At 100 Ma, about midway through the history of the emplacement of the northern part of Peninsular Ranges batholith, a regional-scale deformation zone¹ developed that juxtaposed two major prebatholithic units. Deformation occurred as subduction transitioned from beneath oceanic crust to beneath continental crust. The deformation zone is well exposed at Searl Ridge, where the intensity of deformation attendant to the juxtaposition of the two prebatholithic units progressively increased from west to east over a distance of 5.5 km. In this 5.5-km-wide zone, a series of three progressive structural transpositions is recorded in the metasedimentary rocks. Many outcrops have two intersecting planar fabrics and a linear fabric produced by the intersecting planar fabrics. West to east within the deformation zone, the rock fabric changes from phyllitic to schistose, and the grade of metamorphism increases from greenschist to lower granulite. Plutonic rocks predating the deformation zone at the west edge of Searl Ridge were likewise deformed locally into rocks having a pronounced gneissic fabric.

The transposition displacement produced a 300 °C gradient over a 4-km-wide zone of low-pressure, high-temperature, Buchan-type metamorphic mineral assemblages. The 300 °C gradient is based on biotite forming at a temperature of ~425 °C, and K-feldspar at ~750 °C. A 3–4 kbar pressure gradient across the deformation zone is inferred from a crystallization pressure of 2 kbar for the 120 Ma Domenigoni Valley pluton, located at the western edge of the deformation zone, and a 5–6.3 kbar pressure for the 100 Ma Lakeview Mountains pluton, emplaced in the eastern part, the latter being coeval with the formation of the deformation zone. If temperature increase is attributable entirely to difference in depth, and assuming an idealized 0.3 kbar increase in pressure per 1 km increase in crustal depth, the vertical displacement component of deformation within the deformation zone was 10–13 km.