A convergence model for the SW Gondwana margin during the late Paleozoic based on the structural analysis of accretionary complexes in northern and south-central Chile

This study presents a comprehensive analysis of the convergence style in the accretionary complexes along the Chilean coast, focusing on the structural study of the Maule River section (35° S), to shed light on the complexity of the subductive southwestern Gondwana margin during the late Paleozoic. The Maule River section, part of the Coastal Accretionary Complex of Central Chile, provides insights into the structural configuration of the basal and frontal accretionary wedge, and is compared in this work with the Chañaral mélange, more than 1000 km to the north. The structural analysis reveals significant differences in the convergence parameters (subduction zone boundary azimuth, convergence direction, flow trajectories). The aim of this study is to assess the influence of convergence style on the differences in lithological, metamorphic, and structural characteristics between these two widely separated fragments of the Carboniferous accretionary complex.

The Maule River section is characterized by two well-differentiated sectors: the Eastern Series, comprising low-grade phyllites and quartzites, and the Western Series, consisting of albite-bearing pelitic schists, greenschists and amphibolites, recording medium-high pressure conditions and temperatures below 500 °C. To the east, the Carboniferous magmatic arc and its basement are tectonically juxtaposed to the metasedimentary wedge.

The Eastern Series exhibits structures with opposed vergences: eastward in the eastern area and west-northwestward in the central and western areas. In contrast, the Western Series shows different structural patterns, including isoclinal folds and abundant S-C structures. The main schistosity presents E-W to N-S directions dipping moderately to the south and east respectively. The mineral and stretching lineation trends mostly E-W to NW-SE with low plunges towards the east and southeast, indicating top-to-the-WNW displacement.

A 3D kinematic model applied to the structural data collected in the Maule river section results in a triclinic to monoclinic transpressive shear zone (subduction channel and its influence zone), with a constrictional coaxial component (K = 5) and with the simple shear direction acting parallel to the dip of the subduction zone boundary. The particle flow trajectories (extrusion direction) fluctuated ±60° around the convergence direction of N123E ±24°, orthogonal to the margin azimuth. In the Chañaral area, both the convergence direction (N50-60E) and the extrusion trajectories predicted for the materials in the shear zone, with a low pitch on the boundary of the deformation zone, contrast with the results obtained in the Coastal Accretionary Complex of Central Chile. The differences in the late Paleozoic margin configuration may partially explain the absence of high-pressure rocks in the north (Chañaral area), although other post-accretionary processes such as tectonic erosion and block rotations may accentuate these differences.

Besides, the study concludes that the Maule River section's double vergence can be explained by the pro-wedge and retro-wedge structure established in wide accretionary prisms. The results highlight the importance of performing 3D kinematic studies in tectonic mélanges to obtain a complete tectonic picture of the activity and characteristics of the entire subduction system during the late Paleozoic.