Chaotic Zone Research Articles

The viscoelastic properties and the scatter of elasticity constants of vulcanizates

Determination of the viscoelastic properties on unfilled vulcanizates with known molecular network parameters has shown a well defined correlation between the scatter of elasticity constants and the variance of relaxation period spectra. These correlations indicate that it is possible to assess the scatter of the viscoelastic properties on the basis of existing combinations of ordered and chaotic zones in the cured elastomers.

Anatomy of an Orogen

Mapping a strip of 15′ quadrangles across the northern Coast Ranges of California by student colleagues and myself has resulted in the development of a model for the origin and emplacement of the Franciscan complex and its relation to the coeval Great Valley sequence that differs from earlier models. Rocks of both Franciscan and Great Valley are predominantly graywacke, silty mudstone, and local conglomerate. Most of the Great Valley rocks are graded, and bottom markings are common; these features are present but not characteristic of the Franciscan rocks. Franciscan graywacke and silty mudstone tend to be broken and sheared, or chaotically deformed. The chaotic zones typically contain exotic blocks, mostly of ophiolitic rocks, but also blue-schist, mica schist, metagraywacke, and blocks similar to the enclosing sheared matrix. These chaotic units are best characterized as melanges. Within the Franciscan complex, mappable units have been defined, based on lithology of matrix and content of exotic blocks. The units dip and face easterly and are oldest to the east (Tithonian or older for the South Fork Mountain Schist). Progressively younger rocks occur down section to the west; the youngest are the Coastal Facies largely of Eocene age. The probable origin involves deposition of Franciscan sediments in a trench, followed by thrusting of the sediments against and under the landward side of the trench, while the axis of the trench migrated progressively westward. The process began in Tithonian time, slowed during Late Cretaceous, and ceased in the Eocene. Models are suggested for the formation of melanges during sedimentation and underthrusting. Each melange unit contains a unique assemblage of exotic blocks. It is suggested that these assemblages in part reflect the lithology of the basement rocks on which the sediments were deposited. If true, the composition of the basement varied from dominantly ophiolite-suite rocks to dominantly blueschist. A melange of ophiolite blocks also formed, the basement beneath a part of the Great Valley sediments. Samples of radiolarian chert from the ophiolite suite throughout the Franciscan were determined to be of Tithonian age by Emile Pessagno. If the ophiolite represents ocean floor, formed at a spreading ridge, the ridge must have been located adjacent to the trench in Tithonian time; after the Tithonian, no oceanic rocks were added to the Franciscan pile. A diapiric origin for the Tithonian ophiolite is suggested. Northwest-trending isolated slabs of Great Valley—type rocks and of shallow-water to continental coal-bearing clastic rocks of Cretaceous to Miocene age are scattered within the Franciscan complex. Evidence is presented that these were deposited in separate, probably fault-controlled basins, developed on the surface of the tectonically active Franciscan complex during and following underthrusting. They do not appear to represent a klippen of a Coast Range thrust fault, as has been suggested by others.

Analysis of Gravity Sliding and Orogenic Translation

Gravity sliding, major thrusting, and the displacement of large crustal plates are accomplished because strata with the lowest viscosity in a rock sequence readily deform by simple (rectilinear) shear flow when subjected to loading. These deforming strata are called decollement zones. Strata above these zones are transported without being deformed, provided their viscosity is one or more orders of magnitude larger than that of the decollement zone. Shear stresses acting at the base of these strata, that is, “basal drag,” are commonly much lower than stresses accompanying friction across a sole fault ( compare with Hubbert and Rubey, 1959). Thus, shear flow in decollement zones most likely represents the operative mechanism in natural settings. The speed of gravity sliding and the ease of tectonic transport increase with increasing decollement zone thickness, dip, and depth of burial, and decrease with increasing decollement zone viscosity. End effects, including (in two dimensions) updip attachment of gravity slides and frontal buttressing of both slides and tectonic plates, are as important as basal drag in determining the speed of slides and the resistance to transport of crustal plates. Structures in the strata above a decollement zone develop independently of those within the zone. In the upper plate, pull-apart grabens form to overcome updip attachment; step thrusts and folds form to overcome frontal buttressing. Structures within decollement zones are proposed to include intensely developed passive disharmonic folds, bedding plane faults at contacts between high and low viscosity strata and through high viscosity masses within the zone, chaotic zones comprised of blocks of high viscosity strata, infolds of adjacent high viscosity strata, and penetrative cleavage, phylonitization, and other features of deformation-induced recrystallization. Velocity profiles for simple shear portray the general mode of deformation and permit study of transport in specific geologic settings. Composite velocity profiles are constructed by first drawing individual profiles for each rock in the sequence under study. Segments from these curves proportional to the thickness of each succeeding stratum are then spliced together from the bottom up.

Chaotic Structure and Décollement in Cenozoic Rocks near Stanford University, California

Chaotic structure and possible decollement relations, previously little known in Cenozoic rocks of the Coast Ranges, have recently been exposed 25 miles southeast of San Francisco, California. Orderly Eocene sandstone-mudstone sequences are interrupted by chaotic zones consisting of disordered mudstone with scattered and rotated bodies of sandstone. It is believed that the chaotic structure resulted in part from Eocene submarine sliding. After the Eocene rocks were moderately folded, Miocene strata were deposited unconformably upon them. Pliocene-Pleistocene thrusting and folding moved the Miocene strata northeastward relative to the underlying Eocene rocks, apparently producing a surface of decollement. Continued folding involved the decollement surface itself, increased the structural complexity of the chaotic zones, and locally overturned intact strata.