Dehydration‐related deformation during regional metamorphism, NW Sardinia, Italy

Abstract

The relationship between deformation and dehydration has been investigated in Hercynian regionally metamorphosed rocks exposed on NW Sardinia. Two episodes of prograde mineral growth (M 1 & M 2 ) involving dehydration are recognized: growth of chlorite/phengite porphyroblasts at anchizone metamorphic conditions, contemporaneous with the first phase of deformation, D 1, and growth of biotite from chlorite and phengite coincident with the second phase of deformation, D 2 . Deformation during both episodes of dehydration is characterized by penetrative axial planar foliations defined by well-developed phyllosilicate preferred orientations quantified by XRD textural goniometry, tight to isoclinal similar folds (interlimb angles <40°), and mineral-filled veins (hydrofractures) orientated parallel to axial planar foliations, that formed contemporaneously with the development of the penetrative foliations. No prograde mineral growth occurred during D 2 at chlorite-zone conditions. D 2 deformation in the absence of dehydration is characterized by non-penetrative crenulation cleavages, poorly developed phyllosilicate preferred orientations, relatively open (interlimb angles >40°), low-strain similar folds and minor brittle deformation. Systematic variations in macrofold interlimb angles, with respect to the timing of mineral growth, indicate that enhanced shortening (c. 80%) occurred during dehydration. Microfabrics show that the onset of dehydration is associated with the transition from a crenulation cleavage to a penetrative foliation. The presence of axial planar hydrofractures that formed coevally with dehydration and fabric development requires that supralithostatic fluid pressures and low differential stresses (<c. 20 MPa) accompanied dehydration. These features demonstrate a connection between the timing of dehydration and the style of deformation.

Efficient Bidirectional Photocurrent Generation by Self‐Assembled Monolayer of Penta(aryl)[60]fullerene Phosphonic Acid

“…Two values of 0.36 and 0.5 have been used for λ , assuming a nearly hydrostatic or slightly higher pore‐fluid pressure, respectively, which is reasonable in the absence of widespread layer‐parallel crack seal in meta‐sandstones (e.g. Simpson, 1998). The viscous‐creep behaviour of the metamorphic crust has been modelled by assuming that strength is primarily controlled by the strain‐rate of wet quartz deforming by dislocation creep, as indicated by the equation: where σ is the principal differential stress, $\dot {\varepsilon }_{{\rm disl}}$ is the strain rate of dislocation creep, T is the absolute temperature, R is the gas constant, and A disl , n , and Q diff are a non‐dimensional constant, the stress exponent, and the activation energy for dislocation creep, respectively.…”

A balanced foreland–hinterland deformation model for the Southern Variscan belt of Sardinia, Italy

“…Simpson, 1998). The viscous-creep behaviour of the metamorphic crust has been modelled by assuming that strength is primarily controlled by the strainrate of wet quartz deforming by dislocation creep, as indicated by the equation:…”

Fluid flow in compressive tectonic settings: Implications for midcrustal seismic reflectors and downward fluid migration

“…Since the maximum overpressures would occur at depth z 0 , fracturing would tend to localize within, rather than at the margins of, the overpressured interval. If the maximum sustainable fluid overpressure is equated to rock tensile strength σ T and the curvature of the mean stress profile estimated from the derivative of equation (10) with respect to depth at z 0 , then the interval over which fluids are trapped is For rock tensile strengths typical of those measured experimentally and inferred from structural studies (5–20 MPa [ Gueguen and Palciauskas , 1994; Simpson , 1998]), equation (15) implies aqueous fluids would be trapped over a maximum depth interval of 2–5 km. The extent of this interval would also be constrained by the brittle‐ductile transition, such that for conditions where Δ z = δ z /2 fluid the interval would breach the brittle‐ductile transition and permit fluid to drain into the brittle crust.…”