West-dipping Subduction Zone Research Articles

Spatio–temporal evolution of Mesoproterozoic magmatism in NE Australia: A hybrid tectonic model for final Nuna assembly

The tectonic regimes that drove the 1560–1490 Ma granitic magmatism c. 50 m.yr. after the final assembly of the Proterozoic supercontinent Nuna in NE Australia remain elusive. Collision between NE Australia (Mount Isa Inlier—MTI) and NW Laurentia (Georgetown Inlier—GTI) occurred at c. 1600 Ma and was associated with a west-dipping subduction zone, with the MTI as the upper plate and the GTI as the lower plate.Structural studies in the GTI showed that the collisional event involved 1600 Ma WNW-ESE shortening, followed by 1550 Ma WNW-ESE directed extension. During this later stage, a crustal-scale, west-dipping detachment fault-system juxtaposed middle- to lower-crustal levels, associated with voluminous, 1550 Ma S-type granites against greenschist facies upper crustal rocks. Regionally, post-collisional magmatism defines a westward, chemical, and temporal trend from 1560 to 1550 Ma, dominantly S-type confined to the lower plate (GTI) through c. 1545–1540 Ma I-/A-type (below the Carpenteria Basin) to 1540–1490 Ma A-type granites that intruded further west the Australian upper plate (eastern MTI). This transition from hydrous (S-type) granites in the east to drier (A-type) granites in the west is also supported by increasing zircon saturation temperatures and geochemical discriminators. Recent zircon Lu–Hf and new in-situ monazite Sm–Nd analyses in granites show increasingly radiogenic initial (at the time of crystallization) isotopic ratios from the GTI to the MTI, reflecting a concomitant westward increase in mantle input. Combined, these features suggest a spatio–temporal evolution of hotter and drier crustal conditions westward associated with progressive lithospheric extension. Classical Phanerozoic upper-plate delamination, slab break-off, and slab rollback and/or tearing tectonic models do not account for all the features of this post-collisional magmatic record. Alternatively, a hybrid tectonic scenario between fast–hard Indian and slow–soft Aegean collision better explains the attributes of Mesoproterozoic NE Australia during Nuna assembly.

The “Nazas Arc” is a continental rift province: Implications for Mesozoic tectonic reconstructions of the southwest Cordillera, U.S. and Mexico

Abstract We reject the notion of a Jurassic continental arc in eastern Mexico, termed the “Nazas arc,” on geologic grounds. Instead, we propose that the Jurassic continental arc of the SW Cordilleran U.S. and northern Sonora, Mexico, passed southward into the oceanic realm and is represented by Jurassic arc volcanic and plutonic rocks that fringed the Mexican paleo-Pacific margin, which are currently found in the western Peninsular Ranges of southern California, USA, and Baja California, the Vizcaino Peninsula of Baja California, and western mainland Mexico. To show this, we present a summary of the geologic features of a continental arc, using the geology of the southern end of the Jurassic continental arc, in southern Arizona and northern Sonora. These features include multi-kilometer–thick sections of volcanic rock; large volcanic centers, including silicic calderas; major eruptive units that can be correlated for distances of 100 km or more; abundant, large plutonic suites; and continuity of these features for distances of hundreds of kilometers along the length of the continental arc. Then we show that the “Nazas arc” consists of scattered, small continental rift basins with thin (meters to tens of meters thick) volcanic sections at the base of clastic sections that are hundreds of meters thick. Plutonic rocks are entirely absent from the “Nazas arc,” despite the fact that post-Jurassic tectonic events should have exposed them if they existed. This paper also presents a tabulation of all published U-Pb zircon dates in the Jurassic continental arc of southern Arizona, USA, and northern Sonora (Table 1A), and in the “Nazas arc” of eastern Mexico (Table 1B), with ages, methods, the rock type dated, and notes on geologic relations. We use this to detail the abundance of thick, laterally extensive volcanic sections and large plutonic suites in a continental arc (the Jurassic arc of southern Arizona–northern Sonora), which contrasts sharply with the “Nazas arc.” The term “Nazas arc” has been in widespread usage for volcanic rocks in eastern Mexico for decades in many dozens of papers, and it is portrayed as a 2000-km-long, 250-km-wide belt that extends from Sonora through eastern Mexico to Chiapas. It has been misunderstood to form a subduction-related silicic large igneous province (SLIP), and it has been proposed that the Gulf of Mexico formed as a backarc basin behind the “Nazas arc.” The “Nazas arc” model also requires an east-dipping subduction zone under Mexico, and a separate west-dipping subduction zone under the oceanic arc rocks of western Mexico, which those models portray as an exotic arc, despite the presence of abundant detrital zircon from the Mexican margin. We urge workers to abandon the term “Nazas arc” and replace it with “Nazas rift province,” which represents continental rift basins formed during the breakup of Pangea.

A crucial geologic test of Late Jurassic exotic collision versus endemic re-accretion in the Klamath Mountains Province, western United States, with implications for the assembly of western North America

Abstract Differing interpretations of geophysical and geologic data have led to debate regarding continent-scale plate configuration, subduction polarity, and timing of collisional events on the western North American plate margin in pre–mid-Cretaceous time. One set of models involves collision and accretion of far-traveled “exotic” terranes against the continental margin along a west-dipping subduction zone, whereas a second set of models involves long-lived, east-dipping subduction under the continental margin and a fringing or “endemic” origin for many Mesozoic terranes on the western North American plate margin. Here, we present new detrital zircon U-Pb ages from clastic rocks of the Rattlesnake Creek and Western Klamath terranes in the Klamath Mountains of northern California and southern Oregon that provide a test of these contrasting models. Our data show that portions of the Rattlesnake Creek terrane cover sequence (Salt Creek assemblage) are no older than ca. 170–161 Ma (Middle–early Late Jurassic) and contain 62–83% Precambrian detrital zircon grains. Turbidite sandstone samples of the Galice Formation are no older than ca. 158–153 Ma (middle Late Jurassic) and contain 15–55% Precambrian detrital zircon grains. Based on a comparison of our data to published magmatic and detrital ages representing provenance scenarios predicted by the exotic and endemic models (a crucial geologic test), we show that our samples were likely sourced from the previously accreted, older terranes of the Klamath Mountains and Sierra Nevada, as well as active-arc sources, with some degree of contribution from recycled sources in the continental interior. Our observations are inconsistent with paleogeographic reconstructions that are based on exotic, intra-oceanic arcs formed far offshore of North America. In contrast, the incorporation of recycled detritus from older terranes of the Klamath Mountains and Sierra Nevada, as well as North America, into the Rattlesnake Creek and Western Klamath terranes prior to Late Jurassic deformation adds substantial support to endemic models. Our results suggest that during long-lived, east-dipping subduction, the opening and subsequent closing of the marginal Galice/Josephine basin occurred as a result of in situ extension and subsequent contraction. Our results show that tectonic models invoking exotic, intra-oceanic archipelagos composed of Cordilleran arc terranes fail a crucial geologic test of the terranes’ proposed exotic origin and support the occurrence of east-dipping, pre–mid-Cretaceous subduction beneath the North American continental margin.

A Neoarchean arc-backarc pair in the Linshan Massif, southern North China Craton

The Linshan Massif is located in the southern North China Craton (NCC) and consists mainly of tonalitic (TTG) and dioritic gneisses, and a volcano-sedimentary-dominated assemblage, all of which were intruded by post-collisional potassic granites. The volcano-sedimentary-dominated assemblage is composed mostly of deformed and foliated basaltic amphibolite and meta-sedimentary rocks, with minor interlaying meta-gabbros, forming a stack of tectonically imbricated sedimentary and volcanic rocks. The TTG gneisses formed at 2.6–2.52 Ga, and the meta-gabbros, basaltic amphibolites and diorites formed at 2.51–2.50 Ga. They underwent subsequent metamorphism at 2.49–2.48 Ga. Furthermore, all their igneous zircons possess large positive εHf (t) values (+2.9 to +7.4). The basaltic amphibolites can be divided into two groups: depleted basalts and enriched basalts. Depleted basalts are characterized by flat LREE patterns and low Th/Nb ratios with positive Nb anomalies, similar to N-MORB-type patterns. Enriched basalts display LREE-enriched patterns with high Th/Nb ratios and pronounced negative Nb anomalies, showing an arc-related affinity. The meta-gabbros have LREE-enriched patterns with negative Nb and Zr anomalies, consistent with subduction-related signatures. Considering mixed arc- and MORB- signatures with positive zircon εHf(t) values, we propose that the meta-gabbros and basaltic amphibolites may have formed in a back-arc setting. The tonalites and the diorites showing LREE-enriched patterns and depleted in HFSEs are interpreted to represent arc-related magmatism. Combined with the documented 2.55 Ga Dengfeng fore-arc accretionary complex, we propose the following geodynamic evolution model for the Southern NCC: (1) at 2.6–2.54 Ga, a TTG-dominated oceanic arc developed above a west-dipping subduction zone, forming the 2.55 Ga Dengfeng accretionary complex and related tectonic units; (2) at 2.52–2.50 Ga, slab rollback subsequently drove back-arc rifting of the mature TTG-dominated intra-oceanic arc, resulting in the formation of two arc segments with the intervening Linshan back-arc basin; and (3) at 2.50–2.48 Ga, the TTG-dominated arc collided with the Eastern Block resulting in the closure of the back-arc basin and formation of the accreted arc-back-arc package in the Linshan Massif. We suggest that the Neoarchean Central Orogenic Belt in the southern NCC contains at least two amalgamated oceanic arc terranes, separated by a belt of back-arc basin magmatic rocks caught within an intra-oceanic mélange/suture zone.

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Porphyry Cu fertility of the Loch Lilly-Kars Belt, Western New South Wales, Australia

This study investigates the prospectivity for porphyry Cu mineralisation of the early Paleozoic Loch Lilly-Kars Belt, which follows the southern margin of the Paleoproterozoic Broken Hill (Willyama) Inlier in western New South Wales, Australia. The belt is thought to have resulted from the 515–510 Ma collision of a volcanic passive margin and an intra-oceanic island arc above a west-dipping subduction zone, with subsequent post-collisional magmatism. To identify fertility signals of selected magmatic intrusive rocks, we combined whole-rock analyses with in-situ zircon trace-element and isotopic measurements. Data reveal that both Cambrian (ca 496 Ma) and Devonian (ca 393 Ma) intrusive suites are present in the Loch Lilly-Kars Belt. Cambrian dioritic rocks are largely non-fertile for porphyry Cu mineralisation, whereas Devonian monzodiorites and monzonitic porphyries carry strong fertility signals that include high whole-rock Sr/Y (>35) and V/Sc (>10) values, associated with the occurrence of equant, stubby zircons showing relatively constant Eu/Eu* values without any significant Eu anomalies. Although only few samples were analysed and more intensive exploration is required, the results of this study indicate significant potential for porphyry Cu mineralisation in this poorly known belt.

Biological evidence constraining river drainage evolution across a subduction-transcurrent plate boundary transition, New Zealand

The topography of the southern part of the North Island of New Zealand has been constructed by relative tectonic uplift and subsidence above an active west-dipping subduction zone. In contrast, the nearby northern South Island has been constructed during active transcurrent faulting. There is a ~100 km tectonic transition zone between these contrasting structural regimes, within which the Cook Strait has opened between the islands. There is little preserved geological record of the timing and geometry of past landscape changes within the tectonic transition zone over the past million years. A biological approach to Pleistocene geomorphology can fill this knowledge gap using quantification of genetic divergences of species of freshwater biota that now inhabit that topography. Riverine fish populations, especially Galaxias divergens, show that there must have been extensive freshwater drainage interconnections across the transition zone prior to ~1 Ma. Genetic divergence of populations of G. divergens show that the islands became separated at ~0.7–1 Ma, and this is supported by isolation of populations of freshwater crayfish Paranephrops planifrons. South Island river drainage interconnections and severance events have been dominated by glacial and interglacial erosion and sedimentation over the past 500 ka. North Island drainage evolution has been primarily driven by fault-dominated uplift of ranges that resulted in several river capture events that have been dated by genetic divergences of G. divergens between 700 and 100 ka. Colonisation by South Island subalpine stonefly species constrains timing of development of North Island upland environments to >400 ka. An apparently antecedent gorge on the Manawatu River through rising North Island bedrock ranges has developed after a tectonically-driven river capture event at ~500 ka. Tectonically and geometrically similar post-capture antecedent style gorges have developed elsewhere in the world, including in the Himalaya where geomorphic and biological evidence supporting river capture still persists despite rapid uplift and deep incision.

Subduction geometry controls on dynamic topography: implications for the Jurassic Surat Basin

The Late Jurassic Springbok unconformity in the Surat Basin remains to date at odds with our understanding of basin-forming processes. A previously proposed cause for the unconformity is uplift related to the rifting of the Argo landmass from Western Australia, but the great distance between the Surat Basin and the Argo rift-zone argues against this interpretation. A more likely scenario is that the uplift was related to a west-dipping subduction zone along the east Australian margin. Numerous examples around the world link subduction processes, such as tears and detaching segments of the slab, to variations in surface topography. We used geodynamic forward modelling to generate uplift in the overriding plate of a subduction zone, as well as topographic changes that occur as subduction is disrupted by slab breakoff or tears. Our findings suggest that slab tearing can produce distribution and magnitude of uplift consistent with the Jurassic uplift in the Surat Basin, which potentially adds new insights into the tectonic evolution of eastern Australia.

Chapter 7 Patuki and Croisilles melanges in South Island, New Zealand: genesis related to Permian subduction–accretion processes

Abstract The late Early Permian ( c. 278–270 Ma) supra-subduction zone (SSZ) Dun Mountain ophiolite is bordered to the east by the Pataki Melange and to the NE by the Croisilles Melange. In the south, the ophiolite passes into a dismembered incipient oceanic arc (Otama Complex). The above units represent an oceanic forearc generated above a west-dipping subduction zone. Terrigenous sediment reached the subduction trench after the Mid-Permian(?) docking of the oceanic forearc with the long-lived SE Gondwana active continental margin. Mixed terrigenous–volcaniclastic turbidites accumulated in the trench prior to and during melange accretion. Fragments of the overriding oceanic forearc (and incipient arc, locally) detached and mixed to form melange and broken formation. Despite some individual features (e.g. of the basalt chemistry), the Patuki and Croisilles melanges are interpreted as originally representing a single Permian trench–accretionary complex. The more distal (easterly) part was sliced into the adjacent accretionary complex of the Caples Terrane to form the Croisilles Melange (and equivalent Greenstone Melange) probably after the Triassic. The South Island melanges exemplify accretionary processes in which igneous and sedimentary rocks were detached from the overriding plate by subduction–erosion, together with accretion, including seamount material from the subducting oceanic plate, with implications for melanges elsewhere.

Early Cretaceous origin of the Woyla Arc (Sumatra, Indonesia) on the Australian plate

Key to understanding the plate kinematic evolution of the Neotethys oceanic domain that existed between the Gondwana-derived Indian and Australian continents in the south, and Eurasia in the north, is the reconstruction of oceanic plates that are now entirely lost to subduction. Relics of these oceanic plates exist in the form of ophiolites and island arcs accreted to the orogen that stretches from Tibet and the Himalayas to SE Asia that formed the southern margin of Sundaland. The intra-oceanic Woyla Arc thrusted over western Sundaland – the Eurasian core of SE Asia – in the mid-Cretaceous. The Woyla Arc was previously interpreted to have formed above a west-dipping subduction zone in the Early Cretaceous, synchronous with east-dipping subduction below Sundaland. The oceanic ‘Ngalau Plate’ between the Woyla Arc and Sundaland was lost to subduction. We present paleomagnetic results from Lower Cretaceous limestones and volcaniclastic rocks of the Woyla Arc, Middle Jurassic radiolarian cherts of the intervening Ngalau Plate, and Upper Jurassic–Lower Cretaceous detrital sediments of the Sundaland margin. Our results suggest that the Woyla Arc was formed around equatorial latitudes and only underwent an eastward longitudinal motion relative to Sundaland. This is consistent with a scenario where the Woyla Arc was formed on the edge of the Australian plate. We propose a reconstruction where the Ngalau Plate formed a triangular oceanic basin between the N–S trending Woyla Arc and the NW-SE trending Sundaland margin to account for the absence of accreted arc rocks in the Himalayas. As consequence of this triangular geometry, accretion of the Woyla Arc to the western Sundaland margin was diachronous, accommodated by a southward migrating triple junction. Continuing convergence of the Australia relative to Eurasia was accommodated by subduction polarity reversal behind the Woyla Arc, possibly recorded by Cretaceous ophiolites in the Indo-Burman Ranges and the Andaman-Nicobar Islands.

Reconstructing Plate Boundaries in the Jurassic Neo‐Tethys From the East and West Vardar Ophiolites (Greece and Serbia)

AbstractJurassic subduction initiation in the Neo‐Tethys Ocean eventually led to the collision of the Adria‐Africa and Eurasia continents and the formation of an ~6,000 km long Alpine orogen spanning from Iberia to Iran. Reconstructing the location and geometry of the plate boundaries of the now disappeared Neo‐Tethys during the initial moments of its closure is instrumental to perform more realistic plate reconstructions of this region, of ancient ocean basins in general, and on the process of subduction initiation. Neo‐Tethyan relics are preserved in an ophiolite belt distributed above the Dinaric‐Hellenic fold‐thrust belt. Here we provide the first quantitative constraints on the geometry of the spreading ridges and trenches active in the Jurassic Neo‐Tethys using a paleomagnetically based net tectonic rotation analysis of sheeted dykes and dykes from the West and East Vardar Ophiolites of Serbia (Maljen and Ibar) and Greece (Othris, Pindos, Vourinos, and Guevgueli). Based on our results and existing geological evidence, we show that initial Middle Jurassic (~175 Ma) closure of the western Neo‐Tethys was accommodated at a N‐S trending, west dipping subduction zone initiated near and parallel to the spreading ridge. The West Vardar Ophiolites formed in the forearc parallel to this new trench. Simultaneously, the East Vardar Ophiolites formed above a second N‐S to NW‐SE trending subduction zone located close to the European passive margin. We tentatively propose that this second subduction zone had been active since at least the Middle Triassic, simultaneously accommodating the closure of the Paleo‐Tethys and the back‐arc opening of Neo‐Tethys.

RETRACTED: Early Cretaceous origin of the Woyla Arc (Sumatra, Indonesia) on the Australian plate

Abstract Key to understanding the plate kinematic evolution of the Neotethys oceanic domain that existed between the Gondwana-derived Indian and Australian continents in the south, and Eurasia in the north, is the reconstruction of oceanic plates that are now entirely lost to subduction. Relics of these oceanic plates exist in the form of ophiolites and island arcs accreted to the orogen that stretches from Tibet and the Himalayas to SE Asia that formed the southern margin of Sundaland. The intra-oceanic Woyla Arc thrusted over western Sundaland – the Eurasian core of SE Asia – in the mid-Cretaceous. The Woyla Arc was previously interpreted to have formed above a west-dipping subduction zone in the Early Cretaceous, synchronous with east-dipping subduction below Sundaland. The oceanic ‘Ngalau Plate’ between the Woyla Arc and Sundaland was lost to subduction. We present paleomagnetic results from Lower Cretaceous limestones and volcaniclastic rocks of the Woyla Arc, Middle Jurassic radiolarian cherts of the intervening Ngalau Plate, and Upper Jurassic–Lower Cretaceous detrital sediments of the Sundaland margin. Our results suggest that the Woyla Arc was formed around equatorial latitudes and only underwent an eastward longitudinal motion relative to Sundaland. This is consistent with a scenario where the Woyla Arc was formed on the edge of the Australian plate. We propose a reconstruction where the Ngalau Plate formed a triangular oceanic basin between the N–S trending Woyla Arc and the NW-SE trending Sundaland margin to account for the absence of accreted arc rocks in the Himalayas. As consequence of this triangular geometry, accretion of the Woyla Arc to the western Sundaland margin was diachronous, accommodated by a southward migrating triple junction. Continuing convergence of the Australia relative to Eurasia was accommodated by subduction polarity reversal behind the Woyla Arc, possibly recorded by Cretaceous ophiolites in the Indo-Burman Ranges and the Andaman-Nicobar Islands.

Comment on “U‐Pb and Hf Isotope Analysis of Detrital Zircons From Mesozoic Strata of the Gravina Belt, Southeast Alaska” by Yokelson Et Al. (2015)

AbstractYokelson et al. (2015) present U‐Pb and Hf isotope analyses of detrital zircons from the Gravina belt showing that the western facies was derived from the west and the eastern facies was derived from the east and concluded that this is evidence that a west dipping subduction zone did not exist along the inboard margin of the Alexander‐Wrangellia terrane during Late Jurassic‐Early Cretaceous time as proposed by Sigloch and Mihalynuk (2013). However, the tectonic affinity of the eastern Gravina belt is open to debate, and Yokelson et al.'s (2015) claim that the Sigloch and Mihalynuk (2013) model of west dipping subduction was implausible is based on an incorrect assumption.

Bridging the gap between the foreland and hinterland I: Geochronology and plate tectonic geometry of Ordovician magmatism and terrane accretion on the Laurentian margin of New England

U-Pb dates on magmatic and detrital zircon from samples in the hinterland of the Taconic orogen place new constraints on the timing and plate tectonic geometry of terrane accretion and magmatic arc activity. The Moretown terrane, a Gondwanan-derived exotic block, extends from the Rowe Schist-Moretown Formation contact in the west to the Bronson Hill arc in the east. Arc-related plutonic and volcanic rocks formed above an east-dipping subduction zone under the western leading edge of the Moretown terrane from approximately 500 to 475 Ma, until collision with hyperextended distal fragments of Laurentia, represented by the Rowe Schist, at 475 Ma. Magmatic arc rocks formed during this interval are primarily located in the Shelburne Falls arc, although some are also located in the Bronson Hill arc to the east. Metasedimentary rocks in the Shelburne Falls arc contain detrital zircon derived from mixing of Gondwanan, Laurentian, and arc sources, suggesting that the Moretown terrane was proximal to Laurentia by 475 Ma. Explosive eruptions at 466 to 464 Ma preserved in the Barnard Volcanic Member of the Missisquoi Formation in Vermont and as ash beds in the Indian River Formation in the Taconic allochthons may record slab-breakoff of subducted lithosphere following collision of the Moretown terrane with distal Laurentian crustal fragments. Between 466 and 455 Ma a reversal in subduction polarity lead to a west-dipping subduction zone under Laurentia and the newly accreted Moretown terrane. Magmatic arc rocks in the Bronson Hill arc formed above this west-dipping subduction zone along the eastern trailing edge of the Moretown terrane at approximately 455 to 440 Ma. The western boundary of Ganderia in New England is east of the Bronson Hill arc, buried beneath Silurian and Devonian rocks deformed during the Acadian orogeny.

The zircon record of high-pressure metasedimentary rocks of New Caledonia: Implications for regional tectonics of the south-west Pacific

We report the U-Pb age, and trace element and hafnium isotope composition of zircons recovered from clastic metasedimentary rocks that span a range of metamorphic grades (prehnite-pumpellyite to eclogite facies) across the high-pressure metamorphic belt of northern New Caledonia. We use these data to evaluate the sedimentary source and environment of formation of these rocks, as well as their respective metamorphic evolution.Metasediments from the low-grade Koumac and blueschist-facies Diahot sequences contain zircon with age populations of 1800–1500Ma, 700–500Ma, 300–250Ma and 140–80Ma. These grains have highly variable Hf isotope compositions (εHf −36 to +12), and some have thin metamorphic rim zones dated to ~38Ma. Zircon grains from the high-grade metamorphic section (Pouebo Eclogite Mélange) are mostly Upper Cretaceous and lowermost Eocene in age, with ~40Ma metamorphic overgrowth zones. The Upper Cretaceous zircons from across the belt have similar isotopic compositions, whereas the lowermost Eocene zircons have relatively high εHf of +6 to +13. The zircon record of the eclogites often lacks the diverse old detrital signature that is prevalent in the blueschist-facies rocks; rather the age populations of the eclogite grade zircons reflect derivation from nearby volcanic sources.We suggest that the bulk of Koumac and Diahot sequences represent the erosional products of the Norfolk Ridge that were transported eastwards and accumulated into the East New Caledonia Basin. Paleozoic and Precambrian zircon grains were originally sourced from Paleozoic orogenic belts of the eastern margin of Australia, and possibly from now-submerged ridges of continental character. We suggest that the Mesozoic zircon grains derived from volcanic arc activity on the eastern margin of Gondwana. Extension led to a reduced continental sediment input into the East New Caledonia Basin during the Cretaceous, which explains the limited amounts of ancient detritus in the mid-Cretaceous to Eocene sedimentary protoliths of the Pouebo Eclogite Mélange. The combined Hf isotope and age data for our zircon record point to the existence of a west-dipping subduction zone active throughout the Late Triassic to the Late Cretaceous, possibly with some short-lived periods of volcanic hiatus. In the early Eocene, a new east-dipping subduction formed following a major change in plate kinematics, producing younger metavolcanic zircons and eventually leading to the metamorphism of sedimentary units in the middle Eocene.

The metamorphic sole of the western Tasmanian ophiolite: New insights into the Cambrian tectonic setting of the Gondwana Pacific margin

The Cambrian Ross–Delamerian Orogeny records the first phase of accretional tectonics along the eastern margin of Gondwana following breakup of the supercontinent Rodinia. Western Tasmania represents a key area for understanding the Cambrian tectonic setting of the eastern margin of Gondwana as it is one of the few places where a Tethyan-type ophiolite is preserved and contains the only known exposures of a sub-ophiolitic metamorphic sole associated with the Ross–Delamerian Orogen. This paper presents an integrated study of the field, petrographic, geochemical, and metamorphic characteristics of the metamorphic sole to the western Tasmanian ophiolite. The structurally highest levels of the metamorphic sole consist of granulite–upper amphibolite facies metacumulates and metagabbros. A transition to amphibolite and epidote–amphibolite facies conditions is recorded by metadolerites and metabasalts towards the base of the metamorphic sole. Kinematic indicators in mylonitic amphibolites suggest the metamorphic sole formed in an east-dipping subduction zone located to the east of the Proterozoic continental crust of Tasmania. Major and trace element whole rock and relict igneous spinel geochemistry indicates that the protoliths to the metamorphic sole formed at a back arc basin spreading centre. Our new data supports a model in which east-dipping subduction in Tasmania was driven by collapse of a back arc basin developed above an earlier west-dipping subduction zone outboard of the eastern margin of Gondwana. The proposed model may help to resolve a controversy related to apparent along-strike variations in subduction zone polarity during the Ross-Delamerian Orogeny and suggests a complex geodynamic setting had developed along the eastern margin of Gondwana by the Middle Cambrian. This study highlights the importance of considering the role of multiple subduction zones in generating metamorphic soles and emplacing ophiolites, which are key events associated with the construction of many orogenic belts worldwide.

Chronostratigraphy of the Hottah terrane and Great Bear magmatic zone of Wopmay Orogen, Canada, and exploration of a terrane translation model

The Paleoproterozoic Hottah terrane is the westernmost exposed bedrock of the Canadian Shield and a critical component for understanding the evolution of the Wopmay Orogen. Thirteen new high-precision U–Pb zircon crystallization ages are presented and support field observations of a volcano-plutonic continuum from Hottah terrane through to the end of the Great Bear magmatism, from >1950 to 1850 Ma. The new crystallization ages, new geochemical data, and newly published detrital zircon U–Pb data are used to challenge hitherto accepted models for the evolution of the Hottah terrane as an exotic arc and microcontinent that arrived over a west-dipping subduction zone and collided with the Slave craton at ca. 1.88 Ga. Although the Hottah terrane does have a tectonic history that is distinct from that of the neighbouring Slave craton, it shares a temporal history with a number of domains to the south and east — domains that were tied to the Slave craton by ca. 1.97 Ga. It is interpreted herein that Hottah terrane began to the south of its current position and evolved in an active margin over an always east-dipping subduction system that began prior to ca. 2.0 Ga and continued to ca. 1.85 Ga, and underwent tectonic switching and migration. The stratigraphy of the ca. 1913–1900 Ma Hottah plutonic complex and Bell Island Bay Group includes a subaerial rifting arc sequence, followed by basinal opening represented by marginal marine quartz arenite and overlying ca. 1893 Ma pillowed basalt flows and lesser rhyodacites. We interpret this stratigraphy to record Hottah terrane rifting off its parental arc crust — in essence the birth of the new Hottah terrane. This model is similar to rapidly rifting arcs in active margins — for example, modern Baja California. These rifts generally occur at the transition between subduction zones (e.g., Cocos–Rivera plates) and transtensional shear zones (e.g., San Andreas fault), and we suggest that extension-driven transtensional shearing, or, more simply, terrane translation, was responsible for the evolution of Bell Island Bay Group stratigraphy and that it transported this newly born Hottah terrane laterally (northward in modern coordinates), arriving adjacent to the Slave craton at ca. 1.88 Ga. Renewed east-dipping subduction led to the Great Bear arc flare-up at ca. 1876 Ma, continuing to ca. 1869 Ma. This was followed by voluminous Great Bear plutonism until ca. 1855 Ma. The model implies that it was the westerly Nahanni terrane and its subducting oceanic crust that collided with this active margin, shutting down the >120 million year old, east-dipping subduction system.

The multiply deformed foreland fold-thrust belt of the Balkan orogen, northern Bulgaria

The generally east-west–trending Balkan orogen (eastern Europe) consists of a northern belt of folded and thrusted Mesozoic and Cenozoic strata that forms the external fold-thrust belt of late Mesozoic and early Cenozoic age, and a southern belt that consists of deformed igneous and metamorphic rocks overprinted by Cenozoic extensional basins. Unlike most foreland fold-thrust belts, wherein deformation commonly migrates toward the foreland, the fold-thrust belt within the Balkan orogen is marginal to the Moesian Platform to the north, but was deformed in at least three events related to three different dynamic systems caused by changes in plate interactions. The earliest event of late-Early to early-Late Cretaceous deformed strata deposited within the Moesian continental margin and within a continental rifted belt containing deep-water flysch of Late Jurassic–Early Cretaceous age, a probable eastward extension of oceanic troughs from the Southern Carpathians. The shortening was a consequence of south or southwest synthetic subduction within the Vardar zone along the southern margin of the Balkan orogen. In Late Cretaceous time a backarc and/or intraarc rift zone developed along the southern margin of the fold belt, terminating shortening. The backarc and/or intraarc basin closed in Late Cretaceous–early Paleocene time, deforming the fold-thrust belt for a second time, but antithetically to north or northeast subduction in the Vardar zone. North- and northwest-vergent subduction within the Vardar zone caused magmatism, metamorphism, and deformation within the Rhodope area of southern Bulgaria south of the foreland thrust belt. In Paleogene time the southern part of the Balkan orogen became extensional with development of extensional basins and abundant magmatism due to trench rollback. The time of the final foreland fold-thrust belt deformation was late Eocene extending into Oligocene or early Miocene, contemporaneous with the extension to the south. The deformation within the fold-thrust belt was caused by a transfer of transpressional right shear within north Bulgaria and the Southern Carpathians as crustal units were translated northward west of the Moesian foreland crust and moved northeast and eastward into the eastern Carpathian west-dipping subduction zone. During the third event of deformation crustal units were molded around the Moesian foreland crust. The shortening ceased by early Miocene time and the right shear west of Moesian foreland crust was manifested by discrete right-slip faults to the present. During this third event southern Bulgaria was in an extensional regime that dominated the south- to southwest-vergent Hellenide orogen throughout the Cenozoic, thus dividing the Balkan orogen into two different deformational regions.

The Australo-Antarctic Columbia to Gondwana transition

From the Mesoproterozoic to Cambrian, Australo-Antarctica was characterised by tectonic reconfiguration as part of the supercontinents Columbia, Rodinia and Gondwana. New tectonic knowledge of the Wilkes Land region of Antarctica allows Australo-Antarctic tectonic linkages to be resolved through reconstruction into ca. 160Ma Gondwana. We also resolve 330±30km of sinistral strike-slip offset on the >3000km long Mundrabilla-Frost Shear Zone and 260±20km of dextral offset on the >1000km long Aurora Fault to reconstruct the ca. 1150Ma geometry of Australo-Antarctica. Using this revised geometry, we derive the first model of the Columbia to Gondwana reconfiguration process that is geometrically constrained to ~100km scale. In this model, early Mesoproterozoic tectonics is driven by two opposing subduction systems. A dominantly west-dipping subduction zone existed at the eastern margin of Australo-Antarctica until ca. 1.55–1.50Ga. A predominantly east-dipping subduction zone operated at the western margin of the Mawson Craton from ca. 1.70Ga to ca. 1.42Ga. The latter caused gradual westwards motion and clockwise rotation of the Mawson Craton relative to the West and North Australian Craton and the accretion of a series of continental ribbons now preserved in the Musgrave Province and its southern extensions. A mid-Mesoproterozoic switch to predominantly west-dipping subduction beneath the West Australian Craton brought about the final closure of the Mawson Craton with the North and West Australian Craton along the Rodona-Totten Shear Zone. Convergence was achieved prior to 1.31Ga, but final collision may not have occurred until ca. 1.29Ga. Post-1.29Ga intraplate activity involved prolonged high-temperature orogenesis from 1.22 to 1.12Ga, and significant movement on the Mundrabilla-Frost Shear Zone between 1.13 and 1.09Ga, perhaps in response to the assembly of Rodinia at ca. 1.1Ga. The Australo-Antarctic Craton was amalgamated with Indo-Antarctica along the Indo-Australo-Antarctic Suture (IAAS) and Kuunga Orogeny, probably in the latest Neoproterozoic to early Cambrian.

Fragmented Tasmania: the transition from Rodinia to Gondwana

The origin of the microcontinent VanDieland extends back to the late Paleoproterozoic, where it was positioned between East Antarctica and southwestern Laurentia, within the supercontinent Nuna and Rodinia. Paleo- to Mesoproterozoic events recorded in VanDieland have greater affinities with southwest Laurentia and East Antarctica, suggesting southern VanDieland was part of the Grenville Front, and the central Tasmanian part was adjacent to the Miller Range in the central Transantarctic Mountains. Late in the Neoproterozoic Rodinia break-up, VanDieland separated from East Antarctica and southwestern Laurentia, and moved north along the Terra Australis margin until its southern part was positioned next to the easternmost Robertson Bay Terrane of north Victoria Land. VanDieland comprises up to seven different crustal megaboudins or microcontinental ribbon terranes that likely had amalgamated by the end of the Cambrian; these ribbon terranes are bounded by major faults and suture zones. Some boundaries, such as the Arthur Metamorphic Complex, are well known. However, other boundaries, like the eastern edge of the Tyennan Zone, and the boundary between King Island and northwestern Tasmania, are more cryptic, as they are covered by younger geology or are under water. The boundaries are commonly defined by sedimentary and mafic volcanic infill that has been trapped between the crustal fragments. These rocks have previously been interpreted as allochthonous terranes but are more likely to represent inverted sections of attenuated transitional crust and back arc basin fill that formed along the eastern margin of the Gondwana plate during the Cambrian. This interpretation also provides an explanation for the previous tectonic analysis that suggests that Tasmania's mafic–ultramafic complexes were obducted westward onto older sequences and were subsequently transported southwards as other ribbons collided along the northeastern and western edges of the growing microcontinent, which existed in the overriding plate of a west-dipping subduction zone at the convergent margin between Gondwana and the proto-Pacific plate.