New Zealand is a fragment of Gondwana that, before Late Cretaceous sea floor spreading, was contiguous with Australia and Antarctica. Only about 10% of the area of continental crust in the wider New Zealand region (Zealandia) is emergent above sea level as the North and South Islands. No Precambrian cratonic core is exposed in onland New Zealand. The Cambrian to Early Cretaceous basement can be described in terms of nine major volcano-sedimentary terranes, three composite regional batholiths, and three regional metamorphic-tectonic belts that overprint the terranes and batholiths. The terranes (from west to east) are: Buller, Takaka, Brook Street, Murihiku, Maitai, Caples, Bay of Islands (part of former Waipapa), Rakaia (older Torlesse) and Pahau (younger Torlesse). The western terranes are intruded by three composite batholith (> 100 km(2)) sized belts of plutons: Karamea-Paparoa, Hohonu and Median, as well as by numerous smaller plutons. Median Batholith (including the Median Tectonic Zone) is a recently-recognised Cordilleran batholith that represents the site of subduction-related magmatisin from ca. 375-110 Ma. Parts of the terranes and batholiths are variably metamorphosed and deformed: Devonian and Cretaceous amphibolite-granulite facies gneisses are present in Buller, Takaka, Median and Karamea-Paparoa units; Jurassic-Cretaceous subgreenschist-amphibolite facies Haast Schist overprints the Caples, Bay of Islands and Rakaia Terranes; Cretaceous subgreenschist facies Esk Head and Whakatane Melanges bound the Pahau Terrane. In the South Island, small areas (<5 km(2) total) of Devonian, Permian, Triassic and Jurassic Gondwana sequences have been identified. In the North Island a widespread Late Jurassic overlap sequence, Waipa Supergroup (part of former Waipapa Terrane), has recently been proposed.
A review of the early history of the Cuyania terrane and the numerous pioneering works of the past century provides the present robust framework of evidence supporting a derivation from Laurentia, travel towards Gondwana as an isolated microcontinent, and final amalgamation to the protomargin of western Gondwana in Middle to Late Ordovician times. The major remaining uncertainties and inconsistencies, such as the time of deformation and collision with Gondwana, the lack of evidence of Famatinian-derived zircons, the effects of strike-slip displacements proposed along the suture, as well as the potential sutures defined by ophiolite assemblages, are discussed. The precise boundary along the northern and southern limits is not yet well defined. The most suitable hypothesis based on present data is that Cuyania originated as a conjugate margin of the Ouachita embayment, south of the Appalachian platform during Early Cambrian times. The subsequent travel toward the Gondwana protomargin is clearly depicted by the changing faunal assemblages in the carbonate platform. New geochemical and age data on K-bentonites presented by several authors reinforce the strong connection between Cuyania ash-fall tuffs and Famatina volcanics by 468-470 Ma, indicating Cuyania and Gondwana were in close proximity at that time. Extension related to flexural subsidence, preceded by the drowning of the carbonate platform in early Llanvirnian times, is recorded by abrupt facies changes in the sedimentary cover during late Llanvirnian and early Caradocian times. This episode marked the beginning of contact between Cuyania and Gondwana. The subsequent evolution of the foreland basin indicates that deformation lasted until latest Silurian-Early Devonian times. The time of collision is tracked by the cessation of arc-related magmatic activity in the upper plate (Gondwana protomargin), at about 465 Main western Sierras Pampeanas, and ages around 454 Ma corresponding to syncollisional and postcollisional magmatism. The age of the collision is also preserved in the lower plate (Cuyania), where both angular unconformities in the sedimentary cover and the ages of peak of regional metamorphism in the basement rocks point to 460 Ma as the most probable age for the beginning of the collision. Evidence from the upper plate is essentially identical with an age of 463 Ma. Thermal gradients along this suture vary from 13 degreesC/km in the lower plate, to 18 degreesC/km in the fore arc upper plate, reaching more than 30 degreesC/km along the Famatinian arc. Based on these data, a Llandelian-Caradocian age for the collision can be postulated on firm grounds. Deformation continued through most of the early Paleozoic until amalgamation of the Chilenia terrane by the Late Devonian.
Correlation and synthesis of published and new structural, paleomagnetic and geochronological data from Central Asia show the important role of strike-slip faulting in their evolution. The pattern of major strike-slip faults outlines a terrane collage produced by a Late Devonian-Early Carboniferous collision of the Gondwana-derived Altai-Mongolia-Tuva composite microcontinent with Siberia, and a Late Carboniferous-Permian collision of East Europe and Kazakhstan, with Siberia. The accreted continental margins were cut by strike-slip faults and conjugate thrusts into numerous terranes, which mixed with one another and disturbed the previous structural and facies framework. Those complex and multi-stage deformations resulted from the Late Devonian-Early Carboniferous collision of Gondwana-derived terranes. The deformations reached their peak in the Late Carboniferous-Permian due to the collision of the Kazakhstan, East-European (Baltica) and Siberian continents. A system of sinistral strike-slip faults formed a mosaic-block structure of Central Asia along the margin of the Siberian continent as a result of the Late Carboniferous-Permian collision. This resulted in the formation of the Northern Eurasia continent. Early Mesozoic strike-slip faulting and conjugate thrusting resulted from the rotation of the Siberian and East European cratons.
A new fairly complete and articulated skull of a Peirosauridae crocodylomorph from Bauru Basin (Late Cretaceous), Brazil, is described. The fossil is from a level of clayish sandstone within Serra do Veadinho sequence, Peiropolis, Uberaba County, Minas Gerais State. The sedimentary strata of Serra do Veadinho belong to the Marilia Formation (Serra da Galga Member), Bauru Group, considered to be Campanian-Maastrichtian in age. The species-Uberabasuchus terrificus sp. nov. - is a peirosaurid with moderately narrow snout, large round orbits protected by supraorbital bones of triangular shape and an antorbital fenestra bounded posteriorly by a deep groove. This fossil resembles Peirosaurus tormini Price, 1955 in the size pattern of premaxillary teeth and by showing a similar wedge-like maxillary process in the premaxilla. It also shares some morphological features with the other species of the Peirosauridae, namely the crocodylomorph Lomasuchus palpebrosus Gasparini, Chiappe and Fernandez, 1991 from Argentina. Their common features comprise a moderately narrow snout and the deep lateral groove at the premaxilla and maxilla articulation for the reception of a large mandibular tooth. However, the nasal participates in the external nares and does not divide the nasal aperture, producing a "beak-like" structure at the extremity of the snout which is unique among peirosaurids. The stratigraphic setting suggests that the specimen was buried when a flash flood overflowed the shallow channels of a braided fluvial system. Parsimony analysis of 183 morphological characters is performed for 23 crocodylomorphs. Analysis of the morphological data matrix resulted in three most parsimonious trees (374 steps, CI = 0.679; RI = 0.826). The new species is closely related to Mahajangasuchus and both, in addition to Peirosaurus and Lomasuchus, compose the Peirosauridae.
New U-Pb age determinations confirm earlier interpretations that the strongly deformed and metamorphosed mafic and intermediate igneous rocks of the Pie de Palo Complex represent a Mesoproterozoic fragment of suprasubduction zone oceanic crust. A gabbroic pegmatite, interpreted to have formed during arc rifting or subsequent back-arc spreading, yielded a U-Pb age of 1204 (+5.3)/(-4.7) Ma. Highly tectonized ultramafic-mafic cumulates, occurring at the structural base of the Pie de Palo Complex and previously interpreted to represent remnants of a primitive arc phase, prior to rifting and back-arc spreading, could not be dated, but should be older than 1204 Ma if these inferences are correct. Tabular, sill-like bodies of leucogabbro/diorite and calc-alkaline tonalite/granodiorite sills yielded ages of 1174 43 and 1169(+8)/(-7) Ma respectively They may represent a younger, more evolved arc phase established after arc rifting or a younger, tectonically unrelated Mesoproterozoic arc. SHRIMP-analysis of metamorphic zircon rims with low Th/U ratios in VVL 110 gave a Pb-206/U-238 age of 455 10 Ma similar to lower intercept dates determined by discordia lines. Combined, these data indicate that the bulk of the amphibolite facies metamorphism present in the Pie de Palo Complex was generated during the Famatinian Orogeny. Analysis of six single detrital zircon grains in a metasedimentary, quartzofeldspathic garnet-mica schist tectonically interleaved with the igneous rocks of the Pie de Palo Complex, and tentatively correlated with the Difunta Correa metasedimentary sequence of other workers, yielded three age populations: 1150-1160 Ma; 1050-1080 Ma and 665 Ma, indicating that these sedimentary rocks were deposited during the late Neoproterozoic or Early Paleozoic. In addition they confirm structural evidence that intercalation of rocks of the Pie de Palo Complex with isolated slivers of these sedimentary rocks is due to tectonic imbrications. These ages are also consistent with a Laurentian provenance, and earlier interpretations that these rocks once represented a sedimentary cover to the Pie de Palo Complex. The zircon population of 1050-1080 Ma could be derived from Grenville-age felsic plutons identified elsewhere in the Pie de Palo Complex by other workers. However, no evidence has been found in our samples for a Grenville-age orogenic event, invoked previously to explain accretion of the oceanic Pie de Palo Complex to Laurentia prior to the late Neoproterozoic/Early Cambrian rifting and drift of Cuyania.
Tholeiitic rocks of the Ferrar Large Igneous Province (FLIP) occur in a linear belt from the Theron Mountains to Horn Bluff in the Transantarctic Mountains and extend into southeastern Australasia. The FLIP was emplaced during the initial stages of Gondwana break-up from a source suggested to be in the proto-Weddell Sea region. Magma transport from its source (Weddell triple junction) was controlled by an Early Jurassic zone of extension. The FLIP comprises the Dufek intrusion, Ferrar Dolerite sills and dykes (sheet intrusions), and extrusive rocks consisting of pyroclastic strata overlain by Kirkpatrick Basalt lavas. The Dufek intrusion occurs in deformed supracrustal rocks of the foldbelt along the paleo-Pacific Gondwana margin. A few sills were emplaced in basement rocks, but the majority of the sheet intrusions occur in flat-lying Devonian to Triassic Beacon strata. Only in the central Transantarctic Mountains (CTM) and south and north Victoria Land (SVL, NVL) are extrusive rocks preserved overlying Beacon strata. The greatest cumulative thicknesses of magmatic rocks (ca. 2 km) occur in areas where lavas are preserved (CTM and SVL). Sheet intrusions have complex relationships. Dyke swarms (sensu stricto) are unknown and dykes cutting basement rocks are uncommon. Nevertheless, these dykes, including a 30-m-wide dyke in SVL, suggest that some magmas locally migrated up through basement rocks. In CTM and NVL the outcrop belt has a width of about 160 km. Sills originally extended farther toward the plate margin but have been cut out by erosion and Cenozoic faulting, most clearly in CTM; geophysical data suggest extension under the East Antarctic ice sheet for at least 100 km. Although Early Jurassic extension is documented in CTM, major rift-bounding faults have not been observed. Models for magma emplacement include transport along the axis of the Transantarctic Mountains and off-axis transport from major rift-bounding faults. Contrasts in geochemistry between lavas of NVL (MgO=6-7%) and CTM (MgO=2-4%) and the presence of massive dolerite bodies (CTM, SVL) suggest discrete episodes and locations of magma emplacement, and that there was no long range interconnection along the mountain range in supracrustal rocks.
Detailed geological, geochemical and biostratigraphic studies of rocks from basaltic-sedimentary terranes in the Kurai and Katun accretionary wedges (Vendian-Middle Cambrian units), the Charysh-Terekta strike-slip zone (Late Cambrian-Early Ordovician units), and the Chara ophiolite-bearing strike-slip zone (Late Devonian-Early Carboniferous units) have been undertaken. The Early Cambrian accretionary wedges record a stage of the Kuznetsk-Altai island arc evolution. The Charysh-Terekta strike-slip zone records evidence of the Late Devonian collision of the Gondwana-derived Altai-Mongolian terrane and the Siberian continent. The Chara ophiolitic zone was formed during the Late Carboniferous-Permian collision of the Siberian and Kazakhstan continents. Our study of these fragments of oceanic crust led us to conclude that intra-plate volcanism was active at the early stages of the Paleo-Asian oceanic evolution, in a period from the Vendian to the Early Carboniferous. Fragments of weakly to strongly differentiated oceanic and island-arc basalts have been preserved in accretion-collision zones and give information about chemical composition, petrology and tectonic setting of the oceanic crust at these times. The geochemical data indicate that the Altai and East Kazakhstan metabasalts could have been formed at mid-oceanic ridges, oceanic islands or oceanic plateau of the Paleo-Asian Ocean. Our interpretation of structural, lithological, geochemical and biostratigraphic data shows that the structure and composition of the oceanic lithosphere of the Paleo-Asian Ocean were similar to those of the present Pacific Ocean.
We report here for the first time, the occurrence of sapphirine+quartz assemblage in textural equilibrium from quartzo-feldspathic and pelitic granulites from southern India. The sapphirine-bearing rocks occur as layered gneisses associated with pink granite within massive charnockite in Rajapalaiyam area in the southern part of Madurai Block. Sapphirine occurs in three associations: (i) fine-grained subhedral mineral associated with quartz enclosed in garnet, (ii) intergrowth with Al-rich orthopyroxene (up to 9.7 wt.% Al.sub.2O.sub.3), and (iii) in symplectitic intergrowth with orthopyroxene (Al.sub.2O.sub.3= 5.9-6.7 wt.%) and cordierite surrounding garnet. The sapphirine in association with quartz is slightly magnesian (X.sub.Mg = 0.79-0.80) and low in Si content (1.55-1.56 pfu) as compared with those associated with orthopyroxene and cordierite (X.sub.Mg= 0.77-0.79, Si = 1.59-1.63 pfu). The sapphirine+quartz assemblage suggests that the granulites underwent T>1050 [degrees]C peak metamorphism. Cores of porphyroblastic orthopyroxene in the sapphirine-bearing rocks shows high-Al.sub.2O.sub.3 content of up to 9.7 wt.%, suggesting T = 1040-1060[degrees]C and P = 8 kbar. FMAS reaction of sapphirine+quartz[right arrow]garnet+sillimanite+cordierite indicates a cooling from sapphirine+quartz stability field after the peak ultrahigh-temperature metamorphism. Slightly lower temperature estimates from ternary feldspar and sapphirine-spinel geothermometers (T = 950-1000[degrees]C) also support a post-peak isobaric cooling. Corona textures of orthopyroxene+cordierite ([+ or -]sapphirine), orthopyroxene+sapphirine, and cordierite+spinel around garnet suggest subsequent decompression. The sapphirine-quartz association and related textures reported in this study have important bearing on the ultrahigh-temperature metamorphism and exhumation history of the Madurai Block as well as on the tectonic evolution of the continental deep crust in southern India.
We report here for the first time, the occurrence of sapphirine+quartz assemblage in textural equilibrium from quartzo-feldspathic and pelitic granulites from southern India. The sapphirine-bearing rocks occur as layered gneisses associated with pink granite within massive charnockite in Rajapalaiyam area in the southern part of Madurai Block. Sapphirine occurs in three associations: (i) fine-grained subhedral mineral associated with quartz enclosed in garnet (ii) intergrowth with Al-rich orthopyroxene (up to 9.7 wt.% Al2O3), and (iii) in symplectitic intergrowth with orthopyroxene (Al2O3= 5.9-6.7 wt.%) and cordierite surrounding garnet. The sapphirine in association with quartz is slightly magnesian (X-Mg = 0.79-0.80) and low in Si content (1.55-1.56 pfu) as compared with those associated with orthopyroxene and cordierite (X-Mg = 0.77-0.79, Si = 1.59-1.63 pfu). The sapphirine+quartz assemblage suggests that the granulites underwent T>10510 degreesC peak metamorphism. Cores of porphyroblastic orthopyroxene in the sapphirine-bearing rocks shows high-Al2O3 content of up to 9.7 wt%, suggesting T = 1040-1060 degreesC and P = 8 kbar. FMAS reaction of sapphirine+quartz>garnet+sillimanite+cordierite indicates a cooling from sapphirine+quartz stability field after the peak ultrahigh-temperature metamorphism. Slightly lower temperature estimates from ternary feldspar and sapphirine-spinel geothermometers (T = 950-1000 degreesC) also support a post-peak isobaric cooling. Corona textures of orthopyroxene+cordierite (+/-sapphirine), orthopyroxene+sapphirine, and cordierite+spinel around garnet suggest subsequent decompression. The sapphirine-quartz association and related textures reported in this study have important bearing on the ultrahigh-temperature metamorphism and exhumation history of the Madurai Block as well as on the tectonic evolution of the continental deep crust in southern India.
The Karimmagar Granulite Belt (KGB) and the Bhopalpatnam Granulite Belt (BGB) occur along both flanks of the Pranhita-Godavari (PG) rift basin. We present a state-of-the-art overview on the geochronological and tectonic aspects of these belts and surrounding geologic domains, and report new age data on zircon, monazite and uraninite recovered from granulite facies assemblages from KGB and BGB based on electron microprobe analyses (EPMA). Zircons from KGB charnockites show core ages of up to 3.1 Ga mantled by rims of 2.6 Ga. Zircons from BGB have 1.9 Ga cores mantled by 1.7 Ga rims. Zircons with core ages of 1.6 to 1.7 Ga in BGB rocks suggest new growth at this time. Monazites and uranitite from KGB show clear peaks with well-defined ages in the narrow range between 2.42+/-0.08 Ga and 2.47+/-0.03 Ga. Rims of monazite show mean age of 2.21+/-0.08 Ga. Monazites from BGB define sharp linear trend in PbO vs. ThO2* diagram delineating a clear isochron with age of 1.59+/-0.03 Ga. Age data from KGB and BGB presented in this report negate current models linking these terrains to "Godavari Granulite Belt" and considering them as single and contemporaneous entity. The mid-Archaean to early Palaeoproterozoic signature recognized from KGB is totally missing in BGB. On the other hand, KGB rocks do not record any evidence for major Mesoproterozoic thermal regime. The two granulite belts shouldering the PG rift basin have therefore evolved in different times under distinct P-T conditions and thermal regimes. Our results have important implications in evaluating models of supercontinent assemblies, particularly the older assemblies of Ur, Columbia and Rodinia. While tectonothermal events in KGB broadly match with those of East Dharwar, we propose that BGB represents a 1.6 Ga collisional mobile belt between the Bastar and the Dharwar cratons. The 1.6 Ga collisional mobile belt at the southern margin of the Bastar craton was superposed by rift activity along the PG basin at 1.5 Ga. This sequence of events goes against the existence of a 3.0 Ga old contiguous assembly of Ur but closely matches with the history of accretion and break-up of the Columbia. Further, parts of the PG basin located away from the influence of the Eastern Ghats Mobile Belt, neither recorded any Grenville ages (1.0 Ga) corresponding to the Rodinia accretion nor late Pan-African ages (ca. 550 Ma) relating to the Gondwana amalgamation, indicating that the region did not witness any of these younger tectonic events.
After a prolonged period of convergent margin tectonics in the Late Paleozoic and Mesozoic, resulting in terrane accretion, uplift and erosion of the New Zealand segment of Gondwana, the region saw a rapid change to extensional tectonics in mid-Cretaceous times. The change in regime is commonly marked by a major angular unconformity that separates the older, often strongly-deformed subduction-related 'basement' rocks from the younger, less-deformed 'cover' strata. The youngest 'basement' strata locally contain Albian fossils, and the youngest associated zircons have been radiometrically dated at ca. 100 Ma. In general the oldest strata overlying the unconformity contain fossils of similar Albian age, and the oldest radiometric dates also give similar dates of ca. 100 Ma, indicating a very rapid transition between the two tectonic regimes. The onset of extension resulted in the widespread development of grabens and half grabens, associated in the northwest of the South Island with a metamorphic core complex. In the west and south, on the thicker and more buoyant crust of most of the South Island, the new basins were infilled with mainly non-marine deposits. Non-marine graben infill consists of locally-derived breccia deposited as talus or debris flows on alluvial fans, passing directly as fan deltas or via fluvial deposits into lacustrine deposits. Active faulting continued in some areas until the initiation of sea floor spreading in Santonian times. Post-subduction strata on the thinner continental crust of the northeastern South Island and eastern North Island (East Coast Basin) were mainly marine. Initial sedimentary deposits in the west of the basin, reflecting extensional tectonism, consist of coarse-grained debris-flow deposits or olistostromes, generally fining upwards as tectonic activity waned: those in the east, including allochthonous sediments derived from the northeast, are dominated by turbidites. Early Cenomanian (ca. 96-98 Ma) injection of intraplate alkaline igneous rocks in central New Zealand caused updoming, resulting in shallowing and local uplift of the basin floor above sea level. A long (ca. 10 Ma) period of slow subsidence and transgressive marine sedimentation interrupted by episodic relative sea level changes followed. This pattern changed in the Late Coniacian (ca. 87-86 Ma), with a sudden influx of coarse, transgressive sands in eastern New Zealand. This was immediately preceded in parts of the region by uplift and erosion, probably driven by convective upwelling of the mantle just prior to sea-floor spreading, resulting in a 'break-up' unconformity. in the Late Santonian (ca. 85-84 Ma), development of a new, diachronous, widespread low-relief erosion surface, overlain by fine-grained deposits accompanying a rapid rise in relative sea level, coincided with the beginning of sea-floor spreading, rapid passive margin subsidence, and final separation of New Zealand from Gondwana.
New C- and O-isotopic determinations from the Vendian lower Arroyo del Soldado Group are reported and combined with sedimentologic and biostratigraphic data. On the basis of different geochemical and petrographic criteria, the primary nature of the C- and Sr-isotopic signature is shown. Positive delta(13)C-values characterize the mainly siliciclastic upper Yerbal Formation, which contains oxide-facies BIF and a diverse assemblage of skeletal fossils, including Cloudina riemkeae. A series of positive and negative delta(13)C-excursions occur up-section in the overlying Polanco Formation, mainly composed of limestones and limestone-dolostone rhythmites. The transition to the overlying Barriga Negra and Cerro Espuelitas Formation, which consists of conglomerates and shales/cherts/BlF respectively, is marked by a further negative excursion. On the basis of sedimentary structures, a correlation of delta(13)C with palaeobathymetry is established. Positive delta(13)C-peaks are associated with high sea-level stand, high organic-carbon burial and relatively higher microfossil diversity while negative delta(13)C-excursions occur in carbonates with less organic matter, less microfossil diversity and are always associated to regressions. These observations can be readily explained by palaeoclimatic models which postulate that enhanced bioproductivity due to higher availability of nutrients (P N, Fe) was the key factor controlling Neoproterozoic glaciations. The mentioned models are discussed in view of the new data. The occurrence of at least four cold periods in the upper Vendian is envisaged. These cold periods led to sea-level fall and possibly glaciation at higher latitudes. The absence of glacigenic rocks associated to negative delta(13)C-excursions in the Arroyo del Soldado Group is probably due to the tropical setting of the basin. Finally, the upper Vendian age of the lower Arroyo del Soldado Group is confirmed, on the basis of C- and Sr-isotopes. The onset of carbonate deposition at the base of the Polanco Formation is estimated at 580 Ma by comparison with existing global isotopic curves.
The Cuchilla Dionisio Terrane (CDT) of Uruguay is a tectonostratigraphic unit defined as the block to the east of the Sierra Ballena Shear Zone (SBSZ). It is composed of a Palaeo- to Mesoproterozoic metamorphic basement (Cerro Olivo and Chafalote Complexes) intensely reworked and intruded by syncollisional to late orogenic granites between 680 and 550 Ma. Dacitic and rhyolitic volcanism is recorded around 570-575 Ma (Cerros de Aguirre and Sierra de Rfos Formations). The CDT is correlated with the Pelotas Terrane of southern Brazil. The western boundary of the block in Brazil is defined by the Cangucu and Major Gercino shear zones, which are the northward extension of the SBSZ. The present position of the CDT is not a consequence of orthogonal collision after completion of a Wilson Cycle, but of lateral sinistral accretion along the above mentioned megashear zones. The allochthonous nature of the CDT is postulated on the grounds that: (a) magmatism in the CDT is ca. 120 Ma younger than metamorphism of neighbouring volcanosedimentary successions, (e.g., Porongos Group in Brazil); and (b) while intense volcanism occurred in the CDT in the late Vendian, represented by the Cerros Aguirre and Sierra de Rios Formations, a passive continental margin deepening to the E existed on the eastern edge of the Rio de la Plata Craton, represented by the Arroyo del Soldado Group. Therefore the CDT and Rio del la Plata Craton were separated hundreds or thousands of kilometers by late Vendian times. Accretion of the CDT took place at 530 Ma by tangential collision, and was one of the last events in the complicated amalgamation of W-Gondwana. In Uruguay, the CDT collided with the Nico Perez Terrane, generating the SBSZ and sinistrally reactivating the Sarandi del Yi-Piriapolis Shear Zone. In Brazil, the CDT-Pelotas Terrane collided with the Parana Block and a series of parautochthonous terranes (Curitiba, Apiai) characterized by a Transamazonian basement strongly reworked in the Neoproterozoic. The provenance of the CDT is still uncertain, considering that both Transamazonian and Namaqua-Natal (Kibaran) ages have been reported from its basement. Previously reported Nd model ages (T-DM) strongly suggest an African affinity. The Dom Feliciano Belt is thus a collage of diachronous units formed in different geotectonic settings, and not the product of a single orogenic cycle. Terrane-clocking at different stages mainly by lateral accretion was the dominant process. Therefore usage of the name "Dom Feliciano Belt" should be restricted only to descriptive purposes.
Recent and new faunal data from the Cambrian to Silurian rocks of the Precordillera Famatina and Northwest Argentina basins are used to discriminate between different paleogeographic models, and especially to establish to what extent they are compatible with a previous conclusion that the Precordillera is a Laurentian-derived microcontinent. There is no paleontological evidence to support a para-autochthonous Gondwanan origin of the Precordillera. The strong differences in the Cambrian trilobite faunas and lithologic successions preclude a common origin of the Precordillera terrane eastern Antarctica and South Africa. Recent discoveries of brachiopods and organisms of the Phylum Agmata strengthened Laurentian affinities during the Cambrian. The latest Cambrian-early Ordovician faunas that inhabited the autochthonous Northwest Argentina basin, including the western Puna volcaniclastic successions, are mostly peri-Gondwanan. The early Ordovician brachiopods, ostracods and trilobites display mixed Laurentian, Baltic and Avalonian biogeographical links supporting a drifting of the Precordillera across the Iapetus Ocean. Increasing Gondwanan elements during the Llanvirn, along with varied geological evidence, indicate that the first stages of collision may have begun at that time, involving a major change in the plate kinematics. The distribution of facies and faunas, basin development, and timing of deformation are interpreted as resulting from a north to south diachronous closing of the remnant basin during the last phases of convergence and oblique collision of the Precordillera terrane with the Gondwana margin. The high level of endemism of Caradoc faunas may be a consequence of the rearrangement and partial isolation of sedimentary areas during the strike-slip movement of the colliding Precordillera plate with respect to the Gondwana margin. Suggested relationships between facies distribution, geographic barriers and faunal migrations before and during the collision are depicted in a series of schematic reconstructions at five time slices from late Cambrian to Silurian.
The Malanjkhand granitoids (MG) pluton (about 1500 sq km) occurs in the Balaghat district of Madhya Pradesh. The MG (similar to2400 Ma) represent an episode of Palaeoproterozoic felsic magmatism in Central India and hosts potential Cu (+/-Mo+/-Au) deposits. The enclaves hosted in MG can be broadly classified into two categories: microgranular enclaves (dark-coloured, fine-grained magmatic) and xenoliths of country rocks. The microgranular enclaves (ME) may be rounded, ellipsoidal, discoid, elongated, lenticular or tabular, and their size commonly reaches up to 2 metres across. The ME have sharp and in places, diffuse contacts with their host granitoids. The shape and size of ME indicate contemporaneous flow and mingling of partly crystalline felsic-mafic magmas. Some ME exhibit dark crenulated margins giving them a pillow-like form that has been attributed to undercooling of a ME magma as globules intruded into a granitoid magma. The presence of corroded felsic and mafic minerals (xenocrysts) in ME is interpreted as the result of mechanical transfer during the mafic-felsic magma interaction and mixing event. Mafic minerals (biotite) rim the quartz xenocrysts giving rise to ocellar texture, which exhibit signatures of resorption under hybrid (enclave) magma conditions. All these features suggest an origin for the calc-alkaline intermediate granitoid magma in Malanjkhand involving a magma mixing process.
The Sierra de San Luis constitutes the southernmost tip of the Eastern Sierras Pampeanas. Its Palaeozoic metamorphic basement units define a key location for the understanding of the accretional history along the proto-Andean margin of Gondwana. Although, it is largely accepted that the polyphase accretional history of the Sierras Pampeanas is preluded by the docking of the Pampean Terrane followed by the Famatinian Orogenic Cycle that involves subduction along the margin of Gondwana and the accretion of the Precordillera (Cuyania) Terrane and finally ceased with the collision of the Chilenia terrane,a vast amount of controversial information concerning the timing and mode of collisions as well as the origin of the different involved crustal fragments within the Eastern Sierras Pampeanas is published. In this paper, those different hypothesis are presented and evaluated under the light of new isotopic data of the Sierra de San Luis. Nd-systematics of the metasedimentary sequences of the Sierra de San Luis indicate that the studied sequences were developed on the Pampean Terrane. An Amazonian origin of the Pampean Terrane that was probably detached from the Arequipa Antofalla Craton is proposed. Furthermore, the correlation of two low-grade phyllitic belts (San Luis Formation) with the widespread Puncoviscana Formation is not supported by Sm-Nd data. It is suggested that the sedimentary precursors of the Pringles Metamorphic Complex and the topping phyllites were sourced on the Pampean Orogen and accommodated in a newly formed back arc basin during the early Famatinian. The cooling history of the basement complex is recorded by an extensive amount of K-Ar muscovite and biotite ages. A high variability in muscovite ages is only partly related to different intrusion times of two pegmatoid generations. Post Famatinian to Achalian crustal scale mylonite formation (similar to359 Ma) and a rotational exhumation of the central basement unit are causal for the observed K-Ar muscovite age pattern in the range from 395 Ma to 447 Ma. Therefore, the decrease in metamorphic degree from west to east is the result of the erosion level of a crustal profile from the mid lower crust to the upper crust. An even higher variability in K-Ar biotite cooling ages covering the range from 315 Ma to 418 Ma is related to the slow cooling after the Famatinian Orogenic Cycle or reheating during the Achalian Orogenic Cycle and consequent variable reset of the isotopic system. However, ages recorded by biotite booklets substantiate the hypothesis of a differential exhumation of the basement of the Sierra de San Luis.
The geological interpretation of high-resolution aeromagnetic data over the La Pampa province, in central Argentina, in addition to lower resolution magnetic information from the region of the Neuquen and Colorado basins leads to the definition of the precise boundaries of the Chilenia, Cuyania, Pampia and Patagonia terranes, as well as that of the Rio de la Plata Craton, within the study region. The high-resolution aeromagnetic survey data are compared and studied in conjunction with all the available geological information, to produce a map of the solid geology of this region, which is largely covered by Quaternary sediments. A number of structures of different magnitudes, as well as their relative chronology, are also recognized, i.e., regional faults, sub-regional faults, fractures and shear zones, as well as the most conspicuous magnetic fabric of the basement that reflects its main planar structures. Three different basements are distinguished on the basis of their contrasting magnetic character, and are interpreted to represent the Cuyania and Pampia terranes and the Rio de la Plata Craton, separated from each other by large-scale discontinuities. In the western part of the study region an additional major discontinuity separates the Chilenia and Cuyania terranes. In the southernmost area studied, WNW-trending structures are predominant, particularly a major NNE-vergent thrust that indicates the truncation of the Cuyania-Pampia suture and is regarded to be related to the possible collision of the Patagonia terrane. An E-W-trending magnetic and gravity anomaly traversing the full extra-Andean Argentine territory, located immediately to the south of 39 S, represents a major structure. The activation of this structure during the Mesozoic gave rise to the Huincul Ridge and marks the interruption of the distinct N-S structures of the Chilenia, Cuyania and Pampia terranes, as well as those of the Rio de la Plata Craton, to the north. This E-W represents the suture zone of the Patagonia terrane.
The pre-Early Cambrian Sandikli Basement Complex in western Central Anatolia comprises a low-grade metasedimentary succession (Guvercinoluk Formation) and meta-rhyolites intruded by meta-quartz porphyry rocks (Kestel Cayi Porphyroid Suite). The Guvercinoluk Formation consists of alternation of meta-siltstones and meta-sandstones with olistostromal conglomerates, rare black chert and cherty meta-dolomite lenses. The Kestel Cayi Porphyroid Suite is a deformed, highly sheared dome-shaped rhyolitic body with quartz porphyry rocks. Quartz porphyry dykes intrude both the volcanic carapace and the meta-sedimentary rocks of the Guvercinoluk Formation. Both the meta-quartz porphyry rocks and meta-rhyolites are typically mylonitic with relict igneous textures. Geochemical data indicate that the felsic rocks of the Kestel Cayi Porphyroid Suite are subalkaline and display characteristic features of post-collisional, I-type granitoids. The basement complex is unconformably overlain by variegated conglomerates, mudstones and arkosic sandstones with andesitic lavas, followed by siliciclastic rocks and carbonates that yielded Early Middle Cambrian fossils. Based on the geochemical characteristics of the felsic rocks of Kestel Cayi Porphyroid Suite and the depositional features of the associated sediments it is suggested that the Sandikli Basement Complex is related to a post-collisional extension event in NW Gondwanaland. Similar occurrences elsewhere have been related to a transition from continental plate convergence to continental plate divergence along the Pan-African Belt.
Four major fault systems oriented N-S to NNE-SSW, NE-SW, E-W and NW-SE are identified from Landsat Thematic Mapper (TM) images and a high resolution digital elevation model (DEM) over the Ethiopian Rift Valley and the surrounding plateaus. Most of these faults are the result of Cenozoic - extensional reactivation of pre-existing basement structures. These faults interacted with each other at different geological times under different geodynamic conditions. The Cenozoic interaction under an extensional tectonic regime is the major cause of the actual volcano-tectonic landscape in Ethiopia. The Wonji Fault Belt (WFB), which comprises the N-S to NNE-SSW striking rift floor faults, displays peculiar propagation patterns mainly due to interaction with the other fault systems and the influence of underlying basement structures. The commonly observed patterns are: curvilinear oblique-slip faults forming lip-horsts, sinusoidal faults, intersecting faults and locally splaying faults at their ends. Fault-related open structures such as tail-cracks, releasing bends and extensional relay zones and fault intersections have served as principal eruption sites for monogenetic Plio-Quaternary volcanoes in the Main Ethiopian Rift (MER).
The closure of the Palaeozoic witnessed the greatest biotic crisis in earth history. Surprisingly little is known about the effects and timing of the terrestrial counterpart of the well-described End-Permian mass extinction from known marine successions worldwide. In the present study, reliable paleomagnetic results were obtained from a PT boundary section in the terrestrial Karoo Basin of South Africa. Permo-Triassic aged mudstones from a locality in the Eastern Cape Province yielded two magnetic chrons, reverse followed by normal (with the boundary possibly close to the reversal). This extends to results from a previous study: thereby jointly identifying a R/N/R polarity pattern for this boundary interval. The PTB interval is constrained below the red mudstones of the Beaufort Group at the present locality and within reverse-magnetised green mudstone, implying a diachronic relation between the marine and terrestrial End-Permian mass extinction events.