Lamprophyre dikes as tectonic markers of late orogenic transtension timing and kinematics: a case study from the Central Iberian Zone

摘要

Variscan Central Iberian Zone lamprophyre dikes cut synorogenic to late orogenic peraluminous S-type granitoids marking an important shift in regional tectonic regime from earlier, 350-290 Ma, extension-related crustal melting to later, 265 Ma, transtension-related mantle melting. The mean trend of the camptonite and bostonite dikes strikes similar to 36 degrees counterclockwise of the associated N-S trending Menga-El Pico fault zone. This indicates emplacement under sinistral transtension, D4, that postdates the D1 compressional, D2 main extensional, and D3 compressional phases of Variscan deformation in the Iberian Massif and gives new insight into late orogenic geodynamic evolution of the region. The progression from extension to transtension ensued as transpression, common in many orogens related to oblique plate convergence, waned. In this context, steep, lithospherescale, Riedel fractures permitted transport of early formed, small-scale, hydrous, enriched potassic mantle melts to shallow depths. Formation and location of such crustal-scale shear may be determined by underlying mantle heterogeneities that focus sites of melting resulting in structurally weak zones of lithosphere. So, the mantle heterogeneity localizes strain and acts as a nucleation point for newly developed steep strike-slip faults which then act as conduits for magma with magmatism controlling faulting rather than being a result of it. Throughout the geological record late orogenic K-rich magmatism is associated with strike-slip movement showing that orogenic igneous rocks are tectonomagmatic markers of geodynamic evolution.