Deformed Vein Sets as a Record of Synmetamorphic Volume Change: Quantifying Solution Mass Transfer in Subduction‐Type Metasediments of the Del Puerto Canyon Region, Franciscan Belt, USA

Abstract

AbstractThe volume change component of deformation is often ignored or assumed to be zero in tectonic studies of metamorphic belts. However, when estimating the original geometries of deformed regions, the volume change is just as important as the other two components of deformation, finite strain, and rotation. Major permanent volume change in metamorphic rocks is accomplished by solution transfer facilitated by H2O fluids and estimates of volume change can be combined with solubilities to provide required time‐integrated volumes of fluid flow. Previously applied methods for estimating rock volume change are based on absolute stretch or changes in whole‐rock chemical compositions. Estimates based on absolute stretches generally indicates large volume reductions whereas geochemical measurements generally imply limited volume change. These contrasting results exist even when the two methods are applied to the same region. In this study, we develop a largely unexplored method for estimating volume change using the direction and deformation type of deformed mineral veins. The assumptions in this method are few and appropriate uncertainties can be estimated. Application of the new method to the metagreywacke in the Del Puerto Canyon of the Franciscan belt constrains the syn‐metamorphic volume change to be greater than 7 vol.%, contrasting with previous proposals for large volume‐loss in the same region. The results of previous studies can be modified by taking into account grain rigid body rotation and grain boundary sliding. The final result of our approach yields a volume change of 7–21vol.% and implies large amounts of water‐rich fluid must have passed through the rock.