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The unprecedented and long-lasting seismicity of Triacastela (over 25 years) attracted the interest of the research community on this stable continental region (SCR), particularly after the anomalously high 5.1 and 4.9 Mw earthquakes compared to the regional standards. The high rainfall and tide rates of this region compared to the rest of the Iberian Peninsula, in addition to the recognized existence of thermal springs and crustal fluids, motivated us to test these sources of hydroseismicity as a potential trigger mechanism for the observed seismicity in Triacastela. Based on network upgrades, we have gathered the seismic catalog in two periods for analysis (before and after year 2002). Before 2002, neither a diffusion-type earthquake migration nor any significant statistical correlation between the seismicity rates and rainfall or tides is found. After 2002, some clusters migrate to the south, suggesting the presence of fluid migration during earthquake swarms, but no diffusion-type migration is observed on longer time scales. Furthermore, we find correlation coefficients close to zero, indicating that rainfall and tides can be excluded as driving mechanisms. However, the seismic upward migration, a high b-value (1.2), and a low aftershock-productivity parameter (α = 0.9) observed in this period support the hypothesis of upward fluid migration through fracture zones. The presence of Mantellic helium-3 along the seismogenic faults and the increase of geochemical precursors in the groundwater previously to 1995 and 1997 mainshocks further support deep fluids as a source for the observed induced fluid migration seismicity in Triacastela. [ABSTRACT FROM AUTHOR]

The NW Iberian Peninsula is an intraplate region with significant seismicity, but it has rarely been studied. Despite the increase in seismicity during recent years, which has even forced changes to the building codes, a comprehensive statistical analysis of this seismicity has not been carried out. Thus, this study aims to characterize seismicity clusters on an intraplate region, based on comprehensive qualitative and quantitative analysis. The b value of the Gutenberg–Richter law is estimated and the Epidemic Type Aftershock Sequence model is employed. Additionally, qualitative analysis is carried out using the Mogi classification, and the results of this work are compared with those of other statistical studies in the literature. We identify clustered seismic activity in the NW Iberian Peninsula, which is classified into 18 swarms and eight sequences. A high b value and low efficiency of aftershock generation are characteristic of the swarms. These parameters, along with the similarities to other regions, point to fluid migration as the swarm seismic source mechanism. Earthquakes of higher magnitude than swarms and a b value close to 1 define the sequences of this region. The statistical results in sequences (high k, c, and p values) and the similarity to other regions would indicate a mix of tectonic origin and aseismic processes (such as fluid migration or slow slip events) as the driving force of seismic sequences. The presence of crustal fluids associated with fault activity in the NW Iberian Peninsula also supports the hypothesis of fluid migration as the source of seismic events. [ABSTRACT FROM AUTHOR]

[Display omitted] • Lateritic clays occur in fault contact with granites in a Cenozoic basin. • Geochemical data indicates an allochthonous mafic source for the lateritic clays. • Hydrothermal fluids interacted with the lateritic clays and the granites. • Faults had extensional component and were re-activated with inverse strike-slip. • Faults were not effective seals for the hydrothermal fluids. Continental basin evolution can be influenced not only by tectonic activity, but by hydrothermal and weathering processes. This is displayed on the basement rocks (granites and xenoliths) and sedimentary cover (clays, sands and conglomerates) in the Cenozoic Ferreira basin of NW Spain. In this study, we applied petrological, geochemical and structural proxies to investigate a red clay unit and adjacent granitic basement rocks in a fault contact. Red clays have a lateritic composition (SiO 2 : 38–42 wt%, Fe 2 O 3 : 13–20 wt% and Al 2 O 3 : 24–29 wt%) with moderate to high contents in V (267 – 324 ppm) and Ni (46–71 ppm), and are characterized by (La/Yb) n and TREE that vary from ∼5 to 12 and ∼55 to 110 ppm, respectively. The basement rocks are peraluminous biotite granites (A/CNK: 1.12–13.57) with (La/Yb) n and TREE that vary from 7.7 to 27.42 and from 82.79 to 244.64 ppm, respectively. The xenoliths are peraluminous shales (A/CNK: 3.48–4.48) with (La/Yb) n and TREE varying from 7.66 to 11.10 and from 228.61 to 344.42 ppm, respectively. This geochemistry indicates that the lateritic red clays come from residual weathering of mafic ± intermediate rocks. They are interpreted to have been generated under the tropical climate conditions in the Late Cretaceous (∼70 Ma). Afterwards red clays were remobilized in subsequent tectonic events (Alpine), deposited in faulted grabens, and together with adjacent basement granites, affected by hydrothermal events that caused element mobility (e.g., REE leaching and Ce positive anomalies). Fault reactivations, under strike-slip stresses with a reverse component, channelized these hydrothermal fluids and uplifted part of the landscape. This uplift is interpreted as the trigger of an erosional regime that affected the lateritic red clay unit in the Ferreira basin. The conclusions of this study highlights the important interplay of different geological processes in small continental basins. [ABSTRACT FROM AUTHOR]

Pleistocene fluvial incision acceleration resulting in narrow and deeply entrenched valleys has been widely described and is generally attributed to uplift rate increase or greater climatic severity. In this paper, the long-term downcutting pattern of the Upper Ebro River and driving mechanisms are assessed, and we reconstruct the valley incision recorded by an outstanding sequence of 22 river terraces. Dating of 8 fluvial levels by means of the ESR, TCN, and OSL techniques, spanning the last 1.2 Ma, reveals a long-term incision deceleration pattern. The estimated age-incision model indicates a decrease in the incision rates, showing a long-term deceleration during the final Early Pleistocene (from 0.42 to 0.18 m/ka), and a tendency towards a steady state or base level stabilization from the Middle Pleistocene (0.15–0.03 m/ka) to the Late Pleistocene (0.03 m/ka). This incision pattern does not support climate change as a long-term incision-acceleration driver by itself, demonstrating the need for base level lowering effects to have operated. Upper Ebro deceleration incision is explained by the headward attenuation of the incision wave induced by the opening of the Ebro Cenozoic Basin in a geodynamic context characterized by an absence of significant uplift over the last million years. This trend could have changed the aggressor-victim roles of the rivers involved in fluvial captures at the drainage divide. The documented incision deceleration pattern differs from that reported for other valleys in the Iberian Peninsula, suggesting that the degree of maturity of the fluvial systems depends on their relative position with respect to the capture point, in addition to tectonic and lithostructural factors. In the Late Pleistocene-Holocene, a short-lived incision rate increase is recorded, corresponding to the last terrace dissection phase, during the transition from the last glacial MIS 2 to the interglacial MIS 1. • Incision acceleration is widely reported during Pleistocene valley entrenchment. • However, the Upper Ebro valley contains a complex sequence of fluvial terraces • which were dated within the last 1.2 Ma using ESR, TCN and OSL techniques • and indicate an incision deceleration pattern. • This suggests that the river incision is tending to a steady state. [ABSTRACT FROM AUTHOR]