The missing craton edge: Crustal structure of the East European Craton beneath the Carpathian Orogen revealed by double-difference tomography

Abstract

The Trans-European Suture Zone (TESZ) is the most important and extensive continental suture in Europe, marking the edge of the East European Craton (EEC), from the North Sea to the Black Sea. It corresponds to significant changes in surface geology and deep crustal structure, evident in seismic, gravitational and magnetic studies. However, the TESZ disappears beneath the Eastern Carpathians accretionary nappes and Neotethys ophiolites, thrusted over the subducted EEC passive margin in Romania that may have experienced progressive southward break-off generating post-collisional volcanism and anomalous seismicity in the Vrancea region of the South-East Carpathians. To illuminate the missing TESZ section and investigate the change in crustal properties from the Precambrian EEC across the collisional orogen and the impact of related volcanism, we determined a 3D seismic model of P and S wave velocities of the Eastern Carpathians in Romania. With the advent of new permanent broadband and short-period seismic stations of the Romanian Seismic Network, we were able to lower the earthquake magnitude detection threshold to 0.5 ML, largely expanding the earthquake database and create seismic images of the crust across the Carpathians. Using double-difference tomography and waveform cross-correlation differential times, we relocated local crustal earthquakes between 2010 and 2017 and jointly inverted for the 3D P and S -wave velocity structure down to the Moho discontinuity. Our study provides the highest resolution 3D crustal seismic model of this area to date and emphasizes the manifestation of surface tectonic boundaries at lower crustal depths. The TESZ is highlighted at mid-to lower-crustal depths beneath the Carpathian nappes, east of the post-collisional volcanic belt, as a gently-dipping transition from high Vp in the eastern footwall, the dipping Precambrian basement, to low Vp beneath the Carpathian Orogen to the west and a downward decrease in Vp/Vs. In parts of the volcanic region, Vp/Vs ratios increase suggesting the presence of fluids, mafic material and partial melts that may have altered the seismic structure of the TESZ. Relocated hypocenters are vertically distributed in the velocity transition zone from low to high especially beneath the youngest volcanoes in the south, consistent with a hypothetically active magmatic plumbing system. In the northern segment, where volcanoes are older, relatively high velocities are estimated and seismicity tends to cluster in the top 10 km, likely connected to a recently reactivated fault system. Seismicity in the foreland of the East Carpathians aligns along the NW-SE trending major crustal faults and continues beneath the thrust belt, suggesting the East European Craton basement extends beneath the orogen as far as the volcanic belt.