Morphological rejuvenation on tectonic seamounts: insights from the Gorringe Bank, SW Iberian Margin

<p>Seamounts are spectacular bathymetric features common within volcanic and tectonically active continental margins. During their lifecycles, they evolve through stages of construction and destruction. Seamount chains on the Southwest Iberian Margin are prone to instability and collapse due to regionally complex tectonism with moderate to high seismicity. In this work we investigate collapse episodes during the lifecycle of the tectonic Gorringe Bank (GB), the largest submarine seamount offshore European margins, based on recurrence patterns of MTDs on the active thrust flank. Eight MTDs with relevant expression on the seismic data were analysed, four of estimated Miocene age and four on a Pliocene-Quarternary interval. Miocene MTDs are overall larger and correlate with the main uplift stages of the GB structure. Their distribution and relative timing suggest that failure-triggering earthquakes were common along the whole length of the GB. Pliocene to Quarternary MTDs tend to cluster along the northern half of the GB flank and are generally smaller. Based on our observations, we propose that the lifecycle of tectonic seamounts is marked by morphological rejuvenation episodes driven by tectonic activity between major collapse events or cycles. Tectonic-driven rejuvenation is thus key to hinder or obliterate evidence of past high-magnitude destructive events on tectonic seamount morphology.</p>

Submarine landslide recurrence and links to seismicity on the Gorringe Bank Seamount, Offshore Portugal

<p>Submarine landslides are common features occurring on the flanks of seamounts. Often triggered by earthquakes or volcanic activity, such landslides are potential generators of tsunamis that constitute a dire geohazard for coastal communities. Understanding the recurrence history and geomorphology of seamount-flanking landslides and their link to seismic triggers is crucial for tsunami hazard assessment. This work aims at revealing the recurrence history of the landslides on the Gorringe Bank and their role on regional geohazards. Morphologically, the Gorringe Bank is the largest submarine seamount in Europe, with a length of circa 180 km and a height of 5000 m. It is linked to NW-directed thrusting which led to the exhumation of upper mantle lithologies in this major bathymetric structure. Numerous landslide scars are identified on both its northern and southern flanks, yet there is limited evidence of their presence and morphology on modern bathymetric data. A wealth of 2D seismic reflection profiles from offshore Southwest Portugal is here used to analyse the occurrence, timing and morphology of landslides complexes on the northwestern flank of the Gorringe Bank. A widespread frontal landslide complex of approximate Upper Miocene age is present along the whole flank, likely associated with the main phase of uplift. However, the recurrence of expressive submarine landslides in the Plio-Quarternary sequence is generally focused towards the northern segment of the Gorringe Bank. The geographical correlation between the areas of higher landslide number and clusters of seismicity epicentres suggest a close link between the two. This has direct implications for the assessment of landslide-prone locations on the seamount and to regional tsunami hazard models applicable to the Iberian and Northern African margins.</p><p>This work is supported by the FCT funded project MAGICLAND - MArine Geo-hazards InduCed by underwater LANDslides in the SW Iberian Margin (Ref: PTDC/CTA-GEO/30381/2017),</p>

A Reassessment of the Magnitude of the 1755 Lisbon Earthquake

ABSTRACT The 1755 Lisbon earthquake is still poorly understood due to its offshore location and complex macroseismic intensity pattern. Gutenberg and Richter (1949) tentatively assigned a magnitude between 8¾ and 9 judging from an estimated perceptibility radius of 2500 km. More recent attempts to estimate the magnitude from isoseismal areas led to results in the 8.5–8.7 range. These values have been adopted in several studies of the seismic hazard of southwest Iberia. In this article, I use the available macroseismic data—a total of 1206 data points from Portugal, Spain, and Morocco—to reassess the magnitude of the 1755 Lisbon earthquake. Because a training set of instrumental earthquakes was not available, I apply the technique of Bakun and Wentworth (1997) in conjunction with the ground-motion model of Atkinson and Wald (2007), which was selected through comparison with the 1969 M 7.8 Gorringe bank earthquake data. I obtain a moment magnitude of 7.7±0.5, significantly lower than previous results. The epicentral location obtained with this analysis is offshore southwest Iberia—as expected given the tsunami effects—but poorly constrained. Based on the pattern of intensity data outliers, I suggest that the source was complex and spatially distributed, with part of the rupture taking place onshore or inshore. I propose an explanation for the large tsunamigenic power of the earthquake, which invokes the basal rupture of the Gulf of Cadiz accretionary prism. Finally, I discuss the implications for hazard assessment of the type of complex rupture proposed.

Evidence of Segmentation in the Iberia–Africa Plate Boundary: A Jurassic Heritage?

The present structure of the Iberia–Africa plate boundary between the Gorringe Bank and the Algerian Basin is characterized by a highly segmented geometry and diffused seismicity. Filtered Bouguer gravity data show conspicuous highs coinciding with the Gorringe Bank, the Guadalquivir–Portimao Bank, and the Ronda/Beni–Bousera massifs, reflecting the current geometry of the plate boundary segments. The Africa–Eurasia Alpine convergence produced crustal-scale thrusting in the Atlantic segments and roll-back subduction in the Ligurian–Tethys segments. Despite the growing consensus that the Gorringe and the Guadalquivir–Portimao Banks resulted from tectonic inversion of hyperextended margin structures inherited from the Early Jurassic, this heritage is more debatable for the Ronda/Beni–Bousera massifs lacking models linking the Atlantic and Mediterranean realms. On the basis of gravity analysis combined with plate reconstruction models, geological cross-sections, and recent local tomography, we infer a strong Jurassic heritage of the present-day segmentation and substantiate a comprehensive tectonic evolution model of the Iberia–Africa plate boundary since the Early Jurassic to Recent that includes the Atlantic and the Mediterranean domains.

A new deep-sea Cirripedia of the genus Heteralepas from the northeastern Atlantic

A new species of the sessile deep-sea barnacle, Heteralepas (Crustacea, Cirripedia), Heteralepas gettysburgensis sp. nov., is described. The specimens were collected at a depth of 225 m at the Gettysburg Seamount on the Gorringe Bank, located in the Portuguese Exclusive Economic Zone, approximately 200 km off the southwestern coast of mainland Portugal. Extensive morphological and molecular (COI, 12S and 16S) analyses were carried out to separate the species from its nearest congeners with similar geographic distribution, i.e., Atlantic waters.

The 1755 Lisbon Tsunami at Vila do Bispo Municipality, Portugal

The numerical model of the 1755 Lisbon Tsunami was conducted at Vila do Bispo municipality. The tsunami source area of this historical event is assumed to be on the Gorringe Bank. Numerical model results of historical data have been validated by a combined analysis of eyewitnesses’ accounts, field surveys, geological records and archeological findings (conducted by previous authors). The coastal area has remained a mostly natural landscape since the 18thcentury, with popular beaches that increase the population potentially exposed to a tsunami, especially one occurring in summer. For these reasons, we use local tsunami hazard assessment as a criterion varying between moderate and critical. Results also show that the 16 low ground areas we surveyed were inundated between 15 and 30 minutes after the earthquake. For this reason, persons should act quickly in future to evacuate areas immediately after an earthquake and to move to higher ground. Results also show that safe, swift evacuation may be difficult in 50% of these areas, endangering special populations such as tourists. Tsunami information boards should therefore be put on beaches and evacuation exercises and drills should be implemented and practiced regularly.