Julia K. Morgan

New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190

Moore, G. F., Taira, A., Klaus, A., Becker, L., Boeckel, B., Cragg, B. A., Dean, A., Fergusson, C. L., Henry, P., Hirano, S., Hisamitsu, T. et al. (2001). New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190. Geochemistry, Geophysics, Geosystems, 2, Article No: 2001GC000166.

Numerical simulations of granular shear zones using the distinct element method: 1. Shear zone kinematics and the micromechanics of localization

Two-dimensional numerical simulations were conducted using the distinct element method (DEM) to examine the influences of particle size distribution (PSD) and interparticle friction μp on the nature of deformation in granular fault gouge. Particle fracture was not allowed in this implementation but points in PSD space were examined by constructing assemblages of particles with self-similar size distributions defined by the two-dimensional power law exponent D. For these numerical “experiments,” D ranged from 0.81 to 2.60, where D=1.60 represents the two-dimensional equivalent of a characteristic PSD to which cataclastically deforming gouge is thought to evolve. Experiments presented here used μp values of 0.10 and 0.50 and were conducted using normal stress σn on the shear zone walls of 70 MPa. Shear strain within these simulated assemblages was accommodated by intermittent displacement along discrete slip surfaces, alternating between broadly distributed deformation along multiple slip planes and highly localized deformation along a single, sharply defined, subhorizontal zone of slip. Slip planes corresponded in orientation and sense of shear to shear structures observed in natural gouge zones, specifically Riedel and Y shears; the oblique Riedel shears showed more extreme orientations than typical, but their geometries were consistent with those predicted for low-strength Coulomb materials. The character of deformation in the shear zone varied with PSD due to changes in the relative importance of interparticle slip and rolling as deformation mechanisms. A high degree of frictional coupling between large rolling particles in low D (coarse-grained) assemblages resulted in wide zones of slip and broadly distributed deformation. In higher D assemblages (D >= 1.60), small rolling particles self-organized into columns that separated large rolling particles, causing a reduction in frictional resistance within the deforming assemblage. This unusual particle configuration appears to depend on a critical abundance of small particles achieved at D ≈ 1.60 and may enable strain localization in both real and simulated granular assemblages.

Numerical simulations of granular shear zones using the distinct element method: 2. Effects of particle size distribution and interparticle friction on mechanical behavior

Two dimensional (2-D) numerical simulations were conducted using the distinct element method (DEM) to explore the influences of particle size distribution (PSD), defined by 2-D power law exponent D, and of interparticle friction μp on mechanical behavior and strength of granular shear zones. The value of D ranged from 0.81 (a coarse breccia) to 2.60 (fine-grained gouge); μp was assigned to 0.10, 0.50, or 0.75; normal stresses on the shear zone walls, σn, varied from 40 to 140 MPa; assemblages were sheared to 200% strain. Fault friction, defined as the ratio of shear to normal stress, μf = τ/σn, was quite low for all experiments. Low μp suites yielded μf ≈ 0.20–0.25, while higher μp values resulted in only slightly higher values for μf ≈ 0.25–32. The stress-strain response of the latter experiments was similar to that of overconsolidated granular assemblages: a peak strength was reached by about 10% strain, followed by a period of strain weakening to 30–50% strain, and finally stabilizing at a residual strength for the rest of the experiment. The transitional phases were accompanied by increasing shear zone dilation of up to about 1.5%. The low μp suites behaved more as normally consolidated assemblages; they showed no noticeable strain weakening and relatively minor dilation of about 0.2%. The anomalously low strengths of the simulated assemblages can be explained largely by high degrees of particle rolling. Periodic drops in shear strength during residual deformation phase of the experiments correlated directly with reduced rates of dilation and the localization of strain. Fault strength also showed second-order variations with D: the low μp suites showed a steady decline in maximum residual strength μfmax with increasing D due to the importance of interparticle sliding in all configurations; in the higher μp suites, μfmax decreased for D values less than a characteristic value of 1.60, then leveled out for increasing D. This may be explained by the increasing importance of particle rolling as small particles became more abundant with increasing D; the particles began to self-organize and strain became more localized. Although the simulations lack particle fracture, they offer insight into how micromechanics control the mechanical evolution of granular shear zones.

Structural geology and kinematic history of rocks formed along low-angle normal faults, Death Valley, California

Several late Cenozoic low-angle normal, or detachment, faults on the western flank of the Black Mountains are each characterized by the following, in descending structural order: (1) a hanging wall of upper Tertiary to Quaternary sedimentary and volcanic strata that displays little evidence for fault-related damage other than widely spaced planar or listric normal faults; (2) a sharp, planar, and locally striated principal slip plane forming the lower boundary of the hanging wall; (3) an upper zone—zone I—of very fine grained, fault-generated rocks composed dominantly of gouge; (4) a lower zone—zone II—of coarser-grained fault rocks consisting chiefly of foliated breccia; and (5) a variably damaged footwall, consisting of partly mylonitic Precambrian units or Tertiary plutonic rocks, that has been exhumed from depths of 10–12 km since late Miocene time. The fault rocks, which were mostly derived from the footwalls, preserve evidence for cataclastic and particulate flow. Fault rocks contain authigenic minerals but lack the cyclically deformed, mineral-filled syntectonic veins that are abundant in some other late Cenozoic high-angle and strike-slip faults. Meso- and microscale fabrics in zones I and II indicate that finite shear strains increase progressively upward, toward the principal slip plane. In a conceptual kinematic model of a shear box, displacements of the hanging wall produced a shearing flow in the fault rocks below. Some of the slip on the principal slip plane was also partitioned into localized slip on discrete sliding surfaces in zones I and II. Since 770 ka, during the latest stages of incremental deformation in the brittle shear zones, distributed flow in the fault rocks alternated with slip that was chiefly localized on the principal slip planes. In this respect, the detachment faults differ from inactive segments of the San Andreas system, along which displacements were progressively and irreversibly localized onto a single principal slip surface. The strain gradients and corresponding changes in grain size in the shear zones resemble those in mylonitic shear zones except in their symmetry. Strain gradients in the Black Mountains are notably asymmetric: they are present only below the principal slip planes, not above.

Décollement processes at the Nankai accretionary margin, southeast Japan : propagation, deformation, and dewatering

The decollement zone, expressed on seismic profiles and observed in drill cores from the Nankai accretionary margin off the southeast coast of Japan, reveals several unique characteristics which appear to distinguish it from thrust faults identified in the same setting. Physical manifestations of these include evidence for the asymmetric distribution of deformation structures about the decollement, the extension of this fault zone well in front of the tectonic deformation front, and the absence of features indicative of precursory shear, for example, folded sediments, shear bands, and penetrative mineral fabrics. These characteristics suggest that the mode of formation and evolution of this decollement zone may be unique from that of most thrust faults. We propose that the decollement zone propagates not as a shear fracture controlled by tectonic stress conditions but rather as a subhorizontal tension fracture propagating under high pore pressures. To test this possibility, physical property measurements and clay mineral fabrics were obtained for several samples from the Nankai decollement zone using computed tomography and X ray texture methods. Our findings suggest that deformation within the decollement zone is partitioned into a volumetric component, preserved as reduced porosities within coherent fragments, and a localized shear component, evidenced by mineral preferred orientations along discrete slip surfaces. We suggest that the reduced porosities result from the destruction of “cementation” in the sediments during the early stages of deformation and may arise from cyclic fatiguing of the sediment induced by fluctuating pore pressures. The nonpenetrative shear fabrics probably develop as the tectonic deformation front migrates seaward, and the weakened protodecollement subsequently accommodates shear displacements along discrete fractures.

Kinematics and a balanced and restored cross-section across the toe of the eastern Nankai accretionary prism

Abstract Seismic profiles across the eastern Nankai accretionary prism show evidence for diffuse deformation through stratal thickening and uplift of the accreting sediment package, thought to reflect the combination of small-scale ductile and brittle strains evident within drill cores. Using a kinematic solution based on changes in stratal thickness and porosities, diffuse strains are estimated for a transect across the eastern Nankai accretionary prism toe, in the vicinity of ODP Site 808. Calculated element displacements are used to reconstruct the undeformed configuration of the prism toe, providing a new method for balancing and restoring deformation in accretionary prisms. The results of this analysis demonstrate a heterogeneous distribution of strain within the prism toe, which appears to correlate with the distribution of brittle deformation structures in drill cores. The greatest vertical tectonic thickening and horizontal shortening estimates are obtained within the deepest sediments, which also display abundant brittle shears. Shallower sediments exhibit high volume loss and lower horizontal shortening, and in drill cores display very few deformation structures. This spatially variable strain distribution may result from inferred high pore pressures near the frontal thrust and decollement inducing a brittle overprint of previous ductile strains.

Seismic stratigraphy of the Frontal Hawaiian Moat: implications for sedimentary processes at the leading edge of an oceanic hotspot trace

Multi-channel seismic imaging reveals the seismic stratigraphy and associated sedimentary processes of the Frontal Hawaiian Moat (FHM) to the southeast of the island of Hawaii. Two sedimentary units are defined: (1) a basal layer of pelagic sediment draping the oceanic basement and (2) a wedge of volcaniclastic material infilling the FHM and onlapping the Frontal Hawaiian Arch. Three distinct seismic facies within the volcaniclastic unit are recognized: (A) areas of chaotic or incoherent reflections interpreted as proximal debris avalanche or slump deposits; (B) groups of hummocky and distorted reflections interpreted as distal debris avalanche or debris flow deposits; and (C) regions of parallel and laterally continuous reflections interpreted as turbidite deposits. The distribution of these facies delineates slope apron, proximal basin, and distal basin depositional environments (respectively) within the FHM. The northwest drift of the Pacific Plate over the Hawaiian hot spot results in the apparent southeasterly migration of the Hawaiian chain. Advancement of the depositional environments within the FHM occurs as individual volcanoes evolve, erode, and are superseded by new volcanic centers. The interplay between depositional processes and tectonic forces (plate motion and lithospheric flexure) predicts a coarsening-upward stratigraphy within the FHM. The combined accumulation of pelagic and volcaniclastic sediment defines a heterogeneous, and potentially unstable, layer of low strength material beneath the volcanic edifice that may influence the mobility of the island flanks.

Volcanic spreading and lateral variations in the structure of Olympus Mons, Mars

The Olympus Mons volcano on Mars is notable not only for its immense height and width, but also for substantial asymmetries in its structure. The gently sloped northwest flank extends to a much greater distance from the central caldera complex than the more steeply sloped southeast flank. Furthermore, the northwest flank exhibits lower-flank extensional faults, whereas the southeast shows upper-flank compressional terraces and lower-flank upthrust blocks. However, both the northwest and southeast flanks exhibit characteristic concave-upward profiles and steep bounding scarps, in contrast to other sectors. The NW-SE asymmetries are aligned with the regional slope from the Tharsis rise, but an understanding of the underlying causes has remained elusive. We use particle dynamics models of growing, spreading volcanoes to demonstrate that these flank structures could reflect the properties of the basement materials underlying Olympus Mons. We find that basal slopes alone are insufficient to produce the observed concave-upward slopes and asymmetries in flank extent and deformation style that are observed at Olympus Mons; instead, lateral variations in basal friction are required. These variations are most likely related to the presence of sediments, transported and preferentially accumulated downslope from the Tharsis rise. Such sediments likely correspond to ancient phyllosilicates (clays) recently discovered by the Mars Express mission.

Volcanic spreading on Mauna Loa volcano, Hawaii: Evidence from accretion, alteration, and exhumation of volcaniclastic sediments

A transect of submersible dives across the submarine west flank of Mauna Loa volcano yields compelling evidence for volcanic spreading and associated hydrothermal circulation during volcano growth. A frontal bench at the toe of the flank, formerly thought to be a downdropped block of Mauna Loa, contains a mix of volcaniclastic lithologies, including distally derived siltstone, mudstone, and hyaloclastite. The bench is overlain by bedded gravels and subaerially erupted pillow flows derived from local shoreline-crossing lava flows. The volcaniclastic strata in the bench were offscraped, uplifted, and accreted to the edge of the flank, as it plowed seaward into the surrounding moat. The accreted strata underwent significant diagenesis, through deep burial and circulation of hydrothermal fluids expelled from porous sediments beneath the volcano. Timing constraints for bench growth and breakup suggest that catastrophic failure of the subaerial edifice ca. 250–200 ka triggered volcanic spreading by reducing stresses resisting basal sliding and rift-zone inflation. Increased eruptive activity, and westward migration of Mauna Loa9s southwest rift zone, gradually rebuilt the massive flank, arresting slip prior to detachment of the Alika 2 debris avalanche ca. 120 ka.

The estimation of diffuse strains in the toe of the western Nankai accretionary prism: A kinematic solution

Arcward changes in the vertical spacing of seismically defined stratigraphic reflectors and sediment porosities determined from drill cores and seismic interval velocities suggest a compact kinematic solution for estimating diffuse deformation within accretionary prism toes. The method involves the solution of the Lagrangian form of the conservation of mass equation for the two-dimensional finite deformation gradient field within discrete deforming domains. We apply this technique to the unfaulted protothrust zone (PTZ) along a western transect across the Nankai accretionary prism, in the vicinity of Deep Sea Drilling Project sites 582 and 583. The results yield horizontal shortening estimates of about 1.7 km, or about 20%. This shortening appears to be accommodated through different strain responses depending on the consolidation state of the sediments at the time of accretion. Shallow sediments respond to horizontal shortening dominantly by expelling pore fluids, producing volume strains up to 30%, while deeper sediments accommodate horizontal shortening dominantly through vertical extension reaching 70% in the footwall of the frontal thrust. Horizontal shear strains within the PTZ are not well resolved by this solution due to uncertainties in the porosity distribution, but they generally appear to be quite small.