Comment on: “Interseismic Strain Accumulation near Lisbon (Portugal) from Space Geodesy” by Fonseca et al. (2021)

The paper by Fonseca et al. (2021), hereafter referred as FON21, published in Geophysical Research Letters2 make several conclusions that are not convincingly supported by the evidence of the data that is made available. In this comment we will address the following statements: 1) FON21 “provides new evidence of sinistral simple shear driven by a NNE-SSW first-order tectonic lineament; 2) “PSInSAR vertical velocities corroborate qualitatively the GNSS strain-rate field, showing uplift/subsidence where the GNSS data indicate contraction/extension”; 3) FON21 proposes “the presence of a small block to the W of Lisbon moving independently toward the SW with a relative velocity of 0.96 ± 0.20 mm/yr”; 4) FON21 shows “that the contribution of intraplate faults to the seismic hazard in the LMA is more important than currently assumed”. We conclude that more evidence needs to be collected to confirm or infirm FON21 statements and conclusions. For the moment the proposal of an autonomous crustal block moving with significant velocity in relation to the neighboring domain should be considered speculative and unproved.

Earthquake effects and insights on fault activity at the Beatrice Cenci cave (Abruzzo, Central Apennines)

he focus of this study is the analysis of a cave in Central Italy, the Beatrice Cenci cave, in order to point out and constrain evidence of possible past earthquakes and of fault activity in the area. We performed a survey of seismic related damages within the cave. This included the analysis of broken/collapsed speleothems, the recognition of structural collapse, of tilting/growth alteration in the speleothems, and the mapping of fractures, joints and/or faults. To timely set the occurrence of the recognized damage, organic sediments were dated with 14C radiocarbon method. The results merged toward the recognition of two distinct seismic shaking events affecting the cave environment, one older than 30 kyr and another around 7 kyr. The deformation observed within the cave led us to the hypothesis that the events of damage were possibly linked to the activity of the regional tectonic lineament that crosses the cave, i.e., the Liri normal fault. The morphology and the evolution of the cave appear controlled by the fault zone. These speleoseismological results provided a new contribution on the knowledge of the past activity of the Liri fault and on the earthquake history of this sector of Central Apennines.

Sedimentary Basin Reconstruction and Tectonic Development of Paleocene-Eocene Succession, Southern Iraq, by Geohistory Analysis

The Paleocene-Early Eocene sequence is represented by Aliji and Umm Er Radhuma formations, while the Middle-Late Eocene sequence is represented by Jaddala and Dammam formations. The Rus Formation has been described and its basin was analyzed separately because it was deposited during the regression period (Middle Eocene), which is a transitional period between these two cycles. This study includes analysis of the geohistory of this succession, interpretation of the changes of the accumulation, and calculation of subsidence rates. The results were compared with the space available to explain the basin development. The study site included the boreholes of Garraf-84 and 92, Halfaya-1, Nasirya-13 and 40, and Noor-5 at the Mesopotamian Block, in addition to the Ratawi-8, Tuba-15, Rumaila-217, Zubair-45, and West Qurna-60 at the Basra Block. The Aliji basin was characterized by the decrease in accommodation values to the northeast direction and the increase in all the other parts of the study area. A comparison of the setting of this basin with the Umm Er Radhuma basin gives a clear evidence of the tectonic impact coming from the northeast. During the Middle Eocene stage, we notice that the basin was affected by comprehensive uplifting processes. This led to the generation of a very shallow basin (Rus basin) with the exposure of the northern part of the basin during the regression stage. The Middle-Late Eocene basin is represented by a transgression stage with high subsidence, where the sea level had been raised and covered the northeastern and eastern parts of the studied area by deep sea deposits (Jaddala Formation). While the other parts of the study area were characterized by shallow sediments of Dammam Formation. This period ended with a clear tectonic uplift occurring in the northeastern parts and decreasing towards the southwest. This confirms the reactivation of the tectonic action from the northeast, represented by the continental collision. All these sources of evidence indicate that the study area is divided into a northern part and a southern part. Both of these parts are separated by a major tectonic lineament extending from the West Qurna oil field to the Nasiriya oil field, which confirms the presence of the tectonic boundary between the Mesopotamian block and the Basra block. In addition, there exists a secondary tectonic boundary that divides the Mesopotamian block into two parts, the first is to the east and the other is to the west. The results showed that the eastern side was most affected by the collision of the Iranian Plate with the Arabian Plate, which led to its uplift, while the western side was less affected by this tectonics evidence.

Morphostructural Setting and Tectonic Evolution of the Central Part of the Sicilian Channel (Central Mediterranean)

The Plio-Quaternary tectonic evolution of the central sector of the Sicilian Channel and the resulting morphostructural setting have been analyzed using a large geophysical dataset consisting of multichannel seismic profiles, which some of them never published, and available bathymetric data. This area hosts two regional-scale tectonic domains that registered the complex pattern of deformation occurred since the Early Pliocene: (1) the Sicilian Channel Rift Zone (SCRZ), which can be divided into a western sector formed by the Pantelleria graben (PG) and in a eastern one represented by the Linosa and Malta grabens (LG and MG) and (2) the Capo Granitola-Sciacca Fault Zone (CGSFZ), a NNE-oriented lithospheric transfer zone that crosses the Sicilian Channel from the Sicily coast to the Linosa Island, of which only its northern part has been studied to date. Data interpretation has allowed achieving the following outcomes: (i) the presence of an alternation of basins and structural highs forming a NNE-oriented separation belt between the western and eastern sectors of the SCRZ, and interpreted as the shallow expression of the southern part of the CGSFZ; (ii) a NE-oriented tectonic lineament separating the MG in a northern and southern part, and interpreted as the southern prosecution of the Scicli-Ragusa Fault System; (iii) the presence of syn-rift deposits in the Plio-Quaternary fill of the grabens, suggesting that the opening of the grabens of the SCRZ was coeval, and started since Early Pliocene in the framework of a NW-oriented right-lateral transtensional mega-shear zone; (iv) continental rifting ended around the Early Calabrian, during which extensional tectonics dominated along the separation belt; (v) the CGSFZ conditioned the SCRZ configuration at a regional scale, leading to the development of the PG in the western sector and of the LG and MG in the eastern one; and (vi) after the Early Calabrian, the PG and the southern MG followed a different tectonic evolution with respect to the LG and northern MG. The syn-rift deposits of the PG and southern MG were sealed by an undeformed post-rift succession, while the LG and the northern MG suffered a basin inversion that ended around the Latest Calabrian time. During this stage, the separation belt was affected by a transpressional tectonics. At present, the grabens of the Sicilian Channel seem to be tectonically inactive, while the CGSFZ represents an active tectonic domain.

Volcanism and Volcanogenic Submarine Sedimentation in the Paleogene Foreland Basins of the Alps: Reassessing the Source-to-Sink Systems with an Actualist View

This work critically reviews the Eocene–Oligocene source-to-sink systems accumulating volcanogenic sequences in the basins around the Alps. Through the years, these volcanogenic sequences have been correlated to the plutonic bodies along the Periadriatic Fault System, the main tectonic lineament running from West to East within the axis of the belt. Starting from the large amounts of data present in literature, for the first time we present an integrated 4D model on the evolution of the sediment pathways that once connected the magmatic sources to the basins. The magmatic systems started to develop during the Eocene in the Alps, supplying detritus to the Adriatic Foredeep. The progradation of volcanogenic sequences in the Northern Alpine Foreland Basin is subsequent and probably was favoured by the migration of the magmatic systems to the North and to the West. At around 30 Ma, the Northern Apennine Foredeep also was fed by large volcanogenic inputs, but the palinspastic reconstruction of the Adriatic Foredeep, together with stratigraphic and petrographic data, allows us to safely exclude the Alps as volcanogenic sources. Beyond the regional case, this review underlines the importance of a solid stratigraphic approach in the reconstruction of the source-to-sink system evolution of any basin.

Paleo-Seismicity in the Euganean Hills Province (Northeast Italy): Constraints From Geomechanical and Geophysical Tests in the Schio-Vicenza Fault Area

Gouges and cataclasites within exhumed fault zones are valuable indicators of past seismic events. Gouge layers, 40- to 100-cm-thick and decameters long, have been found within uplifted Cretaceous limestones in the Euganean Hills (SW of Padova, NE Italy), Cenozoic subvolcanic chambers intruded within a Meso-Cenozoic sequence. The main tectonic lineament in the area is the Schio-Vicenza Fault that bounds the Euganean Hills to the East. Micropaleontological analyses reveal that the gouges derive from the fragmentation and pulverization of the adjacent pristine carbonatic rocks. Stress tests on specimens from bedrock associated with gouges yielded a minimum dissipated strain energy of 0.3–0.5 MJ/m3 to shatter them. Henceforth, additional strain energy was necessary to pulverize these rocks within the gouge zones. Global navigation satellite system observations show that the present deformation pattern in this region of Italy is a few tens of nanostrain (10–30 1/y), not enough to generate such gouge layers. Therefore, the seismicity of the Euganean Hills (currently M ≤5) must be reconsidered in the light of the Schio-Vicenza Fault past activity. The gouges may imply that the Schio-Vicenza Fault had a more intense activity in the past, or this area was affected by remote events or tectonic structures. This article provides new clues on the evolution of the tectonic and morphological setting of the area, with relevant consequences on seismic risk assessment of the nearby urbanized area, including the cities of Padova and Vicenza.