From Sharp to Diffuse: How Erosion Level Controls the Architecture of Continental Sutures in the Crust

Continental sutures are fundamental markers of past plate convergence, yet their geological expression varies markedly depending on erosion level and crustal depth. Classical descriptions of sutures emphasize narrow, localized zones characterized by ophiolites, mélanges, arc-related assemblages, and high-pressure metamorphic rocks, often exposed at the surface. However, such features typically reflect shallow-crustal levels of preservation. Here we demonstrate that deeply eroded or buried sutures may lack this diagnostic surface expression and instead form wide, diffuse boundary zones within the middle and lower crust, extending laterally for 100–200 km. This conceptual framework is illustrated with two contrasting examples from Europe: the early Palaeozoic suture between Baltica and Avalonia and the Paleoproterozoic suture between Fennoscandia and Sarmatia within the East European Craton.

The first case examines the German–Polish Caledonides and the Thor Suture separating Avalonia from Baltica. Integration of geological data with reinterpretation of the Basin-9601 deep seismic profile, complemented by newly constructed 2-D forward gravity modelling and regional gravity and magnetic compilations, allows refinement of the crustal architecture across eastern Germany and western Poland. The Caledonian Deformation Front is shown to mark the northern limit of a thin-skinned fold-and-thrust belt, composed of Ordovician metasediments derived from a Caledonian accretionary wedge near Rügen and of deformed foreland-basin sediments incorporated into the orogenic wedge farther east. In contrast, the Thor Suture itself—defined as the thrust of Avalonia’s crystalline basement over Baltica—is located ~120 km farther south, beneath the depocentre of the North German Basin and along the Dolsk Fault Zone in western Poland. At depth, the lower crust of Baltica is underthrust southward to the Flechtingen High and toward the Variscan Rheno–Hercynian suture. This geometry demonstrates that, although the Caledonian suture has a narrow and classical expression in the shallow crust, it broadens downward into a wide lithospheric-scale transition zone, coinciding with mantle lithosphere necking between thick Baltican and thinner Avalonian lithosphere.

The second example addresses the Paleoproterozoic Fennoscandia–Sarmatia Suture (FSS) in eastern Poland. Reassessment of deep reflection seismic data from the PolandSPAN™ survey, combined with 2-D gravity and magnetic modelling and 3-D models of basement depth and crustal thickness, reveals a fundamentally different suture style. Rather than a discrete fault, the FSS is expressed as a 100–120 km wide transitional zone involving the Belarus–Podlasie Granulite Belt and the Okolovo Belt. These domains are characterized by anomalously dense and magnetically susceptible lithologies, interpreted as remnants of arc-related magmatic complexes, mafic igneous suites, and high-pressure metamorphic rocks. Seismic and potential-field data demonstrate that these features continue through the entire crust, indicating a deeply rooted Paleoproterozoic collision that has been subsequently overprinted but not obliterated.

Together, these examples show that sutures preserved at shallow levels are narrow and lithologically distinctive, whereas deeply eroded or ancient sutures are cryptic, broad, and best recognized through integrated seismic and potential-field analyses. At the same time, we acknowledge that differences in Precambrian versus Phanerozoic tectonic regimes—such as lithospheric strength, thermal structure, and strain distribution—may further contribute to the development of especially wide deformation zones in Palaeoproterozoic sutures.