Showing papers by "Ake Fagereng published in 2010"

Mélange rheology and seismic style

Abstract: Shear displacements in crustal fault zones are accommodated by a range of seismic styles, including standard earthquakes, non-volcanic tremor, and continuous and transitory aseismic slip. Subduction channel shear zones, containing highly sheared, fluid-saturated trench-fill sediments intermingled with fragments of oceanic crust, are commonly inferred to occur along active subduction megathrusts. If this interpretation is correct, these plate boundary faults are not discrete planes, but may resemble the melange shear zones commonly found in exhumed subduction-related rock assemblages. Melange deformation appears to depend critically on the ratio of competent to incompetent material, with shear surfaces localized along lithological contacts or within competent domains, while matrix flow accommodates shearing by distributed strain. If the style of strain and/or displacement accommodation in a melange reflects the partitioning between aseismic and seismic slip, the proportion of competent material seems likely to be a significant factor affecting seismic style within subduction channel shear zones, and along comparable mixed-lithology fault zones.

Shear veins observed within anisotropic fabric at high angles to the maximum compressive stress

Abstract: Some faults seem to slip at unusually high angles (>45°) relative to the orientation of the greatest principal compressive stress1, 2, 3, 4, 5. This implies that these faults are extremely weak compared with the surrounding rock6. Laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre-existing frictionally weak surface7, 8, 9, weakening from chemical reactions10, elevated fluid pressure11, 12, 13 or dissolution–precipitation creep14, 15. Here we describe shear veins within the Chrystalls Beach accretionary melange, New Zealand. The melange is a highly sheared assemblage of relatively competent rock within a cleaved, anisotropic mudstone matrix. The orientation of the shear veins—compared with the direction of hydrothermal extension veins that formed contemporaneously—indicates that they were active at an angle of 80°±5° to the greatest principal compressive stress. We show that the shear veins developed incrementally along the cleavage planes of the matrix. Thus, we suggest that episodic slip was facilitated by the anisotropic internal fabric, in a fluid-overpressured, heterogeneous shear zone. A similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings. We therefore conclude that incremental slip along a pre-existing planar fabric, coupled to high fluid pressure and dissolution–precipitation creep, may explain active slip on severely misoriented faults.

The metamorphic history of rocks buried, accreted and exhumed in an accretionary prism: an example from the Otago Schist, New Zealand

Abstract: Although subgreenschist facies metamorphic rocks are widespread in the upper crust, mineralogical processes affecting these rocks are poorly understood. Subgreenschist mineralogical transitions have been invoked as critical controls on the mechanical behaviour of rocks within the crustal seismogenic zone, calling for further study of very low-grade metamorphic assemblages. In this study a multi-technique thermobarometric study of the Chrystalls Beach Complex melange, which is located within the Otago Schist accretion-collision assemblage of the South Island of New Zealand, is presented. The Chrystalls Beach Complex comprises highly sheared trench-fill sedimentary rocks and scattered pillow basalts, and is inferred to have formed during Jurassic subduction under the paleo-Pacific Gondwana margin. Equilibrium mineral assemblages indicate peak P–T conditions in the range 400–550 MPa and 250–300 °C, which is supported by chlorite thermometry. Relatively high pressures of burial and accretion during foliation development are inferred from phengite content and b0 spacing analyses of white mica. Rare lawsonite occurs in a post-foliation vein, and illite ‘crystallinity’ measurements indicate a thermal overprint during exhumation. These P–T estimates and their relative chronology indicate that the mineral assemblages developed along a clockwise P–T path. Based on variability in P–T estimates from different techniques, mineral assemblages developed during burial are largely overprinted during exhumation at similar or higher-T than experienced along the prograde path. Observed subduction-related subgreenschist assemblages are therefore likely to indicate lower-P than experienced during subduction, as higher-P mineral compositions re-equilibrate during exhumation. The P–T path inferred in this study is similar in shape to P–T paths for higher grade parts of the Otago Schist, and other exhumed accretionary prisms around the world, and is therefore probably common for rocks buried, accreted and exhumed in accretionary prisms.

Petrology of metabasalts from the Chrystalls Beach accretionary melange - implications for tectonic setting and terrane origin

Abstract: The Chrystalls Beach Complex is interpreted as an accretionary melange within the Otago Schist on the South Island of New Zealand. Its stratigraphic position within the New Zealand Mesozoic accretionary prism is not well constrained, and the terrane affinity of the complex has remained enigmatic. Previous studies have focused on age relationships and sediment geochemistry. In this contribution we present geochemical data for metabasalts within the complex. Two distinctly different basalt compositions are found in the melange. Metabasalts outcropping at the northern end of the complex have an inferred mid-ocean ridge affinity, while metabasalts in the central part of the assemblage are suggested to represent ocean island basalts. It is speculated that the northern mid-ocean ridge basalts represent blocks from the crust of a subducting slab, while the ocean island basalts are part of a subducting seamount. These tectonic affinities are in agreement with melange formation during subduction of material below the Otago Schist protolith. In this setting, the melange may contain material derived from both the Caples and Rakaia Terranes, or represent a different terrane fragment.