Complex deformation and surface interactions at the base of a massive landslide breccia, lower comondú group, miocene, BCS, México

Studying volcanic debris avalanche deposits (VDADs) and their interaction with the substrate is crucial for understanding deformation processes nearby composite volcanoes. The Late Oligocene – Early Miocene volcano-sedimentary succession exposed in the studied area, preserves exceptional evidence of such interaction, providing a unique opportunity to analyze substrate deformation beneath an ancient VDAD. The substrate succession is low-grade felsic ignimbrites interlayered with andesitic to rhyolitic epiclastic-volcanic rocks, gently inclined to the West. Field mapping, photogeological analysis, and structural data compiled in key areas reveal non-penetrative, complex deformation structures, which include folds, faults, and clastic intrusions within the avalanche substrate, some of which are consistent with liquefaction and load-induced deformation caused by a voluminous VDAD emplacement. We interpret this deformation as caused by an avalanche moving over a wet, loose, and uncemented or poorly lithified substrate. Part of the deformation continued as load-induced deformation after the avalanche movement ceased. U Pb zircon geochronology constrains the timing of VDAD emplacement to 20.56–23.9 Ma. However, not all the documented deformation developed during the avalanche emplacement, as other deformation episodes modified the region over time. Subsequent tectonic events, which include a brief period of local crustal shortening and pulses of transtensional faulting related to the evolution of the Gulf of California, overprinted and reactivated pre-existing structures generated by the VDA. Recognizing the polygenetic nature of substrate deformation in ancient VDAD is essential for distinguishing between syn -emplacement and post-emplacement structures, improving our understanding of mass transport deposits, and refining models of volcanic hazard assessment in active volcanic regions. • Deformation in an unlithified Miocene volcanosedimentary succession is documented. • Volcanic debris avalanche (VDA) formed a structural complex with fluidization/mixing. • The voluminous VDA caused instant deformation. Loading modified earlier structures. • Recognizing deformation's polygenetic nature unravels syn - and post-VDA structures.