Neogene and Pleistocene geodynamics: the paleoseismic evolution of Armorica (Western France)

The evolution of the passive Armorican margin (Western France) during the Neogene and Quaternary was analyzed using field data. The morphology of the margin attests to a late Hercynian shaping, further deformation during the Mesozoic mid-Atlantic opening, during the Alpine Orogeny, and ultimately, a Late Cenozoic uplift, mostly related to an onshore isostatic accommodation in response to erosion and limited tectonic activity. A very limited strike–slip dynamic, with very low seismicity, accommodated the Neogene–Pleistocene N170 strains around the rigid Armorican terrane. The South Armorican domain and English Channel floor include shear zones that adjusted the Alpine convergence, facilitating its transpressive slip to the west. The Permo-Triassic N150 faults were reactivated during the inversion phases that began after the Bartonian under the distal control of the Alpine convergence and the decrease in the Atlantic spreading rate after 34 Ma. The Armorican marine platforms were stable after the late Eocene and slightly subsident, experiencing pulsed episodes of transient lithospheric doming during the Neogene and Quaternary. Co-seismic activity onshore without surface rupture was recorded around ∼5.3 Ma, ∼3.7 Ma, ∼2.4–1.2 Ma, and ∼400–250 ka, in tandem with an inland exhumation driven by isostatic adjustment due to an intensification of periglacial erosion at the onset of the early interstadials or by agriculture. Low-magnitude and ubiquitous shallow seismic activities seem to be related today to an isostatic uplifted old brittle–ductile transition due to the accumulation of shearing strain

GPS constraints on deformation in northern Central America from 1999 to 2017, Part 1 – Time-dependent modelling of large regional earthquakes and their post-seismic effects

We use continuous and campaign measurements from 215 GPS sites in northern Central America and southern Mexico to estimate coseismic and afterslip solutions for the 2009 Mw = 7.3 Swan Islands fault strike-slip earthquake and the 2012 Mw = 7.3 El Salvador and Mw = 7.4 Guatemala thrust-faulting earthquakes on the Middle America trench. Our simultaneous, time-dependent inversion of more than 350 000 daily GPS site positions gives the first jointly consistent estimates of the coseismic slips for all three earthquakes, their combined time-dependent post-seismic effects and secular station velocities corrected for both the coseismic and post-seismic deformation. Our geodetic slip solutions for all three earthquakes agree with previous estimates that were derived via static coseismic-offset modelling. Our time-dependent model, which attributes all transient post-seismic deformation to earthquake afterslip, fits nearly all of the continuous GPS site position time-series within their severalmillimetre position noise. Afterslip moments for the three earthquakes range from 35 to 140 per cent of the geodetic coseismic moments, with the largest afterslip estimated for the 2012 El Salvador earthquake along the weakly coupled El Salvador trench segment. Forward modelling of viscoelastic deformation triggered by all three earthquakes for a range of assumed mantle and lower crustal viscosities suggests that it accounts for under 20 per cent of the observed post-seismic deformation and possibly under 10 per cent

Localization and characterization of an active fault in an urbanized area in central Guatemala by means of geoelectrical imaging

The Polochic and Motagua faults define the active plate boundary between the North American and Caribbean plates in central Guatemala. A splay of the Polochic Fault traverses the rapidly growing city of San Miguel Uspantan that is periodically affected by destructive earthquakes. This fault splay was located using a 2D electrical resistivity tomography (ERT) survey that also characterized the fault damage zone and evaluated the thickness and nature of recent deposits upon which most of the city is built. ERT images show the fault as a similar to 50 m wide, near-vertical low-resistivity anomaly, bounded within a few meters by high resistivity anomalies. Forward modeling reproduces the key aspects of the observed electrical resistivity data with remarkable fidelity thus defining the overall location, geometry, and internal structure of the fault zone as well as the affected lithologies. Our results indicate that the city is constructed on a similar to 20 m thick surficial layer consisting of poorly consolidated, highly porous, water-logged pumice. This soft layer is likely to amplify seismic waves and to liquefy upon moderate to strong ground shaking. The electrical conductivity as well as the major element chemistry of the groundwater provides evidence to suggest that the local aquifer might, at least in part, be fed by water rising along the fault. Therefore, the potential threat posed by this fault splay may not be limited to its seismic activity per se, but could be compounded its potential propensity to enhance seismic site effects by injecting water into the soft surficial sediments. The results of this study provide the basis for a rigorous analysis of seismic hazard and sustainable development of San Miguel Uspantan and illustrate the potential of ERT surveying for paleoseismic studies

Is the machecoul fault the source of the ∼M6 1799 Vendée earthquake (France)?

The ∼M6 1799 Bouin earthquake is considered as one of the largest earthquakes to have struck Western France. However, the seismogenic source potentially responsible for this event remain marginally documented. We present results from a focused offshore-onshore multidisciplinary survey in its meizoseismal area in order to identify the fault segments that potentially ruptured during this earthquake. Based on macroseismic data and the geology, we focused our study on the so-called Machecoul Fault as a potential source of the 1799 Bouin event. Our survey includes extensive high-resolution seismic reflection, high resolution bathymetry and a one-year seismological survey. These data were combined with existing topography, onshore gravity data and drill data to document the geometry of the Marais Breton / Baie de Bourgneuf basin, the past tectonic activity and the current local microearthquakes at depth along its bounding faults. Offshore and onshore observations suggest a recent activity of the segmented Machecoul Fault bounding the basin to the North. Offshore, the planar contact between the Plio-Quaternary sediments and the basement along the fault trace as well as the thickening of these sedimentary units near this contact suggests tectonic control rather than erosion. Onshore, the recent incision of the footwall of the fault suggests a recent tectonic activity. The temporary local seismological experiment deployed between 2016 and 2017 recorded a diffuse micro-seismicity down to the depth of 22 +/-5 km along the southward dipping Machecoul Fault, associated with predominantly normal fault mechanisms. Altogether, these results suggest that the Machecoul Fault is a serious candidate for being the source of the historical Bouin 1799 earthquake

Geomorphologic Indices for Transition from Subduction to Arc-Continent Collision in Sumba Island, Indonesia

International audienc

Analyse géomorphologique de la déformation d’avant-arc au niveau d’une transition subduction océanique/subduction continentale;

International audienc

"Arc-continent collision" of the Aleutian-Komandorsky arc into Kamchatka: Insight into Quaternary tectonic segmentation through Pleistocene marine terraces and morphometric analysis of fluvial drainage

At the NW corner of the Pacific region, just south of the Kamchatsky Peninsula, the northern tip of the Pacific plate subduction and associated volcanic arc interacts with the western end of the Aleutian-Komandorsky dextral transform plate boundary and associated arc. Study of both Holocene and Pleistocene sequences of uplifted marine terraces and also of fluvial drainage patterns on the Kamchatsky Peninsula allows us to highlight active tectonics produced by complex plate interaction. Our results show that the central eastern coast of the peninsula is currently divided into four different zones consisting in uplifted blocks associated with various uplift rates in front of a fold-and-thrust zone to the west. Our main tectonic benchmark—the altitude of the shoreline correlated to the Last Interglacial Maximum (Marine Isotopic Stage 5e)—yields late Pleistocene uplift rates ranging from 0.2 to 2.74 mm/yr. One of the main active faults bounding the coastal blocks is dextral and is interpreted as a prolongation of an offshore fault of the Aleutian-Komandorsky dextral transform plate boundary. We suggest that structures on the Kamchatsky Peninsula accommodate a part of the transform motion, but that mainly, the arc-continent collision of the Aleutian arc against Kamchatka produces a “bulldozer” effect on the Kamchatsky Peninsula

New magnetic fabric data and their comparison with palaeostress markers in the western Fars Arc (Zagros, Iran): tectonic implications

Abstract not availableCharles Aubourg, Brigitte Smith, Ali Eshraghi, Olivier Lacombe, Christine Authemayou, Khaled Amrouch, Olivier Bellier and Frédéric Moutherea

Genesis and Architecture of Sequences of Quaternary Coral Reef Terraces: Insights From Numerical Models

(IF 2.95 [2018]; Q1)International audienceThe variety of coral reefs morphologies highlights their sensitivities to several forcings; fossil reefs stack in sequences that are accordingly diverse. In order to understand their genesis and architectures, we devised a numerical approach, accounting for Quaternary sea level oscillations, vertical land motion, initial slope, wave erosion, and reef growth. We first test our model on the subsiding sequence of Hawaii and on the uplifting sequence of Wangi‐Wangi (Sulawesi) that bears active barriers. We then construct a parametric study that we analyze based on a comprehensive yet compact description of sequences as barcodes, that depict the vertical distribution of a few geometrical characteristics (number, width, and height of the terraces and barriers). We find that geological factors suffice to explain the variety of architectures of reefal sequences at first order, regardless of additional ecosystemic processes. Vertical land motion and foundation slopes are the prime players, while reef growth rates only play a minor role. Barriers may develop both in uplift and subsidence mode, and their preservation attests for the erosional power. Last, we reappraise the genesis of sequences and find that sequences do not fingerprint discrete events of sea level oscillations but a continuous process harrowed by stochastic events: Major sea level fluctuations can be overrepresented by several terraces or conversely absent; reoccupations may yield composite terraces representing multiple events. Overall, sequences shall not be regarded as stacks of reef bodies forming during sea level highstands, which implies that the commonly assumed bijective relationship between sea level highstands and terraces shall be abandoned

Morphometric assessment of uplifting coral reef sequences, Sumba Island, Indonesia.

International audienc