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From a Continental Margin to an Alpine Dome: 4D Geodynamic Evolution of the Aar Massif

Ferdinando Musso Piantelli, Lukas Nibourel, Alfons Berger, Marco Herwegh — Sun, 08 Feb 2026 00:00:00 +0000

The inversion of crystalline basement units of former passive continental margins is a typical feature of late-stage collisional mountain belts. In this study, a new large-scale 3D geological model of the main lithostratigraphic and structural units of the Aar Massif was built to investigate how the 3D geometry of the passive margin and inherited extensional structures influenced the tectonic evolution during late-stage continent-continent collision. Cross-section restoration of such units and of metamorphic peak temperature data allowed us to reconstruct the 4D geodynamic evolution of the massif during the late-stage Alpine orogeny. Our results show that: (i) The Aar Massif results from the inversion of the unevenly stretched proximal zone of the passive European margin. (ii) 20% of the present-day structural relief of the massif is a legacy of the 3D architecture of the Permian to Mesozoic passive continental margin. Mesozoic rifting structures formed larger-scale basins separated by a central topographic high subdividing the Aar Massif into three SW-NE trending basement blocks. (iii) The inherited spatial variations in graben/basins geometry and associated sediment and crustal thickness variations caused an in-sequence, non-cylindrical exhumation of the Aar Massif, controlled by a dense network of reverse and thrust faults. This interplay between crustal dynamics and crustal inheritance resulted in the uplift and emplacement of the Aar Massif. The results of this study highlight the importance of integrating regional-scale 3D geological modelling in the analysis of complex geological systems, such as orogens. By enabling a comprehensive understanding of their 4D evolution and geodynamic processes, this methodology opens the door to a wide range of applications in tectonics, resource exploration, and hazard assessment.

Unraveling the Kinematics and Morphotectonics of the Petrinja Fault (Croatia), Source of the 2020 M 6.4 Earthquake

Mon, 09 Mar 2026 00:00:00 +0000

The 2020 Mw 6.4 Petrinja earthquake in central Croatia is one of the European strongest continental earthquakes in recent decades. This event shed light on the poorly investigated Petrinja-Pokupsko Fault (PPKF) zone, a right-lateral fault system accommodating a fraction of the shortening between the Adria and European plates. Through field observations and high-resolution Lidar-derived digital elevation models of the central section of the PPKF, we precisely mapped the fault trace, revealing a discontinuous geometry with a main fault strand and left-stepping segments in agreement with the position of the 2020 surface ruptures. To the north, the accumulated relief from this transpressive fault system decreases, and the fault trace becomes less distinct, suggesting a northward propagation of the deformation. Cumulative offsets of 4 to 24 meters along the main fault strands in the central and southern sections of the fault attest to its Quaternary activity. Dating results suggest offset markers at the Marine Isotopic Stage 4 (MIS 4), the Late Glacial Maximum (LGM) or the Early Holocene periods, allowing for the first direct estimation of the fault slip rates. Preliminary estimates indicate that, assuming a common MIS 4 or LGM age for the investigated markers, fault slip rates range from 0.2 to 0.7 mm/yr and 0.7 to 1.6 mm/yr, respectively. In contrast, assigning Early Holocene ages would imply much higher - and likely unrealistic - slip rates of 1.6 to 3.9 mm/yr. Although the estimated loading rates vary greatly and depend on strong assumptions regarding the age of the markers abandonment, our results suggest a minimum fault slip-rate of 0.2 mm/yr for local seismic hazard assessments.

Setting the Sequence of Slicing Events Along Deep Subduction Interfaces: 1. The Tectonic and Thermal Structure of the High-P Duplex in Western Crete (Hellenic Margin)

Thu, 09 Apr 2026 00:00:00 +0000

Basal accretion at active subduction margins occurs through a series of tectonic slicing events at varying depths along the plate interface, shaping the forearc domain. To assess the spatial and temporal scale of the accretion-controlled forearc dynamics, it is crucial to constrain the sequence of basal-accretion episodes that form deep accretionary duplexes. This requires identifying the successive tectono-metamorphic units constituting paleo-duplexes and dating the accretion and exhumation events that expose high-pressure rocks at the surface. This first contribution of two companion papers (this issue) presents a detailed reconstruction of the tectonic and thermal structure of a high-pressure/low-temperature paleo-accretionary duplex in western Crete (Greece) that formed along the active Hellenic margin during the Oligocene-Miocene. Combining field observations, structural measurements and Raman spectroscopy on carbonaceous material (RSCM), we identify five tectono-metamorphic slices (i) bounded by shear zones often reworked during exhumation and (ii) characterized by a down-stepping of peak metamorphic temperatures towards lower structural levels. Our geological and structural mapping reveals the overall geometry of the nappe stack forming a dome-like structure, exhumed beneath major top-to-the-N and subordinate top-to-the-S detachments that accommodated N-S-directed crustal extension. This trench-perpendicular extension was intermittently rotated into an E-W direction (trench-parallel), as evidenced by a newly recognized top-to-the-W ductile-brittle detachment. Minor compressional events did not significantly alter the 3D architecture of the paleo-duplex. Reported RSCM peak metamorphic temperatures of ~350-450 °C from the nappe stack align with the typical temperature range for the downdip limit of the seismogenic zones, suggesting a first-order thermo-mechanical control on the depth of basal accretion along the subduction interface. These findings provide crucial constraints for interpreting the deep-accretion and exhumation dynamics that shaped the long-term evolution of the Hellenic forearc domain.

Setting the Sequence of Slicing Events Along Deep Subduction Interfaces: 2. P-T Conditions and Timing of Accretion and Exhumation in Western Crete (Hellenic Margin)

Thu, 09 Apr 2026 00:00:00 +0000

To understand basal-accretion dynamics in subduction zones and forearc crustal response, it is crucial to constrain the timing of slicing events forming high pressure-low temperature accretionary duplexes. This second contribution investigates the pressure-temperature-time history of tectono-metamorphic units in the paleo-duplex of western Crete, accreted along the Hellenic subduction zone during the late Oligocene-Miocene. Petrological characterization, thermodynamic modeling and a data review reveal peak metamorphic conditions evolving from 17-18 kbar and 410-430 °C to 7-8.5 kbar and 310-360 °C from top to base of the nappe stack. These results suggest a decrease in basal-accretion depth from 55-60 km to 25-30 km, likely linked to an increase in the subduction-related geothermal gradient. New Rb/Sr multi-mineral ages show a consistent decrease toward the base of the duplex, except for the lowermost Plattenkalk Unit. These ages, along with the down-stepping of peak conditions, reveal two slicing episodes between ~26 Ma and ~15 Ma, and likely three additional events from the late Oligocene to middle Miocene. (U-Th-Sm)/He thermochronology on zircon indicates rapid exhumation during the middle Miocene, with rates of ~3-11 mm/yr, decreasing to ~2-4 mm/yr at shallow levels. This dynamics was driven by the accelerating southward retreat of the Hellenic subduction, enhanced by slab tearing from ~15 Ma, contributing to the geothermal gradient increase. This study further suggests a sequence of ~2-3-Myr-long deep slicing events, providing a critical timescale for monitoring the tectonic and topographic signatures of deep mass fluxes along active margins worldwide. It also supports ongoing basal-accretion events beneath Crete, contributing to the island’s emergence.