Relationship between magnetic fabrics and deformation of the Miocene Pohorje intrusions and surrounding sediments (Eastern Alps)

Abstract

The Miocene deformation history of magmatic and host metamorphic rocks and surrounding sediments was reconstructed by measuring meso-and microscale structures and anisotropy of magnetic susceptibility (AMS) data in order to constrain the structural evolution of the Pohorje pluton during the onset of lithospheric extension at the Eastern Alps-Pannonian Basin transition. Principal AMS axes, lineation and foliation are very similar to mesoscopic lineation and foliation data from the main intrusive body and from some dykes. Although contribution from syn-magmatic texture is possible, these structures were formed during the cooling of the pluton and associated subvolcanic dykes just shortly after the 18.64 Ma pluton intrusion. Dykes emplaced during progressively younger episodes reflect decreasing amount of ductile strain, while firstly mesoscopic foliation and lineation, and then the tectonic AMS signal gradually disappears. In the structurally highest N-S trending dacite dykes, the AMS fabric only reflects the magmatic flow. The Miocene sediments underwent the same, NE-SW to E-W extension as the magmatic and host metamorphic rocks as indicated by both AMS and fault-slip data. All these events occurred prior to ~ 15 Ma, i.e., during the main syn-rift extension of the Pannonian Basin and during the fastest exhumation of the Tauern and Rechnitz windows, both demonstrating considerable extension of diverse crustal segments of the Alpine nappe pile. After a counterclockwise rotation around ~ 15 Ma, the maximum stress axis changed to a SE-NW orientation, but it was only registered by brittle faulting. During this time, the overprinting of a syn-rift extensional AMS texture was not possible in the cooled or cemented magmatic, metamorphic and sedimentary rocks.

A wealth of P–T–t information from metasediments in the HP–UHP terrane of the Pohorje Mountains, Slovenia, elucidates the evolution of the Eastern Alps

“…Isobaric heating of sample 18Slo39 to 577 ± 23°C (Figure 9c, d) is not in conflict with petrographic observations. The estimated pressure range of 0.66 ± 0.10 GPa (around 23‐km depth) overlaps with previously reported pressures (0.6–0.7 GPa, 16–20 km depth given by Fodor et al, 2021, although densities of upper continental crust, average metasediment, et cetera, scattering around 2.8 g/cm 3 at 1.0 GPa and 600°C according to Massonne et al, 2007, would lead to depths of 21 to 24.5 km) for the deepest intrusion level of the presently exposed Pohorje pluton in the southeastern Pohorje Mts (Altherr et al, 1995; Fodor et al, 2008, 2020, 2021; Márton et al, 2006; Poli et al, 2020). It is, thus, likely that metamorphic rocks close to the intrusion were heated by the pluton at a mean pressure of 0.65 GPa corresponding to 22–23‐km depth.…”

“…In the core of the dome, magmas forming the granodioritic Pohorje pluton and subvolcanic dacite bodies intruded the host metamorphic rocks in the Miocene (Figure 1). The intrusive contact is marked by a thin metamorphic aureole (Fodor et al, 2020). The Pohorje pluton formed at variable depth from ~4 to 16–20 km considering westward tilting with the deepest intrusion depth in the southeastern pluton (Fodor et al, 2021; Poli et al, 2020), which was related to a lithostatic pressure of 0.6–0.7 GPa (Altherr et al, 1995; Fodor et al, 2008, 2020, 2021; Márton et al, 2006; Poli et al, 2020).…”

“…Cooling ages for the pluton were obtained by K‐Ar dating on biotite, amphibole, feldspar and whole rock (18–15 Ma; Fodor et al, 2002, 2008; Trajanova et al, 2008), as well as zircon fission‐track dating (17.7–15.6 Ma; Fodor et al, 2008). Considering these cooling ages and the intrusion age of the Pohorje pluton, it was emplaced and rapidly cooled within a few million years (Fodor et al, 2008, 2020; Poli et al, 2020).…”

Torn Between Two Plates: Exhumation of the Cer Massif (Internal Dinarides) as a Far‐Field Effect of Carpathian Slab Rollback Inferred From 40Ar/39Ar Dating and Cross Section Balancing

“…Continued crustal extension (step 6) induced warping of the exhuming middle crust into a dome (e.g., Wernicke, 1985), associated with the emplacement of S-type melts (e.g., Lister & Baldwin, 1993). Such structures have also been observed in the Pannonian Basin, for example in the Great Hungarian Plain, where the WNW part of the Derecske subbasin exhumes Mesozoic metamorphic rocks (Balázs et al, 2016); as well as in the Pohorje dome at the westernmost margin of the Pannonian Basin (Fodor et al, 2020). The driving forces responsible for the exhumation of Cer are ambiguous but might involve isostatic reequilibration of hot mantle lithosphere due to tectonic denudation (Lister & Davis, 1989;Wernicke, 1985) or lower crustal flow (Block & Royden, 1990).…”

Miocene syn-rift evolution of the North Croatian Basin (Carpathian–Pannonian Region): new constraints from Mts. Kalnik and Požeška gora volcaniclastic record with regional implications

“…However, such interpretations are questionable since petrogenesis and geodynamic setting of the Early-Middle Miocene magmatic rocks occurring in the relative vicinity of PFZ is either generally unresolved (Neubauer et al 2018), or, as is the case of Mt. Pohorje (Slovenia) igneous complex, linked with CPR (PB) evolution (Fodor et al 2008(Fodor et al , 2020Poli et al 2020; see also Schefer et al 2011;Ji et al 2019;Kästle et al 2020).…”