Strain in metamorphosed volcaniclastic rocks and its bearing on the evolution of orogenic belts

Abstract

Orogenic belts commonly contain thick deposits of volcaniclastic and associated rock types that bear abundant strain markers. Detailed mapping of a deformed assemblage of volcaniclastic rocks in a well-exposed part of the Ritter Range (central Sierra Nevada, California) has yielded five types of strain markers: lithic lapilli, accretionary lapilli, tuff-breccia fragments, reduction spots, and “ash-flow ellipsoids.” The axial ratio and angle between the long axis of the markers to a reference line were measured in a large number of specimens, and the strain was determined using the shape factor grid method and data on initial fabrics. Strain in these rocks is markedly heterogeneous over short distances owing mostly to the heterogeneous nature of the stratigraphic section. The mean strain ellipsoid ( X > Y > Z ) for the area has the following values: a 62 percent increase in X , a 15 percent increase in Y , and a decrease of 46 percent in Z (shortening normal to the slaty cleavage). The mean strain magnitude (¯e s ) for the area is ¯e s =0.77, considerably below that of the mean slate (¯e s =1.44); the symmetry of the deformation is principally one of flattening, however, and has a Lode9s value (ν = 0.40) close to that of the mean slate (ν = 0.43). Results indicate that thick stratigraphic sections can be profoundly affected by internal strain: for the volcanogenic assemblage in the Ritter Range, calculations made show that tectonic deformation has thinned the part of the pendant studied by more than 50 percent, from a thickness of 9.0 to 4.3 km. It appears likely that significant portions of the central Sierra Nevada country rock may have undergone comparable thinning of section. A plot of mean strain ellipsoids from a number of orogenic belts ranging in age from Archean to Mesozoic define two mean deformation paths, one clearly a flattening deformation in which extensions of the X and Y axes of the main strain ellipsoids have a ratio of (% X /% Y ) ≈ 4, while the other path lies close to a plane strain ( Y ≈ 1). The elongate shape characteristic of most orogenic belts is probably the most important factor controlling the extension ratios of thick stratigraphic sections during deformation, and hence their mean deformation paths. The fact that mean strain ellipsoids from orogenic belts spanning a considerable period of geologic time (2.5 b.y.) show closely comparable symmetries, suggests that present-day deformation is likely to provide realistic models for interpreting paleostrain found in ancient orogenic belts.