Satellite gravity for defining continuity of hidden crustal scale subduction induced mass only partially visible at the surface- the example of the Caucasian subduction belts.

The objective of this work is to infer evidences of geologic and tectonic units in the area of the Greater and Lesser Caucasian belts, located between the Black Sea and the Caspian Sea. Thermochronological data have been used to define cooling episodes along the Amasia-Sevan-Akera Suture, the northern margin of the Neotethys [1]. The cooling has originated from obduction and continental accretion along the northern branch of the Neotethys, which had closed due to the ocean subduction. The oceanic plate subduction is typically accompanied by back arc magmatism, and lower crustal intrusions on the upper plate outboard of the contact between subducting and upper plate, leading to a long-lasting alteration of the crust through densification. The following continental accretion, obduction and uplift that are documented in the Caucasus are expected to bring the dense rocks associated with the subduction arc to the surface or at mid crustal depths, depending on the amount of uplift. The denser crust is visible through the gravity field, which thanks to satellite observations is globally available, and will be improved with the future gravity missions NGGM/MAGIC (e.g. [2]). The gravity field requires several processing steps, in order to be able to invert for the intracrustal masses, which include correction for topography, and crustal thickness, and isostatic considerations. A clear band of dense crust runs parallel to the Amasia-Sevan-Akera Suture and continues northwestward in the region of the Adjara-Trialeti fold-and-thrust belt. Such band of intracrustal dense material broadly defines the Somkheto-Karabakh  magmatic arc (i.e. the product of northward subduction of the northern Neotethys underneath the Eurasian plate southern margin) over a distance of ca. 800 km. Assuming that our interpretation is correct, a second intracrustal dense band parallel to the former, but displaced 100 km southwards, may suggest the presence of either a second subduction zone or a large crustal duplex. Post-subduction exhumation  is documented by low-temperature thermochronologic ages clustering around 19-16 Ma [1]. Arabian subduction beneath Anatolia along the Bitlis Zagros suture zone some 400 km south of the Amasia-Sevan-Akera Suture is not marked by a band of intracrustal high density, in agreement with the absence of a well developed magmatic arc. Our interpretative model is a working hypothesis, which requires field work to be verified, but is a means to guide the locations where the field work is most efficient to allow to verify our hypothesis. The study is embedded in a major project addressing the “Intraplate deformation, magmatism and topographic evolution of a diffuse collisional belt: Insights into the geodynamics of the Arabia-Eurasia collisional zones” financed by the Italian Ministry (PRIN 2017). The interest in the study lies also in applications of future gravity missions, which are expected to result in improved spatial resolution and improved sensitivity to the gravity field [2, 3, 4].

[1] Cavazza et al. (2019) https://doi.org/10.1016/j.gsf.2019.02.007

[2] Pivetta et al. (2022) https://doi.org/10.3390/rs14174278

[3] Pivetta et al. (2021) https://doi.org/10.1007/s00024-021-02774-3

[4] Migliaccio et al. (2023, Surveys in Geophysics, in press)