Understanding the closure of Alpine Tethys in the Western Carpathians using Receiver Functions

Recent receiver function results from a passive seismic experiment have provided new insights into the geodynamic evolution of the Western Carpathians, the eastern extension of the Alps, formed in part by the closure of the Alpine Tethys. The Pieniny Klippen Belt (PKB) represents this closure at the surface, characterised by a narrow, elongated geometry dividing the external fold-and-thrust belt of the Outer Western Carpathians and the Central Western Carpathians. Unlike typical sutures, the PKB lacks ophiolites or high-pressure metamorphic rocks, instead it consists of resistant limestone blocks within a matrix of non-resistant flysch deposits, forming a distinctive “block-in-matrix” structure. This configuration has traditionally been attributed to the hypothesized Czorsztyn ridge, an island-like feature within the Alpine Tethys, where limestone deposition has been thought to occur. The ridge is supposed to correspond to the Briançonnais unit in the Alps, though evidence for its existence remains tenuous.

The current passive seismic experiment seeks to validate or refute the Czorsztyn ridge hypothesis. In May 2023, 18 broadband seismic stations were deployed along a north-south trending profile, under the umbrella of the Adria Array, complemented by 9 other permanent and temporary stations. This 27-station dense network enabled the extraction of receiver functions and the creation of Common Conversion Point (CCP) stack images to resolve the sub-surface geometry of the region.

Preliminary findings challenge the Czorsztyn ridge model. No distinct continental crustal body – interpretable as the Czorsztyn ridge basement and separate from the northern European platform or ALPCAPA – is evident beneath the PKB. Instead, subsurface structures appear complex, showing similarity to those in the Vienna Basin, located between the Eastern Alps and the Western Carpathians. A blind detachment fault occurs in the deep basement of the Outer Western Carpathians and connects southward with mid-crustal detachments in the Central Western Carpathians. Furthermore, a 40 km wide gap in Moho signature of the receiver functions beneath the PKB may reflect the position of the suture at a lower crustal level. Additionally, the Steimberg Fault in the Vienna basin likely correlates with the PKB, as both exhibit a displacement with partly strike-slip kinematics. Continued data collection and analysis will refine these interpretations and advance the understanding of the tectonic evolution of Western Carpathians.