Ligure-Piemontese Basin Research Articles

Insights into the Subduction of the Ligure-Piemontese Oceanic Basin: New Constraints from the Metamorphism in the Internal Ligurian Units (Northern Apennines, Italy)

In the Northern Apennines, the Internal Ligurian Units are considered deformed and metamorphosed fragments of the Ligure-Piemontese oceanic basin. In this paper, we report on the temperature and pressure conditions of the metamorphic peak for four Internal Ligurian Units, estimated using different geothermometers and geobarometers based on the white mica and chlorite compositions. These minerals were formed during the D1 deformation phase in the pre-Oligocene. The results indicate that the Portello and Gottero units are both characterized by metamorphic conditions pertaining to low blueschists facies, while the Colli-Tavarone and Bracco-Val Graveglia Units show a lower metamorphic imprint that produces assemblages of prehnite-pumpellyite facies. The estimated geothermal gradient for the metamorphic peak achieved by the analyzed Internal Ligurian Units during the D1 phase is 7–15 °C/Km, which is indicative of deformation in a subduction setting. Under these conditions, the D1 phase developed in these units as a result of underplating at the base of the accretionary wedge during the closure of the Ligure-Piemontese basin. These data indicate a close geodynamic correlation among the Internal Ligurian Units and the ophiolite-bearing units of the Alps.

New hermit crabs (Crustacea, Anomura, Paguroidea) from the Lower Oligocene of the Ligure Piemontese Basin, northwest Italy

Extinct hermit crabs from Italy have been recorded by several authors, mainly on the basis of both complete or fragmentary chelipeds. Here we add three new paguroids, Dardanus cherpionensis n. sp.,Paguristes collinsi n. sp. and Paguristes laurae n. sp., from Lower Oligocene (Rupelian) strata in the Ligure-Piemontese Basin of northwest Italy. Paguristes laurae n. sp. is preserved in situ within the internal mould of a indeterminate xenophorid gastropod.

A new homolid crab, Cherpihomola italica gen. nov., sp. nov. (Decapoda, Brachyura), from the Rupelian of the LigurePiemontese Basin (Alessandria, northern Italy)

A new genus and species of homolid from lower Oligocene (Rupelian) strata in the Ligure-Piemontese Basin (northern Italy) is introduced. Cherpihomola italica gen. nov., sp. nov. represents the first record of homolids from Oligocene deposits across Europe and extends the palaeogeographical distribution of extinct homolids.

A new calappid species from the Ligure Piemontese Basin (NW Italy) and reappraisal of the fossil calappid specimens in the Museo “G. Maini”, Ovada

espanolLas especies del Oligoceno, adscritas a los Calappidae H. Milne-Edwards, 1837, depositadas en las colecciones paleontologicas del Museo“G. Maini” de Ovada (Alessandria, Piemonte, NO Italia), procedentes de la Cuenca Ligure Piemontese, son revisadas aqui, con el proposito de verificar su asignacion sistematica. Dos especimenes son adscritos a Calappilia guttata n. sp., en tanto que un especimen adscrito a M. pustulosus, es aqui asignado a Calappilia sp. Esta revision nos permite ofrecer una diagnosis corregida y una nueva descripcion de Mursiopsis pustulosus Ristori, 1889. Finalmente, la asignacion de Stenodromia mainii (Allasinaz, 1987) y Calappilia vicetina Fabiani, 1910, es brevemente discutida. EnglishThe Oligocene species ascribed to the Calappidae H. Milne-Edwards, 1837, housed in the palaeontological collections of the Museum “G. Maini” of Ovada (Alessandria, Piemonte, NW Italy) from the Ligure Piemontese Basin, are herein revised in order to verify their systematic assignment. Two additional specimens are ascribed to Calappilia guttata n. sp., whereas one specimen previously ascribed to M. pustulosus is herein assigned to Calappilia sp. The revision allows us to provide an emended diagnosis and new description of Mursiopsis pustulosus Ristori, 1889. Finally the assignment of Stenodromia mainii (Allasinaz, 1987) and Calappilia vicetina Fabiani, 1910, is briefly discussed.

A new raninoid crab (Decapoda, Brachyura, Raninidae) from the early Oligocene (late Rupelian) of Italy

Pasini & Garassino (2017) did a preliminary systematic review of the worldwide fossil species of Ranina, pointing out that it was used as “basket genus” for almost two centuries to accommodate different questionable taxa pertaining or not to Raninoidea. The result was that just four species were possibly assigned to Ranina compared to 23 species previously listed by Karasawa et al. (2014). Among the studied species, a rich sample of specimens from the early Oligocene of the Ligure-Piemontese Basin (Italy), previously assigned to Ranina speciosa by Ristori (1889) and Allasinaz (1987), has been revised and here discussed. The studied specimens are herein assigned to the new genus Alcespina, with A. ovadaensis n. sp (Raninidae), whereas five specimens of Allasinaz’s original sample are assigned to Raninoides H. Milne Edwards, 1837. Finally, this study allows us to review the questionable species Ranina berglundi Squires & Demetrion, 1992, from the early Eocene of Mexico, herein also assigned to A. berglundi (Squires & Demetrion, 1992) n. comb.

Anatomy of the Ligure-Piemontese subduction system: evidence from Late Cretaceous–middle Eocene convergent margin deposits in the Northern Apennines, Italy

In the Northern Apennines, in contrast to the Western Alps and Alpine Corsica, upper structural levels of the Late Cretaceous–middle Eocene subduction complex are still preserved and well exposed. This subduction complex developed in the Ligure-Piemontese basin since the Late Cretaceous time as a consequence of convergence between the Eurasia and Adria plates. Representative successions of this ancient subduction complex are well preserved in the Ligurian units of the Northern Apennines, where turbidite and mass-gravity deposits showing pristine stratigraphic features are present. Three main domains, represented by different groups of tectonic units, can be identified, each delineating a different domain of the subduction zone. In this article, we first present a brief history of geological research in the Northern Apennines during the last half of the twentieth century and then a comprehensive picture of the stratigraphy and tectonics of the Ligurian units. A new interpretation of the related tectonostratigraphic units is proposed within the conceptual modern geodynamic framework of convergent margins.

Deformation history of the ophiolite sequence from the Balagne Nappe, northern Corsica: insights in the tectonic evolution of Alpine Corsica

AbstractIn Alpine Corsica, the Jurassic ophiolites represent remnants of oceanic lithosphere belonging to the Ligure‐Piemontese Basin located between the Europe/Corsica and Adria continental margins. In the Balagne area, a Jurassic ophiolitic sequence topped by a Late Jurassic–Late Cretaceous sedimentary cover crops out at the top of the nappe pile. The whole ophiolitic succession is affected by polyphase deformation developed under very low‐grade orogenic metamorphic conditions. The original palaeogeographic location and the emplacement mechanisms for the Balagne ophiolites are still a matter of debate and different interpretations for its history have been proposed. The deformation features of the Balagne ophiolites are outlined in order to provide constraints on their history in the framework of the geodynamic evolution of Alpine Corsica. The deformation history reconstructed for the Balagne Nappe includes five different deformation phases, from D1 to D5. The D1 phase was connected with the latest Cretaceous/Palaeocene accretion into the accretionary wedge related to an east‐dipping subduction zone followed by a Late Eocene D2 phase related to emplacement onto the Europe/Corsica continental margin. The subsequent D3 phase was characterized by sinistral strike‐slip faults and related deformations of Late Eocene–Early Oligocene age. The D4 and D5 phases were developed during the Early Oligocene–Late Miocene extensional processes connected with the collapse of the Alpine belt. Copyright © 2003 John Wiley & Sons, Ltd.

Ophiolitic magmatism in the Ligurian Tethys: an ion microprobe study of basaltic clinopyroxenes

Ion microprobe data (REE, Na, Sc, Ti, V, Cr, Sr, Zr) of unaltered clinopyroxenes in the ophiolitic basalts from the Northern Apennines have been used in a epx-based geochemical modelling of MORB magmatism from both External (EL) and Internal (IL) sectors of the Ligurian Tethys (i.e. Jurassic Ligure-Piemontese basin), alternative to the more common whole-rock approach. Clinopyroxenes from EL basalts display slightly fractionated LREE (CeN/SmN∼0.5) and HREE (GdN/ YbN∼1.5) patterns and large variations in the REE composition (up to 6 times from microphenocryst cores to interstitial clinopyroxenes). Interstitial clinopyroxenes in IL basalts are similar to the microphenocrysts from the most primitive EL basalts. By contrast, IL microphenocrysts are characterized by greater LREE (CeN/SmN ∼0.3) and lesser HREE (GdN/YbN<1.2) fractionation. The comparison of trace element variations in wholerocks and clinopyroxenes clearly shows that the olivine and plagioclase portion of the fractionation sequence is poorly represented by the EL and IL basalts. In fact, ophiolitic basalts mainly consist of a minor interstitial glass (now deeply altered) associated with a prevailing plagioclase-clinopyroxene assemblage crystallized from liquids significantly evolved along the olivine-plagioclase-clinopyroxene saturation boundary. Thus, bulk rock chemistry is largely governed by clinopyroxene composition. This, in addition to alteration, indicates that the bulk rock chemistry does not provide reliable chemical information to constrain the composition and the generation of the parental magmas. Unfortunately, most clinopyroxenes are characterized by complex zoning, probably caused by disequilibrium partitioning during crystal growth as a result of kinetic factors. On this ground, estimation of melt chemistry and inferences about the origins of these basalts are only allowed by the core compositions of microphenocrystic clinopyroxenes. Modelling of (Nd/Yb)N and Ti/Zr in the parental magmas, as deduced from the clinopyroxene compositions, indicates thata EL and IL basalts do not represent products of different mantle source composition. Rather, they were generated by varying degrees of fractional melting in the spinel stability field, lower for the EL (a few percent) relative to IL, totalling no more than 10% of an asthenospheric MORB source, and leaving in the residua clinopyroxene with REE patterns similar to those shown by IL suboceanic type peridotites. Accordingly, these latter are interpreted as refractory residua after MORB-generating fractional melting occurred during rifting and opening of the Ligure-Piemontese basin. By contrast, residual clinopyroxenes from the EL subcontinental type peridotites are not consistent with low degrees of fractional melting in agreement with the current interpretation that EL peridotites are unrelated to the MORB magmatism in the Ligure-Piemontese basin and represent lithospheric mantle material already emplaced towards the surface by a tectonic denudation mechanism during the early stages of oceanic rifting.