Active Transpression and Crustal Extrusion at the Adria-Eurasia Junction: Insights from Slovenia GNSS Data

The territory of Slovenia sits in the NE corner of the active collisional boundary between the Eurasian plate and the Adriatic microplate, which at this location near its Euler pole, indents north-northeastward at a velocity of 2-4 mm/yr. Using velocities from more than 40 permanent GNSS stations located within Slovenia and immediate surroundings we analyzed the regional distribution and structural style of active tectonic deformation and assessed activity along and across the main fault systems. (Stations are in the SIGNAL network and the national zero-order geodetic network.) From 2010-2025 GNSS observations, we determined daily coordinates in a Eurasian reference frame, used these to establish coordinate time series, and derived horizontal velocity vectors at each site. Data from more than 70 campaign GNSS stations surveyed between 1994 and 2016 were also similarly analyzed. By interpolating between the obtained point velocity vectors, we calculated a continuous strain-rate field. Velocity profiles were then constructed and analyzed for elastic strain accumulation across major fault systems. Relocated earthquake hypocenters from 2004-2024 and the new high-quality ARSO focal mechanism database were used to enhance our analysis.

Based on the distribution of strain-rates and seismicity, we divide Slovenia into three distinct active deformation domains: 1) The coastal zone, which is on the Adriatic microplate, exhibits minor (< 1 mm/yr) internal deformation and shows a pronounced counterclockwise rotation; 2) A central transpressive zone, which is approximately 70 km wide and extends from NW Slovenia eastward and southeastward into the Sava Folds, accommodates the highest rates of deformation in Slovenia and exhibits the highest shear strain rates, which is reflected by the abundance of strike-slip and oblique-reverse focal mechanisms there. In its western segment, approximately 40% of the deformation occurs along the Periadriatic fault system; the remainder is accommodated by Dinaric faults; 3) NE Slovenia, located east and north of the Periadriatic fault system, behaves as a rigid block translating (extruding) slowly (< 1 mm/yr) toward the NE with negligible internal deformation; this domain probably belongs to the slowly extruding ALCAPA/Pannonian block that has been suggested in earlier studies.