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
The volume change component of deformation is often ignored or assumed
to be zero in tectonic studies of metamorphic belts. However, when
estimating original geometries of deformed regions, 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 flow of
H2O-rich fluids. Therefore, estimates of volume change
can be combined with solubilities to estimate volumes of fluid flow.
Previously applied methods for estimating rock volume change are based
on estimates of absolute stretch, or changes in whole-rock chemical
compositions. Estimates based on these approaches give large
discrepancies even when 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%, contrasting with previous
proposals for large volume-loss in the same region. The results of
previous studies can be modified taking into account grain rigid body
rotation and grain boundary sliding. The final result of our approach
yields a volume change of 7–21% vol.% and implies large amounts of
water-rich fluid must have passed through the rock.