ALBERT

Description: Gulf of Mexico Loop Current (LC) interactions with the West Florida Shelf (WFS) slope play an important role in shelf ecology through the upwelling of new inorganic nutrients across the shelf break. This is particularly the case when the LC impinges upon the shelf slope in the southwest portion of the WFS near the Dry Tortugas. By contacting shallow water isobaths at this “pressure point” the LC forcing sets the entire shelf into motion. Characteristic patterns of LC interactions with the WFS and their occurrences are identified using unsupervised neural network, Self-Organizing Map, from 23 years (1993 – 2015) of altimetry data. The duration of the occurrences of such LC patterns is used as an indicator of offshore forcing of anomalous upwelling. Consistency is found between the altimetry-derived offshore forcing and the occurrence and severity of WFS coastal blooms of the toxic dinoflagellate, Karenia brevis : years without major blooms tend to have prolonged LC contact at the “pressure point,” whereas years with major blooms tend not to have prolonged offshore forcing. Resetting the nutrient state of the shelf by the coastal ocean circulation in response to deep-ocean forcing demonstrates the importance of physical oceanography in shelf ecology. A satellite altimetry-derived seasonal predictor for major K. brevis blooms is also proposed. This article is protected by copyright. All rights reserved.

Description: Abstract The Dras Arc in NW India Himalaya is a belt of basaltic‐andesites intercalated with arkose‐dominated volcaniclastic rocks of the Nindam Formation situated along the Indus Suture between India and Eurasia. Debates exist as to whether these rocks developed in a forearc basin to the Eurasian margin or as part of an intraoceanic island arc system that collided with either India or Eurasia before final continental collision. Detrital zircons from the Nindam Formation yield U‐Pb age spectra with dominant youngest age populations of ~84–125 Ma, corresponding with arc magmatism. Sandstone provenance analysis from the Nindam Formation indicates that the Dras Arc evolved from an undissected arc to dissected arc over a period of ~41 Myr. Slightly older, smaller populations occur at ~135–185 Ma, corresponding with reported ages of Neotethyan ophiolites (e.g., Spongtang). The basal section of the Nindam Formation reveals the presence of arc‐derived basaltic‐andesite and tonalite clasts, plus ophiolitic components sourced from an adjacent accretionary complex. There is a distinct absence of quartz or felsic granitic clasts, suggesting that the Nindam Formation did not develop as a forearc basin to the Ladakh Batholith of southern Eurasia but rather as separate intraoceanic island arc. A distinct “Gondwanan” signature occurs in all samples, with zircon age peaks at ~514–988, ~1000–1588, ~1627–2444, and ~2500 Ma. We suggest that the Dras and Spong arcs are the same intraoceanic island arc system that developed as a result of subduction initiation along NNE‐SSW transform faults perpendicular to the Indian and Eurasia continents.

Description: Abstract The Porcupine Basin is a Mesozoic failed rift located in the North Atlantic margin, SW of Ireland, in which a postrift phase of extensional faulting and reactivation of synrift faults occurred during the Mid–Late Eocene. Fault zones are known to act as either conduits or barriers for fluid flow and to contribute to overpressure. Yet, little is known about the distribution of fluids and their relation to the tectono‐stratigraphic architecture of the Porcupine Basin. One way to tackle this aspect is by assessing seismic (Vp) and petrophysical (e.g., porosity) properties of the basin stratigraphy. Here, we use for the first time in the Porcupine Basin 10‐km‐long‐streamer data to perform traveltime tomography of first arrivals and retrieve the 2D Vp structure of the postrift sequence along a ~130‐km‐long EW profile across the northern Porcupine Basin. A new Vp–density relationship is derived from the exploration wells tied to the seismic line to estimate density and bulk porosity of the Cenozoic postrift sequence from the tomographic result. The Vp model covers the shallowest 4 km of the basin and reveals a steeper vertical velocity gradient in the centre of the basin than in the flanks. This variation together with a relatively thick Neogene and Quaternary sediment accumulation in the centre of the basin suggests higher overburden pressure and compaction compared to the margins, implying fluid flow towards the edges of the basin driven by differential compaction. The Vp model also reveals two prominent subvertical low‐velocity bodies on the western margin of the basin. The tomographic model in combination with the time‐migrated seismic section shows that whereas the first anomaly spatially coincides with the western basin‐bounding fault, the second body occurs within the hangingwall of the fault, where no major faulting is observed. Porosity estimates suggest that this latter anomaly indicates pore overpressure of sandier Early–Mid Eocene units. Lithological well control together with fault displacement analysis suggests that the western basin‐bounding fault can act as a hydraulic barrier for fluids migrating from the centre of the basin towards its flanks, favouring fluid compartmentalization and overpressure of sandier units of its hangingwall.

Description: We describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host Littleton Schist near Claremont, New Hampshire. Here, Na-poor muscovite and chlorite progressively replace first staurolite, then garnet, and finally biotite porphyroblasts as the core of the fault zone in approached. Across the transect, higher-grade fabric-forming Na-rich muscovite is also progressively replaced by fabric forming Na-poor muscovite. The mineralogy of the new phyllonitic fault-rock produced is dominated by Na-poor muscovite and chlorite together with late albite porphyroblasts. The replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement. In contrast, element mapping shows that fabric-forming Na-rich muscovite is selectively replaced at high-strain microstructural sites, indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite. The replacement of strong, high-grade porphyroblasts by weaker Na-poor muscovite and chlorite constitutes reaction softening. The crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated Na-rich muscovite-defined foliation destroys not only the metastable high-grade mineralogy, but also its stronger geometry. This process constitutes both reaction and microstructural softening. The deformation mechanism here was thus one of dissolution-precipitation creep, activated at considerably lower stresses than might be predicted in quartzo-feldspathic rocks at the same lower greenschist facies conditions. This article is protected by copyright. All rights reserved.

Description: Abstract The Celtic Sea basins lie on the continental shelf between Ireland and northwest France and consist of a series of ENE ‐ WSW trending elongate basins that extend from St George's Channel Basin in the east to the Fastnet Basin in the west. The basins, which contain Triassic to Neogene stratigraphic sequences, evolved through a complex geological history that includes multiple Mesozoic rift stages and later Cenozoic inversion. The Mizen Basin represents the NW termination of the Celtic Sea basins and consists of two NE‐SW trending half‐grabens developed as a result of the reactivation of Caledonian and Variscan faults. The faults bounding the Mizen Basin were active as normal faults from Early Triassic to Late Cretaceous times. Most of the fault displacement took place during Berriasian to Hauterivian (Early Cretaceous) times, with a NW‐SE direction of extension. A later phase of Aptian to Cenomanian (Early to Late Cretaceous) N‐S oriented extension gave rise to E‐W‐striking minor normal faults and reactivation of the pre‐existing basin‐bounding faults that propagated upwards as left‐stepping arrays of segmented normal faults. In common with most of the Celtic Sea basins, the Mizen Basin experienced a period of major erosion, attributed to tectonic uplift, during the Paleocene. Approximately N‐S Alpine regional compression causing basin inversion is dated as Middle Eocene to Miocene by a well preserved syn‐inversion stratigraphy. Reverse reactivation of the basin bounding faults was broadly synchronous with the formation of a set of near‐orthogonal NW‐SE dextral strike‐slip faults so that compression was partitioned onto two fault sets the geometrical configuration of which is partly inherited from Palaeozoic basement structure. The segmented character of the fault forming the southern boundary of the Mizen Basin was preserved during Alpine inversion so that Cenozoic reverse displacement distribution on syn‐inversion horizons mirrors the earlier extensional displacements. Segmentation of normal faults therefore controls the geometry and location of inversion structures, including inversion anticlines and the back rotation of earlier relay ramps.

Description: Seven years of measurements from the Polar spacecraft are surveyed to monitor the variations of plasma density within the magnetospheric cusps. The spacecraft's orbital precession from 1998 through 2005 allows for coverage of both the northern and southern cusps from low altitude out to the magnetopause. In the mid- and high- altitude cusp, plasma density scales well with the solar wind density ( n c u s p / n s w ∼0.8). This trend is fairly steady for radial distances greater then 4 R E . At low altitudes ( r 〈4 R E ) the density increases with decreasing altitude and even exceeds the solar wind density due to contributions from the ionosphere. The density of high charge-state Oxygen (O 〉+2 ) also displays a positive trend with solar wind density within the cusp. A multifluid simulation with the BATSRUS MHD model was run to monitor the relative contributions of the ionosphere and solar wind plasma within the cusp. The simulation provides similar results to the statistical measurements from Polar and confirms the presence of ionospheric plasma at low altitudes.