ALBERT

Description: Jurassic dykes ( c . 182 Ma) are widespread across the Falkland Islands and exhibit considerable geochemical variability. Orthopyroxene-bearing NW–SE-oriented quartz-tholeiite dykes underwent fractional crystallization at 〉1 GPa, and major element constraints suggest that they were derived by melting of a pyroxenite-rich source. They have Nd 182 in the range –6 to –11 and 87 Sr/ 86 Sr 182 〉0.710 and therefore require an old lithospheric component in their source. A suite of basaltic-andesites and andesites exhibit geochemical compositions transitional between Ferrar and Karoo magma types, and are similar to those seen in the KwaZulu-Natal region of southern Africa and the Theron Mountains of Antarctica. Olivine-phyric intrusions equilibrated at 〈0.5 GPa, and have isotopic compositions (Nd 182 1.6–3.6 and 87 Sr/ 86 Sr 182 0.7036–0.7058) that require limited interaction with old continental lithosphere. A suite of plagioclase-phyric intrusions with 87 Sr/ 86 Sr 182 c . 0.7035 and Nd 182 c . +4, and low Th/Ta and La/Ta ratios ( c . 1 and c . 15, respectively) also largely escaped interaction with the lithosphere. These isotopically depleted intrusions were probably emplaced synchronously with Gondwana fragmentation and the formation of new oceanic lithosphere. Estimates of mantle potential temperature from olivine equilibration temperatures do not provide unequivocal evidence for the presence of a plume thermal anomaly beneath the Falkland Islands at 182 Ma. Supplementary materials: Mineralogical data, XRF major and trace element data and Ar-Ar results for Falkland Islands intrusions are available at http://www.geolsoc.org.uk/sup18873 .

Description: The origin and age of topography along the west Greenland margin is a matter of continued debate. Evidence for tectonically driven Neogene uplift has been argued from interpretations of offshore seismic surveys, onshore fission-track data and inferred episodes of cooling. Here, analysis of seismic reflection profiles and 1D modelling of exploration wells along the Greenland margin of Davis Strait demonstrate that the data are consistent with a model of ancient continental topography affected by late Cretaceous–early Palaeocene rifting followed by thermal subsidence where offshore Neogene tectonic uplift is not required. This interpretation for the offshore evolution of the west Greenland margin has implications for the adjacent onshore evolution and for other continental margins developed throughout the Atlantic–Arctic rift system.

Description: 〈p〉The Colorado River in the SW of the USA is one of Earth's few continental-scale rivers with an active margin delta. Deformation along this transform margin, as well as associated intra-plate strain, has resulted in significant changes in sediment routing from the continental interior and post-depositional translation of older deltaic units. The oldest candidate deposits, fluvial sandstones of the Eocene Sespe Group, are now exposed in the Santa Monica Mountains, 300 km to the north of the Colorado River. Heavy mineral data from this basin indicate that sediment was sourced by a large river system, with some affinity to the early Pliocene Colorado River, but was unlikely to have been integrated across the Colorado Plateau. Sedimentological and mineralogical evidence from the earliest (〈i〉c.〈/i〉 5.3 Ma) unequivocal Colorado River-derived sediments in the Salton Trough provide evidence for a rapid transition from locally derived sedimentation. Lack of evidence for a precursor phase of suspended-load sediment suggests that drainage capture took place in a proximal position, favouring a ‘top-down’ process of lake spillover. Following drainage integration, significant changes in heavy mineral assemblages of fluvio-deltaic sediments, particularly evident from apatite–tourmaline and garnet–zircon indices, as well as U–Pb ages of detrital zircons, document the integration of the fluvial system to its present form and progressive incision of the Colorado Plateau from the Miocene to the present.〈/p〉 〈p〉〈b〉Supplementary material〈/b〉: Heavy mineral data, stable mineral data and U–Pb zircon ages are available at 〈a href="https://doi.org/10.6084/m9.figshare.c.4354814"〉https://doi.org/10.6084/m9.figshare.c.4354814〈/a〉〈/p〉

Description: Cratonic basins comprise a significant component of the Earth's continental crust and surface geology. Their subcircular form and large areas of flat-lying, largely undeformed sedimentary rocks characterize the central regions of many continents, and are also a significant habitat for water, mineral and petroleum resources. These basinal regions have been extensively studied, yet there is little consensus on the driving mechanism of their subsidence or their greater tectonic context. Here we present the results of an integrated basin analysis of the Paleozoic–Early Mesozoic Parnaíba cratonic basin of NE Brazil. The analysis integrates existing geological and geophysical data, and a new deep-crustal geophysical dataset, to determine the deep structure of the basin and the underlying crust and mantle. Several major features have emerged from this which constrain the basins genesis: (1) continental–shallow-marine stratigraphy characterized by an exponentially decreasing tectonic subsidence with a relatively long time constant of the order of 70–90 myr; (2) a complex Proterozoic–Early Paleozoic basement that comprises at least three major crustal blocks defined by seismic facies and conductivity contrasts with no evidence of an extensive rift system beneath the basin; (3) a mid-crustal fabric that appears to define the top of a dense and seismically fast lower crust ( V p 6.7–6.8 km s –1 and V s 3.7–3.8 km s –1 ) and upper mantle ( V p 8.2–8.4 km s –1 ) directly beneath the basin, and which correlates with a sediment-corrected Bouger gravity anomaly high of +40–60 mGal; (4) a Moho that is generally as deep or deeper beneath the basin (40–45 km) than its surrounding region (34–40 km), and which appears stepped at the terrane boundaries; (5) a relatively conductive crust and upper mantle beneath the basin, and relatively resistive crust along the boundaries of the basement blocks; and (6) igneous events immediately before and after formation of the cratonic megasequence and a geochemically enriched mantle beneath the basin that sourced two major episodes of Mesozoic igneous intrusions. These latter events are responsible for the development of an atypical gas-prone petroleum system dependent on local magmatic events for heat generation and trapping configurations. The data describing these features are presented and discussed, and their implications used to draw conclusions about the formation of the Parnaíba Basin specifically and cratonic basins more generally.

Description: This paper uses an extensive dataset from more than 200 samples to provide a comprehensive source-to-sink analysis of the Amur River and its delta in the Russian Far East. The majority of sand-sized sediment in the Amur River and its former delta comes from upstream of the Lesser Khingan Ridge, shown by uniformity of sediment composition in the lower 1700 km of the river. Stable mineral ratios, U–Pb age spectra and garnet geochemistry show little stratigraphic provenance-specific variation in the Neogene delta. This renders Miocene–Pliocene drainage capture models unlikely. The onset of uplift in the delta is marked by a decrease in the apatite–tourmaline index (ATi) in Upper Pliocene offshore well samples, caused by dissolution of apatite as sediments were uplifted and eroded onshore Sakhalin. These wells also show variable ATi and garnet–zircon index (GZi) values in Lower Miocene samples, which could potentially be used for stratigraphic correlation. A positive correlation between GZi values and distance from the river mouth is attributed to hydrodynamic sorting across the delta system. This has negative implications for the use of this stable mineral index and others of a similar hydraulic equivalence as regional correlation tools on a basin scale (〉100 km). Supplementary material: Heavy mineral data, petrographic data, geochronometric data, sample locations available at www.geolsoc.org.uk/SUP18643

Description: This study focuses on the fluvial and tectonic landscape of the North Sakhalin Basin (Russia), where 5 km of Neogene deltaic sediments were deposited across the Okhotsk-Amur plate boundary. The homogeneous, poorly lithified sedimentary sequence created a flat landscape without structural inheritance. These sediments are now being actively deformed by oblique compression. This allows us to investigate the early stages of orogenesis in a strike-slip plate boundary and the response of drainage networks in such a setting, and to construct a model for the topographic evolution along 220 km of the plate boundary. We use fluvial geomorphological indicators (planform morphology, concavity, steepness indices, and knickpoint distribution) as evidence for active landscape deformation. Tectonics and topography are strongly coupled, and neotectonic activity can be observed directly from the landscape. Knickpoints are mostly located on fault planes, suggesting geologically recent activity, or in areas of drainage capture, where they are associated with low concavity indices. Geomorphic indications from longitudinal river profiles and planform morphology suggest that creation of anticlines and disruption of drainage patterns appears to be diachronous in the North Sakhalin Basin, with deformation propagating north and east through time. Minimum uplift and strike-slip displacement rates in the northeast of Sakhalin are 0.63 mm a –1 and 1.95 mm a –1 , based on exhumed stratigraphy and offset drainage networks, respectively.

Description: The intracratonic Paleozoic Parnaíba Basin, NE Brazil covers c. 600 000 km 2 . Lithostratigraphic, mineralogical and geochemical approaches were used to deduce the changing environments of deposition of the Parnaíba Basin based on 150, mostly argillite, samples (Silurian–earliest Triassic). The results indicate that brackish conditions and mixed kerogen sources were important and that the palaeowater depth and sources of meteoric water varied from west to east. The chemical index of alteration results, together with other elemental and mineralogical data, indicate that the western margin was humid until the end of the Carboniferous and then became arid, whereas the eastern margin showed the reverse pattern. This supports observations of major stratigraphic differences between the two margins. There were also major variations in the palaeoredox and palaeosalinity conditions (oxic and fresh to brackish water systems in the east v. suboxic to dysoxic and saline water systems in the west). The clay mineral assemblages are characteristically dominated by kaolinite, which is considered to be inherited from weathering horizons developed in silicic granitic terranes. Regional palaeoclimate reconstructions were carried out in the Parnaíba Basin by linking GPlates software with the South America plate motion model (northwards drift during the Paleozoic coupled with a latest Paleozoic rotation phase) and with the regional database of information on the location, formation name and original references for each locality. Supplementary material: Chemical composition data and geographical coordinates from the 177 outcrops investigated as part of this study: a detailed description of the geochemical and mineralogical analyses with calculation of the CIA value and the GPlates Markup Language File (Animation 1). Supplementary material available online at https://doi.org/10.6084/m9.figshare.c.4100348

Description: The intracratonic Paleozoic Parnaíba Basin, NE Brazil covers c. 600 000 km 2 . Lithostratigraphic, mineralogical and geochemical approaches were used to deduce the changing environments of deposition of the Parnaíba Basin based on 150, mostly argillite, samples (Silurian–earliest Triassic). The results indicate that brackish conditions and mixed kerogen sources were important and that the palaeowater depth and sources of meteoric water varied from west to east. The chemical index of alteration results, together with other elemental and mineralogical data, indicate that the western margin was humid until the end of the Carboniferous and then became arid, whereas the eastern margin showed the reverse pattern. This supports observations of major stratigraphic differences between the two margins. There were also major variations in the palaeoredox and palaeosalinity conditions (oxic and fresh to brackish water systems in the east v. suboxic to dysoxic and saline water systems in the west). The clay mineral assemblages are characteristically dominated by kaolinite, which is considered to be inherited from weathering horizons developed in silicic granitic terranes. Regional palaeoclimate reconstructions were carried out in the Parnaíba Basin by linking GPlates software with the South America plate motion model (northwards drift during the Paleozoic coupled with a latest Paleozoic rotation phase) and with the regional database of information on the location, formation name and original references for each locality. Supplementary material: Chemical composition data and geographical coordinates from the 177 outcrops investigated as part of this study: a detailed description of the geochemical and mineralogical analyses with calculation of the CIA value and the GPlates Markup Language File (Animation 1). Supplementary material available online at https://doi.org/10.6084/m9.figshare.c.4100348

Description: Plate tectonic reconstructions are usually constrained by the correlation of lineaments of surface geology and crustal structures. This procedure is, however, largely dependent on and complicated by assumptions on crustal structure and thinning and the identification of the continent-ocean transition. We identify two geophysically and geometrically similar upper mantle structures in the North Atlantic and suggest that these represent remnants of the same Caledonian collision event. The identification of this structural lineament provides a sub-crustal piercing point and hence a novel opportunity to tie plate tectonic reconstructions. Further, this structure coincides with the location of some major tectonic events of the North Atlantic post-orogenic evolution such as the occurrence of the Iceland Melt Anomaly and the separation of the Jan Mayen microcontinent. We suggest that this inherited orogenic structure played a major role in the control of North Atlantic tectonic processes.