A volcanotectonic cascade: Activation of range front faulting and eruptions by dike intrusion, Mono Basin-Long Valley Caldera, California

Stratigraphic data suggest that during the North Mono-Inyo eruption sequence of ∼1350 A.D. a series of strong earthquakes occurred near the end of the North Mono explosive phases and the beginning of the Inyo explosive phases. The temporal proximity of these events suggests the possibility of a causal relationship. Geological and geomorphic features of the Hartley Springs Fault are consistent with rupture of the fault during the eruption sequence. These features include steep central slope segments on several fault scarps and a fault scarp and stratigraphic offset since the deposition of ∼1200-year-old tephra. We hypothesize that the Inyo Dike, found by drilling underneath the main Inyo vents, neared the Hartley Springs Fault as it propagated southward from the Mono Basin circa 1350 A.D. We demonstrate that once the lateral distance between dike and fault was sufficiently small, the mechanical interaction between them could have triggered the slip observed on the fault. The slip, in turn, could have reduced the horizontal confining pressure in a region near the southern tip of the fault. The presence of the main Inyo vents in this region suggests that the reduction in confining stress was sufficient to allow magma to propagate to the surface. The results suggest that a volcanotectonic "cascade" of eruptions and earthquakes is a possible mechanism by which a large section of a range front or rift system can be activated because of the positive feedback provided by each element to continued activity.