Peter Geiser

Fault-related rocks: Suggestions for terminology

Many traditional terms for fault-related rocks have undergone recent dynamic metamorphism under high-pressure discussions by various groups of specialists. A generally acceptable simplified framework encompassing these and associated structural terms is now needed for many geologic, engineering, and legal purposes. Such a framework is proposed here, focusing on a rate-of-strain versus rate-of-recovery diagram and relating this framework to the products of brittle and ductile deformation along faults.

Classification of solution cleavage in pelagic limestones

Spaced cleavage formed by rock dissolution can represent major amounts of shortening parallel to bedding; much so-called fracture cleavage is of this origin. We classify the solution cleavage developed in Mesozoic pelagic limestones of the Umbrian Apennines into four intensity types ( weak, moderate, strong, very strong ) on the basis of qualitative attributes and mean spacing of cleavage surfaces. Shortening can be determined from imbricated chert beds and reaches 50% in rocks with very strong cleavage. In the Umbrian Apennines, solution cleavage is commonly associated with detachment thrusts. We describe an example in which the dissolution mechanism “damaged” the rock beneath a thrust by creating closely spaced discontinuities; fragments bounded by these discontinuities were torn up and incorporated in a nearly chaotic shear zone as the thrust sheet moved forward.

Mechanisms of thrust propagation: some examples and implications for the analysis of overthrust terranes

Abstract Evidence is presented for the existence of three fundamental mechanisms by which thrust sheets move. The mechanisms are: (1) failure of a stiff layer, to form ramp-flat geometry (‘imbricated’ thrust sheets); (2) detachment of a layer by folding (‘decollement” thrust sheets); (3) differential layer-parallel shortening (‘LPS’ thrust sheets). The mechanisms are independent but may operate interactively to form families of ‘hybrid” thrust sheets. Since LPS thrust sheets have not been previously described or documented, data from the New York plateau, deformed almost exclusively by differential LPS, is presented to demonstrate the physical characteristics of this thrust mechanism. Information from the plateau indicates that finite-strain behavior closely reflects the geometric boundary conditions and is independent of temperature and depth of burial. Finite-strain data are used to construct a set of iso-strain maps in both the deformed and undeformed states. The iso-strain maps are in turn used to determine the displacement field for the thrust sheet. The displacement field allows visualization of the effects of both Lagrangian and Eulerian transformations on an initially orthogonal grid. Utilizing sections from a number of overthrust belts, it is shown that the three mechanisms occur universally although in various proportions. LPS is regionally developed throughout the central and northern Appalachians. The presence of the LPS thrust sheets probably accounts for the failure of structural cross-sections to bed-length balance in these blind thrust terranes. Finally a series of examples drawn from the field as well as the literature are used to illustrate both the characteristics of hybrid thrust sheet families as well as to indicate how knowledge of their behavior can be utilized in developing strategies for the construction and balancing of structural cross-sections.

The relationship between pencil cleavage and lateral shortening within the Devonian section of the Appalachian Plateau, New York

Here we call attention to a weakly developed pencil cleavage as an indicator of the pervasiveness of lateral shortening within the sedimentary sequence of the Appalachian Plateau. The pencil cleavage appears in Upper Devonian shales as a set of closely spaced vertical partings, and it gives the shale a crude lineation similar to the pencil structures in slate districts. The strike of the shale pencil structures parallels the strike of stylolitic solution cleavage in limestones and is normal to the direction of lateral shortening indicated by deformed fossils. These deformation markers generally conform with the variation in trend of the Appalachian Plateau folds. Where the strike of the pencil cleavage diverges from the fold axes, the relation between cleavage and folding suggests that the cleavage formed first.

Cleavage in Some Sedimentary Rocks of the Central Valley and Ridge Province, Maryland

The Cacapon Mountain anticline of the folded Appalachian Mountains contains rocks ranging in age from Early Silurian to Early Devonian. The Middle Silurian Bloomsburg Formation approximates a thin, viscous plate embedded in a less viscous medium. The presence of scolithuslike burrows, useful as strain markers, makes the Bloomsburg Formation an excellent unit for studying the folding of a sedimentary plate. Examination of the microscopic fabric of rock containing a so-called “fracture cleavage” reveals the presence of discontinuous, platy zones containing tectonically oriented quartz and mica fragments. It is deduced that the shape and orientation of the quartz fragments is largely due to differential solution; however, there is some evidence of mechanical rotation of the fragments. As the “fracture cleavage” does not fit the classical definition (being locally penetrative), it has been designated S p cleavage for the purposes of this paper. Finite strain analysis shows that the cleavage postdates lithification, that it formed perpendicular to bedding and parallel to the plane of the finite strain ellipsoid containing λ 1 and λ 2 , and that it is a plane of flattening. The S p cleavage is interpreted as a phenomenon marking a transition point in the material behavior of the rock. The transition point was reached after the rock underwent a maximum two-dimensional irreversible strain of e = 0.06. This small strain is in contrast to the values of e = 0.30 that have been suggested for slaty cleavage. During progressive deformation, the S p cleavage became structurally passive, permitting it to be used as a strain marker in the determination of strain about fold profiles (ac surface). Limited field evidence suggests that the preferred orientation shown by burrow cross sections may be controlled by paleocurrent directions, making the burrows a possible current indicator. Finally, it is suggested that the fabric imposed by the burrowing organisms may have localized and controlled the development of the S p cleavage.

Mesoscopic fault array of the northern Umbrian Apennine fold belt, Italy: Geometry of conjugate shear by pressure-solution slip

The strata of the northern Umbrian Apennine fold belt are cut by an array of mesoscopic faults that generally display strike- or oblique-slip offset. The majority of these faults have traces less than a few metres long and represent displacements of < 10 cm. Fault surfaces are associated with stylolites and are coated with elongate calcite fibers, suggesting that movement occurred by the mechanism of pressure-solution slip. Crosscutting relationships indicate that faulting occurred before, during, and perhaps after regional folding. The slip on the faults permitted translations of mesoscopic blocks with respect to one another, thereby accommodating regional strain. There is a great range among fault attitudes, but two clusters forming a conjugate set with about a 90° dihedral angle stand out. The mean trend of left-lateral faults of this pair is N72°E, whereas the mean trend of right-lateral faults is N16°W. The bisector between these two fault clusters is about 15° away from the normal to the regional fold axes. This unusual orientation pattern of mesoscopic faults of the study area may indicate that the mechanics of initiating faults in rocks undergoing pressure-solution deformation is different from that in rocks undergoing purely brittle deformation. Alternatively, the fault pattern may indicate that the faults represent slip on pre-existing fractures. If this latter situation is true, the geometry of the fault array may merely reflect the geometry of the pre-existing joint array.

Joints, microfractures, and the formation of solution cleavage in limestone

The geometry of solution cleavage surfaces in limestones indicates a close connection between cleavage formation and joint and microfracture surfaces. These relations suggest that networks of microfractures and joints act both as conduits for transport of dissolved phases and as surfaces on which the cleavage nucleates. Material transport is by Darcyian-type bulk flow and is thus not a rate-limiting process; this contrasts with metamorphic terranes, where the transport mechanism (diffusion) has been identified as a rate-limiting step. Cleavage surfaces of weak to moderate intensities or spacing may form parallel to nonprincipal planes of finite strain.

Evidence for aseismic deformation in the western Transverse Ranges, southern California: Implications for seismic risk assessment

Recent studies in southern California suggest that long-term deformation rates are far in excess of that which can be accounted for by historical seismicity, and thus, a deficit of moderate and/or large earthquakes exists in southern California. Although possible, this conclusion is not unique because aseismic deformation may have contributed to bulk regional strain. We examined Cretaceous to Pleistocene sedimentary rocks exposed in the Ventura basin along four cross-strike traverses to evaluate the possibility that aseismic deformation contributed to regional shortening. Our field and microstructural investigations suggest that aseismic deformational mechanisms, particularly pressure solution, contributed significantly to permanent shortening strain during the late Neogene and that the proposed seismic deficiency may be overestimated.

Slaty cleavage and the dewatering hypothesis — An examination of some critical evidence

I have examined the evidence cited in support of Maxwells hypothesis of dewatering as the origin of slaty cleavage. Study of critical exposures of cleavage associated with clastic dikes cited by Maxwell fails to show the necessary parallelism between the two structures. Measurements made on the cleavage-dike pairs gives a mean dihedral angle of 14°. Other evidence cited in support of dewatering is ambiguous either in terms of finite strain considerations or alternative explanations offered by the pressure-solution hypothesis. I conclude that as yet no evidence of substance has been produced in support of the dewatering hypothesis.

Characterization of Appalachian faults

This study presents a classification/characterization of Appalachian faults. Characterization factors include timing of movement relative to folding, metamorphism, and plutonism; tectonic position in the orogen; relations to existing anisotropies in the rock masses; involvement of particular rock units and their ages, as well as the standard Andersonian distinctions. Categories include faults with demonstrable Cenozoic activity, wildflysch-associated thrusts, foreland bedding-plane thrusts, premetamorphic to synmetamorphic thrusts in medium- to high-grade terranes, postmetamorphic thrusts in medium- to high- grade terranes, thrusts rooted in Precambrian basement, reverse faults, strike-slip faults, normal (block) faults, compound faults, structural lineaments, faults associated with local centers of disturbance, and geomorphic (nontectonic) faults.