Pressure solution inhibition in a limestone-chert composite multilayer: Implications for the seismic cycle and fluid flow

Pressure solution seams (PSSs) are frequent features in carbonate rocks undergoing tectonic shortening. In particular, pervasive, anticline-axis-parallel, bed-normal PSSs are known to develop during layer-parallel-shortening of (marly) carbonate rocks in fold-thrust belts. These pressure solution features can impact subsequent fracture development, fluid circulation, and strain localization including the seismic cycle. It is here demonstrated that the occurrence of frequent and continuous chert layers may strengthen a limestone sequence and inhibit pressure solution under layer-parallel-shortening. Field observations and laboratory determinations are reported from marly limestone with continuous chert layers of the Scaglia Fm. (Cingoli anticline, northern Apennines, Italy) exhumed from a depth of c. 1 km. In these outcrops, bed-normal solution seams do not occur or they occur only where infrequent chert layers have been shortened by small thrusts. In analogy with laminae-reinforced composite materials, a model is developed explaining the field observations with the strengthening effect of chert in the chert-limestone composite multilayer. During layer-parallel-shortening, the composite multilayer deforms under equal strain boundary conditions. In this situation, the tectonic load is mostly supported by the stiff and frequent chert layers and the strain of the whole chert-limestone composite remains in the elastic field, so that pressure solution seam development is prevented in the limestone beds. Our model may be applied down to a depth of a few kilometers in the upper crust that is relevant for the seismic cycle and fluid flow.