The Managua Graben and Las Sierras-Masaya volcanic complex (Nicaragua); pull-apart localization by an intrusive complex: results from analogue modeling

There is a well-documented association between pull-apart basins in strike-slip zones and large volcanic caldera complexes. Las Sierras-Masaya volcanic complex, Nicaragua, is a large basaltic lava and ignimbrite shield with nested calderas. The widest caldera is partly underlain by a large and dense intrusive complex, evidenced by a positive gravity anomaly. The Las Sierras Caldera, relating to this cumulate complex, is less than 30,000 years old and is probably still hot and ductile. The inner Masaya Caldera hosts an active magmatic system. The volcanic complex is in a dextral transtensional tectonic context of the Nicaraguan Depression. The highly active Managua Graben is on the northern part of the volcano. We speculate that the graben and volcano are linked tectonically, with the graben initiating in response of a regional stress field modified around the dense, ductile intrusive complex. Previous field work and reappraisal of structures with digital elevation model morphological analysis show that the volcano is surrounded by a rhombic fault pattern that may form a nascent pull-apart basin. We have done scaled analogue models to test the effect of intrusion density and rheology anomalies on strike-slip fault geometries. The models show that intrusion density variations alone do not significantly change fault patterns. In contrast, ductile rocks (silicone in the models as analogue for hot mafic intrusive rocks) markedly alter strike-slip fault patterns. In transtension the presence of a ductile intrusion causes the formation of a pull-apart, while in pure strike-slip or transpression, uplift and thrusting is generated. Pull-apart and uplift structures are rhomb-shaped even when the ductile inclusion is circular. We conclude that a pull-apart is developing at Las Sierras-Masaya volcanic complex in response to the transtensive regional deformation regime and the dense, ductile intrusive complex. We suggest that the volcano and graben are one dynamic system and should be monitored as one entity.