Craton Edge Research Articles

EVIDENCE FOR FORMATION AND MODIFICATION OF THE KAAPVAAL CRATON (SOUTHERN AFRICA) IN ARCHAEAN SUBDUCTION ZONES

Considerable progress has been made in the understanding of craton formation and evolution in southern Africa in recent years, thanks to newly available geophysical and geochemical data. Recent tomographic studies and Re-Os analyses of on- and off-craton mantle xenoliths have clearly shown that the Kaapvaal craton is underlain by a, cold, depleted lithosphere at least 200 km thick that was stabilized in the Archaean. Several important questions regarding the consolidation and modification of the lithosphere, however, remain unanswered or highly debated. Firstly, the sub-cratonic lithospheric mantle (SCLM) is distinct from oceanic and most continental mantle in that it is more depleted in magmaphile elements (Fe, Al, Ca, HREE), but at the same time strongly enriched in incompatible trace elements like LILE and LREE. The Kaapvaal SCLM also has a distinctly higher Si/Mg ratio and therefore higher modal orthopyroxene content (at a given Mg/Fe) than oceanic and most continental upper mantle. Resolution of this issue is critical to understanding the mechanism of lithosphere formation. We carried out a detailed petrological and geochemical study on low-temperature peridotite xenoliths from Kimberley (located at the craton core) and northern Lesotho (at the south-eastern craton edge) in order to obtain information about the processes that led to the depletion and re-enrichment of the Kaapvaal SCLM. Samples have been characterized for Re-Os isotope systematics, major and trace element concentrations in whole rocks and minerals, and garnet and clinopyroxene Lu-Hf, Sm-Nd and Rb-Sr isotopes. The combined results require a multistage history of the Kaapvaal mantle. They are most consistent with a model that involves introduction of orthopyroxene and trace element enrichment of the garnets by aqueous fluids, possibly derived from a subducting oceanic slab. This is consistent with the observed enrichment in Si (orthopyroxene) and the Re-Os systematics. Infiltration of hydrous fluids would also increase the degree of mantle melting and could therefore explain the strong major and HREE depletion of the SCLM. The Nd-Hf isotope characteristics of the garnets require the trace element enrichment to be ancient. We therefore suggest that melting and metasomatism of the Kaapvaal SCLM took place in subduction zone settings, probably during amalgamation of smaller pre-existing terranes in the Late Archaean (~2.9 Ga). Trace element and Nd and Hf isotope disequilibrium between garnet and clinopyroxene is preserved in many samples and indicates that garnet and clinopyroxene are not cogenetic. Calculated equilibrium liquids and Hf-Nd isotopic compositions for clinopyroxene suggest that most diopside in the xenoliths studied crystallized from an infiltrating kimberlite- like melt, either during group II kimberlite magmatism at ~200-110 Ma ago (Kimberley) or shortly prior to eruption of the host kimberlite around 90 Ma (N. Lesotho).

Pampean Orogen: An Intraplate Component of Early Paleozoic Deformation?

After accretion of the Pampean continental fragment to the western Rio de la Plata craton margin (530 Ma), subsequent deformation, crustal anatexis and plutonism may have been intraplate responses to Brasiliano-PanAfrican collisional tectonism on the eastern margin during the amalgamation of Gondwana. Investigations of intraplate orogens such as the Tien Shan and the Ancestral Rocky Mountains, as well as of analogue and numerical models, permit discrimination of two early Paleozoic tectonomagmatic phases in the Sierras Pampeanas. The first involved marginal trough subduction and calc-alkaline magmatism, culminating in accretion of the Pampean terrane to the western craton edge; the second was characterized by crustal anatexis and peraluminous plutonism, penetrative deformation and high-angle reverse faulting resulting from continental collision on the eastern craton margin. Field observations from modern (Tien Shan) and ancient (Ancestral Rocky Mountains) intraplate chains, deep seismic and borehole data, radiometric and fission-track data constitute control for analogue and numerical models of intraplate deformation resulting from continental collision. Near-simultaneous continent-wide deformation, regularly spaced ranges/buckles, reverse-fault initiation at fold hinges of buckles, and doubling of crustal thickness are replicated in structural arrays formed in four-layer analogue models of lithospheric buckling. These data have significant implications for the ductile deformation, crustral thickening and post-subduction plutonism that spanned central South America in Late Cambrian time.

Production and preservation of organic matter during deposition of the Bakken Formation (Late Devonian and Early Mississippian), Williston Basin

The organic-rich (up to 35% TOC) lower and upper black mudstone members of the Bakken Formation (Late Devonian and Early Mississippian) accumulated in response to both increased productivity and enhanced preservation that resulted from unique palaeogeographic, palaeoceanographic and palaeoclimatic conditions. During Bakken deposition the semi-isolated Williston Basin was located close to the Equator (5° to 10°N) in a zone of prevailing east to west winds. A pattern of estuarine-like marine circulation between the Williston Basin and open-ocean conditions at the western craton edge of North America is proposed. Surface water discharged from the basin was replaced by nutrient-rich deeper water sourced from the equatorial undercurrent in the Pacific Basin. Undercurrent flow into the Williston Basin upwelled providing a potentially sustainable source of nutrients to enhance productivity in the surface photic zone during black mudstone deposition. In response to increased productivity, that led to a high rate of organic sedimentation, and quiescent, deep-water conditions, due primarily to the geographic isolation of the basin and the restriction of estuarine-like circulation to the upper water column, anoxic bottom water developed on the basin floor. As sea-level rose during lower and upper black mudstone deposition, anoxic bottom water expanded across the basin enhancing organic matter preservation. Conditions in the Williston Basin demonstrate that both increased productivity and improved preservation can be essential and complementary factors affecting organic-rich mud deposition. These results have potentially important implications for the accumulation of similarly organic-rich deposits across the North American interior during Late Devonian and Early Mississippian time.

Tectonic controls on the petroleum geology of NE Africa

Abstract During the Palaeozoic and early Mesozoic, the NE sector of the African craton remained an entire, integral part of Gondwanaland. It experienced two episodes of intracratonic sagging and marginal cratonic rifting, linked to episodes of strike-slip affecting both the craton interior and its northern margins. Source kerogens accumulated during Palaeotethyan highstands within these sags, across which reservoir clastics prograded during the corresponding lowstands. Such source-reservoir associations are known from the Silurian, Devonian and Carboniferous. The fragmentation of the NE African margin with Mesotethys took place over the period Mid Triassic-Bathonian. From Palestine to Cyrenaica, a rift propagated westwards, detaching crustal terranes along shears that later formed Asia Minor. Deep marine shales within this rift have some proven oil-generating potential. However, the principal Jurassic oil kerogen is found associated with coal swamp-delta clastics, deposited during the Bathonian lowstand, in basins formed between the shears along the new cratonic edge. There is also now evidence for a Mid-Triassic rift penetrating the Sirt Basin from the northwest. Sinistral strike-slip associated with these Mesotethyan shears penetrated deeper into NE Africa during the Late Cretaceous, with subsidence reaching a maximum in the Jebel el Akhdar, Abu Gharadiq and proto-Clysmic Basins, in which rich oil-prone marine source rocks accumulated during the Turonian-Coniacian lowstand. Inversion followed, as the sense of strike-slip reversed with the onset of ‘Syrian Arc’ movements, starting in the latest Cretaceous, and proceeding as a series of pulses into the Late Oligocene. The final phase of ‘Syrian Arc’ tectonics was linked to the opening of the Red Sea and Gulf of Suez Basins, and transtension along the Gulf of Aqaba in the Early Miocene. Falling Miocene sea levels then permitted the accumulation of the restricted ‘Globigerina Marls’ facies within the active graben system of the Gulf of Suez.

Plan‐view curvature of foreland basins and its implications for the palaeostrength of the lithosphere underlying the western Alps

In zones of continental collision, three methods can be used to calculate the strength of the underthrust lithosphere: (1) a forward model approach to the Bouguer gravity field, (2) an inverse model of the gravity and topography using admittance techniques, or (3) a forward model of the stratigraphic infill of the foreland basin to estimate the cross‐sectional profile of the downflexed plate. The use of reconstructed stratigraphy has the potential to yield values for the equivalent elastic thickness (Te) of the cratonic lithosphere at varying slices in geological time, and hence enable an insight into the longer term (10–50 Myr) mechanical behaviour of the continental lithosphere.Calculations of Te based on isopachs of foreland basin stratigraphy use sea level as a reference line to estimate the basement deflection, and therefore are limited to using stratigraphy which records shallow marine or coastal sedimentation. A new empirical approach is applied to evaluating ancient Te values using the reconstructed palaeocurvature of the basin in plan view. The radius of curvature of 12 curvilinear foreland basins is plotted against their documented Te values and shows a linear relationship. The maximum Te value for a given radius of curvature can also be plotted as a straight line. The palaeocurvature of reconstructed basins can then be compared with the plots, and estimates of likely maximum Te values may be obtained.During Eocene times, the underfilled foreland basin of the Alps was characterized on its cratonic edge by the deposition of Nummulite‐rich limestones. Palaeogeographical reconstructions of the Nummulitic Limestones enable estimates of the palaeocurvature of the cratonic margin of the Alpine foreland basin during the Eocene. By comparing this value with the curvature of documented basins, it is possible to suggest that the European lithosphere underlying the western Alps had an effective elastic thickness of no greater than 17 km during the Eocene. It has been suggested that the transition in the depositional state of the Alpine foreland basin from an underfilled to a filled state during middle Oligocene times was linked to a thickening of the continental lithosphere associated with the effective ramp of the Tethyan passive margin. The Te value of less than 17 km during the underfilled stage combined with a value of 10±5 km for the later filled stage at 17 Ma does not lend support to this hypothesis.

Late‐ to post‐orogenic basins of the Pan‐African – Brasiliano collision orogen in southern Africa and southern Brazil

Late‐ to post‐orogenic basins formed on both sides of the Pan‐African – Brasiliano orogen when the Congo and Kalahari Cratons collided with the Rio de la Plata Craton during the formation of western Gondwana. Trace fossil evidence and radiometric age dating indicate that deposits on both sides are coeval and span the Cambrian–Precambrian boundary.A peripheral foreland basin, the Nama Basin, developed on the subducting southern African plate. Lower, craton‐derived fluviomarine clastics are overlain by marine platform carbonates and deltaic flysch derived in part from the rising subduction complex along the northern (Damara Belt) and western (Gariep Belt) orogenic margins. Rare, thin volcanic ash layers (tuffs and cherts) are present. Upper sediments consist of unconformable red molasse related to collisional orogenesis. Orogenic loading from the north and west led to crustal flexure and the formation of a remnant ocean that drained to the south and closed progressively from north to south. During final collision SE‐, E‐ and NE‐verging nappes overrode the active basin margins. Although younger than most of the post‐orogenic magmatism, its setting on the cratonic edge of the subducting plate precluded marked volcanism or granitic intrusion, the only exception being the youngest intrusions of the Kuboos‐Bremen Suite dated at 521±6 Ma to 491±8 Ma.Two foreland‐type basins, perhaps faulted remnants of a much larger NE–SW elongated retroarc foreland basin, are found west of the Dom Feliciano Belt on the edge of the Rio de la Plata Craton in southern Brazil. In the southern Camaqua Basin, basal fluvial deposits are followed by cyclical marine and coarsening‐up deltaic deposits with a notable volcanic and volcaniclastic component. This lower deformed succession, comprising mainly red beds, contain stratabound Cu and Pb–Zn deposits and is overlain unconformably by a fluviodeltaic to aeolian succession of sandstones and conglomerates (with minor andesitic volcanics), derived primarily from an eastern orogenic source and showing southerly longitudinal transport. In the northern Itajaí Basin, sediments range from basal fluvial and platform sediments to fining‐up submarine fan and turbidite deposits with intercalated acid tuffs. The Brazilian basins had faulted margins off which alluvial fans were shed. They also overlie parts of the Ribeira Belt. Thrust deformation along the orogenic margin bordering the Dom Feliciano Belt was directed westward in the Camaqua and Itajaí basins, but reactivated strike‐slip and normal faults are also present. Late‐ to post‐orogenic granitoids and volcanics of the Dom Feliciano Belt, ranging in age from 568±6 Ma to 529±4 Ma, occur in the foreland basins and are geochemically related to some of the synsedimentary volcanics.

High-Frequency Cyclicity during Eustatic Sea-Level Rise: Edwards Group of the Balcones Fault Zone: ABSTRACT

ABSTRACT Depositional and diagenetic patterns in the Edwards Group (Albian) .of South Texas were strongly influenced by interactions among (1) Cretaceous eustatic sea-level rise, (2) high-frequency cyclicity, and (3) gentle warping at the craton edge. The Balcones Fault Zone study area extends from the San Marcos Platform to the Maverick Basin. High-frequency, upward-shoaling cycles on the San Marcos Platform are composed of basal transgressive condensed packstones; highstand sequences of burrowed wackestone, grain-dominated bar-flank packstone, grainstone bar deposits, and subtidal gypsum; and relative lowstand tidal-flat caps. Ten high-frequency cycles are stacked in two progradational-retrogradational intermediate-frequency cycles to form the Kainer and Person Formations. Strong world wide Lower Cretaceous sea-level rise created accommodation even during lowstand and resulted in deposition of thick tidal-flat deposits. Prolonged exposure is recognized only at the top of the upper intermediate-frequency cycle (Person Formation). In the Maverick Basin, changing cycle patterns and lithology during Edwards Group deposition document greater relative subsidence than occurred on the San Marcos Platform. All cycles are subtidal. High-frequency cycles in the West Nueces Formation are wackestone-packstone alternations, shoaling toward grainstone at the top. The overlying McKnight Formation is composed of high-frequency alternations of organic argillaceous limestone and subtidal gypsum. Stacking patterns show two intermediate-frequency upward-shoaling cycles. Cycle patterns are poorly expressed in the Salmon Peak Formation, indicating basin deepening and increased accommodation. Poor match in intermediate-frequency cycle patterns between the San Marcos Platform and Maverick Basin is attributed to low-amplitude tectonic effects, although correlation uncertainties and facies contrast may also contribute.

The Gariep belt: stratigraphic-structural evidence for obliquely transformed grabens and back-folded thrust stacks in a combined thick-skin thin-skin structural setting

This paper deals with the tectonics of the Port Nolloth Zone (PNZ) of the Gariep belt, one of the Pan-African tectono-metamorphic belts along the southwest coast of Africa. Stratigraphical and sedimentological evidence indicates that deposition in the sub-basins was controlled by step and/or graben faults parallel to the craton edge. Some of these structures were inverted during the compressional phase to form oblique wrenches in a subzone proximal to the craton edge, whereas others developed into stacked thrusts, blind ramps and a refolded duplex in a distal subzone. A near vertical ZX plane of deformation strikes north-northwest to south-southeast for the entire PNZ and the shear planes are near horizontal in the distal subzone and near vertical in the proximal area. Because metamorphic grades are very similar everywhere, the difference in style is probably related to the original shape of the basement and the varying lithology of the Gariep rocks. Two-dimensional modelling shows that oblique obduction of an oceanic crustal slab to the southeast, followed by relaxation and renewed compression in three pulses, probably produced a combination of thick-skin and thin-skin elements in a zoned tectonite.

Apatite fission track thermochronometry from central Alberta: Implications for the thermal history of the Western Canada Sedimentary Basin

Apatite separated from 47 Cretaceous and Tertiary drill core and surface samples from the foreland basin strata of the Western Canada Sedimentary Basin (WCSB) have been analyzed for their apatite fission track (AFT) age and confined track length distribution. Thermal histories of the fission track data were then estimated using a constrained random search inversion technique based on the Durango apatite annealing model. Most importantly, this technique provides an estimate of the peak postdepositional temperature experienced by each sample with error bounds determined by the precision of the fission track data. Most apatite samples retain at least some tracks that formed prior to the time of maximum Cenozoic burial, assumed to be coincident with maximum temperature. Seven apatite samples were fully annealed during burial, during or following the Laramide Orogeny, and provide a minimum age of 42 Ma for heating. Lower Cretaceous samples from near the deformation front were fully annealed and attained temperatures of at least 119°–138°C during maximum burial. In contrast, stratigraphically equivalent samples at the northeastern end of the transect near the Cold Lake heavy oil and tar sand deposits did not exceed 80°C. Paleogeothermal gradients calculated using bounded estimates of the peak temperatures experienced by the samples, together with maximum burial estimates from coal moisture studies, range from ∼20°C/km near the deformation front to as high as 60°C/km near the cratonic edge of the basin. This variation has the same trend as present geothermal gradients, which range from ∼30°C/km to 45°C/km, but indicates a greater contrast at the end of the Laramide. The results are consistent with the concept of heat transport by basinal scale fluid flow.

Thermal comparison of mississippi valley-type lead-zinc deposits and their host rocks using fluid inclusion and conodont color alteration index data

Fifteen North American Mississippi Valley-type districts, distributed from the Arctic archipelago to Arkansas and in host rocks ranging in age from Late Cambrian to early Carboniferous, were selected for abroad test survey of Mississippi Valley-type district thermal histories. For each district in which data were collected, histograms of homogenization temperatures for the ore minerals were compared with the temperature range of the corresponding host rocks, as indicated by color alteration index (CAI) determinations on conodonts in the host-rock carbonates. Although a special effort was made to collect conodonts from the Upper Cambrian Bonneterre Formation, host rock to the Southeast Missouri district, all samples were devoid of conodonts. Thus only 14 districts were used in the final analysis.Comparison of fluid inclusion homogenization temperatures in main-stage ore minerals with color alteration index-determined host-rock temperatures reveals that a majority of Mississippi Valley-type districts (Pine Point, Newfoundland Zinc, Mascot-Jefferson City, Copper Ridge, Sweetwater, Central Missouri, Northern Arkansas, and Tri-State) are representative of group 1 (i.e., they are in thermal equilibrium with their host rocks insofar as could be determined by the color alteration index method). Ore minerals in group 2 districts (Upper Mississippi Valley, Polaris, and central Tennessee) are significantly hotter than surrounding host rocks and, as such, define a positive thermal anomaly. Host rocks to the Austinville-Ivanhoe, northern Newfoundland, and Robb Lake deposits (group 3) revealed the highest color alteration index values of North American Mississippi Valley-type districts, with host-rock temperatures greatly exceeding ore mineral fluid inclusion temperatures (negative thermal anomaly).With respect to major tectonic elements of North America, group 3 deposits (cool ore in hot rocks) are positioned within orogenic belts but on the continental side. Some group 1 deposits (ore temperature = host-rock temperature) are well within the continental interior but others are adjacent to orogenic belts on the continental side. Group 2 deposits (hot ore in cool rocks) are all within the continental interior.Current hypotheses regarding the formation of Mississippi Valley-type deposits and simultaneous heating of their host rocks invoke the migration of hot brines set into motion craton-ward by the hydraulic gradient produced by orogenic uplift at the craton edge. Once the ore-bearing fluids entered the foreland carbonate platforms, they traveled through sedimentary aquifers of the continental interior raising the temperature of the thinly covered carbonate rocks. The fluids left, as evidence of their passage, elevated color alteration index values and group 1 Mississippi Valley-type deposits distributed with decreasing homogenization temperatures away from the orogenic front. The correspondence between host-rock and ore temperatures suggests that group 1 deposits were deposited essentially within their own aquifers.Group 2 deposits were formed when the same, or genetically similar, fluids rose from the regional aquifer(s) into structurally controlled conduits in the continental interior, possibly developed as a result of synorogenic doming and arching. This upward channeling of the fluids resulted in rapid and localized fluid flow which precluded or at least inhibited wide-spread heating of the host rocks by ore fluids; consequently, color alteration index values in group 2 districts record only maximum burial depths of the host rocks and bear little relation to temperatures of the ore fluids. Although the cause of the unusually high color alteration index values (4-5) of group 3 districts is not fully understood, it is noted that these districts occur near the extreme outward edge of foreland platforms, near or within the leading margins of orogenic belts. Whether these group 3 host rocks were heated by simple tectonic burial or by passage of heated fluids under shallower conditions is not known.

Post-collisional oblique convergence along the Thelon Tectonic Zone, north of the Bathurst Fault, NWT, Canada

The Thelon Tectonic Zone (TTZ) separates the Slave from the Churchill (Rae) structural provinces of the northwest Canadian Shield. Along the TTZ in the Tinney Hills-Overby Lake area, collision was initiated with northwestward overthrusting of granulite-facies gneiss onto the Slave province accompanied by thinskinned shortening of the Proterozoic cover. In the succeeding transpressive stage, shortening structures assumed a SE-, rather than NW-, vergence; a deformation-front migrated westward into the Slave craton-edge, causing thick-skinned folding of the cover, probably including the detachment; and transpression was partitioned, in craton edge and allochthon, into zones of shortening, shortening + strike-slip, and strike-slip. Ductile deformation along the trace of the late NW-striking brittle Bathurst Fault occurred at this time; this zone links dextral NNE-trending transpressive zones, in which the shortening structures have opposite vergence (SE north of the fault, and NW to the south). The last episode of convergence, tectonic escape, may have been accomplished during brittle deformation on the Bathurst Fault. In the Slave Foreland and the reworked Slave craton-edge, the role of heterogeneous basement deformation is very important. The basement had a complex Archean structure, which was overprinted by systems of Proterozoic shear zones. The bulk strains accomplished by these are expressed as upright folds in the stratiform cover, causing characteristic synclines.

Tectonic interpretation of electrical structures beneath the West African Craton edge in eastern Senegal

Tectonic interpretation of electrical structures beneath the West African Craton edge in eastern Senegal

Structural morphology and associated strain in the central Cordillera (British Columbia and Washington): Evidence of oblique tectonics

The central Cordillera developed during Early Jurassic to Late Cretaceous time as the continental edge was subducted under the exotic terranes of Quesnellia and Stikinia. The earliest Jurassic structures formed during prograde metamorphism and include cylindrical and double-verging folds that trend subparallel to the orogen. These structures are associated with mineral lineations and stretch values between 120% and 400% parallel to the fold axes. Later, disharmonic structures formed during the emergence of the crust, after the metamorphic peak and in response to decreasing P/T conditions. Present tectonic models do not incorporate the kinematics capable of forming the early structures, and thereby only address the emergent phase. A scenario is offered which involves oblique subduction of the craton edge and which is kinematically compatible with the early structural morphology and associated strains.

Tectonics of Brooks Range Ophiolites, Alaska: GEOLOGIC NOTES

Ophiolites in the Alaska Brooks Range of probable Paleozoic crustal age are erosional remnants of stacked thrust sheets and are composed of two complexes formed at different sites and tectonically assembled during orogeny. An upper complex is an incomplete ophiolite suite with mafic and ultramafic cumulates and small remnants of a sheeted-dike complex. A lower thrust sheet consists of basaltic flows which probably were extruded in the lower plate during the Jurassic to Neocomian obduction of the ophiolites over the Arctic Alaskan craton and its late Paleozoic south margin composed of cratonic shelf edges and rifts or basins with thinner crust.

Miogeoclines (Miogeosynclines in Space and Time): A Reply

Previous articleNext article No AccessDiscussionMiogeoclines (Miogeosynclines in Space and Time): A ReplyRobert S. Dietz and Walter P. SprollRobert S. Dietz Search for more articles by this author and Walter P. Sproll Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 76, Number 1Jan., 1968 Article DOIhttps://doi.org/10.1086/627313 Views: 2Total views on this site Copyright 1968 University of ChicagoPDF download Crossref reports no articles citing this article.