AIN'T NO SHEAR ZONE CLOSE ENOUGH: IT MATTERS WHERE YOU ARE, IT MATTERS HOW FAR

Observations of spatial coincidence between geologic features and more generally, spatial and temporal correlations between sets of features, underlie proposed models of causative relationships between varied geologic phenomena. Only rarely is the statistical significance of apparent spatiotemporal relationships evaluated. Confounding effects of the scale and extent of observation can easily produce spurious correlations.

In the central Sierra Nevada (CA), direct tectonomagmatic links between coeval mid-Cretaceous shear zones and plutons are invoked to explain voluminous magma emplacement. In these models, space is made directly by strain accumulation across dilational accommodation zones within transpressional shear zone systems. The ~10 myr-long construction of the ~1100 sq. km Tuolumne Intrusive Suite would require strain rates of ~5-7 mm/yr along bounding shear zones. The northern Sierra Crest shear zone borders the eastern margin of the Tuolumne, with ~20-80 km of displacement estimated over ~20 myr of ductile activity along the ~5 km wide structure. Emplacement of the Tuolumne would require most of the total strain accumulated, necessitating near perfect coincidence of the pluton intrusive margin with the shear zone core.

We test this presumed coincidence with a data-rich, digital synthesis of detailed mapping along the entire length of the Tuolumne margin and northern Sierra Crest shear zone. Distance between the mapped shear zone core and intrusive margin is measured normal to the mean shear zone trend continuously along its ~60 km length, with negative values where the shear zone core is east of the Tuolumne margin. Space making dominated by the above mechanism should predict an essentially normal distribution of distances centered about zero. The distribution of the measured distances is highly dispersed and right-skewed: the Tuolumne intrusive margin is 100s of meters or more away from the core of the shear zone along more than half its length. Thus, available data are inconsistent with proposed models of direct tectonomagmatics link when considered at the appropriate scale and extent of observation. Although highly influential, such models and interpretations dependent upon them in both active and ancient arcs across the world need to be critically reevaluated with data-driven analyses.