Birth and closure of the Kallipetra Basin: Late Cretaceous reworking of the Jurassic Pelagonian-Axios/Vardar contact (northern Greece)

Some 20 Myr after the Late Jurassic to Early Cretaceous obduction and collision at the eastern margin of Adria, the eroded Pelagonia (Adria) Axios/Vardar (oceanic complex) contact collapsed, forming the Kallipetra Basin, described around the Aliakmon River near Veroia (northern Greece). Clastic and carbonate marine sediments deposited from the early Cenomanian to the end of the Turonian, with abundant olistoliths and slope failures at the base due to active normal faults. The middle part of the series is characterized by red and green pelagic limestones, with a minimal contribution of terrigenous debris. Rudist mounds in the upper part of the basin started forming on the southwestern slope, and their growth competed with a flux of ophiolitic debris, documenting the new fault scarps affecting the Vardar oceanic complex (VOC). Eventually, the basin was closed by overthrusting of the VOC towards the northeast and was buried and heated up to ~ 180 °C. A strong reverse geothermal gradient with temperatures increasing up-section to near 300 °C is recorded beneath the VOC by illite crystallinity and by the crystallization of chlorite during deformation. This syntectonic heat partially reset the zircon fission track ages bracketing the timing of closure just after the deposition of the ophiolitic debris in the Turonian. This study documents the reworking of the Pelagonian Axios/Vardar contact, with Cenomanian extension and basin widening followed by Turonian compression and basin inversion. Thrusting occurred earlier than previously reported in the literature for the eastern Adria and shows a vergence toward the northeast, at odds with the regional southwest vergence of the whole margin but in accordance to some reports about 50 km north

Carbonate deformation through the brittle-ductile transition: The case of the SW Helvetic nappes, Switzerland

Carbonate deformation through the brittle-ductile transition (BDT) remains incompletely documented in the field. We therefore investigate the exhumation of the SW-Helvetic nappe stack using a new multi-method approach that integrates optical observations with a revised nomenclature, thermochronology, stable isotopes, and clumped isotope thermometry, aiming to constrain the time-temperature history of BDT deformation processes in carbonates. Single grain (U–Th)/He zircon and apatite fission track ages establish new burial/exhumation trajectories from different nappes, allowing us to infer the thermal history of the Rawil Depression. This prominent doubly-plunging hinge zone between the Mont Blanc and Aar Crystalline Massifs underwent post-nappe faulting, now constrained to the Tortonian-Early Pliocene, reflected in differential exhumation rates of ∼0.2 km/Myr between the most depressed area and its easternmost side. Calculating and modelling the rock-buffered clumped isotope temperatures (ranging from ∼250 °C to 55 °C), we indirectly date the BDT processes, exemplified by the Rezli Fault. On this structure, possible shear heating during mylonitisation is indirectly dated at 18–15.5 Ma. Progressive embrittlement started around 11–9 Ma at temperatures of about 150–110 °C. Lower clumped isotope temperatures correspond to recent brittle faulting between 9 and 5 Ma. Our results on the regional evolution can be applied to analogous seismogenic carbonate-rich crustal sections.ISSN:0191-814

Tectono-thermal evolution of a distal rifted margin: constraints from the Calizzano Massif (Prepiedmont-Briançonnais domain, Ligurian Alps)

The thermal evolution of distal domains along rifted margins is at present poorly constrained. In this study, we show that a thermal pulse, most likely triggered by lithospheric thinning and asthenospheric rise, is recorded at upper crustal levels and may also influence the diagenetic processes in the overlying sediments, thus representing a critical aspect for the evaluation of hydrocarbon systems. The thermal history of a distal sector of the Alpine Tethys rifted margin preserved in the Ligurian Alps (Case Tuberto-Calizzano unit) is investigated with thermochronological methods and petrologic observations. The studied unit is composed of a polymetamorphic basement and a sedimentary cover, providing a complete section through the prerift, synrift, and postrift system. Zircon fission track analyses on basement rocks samples suggest that temperatures exceeding ~240\uc2\ua0\uc2\ub1\uc2\ua025\uc2\ub0C were reached before ~150\ue2\u80\u93160\uc2\ua0Ma (Upper Jurassic) at few kilometer depth. Neoformation of green biotite, stable at temperatures of ~350 to 450\uc2\ub0C, was synkinematic with this event. The tectonic setting of the studied unit suggests that the heating-cooling cycle took place during the formation of the distal rifted margin and terminated during Late Jurassic (150\ue2\u80\u93160\uc2\ua0Ma). Major crustal and lithospheric thinning likely promoted high geothermal gradients (~60\ue2\u80\u9390\uc2\ub0C/km) and triggered the circulation of hot, deep-seated fluids along brittle faults, causing the observed thermal anomaly. Our results suggest that rifting can generate thermal perturbations at relatively high temperatures (between ~240 and 450\uc2\ub0C) at less than 3\uc2\ua0km depth in the distal domains during major crustal thinning preceding breakup and onset of seafloor spreading

Thermochronological Record of a Jurassic Heating-Cooling Cycle Within a Distal Rifted Margin (Calizzano Massif, Ligurian Alps)

The aim of the present study is to analyse, through thermochronological investigations, the thermal evolution of a fossil distal margin owing to the Alpine Tethys rifting system. The studied distal margin section consists of a polymetamorphic basement (Calizzano basement) and of a well-developed Mesozoic sedimentary cover (Case Tuberto unit) of the Ligurian Alps (NW Italy). The incomplete reset of zircon (U-Th)/He ages and the non-reset of the zircon fission track ages during the Alpine metamorphism indicate that during the subduction and the orogenic stages these rocks were subjected to temperatures lower than ~200 ºC. Thus, the Alpine metamorphic overprint occurred during a short-lived, low temperature pulse. The lack of a pervasive orogenic reset, allowed the preservation of an older heating-cooling event that occurred during Alpine Tethys rifting. Zircon fission-track data indicate, in fact, that the Calizzano basement records a cooling under ~240 °C, at ~156 Ma (early Upper Jurassic). This cooling followed a Middle Jurassic syn-rift heating at temperatures of about 300-350°C, typical of greenschist facies conditions occurred at few kilometres depth, as indicated by stratigraphic and petrologic constraints. Thus, in our interpretation, major crustal thinning likely promoted high geothermal gradients (~60-90°C/km) triggering the circulation of hot, deep-seated fluids along brittle faults, causing the observed thermal anomaly at shallow crustal level