Tectonic Reconstructions Research Articles

Episodes of Abnormally High Intensity of Tectonic Dislocations

It is shown for the structures of the Late Permian–Early Triassic, Early–Late Cretaceous, and Early–Middle Miocene stages in the formation of the eastern margin of Asia that the evolution of the Earth’s crust included periods of tectonic reconstructions, which occurred practically instantaneously from the geological viewpoint. These periods are often shorter than the resolution of both local isotope and paleontological methods of dating.

Late Cretaceous to Oligocene burial and collision in western Papua New Guinea: Indications from low-temperature thermochronology and thermal modelling

Recent field and analytical work have considerably improved our understanding of the structure of the Muller Range along the frontal trend of the Papuan Fold and Thrust Belt (PFTB) in western Papua New Guinea (PNG). However numerous questions remain unresolved concerning the geological evolution of the region. In particular, the Late Cretaceous to Oligocene history of the region is largely unknown due to the absence of a continuous stratigraphic record. Here, we use both new and existing low-temperature thermochronology data to investigate the geological history of the Muller Range. Thermal history models based on these data suggest two major Cenozoic cooling episodes. The youngest, and best constrained, is clearly recorded in the stratigraphic record and relates to Neogene collision at the northern margin of the Australian continent. An older episode of comparable or greater magnitude occurred in the Eocene to Oligocene and may relate to the removal of 1500–3000 m of Late Cretaceous to Eocene section across the Muller Range prior to the widespread deposition of the shelfal Darai Limestone. We suggest that extension along major faults beneath the Muller Range accommodated sedimentation from the Late Cretaceous to the Eocene, consistent with long-lived extensional structures observed in neighbouring regions. In contrast to the Muller Range area, an almost continuous Late Cretaceous to Eocene stratigraphic sequence is preserved in the hinterland <50 km to the northeast. The selective removal of this sequence across the Muller Range suggests it was uplifted in the Eocene to Oligocene, possibly in part facilitated by the inversion of extensional faults in the Muller Range area. We suggest that this inversion was related to the Eocene to Oligocene collision of the expansive Sepik Terrane to the northwest of the PNG margin. The new data and interpretations presented here have significant implications for the evolution of the PFTB and for tectonic reconstructions of PNG.

Thinking about LIPs: A brief history of ideas in Large igneous province research

Large igneous provinces (LIPs) are extraordinary igneous and tectonic events that have influenced the planet in profound ways, including the major turnovers in the history of life. The LIP concept, definitions, and terminology first nucleated in a 1990 workshop jointly arranged by Joint Oceanographic Institutions and U.S. Science Support Program, subsequently presented in a series of seminal papers by Millard F. Coffin and Olav Eldholm in the early 1990's. They combined existing data and information from continental flood basalts with the emerging geophysical understanding of oceanic plateaus and rifted continental margins. The new terminology had major implications within the geosciences and beyond. However, when investigating how geoscientists described and interpreted these vast provinces prior to the 1990's, we are left with a series of questions. Who first realized that LIPs represent extraordinary events in the history of the Earth, what terminology was used, and why were geologists interested in this type of events? Here we discuss some of the key developments in LIP research, including mapping projects, expeditions, international programs, correlations across continents, and the history of the terminology. We focus our discussion on four cases, the Karoo LIP, the Siberian Traps, the North Atlantic Igneous Province, and the Ontong Java Plateau, comprising key examples of LIPs with long histories of study and exploration. We conclude that the past 150 years of LIP-related research and endeavor was largely driven by a need to understand fundamental aspects of Earth evolution, including continental drift, plate tectonic reconstructions, the origin of basalt, mantle processes and plumes, and the role of volcanism in driving mass extinctions and climatic changes.

Tectonic reconstruction of Cyprus reveals Late Miocene continental collision of Africa and Anatolia

The extended northern continental margin of Africa is currently colliding with and under-thrusting northward below Cyprus. The age of onset of this collision is poorly constrained, but is critical if we are to quantify continental subduction and evaluate its role during spectacular recent uplift of southern Turkey from ~7 Ma to the present. Here, we reevaluate the evolution of northern Cyprus and document the modern structure of the Kyrenia fold-thrust belt in a balanced cross section for the first time, to determine the timing and amount of shortening. The belt deformed an Upper Cretaceous to Miocene stratigraphy, which was deposited onto a metamorphic basement. The fold-thrust belt was previously proposed to have formed during two stages: in Eocene and late Miocene time, based on stratigraphic and limited structural evidence. We revaluate evidence for an Eocene phase of thrusting in Kyrenia and find that 1) repetitions of Eocene and older rocks are explained by Miocene thrusting, and newly documented olistoliths were misinterpreted as pre-Miocene thrust slices 2) uplift in Eocene time was not isolated to Kyrenia but also affected northern Arabia and results from regional dynamic topography or forebulge formation. Based on kinematic constraints from a plate reconstruction, we interpret that metamorphic rocks in Kyrenia were metamorphosed by Cretaceous burial below the Troodos ophiolite and were subsequently exhumed by upper plate extension during latest Cretaceous obduction onto the African margin. The latest Cretaceous-Miocene plate boundary between Africa and the Taurides was located to the north of Kyrenia. The Kyrenia fold-thrust belt accommodated a minimum of 17.5 km of shortening as a result of ~9–6 Ma thrusting as a result of initial Africa-Taurides collision. The northern edge of African continental crust arrived below the Taurides, where it has been seismologically imaged, in late Pleistocene time and may have caused or contributed to recently recognised major Tauride uplift since ~0.5 Ma.

Tectonics and geodynamics of the eastern Tethys and northern Gondwana since the Jurassic

Southeast Asia experienced a complex tectonic and geodynamic history related to the subduction of the eastern Tethyan ocean basins, resulting from the long-term convergence between the Indo-Australian, Eurasian, and Pacific plates since Pangea breakup. The complex collage of continental and island arc terranes can be reconstructed into an estimated ancient arrangement using plate tectonic reconstruction approaches based on a synthesis of continental and marine geological and geophysical data. We use the open-source and cross-platform software GPlates (www.gplates.org) to refine the evolution of the eastern Neo-Tethys since the latest Jurassic rifting episodes along northern Gondwana. We apply the resulting plate motions to drive numerical models of mantle flow in order to predict the evolving mantle structure. New Guinea’s northward motion over subducted slabs, related to the Sepik back-arc basin and the Maramuni subduction system, resulted in long-term flooding of the margin since ~20 Ma, despite falling long-term global sea levels. The Sundaland continental promontory experienced dynamic uplift in the latest Cretaceous to Eocene times due to the accretion of the Woyla Arc at ~80 Ma, leading to slab breakoff and a temporary interruption of subduction. However, renewed subduction along the Sunda margin resulted in renewed dynamic subsidence from ~30 Ma, which was amplified by regional basin rifting events. In addition, the sinking Sunda slab likely triggered a mantle slab avalanche, resulting in a counterintuitive combination of contemporaneous basin inversion and strong dynamic subsidence from ~15 Ma. The evolution of the eastern Tethyan oceanic gateway provides an important framework for understanding the role of plate tectonics in controlling long-term oceanic circulation and climate, as well as shedding light on the complex interplay between deep Earth and surface processes in driving basin formation and evolution. These results provide new avenues for reconciling stratigraphic and tectonic processes, as well as contributing new approaches for basin analysis and hydrocarbon exploration.

The Dynamic History of 220 Million Years of Subduction Below Mexico: A Correlation Between Slab Geometry and Overriding Plate Deformation Based on Geology, Paleomagnetism, and Seismic Tomography

AbstractGlobal tectonic reconstructions of pre‐Cenozoic plate motions rely primarily on paleomagnetic and geological data from the continents, and uncertainties increase significantly with deepening geological time. In attempting to improve such deep‐time plate kinematic reconstructions, restoring lost oceanic plates through the use of geological and seismic tomographical evidence for past subduction is key. The North American Cordillera holds a record of subduction of oceanic plates that composed the northeastern Panthalassa Ocean, the large oceanic realm surrounding Pangea in Mesozoic times. Here we present new paleomagnetic data from subduction‐related rock assemblages of the Vizcaíno Peninsula of Baja California, Mexico, which yield a paleolatitudinal plate motion history equal to that of the North American continent since Late Triassic time. This indicates that the basement rocks of the Vizcaíno Peninsula formed in the forearc of the North American Plate, adjacent to long‐lived eastward dipping subduction at the southern part of the western North American continental margin. Tomographic images confirm long‐lived, uninterrupted eastward subduction. We correlate episodes of overriding plate shortening and extension to flat and steep segments of the imaged slab. By integrating paleomagnetic, geological, and tomographic evidence, we provide a first‐order model that reconciles absolute North American plate motion and the deformation history of Mexico since Late Triassic time with modern slab structure.

Multiphase magmatic and tectonic evolution of a large igneous province - Evidence from the crustal structure of the Manihiki Plateau, western Pacific

The Manihiki Plateau, a Large Igneous Province (LIP) in the western Pacific, has been proposed to be emplaced as part of the “Greater Ontong Java Event” during the early Cretaceous. Shortly after its formation, the Manihiki Plateau fragmented into multiple sub-provinces. Plate tectonic reconstructions ignore this fragmentation, treating the Manihiki Plateau as a single crustal block. By analyzing two seismic refraction/wide-angle reflection profiles crossing the two largest sub-provinces of the Manihiki Plateau, we provide new insight into their deep crustal structure and magmatic evolution. Our data indicate that the High Plateau and the Western Plateaus were emplaced as a single unit during an initial phase of massive magmatic activity, but later magmatic stages altered the individual sub-provinces considerably.The High Plateau has a crustal thickness of 20 km and its P-wave velocity distribution is comparable to previously surveyed oceanic LIPs. Strong secondary magmatic phases are visible by eruptive centers and former magmatic pathways in the middle and lower crust. The Western Plateaus, which have a crustal thickness decreasing gradually from 17.3 km (East) to 9.2 km (West), experienced smaller amounts of magmatism mainly along fault zones and on local seamounts.Therefore, we propose a distinct development of the two main sub-provinces of the Manihiki Plateau after their initial joint emplacement. The High Plateau experienced voluminous multi-phase magmatic accretion and extrusion, whereas the thinner Western Plateaus exhibit only relatively minor magmatic growth. Such a large difference in the evolution process has not been reported from any other oceanic LIP so far.

Evaluating the influence of meteorite impact events on global potassium feldspar availability to the atmosphere since 600 Ma

Potassium feldspar present in global mineral aerosol (&lt;5%) plays a disproportionate role in modulating the microphysics of mixed-phase cloud. Via exceptional ice nucleation properties, it is capable of changing cloud properties and behaviour. Here we identify times of substantial and abrupt change in the global availability of potassium feldspar since 600 Ma. Normally, weathering and vegetation cover contribute to low availability, with clay dominating mineral aerosol. Periods of maximum availability are reasoned to follow the emplacement and remobilization of ejecta blankets from major meteorite impact events, before returning to background after some hundreds to thousands of years. We review the 44 largest confirmed craters and evaluate the potassium feldspar content of their target rocks, which range from c. 0 to &gt;30%. By combining crater size and tectonic reconstructions, we are able to provide a quantitative and self-consistent assessment of changes to global potassium feldspar availability. Considerable differences in potassium feldspar availability following meteorite impact events are revealed. Different impact events generated dust containing different amounts of potassium feldspar. Differing levels of influence upon climate are hypothesized, and should now be tested by looking at stratigraphic records of these events to reveal the sensitivity of climate to different dust mineralogy. Supplementary material: Figures showing palaeolatitude determinations using various methods and reconstruction models, and estimates of alkali feldspar abundance and distribution across contemporary continental landmasses for meteorite impact events are available at: https://doi.org/10.6084/m9.figshare.c.4253312

On Stress Distribution in Layered Rock Masses

Spatial distribution of principal stresses in a layered elastic medium is considered in the paper. Transformations of the principal stresses across an interface between the layers are analyzed. A method for reconstructing the spatial stress distributions in layered medium from several spot measurements is proposed. The introduced approach is capable of obtaining the stable solution of the stress reconstruction problem with respect to the noise in input data. A typical layered medium is considered for which the stresses are reconstructed from few independent spot measurements of the stress state. The proposed approach may be used for solving an inverse problem of tectonic stresses reconstruction from the borehole logging data.

Oblique rifting: the rule, not the exception

Abstract. Movements of tectonic plates often induce oblique deformation at divergent plate boundaries. This is in striking contrast with traditional conceptual models of rifting and rifted margin formation, which often assume 2-D deformation where the rift velocity is oriented perpendicular to the plate boundary. Here we quantify the validity of this assumption by analysing the kinematics of major continent-scale rift systems in a global plate tectonic reconstruction from the onset of Pangea breakup until the present day. We evaluate rift obliquity by joint examination of relative extension velocity and local rift trend using the script-based plate reconstruction software pyGPlates. Our results show that the global mean rift obliquity since 230 Ma amounts to 34° with a standard deviation of 24°, using the convention that the angle of obliquity is spanned by extension direction and rift trend normal. We find that more than ∼ 70 % of all rift segments exceeded an obliquity of 20° demonstrating that oblique rifting should be considered the rule, not the exception. In many cases, rift obliquity and extension velocity increase during rift evolution (e.g. Australia-Antarctica, Gulf of California, South Atlantic, India-Antarctica), which suggests an underlying geodynamic correlation via obliquity-dependent rift strength. Oblique rifting produces 3-D stress and strain fields that cannot be accounted for in simplified 2-D plane strain analysis. We therefore highlight the importance of 3-D approaches in modelling, surveying, and interpretation of most rift segments on Earth where oblique rifting is the dominant mode of deformation.

Pleistocene-Holocene tectonic reconstruction of the Ballık travertine (Denizli Graben, SW Turkey): (De)formation of large travertine geobodies at intersecting grabens

Travertine geobodies have been identified as potential reservoir analogues to carbonate build-ups in pre-salt hydrocarbon systems. To investigate travertine geobody deformation, faults were mapped in 35 travertine quarries that excavate the Ballık travertine, i.e. a c. 12.5 km2 large travertine geobody that precipitated at the intersection of the NE margin of the Denizli Basin and neighbouring Baklan Graben (SW Turkey). This travertine precipitated from cooling carbonate-saturated thermal spring waters that resurfaced along the margin fracture/fault network and through Neogene unconsolidated underlying sediments. From the Denizli basin floor to the uplifted graben shoulders, fault orientation is dominantly WNW-ESE oriented with major basin faults showing a left-stepping trend. Along the upper Denizli margin, travertine is only deformed by extensional normal faults. Along the lower margin, travertine starts with a subhorizontal facies but evolves to a travertine facies formed by a sloping topography with a domal architecture. Paleostress inversion of fault-slip data reveals that an Early Pleistocene NNE-SSW extensional-transtensional phase initiated the WNW-ESE oriented, graben-facing normal fault network. In the Middle Pleistocene, the Ballık fault network was left-lateral strike-slip reactivated because it acted as a transfer zone between the NW-SE extending neighbouring Baklan Basin and NW-SE extension along NE-SW oriented margin faults of the DGHS. In this stress configuration, travertine precipitated along the SW margin fault of the Baklan Graben. After strike-slip reactivation, a Late Pleistocene-to-current NNE-SSW extensional stress regime reinstalled during which margin faults widened and active travertine precipitation moved to more central parts of the DGHS. As different tectonic regimes affect graben intersections, reservoir analogues can have a complex deformation history driven by fault reactivation and recurrent stress permutations. This study concludes that large travertine geobodies can form at graben intersections because of their susceptibility to enhanced fluid flow through the complex fault-fracture network.

Geodynamic reconstruction of an accreted Cretaceous back-arc basin in the Northern Andes

A complex history of subduction, back-arc basin formation, terrane accretion and transpressional shearing characterizes the evolution of the Caribbean and northern South American margin since Jurassic times. Quantitative plate tectonic reconstructions of the area do not include Jurassic-Cretaceous back-arc terranes of which there are both geological and geophysical observations. We developed a revised plate tectonic reconstruction based on geological observations and seismic tomography models to constrain the Jurassic-Cretaceous subduction history of eastern Panthalassa, along the western margin of the Caribbean region. This reconstruction considers the opening of a Northern Andean back-arc basin at 145 Ma, the Quebradagrande back-arc, closing at 120 Ma and followed by terrane accretion and northward translation along the South American margin starting at 100 Ma. This kinematic reconstruction is tested against two previously published tectonic reconstructions via coupling with global numerical mantle convection models using CitcomS. A comparison of modelled versus tomographically imaged mantle structure reveals that subduction outboard of the South American margin, lacking in previous tectonic models, is required to reproduce mid-mantle positive seismic anomalies imaged in P- and S-wave seismic tomography beneath South America, 500–2000 km in depth. Furthermore, we show that this subduction zone is likely produced by a back-arc basin that developed along the northern Andes during the Cretaceous via trench roll-back from 145 Ma and was closed at 100 Ma. The contemporaneous opening of the Quebradagrande back-arc basin with the Rocas Verdes back-arc basin in the southern Andes is consistent with a model that invokes return flow of mantle material behind a retreating slab and may explain why extension along the Peruvian and Chilean sections of the Andean margin did not experience full crustal break-up and back-arc opening during the late Jurassic-early Cretaceous Period.

Pre-mid-Frasnian angular unconformity on Kotel’ny Island (New Siberian Islands archipelago): evidence of mid-Paleozoic deformation in the Russian High Arctic

We present detailed structural studies which reveal for the first time the existence of an angular unconformity at the base of the Middle Frasnian deposits across the western part of Kotel’ny Island (New Siberian Islands, Russian High Arctic). Pre-Mesozoic convergent structures are characterized by sublatitudinal folds and south-verging thrusts. Based on the age of the rock units above and below the unconformity, the age of the deformation event can be described as post-Givetian but pre-mid-Frasnian. Based on the vergence direction of thrusts deforming pre-Frasnian deposits on Kotel’ny Island, shortening occurred from north to south (in the present day coordinates). The small scale of the structures suggests that this part of the New Siberian Islands formed a distal part of an orogenic belt in the Middle Paleozoic. The angular unconformity described on Kotel’ny Island can be tentatively correlated with the Ellesmerian Orogeny. However, due to a paucity of detailed geological data from the neighboring broad Arctic continental shelves, a precise correlation with known tectonic events of the circum-Arctic cannot be achieved. The subsequent Mesozoic tectonic structures with NW-trending folds and faults were superimposed on pre-existing Paleozoic and older structures. Thus, the data presented here provide additional constraints on the Paleozoic geodynamic affinity of the New Siberian Islands and provide a regional link to other Arctic regions, aiding future tectonic reconstructions of the circum-Arctic.

Breakup volcanism and plate tectonics in the NW Atlantic

Passive margins are the locus of tectonic and magmatic processes leading to the formation of highly variable along-strike and conjugate margins structures. Using extensive new seismic, gravity, and magnetic datasets, complemented by seabed samples and field work, we investigate the tectonomagmatic evolution of the northwest (NW) Atlantic where breakup-related igneous rocks were emplaced during several Paleogene events associated with lithospheric stretching, continental breakup, and the formation of new oceanic basins. Interpretational methods include integrated seismic-gravity-magnetic (SGM) interpretation and seismic volcanostratigraphy. In addition, seabed and field samples were collected and analyzed to constrain the basin stratigraphy, hydrocarbon potential, and the geochronology and geochemistry of the volcanic sequences. Offshore, 2D seismic data reveal several Seaward Dipping Reflector (SDR) wedges and escarpments in the Labrador Sea, Davis Strait, and Baffin Bay. Onshore, eastward prograding foreset-bedded hyaloclastite delta deposits and overlying horizontal lava successions outcrop on Nuussuaq. These hyaloclastites and lava successions are world class analogues to the Lava Delta and Landward Flows volcanic seismic facies units identified offshore. Our mapping results document an aerial extent of the Paleogene breakup-related volcanics of 0.3 × 106 km2, with an estimated volume of 0.5–0.6 × 106 km3. Basalt samples recovered by dredging the Upernavik Escarpment have late Paleocene to/early Eocene ages, whereas the sedimentary samples provide an excellent seismic tie with the stratigraphy and the geology in this frontier area. From the integrated SGM interpretation, we identify a rapidly thinning crust and changes in top and intra-basement seismic reflection characteristics in the oceanic domain correlated with transition between different magnetic domains. The mapping results were subsequently integrated in a plate tectonic model. The plate tectonic reconstruction and basalt geochronology suggest that the majority of the volcanism in the NW Atlantic occurred between ~62 and ~58 Ma, associated with an increased spreading rate in the Labrador Sea, starting from the onset of the Selandian (~61.6 Ma). A change in the spreading direction during the Eocene (~56 Ma), synchronously with a shift of volcanic activity from the NW to the NE Atlantic, correspond to a northward drift of Greenland and the initiation of the Eurekan Orogeny. Finally, our interpretations reveal a complex rift configuration along the NW Atlantic conjugate margins both prior to and during breakup.

Constraining the depositional age of an Upper Cretaceous non-marine and shallow marine siliciclastic succession, Kuji Group, northeastern Japan, based on carbon isotope stratigraphy and U−Pb radiometric dating

Determining detailed geologic ages for pre-Quaternary non-marine and shallow marine siliciclastic successions has generally been difficult due to the rare occurrence of age-diagnostic fossils. In this study, we present the first record of stable carbon isotope stratigraphy derived from detrital woods with U–Pb radiometric dating for felsic tuffs in the Upper Cretaceous Kuji Group, northeast Japan, for which the geologic age has been poorly constrained to date. The radiometric ages for each tuff, obtained by laser ablation–inductively coupled plasma mass spectrometry (LA–ICPMS) analyses, range from 92.6 ± 0.6 to 79.1 ± 1.2 Ma (95% confidence interval). The composite δ13Cwood curve in the Kuji Group is correlated to the Upper Cretaceous reference sections in the Northwest Pacific, North America and Northwest Europe. Based on our correlations, several global carbon isotope excursion events in the late Turonian–early Campanian interval were recognized, and the Upper Cretaceous stage boundaries in the Kuji Group were partially identified, which agree well with previous biostratigraphic assumptions. Further, the depositional age of the dinosaur-bearing bone bed in the Kuji Group was estimated to be restricted to the early–middle Santonian. Our detailed chronostratigraphic framework provides significant implications for sequence stratigraphy, regional tectonic reconstruction and further exploration into the terrestrial ecosystem of the Late Cretaceous East Asian continental margin.

Petrogenesis of Late Silurian volcanism in SW Yunnan (China) and implications for the tectonic reconstruction of northern Gondwana

ABSTRACTEarly Palaeozoic magmatic records and tectonic reconstructions along the northern margin of Gondwana are still pending problems. In this paper, Late Silurian Dawazi and Dazhonghe volcanics in SW Yunnan Province (China) were studied. The Dazhonghe volcanics (419 Ma) have variable chemical compositions with SiO2 ranging between 49.8 and 79.5 wt.%, whereas, the Dawazi volcanics (417–429 Ma) form a bimodal volcanic suite consisting mainly of silicic rocks and subordinate basaltic rocks with a SiO2 content gap of ca. 15 wt.%. The Dazhonghe volcanics display calc-alkaline elemental compositions with enrichment in light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs) (e.g. Nb, Ta and Ti) and positive εNd(t) values (+ 4.0 to + 5.5). The Dawazi basaltic rocks are calc-alkaline, depleted in HFSEs, enriched in large ion lithophile elements (LILEs) (e.g. Cs, Rb, U and K), and have high εNd(t) values of −1.7 to + 5.4. The Dawazi silicic rocks have high Na2O/K2O ratios and positive εNd(t) values of + 2.4 to + 5.0, which are equivalents of calc-alkaline I-type granites. The Dazhonghe volcanics are dominated by fractional crystallization (FC) from a calc-alkaline primary magma which originated from an enriched mantle source metasomatized by subduction-related, sediment-derived fluid. The Dawazi basaltic rocks were derived from partial melting of an enriched mantle source metasomatized by subducted oceanic sediment/slab-derived fluids; the Dawazi silicic rocks originated from partial melting of the juvenile mafic lower crust with extensive FC. Both the Dazhonghe and Dawazi volcanics were generated in a continental back-arc extension setting. Combined with previous geological observations, a Late Silurian Prototethyan arc and back-arc extension system is proposed along the northern margin of Gondwana in SW China.

A subduction and mantle plume origin for Samoan volcanism

The origin of Samoan volcanism in the southwest Pacific remains enigmatic. Whether mantle melting is solely caused by a mantle plume is questionable because some volcanism, here referred to as non-hotspot volcanism, defies the plume model and its linear age-progression trend. Indeed, non-hotspot volcanism occurred as far as 740 km west of the predicted Samoan hotspot after 5 Ma. Here we use fully-dynamic laboratory subduction models and a tectonic reconstruction to show that the nearby Tonga-Kermadec-Hikurangi (TKH) subduction zone induces a broad mantle upwelling around the northern slab edge that coincides with the non-hotspot volcanic activity after 5 Ma. Using published potential mantle temperatures for the ambient mantle and Samoan mantle plume, we find that two geodynamic processes can explain mantle melting responsible for intraplate volcanism in the Samoan region. We propose that before 5 Ma, the volcanism is consistent with the plume model, whereas afterwards non-hotspot volcanism resulted from interaction between the Subduction-Induced Mantle Upwelling (SIMU) and Samoan mantle plume material that propagated west from the hotspot due to the toroidal component of slab rollback-induced mantle flow. In this geodynamic scenario, the SIMU drives decompression melting in the westward-swept plume material, thus producing the non-hotpot volcanism.

Heterogeneity of conodont faunas in the Cache Creek Terrane, Canada; significance for tectonic reconstructions of the North American Cordillera

The oceanic Cache Creek Terrane has long been interpreted to be exotic relative to the neighbouring Stikine and Quesnel terranes, on the basis of its unusual fossil faunas. Current tectonic models for the amalgamation of these three terranes have to account for the position of the exotic Cache Creek Terrane between two volcanic island arc terranes, and competing models invoke either terrane entrapment or strike-slip motion to explain this configuration. Although previous paleobiogeographic studies have attempted to assess the degree of faunal variability among terranes of the North American Cordillera, none have yet measured the amount of internal faunal heterogeneity and considered the effect that this may have on tectonic reconstructions. The use of multivariate statistics demonstrates that the Late Paleozoic to Triassic conodont faunas from the Cache Creek Terrane show distinct heterogeneity. The degree of heterogeneity among faunas from different regions of the terrane varies with time, and does not correlate with temporal changes in depositional environment, conodont ecology, or species endemism. Instead, it is proposed that the Cache Creek Terrane may consist of a number of sub-terranes which moved independently of each other prior to the final accretion of the Intermontane terranes. Periods of low faunal heterogeneity may correspond to times when the sub-terranes were close together, whereas periods of high heterogeneity may indicate that the sub-terranes had moved further apart. This interpretation requires that tectonic models for the formation of this part of the Cordillera emphasise lateral movement both within and between terranes prior to accretion.

Formation and evolution of the Chain-Kairali Escarpment and the Vishnu Fracture Zone in the Western Indian Ocean

Published models for the plate tectonic evolution of the Western Indian Ocean suggest that the Southern Mascarene Basin opened by oceanic crustal accretion between the continental margins of southwestern India and southeastern Madagascar. However, with the cessation of the Mascarene Basin spreading centre followed by a ridge jump resulting in the opening of the Carlsberg Ridge, almost all the traces of India-Madagascar divergence were carved away from the Indian Plate and attached to the African Plate. According to some recent studies, the Chain-Kairali Escarpment, a prominent feature on the southwestern continental margin of India, is possibly the only trace of India-Madagascar divergence that remained on the Indian Plate. But the exact conjugate correspondence of this feature on the Madagascar side is uncertain. Published plate tectonic reconstructions imply that the Vishnu Fracture Zone on the Indian side and the Mauritius Fracture Zone on the Madagascar side are aligned at chron C22ny (∼49.04 Ma). Based on the near collinearity of gravity anomaly trends, the Chain-Kairali Escarpment appears to be the landward extension of the Vishnu FZ. However, at chron C34ny (∼83.0 Ma), the Chain-Kairali Escarpment was in close proximity to the incipient Mahanoro Fracture Zone. In this study we investigate this incompatibility, using an up-to-date compilation of the tectonic elements from the conjugate regions of India and Madagascar and the latest available rotation parameters that describe India-Madagascar separation through a direct India-Seychelles-Madagascar plate circuit. Our revised plate reconstruction model suggests that the Chain-Kairali Escarpment was formed due to the strike-slip motion between the southeast coast of Madagascar and the then southwest coast of India during the initial stages of India-Madagascar breakup. The migration of the Chain-Kairali Escarpment from the proximity of the Mahanoro FZ and aligning with the Vishnu FZ was the result of several successive events. The first among those events was asymmetric crustal accretion in the Mascarene Basin due to ridge propagation, between chrons C34ny (83.0 Ma) to C33ny (∼73.62 Ma). The Chain-Kairali Escarpment and associated crustal weak zones offshore India appear to have facilitated subsequent initiation of the Mauritius FZ and its conjugate Vishnu FZ during a plate reorganization at about chron C33ny (∼73.62 Ma). The cessation of spreading in the Mascarene Basin and development of full extent of the Carlsberg Ridge, shortly after chron C27ny (60.92 Ma), resulted in the initiation of a long transform fault, coinciding with the Vishnu FZ, which connected the Carlsberg Ridge with the spreading centre of the Madagascar Basin. Therefore, the Vishnu FZ and the Chain-Kairali Escarpment are two independent features created during different episodes of evolution of the Western Indian Ocean and the Chain-Kairali Escarpment is not a landward extension of the Vishnu FZ.

Crustal evolution of the Central Tianshan Block: Insights from zircon U-Pb isotopic and structural data from meta-sedimentary and meta-igneous rocks along the Wulasitai – Wulanmoren shear zone

The Central Tianshan Block is one of the major continental constituents of the Central Asian Orogenic Belt (CAOB) and contains vital information for understanding the amalgamation of the Eurasia continent. Its tectonic affinity remains controversial hindering a better reconstruction of the paleogeographic evolution of the Paleo-Asian Ocean and geodynamic processes of the CAOB. In order to put more constraints on the Precambrian basement composition and crustal evolution of the Central Tianshan Block, we conducted field structural investigations and zircon U-Pb dating on the meta-sedimentary and meta-igneous rocks along the Wulasitai – Wulanmoren ductile shear zone (WWSZ), northern Baluntai area (NW China). The WWSZ is geometrically a SE-NW-striking high-strain deformation zone of ∼1–5 km wide. It is lithologically composed of greenschists, paragneiss, amphibolitic metabasalts, migmatitic granites and mylonitic granites. The greenschist to amphibolite facies meta-sandstones and meta-volcanic rocks belong to the Ahebulake Group, in which NE- or SW-dipping steep mylonitic foliations and sub-horizontal lineations are well developed. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb dating of detrital zircons from four representative meta-sedimentary rocks from the Ahebulake Group yielded several major age groups of 3130–2530 Ma, 2450–2150 Ma, 2000–1700 Ma, 1620–1433 Ma, 1170–655 Ma, ∼450 Ma, ∼425 Ma, ∼400 Ma, ∼360 Ma and ∼312 Ma. Three ductilely deformed and metamorphosed granitic samples show four major zircon U-Pb age peaks at ∼430 Ma, ∼400–390 Ma, ∼360 Ma and ∼330 Ma. On the basis of a careful analysis of the texture of the dated zircons and the structural features of their host rocks, and combined with the available isotopic ages and regional geological data, we propose that: (1) meta-sedimentary rocks from the Ahebulake Group were likely deposited later than about 400 Ma instead of in the Silurian, as previously assumed; (2) the distinct Neoproterozoic detrital zircon age population in these meta-sediments probably reflects the widespread occurrence of coeval in-situ basement rocks as source; (3) the Central Tianshan Block may have close tectonic affinities with both the Yili Block and the Tarim Craton during the Proterozoic; and (4) the ductile shearing along the WWSZ is likely later than ∼330 Ma, probably initiated at ∼312–299 Ma. Our new results provide further arguments for the paleogeographic linkage of the microcontinents within the Paleo-Asian Ocean and tectonic reconstruction of the southwestern CAOB.