Calcite twinning strains in Alpine orogen flysch: Implications for thrust-nappe mechanics and the geodynamics of Crete

Abstract

Recent uplift in Crete, along the Hellenic trench, has exposed the internal portions of Alpine nappes that were transported south along north-dipping thrusts from the middle Cretaceous to the Oligocene (100-20 Ma). Flysch deposited along the leading, southern edge of the Alpine foreland basin dates the timing of nappe transport and contains twinned calcite in synorogenic limestones and calcite veins that record the stress-strain fi elds associated with nappe transport and stacking for a 17 Ma period between 35 and 18 Ma. The fl ysch ages young away from the craton thereby documenting thin-skin thrusting and nappe motion, for the fi rst time, toward the trench.Fossiliferous marine limestones and calcareous shales were deposited in marginal basins on the leading edges of four thrust-nappes in Crete: the higher and older fl ysch deposits (Asteroussia, Pindos) are underlain by younger fl ysch deposits of the Tripolitza and Plattenkalk nappes. The ages of these fl ysch deposits are well constrained and the fl ysch sequences are now found deformed between nappes without visible signs of Alpine metamorphism, except the lowest Plattenkalk fl ysch. Field observations in the fl ysch deposits include variable bedding, vein, fold axis, and kinematic orientations. Twinned calcite in the fl ysch limestones (16 samples, n = 425) and veins (23 samples, n = 612) generally preserve subhorizontal, in-transport shortening and vertical extension; where there is a strain overprint (high negative expected values [NEV]), a vertical shortening strain with transport-parallel extension (~N-S) is preserved. Strain magnitudes are greater in the vein sample suite, and differential stress magnitudes responsible for twinning are −230 bars for the entire sample suite. The tectonic evolution of the region involved thin-skinned shortening with south-vergent nappe formation, from north to south, followed by thrust motion on the Cretan detachment. Today, metamorphosed rocks (i.e., Phyllite-Quartzite unit [PQU]) overlie the basal Plattenkalk nappe placing earlier exhumed rocks of the Asteroussia, Tripolitza, and Pindos nappes on top of high-pressure (HP)-metamorphic rocks of the PQU without the deposition of any fl ysch.

Geodynamic significance of the Santorini Detachment System (Cyclades, Greece)

“…Commonly the pre‐volcanic rocks exposed on Santorini have been correlated with the Phyllite‐Quartzite and Tripolitza units mainly based on the exposure of several hundreds of metres of Upper Triassic shallow water carbonates at Profitis Ilias and Mésa Vounó (Bonneau, ; Craddock et al., ; Kilias et al., ). However, based on the observation that the rocks at Athinios experienced high‐pressure/low‐temperature conditions overprinted by greenschist facies metamorphism, Skarpelis and Liati () suggested that these rocks belong to the CBU.…”

“…Although excellent maps of the volcanic units exist (e.g., Druitt et al., ), the pre‐volcanic basement (Figure ) exposed at Athinios, on Profitis Ilias‐Mésa Vounó and a few other outcrops is barely mapped in detail (Skarpelis & Liati, ). The longstanding view is that metasedimentary and metavolcanic rocks at Athinios belong to the Phyllite‐Quartzite Unit and the marbles at Profitis Ilias correlate with the Tripolitza limestones on Crete (Bonneau, ; Craddock, Klein, Kowalczyk, & Zulauf, ; Kilias, Mountrakis, Tranos, & Pavlides, ), a view which has been supported by reports of Megalodons (bivalve molluscs) within the marbles (Papastamatiou, ). Based on other fossil assemblages, an early Palaeocene age has been suggested for flysch‐type sediments that occur within the marbles (Tataris, ).…”

Upper crustal emplacement and deformation of granitoids inside the Uppermost Unit of the Cretan nappe stack: constraints from U–Pb zircon dating, microfabrics and paleostress analyses

“…A decreasing trend in grain size is obvious with increasing distance from the intrusive contact. n number of measured grains (Craddock et al 2009). The calcite twinning fabrics and striation kinematics preserve a transport-parallel shortening strain and top-to-south motion for a deformational period between 35 and 18 Ma (Craddock et al 2009).…”

Back to Normal: Direct Evidence of the Cretan Detachment as a North‐Directed Normal Fault During the Miocene

“…Other investigations favor that the Cretan Detachment is a normal fault on top of an extruding wedge, which was active during convergence (e.g., Chatzaras et al, 2006;Ring & Reischmann, 2002;Xypolias & Kokkalas, 2006). Except for a few earlier studies, which clearly lack modern structural and tectonic concepts (Greiling, 1982;Wachendorf et al, 1975), several recent studies argue that the tectonic contact at the structural level of the proposed Cretan Detachment is a top-to-the S thrust fault (Chatzaras et al, 2006;Craddock et al, 2009;Klein et al, 2012;Ring & Yngwe, 2018;Zulauf et al, 2002Zulauf et al, , 2008.…”