Breakup Of Gondwana Research Articles

Geochemical constraints on Cenozoic intraplate magmatism and their relation to Jurassic dolerites in Tasmania, using Sr-Nd-Pb isotopes

Tasmania hosts two prominent Meso-Cenozoic mafic magmatic provinces: abundant Jurassic dolerites that belong to the Ferrar Magmatic Province that formed during Gondwana break-up; and Cenozoic, mainly basaltic volcanics, which represent intraplate magmatism, post-Gondwana break-up. The origin of the Cenozoic volcanics is still controversial, in that the detailed role of competing asthenospheric and lithospheric sources remains unclear. This study presents new geochemical and isotopic data which is used to identify and contrast the magmatic sources and their relationship to Jurassic and Cenozoic magmas. The compositions of the Cenozoic lavas span a wide range from highly silica-undersaturated olivine melilitites and nephelinites to basanites, ne-hawaiites, olivine basalts and quartz-tholeiites. New Sr-Nd-Pb isotopic data combined with major and trace element geochemistry show that the most silica-undersaturated rocks are most enriched in incompatible elements and also have the least radiogenic Sr and most radiogenic Nd. Sr-Nd-Pb isotope ratios for the most silica-rich lavas trend towards values for the Ferrar Jurassic dolerite source. More generally, Sr, Nd and Pb isotopes in the Cenozoic lavas can be modelled by a three component mixture of Pacific Mid-Ocean Ridge Basalts (MORB), HIMU-like (characterized by high time-integrated μ (238U/204Pb)) and Jurassic dolerite sources. Pb isotope values confirm the HIMU-like component in the array. Various degrees of melting during decompression melting has also contributed to the wide range of compositions observed, with most silica-undersaturated rocks originating at the highest pressures (>20 kbar) and by the smallest percentages of melting. We suggest that this process, combined with variable interaction of MORB and HIMU-like asthenospheric sources with lithospheric mantle containing remnant Jurassic signatures, resulted in the wide range of Cenozoic lava compositions observed today.

Distributional patterns of Aptian-Albian paleoichthyofauna of Brazil and Africa based on Track Analysis.

The Aptian-Albian paleoichthyofauna in Brazil and Africa is quite diverse, yet their geographic patterns still require accounts. This work consists of a panbiogeographical analysis of paleoichthyofauna from Aptian-Albian through track analysis. Geographic coordinates were obtained and converted into paleocoordinates. Individual Tracks (ITs) and Generalized Tracks (GTs) were built with the aid of ArcView GIS v3.2 program using Trazos2004 extension. As a result, four GTs and tree Biogeographic Nodes (A, B and C) had been identified. GTs 1 and 2 shows two different faunas. GT3 indicates similarity between Brazil and Africa paleoichthyofauna suggesting the break-up of Gondwana as the event that separated it. GT4 represents a probable local seaway produced by transgressions. In the Node A a unique fauna was found indicating the presence of a third biota with its own characteristics. Node B is associated with the break-up of the supercontinent Gondwana. Node C was related to fauna exchange promoted by marine ingression and epicontinental seas that occurred during Aptian-Albian. We concluded that there is a relationship between the distributional patterns of biota and events such as the break-up of Gondwana, the formation of Atlantic Ocean and eustatic movements that affected South America and Africa.

Paleomagnetic and Geochronological Results From the Zhela and Weimei Formations Lava Flows of the Eastern Tethyan Himalaya: New Insights Into the Breakup of Eastern Gondwana

AbstractThe breakup of eastern Gondwana is among the hottest topics in the Earth sciences because of its effect on global climate during the Jurassic‐Cretaceous, its influence on the evolution of life, and its importance to paleogeographic reconstruction. To better constrain the Jurassic and Cretaceous paleogeographic position of the Tethyan Himalaya and the breakup of eastern Gondwana, a combined paleomagnetic and geochronological study was performed on the Zhela and Weimei Formations lava flows, dated at ~138–135 Ma, in the Luozha area of the eastern Tethyan Himalaya. Both positive fold and reversal tests together with a maximum grouping at 100% unfolding indicate that the characteristic remanent magnetization directions are primary magnetizations acquired before folding. The tilt‐corrected directions yielded a paleopole at 0.9°N, 293.4°E with A95 = 7.0° and a corresponding paleolatitude of 53.5°S ± 7.0°S for the Luozha sampling area (28.9°N, 91.3°E), validating that the original erupted position of the Zhela and Weimei Formations lava flows was located in the center of the Kerguelen mantle plume. Our new results, together with the published paleomagnetic, geochronological, and geochemical results, demonstrate that the Comei‐Bunbury large igneous province originated from the Kerguelen mantle plume. The temporal and spatial relationships between the Comei‐Bunbury large igneous province and the Kerguelen mantle plume indicate that eastern Gondwana initially rifted at ~147 Ma and that the Indian Plate fully separated from the Australian‐Antarctic Plate before ~124 Ma.

Multi-cycle evolution of the intracontinental passive rift basins and its controlling on accumulation of oil & gas: Taking Muglad Basin in Africa as an example

Muglad Basin, one of the interior passive rift basins in Africa, has experienced multi-cycle passive rifting and superimposition evolution, which distinguishes itself from the active rift or single-cycle passive rift or typical multi-cycle superimposed basins. By application of analysis workflows & approaches to the superimposed basins, including the identification of key geological events during the evolutionary processes and stage division of the evolutionary history, the proto-type basins at the key periods were restored. And based on the analysis on the spatiotemporal difference in rifting intensities, superimposition processes & styles, the superimposition types of each depression or sag as well as its accumulation modes of oil & gas were established. Results show that accompanying with the breakup of Gondwana, and evolution of Atlantic Ocean, Indian Ocean and Red Sea surrounding the Africa Plate, Muglad Basin has experienced three evolutionary periods, namely, the deposition period of Abu Gabra Formation (AG Formation for short) in the Early Cretaceous, the deposition period of Darfur Group in the Late Cretaceous, and the deposition period of Tendi Formation in the Cenozoic. The proto-type basin at Early Cretaceous features several separate grabens and half-grabens, which was produced under the extensional environment having a close relationship with the spreading of Atlantic Ocean; the pro-type basin at Late Cretaceous features the successive development of those early grabens and half-grabens, which was produced under the extensional environment having a close relationship with the rapid movement of Indian Plate. And the proto-type basins at the Cenozoic were mainly confined within the grabens and half-grabens in Kaikang Depression, which was largely affected by the opening of Red Sea. According to the different intensities of three-cycle rifting in each depression or sag and the distinctive processes of tectonic subsidence and sediment filling, the superimposition types of three-cycle rifting were divided into three types, namely, the early-developed, the successive and the dynamic. And the early-developed sag could be represented by the Sufyan Sag, which features the drastically decreasing intensity of three-cycle rifting. In those sags, the amount of basement subsidences and sediment fillings during the first-cycle rifting and post-rifting could account for 79% similar to 82% of the total subsidences and fillings. And the third cycle of rifting and post-rifting was so weak that it was difficult to interpret the layers in seismic section, and even in the logging, it was always omitted. Fula Sag was the most typical sag of the successive. Those sags feature the gradually decreasing intensity of three-cycle rifting & post-rifting. Unlike the former one, the second cycle of rifting & post-rifting still remains quite strong in intensity, which could be proved by the fact the thickness of Darfur Group (Late Cretaceous) in Fula Sag is larger than that of Bentiu Formation (Early Cretaceous) which was deposited during the first cycle of post-rifting. Kaikang Depression was the most typical depression of the dynamic, which features the relatively continuous intensity of the three cycles. When comparing with the formers, every cycle of the three cycles in the Kaikang Depression was larger than any of the early-developed and the successive. Even in the third cycle of rifting & post-rifting, the amount of subsidence and sediment fillings was as much as 2,000m. The spatiotemporal differences in intensities of three-cycle rifting & post rifting determine the differences in petroleum geological conditions for hydrocarbon accumulation and distribution. The dominant play in the early-developed sags is the lower play, and that in the successive sags is the middle play. However, the upper plays were the most dominant plays in those dynamic sags. The study of multi-cycle superimposed passive rift basin in this paper would not only contribute and enrich the research on the structural feature, evolutionary history as well as its effect on the hydrocarbon accumulation of other rift basins in the world, but also had a practical significance in making further strategies for exploration in these basins.

Chapter 1 Introduction to Paleozoic–Mesozoic geology of South Island, New Zealand: subduction-related processes adjacent to SE Gondwana

This Memoir presents and discusses recent research mainly concerning the Permian and Triassic geological development of South Island in its regional context, which includes New Zealand as a whole, the continent of Zealandia, eastern Australia and Antarctica. The Permian and Triassic geology of the South Island encompasses several geological units which shed light on fundamental geological processes. These include a well-developed oceanic volcanic arc, a classic subduction-related ophiolite (oceanic crust on land) and a very thick succession of continental margin forearc sediments. These, and other related geological units, constitute the oldest extensively outcropping basement of Zealandia, which can be considered as the world's seventh largest continent, albeit 94% submerged (Mortimer et al. 2017). The supercontinent, Gondwana, formed during the Late Neoproterozoic and remained until its progressive break-up into crustal fragments during the Mesozoic. During this prolonged period, New Zealand and adjacent crustal units were located along the SE edge of Gondwana. Subduction took place beneath this margin during the Paleozoic–Mesozoic, until the end of Early Cretaceous time ( c. 100 Ma) when it halted, followed by rifting of the Tasman Sea Basin. This volume consists of 14 contributions, comprising a mix of new-data-based papers (and one note) and several review and synthesis papers. The contributions largely concern igneous and sedimentary geology, tectonics and biostratigraphy. Particular emphasis is placed on field geology, petrography and geochemistry. Some aspects are less well covered (e.g. Permian–Triassic geology of New Zealand's Cordilleran-style Median Batholith; metamorphic geology), primarily because there is a large amount of existing and/or recently published data. While the Permian–Triassic terranes bear imprints of Gondwana break-up and Neogene Alpine Fault development, these topics are outside the scope of this volume. The final chapter discusses various tectonic-based interpretations for the Permian–Triassic development of Zealandia and points the way to an overall synthesis. In this introductory …

Nature and origin of the Mozambique Ridge, SW Indian Ocean

The Mozambique Ridge (MOZR) is one of several bathymetric highs formed in the South African gateway shortly after the breakup of the supercontinent Gondwana. Two major models have been proposed for its formation - volcanic plateau and continental raft. In order to gain new insights into the genesis of the Mozambique Ridge, R/V SONNE cruise SO232 carried out bathymetric mapping, seismic reflection studies and comprehensive rock sampling of the igneous plateau basement. In this study, geochemical data are presented for 55 dredged samples, confirming the volcanic origin of at least the upper (exposed) part of the plateau. The samples have DUPAL-like geochemical compositions with high initial 87Sr/86Sr (0.7024–0.7050), low initial 143Nd/144Nd (0.5123–0.5128) and low initial 176Hf/177Hf (0.2827–0.2831), and elevated initial 207Pb/204Pb and 208Pb/204Pb at a given 206Pb/204Pb (Δ7/4 = 2–16; Δ8/4 = 13–167). The geochemistry, however, is not consistent with exclusive derivation from an Indian MORB-type mantle source and requires a large contribution from at least two components. Ratios of fluid-immobile incompatible elements suggest the addition of an OIB-type mantle to the ambient upper mantle. The MOZR shares similar isotopic compositions similar to mixtures of sub-continental lithospheric mantle end members but also to long-lived, mantle-plume-related volcanic structures such as the Walvis Ridge, Discovery Seamounts and Shona hotspot track in the South Atlantic Ocean, which have been proposed to ascend from the African Large Low Shear Velocity Province (LLSVP), a possible source for DUPAL-type mantle located at the core-mantle boundary. Interestingly, the MOZR also overlaps compositionally with the nearby Karoo-Vestfjella Continental Flood Basalt province after filtering for the effect of interaction with the continental lithosphere. This geochemical similarity suggests that both volcanic provinces may be derived from a common deep source. Since a continuous hotspot track connecting the Karoo with the MOZR has not been found, there is some question about derivation of both provinces from the same plume. In conclusion, two possible models arise: (1) formation by a second mantle upwelling (blob or mantle plume), possibly reflecting a pulsating plume, or (2) melting of subcontinental lithospheric material transferred by channelized flow to the mid-ocean ridge shortly after continental break-up. Based on geological, geophysical and geochemical observations from this study and recent published literature, the mantle-plume model is favored.

The Falkland Plateau in the context of Gondwana breakup

During the Jurassic, the Falkland Plateau was part of Gondwana and occupied a position between the African and Antarctic plates. Several contrasting models exist for the breakup of Gondwana that depend on assumptions about the currently unknown crustal structure of the Falkland Plateau. Here, we present the results of recently acquired wide-angle seismic data along the entire plateau that provide sound constraints on its role in geodynamic reconstructions. In contrast to published crustal models, the new data show that the Falkland Plateau Basin consists of up to 20 km thick oceanic crust, which is bounded to the east by a continental fragment, the Maurice Ewing Bank. In a refined geodynamic model, rifting started between the Falkland Islands and the Maurice Ewing Bank at ~178 Ma and ceased at around ~154 Ma. The plateau's exceptionally thick oceanic crust likely results from its position in an extensional back-arc-regime situated over a mantle thermal anomaly that was also responsible for the extensive onshore Karoo-Ferrar and Chon Aike volcanic provinces.

Early Cretaceous bimodal magmatism in the eastern Tethyan Himalayas, Tibet: Indicative of records on precursory continental rifting and initial breakup of eastern Gondwana

In this study, we report a bimodal magmatic suite of gabbroic-granitic rocks in the Cuona area of the eastern Tethyan Himalayas, which is related to the rifting event between Greater India and western Australia in eastern Gondwana. Whole-rock major and trace elements, SHRIMP zircon U-Pb geochronology and Hf isotopic analysis were conducted to reveal the petrogenesis of the bimodal suite and through this to explore the tectonic implications. Our main findings include the following. First, the gabbros and granites were emplaced synchronously at 140–137 Ma, considering the field relationships they comprise a bimodal suite. Second, the gabbros are enriched in light rare-earth-elements (LREE), depleted in heavy rare-earth-elements (HREE) with slight Eu anomalies, and have εHf(t) values of +0.04 − +8.30, which as a whole, suggests that they were derived from an ocean island basalt (OIB)-like mantle source. Third, the granites display low MgO, CaO, and Al2O3, marked enrichment in LREE, highly refracted HREE with strongly negative Eu anomalies and εHf(t) values of −2.01 to +1.71, all of which ultimately gives the characteristics of A1-type granites. Based on these observations, we propose that the bimodal suite is formed in a continental rift. Specifically, the gabbros originated from the mantle plume, whereas the granites derived from the upper crust with garnet-bearing metapelite. This rifting event initiated at 140 Ma, which places bounds on the initial breakup of eastern Gondwana, specifically the separation of Greater India from western Australia.

The initial Gondwana break-up: A synthesis based on new potential field data of the Africa-Antarctica Corridor

The initial opening of the Africa-Antarctica Corridor, in the heart of Gondwana, is still enigmatic due to missing information on the origin of major crustal features and the exact timing of the onset of the first oceanic crust in the Jurassic. Therefore, in 2014, new ship-borne magnetic data were systematically acquired in the northern Mozambique Basin and across Beira High, which we merged with all accessible magnetic data in the Mozambique Basin. Herein, distinct magnetic lineations are observed, which allow a refined identification of a whole set of Jurassic magnetic spreading anomalies, constraining the timing of the onset of oceanization, beginning at M38n.2n (164.1 Ma). In combination with high-resolution potential field data from the conjugate Antarctic margin, well-expressed fracture zones can be traced throughout the Africa-Antarctica Corridor and allow the precise rotation of Antarctica back to Africa. The initial fit depicts striking continuations of onshore tectonic features across the plate boundaries taking onshore aeromagnetic data of both margins into account. Within a tight Gondwana fit, the Beira High can be restored along the major sinistral Namama-Orvin Shear Zone of the East African-Antarctic Orogen. The Beira High represents a continental block, which was detached from Antarctica, by 157 Ma at the latest. Simultaneously, the Antarctic plate cleared the area of the MCP. However, the crustal nature of the southern MCP remains ambiguous. The Northern Natal Valley and the Mozambique Ridge consist of thick oceanic crust, being emplaced between M26r-M18n (157.1–144 Ma) and M18n-M6n (144–131.7 Ma), respectively. About the half of this crust was won from the Antarctic plate by a series of southwards directed ridge jumps to the northern boundary of the Explora Wedge. A refined kinematic break-up model constrained by the most extensive magnetic dataset is presented describing consistently the initial opening of the Africa-Antarctica Corridor and the Somali Basin.

Long-term reactivation and morphotectonic history of the Zambezi Belt, northern Zimbabwe, revealed by multi-method thermochronometry

Neoproterozoic-early Paleozoic Pan-African mobile belts that formed during the amalgamation of Gondwana, such as the Zambezi Belt, are inherently weak zones that are susceptible to reactivation by later tectonism. With the exception of Karoo rifting, however, the post-Pan-African morphotectonic history of the Zambezi Belt is poorly constrained. Here, we use multiple low-temperature thermochronometers on samples collected across major structures in northern Zimbabwe to reveal the temporal and spatial pattern of tectonism and denudation in this portion of the Zambezi Belt. Thermal history modelling suggests that a large crustal block encompassing part of the Zambezi Belt and northern margin of the Zimbabwe Craton experienced differential denudation during three main Phanerozoic episodes. This denudation of the Archean-Proterozoic basement was associated with reactivation of the Zambezi Escarpment Fault, which demarcates the southern margin of the Karoo Cabora Bassa Basin. In contrast, other major structures within the region remained relatively stable. The results highlight the value of using a multi-thermochronometer approach, where essentially zircon (U-Th)/He data preserve evidence of late Carboniferous Karoo rifting, apatite fission track data record the thermal effects of Jurassic tectonism associated with Gondwana breakup, and the apatite (U-Th-Sm)/He data reveal Paleogene reactivation of the basin-bounding fault. Thermal history modelling also suggests that the Cabora Bassa Basin experienced Gondwana breakup-related denudation, potentially associated with a proposed concurrent major regional drainage reversal. Since the Cretaceous, the basin has experienced limited sedimentation and erosion.

Normal fault linkage and reactivation, Dampier Sub-basin, Western Australia

The Northern Carnarvon Basin of Western Australia has experienced a polyphase deformation history during the breakup of Gondwana. Extension during the Carboniferous–Permian and a subsequent Early Jurassic rift event imposed two distinct fault systems, separated by a several kilometre-thick Triassic sedimentary sequence. Inboard areas, where Triassic sequences are thinner, Jurassic faults both detach above and also penetrate into Permian sequences. Other large-scale faults demonstrate a vertical hard/soft linkage between the two fault systems. In outboard areas where the Triassic is thicker, the relationship is less clear owing to the lower resolution of Permian sequences in seismic data. Here we undertake fault displacement analysis on three faults on the southern margin of the Exmouth Plateau to investigate the growth mechanism of Jurassic-aged faults and possible structural influence of deeper Permian faults. We find evidence of low-throw faults restricted to Mesozoic strata as more complex-segmented faults that have nucleated at a depth below that resolvable on seismic data. When considered in a regional context, the nature of faults in this study suggest oblique reactivation of the NE-trending Permian fabric, under east–west-oriented extension.

Thermochronological insights into reactivation of a continental shear zone in response to Equatorial Atlantic rifting (northern Ghana)

West Africa was subjected to deformation and exhumation in response to Gondwana break-up. The timing and extent of these events are recorded in the thermal history of the margin. This study reports new apatite fission track (AFT) data from Palaeoproterozoic basement along the primary NE-SW structural trend of the Bole-Nangodi shear zone in northwestern Ghana. The results display bimodality in AFT age (populations of ~210-180 Ma and ~115-105 Ma) and length distributions (populations of 12.2 ± 1.6 and 13.1 ± 1.4 µm), supported by differences in apatite chemistry (U concentrations). The bimodal AFT results and associated QTQt thermal history models provide evidence for multiple cooling phases. Late Triassic – Early Jurassic cooling is interpreted to be related with thermal relaxation after the emplacement of the Central Atlantic Magmatic Province (CAMP). Early to middle Cretaceous cooling is thought to be associated with exhumation during the Cretaceous onset of rifting between West Africa and Brazil. Late Cretaceous – Cenozoic cooling can be related with exhumation of the Ivory Coast – Ghana margin and NNW-SSE shortening through western Africa. Furthermore, our data record differential exhumation of the crust with respect to the Bole-Nangodi shear zone, preserving older (CAMP) cooling ages to the south and younger (rifting) cooling ages to the north of the shear zone, respectively. This suggests that the Palaeoproterozoic BN shear zone was reactivated during the Cretaceous as a result of deformation in the Equatorial Atlantic region of Africa.

Tectonic controls on sediment provenance evolution in rift basins: Detrital zircon U–Pb and Hf isotope analysis from the Perth Basin, Western Australia

The role of tectonics in controlling temporal and spatial variations in sediment provenance during the evolution of extensional basins from initial rifting to continental breakup and passive margin development are not well established. We test the influence of tectonics in a rift basin that has experienced minimal uplift but significant extension throughout its history: the Perth Basin, Western Australia. We use published zircon U–Pb and Hf isotope data from basin inception through to continental drift and complement this with new data from samples deposited synchronously with the continental breakup of eastern Gondwana. Three primary source regions are inferred, namely the Archean Yilgarn Craton to the east, the Paleo- and Mesoproterozoic Albany–Fraser–Wilkes Orogen to the south and east, and the Mesoproterozoic and Ediacaran–Cambrian Pinjarra Orogen underlying the rift basin and comprising the dominant crustal components to the west and southwest. From mid-Paleozoic basin inception to Early Cretaceous breakup of eastern Gondwana, drainage in the Perth Basin was primarily north- to northwest-directed as evidenced by the dominant Mesoproterozoic detrital zircon cargo, paleodrainage patterns and paleocurrent directions. Thus, provenance was primarily parallel to the rift axis and perpendicular to the extension direction, particularly during periods of thermal subsidence. During episodes of mechanical extension, detrital zircon ages are polymodal and consistently dominated by Paleo- and Mesoproterozoic grains derived from the Albany–Fraser–Wilkes Orogen, but with significant Archean and Neoproterozoic inputs from the rift margins. It is inferred that during mechanical extension the rate of subsidence exceeded sediment supply, which generated basin-margin scarps and enhanced direct input from the rift shoulders. Detrital zircon spectra from temporally-equivalent samples at the rift margin and in the rift axis reveal that distinct sedimentary routing operated on the flanks. In summary, sediment provenance in the Perth Basin (and probably other rift basins) is tectonically controlled by: (1) extension direction, (2) episodes of mechanical extension (rift) or thermal subsidence (post-rift), and (3) proximity to rift axis or rift margin.

Cenomanian–Turonian depositional history of a post–Gondwana rift succession in the West Beni Suef Basin, Egypt

The structural and stratigraphic evolution of rift basins has been documented globally from modern systems, outcrop and subsurface datasets. Extensive fragmentation and break-up of Gondwana during the Cretaceous created a complex suite of rift basins in the Northern Africa. The Beni Suef rift basin straddles the River Nile and is infilled by a thick Mesozoic–Tertiary succession. Cenomanian–Turonian deposition commenced with the sandstone-dominated Bahariya Formation. Depositional environments transitioned stratigraphically from fluvial-marine to shallow marine and finally to mixed clastic and non-clastic shelfal deposits of the Abu Roash Formation. Sedimentation styles were tectonically controlled showing transgressive-to-regressive depositional cycles developed over the Gondwana rift systems. Cycles were controlled by short-lived, small-scale tectonic oscillations of the platform, resulting in four depositional sequences. Deposition environments in the Cenomanian transitioned from shallow marine to deeper marine shelfal deposits and finally to a coastal environment. During Late Cenomanian–Early Turonian, rapid platform submergence resulted in a transgressive facies succession, topped by deltaic deposits. In the Middle–Upper Turonian, two sequences formed with an intervening short-lived period of lower relative sea level. Relative sea-level oscillations were partly in phase with the global sea-level changes, controlled by local tectonics induced by the fragmentation of Gondwana during the Late Cretaceous.

Long-term evolution of the West African transform margin: estimates of denudation from Benin using apatite thermochronology

The Benin continental margin was formed during the breakup of Gondwana through oblique rifting along transform faults. The evolution of topography following breakup directly affects the evolution of sedimentary basins, which has major implications for hydrocarbon exploration in the region. Quantitative constraints on erosion across Benin are limited to the Cenozoic, based on analysis of dissected lateritic palaeolandscapes. To resolve the Mesozoic erosion history, we have obtained apatite fission-track and single-grain (U–Th–Sm)/He data from 18 samples collected across a 600 km long transect through Benin. We invert these data, including available geological and geomorphological constraints, to obtain time–temperature paths, which are used to estimate magnitudes of denudation over the last 200 myr. Our study suggests that denudation was focused over a c. 300 km long seaward sloping limb of the marginal upwarp and at the southern margin of the interior Iullemmeden Basin from 140 to 100 Ma with lower magnitudes of denudation characterizing the continental interior and post-Cretaceous evolution of the margin. Models are consistent with modest burial ( c. 1 km) of the Iullemmeden Basin between 120 and 85 Ma, and of the continental margin between 85 and 45 Ma. By the Eocene the first-order relief of Benin had developed, with regional erosion rates <20 m Ma −1 since then. Supplementary information: Full details of the analytical data and modelling results, including the various constraints, and further data on denudation and burial magnitudes and rates are available at https://doi.org/10.6084/m9.figshare.c.4220804

The pre-glacial landscape of Antarctica

ABSTRACTThe geomorphology of the hidden subglacial landscape of Antarctica is relevant to our understanding of the stability of the Antarctic Ice Sheet and also to that of global interactions between plate tectonics and surface processes. We believe that geomorphology has much to contribute, but that the lack of coherent hypotheses about the origins of the subglacial landscape is holding back understanding. This paper approaches the problem by using southern hemisphere land masses in Africa and Madagascar as analogues. We find that the Antarctic landscape evolved in a similar way to passive margin evolution in southern Africa. Rifting associated with the breakup of Gondwana changed river base levels and caused rapid erosion on the flanks of rifts and was accompanied by the uplift of rift-margin mountains. Rift-margin plains, often coastal or extending inland along large rivers, are backed by an escarpment, while low-gradient continental river basins characterised the interior of Antarctica. In East Antarctica ice has removed pre-existing regolith from lowlands and excavated 2–3 km troughs below sea level along the course of major trunk rivers. The micro-continents of West Antarctica are comparable to Madagascar and apparently share a similar topography with coastal plains, backing escarpments and interior plateaux.

Geochemical signature and reservoir conditions of Early Jurassic calc-alkaline volcanic rocks from Lonco Trapial Formation, Central Patagonia

Central Patagonia is traversed by a belt of Early to Middle Jurassic calc-alkaline intermediate volcanic rocks interspersed with more felsic volcanic rocks which are associated with the widespread magmatism that took place during Gondwana break-up times. This work uses KAr and ArAr dating and whole-rock and phenocryst (plagioclase, amphibole, clinopyroxene and titanomagnetite) compositional data to refine the age, geochemical signature and reservoir conditions of these volcanic rocks, which are known as Lonco Trapial Formation. The andesites, dacites and trachydacites which were the object of this study have either amphibole or clinopyroxene as the main mafic phenocryst (amphibole-bearing and clinopyroxene-bearing volcanic rocks, respectively), though amphibole is the main mafic phase. Despite the calc-alkaline signature a mild alkaline affinity emerges from some whole-rock trace elements content and from mineral chemistry (amphibole, clinopyroxene and titanomagnetite compositions). The magmatic evolution of the Lonco Trapial andesites, dacites and trachydacites was governed by fractionation of amphibole, clinopyroxene, plagioclase, titanite, titanomagnetite and apatite. Amphibole phenocrysts show an overall normal chemical zoning. The cores of the amphiboles crystallized over a temperature range of 869–916 °C, whereas the rims crystallized over a temperature range of 826–867 °C. Shallow to intermediate depths (2–8 kbar, ∼7–26 km) were inferred from geobarometric calculations. Crystallization temperatures are slightly higher in the clinopyroxene-bearing volcanic rocks, consistent with their more primitive character. The geobarometric estimations of this work are coherent with the lack of marine ingressions and with geophysical estimations which suggest that the Early Jurassic Moho depth would have been ≥35 km The combination of whole-rock and mineral geochemistry is consistent with an extensional affinity for this paleo-volcanic belt.

Late Cretaceous oceanic plate reorganization and the breakup of Zealandia and Gondwana

New 40Ar/39Ar ages of igneous rocks clarify the nature, timing and rates of movement of the oceanic Pacific, Phoenix, Farallon and Hikurangi plates against Gondwana and Zealandia in the Late Cretaceous. With some qualifications, cessation of spreading at the Osbourn Trough is dated c. 79 Ma, i.e. 30–20 m.y. later than 110–100 Ma Hikurangi Plateau-Gondwana collision. Oceanic crust of pre-84 Ma is confirmed to be present at the eastern end of the Chatham Rise, and a 99–78 Ma intraplate lava province erupted across juxtaposed Zealandia, Hikurangi Plateau and oceanic crust. We propose a new regional tectonic model in which a mechanically jammed Hikurangi Plateau resulted in the dynamic propagation of small, kinematically misaligned short-length 110–84 Ma spreading centres and long-offset fracture zones. It is only from c. 84 Ma that geometrically stable spreading became localized at what is now the Pacific-Antarctic Ridge, as Zealandia started to split from Gondwana.

Formation and Evolution of Microcontinents of the Kerguelen Plateau, Southern Indian Ocean

The paper considers geological and geophysical data on the crustal structure of the volcanic Kerguelen Plateau and nearby Indian Ocean, as well as evolution of the Kerguelen Plateau during the Gondwana breakup. It is assumed that three isolated continental blocks (microcontinents) evolved within the Kerguelen Plateau. In order to provide insight into the mechanisms for the microcontinent formation, physical modeling under three different conditions has been carried out. The first experiment simulates a homogeneous lithosphere with two rift fractures propagating toward each one other; in the second experiment, the propagating rift fractures collide with a structural “barrier” having a stronger lithosphere; the third experiment simulated the effect of a local heat source (a hot spot). Based on these experiments and taking into account the available structural and geophysical data, it is suggested that isolation of continental block in the southern Kerguelen Plateau could have begun due to counterpropagation of rifting branches: oceanic from east to west and continental from west to east. The isolation of the southern block with attachment to the Antarctic Plate took place 120 Ma ago after extinction of the spreading ridge in the Princess Elizabeth Trough. Because of the structural inhomogeneity in the prebreakup lithosphere, which is represented by a zone of intracontinental Lambert–Mahanadi rifts, opening of the ocean could have developed in a more complex regime. Under the effect of a hot spot, a jump of the spreading axis occurred at the moment of its emplacement into the lithosphere.

The chemistry of Karoo-age andesitic lavas along the northern Mozambique coast, southern Africa and possible implications for Gondwana breakup

The chemistry of Karoo-age andesitic lavas along the northern Mozambique coast, southern Africa and possible implications for Gondwana breakup