Article

Early extensional detachments in a contractional orogen: Coherent, map-scale, submarine slides (mass transport complexes) on the outer slope of an Ediacaran Collisional Foredeep, Eastern Kaoko Belt, Namibia

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Abstract

The existence of coherent, large-scale, submarine landslides on modern continental margins implies that their apparent rarity in ancient orogenic belts is due to non-recognition. Two map-scale, coherent, pre-orogenic, normal-sense detachment structures of Ediacaran age are present in the Kaoko belt, a well-exposed arc–continent collision zone in northwestern Namibia. The structures occur within the Otavi Group, a Neoproterozoic carbonate shelf succession. They are brittle structures, evident only through stratigraphic omissions of 400mor more, that ramp down to the west with overall ramp angles of 1.1° and 1.3° with respect to stratigraphic horizons. The separations of matching footwall and hangingwall stratigraphic cut-offs require horizontal translations >20 km for each detachment. One of the detachments is remarkably narrow (5 km) in the up-dip direction, just one fourth of its translation. The other detachment is stratigraphically dated at the shelf–foredeep transition, when the passive margin was abortively subducted westward, in the direction of submarine sliding. Trenchward sliding on the foreslope occurred concurrently with deep karstification of the autochthonous carbonate succession to the east, presumably due to forebulge uplift and (or) conjectural basin-scale base-level fall. We expect that similar detachments exist in other orogenic belts, and failure to recognize them can lead to misinterpretations of stratigraphy, sedimentary facies, and paleogeography.

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... If this were the case, an explanation for the absence of Ediacaran carbonate (Karibib Formation) in the Austerlitz Anticlinorium is re- quired. Previous mapping in the Northern Damara and Eastern Kaoko Belts has shown that when a chunk of stratigraphy is inexplicably ab- sent locally (e.g., on the scale of Austerlitz Anticlinorium), the culprit is generally a map-scale submarine slide (Clifford 2008;Hoffman and Hartz 1999;Hoffman et al. 2016b). ...
... (7) Hoffman et al. (2016) describe the occurrence of extensional, brittle detachments formed as submarine slides on the outer slope of an Ediacaran collisional foredeep, eastern Kaoko belt, Namibia. The authors argue that the existence of coherent, largescale, submarine landslides on modern continental margins implies that their apparent rarity in ancient orogenic belts is due to non-recognition. ...
Article
Otavi Group is a 1.5−3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran−Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S−N crustal stretching (770−655 Ma), followed by post-rift thermal subsidence (655−600 Ma). Rifting under southern Swakop Group continued until 650−635 Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S−N plate divergence. Rift related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746 Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770−717 Ma; (2) glacial early Cryogenian/Sturtian, 717−661 Ma; (3) non-glacial middle Cryogenian, 661−646±5 Ma; (4) glacial late Cryogenian/Marinoan, 646±5−635 Ma; and (5) non-glacial early Ediacaran, 635−600±5 Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585 measured sections (totaling >190 km of strata) and >8,764 pairs of δ13C/δ18Ocarb analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5−9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3−8.8 m Myr−1, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ≤1.6 km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9 m regional average thickness, n=140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ≤90 m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ≥0.6 km of paleobathymetric relief. Late Tonian shallow-neritic δ13Ccarb records were obtained from the 0.4-km-thick Devede Fm (~770−760 Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737−717 Ma) in Swakop Group. Devede Fm is isotopically heavy, +4−8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746 Ma volcanics and shows two negative excursions bridged by heavy δ13C values. The negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic δ13C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0−3.5‰. Early Ediacaran neritic δ13C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (≤+11‰) with extreme point-to-point variability (≤10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform δ13C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9±1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60−0.58 Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ≥1.85 km of megakarstic relief, and kmthick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.
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The Kaoko Belt in Namibia represents the deeply eroded core of a classic sinistral transpressional orogen with a half flower structure centred on the crustal-scale Purros Mylonite Zone. The Kaoko Belt consists of three NW-trending structural zones each with distinct kinemetamorphic style. The Eastern Kaoko Zone contains upright-folded, Neoproterozoic Damara Sequence shelf carbonates. The Central Kaoko Zone comprises an inverted Barrovian metamorphic series within large-scale east-vergent nappes, whereas the Western Kaoko Zone is predominantly of high metamorphic grade and intruded by numerous granitoids. The Western Kaoko Zone has orogen-parallel panels of distinctly different metamorphic grade separated by strike-slip ductile shear zones with overall isograd pattern being indicative of extrusional tectonics in the orogen core. The Kaoko Belt evolved through three distinct phases of a protracted Pan-African Orogeny in the late Neoproterozoic to Cambrian: (1) an early Thermal Phase (early M 2 ) was responsible for pervasive partial melting, high-grade parageneses and granite emplacement between 580 and 570 Ma; (2) the main deformation Transpressional Phase (580–550 Ma) reworked early M 2 parageneses in the pervasive orogenic fabric producing M 2 assemblages that formed as a result of progressive sinistral transpression that evolved from wrench-style to high-angle convergence accompanying foreland-vergent thrusts and nappes; (3) the post-transpression Shortening Phase generated upright, open folds during north–south shortening (530–510 Ma). In the Western Kaoko Zone, peak metamorphic conditions were attained during early M 2 at moderate to high average thermal gradients (29–40°C/km) and were intensely reworked by lower-grade pervasive fabrics during M 2 . (Average thermal gradient is simply the calculated metamorphic temperature divided by the calculated depth assuming a density of 2·8 g/cm3. It should not be confused with the instantaneous thermal gradients in the vicinity that an assemblage formed, or imply that the thermal gradients are time equivalent.) In the northern part of the Western Kaoko Zone, immediately adjacent to the Purros Mylonite Zone, the amphibolite-grade Khumib Terrane experienced peak M 2 metamorphism at 573°C and 5·4 kbar. Along strike to the south the granulite-grade Hoarusib Terrane experienced peak early M 2 conditions at 843°C and 8·1 kbar and M 2 reworking at approximately 560–580°C and 4·8 kbar. In the western margin of the orogen, the Coastal Terrane experienced early M 2 metamorphism at sillimanite–K-feldspar–melt grades and was reworked during M 2 at muscovite–biotite grade. In the Central Kaoko Zone, metamorphic grade increases towards the west to higher structural levels. Peak metamorphic matrix assemblages formed during pervasive deformation in the Transpressional Phase ( M 2 ) at conditions in the range of 530–690°C and 8·5–9·0 kbar with consistently low average thermal gradients (17–23°C/km). Clockwise P – T paths were experienced in both the Central Kaoko Zone and Western Kaoko Zone. Garnet Sm–Nd geochronology indicates that matrix parageneses, early M 2 in the Western Kaoko Zone and M 2 in the Central Kaoko Zone, formed at the same time within uncertainties (576 ± 15 Ma). This indicates that the thermal peak was contemporaneous across the belt, even though deformational phases of equivalent structural style were diachronous across the Kaoko Belt.
Article
The Pan-African Orogen formed by convergence of numerous continental blocks during the Neoproterozoic to early Cambrian. This convergence eventually led to amalgamation of Gondwana, a supercontinent crosscut by a network of highly oblique linear orogenic belts that locally intersect each other, as in NW Namibia, where the NNW trending Kaoko Belt joins the NE trending Damara Belt. The northern Damara Belt has preserved well three regional Pan-African tectonic events due to the dominance of weak Neoproterozoic marine sediments (Damara Supergroup) that have been affected by low-grade metamorphism. A newly discovered early N-S horizontal contraction, dated by 40Ar/39Ar at ~590Ma, is tentatively linked to convergence between the Congo and Kalahari cratons. This was superseded by collision between the Congo and Rio de la Plata cratons between 580 and 530Ma that thickened and exhumed the orogenic crust of the Kaoko Belt and produce upper crustal N-S oriented folds of earlier fold trains and associated axial planar schistosities in the northern Damara Belt. A switch from E-W to NW-SE horizontal shortening occurred at ~530Ma as a result of collision with the Kalahari Craton, triggering extensive syn-orogenic magmatism in the entire Damara Belt. During this last event, southward indentation and underthrusting of the Congo Craton promontory below the Neoproterozoic cover sequences produced a deformation front in the northern Damara Belt. Our results show that highly oblique convergent processes competed over a period of ~120Ma to build Gondwana in Namibia during the late Neoproterozoic to early Cambrian.
Article
The Embu Domain represents the central part of the Ribeira Fold Belt in São Paulo, Brazil. It hosts several granitic occurrences of varied composition ranging from small granitic bodies that outcrop in a domain bounded by the intersection of two major sutures (Taxaquara and Guararema Faults) and batholitic masses outcropping to the east. Understanding the evolution of such granites is of vital importance to better constrain evolutionary models for the Ribeira Belt. In the present study, a set of samples from eleven main granite occurrences from the east of São Paulo state was selected for geochronological investigation using laser ablation-multicollector-inductively coupled plasma spectrometry (LA-MC-ICP-MS) and thermal ionization mass spectrometry (TIMS) U–Pb dating of zircon and monazite crystals, respectively. The results indicate a remarkable cluster of ages around 590 Ma with older events of granite magmatism between 660 and 600 Ma registered for four plutons, indicating a long history of crustal reworking and magma generation.The ages of reworked sources were evaluated from inherited zircon cores. Although highly discordant these point to the predominance of Paleoproterozoic (2.4–2.1 Ga) sources, with minor contributions from Mesoproterozoic (1.1–0.9 Ga) and Archean sources (∼3.1 Ga). The new data bring important insights into the role played by the Embu Domain on the paleogeography and evolution of the Ribeira Belt.
Article
The Damara Orogeny is a late Neoproterozoic to Cambrian (ca. 570–480 Ma) intracratonic event that affected the Kaoko Belt, the inland branch of the Damara orogen and the Gariep Belt in Namibia and South Africa. This study focuses on the Pan-African evolution of part of the Kaoko Belt between the Puros shear zone and the Village mylonite zone which consists of Mesoproterozoic migmatitic para- and orthogneisses with minor granulite and amphibolite. Pseudosection modeling combined with thermobarometric calculations indicate that the para- and orthogneisses equilibrated at about 670–800 °C and ca. 0.6–0.8 GPa. Some garnets display a pronounced bell-shaped Ca, HREE, Y and Sr zoning, flat zoning profiles of Mn and Fe and concave upward concentration profiles of Sm and Nd. Pressure–temperature estimates obtained on these garnets reveal similar temperatures of 700–750 °C but slightly higher pressures of ca. 0.9 GPa. The preservation of distinct major and trace element zoning in garnet and the existence of broadly similar (near prograde) Sm–Nd and Lu–Hf garnet–whole rock ages of ca. 525 Ma obtained on the same sample indicate an extremely fast cooling path. Retrograde conditions persisted until ca. 490 Ma indicating a slow, late stage near isobaric cooling path. The resulting clockwise P–T–t path is consistent with crustal thickening through continent–continent collision followed by post-collisional extension and suggests that the upper amphibolite to granulite facies terrain of the central Kaoko Belt formed initially in a metamorphic field gradient of ca. 25–35 °C km− 1 at moderately high pressures.
Article
Isotopic dating of detrital zircon populations from metasediments and of magmatic zircon from intercalated metavolcanic layers in the medium- to high-grade part of the Kaoko Belt in Namibia provides first robust constraints on the age of Neoproterozoic sedimentation along the southwestern margin of the Congo Craton. Dating of detrital zircons from metasediments directly overlying the cratonic basement shows maximum sedimentary ages of c. 1.00 and c. 1.45 Ga, and age populations comparable with known protolith ages from the gneisses of the Congo/Kalahari cratons. Dating of zircon from associated metavolcanics constrains the age of the earliest preserved sediments at c. 740-710 Ma. Detrital zircon populations from the samples collected from the upper parts of the metasedimentary succession contain only small proportion of grains with ages similar to those from the Congo Craton. These samples show dominance of c. 1.00 Ga, c 750 Ma and c. 650 Ma old zircon grains that are probably derived from the Punta del Este - Coastal Terrane (Dom Feliciano and Kaoko belts) that acted as an arc/back-arc domain at c. 650-630 Ma. Neodymium model ages for the samples of metasedimentary cover of the cratonic basement provide another evidence that the youngest preserved sediments could not have been derived from the Congo Craton. Recognition of the Punta del Este - Coastal Terrane crust as a source region for the youngest pre-collisional sediments of the Kaoko Belt suggests that this terrane must have been in close proximity to the Congo Craton passive margin already some time prior to their mutual collision at c. 570-550 Ma. This is in accord with an interpretation that the c. 650-630 Ma arc/back-arc Punta del Este - Coastal Terrane has developed directly on top of, or very close to, the attenuated Congo Craton passive margin.
Book
Arc-continent collision has been one of the important tectonic processes in the formation of mountain belts throughout geological time, and it continues to be so today along tectonically active plate boundaries such as those in the SW Pacific or the Caribbean. Arc-continent collision is thought to have been one of the most important process involved in the growth of the continental crust over geological time, and may also play an important role in its recycling back into the mantle via subduction. Understanding the geological processes that take place during arc-continent collision is therefore of importance for our understanding of how collisional orogens evolve and how the continental crust grows or is destroyed. Furthermore, zones of arc-continent collision are producers of much of the worlds primary economic wealth in the form of minerals, so understanding the processes that take place during these tectonic events is of importance in modeling how this mineral wealth is formed and preserved. This book brings together seventeen papers that are dedicated to the investigation of the tectonic processes that take place during arc-continent collision. It is divided into four sections that deal firstly with the main players involved in any arc-continent collision; the continental margin, the subduction zone, and finally the volcanic arc and its mineral deposits. The second section presents eight examples of arc-continent collisions that range from being currently active through to Palaeoproterozoic in age. The third section contains two papers, one that deals with the obduction of large-slab ophiolites and a second that presents a wide range of physical models of arc-continent collision. The fourth section brings everything that comes before together into a discussion of the processes of arc-continent collision.
Article
In the Daqingshan area within the Khondalite Belt of the North China Craton, Paleoproterozoic gabbro and dolerite intrusions show amphibolite to granulite-facies metamorphism. U–Pb zircon dating and whole-rock geochemistry were undertaken on these rocks in order to understand the timing of mantle magmatism and thermal processes. Ten samples yielded zircon U–Pb formation ages of 2.46–2.44 Ga, 1.97–1.92 Ga and 1.84 Ga and metamorphic ages of 1.95–1.83 Ga. They are variable in geochemical composition with some being enriched in light rare earth and large ion lithosphere elements but depleted in Nb–Ta. Combined with previous results, it is concluded that mafic magmatism at 2.45–2.37 Ga and 1.97–1.92 Ga indicates two Paleoproterozoic extensional phases, with the latter accompanied by HT–UHT metamorphism as a result of underplating of mantle magma.
Article
The Neoproterozoic Kaoko belt of northwestern Namibia, to the west of the Congo craton, consists of basement gneisses and a sequence of highly deformed metasedimentary and metavolcanic rocks. The metasedimentary rocks are interpreted as the infill of a narrow north-trending basin floored by attenuated continental crust. Similar basins are represented by the western continuation of the Kaoko belt---the Dom Feliciano and Ribeira belts of eastern Brazil. On the basis of consideration of data from these belts, the Kaoko belt rocks indicate the absence of a large Neoproterozoic oceanic basin between the Congo and southwestern American cratons. Deformation structures in the Kaoko belt show transpressional kinematics with strain partitioning at around 550 Ma. North-northwest south-southeast, sinistral transcurrent shear in the west passes eastward into east-southeast directed thrusting. The transcurrent shear component dominates temporally and in strain intensity. Dextral transcurrent motion at 550 Ma is recorded in the east-northeast trending Schlesien-Mwembeshi shear zone, bordering the Congo craton to the south. Accordingly, the Congo craton extruded northeastward along the Kaoko belt and the Schlesien-Mwembeshi shear zone relative to the Kalahari craton in the southeast and the South American cratons to the west. This extrusion probably resulted from a final phase of convergence between east and west proto-Gondwana.
Article
The ca. 790–600Ma Rio Negro Complex (RNC) of the Ribeira belt (Brazil) consists of a plutonic portion of a magmatic arc built by the E-vergent subduction of the ESE border of the São Francisco paleoplate during the amalgamation of Western Gondwana.The plutonic series comprises low- to medium-K granitoids (ca. 790–620Ma) and high-K granitoids and shoshonite rocks (ca. 610–605). The age span of 185m.y. is suggestive of a long history of arc-related magmatism, continuously or not in time. The Nd isotopic signatures of the RNC consist of εNd(t) ratios from −3 to +5 for the medium-K series shoshonite series and from −14 to −3 for the younger high-K group. This time-dependent trend of Nd isotopes is indicative of progressive maturity of the arc over time. The same evolution is indicated by Sr data, as the medium-K rocks have 87Sr/86Sr initial ratios
Article
The Neoproterozoic succession in the Outjo District of northern Namibia is a mainly sedimentary sequence that was deposited in the time-interval of approximately 800 to 600 Ma ago and was deformed and suffered mild greenschist facies regional metamorphism during Damaran times ca. 550 Ma ago. The map area occupies the lithofacies and tectonic transition from the highly deformed more eugeoclinal Swakop Group sequence to the south to the coeval, uniformly southward-dipping, miogeoclinal Otavi Group of the Fransfontein Ridge to the north. The oldest unit is a largely sandstone and biotite-muscovite phyllite succession tentatively equated with the volcanogenic Naauwpoort Formation (Nosib Group) that pre-dates the Swakop Group in the Summas Mountains to the southwest. The overlying Saturn Formation comprises two distinct but coeval lateral carbonate facies: a 300 m thick sequence of bedded dolomite with a 20 m thick median limestone; and massive biohermal dolomite of undetermined thickness. The conformably overlying Landeck Formation comprises a lower succession of interbedded sandstone, phyllite and local carbonate rocks, a distinctive middle iron formation marker horizon with rather rare overlying dolomite, followed by a siliciclastic-carbonate sequence. The combined succession is up to 450 m in thickness, and glacial diamictite is recorded at 30+ localities sited below, within and above the iron formation. However, in the more distal parts of the Landeck Formation in the Saturn Dome, glacial diamictictite has not been identified, the iron formation is overlain by a distinctive dolomite, and the uppermost 200 to 300 m of the Landeck Formation is characterized by four horizons of siliciclastic sediments with three intervening horizons of blue limestone that reflect the concordant transition to the overlying Bergfriede Formation. New chemical analyses are reported for graphitic dolomite, magnetite-calcite schist, iron formation, ferruginous limestone, dolomitic sandstone and biotite-muscovite phyllite of the Landeck Formation. The Bergfriede Formation consists almost entirely of limestone and dolomite and is characterized by major changes in facies and thickness, and five lateral facies end-members are recognized: I. Hankow Facies, a five-fold succession of limestone, dolomite and calcarenite up to 500 m thick; II. Tsuwandes-Zuwitsaub Facies, a three-fold sequence of limestone-dolomite-limestone >300 m thick; III. Uranus Facies, entirely dolomite generally 150 m thick; IV. Belina-Sophienhof Facies, a limestone-dolomite sequence, with a distinctive arenaceous limestone at the top, 100 to 400 m thick; and V. Mooilagte-Mooihoek Facies, largely dolomite-chert-limestone ≥1500 m thick, that is transitional to the dolomite-dominated upper Otavi Tsumeb Subgroup of the Fransfontein Ridge to the north. The overlying Okaua Formation is composed of a >3000 m thick sequence of muscovite-chlorite phyllite and calcareous sandstone that occupy, inter alia, the troughs of the regional Ugab and Uranus-Neuland Synclines; it is most likely correlative with the Mulden Group of the Otavi Mountainland. The Damaran regional structure of area is characterized by: D1 an early phase of southward-directed gravity nappes; D2 the regionally dominant northeast-southwest to east-west trending tight compressional upright or northward-verging folds, and local thrusts; and D3 late strike-slip faulting.
Article
Deformation styles due to the collisional convention of Gondwana that ended ∼500 Ma are reassessed in the Kaoko fold-thrust belt of the NE Damaran orogen. Downward facing sequences on the overturned limb of a major recumbent fold nappe are exposed in the Hoanib valley beneath pre-Damara basement, initially identified in a D2/D3 antiformal refold core, constituting the Obias River Window. Basement paragneisses, psammitic and pelitic protoliths, were infolded into the core of a major recumbent anticlinal Di fold nappe structure (dimensions 55 km across x 150 km+ along tectonic strike), comparable in dimensions to other major fold nappes (.e.g. the Loch Tay nappe of the Scottish Highlands). D2/D3 refolding produced large-scale polyphase fold traces later affected by a D4 phase of open cross folding. The nappes detached along flat lying sole thrusts (Eastern Zone), soled by the Sesfontein master thrust, and are backed by a structurally steep root zone (Central Zone). Farther west ramped-up syn-orogenic granitoids intruded into the Damara sediments (Western Zone). Some thrust zones mark major facies changes and imply a precompressional identity, with most complete stratigraphic sequences immediately to their western side. The latter Damaran (750 to 600 Ma) sequences progressively on-lap westwards onto basement, to exclude successively lowest stratigraphic units. Prior to collision half-graben sub-basin fills were accommodated by extensional fault systems. Subsequent Damaran compression reversed extensional faults as thrust zones. Intra-Pangean extension and break-up later reactivated Damaran thrusts, expressed as rift basins that developed during the latest Palaeozoic and culminated during the Early Cretaceous with the opening of the South Atlantic. Karoo to post-Karoo sediments and volcanics were accommodated in the resulting half-grabens. Indeed the Damaran continental suture probably inverted as the southern South Atlantic breakaway zone between Africa and South America. Atlantic shelf sediments now cover the suture apart from in southernmost Namibia, where it is marked by the Damaran thrust-floored Oranjemund Complex of the Gariep Belt. The Oranjemund Complex incorporates an exotic Adamastor Oceanic volcanic prominence thrust onto the proto-African foreland.
Article
Granitoid intrusions of the Boundary Igneous Complex separate segments with different ages of high-grade metamorphism in the Kaoko Belt, NW Namibia. Two granitoids of this complex were dated at 575 +/- 10 Ma (secondary ionization mass spectrometry; SIMS) or 571 +/- 9 Ma (laser ablation inductively coupled plasma mass spectrometry; LA-ICP-MS) and 562 +/- 11 Ma (SIMS) or 572 +/- 4 Ma (LA-ICP-MS), respectively. The age of granulite-facies metamorphism in the eastern part of the Western Kaoko Zone was established at 549 +/- 5 Ma (SIMS) by analysing metamorphic overgrowths of older (c. 1850-1000 Ma) zircons from melt segregations in amphibolites. The coastal part of the Western Kaoko Zone consists of horizons of migmatitic metasedimentary rocks that are intercalated with fine-grained orthogneisses and amphibolites resembling metamorphosed sequences of bimodal volcanic rocks. Zircons from felsic members of two bimodal suites have SIMS ages of 805 +/- 4 Ma and 810-840 Ma, respectively, that are interpreted as dating their respective igneous protoliths. Melt segregations in the mafic member of the lower bimodal suite contain two populations of zircon dated at 650 +/- 5 Ma (SIMS) or 645 +/- 5 Ma (LA-ICP-MS) and 629 +/- 6 Ma (SIMS) or 630 +/- 5 Ma (LA-ICP-MS), respectively. The later age is indistinguishable from the age of 630 +/- 4 Ma (SIMS) or 625 +/- 10 Ma (LA-ICP-MS) obtained from melt patches present in overlying metagreywackes. The available age data suggest that the Boundary Igneous Complex masks the suture between the Coastal Terrane and the rest of the Kaoko Belt. Ages of granitoid intrusions in this igneous complex are indicative of magmatic activity between 580 and 550 Ma.
Article
The morphology and structure of a large submarine slump (Agulhas Slump) on the sheared continental margin off SE Africa are described from bathymetric and continuous seismic reflection records. It is a composite feature consisting of proximal and distal allochthonous sediment masses separated by a large glide plane scar. The locations of the various structural elements of the slump are related to underlying features: in the head region these are controlled by large-scale Cretaceous and Palaeogene depositional features, and in the toe region by older basement ridges. In the western part of the slump, the basement ridges have dammed the slump over continental crust, whereas in the east, allochthonous material has spread into the oceanic Transkei Basin. Characteristics related to a structural setting on a sheared continental margin are emphasized and discussed. The Agulhas Slump is probably the largest slumped mass so far recognized from modem oceans (750 km long, 106 km wide, with a volume of over 20,000 km 3) and is post-Pliocene in age. A seismic triggering mechanism is tentatively proposed: the slump lies on two major fault zones whose extensions are known to be seismically active (the Cape Fold Belt and the Agulhas marginal fracture zone).
Article
Landslides are common on inclined areas of the seafloor, particularly in environments where weak geologic materials such as rapidly deposited, fine-grained sediment or fractured rock are subjected to strong environmental stresses such as earthquakes, large storm waves, and high internal pore pressures. Submarine landslides can involve huge amounts of material and can move great distances: slide volumes as large as 20,000 km³ and runout distances in excess of 140 km have been reported. They occur at locations where the downslope component of stress exceeds the resisting stress, causing movement along one or several concave to planar rupture surfaces. Some recent slides that originated nearshore and retrogressed back across the shoreline were conspicuous by their direct impact on human life and activities. Most known slides, however, occurred far from land in prehistoric time and were discovered by noting distinct to subtle characteristics, such as headwall scarps and displaced sediment or rock masses, on acoustic-reflection profiles and side-scan sonar images. Submarine landslides can be analyzed using the same mechanics principles as are used for occurrences on land. However, some loading mechanisms are unique, for example, storm waves, and some, such as earthquakes, can have greater impact. The potential for limited-deformation landslides to transform into sediment flows that can travel exceedingly long distances is related to the density of the slope-forming material and the amount of shear strength that is lost when the slope fails.
Article
In the Damara Orogen sedimentary basinal responses are important in recording the evolution of the fold belt. Here we integrate sedimentological patterns and tectonics to characterise the basin development of both the pre- to syn-orogenic Damara Sequence and the syn- to post-orogenic Nama Group. The evolution of an entire Late Proterozoic Wilson Cycle involved initial rifting, with the opening of two oceanic arms through convergence to collision and foreland basin development.Rift initiation (stage 1) took place along old tectonic weaknesses and extensional rift basins (stage 2) were filled by continental sediments and alkaline/bimodal volcanics. Two oceanic openings occurred: (i) the Adamastor Ocean (stage 3) produced a break-up unconformity and eastward transgression over the Kalahari and Congo Cratons; and (ii) the Khomas Sea gulf subsequently developed betwen the two cratons (stage 4) and is associated with break-up unconformities, and ultimately the development of mature shleves (stage 5). In the latter opening, we envisage an anticlockwise rotation of the Kalahari Craton with respect to the Congo Craton.During convergence the closing Khomas Sea produced an accretionary prism/arc/retro-arc system (stage 6) and the first deformation phase in the Southern Zone. The Khomas Orogeny records the collision between the Kalahari and Congo Cratons (stage 7) including the obduction of oceanic elements onto the Kalahari Craton foreland, and caused the second and third deformation phases in the Southern Zone and first and second deformation phases in the Central Zone. A peripheral foreland basin and peripheral bulge on the Kalahari Craton resulted, which respectively contained and affected the marine and fluvial Nama Group sedimentation. A complementary hinterland basin accepted Mulden Group sediments on the Congo Craton. Ultimately the collision of the South American continent with the newly reconstituted African foreland (stage 8) caused the Adamastor Orogeny and produced a peripheral foreland basin. This basin was divided into two parts by the extant Khomas mountain belt. This event resulted in a dominant westerly source for the fluvial Fish River Subgroup and exclusion of previous marine conditions.We distinguish for the first time, the temporal separation of the Khomas and Khomas and Adamstor Orogenesis. The Khomas Orogeny involved the subduction of hot young oceanic crust associated with the relatively short residence time of the Khomas Sea, and can be dated at just before the Precambrian-Cambrian boundary. In contrast, the Adamastor Ocean had a residence time of about 200 M.a. Convergence therefore involved the subduction of cooled oceanic crust and incorporation of exotic terranes. Collision was associated with relatively major translation of tectonostratigraphic units. The Adamastor Orogeny occurred close to 500 M.a.
Article
Early structures in the central part of the Kaoko orogenic belt of NW Namibia suggest that the initial stage of collision was governed by underthrusting of the medium-grade Central Kaoko zone below the high-grade Western Kaoko zone, resulting in the development of an inverted metamorphic gradient. In the Western zone, early structures were overprinted by a second phase of deformation, which is associated with localization of the transcurrent Puros shear zone along the contact between the Western and Central zones. During this second phase, extensive partial melting and intrusion of ∼550 Ma granitic bodies occurred in the high-grade Western zone. In the Central zone, the second phase of deformation led to complete overprinting of the early foliation in the zone adjacent to the Puros shear zone, and to the development of kilometre-scale folds in the more distal parts. Strain partitioning into transcurrent deformation along the Puros shear zone and NE–SW oriented shortening in the Central zone is consistent with a sinistral transpressional regime during the second phase of deformation. Transcurrent deformation continued during cooling of the entire belt, giving rise to the localized low-temperature Village Mylonite Zone that separates a segment of elevated Mesoproterozoic basement from the rest of the Western zone in which only Pan-African ages have so far been observed. The data suggest that the boundary between the Western and Central Kaoko zones represents a modified thrust zone controlling the tectonic evolution of the Pan-African Kaoko belt. (J. Konopásek).
Article
The Curitiba Terrane represents a major segment of the southern Ribeira Belt (SE Brazil), which was derived from the collision between the São Francisco, Congo, Paranapanema and Luís Alves Cratons during the Neoproterozoic (Brasiliano/Pan-African Orogeny). The tectonic setting and the metamorphic records of two major juxtaposed units from the Curitiba Terrane, a Neoproterozoic shallow continental-shelf metasedimentary assemblage (Turvo-Cajati Formation) and an Archaean to Paleoproterozoic TTG-type orthogneiss assemblage (Atuba Complex), were investigated. Migmatitic paragneisses from the Turvo-Cajati Formation underwent a deep collision metamorphism. Conventional geothermobarometry and petrological modelling in the system NCKFMASHTi indicate peak metamorphic conditions between 670 and 810 °C at 9.5–12 kbar. Metamorphic paths calculated from zoned garnet and plagioclase using the Gibbs method of differential thermodynamics indicate distinct evolution for two major groups of migmatites from the Turvo-Cajati Formation: (i) kyanite migmatites evolved from low-temperature eclogite to high-pressure granulite facies conditions following near isobaric heating; (ii) sillimanite migmatites underwent near isothermal decompression and apparently evolved from high-temperature eclogite facies conditions. Chemical dating of monazite indicates that the peak metamorphism of the Turvo-Cajati Formation occurred at 589 ± 12 Ma, followed by a greenschist facies metamorphic overprint at 579 ± 8 Ma related with late transcurrent shear zones. 40Ar–39Ar biotite ages indicate that the Turvo-Cajati Formation cooled below 250–300 °C at 555 ± 4 Ma. P–T data and petrological evidence of rocks from the Atuba Complex suggest a retrograde metamorphic path with cooling from 730 to 630–650 °C at 6–7 kbar. Available K–Ar and 40Ar–39Ar data indicate that the Atuba Complex had cooled to below 300–500 °C between ca. 590 and 580 Ma. Geochronological data indicate that the main metamorphism of the Turvo-Cajati Formation and the Atuba Complex are coeval, but very contrasting metamorphic signatures reflect formation in different parts of a collisional suture. The integration of structural and petrological data indicates that the structural pattern of the Curitiba Terrane is related to Ediacaran westward directioned nappes during the late- to postmetamorphic period. This is concomitant with a main, crustal-scale, strike-slip regime, dominant throughout the Ribeira Belt. The nappe stack was later deformed by cylindrical folds with E–W trending sub-horizontal axes parallel to the synthrusting stretching lineation and was dismembered and dispersed by late sinistral strike-slip shear zones. The late tectonic assembly of the Ribeira Belt was controlled by significant postcollision terrane dispersion along major strike-slip shear zones.Highlights► Collision metamorphism and anatexis occurred at 600–590 Ma in southern Ribeira Belt. ► Metasedimentary rocks were buried to depths of 40–47 km. ► Terranes with distinct histories were juxtaposed during postmetamorphic events. ► The late assembly was controlled by terrane dispersion along strike-slip shear zones.
Article
Deepwater fold and thrust belts (DWFTBs) are classified into near-field stress-driven Type 1 systems confined to the sedimentary section, and Type 2 systems deformed by either far-field stresses alone, or mixed near- and far-field stresses. DWFTBs can occur at all stages of the Wilson cycle up to early stage continent continent collision. Type 1 systems have either weak shale or salt detachments, they occur predominantly on passive margins but can also be found in convergent-related areas such as the Mediterranean and N. Borneo. Examples include the Niger and Nile deltas, the west coast of Africa, and the Gulf of Mexico. Type 2 systems are subdivided on a tectonic setting basis into continent convergence zones and active margin DWFTBs. Continent convergence zones cover DWFTBs developed during continent–arc or continent–continent collision, and those in a deepwater intracontinental setting (e.g. W. Sulawesi, Makassar Straits). Active margins include accretionary prisms and transform margins. The greatest variability in DWFTB structural style occurs between salt and shale detachments, and not between tectonic settings. Changes in fold amplitude and wavelength appear to be more related to thickness of the sedimentary section than to DWFTB type. In comparison with shale, salt detachment DWFTBS display a lower critical wedge taper, more detachment folds, long and episodic duration of deformation and more variation in vergence. Structures unique to salt include canopies and nappes. Accretionary prisms also standout from other DWFTBs due to their relatively long, continuous duration, rapid offshore propagation of the thrust front, and large amount of shortening. In terms of petroleum systems, many similar issues affect all DWFTBs, these include: the oceanward decrease in heat flow, offshore increase in age of mature source rock, and causes of trap failure (e.g. leaky oblique and frontal thrust faults, breach of top seal by fluid pipes). One major difference between Type 1 and Type 2 systems is reservoir rock. High quality, continent-derived, quartz-rich sandstones are generally prevalent in Type 1 systems. More diagenetically reactive minerals derived from igneous and ophiolitic sources are commonly present in Type 2 systems, or many are simply poor in well-developed turbidite sandstone units. However, some Type 2 systems, particularly those adjacent to active orogenic belts are partially sourced by high quality continent-derived sandstones (e.g. NW Borneo, S. Caspian Sea, Columbus Basin). In some cases very high rates of deposition in accretionary prisms adjacent to orogenic belts, coupled with uplift due to collision, results in accretionary prism related fold belts that pass laterally from sub-aerial to deepwater conditions (e.g. S. Caspian Sea, Indo-Burma Ranges). The six major hydrocarbon producing regions of DWFTBs worldwide (Gulf of Mexico, Niger Delta, NW Borneo, Brazil, West Africa, S. Caspian Sea) stand out as differing from most other DWFTBs in certain fundamental ways, particularly the very large volume of sediment deposited in the basins, and/or the great thickness and extent of salt or overpressured shale sdetachments.
Article
Tectonic fold tests conducted in Namibia demonstrate that the inclination with respect to bedding of geoplumb (palaeovertical) tubular structures in the Marinoan (635 Ma) syndeglacial cap dolostone is mainly the result of tectonic strain. Therefore, tubestone inclination data cannot be used to estimate the gradient of the sea floor on the foreslope of the Otavi carbonate platform during the Marinoan glaciation. A gradient steeper than 0·1 (slope angle ca 5·7°), implying a glacial base-level fall ≥0·5 km, is nevertheless supported by boulder-size intraclast debrite in the falling-stand wedge directly beneath the glacigenic sequence. Cryogenian oceans lacked skeletal carbonate production, raising the carbonate saturation state and persistent deep water anoxia excluded acid-producing aerobic respiration, facilitating early diagenetic carbonate precipitation, lithification and steep submarine slopes.
Article
This paper uses three-dimensional (3D) seismic data from the continental margin of Israel (Eastern Mediterranean) to describe a series of slump deposits within the Pliocene and Holocene succession. These slumps are linked to the dynamics of subsidence and deformation of the transform margin of the eastern Mediterranean. Repeated slope failure occurred during the post-Messinian, when a clay-dominated progradational succession was forming. This resulted in large-scale slump deposits accumulating in the mid-lower slope region of the basin at different stratigraphic levels. It is probable that the slumps were triggered by a combination of slope oversteepening, seismic activity and gas migration.The high spatial resolution provided by the 3D seismic data has been used to define a spectrum of internal and external geometries within slump deposits. Importantly, we recognise two main zones for many of the slumps on this margin: a depletion zone and an accumulation zone. The former is characterised by extension and translation, and the latter by complex imbricate thrusts and fold systems. Volume-based seismic attribute analysis reveals transport directions within the slump deposits, which are predominately downslope, but with subtle variations particularly at the lateral margins. Basal shear surfaces are observed to ramp both up and down stratigraphy. Slump evolution occurs both by retrogressive upslope failure, and by downslope propagation (out-of-sequence) failure. Slump anatomy and the combination of factors responsible for slump failure and transport are relatively poorly understood, mainly because of the limited 3D of outcrop control; hence, this subsurface study is an example of how improved understanding of the mechanisms and products can be obtained using this 3D seismic methodology in unstable margin areas.
Article
The Coastal Terrane, or westernmost part of the Kaoko Belt outboard of the Three Palms Mylonite Zone, has distinct feldspathic arenite sedimentary sequences, no basement, distinct ɛNd sediment signatures excluding the Archaean and suggesting Mesoproterozoic–Neoproterozoic provenance sources, and primitive arc-like geochemical signatures for I-type granitoids. It contains evidence for an older metamorphic event at ∼650–645 Ma not present in any other part of the Kaoko Belt. This metamorphism (M1) was of high-T/low-P granulite to upper-amphibolite facies and includes migmatisation associated with I-type granitic magmatism of arc affinity. Arc growth at 650–640 Ma took place outboard within the Adamastor Ocean while Swakop Group facies turbidite sedimentation continued inboard along the passive margin. Overprinting M1 and M2 fabrics and metamorphic assemblages constrain the docking to have occurred between 650 and 580 Ma. Lack of ophiolite fragments and evidence of intermediate- to high-P metamorphism along the Three Palms Mylonite Zone suggest that it is not a suture and more likely to be part of an arc–backarc wrench–shear system that developed inboard of an E-dipping subduction system where the Adamastor Ocean was subducted beneath the leading edge of the attenuated Congo Craton. Oblique collision between 650 and 580 Ma, due to docking of this outboard Coastal Terrane with arc affinities, caused: (i) crustal scale oblique-overriding of the arc terrane over the passive margin of the Congo Craton; (ii) crustal scale sinistral shear partitioned into two major ductile shear zones; (iii) high-T granulite facies metamorphism in an extruded, shear zone-bounded core; and (iv) outwards- or cratonwards-verging basement-cored fold nappes.
Article
Zircon and monazite U–Pb dates, garnet Sm–Nd dates and hornblende 40 Ar/ 39 Ar data from the transpressional Kaoko Belt of the late Neoproterozoic Pan-African Orogenic system confirm three distinct tectono-metamorphic cycles: M1 (655–645 Ma), M2 (580–550 Ma) and M3 (535–505 Ma). The high-grade M1 metamorphic cycle and associated intrusive complexes are evident only within the westernmost Coastal Terrane. The isotopic data record a progressive and protracted history for the M2 metamorphic cycle that is initiated by collision and terrane docking, but with three distinct tectono-thermal periods including (1) peak metamorphic parageneses and voluminous granitoid emplacement at 580–570 Ma, (2) overlapping whole-scale transpressional orogenesis and reworking dominated by crustal-scale shear zones, throughout the period 575–550 Ma, and (3) cessation of transpressional strain before 530–508 Ma, the age of late-kinematic pegmatite dykes that cross cut the major shear zones. M1 metamorphism of the exotic Coastal Terrane at 650 Ma must have occurred out-board from the Kaoko Belt passive margin, where M1 intrusives and metamorphic mineral parageneses have not been recognised. Accretion of the Coastal Terrane to the Kaoko Belt proper must have occurred between 645 Ma (M1) and 580 Ma, prior to the peak of M2 metamorphism accompanying transpressional orogenesis. Low-grade buckling of the Kaoko Belt, minor post-kinematic granite and pegmatite intrusions and post-metamorphic cooling occurred between 535 and 505 Ma during the M3 metamorphic cycle accompanying NNE–SSW directed, high-angle convergence between the Congo and Kalahari Cratons. (B. Goscombe).
Article
Plate theory has successfully related sea floor spreading to the focal mechanisms of earthquakes and the deep structure of island arcs. It is used here to calculate the temperature distribution in the lithosphere thrust beneath island arcs, and to determine the flow and the stress elsewhere in the mantle. Comparison with observations demonstrates that earthquakes are restricted to those regions of the mantle which are colder than a definite temperature. The flow and the stress heating in the mantle can maintain the high heat flow anomaly observed behind island arcs. Plate theory also suggests a new approach to the convection problem. The most obvious mechanism causing surface motion is the force on the plates due to the sinking lithosphere. This does not appear to be the way in which the motions are maintained. However, the input of large volumes of cold material can control convection and cause general downward movements in the mantle near island arcs. This input of cold lithosphere must cease when the island arc tries to consume a continent, since the light continental crust cannot sink through the denser mantle. Attempts to assimilate continental crust in this way can produce fold mountains, and also permit a rearrangement of convection cells.
Article
The Kaoko Belt portion of the Damara Orogen, Namibia, is the deeply eroded core of a sinistral transpressional orogen that has half-flower structure geometry centred on the major, 4–5-km-wide Purros Mylonite Zone. Formed between the Congo Craton in the east and Rio De La Plata Craton in Brazil, the Kaoko Belt represents the northern coastal arm of a triple junction within the Pan-African Orogenic System. Consisting of reworked Archaean, Palaeoproterozoic and Mesoproterozoic basement and a cover of Neoproterozoic Damara Sequence, the Kaoko Belt can be sub-divided structurally into three parallel NNW-trending zones. The Eastern Kaoko Zone comprises sub-greenschist facies shelf carbonates that have been uprightly folded. The Central Kaoko Zone contains a slope and deep basin facies succession that has experienced intense deformation, including pervasive reworking of basement into large-scale east-vergent nappes. The Western Kaoko Zone is predominantly deep basin facies of high metamorphic grade intruded by numerous granites. It has experienced intense wrench-style deformation with formation of upright isoclines and steep, crustal-scale shear zones. The Kaoko Belt evolved through three distinct phases of a protracted Pan-African Orogeny in the late Neoproterozoic to Cambrian. (1) An early Thermal Phase (M1) was responsible for pervasive partial melting and granite emplacement in the Western Kaoko Zone from 656 Ma. (2) The Transpressional Phase produced the geometry of the belt by progressive sinistral shearing between 580 and 550 Ma. Deformation was continuously progressive through two stages and involved both temporal and spatial migration of deformation outwards towards the margin. The early strike-slip Wrench-Stage produced a high-strain L–S fabric by sub-horizontal transport. Deformation became progressively more transpressive, with high-angle convergence and flattening strains during the Convergent-Stage. In this stage, strike-slip movements evolved through multiple fold generations, progressively steeper stretching lineations, west over east verging large-scale nappes and overfolds and ultimately thrusts with shortening at a high-angle to the orogen. The pervasive L-S fabric was continually reworked and was both folded by nappes and partitioned into sub-vertical crustal-scale shear zones forming at the same time in the core of the orogen. (3) A post-transpression Shortening Phase, with large-scale, upright, open folds formed during minor N–S shortening along the length of the belt (a phase of deformation correlated with high-angle convergence in the Inland Branch of the Damara Orogen at 530–510 Ma).
Article
Geological structures and Precambrian rock units thought to be related to Rodinia Supercontinent evolution were recognized in three main domains of South America: (i) Mesoproterozoic fold belts ca. 1.5–1.1 Ga old and corresponding foreland cover successions and coeval cratonic intrusions exposed in the southwestern portion of the Amazonian Craton make up the most complete and best preserved record of interpreted Rodinia amalgamation in South America. Recently obtained paleomagnetic data place this part of the Amazonian Craton close to the southernmost segment of Laurentia's Grenville margin. Inferred collision of both continents is reflected in the Nova Brasilândia and Aguapeí-Sunsas fold belts, as well as in the Llano Uplift area. (ii) In eastern South America small crustal fragments of inferred Rodinia ascent were variably reworked during Neoproterozoic Brasiliano orogenic events, rendering it difficult to recognize and map Meso-Neoproterozoic (Grenvillian) mobile belts. So far, the best candidates to represent possible fragments of such mobile belts were recognized in the Punta del Este, Uruguay, terrain, in the Serra do Itaberaba, São Paulo, eastern Brazil area and in the Cariris Velhos, northeastern Brazil area. (iii) The third domain comprises a number of scattered basement exposures within the Andean Cordillera, from Venezuela and Colombia (Guajira, Santa Marta) in the north to northwest Argentina (Pampia, Arequipa-Antofalla) in southern South America. Although deeply reworked and fragmentary in exposure, these basement inliers seem to represent the largest litho-structural record of the Meso-Neoproterozoic orogenic collage in South America, apparently making up the western border of the South American Platform.
Article
The Neoproterozoic–Cambrian Kaoko Belt is an orogen-scale (800 × 180 km) transpressional system important in the amalgamation of West Gondwana. Mid-crustal transpression at amphibolite to granulite facies conditions is dominated by two major, > 400 km exposed, strike-slip shear zones bounding a 20–40 km wide high-grade Orogen Core. To the east, a deeply buried nappe-dominated Escape Zone has inverted metamorphic sequence and verges outwards onto a platformal foreland. To the west, an arc-like Neoproterozoic Coastal Terrane was amalgamated and variably reworked during transpression. The major Purros and Three Palms Mylonite Zones have calculated shear displacements on the order of 120–180 km. These shear zones are moderately to steeply dipping mylonite zones of 1–5 km width, are arcuate and curvilinear in map view and show along-strike variation in slip kinematics. Also highly curved in vertical section, the shear zones define a flower to half-flower geometry for the Orogen Core. An oblique network of mylonitic shear zones, akin to Riedel shears, links the major shear zones and defines regional-scale shear lozenges internally deformed by tight upright folding and shear fabrics. These shear zones create domains in the Orogen Core with varying dominance of pure shear (in shear lozenges) and simple shear (in shear zones). However, absence of dip-slip domains and the smoothly continuous traces of sub-horizontal to shallow and acute, oblique stretching lineations across all parts of the belt, preclude marked kinematic partitioning and the internal part of the belt resembles large-scale triclinic shear. Clast aspect ratios, boudin train extension, sheath fold aspect geometry, degree of rotation of planes producing flanking folds, composite S–C foliations, pressure fringes on pyrite and garnet porphyroclasts provide a semi-quantitative measure of strain intensity. Average strain ratios are X/Z > 40:1 for the major shear zones, X/Z > 12:1 for the Orogen Core, X/Z > 8:1 for the Escape Zone and X/Z > 3:1 for the Coastal Terrane. A more continuous pattern of strain intensity across the whole belt is mapped using a qualitative foliation intensity index. Foliation traces have a sigmoidal pattern in the Orogen Core, swinging from sub-parallel to the boundary shear zones to higher acute angles in the internal parts. Deformation character also varies from upright open folding in amphibolite facies domains in the north, upright tight chevron folding in a low-grade central domain, to a high-grade domain of tight to isoclinal inter-folded basement and cover, with inclination decreasing towards the south.The Kaoko Belt is a well-exposed sector of an extensive (3000 km long), broad (400 km) arcuate orogenic system “Adamastor Orogen” that amalgamated West Gondwana, bringing the South American (Sao Francisco and Rio de la Plata Cratons) and African (Kalahari and Congo Cratons) components together. Though a complex system, most sectors involved oblique collision and accretion of magmatic arcs of 660–610 Ma age, followed by peak metamorphism and main phase transpressional orogenesis between 585 and 560 Ma, with shear zones remaining active until ∼ 530 Ma. This E–W amalgamation immediately pre-dates the final N–S amalgamation of Gondwana along the Kuunga Orogen between 535 and 510 Ma. The large-scale Adamastor Orogen, consisting of Kaoko, Dom Feliciano, Ribeira, Araçuai and West-Congo mobile belts, also shows broadly similar and symmetric structural architecture throughout. The high-grade thermally softened core partitioned intense wrench dominated strains and networks of transcurrent shear zones that dip inwards with listric form. Either side of the internal zone containing amalgamated arcs and high-grade core, are nappe-fold and thrust belts that rework attenuated passive margin basement and Adamastor Ocean sediments and structures verge outward at moderate to high-angles onto both foreland margins. The Kaoko Belt well illustrates the highly partitioned nature of transpressional systems in general and patterns in common throughout the greater “Adamastor Orogen”; such as metamorphic zonation and heterogeneous distribution of deformation style, flow regime and highly variable degrees of reworking strain and recrystallization. This highly partitioned and steep structural grain localized lithospheric extension and rifting 415 Ma later during breakup and dispersal of Gondwana.
Article
Huge landslides, mobilizing hundreds to thousands of km(3) of sediment and rock are ubiquitous in submarine settings ranging from the steepest volcanic island slopes to the gentlest muddy slopes of submarine deltas. Here, we summarize current knowledge of such landslides and the problems of assessing their hazard potential. The major hazards related to submarine landslides include destruction of seabed infrastructure, collapse of coastal areas into the sea and landslide-generated tsunamis. Most submarine slopes are inherently stable. Elevated pore pressures (leading to decreased frictional resistance to sliding) and specific weak layers within stratified sequences appear to be the key factors influencing landslide occurrence. Elevated pore pressures can result from normal depositional processes or from transient processes such as earthquake shaking; historical evidence suggests that the majority of large submarine landslides are triggered by earthquakes. Because of their tsunamigenic potential, ocean-island flank collapses and rockslides in fjords have been identified as the most dangerous of all landslide related hazards. Published models of ocean-island landslides mainly examine 'worst-case scenarios' that have a low probability of occurrence. Areas prone to submarine landsliding are relatively easy to identify, but we are still some way from being able to forecast individual events with precision. Monitoring of critical areas where landslides might be imminent and modelling landslide consequences so that appropriate mitigation strategies can be developed would appear to be areas where advances on current practice are possible.
Gravity-driven fold belts on passive margins. Thrust Tectonics and Hydrocarbon Systems
  • M G Rowan
  • F J Peel
  • B C K R Vendeville
  • Mcclay
The Geology of Namibia Neoproterozoic to Lower Palaeozoic. Geological Survey of Namibia
  • R Miller
  • Mcg
Geology and structure of the Huab-Welwitschia area
  • D C Frets
Neoproterozoic to Lower Palaeozoic. Geological Survey of Namibia
  • R Miller
  • Mcg
Gravity-driven fold belts on passive margins. Thrust Tectonics and Hydrocarbon Systems. K.R. McClay
  • M G Rowan
  • F J Peel
  • B C Vendeville