Tania Mochales

Chronostratigraphy of the Boltaña anticline and the Ainsa Basin (southern Pyrenees)

The Ainsa Basin (south-central Pyrenees) is an exceptional example of a syntectonic foreland basin where structures oblique to the Pyrenean trend are well preserved. The absence of a complete chronostratigraphic frame motivated us to perform a detailed magnetostratigraphic study in the Ainsa Basin, including the shallow-marine–continental transition. Three sections covering almost the entire Eocene sedimentary fill were sampled (Ara, Coscollar, and Mondot), covering 2450 m of the sedimentary pile, with more than 1000 demagnetized specimens (sample spacing of 2.7 m). Data from previous magnetostratigraphic studies (Eripol section) allow us to complete the Eocene record with an 840-m-long profile within the Bartonian–Priabonian interval overlying the sampled sequence. Characteristic remanent magnetization (ChRM) directions were effectively isolated between 300 and 500 °C. Magnetite is the main magnetic carrier, with variable iron sulfide content and occasional hematite. Fold tests indicate a prefolding magnetization. Biostratigraphy based on shallow benthic foraminifera (15 localities) and a locality with abundant charophyte gyrogonites help to constrain the local magnetostratigraphic record (20 reversals) between the early Ilerdian and the middle Priabonian. A global correlation based on a magnetostratigraphic composite section derived from this work allows us to propose a complete chronostratigraphic frame for the Ainsa Basin infill between 55 and 45 Ma. Accumulation rates range from 2 to 53 cm/k.y., responding to retrogradational to progradational features during the early Lutetian, and a progressive increase from the middle Lutetian onward related to overall continentalization of the basin. Westward migration of subsidence is associated with progressively younger synsedimentary structures nucleating westward of the older ones in the South Pyrenean Basin.

Unraveling the geometry of the New England oroclines (Eastern Australia): Constraints from magnetic fabrics

The southern New England Orogen (NEO) in eastern Australia is characterized by tight curvatures (oroclines), but the exact geometry of the oroclines and their kinematic evolution are controversial. Here we present new data on the anisotropy of magnetic susceptibility (AMS), which provide a petrofabric proxy for the finite strain associated with the oroclines. We focus on a series of preoroclinal Devonian-Carboniferous fore-arc basin rocks, which are aligned parallel to the oroclinal structure, and by examining structural domains, we test whether or not the magnetic fabric is consistent with the strain axes. AMS data show a first-order consistency with the shape of the oroclines, characterized, in most of structural domains, by subparallelism between magnetic lineations, “structural axis” and bedding. With the exception of the Gresford and west Hastings domains, our results are relatively consistent with the existence of the Manning and Nambucca (Hastings) Oroclines. Reconstruction of magnetic lineations to a prerotation (i.e., pre–late Carboniferous) stage, considering available paleomagnetic results, yields a consistent and rather rectilinear NE-SW predeformation fore-arc basin. This supports the validity of AMS as a strain proxy in complex orogens, such as the NEO. In the Hastings Block, magnetic lineations are suborthogonal to bedding, possibly indicating a different deformational history with respect to the rest of the NEO.Three decades after declaration of World Heritage status for Shark Bay new research findings are being reported on the specialised microbial habitats that characterise its hypersaline settings, the composition of microbial communities, tidal flat evolution, stromatolite geochronology and subtidal microbial systems. In the stable, semiarid and evaporative setting within the intertidal–subtidal environment the microbial ecosystem is trapping, binding and biologically inducing carbonate precipitation within laminated stromatolites, non-laminated thrombolitic forms and cryptomicrobial non-laminated forms. Filamentous microbes constitute the dominant group in the blister, tufted and smooth mat types, and coccoid microbes dominate the pustular, colloform and microbial pavement deposit types. Detailed georeferenced substrate mapping has revealed extensive subtidal microbial deposits occupying ~300 km of the total Holocene 1400 km area of Hamelin Pool. The microbial pavement covers 227 km of the subtidal substrate, which together with columnar structures reveals a subtidal microbial habitat that occupies an area several times larger than the area of the intertidal deposits. Oldest dated stromatolite heads are 1915 C years BP, and the overall system was deposited in two stages: the first between 2000 and 1200 and the last from 900 years BP to the present. Slow accretion rates vary from less than 0.1 to 0.5 mm/year. Different internal fabrics were constructed according to their position in relation to the littoral zone by distinct microbial communities, and lateral fabric relations have been established. Evidence of shallowing-upward fabric sequences of microbial origin reflects relative falling sea levels during the late Holocene and is likely useful in ancient environmental interpretation. A new substrate map and depositional history for this distinctive microbial habitat has established the significance of subtidal structures and emphasises the geoscientific importance of Hamelin Pool, especially with respect to early life studies and ancient analogues for understanding microbial activity, deposit characteristics, fenestral fabrics and distribution.

Representation of paleomagnetic data in virtual globes

Virtual globes allow geo-referencing and visualization of diverse geologic datasets. A vertical axis paleomagnetic rotation study in the Southern Pyrenees, Spain, is used to illustrate the potential of virtual globes for representing paleomagnetic data. A macro-enabled workbook that we call P2K, allows KML files to be generated from conventional paleomagnetic datasets. Cones and arch models are used to represent the paleomagnetic vector, and the rotation with regard to the local reference direction, respectively. This visualization provides simultaneous representation of local magnetic declination, inclination and precise confidence cones, shown in their geographic position from diverse perspectives and scales. Display Omitted We propose 3D models for the visualization of paleomagnetic data in virtual globes.A macro-enabled spreadsheet (P2K) generates KML files from paleomagnetic datasets.This tool allows accurate representation of declination, inclination and α95.

Tethyan versus Iberian extension during the Cretaceous period in the eastern Iberian Peninsula: insights from magnetic fabrics

This work investigates how anisotropy of magnetic susceptibility (AMS) recorded the strain related to the Early Cretaceous extensional processes in synrift sediments of the Maestrat basin (eastern Spain). Forty-two sites, distributed throughout the Lower Cretaceous sequence with dominant gentle dips, were sampled. Minerals contributing to the AMS are mainly phyllosilicates. The parallelism between magnetic and sedimentary foliation seems to indicate that a primary (synsedimentary and early diagenetic) magnetic fabric was preserved at 84% of sites. Consequently, preferred orientations of magnetic lineations are interpreted to record the effect of extensional processes coeval with sedimentation and diagenesis during this period. At these 35 sites, two main magnetic lineation orientations are found, delimiting two large domains: a NE–SW orientation prevailing in the NW sector of the basin (parallel to the extension direction of the Iberian basin), and NW–SE to NNW–SSE orientations to the SE (parallel to the extension direction controlling the western Tethys margin). Directional variability demonstrates that the Maestrat basin is located at the boundary between two domains (Iberian and Tethyan) undergoing different plate-scale extensional processes. The subsequent Cenozoic tectonic inversion affected the synsedimentary magnetic fabrics at only a few sites at the borders of the basin, where compressive features are more developed.

Interpretation of gravimetric and magnetic anomalies in the Cameros Basin (North Spain): combination of deep and shallow sources

Outstanding potential field anomalies (gravimetric and magnetic) in the Cameros Basin (N Spain) follow a WNW-ESE trend, parallel to the geological structures resulting from Mesozoic extension and Tertiary basin inversion. The positive Bouguer gravity anomaly (15 mGal) is interpreted as the result of a strong contrast between the density of Tertiary rocks of the foreland basin and the Paleozoic and Mesozoic rocks, combined with crustal thickening in the Iberian Chain with respect to the Ebro Basin. The dipolar magnetic anomaly, slightly shifted to the south with respect to the relative maximum of the Bouguer anomaly, can be interpreted as related to volcanic rocks within the basement, which are linked to Triassic rifting as witnessed by outcrops of basalts along the basin margins. An exhaustive analysis of rock properties (density, magnetic susceptibility and remanence) and basin geometry from other sources (seismic reflection profiles) allow to constrain variations in crustal thickness and the location of large-scale basement faults.

High-resolution aeromagnetic survey of Calabria (Southern Italy)

ABSTRACT We present a 1:350,000 high-resolution magnetic anomaly map of Calabria (Southern Italy), obtained by merging the results from two low-altitude aeromagnetic surveys performed in southern and northern Calabria. Magnetic anomalies of Calabria are of low intensity, and mostly range from 11 to –9 nT. Northern Calabria is characterized by positive anomalies in the Tyrrhenian margin (Coastal Chain) that turn into negative values moving eastward in the Sila Massif. Southern Calabria is characterized by slightly positive anomaly values, interrupted by a null magnetic anomaly corridor roughly corresponding to the eastern margin of the Gioia Tauro basin. Finally, anomaly values turn systematically negative in the Messina Straits. Due to the unprecedented resolution (low flying height, spatial sampling along the flight line of ∼5 m and 1–2 km flight line spacing), the new map highlights, in detail, the geometry and setting of the upper crustal features. As Calabria is one of the most seismically active regions in Italy, hit by several high-magnitude earthquakes in recent centuries, the interpretation of this new map will hopefully contribute to new insights into the crustal geological setting, location and dimension of the main seismogenic sources.