Spatial extent of deep slab slicing events: Insights from the Phyllite-Quartzite paleo-accretionary wedge (Hellenic subduction zone)

The transfer of material from the downgoing plate to the overriding plate at depth exerts a first-order control on the mechanical and thermal evolution of subduction zones. However, the geometry, size, and temporal organization of deep tectonic slices formed during basal accretion remain poorly constrained, due to the limited resolution of geophysical imaging and the rarity of geological analogues preserving deep accretionary architectures formed during continental subduction.

Here, we investigate the spatial extent and stacking dynamics of tectonic slices by reconstructing the architecture of the deep paleo-accretionary wedge through the study of the exhumed Phyllite-Quartzite (PQ) nappe, derived from microcontinental protoliths, which was formed by basal accretion during the Oligo-Miocene along the Hellenic subduction zone. We carried out a multidisciplinary study of this now-exhumed PQ nappe, which crops out discontinuously from Crete to the Peloponnese (Greece). Preserved in a fore-arc position and weakly overprinted by later tectonic events, this natural laboratory provides direct access to deep accretionary processes.

An integrated petro-structural study conducted across southeastern Peloponnese and Kythira combines detailed mapping, structural analysis, petrological observations, Raman spectroscopy of carbonaceous material, and thermobarometric modeling. This approach allows us to distinguish several tectono-metamorphic sub-units within the PQ nappe stack, each recording a distinct P-T evolution that constrains the depth of basal accretion for successive episodes. Hypotheses of lateral continuity between these sub-units provide first-order constraints on their present-day spatial extent and on the minimum size of individual accretionary slices.

In southeastern Peloponnese, two HP-LT sub-units are identified within the PQ nappe stack, while at least two equivalent sub-units are recognized on Kythira. These sub-units record a systematic increase in peak temperature from the base to the top of the HP-LT nappe stack, consistent with successive episodes of basal accretion. Reconstructed P-T conditions indicate that basal accretion occurred at depths of ~50-60 km along the subduction interface. Based on spatial correlations between structurally equivalent HP–LT sub-units exposed in neighboring regions along strike, we infer a minimum present-day lateral continuity of individual accretionary slices. On this basis, deep tectonic slices formed during basal accretion are inferred to currently extend over several tens of kilometers in the trench-perpendicular direction and up to a hundred kilometers along strike.

This study provides new quantitative constraints on the depth, lateral extent, and dynamics of tectonic underplating, with direct relevance for the Hellenic margin, where such processes may still be active, and for active subduction zones worldwide.