The mechanical anisotropy of Four‐mile gneiss has been investigated in a series of uniaxial and triaxial, compression and extension experiments performed at confining pressures P c up to 400 MPa, constant strain rates ε from 1.6×10 −6 to 1.5×10 −4 s −1 , and temperatures T from 25° to 800°C on cylindrical and notched samples oriented with respect to foliation ( S ) and lineation ( L ). Differential stresses measured both at the onset of yielding and at failure vary with specimen orientation, with maximum compressive strengths exhibited by samples cored perpendicular to S and minimum strengths exhibited by samples cored at 45° to both S and L . While failure strengths are influenced most strongly by the orientation of S , they appear to depend upon the orientation of L as well. An orthorhombic failure criterion, generalized from a nonlinear Mohr‐Coulomb relation, has been considered with quadratic and linear stress terms resembling those of invariants J 2 and I 1 , respectively, and material parameters estimated by nonlinear regression methods. Satisfactory fits were achieved for results at T = 25°C as well as T = 700°C. Fracture strengths are relatively insensitive to changes in T and ε and the anisotropy exhibited at T = 700°C is remarkably similar to that measured at T = 25°C. Relatively small reductions in strength observed at elevated temperatures are probably due to the influence of thermally induced microcracks. Mechanisms of deformation and sources of anisotropy have been identified by examining microstructures developed in deformed specimens and observing their relationships to those fabric elements initially present in the starting material. Throughgoing shear fractures developed in samples shortened in all orientations with respect to S and L by the coalescence of microcracks in feldspar and quartz grains, as reported for isotropic granites. However, inelastic strains within mica grains were accommodated by slip, frictional sliding, and kinking, and deformation of favorably oriented micas appears to have led to local stress concentrations in neighboring phases that result in nucleation of tensile microcracks. Both S and L are defined by the preferred orientations of micas, and a simple model involving crack nucleation around oriented mica grains is proposed to explain the anisotropy observed.

Using screw-driven machines with exceptionally reliable data-acquisition and confining-pressure systems and an uninterruptible power-supply, we have tested 10 by 20-cm specimens of Avery Island rocksalt at constant strain-rates down to 10−9/s. At strains between 0.05 and 0.10, steady-state flow is achieved at about 6.8 MPa at 200°C and 10−7/s and 9.4 MPa at 100°C and 10−8/s. At 10−9/s a strain of only 0.03 has so far been reached after 3 × 107s (about 350 days); tests at 50° and 100°C are to continue for another 100 days unless steady-state flow is surely observed sooner.

Notched samples of Sioux quartzite have been extended to failure at temperatures T between 25° and 710° C, strain rates ⋗e from 5.2 × 10−6 to 6.7 × 10−4 s−1 and 100 MPa confining pressure, and fracture morphologies examined by optical and scanning electron microscopy. In all the extension tests performed, sample failure occurred by the formation of a tensile fracture in or near the notch mid-plane. Differential stress magnitudes at failure are insensitive to variations in T and ⋗e and calculated tensile strengths are comparable to previously reported values for unconfined tests at room temperature. Force-displacement records show a change in mechanical response, from linear elastic behavior at temperatures between 25° and 412°C to significant yielding in compression prior to failure in the 500° to 710°C tests. In all cases, fractures consist almost entirely of intragranular cracks (IGC) which exhibit characteristic surface features which can be used to infer local crack propagation directions. In one test in which fracturing was initiated at an edge flaw, IGC propagation directions show no overall trend. The observations indicate that the mechanism of fracture propagation involves nucleation, growth and coalescence of IGC which are themselves propagating in all directions in the plane of the main fracture. The onset of inelastic yielding prior to failure at elevated temperatures is attributed to the formation of thermally induced GBC which act as nucleation sites for IGC during extension.

During the final year of the grant, we have investigated (1) why the strengths of rocks decrease with increasing temperature and in the presence of water through study of the fracture process in Westerly granite and Sioux quartzite specimens deformed in extension (some in true tension), (2) frictional strengths of rocks at high temperatures, (3) the stability of boreholes in fractured rock, and (4) slip in biotite single crystals (in that biotite is probably the weakest and most ductile of the common constituents of crystalline rocks.

Because virtually all tectonophysical processes are masked by the overburden, or occur too slowly for adequate observation in anthropocentric time, or both, they must be studied in carefully controlled laboratory experiments that simulate the natural environment as realistically as is practicable. Extrapolations of laboratory data in space and time are invalid unless the experimental and natural phenomenologies are essentially the same. The size of conventional specimens is of the order of 10 cm, whereas the discontinuities (defects in a continuum) in real rock‐masses are often much larger, of the order of l m of more. Furthermore, such discontinuities as macrofractures (joints) may well dominate the mechanical and fluid‐transport properties in nature. Adequate sampling of rock‐mass properties will probably always require in‐situ testing, but testing machines much larger than any now available could provide useful data at least at intermediate scale.

The short-term failure strengths and strains at failure of room-dry and water-saturated, cylindrical specimens (2 by 4 cm) of Charcoal Granodiorite (CG), Mt. Hood Andesite (MHA), and Cuerbio Basalt (CB) at a strain rate of 10/sup -4/s/sup -1/, at effective confining pressures of 0, 50, and 100 MPa and at temperatures to partial melting were investigated. Data from water-saturated specimens of the granodiorite and andesite, compared to room-dry counterparts, indicate (1) the pore pressures are essentially communicated throughout each test specimen so that they are fully effective; (2) at P/sub e/ = 0 and 50 MPa the granodiorite does not water-weaken; (3) at these same effective pressures the more porous and finer-grained andesite begins to exhibit water-weakening at about 600/sup 0/C; (4) at P/sub e/ = 0 and 870 to 900/sup 0/C the andesite's strength averages 20 MPa while the strength of dry specimens at the same P and T exhibit a strength of 100 MPa; (5) at P/sub e/ = 50 MPa compared to 160 MPa dry; (6) the basalt at P/sub e/ = 0, appears to be water-weakened at 800/sup 0/C; (7) water saturated specimens deformed at temperatures less than that of melting exhibit ultimate strengths at less than 2% shortening and then work-soften along faults; (8) again as do the dry counterparts, the wet specimens deform primarily by microscopic fracturing that coalesces into one or more macroscopic faults; and (9) the temperature for incipient melting of the andesite is decreased >150/sup 0/C in the water-saturated tests.

The stick-slip of frictional sliding depends not only on material properties but also on the elastic and inertial properties of the loading system. To compare data from different testing machines or to apply them to the problem of natural seismogenic faulting, one must account for the differences in stiffness and mass. We develop a simple mechanical model to describe the stick-slip oscillation during frictional sliding in a triaxial-compression machine. The experimental system, the loading frame and rock specimen with precut sliding surface, is divided into two subsystems across this surface. The model is based upon two key assumptions: the kinetic friction is constant regardless of the relative motion of the subsystems, and the elastic restoring force is uniform throughout each subsystem. The first assumption leads to the decoupling of the subsystems, and the behavior of each becomes mathematically analogous to that of a simple spring/mass/slider-block model, owing to the second assumption. The theory agrees well with the experimental data from the dynamic measurements of stick-slip. The displacement-time function is of cosine form, the rise time of stick-slip is constant, and the relation between force drop and average displacement rate is linear. From this model we argue that the differences in the frictional behavior of experimental fault-gouges may indeed be ascribed to differences in the material properties of their specimens because the elastic and inertial properties of a particular testing machine are little influenced by the specimen itself, so long as all specimens are of about the same size. However, interlaboratory correlations may well be invalid unless machine effects are properly accounted for.

Research Article| May 01, 1980 Experimental folding of rocks under confining pressure, Part VIII—Forced folding of unconsolidated sand and of lubricated layers of limestone and sandstone M. FRIEDMAN; M. FRIEDMAN 1Center for Tectonophysics, Texas A&M University, College Station, Texas 77843 Search for other works by this author on: GSW Google Scholar R.H.H. HUGMAN, III; R.H.H. HUGMAN, III 1Center for Tectonophysics, Texas A&M University, College Station, Texas 77843 Search for other works by this author on: GSW Google Scholar J. HANDIN J. HANDIN 1Center for Tectonophysics, Texas A&M University, College Station, Texas 77843 Search for other works by this author on: GSW Google Scholar Author and Article Information M. FRIEDMAN 1Center for Tectonophysics, Texas A&M University, College Station, Texas 77843 R.H.H. HUGMAN, III 1Center for Tectonophysics, Texas A&M University, College Station, Texas 77843 J. HANDIN 1Center for Tectonophysics, Texas A&M University, College Station, Texas 77843 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1980) 91 (5): 307–312. https://doi.org/10.1130/0016-7606(1980)91<307:EFORUC>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. FRIEDMAN, R.H.H. HUGMAN, J. HANDIN; Experimental folding of rocks under confining pressure, Part VIII—Forced folding of unconsolidated sand and of lubricated layers of limestone and sandstone. GSA Bulletin 1980;; 91 (5): 307–312. doi: https://doi.org/10.1130/0016-7606(1980)91<307:EFORUC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Field and laboratory data suggest that variations in structural style are associated with differences in lithologic composition, stratigraphic sequence, and the mechanical behavior of the layers that are drape (forced) folded by differential vertical movements of underlying essentially rigid blocks. This hypothesis is tested by study of experimental, faulted, drape folds in veneers of loose, dry, unconsolidated sand and in multilithologic layered sequences with lubricated interfaces produced under confining pressures to 200 MPa (2 kb) at 25 °C. Deformation of the sand veneer provides a classic example of cataclastic flow. Forced folds develop as a result of microfracturing, rigid-body rotation of grains and fragments, and faulting and gouge development. The sand veneer is thinned drastically in the zone of faulting. The multilithologic, layered veneers with lubricated interfaces exhibit the same magnitudes of "bedding-plane" slip and somewhat more variability in the senses of slip than do specimens not lubricated. With lubrication, however, there is less deformation of the leading edge of the forcing block, less extensile faulting in the upthrown block, and more folding without faulting in the veneer. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Fluid flow through fractured porous subsurface reservoirs is an important but often unquantified property. The necessary quantification of this flow is achieved by both laboratory and field measurements. Laboratory experiments of the effect of temperature and confining pressure on permeability in Navajo Sandstone indicate that simulated fractures in porous rock (1) have a higher percentage rate of permeability decline with depth than whole rock, (2) experience a greater degree of permanent deformation with depth than whole rock, (3) are healed effectively when fracture permeabilty approaches that of the whole rock, and (4) experience a reduction in permeability dependent on the macroscopic ductility and previous maximum depth of burial of the host sandstone.

Specimens composed of as many as five layers of various combinations of dry Coconino Sandstone (brittle) and Indiana Limestone (ductile) are folded at 1-kb confining pressure. Stress-shortening curves for specimens with aspect ratios ≥20 are nearly linear up to a maximum stress (critical buckling stress) and then show pronounced postbuckling work-softening. For specimens of equal total thickness, the average maximum stress decreases with an increase in the number (and thickness) of limestone relative to sandstone layers. A single beam of limestone or sandstone has larger maximum stress than does the corresponding three-layer specimen of equal thickness. Curves for limestone specimens composed of from one to five layers and with aspect ratios ≤3 are monotonic and show poorly defined yield regions and pronounced work-hardening. Both thick-beam (aspect ratios of 7 to 13) and thin-beam (aspects ratios of 20 or more) folds tentatively are regarded as buckles because instabilities probably are involved in the folding. Fold (anticlinal) shape depends on the mechanical behavior of the bottom layer. When the ductile limestone is lowest, the fold shape is nearly sinusoidal; when the brittle sandstone is lowest, the anticline has a chevron shape. “Bedding-plane” slip is a maximum in the inflection region on the flanks, and the sense of slip is the reverse of that in classical flexural-slip folding. Dynamic petrofabric interpretations of small thrust and normal faults, macrofractures and microfractures, compression and extension axes derived from calcite twin lamellae, and the twin-lamellae spacing index indicate that although bedding-plane slip occurs, the entire layered specimen acts as a single mechanical beam throughout most of the folding. In thin, multilithologic folds, the apparent neutral surface is displaced from the center toward the compressed side. In thick, multilayered limestone specimens, large axial shortening prior to bending displaces the neutral surface toward the region of extension, so that even the uppermost layer in the anticlinal hinge can be in the zone of layer-parallel compression.