Origin of the Coloured Karst Fills in the Neogene Extensional System of NE Iberia (Spain)

Karst fills from the onshore Penedès Basin and offshore València Trough display red, pink, orange and ochre colours. Their Mössbauer spectra indicate that Fe3+ contained in goethite is the dominant species in reddish-pink fills, whereas Fe2+ contained in dolomite and clays is more dominant in the orange and ochre ones. The lower δ13C values and higher 87Sr/86Sr ratios of the karst fills with respect to their host carbonates can reflect the input of soil-derived CO2 and an external radiogenic source into the karst system. This geochemical composition, together with the non-carbonate fraction of the fills, consists of authigenic and transported illite, illite-smectite interlayers, as well as kaolinite, chlorite, pyrite, quartz, ilmenite, magnetite, apatite and feldspar, account for a mixed residual-detrital origin of fills. This polygenic origin agrees with that of the terra rossa sediments described worldwide. The different colours of karst fills are attributed to fluctuations in the water table, which control the Eh/pH conditions in the karst system. Thus, reddish colours reflect low water table levels and oxidising episodes, and orange and ochre ones reflect high water table levels and more reducing episodes. The greenish colours of fills could be related to fluctuations in the Fe3+/Fe2+ ratio.

Subsidence discrepancy in the Valencia Trough revealed from reflection seismic observations and backstripping results

<p>    The Valencia Trough is commonly included as part of the set of western Mediterranean Cenozoic extensional basins that formed in relation with the Tethyan oceanic slab rollback during the latest Oligocene to early Miocene. It lies in a complex tectonic setting between the Gulf of Lions to the North-West, the Catalan Coastal Range and the Iberian chain to the West, the Balearic promontory to the East and the Betic orogenic system to the South. This rifting period is coeval with or directly followed by the development of the external Betics fold and thrust belts at the southern tip of the Valencia Trough. Recent investigations suggest that the Valencia Trough is segmented into two main domains exhibiting different geological and geophysical characteristics between its northeastern and southwestern parts. The presence of numerous Cenozoic normal faults and the well-studied subsidence pattern evolution of the NE part of the Valencia Trough suggest that it mainly formed coevally with the rifting of Gulf of Lion. However, if a significant post-Oligocene subsidence is also evidenced in its SW part; fewer Cenozoic rift structures are observed suggesting that the subsidence pattern likely results from the interference of different processes.</p><p>    In this presentation, we quantify the post-Oligocene subsidence history of the SW part of the Valencia Trough with the aim of evaluating the potential mechanisms explaining this apparent subsidence discrepancy. We analyzed the spatial and temporal distribution of the post-Oligocene subsidence using the interpretation of a dense grid of high-quality multi-channel seismic profiles, also integrating drill-hole results and velocity information from expanding spread profiles (ESP). We used the mapping of the main unconformities, especially the so-called Oligocene unconformity, to perform a 3D flexural backstripping, which permits the prediction of the post-Oligocene water-loaded subsidence. Our results confirm that the post-Oligocene subsidence of the SW part of the Valencia Trough cannot be explained by the rifting of the Gulf of Lions. Previous works already showed that the extreme crustal thinning observed to the SW is related to a previous Mesozoic rift event. Here, we further highlight that if few Cenozoic extensional structures are observed, they can be interpreted as gravitational features rooting at the regionally identified Upper Triassic evaporite level. Backstripping results combined with the mapping of the first sediments deposited on top of the Oligocene unconformity show that they are largely controlled by the shape of Betic front with a possible additional effect of preserved Mesozoic structures. At larger scale, we compare the mechanisms accounting for the origin and subsidence at the SW part of the Valencia Trough with those responsible for the subsidence of its NE part and the Gulf of Lions.</p>

Comparison of the crust and upper mantle structure of the Alboran and Algerian domains (Western Mediterranean): Tectonic significance

<p>We present a comparison of the present-day crust to upper-mantle structure in the Western Mediterranean along two NW-SE oriented geo-transects in the Alboran and Algerian basins. The Alboran domain geo-transect traverses the Iberian Massif, the Betics, the Alboran Basin and ends in the northern margin of Africa between the Tell and Rif mountains. The Algerian domain geo-transect traverses the Catalan Coast Ranges, the Valencia Trough, the Balearic Promontory, the Algerian basin, the Greater Kabylies and ends in the Tell-Atlas Mountains in the northern margin of Africa. We model the thermal, density (i.e. compositional) and seismic velocity structure by integrating geophysical and geochemical dataset in a self-consistent thermodynamic framework. The crustal structure is constrained by seismic experiments and geological cross-sections, whereas seismic tomography models and mantle xenoliths constrain the upper mantle structure and composition. The Algerian Basin lithosphere shows a typical oceanic lithosphere composition, whereas the Alboran Basin lithosphere is slightly fertile. The lithospheric mantle beneath the Betics and Greater Kabylies are also fertile compared to the Iberian and African lithospheres showing the involvement of the fertile sublithosphere mantle during the later stages of subduction. In the Valencia Trough, the lithosphere is fertile in comparison to the Balearic Promontory lithosphere, which is similar to Iberian lithosphere. A lithosphere-scale thickening is observed in the Betics, and the Greater Kabylies, and thinning follows towards the Alboran and Algerian back-arc basins. Detached slabs with anomalous temperature of-320 <sup>o</sup>C, with oceanic lithosphere composition beneath the Greater Kabylies, and Iberian lithosphere composition beneath the Betics, are required to fit the geoid height. Our results impose important constraints for the geodynamic evolution models of the Western Mediterranean.</p><p>This work has been supported by SUBTETIS (PIE-201830E039) project, EU Marie Curie Initial Training Network ‘SUBITOP’ (674899-SUBITOP-H2020-MSCA-ITN-2015), the Agencia Estatal de Investigación through projects MITE (CGL2014-59516) and GeoCAM (PGC2018-095154-B-100), and the Agency for Management of University and Research Grants of Catalonia (AGAUR-2017-SGR-847).</p>

Fault reactivation by gas injection at an underground gas storage off the east coast of Spain

During September–October of 2013 an intense swarm of earthquakes occurred off the east coast of Spain associated with the injection of the base gas in an offshore underground gas storage. Two weeks after the end of the injection operations, three moderate-sized earthquakes (Mw 4.0–4.1) occurred near the storage. These events were widely felt by the nearby population, leading to the indefinite shut-down of the facility. Here we investigate the source parameters (focal depth and mechanism) of the largest earthquakes in the sequence in order to identify the faults reactivated by the gas injection and to help understand the processes that caused the earthquakes. Our waveform modeling results indicate that the largest earthquakes occurred at depths of 6–8 km beneath the sea floor, significantly deeper than the injection depth (∼1800 m). Although we cannot undoubtedly discriminate the fault plane from the two nodal planes of the mechanisms, most evidence seems to favor a NW–SE-striking fault plane. We propose that the gas injection reactivated faults in the Paleozoic basement, with regional orientation possibly inherited from the opening of the Valencia Trough.

Fault reactivation by gas injection at an underground gas storage off the east coast of Spain

During September–October of 2013 an intense swarm of earthquakes occurred off the east coast of Spain associated with the injection of the base gas in an offshore underground gas storage. Two weeks after the end of the injection operations, three moderate-sized earthquakes (Mw 4.0–4.1) occurred near the storage. These events were widely felt by the nearby population, leading to the indefinite shut-down of the facility. Here we investigate the source parameters (focal depth and mechanism) of the largest earthquakes in the sequence in order to identify the faults reactivated by the gas injection, and to help understand the processes that caused the earthquakes. Our waveform modeling results indicate that the largest earthquakes occurred at depths of 6–8 km beneath the sea floor, significantly deeper than the injection depth (~ 1800 m). Although we cannot undoubtedly discriminate the fault plane from the two nodal planes of the mechanisms, most evidence seems to favor a NW-SE striking fault plane. We propose that the gas injection reactivated unmapped faults in the Paleozoic basement, with regional orientation possibly inherited from the opening of the Valencia Trough.