Paleoseismic origin of cone-in-cone structures: Evidence from volcanic-associated strata in the Anti-Atlas, Morocco

Cone-in-cone (CIC) structures have long been a subject of debate, with proposed origins ranging from primary (e.g., force of crystallization) to secondary (e.g., fluid overpressure or tectonic shearing). We report new siliceous (Middle Ordovician) and calcareous (Lower Devonian) CIC occurrences in Morocco’s Anti-Atlas region that exhibit direct stratigraphic and temporal associations with coeval basalt and tuff deposits. Morphological analyses reveal striking similarities between CICs and seismically induced sand and/or mud volcanoes, including nailhead terminations and funnel-shaped conduits. Microstructural observations further confirm diagnostic features of seismic fluid escape, such as concentric zoned fluid-escape structures (siliceous CICs) and clay-rich laminae reflecting wall-rock entrainment (calcareous CICs). Integrating field, microstructural, and global analog data, we propose a unified four-stage seismo-volcanic formation model: (1) sedimentary preconditioning (low-permeability cap rocks confining fluid-rich soft sediments); (2) pore pressure buildup triggered by volcanic seismicity; (3) development of fluid-escape structures; and (4) abiotic fibrous mineral crystallization. This model is supported by the first documented direct stratigraphic link between CICs and volcanic deposits, resolving a critical gap in previous paleoseismic hypotheses that lacked volcanic trigger evidence. Our findings explain the global occurrence of CICs across diverse lithologies and establish them as a novel, underutilized paleoseismic proxy—particularly valuable in fine-grained successions where conventional seismites (e.g., sand dikes) are scarce—advancing insights into tectono-volcanic−fluid interactions in ancient sedimentary basins.