Rift Research Articles

Curie point depth, thermal gradient and heat flow along the Ethiopia Rift System and adjacent plateaus using spectral evaluation approach: implications for geothermal resources

The Ethiopia Rift System (ERS) is a section of the East African Rift System within Ethiopia extending from the Afar in the northeast to the Kenya border in the southwest. It is apparent that magmatism and magmatic intrusions influence the crustal shape in the ERS resulting in its thinning and the shallowing of magmatic sources at various locations within it. As a consequence, more than 31 volcanoes hosting hydrothermal structures with a conceivable potential to generate massive quantities of geothermal energy have been identified along the ERS. In this study, we map the Curie Point Depth (CDP) over the ERS based on the analysis of aeromagnetic data extracted from the World Digital Magnetic Anomaly Map. Spectral evaluation method was used to estimate the boundaries (top and bottom) of the magnetized crust. Reduced-to-pole (RTP) aeromagnetic records have been divided into 105 (50% overlap) square blocks of 200 × 200 km size. The Curie temperature (580 °C) of magnetite was used to determine the thermal gradient and the heat drift in the area. The depths obtained for the bottom of the magnetized crust are assumed to correspond to the Curie Depths, where the magnetic layer loses all its magnetization. The determined values of Curie Point Depth, geothermal gradient and heat flow for the 50% overlapped 105 blocks, respectively, range from 8.85 to 55.85 km, 10.38 to 65.54 °C/km and 25.96 to 163.84 mW/m2. Lower CPD (< 20 km) in the ERS was obtained between Mille and Gewane (southwest Afar), between Adama (Nazret) and Yerer (NMER) and between Wendo Genet and Koti (SMER) localities. These areas, showing low CPD, exhibit excessive geothermal gradient and high heat flow all of which indicate the presence of significant geothermal potential.

Zircon trace element fingerprint of changing tectonic regimes in Permian rhyolites from the Central European Lowlands

AbstractThe late Carboniferous/early Permian post-collisional rhyolites (305–285 Ma) that formed in Central Europe have generally similar whole rock compositions to that of older Late-Variscan rhyolites (330–310 Ma). However, data compilation combining zircon age with the chemical composition of rhyolites from 20 units shows a trend of increasing zircon saturation temperature with decreasing age. This trend is particularly well identified in rhyolites from the Central European Lowlands (CEL)—consisting of the NE German and NW Polish Basin—and also correlates their location with the zircon saturation temperature increasing from SE to NW from 750°C to 850°C. We infer that these higher temperatures of zircon saturation reflect a contemporaneous change in the tectonic setting from collisional to divergent, reflecting the onset of the Central European continental rifting. This interpretation is further corroborated by the trace element compositions of the CEL zircons, which resembles zircon crystallized in a divergent setting. Interestingly, the zircon formed globally in this type of setting is chemically diverse, especially considering uranium concentration. For example, zircon from locations dominated by mafic magma fractionation, such as rhyolites from Iceland, have low U concentrations and low U/Yb ratios. On the other hand, zircon formed in rhyolites in rifted margins, like western North America, tends to have much higher U and U/Yb ratios. Such high concentrations are not observed in zircon from the CEL, suggesting that the mantle input could be higher and residence times within continental crust shorter than those for rhyolites from the Cenozoic western USA. This may, in turn, suggest that the region might have been affected by a hot spot, similar to that responsible for rhyolite formation of the Snake River Plain. Graphical abstract

Transferability of Machine Learning Models for Geogenic Contaminated Groundwaters.

Machine learning models show promise in identifying geogenic contaminated groundwaters. Modeling in regions with no or limited samples is challenging due to the need for large training sets. One potential solution is transferring existing models to such regions. This study explores the transferability of high fluoride groundwater models between basins in the Shanxi Rift System, considering six factors, including modeling methods, predictor types, data size, sample/predictor ratio (SPR), predictor range, and data informing. Results show that transferability is achieved only when model predictors are based on hydrochemical parameters rather than surface parameters. Data informing, i.e., adding samples from challenging regions to the training set, further enhances the transferability. Stepwise regression shows that hydrochemical predictors and data informing significantly improve transferability, while data size, SPR, and predictor range have no significant effects. Additionally, despite their stronger nonlinear capabilities, random forests and artificial neural networks do not necessarily surpass logistic regression in transferability. Lastly, we utilize the t-SNE algorithm to generate low-dimensional representations of data from different basins and compare these representations to elucidate the critical role of predictor types in transferability.

Extensive palaeo-surfaces beneath the Evans–Rutford region of the West Antarctic Ice Sheet control modern and past ice flow

Abstract. The subglacial landscape of Antarctica records and influences the behaviour of its overlying ice sheet. However, in many places, the evolution of the landscape and its control on ice sheet behaviour have not been investigated in detail. Using recently released radio-echo sounding data, we investigate the subglacial landscape of the Evans–Rutford region of West Antarctica. Following quantitative analysis of the landscape morphology under ice-loaded and ice-unloaded conditions, we identify 10 flat surfaces distributed across the region. Across these 10 surfaces, we identify two distinct populations based on clustering of elevations, which potentially represent remnants of regionally coherent pre-glacial surfaces underlying the West Antarctic Ice Sheet (WAIS). The surfaces are bounded by deeply incised glacial troughs, some of which have potential tectonic controls. We assess two hypotheses for the evolution of the regional landscape: (1) passive-margin evolution associated with the break-up of the Gondwana supercontinent or (2) an extensive planation surface that may have been uplifted in association with either the West Antarctic Rift System or cessation of subduction at the base of the Antarctic Peninsula. We suggest that passive-margin evolution is the most likely of these two mechanisms, with the erosion of glacial troughs adjacent to, and incising, the flat surfaces likely having coincided with the growth of the WAIS. These flat surfaces also demonstrate similarities to other identified surfaces, indicating that a similar formational process may have been acting more widely around the Weddell Sea embayment. The subsequent fluctuations of ice flow, basal thermal regime, and erosion patterns of the WAIS are therefore controlled by the regional tectonic structures.

The Cretaceous World: Plate Tectonics, Paleogeography, and Paleoclimate

Abstract The tectonics, geography, and climate of the Cretaceous world was a very different from the modern world. At the start of the Cretaceous, the supercontinent of Pangea had just begun to break apart and only a few small ocean basins separated Laurasia, West Gondwana, and East Gondwana. Unlike the modern world, there were no significant continent-continent collisions during the Cretaceous and the continents were low-lying and easily flooded. The transition from a Pangea-like configuration to a more dispersed continental arrangement had important effects on global sea level and climate. During the Early Cretaceous, as the continents rifted apart, the new continental rifts were transformed into young ocean basins. The oceanic lithosphere in these young ocean basins was thermally elevated, which boosted sea level. Sea level, on average, was ∼70 m higher than the present-day. Sea level was highest during the mid-Cretaceous (90 Ma – 80 Ma), with a subsidiary peak ∼ 120 million years ago (early Aptian). Overall, the Cretaceous was much warmer than the present-day (&gt; 10˚C warmer). These very warm times produced oceanic anoxic events (OAEs) and high temperatures in equatorial regions sometimes made terrestrial and shallow marine ecosystems uninhabitable (temperatures &gt; 40˚C). This is unlike anything we have seen in the last 35 million years and may presage the eventual results of man-made global warming. This mostly stable, hot climate regime endured for nearly 80 million years before dramatically terminating with the Chicxulub bolide impact 66 million years ago. Temperatures plummeted to icehouse levels in the “impact winter” resulting from sunlight-absorbing dust and aerosols. As a consequence of the collapse of the food chain, ∼75% of all species were wiped out (Sepkoski, 1996). The effect of this extinction event on global ecosystems was second only to the great Permo-Triassic Extinction (McGhee et al., 2013).

2‐D Geodynamic Modeling of the Central South Atlantic Wide Rifted Margins, Implications for Evaporite Deposition

AbstractThe thick late syn‐ to early post‐rift shallow water evaporites in the most distal part of wide rifted margins is paradoxical with the deep depression at crustal breakup time predicted by isostatically compensated lithospheric thinning. Elevation of the distal margin and water depth during deposition of the late syn‐rift evaporites in the central South Atlantic are not well constrained and remain to be quantified. We use forward 2‐D thermo‐mechanical modeling coupled with melt prediction and surface processes to assess the contribution of lithospheric and mantle processes on the distal margin topography and subsidence history during continental rifting. Models show that (a) counter‐flow of depleted lower lithospheric mantle during rifting explains the magma‐poor nature of these margins and (b) weak crust and syn‐rift sediment control the wide crustal necking and subsidence history of the distal margin. Integration of our modeling results with quantified geophysical and geological observations suggests that (a) base level was down to −600 m below present‐day global sea level (bsl) during distal margin formation in the Aptian before sag and evaporite deposition, (b) base level was about −300/−400 m bsl at the end of evaporite deposition, and (c) scenarios with a fixed shallow base level (−400 m bsl) or with an increasing base level from an initially deep position (−1,600 m bsl) during evaporite deposition can both fit the observed evaporite distribution. However, erosional features along the base of evaporites suggest a deep initial base level.

Tectono-metamorphic interaction of upper mantle peridotites and lower crustal units during continental rifting in the western Betic Cordillera

Recent geological mapping conducted in the Ronda Peridotites (Betic Cordillera, S Spain) has revealed a systematic field correlation between lower crustal metamorphic units and different tectono-metamorphic domains of the ultramafic massif. Mylonitic and highly tectonized Spl ± Grt peridotites (i.e., Grt-Spl mylonite and Spl tectonite domains), which are considered to be derived from a thick continental lithosphere, are in contact with garnet-bearing gneisses (i.e., kinzigites) of the Jubrique unit along a narrow but continuous mylonitic shear zone. Phase equilibrium calculations indicate that the metamorphic rocks of the Jubrique unit are consistent with an initial continental setting characterized by normal crustal thicknesses, which underwent two melting events. The first melting occurred at 0.9–1.0 GPa / 770–800 °C and resulted in 13 %melt, while the second one took place at shallower crustal conditions (0.4–0.5 GPa and 710–765 °C) and led to more restricted melt production (2–3 %melt). In contrast, the Spl ± Pl peridotites (Pl-tectonite domain), which are stable only at shallowest mantle levels within a highly extended continental lithosphere, are consistently found exposed in contact with heterogeneous granites and migmatites of the Guadaiza unit. According to new thermodynamic modeling, the Guadaiza metamorphic rocks record a single melting event characterized by a theoretical melt production of 6 to 11 % at the base of a very thin continental crust (ca. 0.3 GPa and 675–710 °C). This process was likely facilitated by influx of external water, necessary to generate high melt fractions observed in the diatexites. The systematic correlation observed between crustal metamorphic units, consistently overlaying the mantle rocks, and specific ultramafic domains of the Ronda massif, suggests that their juxtaposition primarily resulted from the severe extension of the continental lithosphere.U-Pb radiometric dating of zircons from gneisses, migmatites, and heterogeneous granites in the middle crustal rocks of the Guadaiza unit indicates that extensional processes, crustal anatexis, and melt stagnation occurred at around 280 Ma. Comparison of these new radiometric ages with previous results from the Jubrique unit suggests that a Permian high-temperature / low- to medium-pressure event uniformly affected the crustal units over the Ronda Peridotites. This event coincided with the formation of characteristic ultramafic mineral assemblages within the Ronda massif and provides evidence for the interaction between upper mantle rocks and lower- to mid-crustal metamorphic rocks during this period.

New insights into the origin and depositional setting of the Haerdaban Pb[sbnd]Zn deposit, Chinese western Tianshan: Evidence from geology, chert geochemistry, and detrital zircon U[sbnd]Pb geochronology

The Haerdaban PbZn deposit (with an ore reserve of 10.93 Mt. at 1.0–25.65 % Zn and 0.7–12.29 % Pb) is hosted in weakly metamorphosed clastic‑carbonate rocks from the Proterozoic Haerdaban Group. It represents a significant addition of the sediment-hosted PbZn deposits in the Yili block, Chinese western Tianshan. Currently, there are ongoing debates regarding its genesis, with a particular focus on the crucial metallogenic mechanism (syngenetic sedimentary exhalation or epigenetic reworking) responsible for the primary sulfide mineralization. Mineralization at Haerdaban primarily occurs as banded to stratiform ore layers or lenses conformably sandwiched in their host rocks. Vein and stockwork ores occur locally below the stratiform ore layers. A syn-sedimentary fault trending SN was identified based on abrupt lateral changes in lithofacies and thickness of the stratigraphic units. The ore mineralogy is dominated by sphalerite, galena, quartz, and dolomite, with a small amount of pyrite, barite, and organic matter. Detrital zircon LA-ICP-MS UPb dating of the Haerdaban siltstones obtained a maximum depositional age of about 604 Ma. Their geochemical composition similar to the passive continental margin signatures, with rare earth element (REE) patterns enriched in LREE and negative Eu anomalies (Eu/Eu* = 0.50–1.14). Stratiform beds of chert that host disseminated ores have relatively high contents of hydrothermal components (e.g., Ba, Zn), with apparent positive Eu anomalies (Eu/Eu* = 7.38–49.34) and negligible negative Ce anomalies (Ce/Ce* = 0.85–0.98). They are thus interpreted to be hydrothermal sedimentary rocks (exhalites) deposited in a suboxic-anoxic environment proximal to the hydrothermal vents. Integrated geological and geochemical evidence indicates that the Haerdaban PbZn deposit is a typical vent-proximal sedimentary exhalative (SEDEX) deposit formed in a Neoproterozoic Sinian (Ediacaran) passive continental margin rift basin. Post-depositional metamorphism and deformation in the Paleozoic may have caused partial remobilization of primary ores but did not significantly alter the morphology of the orebodies. Furthermore, establishing a genetic model for the Haerdaban deposit has important implications for the exploration of similar deposits preserved in the equivalent stratigraphy within the Chinese western Tianshan region.

Petrogenesis and Tectonic Setting of the Baluogenguole Mafic Dykes, Zongwulong Belt: Implications for Evolution of the Northern East Paleo-Tethys Ocean

The late Paleozoic tectonic setting of the Zongwulong Belt (ZWLB), a significant unit located in the northern Qaidam margin, Qinghai province, remains uncertain. Diabase dykes in the western part of the Zongwulong Belt offer insights into this issue. Field investigations reveal that the dips of the dykes are almost vertical, and they have sharp boundaries with the host rocks. These dykes consist of plagioclase, clinopyroxene, and opaque minerals exhibiting a characteristic porphyritic texture and massive structure. Zircon U-Pb dating of the dykes yields a weighted 206Pb/238U age of 289 ± 1 Ma. The dykes exhibit relatively high concentrations of TFeO, K2O + Na2O, and TiO2, while the SiO2 and MgO concentrations are relatively low. They display relative light-over-heavy REE enrichment, and lack negative Nb-Ta and Eu anomalies. The dykes underwent negligible crustal contamination, and experienced extensive fractional crystallization of olivine, clinopyroxene, and Fe-Ti oxides. Originating from the spinel–garnet transition zone at depths of approximately 75 km, the dykes result from garnet facies low-degree melting (5%–10%) in a continental rift setting. Combining these findings with regional geological data, we propose that the ZWLB likely experienced a continental rift in the west and exhibited a narrow oceanic environment in the east in the late Paleozoic period, potentially representing the most distant north branch of the East Paleo-Tethys Ocean.

Nundorite: a geological oddity of critical minerals significance from the Mount Arrowsmith Volcanics, northwestern NSW

Nundorite is a unique and enigmatic rock type, known only from its type locality within the Koonenberry Belt of western NSW. The rock is highly anomalous in geochemical composition being peralkaline, with low silica and high alumina, but enriched in rare metals such as Zr, Nb and rare earth elements. This study presents a comprehensive mineralogical and geochemical study of nundorite to resolve its origins and its potential to inform on the geological processes that lead to critical metal concentration in the crust. Nundorite is found only at a single outcrop but shows a progression of alteration and deformation over ∼250 m from the northwest to southeast. The least deformed varieties consist of coarse aegirine grains within a matrix composed predominantly of natrolite and microcline with minor albite, nepheline and schizolite. With increasing deformation, the proportion of albite and aegirine decreases, the primary zeolite changes from natrolite to analcime, and the pyroxene and schizolite shift towards the aegirine and serandite (Mn-rich) endmember compositions, respectively. These transitions can be related to progressive hydration and recrystallisation of the rock in response to hydrothermal alteration associated with deformation. Nundorite also contains a diverse suite of accessory rare metal phases such as zircon, monazite, cerite-group minerals, fergusonite, fersmite, natroniobite, galgenbergite, epidote–allanite and a range of Zr alkali silicate minerals. Samarium–Nd isotopic compositions of nundorite (εNd ∼+5.5 at 585 Ma) are comparable with mafic volcanic rocks of the surrounding Mount Arrowsmith Volcanics and are consistent with derivation from a mantle source. These data, together with the uniform peralkaline composition, depletion in Ti and P, and enrichment in incompatible elements, are consistent with an origin as highly fractionated phonolite. Phonolites of similar composition are rare but have been reported from continental rift and intraplate alkaline provinces worldwide. Nundorite, therefore is not just a geological oddity, but also provides reaffirmation of the continental rift setting for the Mount Arrowsmith Volcanics and highlights the critical mineral resource potential of the region, as extensive crystal fractionation is a key mechanism to produce ore grade levels of rare metals in alkaline magmatic systems.

Neoproterozoic (ca. 830 Ma) carbonatite dykes from Qianlishan in the western North China Craton: Petrogenesis and metallogenic implications

Carbonatite and the associated alkaline intrusive complexes are the primary sources of light rare earth element (REEs). Our field investigations in the western margin of the North China Craton (NCC) have firstly identified over 20 carbonatite dykes and minor coeval carbonate-rich lamprophyre dykes intruding the Paleoproterozoic Qianlishan Group in the southern Qianlishan area. Zircon LA-ICP-MS U-Pb dating on six carbonatite samples yielded crystallization ages ranging from 835.2 ± 3.7 Ma to 828.9 ± 3.8 Ma, indicating emplacement of the carbonatite dykes at ca. 830 Ma during the Neoproterozoic. Geochemical data show that most of the carbonatite dykes are rich in light REEs with total light REEs > 600 ppm to 4932 ppm, and the carbonate-rich lamprophyre dykes are characterized by high Cr (1219–1267 ppm) and Ni concentrations (857–1076 ppm). Petrological and scanning electron microprobe (SEM) analytical results show that the dominant REE-bearing minerals in the carbonatites are monazite, and other REE-bearing minerals are parisite and bastnäsite. Geochemical and carbon–oxygen isotopic results indicate that the ca. 830 Ma carbonatite dykes were produced by low-degree partial melting of a previously metasomatised subcontinental lithospheric mantle with assimilation of crustal material. Emplacement of the ca. 830 Ma carbonatite and carbonate-rich lamprophyre dykes in the southern Qianlishan area and the previously reported ca. 810 Ma mafic dykes in the middle Qianlishan area occurred in a continental rift environment. Simultaneous development of the Neoproterozoic (830 − 810 Ma) continental rifting events in the western margin of the NCC and the eastern margin of the Alxa Block as previously suggested may indicate a connection between the eastern Alxa Block and the western NCC during and before the middle Neoproterozoic.

Magnitudes and surface rupture lengths of paleo-earthquakes at the NW-part of the Peel Boundary fault zone, Roer Valley Rift System

The Peel Boundary Fault zone (PBFZ) is the 125 km long, seismically active, northern bounding fault zone of the Roer Valley Rift System (RVRS). The last damaging earthquake along the PBFZ was the Roermond earthquake of 1992. It had a magnitude of Mw 5.3 and no surface rupture. Previous results from two trenching studies located in the central and southeastern parts of the PBFZ provided evidence for two surface rupturing paleo-earthquakes. The largest earthquake had an estimated magnitude of Mw ∼6.8 and a surface rupture length of at least 35 km. As it took place around the Late Pleniglacial – Late Glacial transition a link to glacio-isostatic motions is likely. Results from a new trench situated at the northwestern part of the PBFZ shows evidence for three to four paleo-earthquakes, of which three were surface rupturing. These comprise two normal faulting- and one, younger trans-tensional displacement. The normal faulting events have ∼1 m vertical displacements each, which translate into magnitudes of Mw ∼7. Like the previous results, they occurred during the Late Pleniglacial-Late Glacial transition, at ∼15 ka and ∼ 14 ka. The younger trans-tensional event occurred sometime during the Holocene, pre-dating an unaffected, 13th century man-made paleo-channel on the hangingwall. The potential fourth, non-surface rupturing earthquake is indirectly evidenced by loading deformations of a sand layer and a collapsed brick-wall in the infill of the paleo-channel. Comparison of our trenching results to those from the two previous studies, which were located farther to the southeast along the PBFZ, shows that for one event a correlation is possible. The correlation would indicate a surface rupture length of at least 55 km. Combined, all trenching results indicate that the characteristic maximum rupturing displacement is ∼1 m, and thus that Mw ∼7 is the maximum magnitude of paleo-earthquakes along the PBFZ.

U‐Pb carbonate dating reveals long‐lived activity of proximal margin extensional faults during the Alpine Tethys rifting

AbstractSyn‐rift extensional faults play a significant role during the early stage of rifting. Constraining the age of faulting, and how and when deformation shifts from the proximal to the distal margin areas, is crucial for the reconstruction of the rifting process. Previous assessments of the structural and temporal evolution of rift‐related faults of the Adria proximal margin have primarily relied on indirect biostratigraphic evidence. Additionally, the majority of rift‐related faults underwent tectonic inversion during the Alpine orogeny. In this study, in‐situ U‐Pb geochronology was applied on syn‐kinematic calcites to unravel the activity of the Amora Fault, a remarkable example of a Jurassic growth fault unaffected by the Alpine orogeny. The obtained ages, spanning from Hettangian to Callovian, extend the AF activity beyond the previously established Early Jurassic time. This chrono‐structural model has significant implications on the role of major extensional faults in focusing deformation throughout the rift system's evolution.

The North Sea rift basement records extensional collapse of the Caledonian orogen

Our knowledge of crystalline basement rocks flooring continental rift basins, rifted margin deposits, and foreland basins is poor, yet important for understanding basin evolution, crustal heat production, petroleum maturation, and for offshore mineral exploration. Using the northern North Sea rift as an example, we demonstrate how modern broadband seismic data can be utilized to unravel basement lithology and structure at a previously unprecedented level of detail. In our case study, we have discovered a pre-rift Caledonian basement with a folded structural pattern very similar to Devonian collapse-related structures onshore Western Norway. The data indicate that the collapse-generated Caledonian infrastructure extends >100 km into the northern North Sea, defining a wide zone of Caledonian crust that turned hot and mobilized vertically by upright folding and detachment faulting during the post-collisional stage. This extensive post-orogenic thermally induced collapse makes the south Scandinavian Caledonides unique among Phanerozoic orogens.

A Multi-Faceted Approach to Determining the Provenance of the Lacustrine Rift Basin in the Initial Rifting Stage: A Case Study of the Paleocene Qintong Sag, Subei Basin, East China

A Multi-Faceted Approach to Determining the Provenance of the Lacustrine Rift Basin in the Initial Rifting Stage: A Case Study of the Paleocene Qintong Sag, Subei Basin, East China

Development of fluvial‐lacustrine systems in the rift basins fostered Jehol Biota origin: Palaeobiogeographic insights on Dabeigou‐type ostracod fauna

The Lower Cretaceous Dabeigou Formation (135–130 Ma) in the Luanping Basin, associated with its contemporary strata in northeastern China, contains the earliest documented fossils of the Jehol Biota. This includes the Luanpingella—Eoparacypris—Ocrocypris ostracod assemblage, referred to in this study as the Dabeigou‐type ostracod fauna (DOF). Ostracods, which are small (0.5–2 mm) aquatic crustaceans, are abundant and easily preserved as fossils. Outlining a clear palaeobiogeographic range of the DOF can help expand the distribution range of the early Jehol Biota, providing valuable information on its origin. Results show that the DOF is mainly distributed in a series of rift basins along the Yanliao area of China to the eastern Transbaikal region of Russia, with a roughly narrow north‐south distribution. Rift basin development in the eastern part of the North China Craton, combined with humid climatic conditions, provided a suitable environment (e.g., fluvial‐lacustrine systems) for the emergence of the Jehol Biota.

Middle Jurassic to Early Cretaceous orogenesis in the Klamath Mountains Province (Northern California–southern Oregon, USA) occurred by tectonic switching: Insights from detrital zircon U-Pb geochronology of the Condrey Mountain schist

The Klamath Mountains Province of Northern California and southern Oregon, USA, consists of generally east-dipping terranes assembled via Paleozoic to Mesozoic subduction along the western margin of North America. The Klamath Mountains Province more than doubled in mass from Middle Jurassic to Early Cretaceous time, due to alternating episodes of extension (e.g., rifting and formation of the Josephine ophiolite) and shortening (e.g., Siskiyou and Nevadan events). However, the tectonic mechanisms driving this profound Mesozoic growth of the Klamath Mountains Province are poorly understood. In this paper, we show that formation of the Condrey Mountain schist (CMS) of the central Klamath Mountains Province spanned this critical time period and use the archive contained within the CMS as a key to deciphering the Mesozoic tectonics of the Klamath Mountains Province. Igneous samples from the outer CMS subunit yield U-Pb zircon ages of ca. 175–170 Ma, which reflect volcanic protolith eruptive timing. One detrital sample from the same subunit contains abundant (~54% of zircon grains analyzed) Middle Jurassic ages with Paleozoic and Proterozoic grains comprising the remainder and yields a maximum depositional age (MDA) of ca. 170 Ma. These ages, in the context of lithologic and thermochronologic relations, suggest that outer CMS protoliths accumulated in an outboard rift basin and subsequently underthrust the Klamath Mountains Province during the Late Jurassic Nevadan orogeny. Five samples of the chiefly metasedimentary inner CMS yield MDAs ranging from 160 Ma to 130 Ma, with younger ages corresponding to deeper structural levels. Such inverted age zonation is common in subduction complexes and, considering existing K-Ar ages, suggests that the inner CMS was assembled by progressive underplating over a &amp;gt;10 m.y. timespan. Despite this age zonation, age spectra derived from structurally shallow and deep portions of the inner CMS closely overlap those derived from the oldest section of the Franciscan subduction complex (South Fork Mountain schist). These relations suggest that the inner CMS is a composite of South Fork Mountain schist slices that were sequentially underplated beneath the Klamath Mountains Province. The age, inboard position, and structural position (i.e., the CMS resides directly beneath Jurassic arc assemblages with no intervening mantle) of the CMS suggest that these rocks were emplaced during one or more previously unrecognized episodes of shallow-angle subduction restricted to the Klamath Mountains Province. Furthermore, emplacement of the deepest portions of the CMS corresponds with the ca. 136 Ma termination of magmatism in the Klamath Mountains Province, which we relate to the disruption of asthenospheric flow during slab shallowing. The timing of shallow-angle subduction shortly precedes that of the westward translation of the Klamath Mountains Province relative to correlative rocks in the northern Sierra Nevada Range, which suggests that subduction dynamics were responsible for relocating the Klamath Mountains Province from the arc to the forearc. In aggregate, the above relations require at least three distinct phases of extension and/or rifting, each followed by an episode of shallow-angle underthrusting. The dynamic upper-plate deformation envisioned here is best interpreted in the context of tectonic switching, whereby slab steepening and trench retreat alternate with slab shallowing due to recurrent subduction of buoyant oceanic features.

Linking the terrestrial environmental record at Mason Spur volcanic complex with the middle Miocene−Pleistocene Ross Sea marine record, Antarctica: A history of subaerial (ice-free) eruptions and glaciovolcanism under variable ice thicknesses

Research into volcanism in southern Victoria Land, Antarctica, within the West Antarctic rift system, has focussed historically on the geochemistry and chronology of its volcanic centers. However, volcanoes in the West Antarctic rift system have also been dramatically influenced by the prevailing eruptive environment. Mason Spur is a middle Miocene to Pleistocene volcanic complex in the Erebus Volcanic Province. The deep interior of the complex is revealed and is used to assess its development under different environmental conditions. Many of the volcanic sequences erupted within ice and are thus ultraproximal compared with marine sediments, which are the usual source of environmental information. The volcanic rocks provide a unique, well-dated record of the terrestrial environmental conditions independent of the marine record, to which they are an important counterbalance. Evidence is provided for conditions varying between ice-free, alpine ice, and regional ice sheets, and direct comparisons are made with the marine record. The first contemporary ice thicknesses are also deduced for the Erebus Volcanic Province. The results significantly advance our understanding of the middle Miocene−Pleistocene Antarctic environment in the coastal Victoria Land region.

Unfolding rotational tectonics and topographic evolution from localized verses diffuse plate boundary counterparts

We present a kinematic model developed from geodetic observations, topography analysis and analogue tectonic modelling results, which reveals a striking similarity between the rotational tectonic settings of the Gakkel Ridge-Chersky Range system in the Arctic, and the Central Indian Tectonic Zone within the Indian subcontinent. A crucial aspect of large-scale extensional rift systems is the gradual variation of extension along the rift axis, due to plate rotation about a Euler pole, which may lead to contraction on the opposite side of the Euler pole to form a rotational tectonic system. Our geodetic and topographic analysis, combined with the reanalysis of analogue tectonic modelling results demonstrates such rotational tectonic plate motion in both the Arctic and Indian case. However, the plate boundary between the North American and Eurasian Plates as represented by the Arctic Gakkel Ridge-Chersky Range system is strongly localized, whereas the Central Indian Tectonic Zone that separates the North and South India Plates involves diffuse deformation instead. Furthermore, in both the Arctic and Central Indian we find that the relative Euler rotation pole is located near an indenter-like feature, which possibly controls the present-day rotational tectonics and contrasting topography on opposite sides of the Euler pole.

Facies architecture, geochemistry and petrogenesis of Middle Triassic volcaniclastic deposits of Mt. Ivanščica (NW Croatia): evidence of bimodal volcanism in the Alpine-Dinaridic transitional zone

During the Middle Triassic, intensive volcanic activity took place along the eastern margin of Pangea, including the Greater Adria promontory, due to the Neotethyan oceanization. This resulted in the formation of various volcanic and volcaniclastic rock types. The region of NW Croatia, acting as a transition zone between the Southern Alps and the Dinarides, showcases the outcrops of these rocks. The present study investigates the facies of volcaniclastic rocks, the distribution of those facies, formation processes, as well as the genesis of the primary magma to gain a better understanding of the complex geodynamics of this region during the Middle Triassic. Six profiles across the Vudelja quarry front were surveyed using drone imaging and samples were collected for detailed petrographic and geochemical analyses. Two groups of volcaniclastic rocks were identified—mafic and intermediate/felsic. The former is represented by (I) autoclastic effusive facies and (II) resedimented autoclastic facies, while the latter is represented by (III) secondary pyroclastic facies. Mafic volcaniclastics were generated through basaltic effusions in marine environments, fragmentation in contact with seawater, mixing with shallow marine carbonate clasts, and subsequent redeposition in deeper marine areas. The secondary pyroclastic facies (III) consists of a regionally distributed felsic Pietra Verde tuff whose deposits may be related to pyroclastic density currents and syn-eruptive resedimentation by turbidite-like currents. Geochemical data indicate that parental magmas responsible for generating the mafic volcaniclastics had a calc-alkaline composition and originated in ensialic and mature arc settings of an active continental margin. The observed chemical composition is likely inherited from older, arc-related lithologies, associated with the subduction of the Paleotethys Ocean. Parental magmas are thought to have formed during continental rifting of the passive Middle Triassic margins of the Greater Adria through (i) partial melting of the heterogeneous lithospheric mantle, which had been metasomatized during an earlier Hercynian subduction, and (ii) subordinate processes related to the melting of the upper continental crust and subsequent fractionation. Ar/Ar dating on plagioclase separates yielded an age of 244.5 ± 2.8 Ma for mafic volcaniclastics. This aligns well with biostratigraphic ages of felsic tuffs which crop out on a broader regional scale of the Dinarides, the Southern Alps, and the Transdanubian Range. The overlapping ages obtained from radiometric dating of mafic volcaniclastics and biostratigraphic ages of the felsic Pietra Verde tuffs strongly suggest that the Greater Adria region experienced concurrent bimodal volcanism during the Middle Triassic.