide thrust faults at several locations. This study, however, shows that the canyon's structural pattern is characterized by much extensional faulting. No evidence of thrust faults or other compressional features were found. Steep, northeast-dipping (55-80{degree}) normal faults, which strike N60W and N10W, are the primary structures within the canyon. These faults are oblique to the general north-south trend of the range and have a maximum vertical offset, as measured by stratigraphic separation, of approximately 650 m. Strain values obtained from oriented limestone samples collected from the field suggest an early Rio Grande rift origin of the sturctures found in this area. Additionally, paleokarst collapse breccias occur in abundance throughout the study area and are confined to the McKelligon Canyon Formation and the lower 2-3 m of the Scenic Drive Formation. The presence of karsting in the upper partmore » of the Scenic Drive Formation and the overlying Florida Mountains Formation could not be verified in the field. These karst features appear to be randomly distributed as seen on the surface, however, this may be due to preferred differential erosion.« less

The riphean sedimentary basins of the Eastern Siberia province and their petroleum potential

From their location in the regional pattern of the Eastern Siberia Province, their style and their evolutionary history, the Riphean sedimentary basins are divided into two groups: “marginal cratonic” and “intracratonic”. The first group includes basins of “pericratonic troughs”, “fore-deeps” and “marginal transverse aulacogens”. They are characterized by a definite division into structural stages according to both characteristics of the individual formations and interbedding of rocks with different lithology. Within sequences, there is a complex combination of formations such as black-shale, and flyschlike, volcanoclastic-terrigenous, carbonate and others. The second group incorporates “aulacogenic” and platform intracratonic sedimentary basins. The main features of aulacogen-type sedimentary basin structure are due to their rift origin and style of subsequent development. The early stages are marked by an alternation of volcanoclastic-terrigenous and volcanoclastic-carbonate rocks. There is rythmic interbedding of terrigenous and terrigenous-carbonate rocks up-section. Depositional environments in sedimentary basins of platform depression have been controlled to a large extent by the structural-dynamic setting of the final stages in the formation of adjacent pericratonic troughs and intracratonic aulacogens. The succession consists mainly of terrigenous-carbonate and carbonate-terrigenous units, i.e., products of erosion and re-deposition of Early Riphean complexes and ancient basement. Distribution of particulate organic matter and its catagenetic transformation indicate rather high petroleum-generating potential for the Riphean. Regional development of high-quality seals (upper Vendian argillaceous evaporite-carbonate units and Lower Cambrian salts) greatly favors preservation of hydrocarbon accumulation. Three groups of hydrocarbon reservoirs are differentiated based on structural-tectonic, lithologic-formational and geochemical features. Hydrocarbon fields of considerable commercial importance have been discovered for the first time in world exploration practice in the Riphean of the East Siberian Province. These should lead to the re-evaluation of the hydrocarbon potential of late Proterozoic basins on other continents.

Geothermal Regime and Maturation in the Peace River Arch Area, Canada

A comprehensive approach was used in the study of geothermal regime and maturation in the Peace River arch area, western Canada, with the purpose of evaluating the potential for hydrocarbon resources. The present-day geothermal regime was determined based on bottom hole temperature, thermal conductivity, and radiogenic heat production data. Hydrogeological studies show that the heat transfer in the region is dominated by thermal conduction. Since the study area is relatively undisturbed and there are no large-scale changes in the thermal conductivity of sediments, the regional distribution of the geothermal gradient is controlled by the heat flow from the crystalline basement. The temperature distribution is controlled in addition by the thickness and thermal conductivity of the sediments. The good control on the present-day geothermal regime allows a good evaluation of thermal maturation and calibration of models for thermal history reconstruction. A thermal inverse method, in which the paleoheat flux is estimated by optimization of the calculated to the measured vitrinite reflectance data, was used to reconstruct the thermal history of the area. The paleoheat flow in most of the region shows a warming-up trend with geological time. The paleoheat flow values on the arch were relatively higher than those off themore » arch, supporting the proposed rifting origin of the arch. Two regional thermal maturation trends are present for most of the source rocks in the area, one increasing from northeast toward the disturbed belt and another increasing from north to south. Both of these trends reflect the structural control of the regional thermal maturation.« less

Geochronological evidence from discordant plutons for a late Proterozoic orogen in the Caledonides of Finnmark, northern Norway

Rb-Sr whole-rock, Sm-Nd mineral and U-Pb zircon dates are reported for minor 0granites and Seiland Igneous Province gabbros in the Kalak Nappe complex (Middle Allochthon), northern Norwegian Caledonides. The Litlefjord (Rb-Sr WR 813 ± 62 Ma, MSWD 2.7; U-Pb zircon 804 ± 19 Ma, MSWD 5.5) and Repvig (Rb-Sr WR 851± 130 Ma, MSWD 5.1) granites and the Hasvik gabbro (Sm-Nd WR/minerals 700 ± 33 Ma, MSWD 0.7) cut previously-deformed metasediments of the Scbrcby Succession while the Kvalfjord (Sm-Nd WR/minerals 612 f ± 33 Ma, MSWD 0.1) and Storvik (Sm-Nd WR/minerals 604 ± 44 Ma, MSWD 0.2) gabbros cut ‘basement' paragneisses. The main (Dl–D2) deformation affecting the Sbrcby Succession in the Kalak Nappe complex, which was previously attributed to the early Ordovician Finnmarkian phase of the Caledonian orogeny, took place before c. 800 Ma in the pre-Caledonian 'Porsanger Orogeny'. Early ‘Sørøy’ plutons of the Seiland Igneous Province were apparently emplaced during this event. However, the later, more voluminous ‘Stjernøy’ intrusions are c . 700–520 Ma old and post-date the Porsanger orogeny. These pre-Caledonian intrusions must be inter-, rather than syn-orogenic as previously claimed. This obviates the problem of accomodating the voluminous Seiland magmatism during orogenic compression and is consistent with an early Caledonian rift origin for these rocks, probably related to the opening of Iapetus.

Sedimentation and Tectonics in the Southern Bida Basin, Nigeria: Depositional Response to Varying Tectonic Context: ABSTRACT

The Upper Cretaceous Bida basin of central Nigeria is sandwiched between the Precambrian schist belts of the Northern Nigerian massif and the West African craton. Of interest is the southern part of the basin, which developed in continental settings, because the facies architecture of the sedimentary fill suggests a close relation between sedimentation dynamics and basin margin tectonics. This relationship is significant to an understanding of the basin's origin, which has been controversial. A simple sag and rift origin has been suggested, and consequently dominated the negative thinking on the hydrocarbon prospects of the basin which were considered poor. This detailed study of the facies indicates rapid basin-wide changes from various alluvial fan facies through flood-basin and deltaic facies to lacustrine facies. Paleogeographic reconstruction suggests lacustrine environments were widespread and elongate. Lacustrine environments occurred at the basin's axis and close to the margins. This suggests the depocenter must have migrated during the basin's depositional history and subsided rapidly to accommodate the 3.5-km-thick sedimentary fill. Although distinguishing pull-apart basins from rift basins, based solely on sedimentologic grounds, may be difficult, the temporal migration of the depocenter, as well as the basin architecture of upward coarsening cyclicity, show a strong tectonic andmore » structural overprint that suggests a tectonic framework for the Southern Bida basin similar in origin to a pull-apart basin.« less

Geochemistry of Ordovician Keli Group basalts associated with Besshi-type Cu-Zn deposits from the southern Trondheim and Sulitjelma mining districts of Norway

Besshi-type volcanogenic Cu-Zn deposits in the Scandinavian Caledonides are hosted by Ordovician metabasalts and clastic sediments of the Storen, Fundsjo and Sulitjelma groups. The basalts are transitional between T-MORB and marginal basin tholeiites in composition and are characterised by Nd and Pb isotopic compositions which overlap the more radiogenic values of Lower Palaeozoic MORB. These features, along with the intercalation of the basalts with tuffs and continentally derived sediments, indicate an epicontinental rift or marginal basin origin, possibly analogous to the present Red Sea and Gulf of Aden rifts. This implies the development of a restricted ocean basin in the north of Iapetus between the Laurentian and Baltoscandian microcontinents during the Cambrian and Early Ordovician.

The timing of uplift, volcanism, and rifting peripheral to the Red Sea: A case for passive rifting?

Prior to the formation of the Red Sea the northeastern Afro/Arabian continent had low relief and was largely below sea level from the Late Cretaceous to the early Oligocene. The events leading to the formation of the Red Sea followed the sequence (1) alkaline volcanism and rifting beginning about 30–32 Ma affecting a narrow linear zone in the continent, (2) rotational block faulting and detachment faulting, well underway by 25 Ma, (3) gabbro and diorite magmatism, andesite to rhyolite volcanism, and fine‐grained nonmarine sedimentation in the rift between 20 and 25 Ma, (4) fine‐grained marine sedimentation in the rift as the early shelves started to subside in the middle Miocene, and (5) uplift of the adjacent continents (about 3 km) and subsidence of the shelves (about 4 km) between 13.8 and 5 Ma. The youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the Proterozoic Arabian Shield that range in age from 13.8 to 568 Ma. The youngest of these ages, coupled with the present high relief along the Arabian escarpment and published heat flow measurements, indicate that 2.5–4 km uplift has occurred in the last 13.8 m.y. The sequence volcanism/rifting followed by uplift leads to our adoption of a passive mantle model for rift origin. Models that require uplift to create the rift are rejected, because of the late uplift. We advocate a model of lithospheric extension caused by two‐dimensional plate stress over those requiring tractional drag at the base of the lithosphere caused by vigorous flow in the asthenosphere. It is acknowledged that traction models could explain the observed data, but they imply a rigid, static lithosphere and seem to require a link between the direction of flow in the asthenosphere and plate motions. Neither requirement is necessary in the extension model. The rift starts with mechanical extension in a narrow zone of lithosphere between 25–32 Ma in our model. The thinned lithosphere is replaced by upwelling asthenosphere and by rocks from the adjacent deep continental lithosphere which flow into the rift. Ductile flow of the deep continental lithosphere is accelerated by partial melting as rocks flow upward toward the rift axis. Once partially melted, rocks formerly part of the continental lithosphere join the upwelling asthenosphere, resulting in a rapid erosion of the lithospheric mantle beneath the continent near the rift edge. The resulting density decrease explains the uplift. We think that the Red Sea began as a consequence of changing plate geometries resulting from the collision of India and Eurasia. After the collision, the segment of the Owens fracture zone north of the Carlsberg Ridge became locked, forcing the northeast corner of Afro/Arabia to rotate with the Indian plate away from the rest of Africa.

Theoretical and experimental studies of mantle upwelling, penetrative magmatism, and development of rifts in continental and oceanic crusts

The relationship between mantle perturbation, penetrative magmatism, crustal uplift and the processes of rifting and rift propagation was studied. Quasi-scale model experiments and theoretical analyses by Boussinesq's three function method were used. Three dimensional stress analyses around a rising mantle intrusion and semiquantitative model experiments on mantle upwelling and magma penetration into brittle and brittle-ductile lithospheric materials indicate the origin of ridges, rifts, and rift-basin structures. These structures are commonly initiated by broad domal or plateau uplift. The magnitude and nature of the uplift, fissure or rift development, branching and pattern(s) are controlled primarily by flexural rigidity, magnitude of stress development due to mantle intrusion and magma penetration, and the degree of preferred orientation of weakness (anisotropy) in the lithosphere. Continued magma penetration into the crust produces en echelon fissures, vertical and lateral dike propagation, graben and graben and horst structures with step or block faulting. This is followed by axial subsidence, fault opening and periodic volcanism with continued crustal extension, stretching and slow spreading. Mantle and magma penetration into the crust are shown to be important in developing in situ lateral compressive stress adjacent to and around the active rift zones. Transverse fissures, dike swarms, volcanoes and anticlines between two rifts (as in Iceland) result from such lateral compressive stress. This stress is also important in the development of marginal fissures and faults (as in fast-spreading centers). Time progressive model experiments on magma formation and magma penetration from the asthenosphere-lithosphere boundary show the importance of magma chambers. These chambers cause thinning and stretching of the composite lithosphere, vertical and lateral propagation of dikes and dike swarms, and initiate rifts with or without significant uplift. This study also suggests that multiple dike wedging with rapid extension fracturing can generate excess magma pressure and push force, and may contribute seismicity to spreading centers.