The Wishbone Ridge at the Chatham Rise Intersection: Structural Characteristics and Tectonic Implications

<p>Geophysical data show that the West Wishbone Ridge, offshore of eastern New Zealand, is best described as having previously been a crustal transform fault, which first propagated along the eastern margin of the Hikurangi Plateau as subduction along the New Zealand sector of the Gondwana margin began to slow and reorientate between 105 and 101 Ma. Variation in the strike of the West Wishbone Ridge has resulted in contrasting compressional and extensional zones along the ridge. These regimes reflect the direction of strike offset from the direction of fault propagation, and constrain the sense of motion along the West Wishbone Ridge as having been dextral. We find evidence that Cretaceous subduction along the Chatham Rise margin extended east of the margin offset at 174°W that marks the edge of Hikurangi Plateau subduction beneath the margin. Rotation of the Chatham Rise margin between 105 and 101 Ma was accommodated by westward broadening of the extensional zone of deformation associated with the West Wishbone Ridge near its intersection with the Chatham Rise. The amount of offset along the ridge indicates that significant transform motion along the West Wishbone Ridge south of ~40.5°S ceased ca. 101 Ma, coeval with the cessation of spreading of the Osbourn Trough, and of subduction of the Hikurangi Plateau. Additionally, we find anomalously thick oceanic crust adjacent to the WWR and north of the Hikurangi Plateau (>12 km thick). This is attributed to the proximity of this crust to the Hikurangi Plateau Large Igneous Province. The results of this study are based on seismic reflection and magnetic data recently collected during the 2016 R/V Sonne survey SO-246, as well as previously collected seismic reflection profiles and satellite gravity data.</p>

The geology of Pegasus Basin based on outcrop correlatives in southern Wairarapa and northeastern Marlborough, New Zealand

<p>Acquisition of high quality 2D seismic data by the New Zealand Government in 2009-10 (the PEG09 Survey) sparked new interest in Pegasus Basin, an offshore frontier basin situated east of central New Zealand. Although no wells have been drilled in Pegasus Basin, strata exposed onshore in southern Wairarapa and northeastern Marlborough provide useful analogues for the sedimentary fill of the basin. Using field observations in combination with petrographic analysis and seismic interpretation, this study provides a more complete understanding of the geology of Pegasus Basin. 13 outcrop localities are described from the surrounding southern Wairarapa and northern Marlborough regions, which are inferred to have been deposited in a range of depositional environments including fluvial, terrestrial and shallow marine deposits, through to inner – mid shelf, and deep marine channel-levee and submarine fans, with fine-grained sedimentation at bathyal depths. These outcrops provide representative and well-exposed examples of facies and lithologies typical of the depositional environments that are likely to exist in Pegasus Basin. Petrographic analysis of six Cretaceous and six Neogene sandstones from Marlborough and Wairarapa regions has revealed that they are compositionally classified as litharenites and feldspathic litharenites, derived from the Torlesse Supergroup. Primary porosity is best preserved in Neogene sandstones, whilst Cretaceous sandstones only tend to preserve secondary porosity, in the form of fractures or dissolution of framework grains. Carbonate cementation, compaction and authigenic clay formation are the biggest contributing factors that degrade reservoir quality. Seismic interpretation of the PEG09 survey has revealed that Pegasus Basin contains a sedimentary succession over 10,000 m thick, that mantles Early Cretaceous syn-tectonic strata in various states of deformation attained during mid-Cretaceous subduction at the eastern Gondwana margin. Key horizons mapped extensively over the basin highlight seismic reflection packages, which are linked to described outcrop localities onshore, based on reflection characteristics and geometries. The Miocene succession contains up to 4,000 m of sediments that are likely to include promising reservoir lithologies akin to the Great Marlborough Conglomerate of Marlborough, or the Whakataki Formation of Wairarapa.</p>

The geology of Pegasus Basin based on outcrop correlatives in southern Wairarapa and northeastern Marlborough, New Zealand

<p>Acquisition of high quality 2D seismic data by the New Zealand Government in 2009-10 (the PEG09 Survey) sparked new interest in Pegasus Basin, an offshore frontier basin situated east of central New Zealand. Although no wells have been drilled in Pegasus Basin, strata exposed onshore in southern Wairarapa and northeastern Marlborough provide useful analogues for the sedimentary fill of the basin. Using field observations in combination with petrographic analysis and seismic interpretation, this study provides a more complete understanding of the geology of Pegasus Basin. 13 outcrop localities are described from the surrounding southern Wairarapa and northern Marlborough regions, which are inferred to have been deposited in a range of depositional environments including fluvial, terrestrial and shallow marine deposits, through to inner – mid shelf, and deep marine channel-levee and submarine fans, with fine-grained sedimentation at bathyal depths. These outcrops provide representative and well-exposed examples of facies and lithologies typical of the depositional environments that are likely to exist in Pegasus Basin. Petrographic analysis of six Cretaceous and six Neogene sandstones from Marlborough and Wairarapa regions has revealed that they are compositionally classified as litharenites and feldspathic litharenites, derived from the Torlesse Supergroup. Primary porosity is best preserved in Neogene sandstones, whilst Cretaceous sandstones only tend to preserve secondary porosity, in the form of fractures or dissolution of framework grains. Carbonate cementation, compaction and authigenic clay formation are the biggest contributing factors that degrade reservoir quality. Seismic interpretation of the PEG09 survey has revealed that Pegasus Basin contains a sedimentary succession over 10,000 m thick, that mantles Early Cretaceous syn-tectonic strata in various states of deformation attained during mid-Cretaceous subduction at the eastern Gondwana margin. Key horizons mapped extensively over the basin highlight seismic reflection packages, which are linked to described outcrop localities onshore, based on reflection characteristics and geometries. The Miocene succession contains up to 4,000 m of sediments that are likely to include promising reservoir lithologies akin to the Great Marlborough Conglomerate of Marlborough, or the Whakataki Formation of Wairarapa.</p>

Metamorphism of the Sierra de Maz and implications for the tectonic evolution of the MARA terrane

The Mesoproterozoic MARA terrane of western South America is a composite igneous-metamorphic complex that is important for Paleozoic paleogeographic reconstructions and the relative positions of Laurentia and Gondwana. The magmatic and detrital records of the MARA terrane are consistent with a Laurentian origin; however, the metamorphic and deformation records lack sufficient detail to constrain the correlation of units within the MARA terrane and the timing and mechanisms of accretion to the Gondwana margin. Combined regional mapping, metamorphic petrology, and garnet and monazite geochronology from the Sierra de Maz of northwest Argentina sug- gest that the region preserves four distinct litho-tectonic units of varying age and metamorphic conditions that are separated by middle- to lower-crustal ductile shear zones. The Zaino and Maz Complexes preserve Barrovian metamorphism and ages that are distinct from other units within the region. The Zaino and Maz Complexes both record metamorphism ca. 430–410 Ma and show no evidence of the regional Famatinian orogeny (ca. 490–455 Ma). In addition, the Maz Complex records an earlier granulite facies event at ca. 1.2 Ga. The Taco and Ramaditas Complexes, in contrast, experienced medium- and low-pressure upper amphibolite to granulite facies metamorphism, respectively, between ca. 470–460 Ma and were later deformed at ca. 440–420 Ma. The Maz shear zone that bounds the Zaino and Maz Complexes records sinistral oblique to sinistral deformation between ca. 430–410 Ma. The data suggest that at least some units in the MARA terrane were accreted by translation, and the Gondwana margin of northwest Argentina transitioned from a dominantly convergent margin to a highly oblique margin in the Silurian.

New detrital zircon U–Pb insights on the palaeogeographic origin of the central Sanandaj–Sirjan zone, Iran

New detrital U–Pb zircon ages from the Sanandaj–Sirjan metamorphic zone in the Zagros orogenic belt allow discussion of models of the late Neoproterozoic to early Palaeozoic plate tectonic evolution and position of the Iranian microcontinent within a global framework. A total of 194 valid age values from 362 zircon grains were obtained from three garnet-micaschist samples. The most abundant detrital zircon population included Ediacaran ages, with the main age peak at 0.60 Ga. Other significant age peaks are at c. 0.64–0.78 Ga, 0.80–0.91 Ga, 0.94–1.1 Ga, 1.8–2.0 Ga and 2.1–2.5 Ga. The various Palaeozoic zircon age peaks could be explained by sediment supply from sources within the Iranian microcontinent. However, Precambrian ages were found, implying a non-Iranian provenance or recycling of upper Ediacaran–Palaeozoic clastic rocks. Trace-element geochemical fingerprints show that most detrital zircons were sourced from continental magmatic settings. In this study, the late Grenvillian age population at c. 0.94–1.1 Ga is used to unravel the palaeogeographic origin of the Sanandaj–Sirjan metamorphic zone. This Grenvillian detrital age population relates to the ‘Gondwana superfan’ sediments, as found in many Gondwana-derived terranes within the European Variscides and Turkish terranes, but also to units further east, e.g. in the South China block. Biogeographic evidence proves that the Iranian microcontinent developed on the same North Gondwana margin extending from the South China block via Iran further to the west.

The birth of the Alps: Ediacaran to Paleozoic accretionary processes and crustal growth along the northern Gondwana margin

New whole-rock geochemical and coupled U–Pb and Lu–Hf LA-ICP-MS zircon data of metasedimentary rocks of the Austroalpine, South Alpine and Penninic basement domains are presented, to disentangle the pre-Variscan tectonic evolution of the proto-Alps. The studied units seem to record distinct stages of protracted Late Ediacaran to Carboniferous tectonosedimentary processes prior to the Variscan collision. In the case of Austroalpine and South Alpine units, nevertheless, no major differences in terms of provenance are observed, since most detrital zircon samples are characterized by a major Pan-African peak. Their detrital zircon spectra record a provenance from the northeastern Saharan Metacraton and the Sinai basement at the northern Arabian-Nubian Shield, being thus located along the eastern Early Paleozoic northern Gondwana margin, whereas sources located further west are inferred for the Penninic Unit, which might have been placed close to the Moldanubian Unit of the Bohemian Massif. In any case, it is thus clear that the Alpine basement remained in a close position to the Gondwana mainland at least during the Early Paleozoic. The Late Ediacaran to Silurian tectonic evolution, which includes Cadomian and Cenerian tectonometamorphic and magmatic processes, seem thus to record a continuum related to a retreating-mode accretionary orogen, with diachronous back-arc basin opening and possibly discrete compressional/transpressional pulses linked to changes in subduction zone dynamics. On the other hand, it is inferred that the Alpine basement essentially comprises Pan-African metasedimentary and subordinate metaigneous rocks, possibly with very few Early Neoproterozoic relics. This basement was significantly reworked during the protracted Paleozoic orogenic evolution, due to anatexis and/or assimilation by mantle-derived juvenile magmatism.

Geochronology and Geochemistry of the Zengudi and Tuobake Granite Porphyries in the Sanjiang Region, SW China: Petrogenesis and Tectonic Significance

The boundary between the Gondwana and Yangtze plate is still controversial. In southwest China, the Sanjiang region marks the collision zone which accreted several blocks coming from the northern Gondwana margin. In this region, subduction of the Paleo-Tethys Ocean and associated continental blocks during the Triassic Period led to the formation of an N–S trending complex involving intrusive and volcanic rocks. The intrusive rocks are important for constraining the evolution of the Paleo-Tethyan in southwestern China. This study presents new geochronological, geochemical, and Sr-Nd-Hf isotopic data of granite porphyries from northern Lancangjiang, in order to discuss the origin of these granites and their tectonic significance. Representative samples of the Zengudi and the Tuobake granite porphyries from the Yezhi area yielded weighted mean 206Pb/238U ages of 247–254 Ma and 246 Ma, respectively. The Zengudi granite porphyries display zircon ԐHf(t) values of −12.94 to −2.63, ԐNd(t) values of −14.5 to −9.35, and initial 87Sr/86Sr ratios of 0.708 to 0.716. The Tuobake granite porphyries have zircon ԐHf(t) values of −14.06 to −6.55, ԐNd(t) values of −10.9 to −9.41, and initial 87Sr/86Sr ratios of 0.716 to 0.731. Both the Zengudi and Tuobake granite porphyries exhibit strongly peraluminous signatures with high A/CNK nAl2O3/(K2O + Na2O + K2O) ratios (1.07–1.86 and 0.83–1.33, respectively). These granites are enriched in Rb and Th, and depleted in Ti, Nb, Ta, Sr, and P, with negative Eu anomalies (Eu/Eu* < 0.61). These geochemical and isotopic data indicate that the primary magma of the granite porphyries originated from partial melting of ancient continental crust as a result of basaltic magma underplating and underwent fractionation crystallization during their emplacement. We propose that the Triassic subduction of the Paleo-Tethys Ocean led to crust shortening and thickening in the Sanjiang region, while the northern Lancangjiang area was involved in the continental collision after the subduction of the Paleo-Tethys Ocean before 254 Ma.