Cooling and Vertical Motions of Crustal Wedges Prior to, During, and After Lateral Extrusion in the Eastern Alps: New Field Kinematic and Fission Track Data from the Mur‐Mürz Fault System

Abstract

New structural and thermochronological (zircon and apatite fission track) data from the eastern most Alps highlight distinct deformation phases affecting the Austroalpine unit along a major sinistral strike-slip fault system, the Mur-Mürz fault (MMF). The data link deformation to vertical motions prior to, during, and after the main phase of lateral extrusion of the orogen. Zircon fission track ages document rapid (ca. 15°C/Myr) and diachronous (eastward younging) cooling and rock exhumation during the latest Cretaceous to Paleocene. Subsequent regional Eocene to early Miocene cooling below the closure temperature of the apatite fission track system occurred at slow rates (ca. 2°C/Myr), suggesting that the region was not subject to major surface uplift and erosion during that period. Fault kinematic analysis along the MMF document pre-extrusion NNW-SSE contraction, middle Miocene syn-extrusion NE-SW to NNE-SSW directed shortening, and Late Miocene E-W contraction. All phases are characterized by strike-slip fault regimes. Formation of the complex MMF zone triggered the exhumation of small, fault-bound crustal blocks within the fault zone as documented by middle Miocene apatite fission track ages. Overall, ages are similar on both sides of the fault suggesting that lateral extrusion along the MMF was not associated with significant differential vertical motions. Local Pliocene rock cooling and exhumation was probably related to the buttressing effect of the underthrust Bohemian basement spur. Whereas large-scale, post-extrusion surface uplift of the extruding crustal wedges, such as the "Styrian block," must have been related to long-wavelength deformation processes affecting the easternmost Alps.

Post-Miocene tectonics of the Northern Calcareous Alps

“…Along SEMP and KLT, Late Miocene E-W compression was followed by N-S extension interpreted as a response to the growing topography 2 , 3 . Fault kinematic analysis and apatite fission track ages suggest a Late Pannonian-Pontian E-W contraction along the MM 6 . The 500 m of uplift, during the last 4 Ma, south from MM was likely to result from renewed N-S compression 26 .…”

“…1 ). To the south, the sinistral Mur-Mürz (MM) fault system continues into the Vienna Basin Transfer fault system (VBT) accommodating eastward extrusion 6 (Fig. 1 ).…”

“…After the subduction of the Penninic ocean followed by the collision of Adria with Europe in the Eocene, the Eastern Alps experienced an eastward lateral motion of crustal blocks between the sinistral Salzachtal-Ennstal-Mariazell-Puchberg fault (SEMP) to the north, and the dextral Periadriatic Fault (PA) to the south, since the Miocene, generally referred to as lateral extrusion 1 – 6 (Fig. 1 ).…”

Strain partitioning during formation of oroclines and indentation

“…5.1b; e.g., Lyberis et al, 1992;Mantovani et al, 2006;Martinod et al, 2000;Perinçek and Çemen, 1990;Smit et al, 2010) or Adria-Europe convergence (Fig. 5.1c; e.g., Frisch et al, 1998;Neubauer et al, 2000;Ratschbacher et al, 1991b;Rosenberg et al, 2004;van Gelder et al, 2020;van Unen et al, 2019a;Wölfler et al, 2011). In all these situations, indentation-induced deformation is affected by extension driven by the slab roll-back of another subduction zone (Fig.…”

“…Indentation tectonics is observed in many orogenic systems (e.g., Argand, 1924;McKenzie, 1972;Ratschbacher et al, 1991a, b). For instance, the indentation of the Indian indenter is accommodated by shortening and uplift of the Himalayas and Tibet, and lateral extrusion towards SE Asia (Chen et al, 2000;Molnar and Tapponier, 1975;Searle et al, 2011;Shen et al, 2001;Sternai et al, 2016;Tapponier et al, 1986), shortening in front of the Arabian indenter is accompanied by Anatolian extrusion towards the Aegean (Kaymakci et al, 2010;Mantovani et al, 2006;Martinod et al, 2000;Regard et al, 2005), or the post-Oligocene shortening in the Eastern Alps is accompanied by extrusion towards the Pannonian-Carpathians region (Frisch et al, 1998;Neubauer et al, 2000;Ratschbacher et al, 1991b;Rosenberg et al, 2018;van Gelder et al, 2020, Wölfler et al, 2011. The lateral transfer of collisional shortening to other subduction systems is often facilitated by a complex strike-slip and transpressional/transtensional deformation associated with significant strain partitioning (e.g., Jolivet et al, 2018;van Hinsbergen et al, 2020;van Unen et al, 2019a).…”