Kinematics and Timing of Superposed Deformation in the Funeral Mountains Metamorphic Core Complex, Death Valley, CA

The Funeral Mountains metamorphic core complex (FMMCC) in Death Valley, California exposes middle to lower crustal rocks of the Sevier-Laramide orogen in the footwall of the Miocene Boundary Canyon detachment. The structurally deepest rocks in the FMMCC are exposed in Monarch Canyon, where the Meso- to Neoproterozoic metasedimentary rocks record upper amphibolite facies metamorphism with migmatites developed at the deepest levels. Distributed ductile deformation and stratigraphically-localized high-strain zones, termed intracore shear zones, are responsible for attenuation and local stratigraphic omission during top-northwest non-coaxial deformation. The structurally deepest Monarch Spring shear zone (MSSZ) juxtaposes the migmatitic paragneisses below against greenschist to amphibolite facies marbles, pelitic and calcsilicate schists above. Below the MSSZ, the migmatitic paragneisses lack the top-northwest fabrics and instead exhibit a northeast-trending mineral lineation and local, strong fabric asymmetry indicative of top-southwest shear. We hypothesize that the MSSZ represents a deformed anatectic front, and an apparent zone of structural decoupling between orthogonally-directed shear fabrics at deep crustal levels during the Sevier-Laramide orogen. U-Pb zircon geochronology on leucogranite sills that are folded with the top-SW fabric yield crystallization ages of 68 Ma, while a dike cutting the top-SW fabric and another cutting isoclinal folds yield ages of 61 Ma and 57 Ma, respectively. This geochronology along with EBSD and structural observations indicate that the orthogonally-directed flow above and below the MSSZ may have developed coevally and synchronous with continued regional compression during the Sevier-Laramide orogeny, supporting the hypothesis of synorogenic extension. The timing of extension within Cordilleran metamorphic core complexes remains controversial, with the opposing views of Tertiary extension during a single tectonic event, or a polyphase extensional history beginning in the Late Cretaceous and continuing through the Miocene. The relative contributions of Late Cretaceous-early Tertiary and Miocene extensional strains in the FMMCC, which manifest in the top-northwest fabric, are addressed here using thermochronologic, microstructural, and EBSD studies. The EBSD and microstructural data show predominantly mixed to prism slip and subgrain rotation to grain boundary migration, suggesting deformation temperatures ca. 400-550° C. One quartzite lies adjacent to a marble that yields an 40Ar/39Ar muscovite cooling age of 44 Ma, suggesting pre-44 Ma deformation. The timing of deformation along the intracore shear zones remains elusive, although an 8 m.y. discrepancy (78 Ma versus 86 Ma) across the Monarch Canyon shear zone may indicate extension along the MCSZ occurring in the Late Cretaceous. Groupings of 40Ar/39Ar muscovite ages in the proximity of Monarch Canyon indicate cooling during the Oligocene and Eocene, hinting at the potential for an early Tertiary phase of extension that has not been previously recognized. Regardless, the thermochronologic and microstructural data support a polyphase extensional history for the Funeral Mountains metamorphic core complex, beginning in the Late Cretaceous and extending to the Miocene

Surface rupture of the Hundalee fault during the 2016 Mw 7.8 Kaikōura earthquake

The Hundalee fault is one of at least 20 faults that ruptured during the 2016 M w Mw 7.8 Kaikōura earthquake in the northeast of the South Island of New Zealand. Here, we document a 12‐km onshore section of the Hundalee fault that exhibited surface rupture from this event. To the northeast of our observations, the fault crosses the coast, and independent seabed surveys show that the 2016 rupture continued at least 2 km offshore. No surface rupture was observed across the southwestern section of the Hundalee fault, which crosses hilly vegetated terrain and poorly consolidated valley‐floor sediment. However, previous Interferometric Synthetic Aperture Radar (InSAR) analyses suggest that a 9‐km‐long section of the fault did rupture. Hence, the minimum length of the 2016 rupture along the Hundalee fault is 23 km. Field measurements indicate oblique dextral‐reverse slip along northeast‐trending Hundalee fault sections and reverse‐sinistral slip along north to north‐northeast‐trending sections. This is consistent with the regional principal horizontal shortening direction. Maximum vertical and horizontal offset measurements are 2.5±0.5 2.5±0.5 and 3.7±0.5  m 3.7±0.5  m , respectively. The discontinuous and irregular surface ruptures we observed are characteristic of a structurally immature fault, yet previous geological mapping indicates that the Hundalee fault is a regionally significant fault with >1‐km >1‐km late Cenozoic throw. Furthermore, a 60‐m‐wide sequence of fault rocks exposed by the rupture indicates that slip has localized into <10‐cm‐thick <10‐cm‐thick gouge zones, as anticipated for a mature fault. Therefore, a discrepancy exists between geological evidence of the Hundalee fault being a structurally mature fault and the characteristics of the 2016 rupture. We speculate that this signifies that the 2016 rupture was imposed on the Hundalee fault by movement across an inefficient multifault network rather than independent rupture of the Hundalee fault itself

Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

<p>During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.</p

Surface rupture of multiple crustal faults in the 2016 Mw 7.8 Kaikōura, New Zealand, earthquake

Multiple (>20 >20 ) crustal faults ruptured to the ground surface and seafloor in the 14 November 2016 M w Mw 7.8 Kaikōura earthquake, and many have been documented in detail, providing an opportunity to understand the factors controlling multifault ruptures, including the role of the subduction interface. We present a summary of the surface ruptures, as well as previous knowledge including paleoseismic data, and use these data and a 3D geological model to calculate cumulative geological moment magnitudes (M G w MwG ) and seismic moments for comparison with those from geophysical datasets. The earthquake ruptured faults with a wide range of orientations, sense of movement, slip rates, and recurrence intervals, and crossed a tectonic domain boundary, the Hope fault. The maximum net surface displacement was ∼12  m ∼12  m on the Kekerengu and the Papatea faults, and average displacements for the major faults were 0.7–1.5 m south of the Hope fault, and 5.5–6.4 m to the north. M G w MwG using two different methods are M G w MwG 7.7 +0.3 −0.2 7.7−0.2+0.3 and the seismic moment is 33%–67% of geophysical datasets. However, these are minimum values and a best estimate M G w MwG incorporating probable larger slip at depth, a 20 km seismogenic depth, and likely listric geometry is M G w MwG 7.8±0.2 7.8±0.2 , suggests ≤32% ≤32% of the moment may be attributed to slip on the subduction interface and/or a midcrustal detachment. Likely factors contributing to multifault rupture in the Kaikōura earthquake include (1) the presence of the subduction interface, (2) physical linkages between faults, (3) rupture of geologically immature faults in the south, and (4) inherited geological structure. The estimated recurrence interval for the Kaikōura earthquake is ≥5,000–10,000  yrs ≥5,000–10,000  yrs , and so it is a relatively rare event. Nevertheless, these findings support the need for continued advances in seismic hazard modeling to ensure that they incorporate multifault ruptures that cross tectonic domain boundaries

Petrophysical, Geochemical, and Hydrological Evidence for Extensive Fracture-Mediated Fluid and Heat Transport in the Alpine Fault's Hanging-Wall Damage Zone

International audienceFault rock assemblages reflect interaction between deformation, stress, temperature, fluid, and chemical regimes on distinct spatial and temporal scales at various positions in the crust. Here we interpret measurements made in the hanging‐wall of the Alpine Fault during the second stage of the Deep Fault Drilling Project (DFDP‐2). We present observational evidence for extensive fracturing and high hanging‐wall hydraulic conductivity (∼10−9 to 10−7 m/s, corresponding to permeability of ∼10−16 to 10−14 m2) extending several hundred meters from the fault's principal slip zone. Mud losses, gas chemistry anomalies, and petrophysical data indicate that a subset of fractures intersected by the borehole are capable of transmitting fluid volumes of several cubic meters on time scales of hours. DFDP‐2 observations and other data suggest that this hydrogeologically active portion of the fault zone in the hanging‐wall is several kilometers wide in the uppermost crust. This finding is consistent with numerical models of earthquake rupture and off‐fault damage. We conclude that the mechanically and hydrogeologically active part of the Alpine Fault is a more dynamic and extensive feature than commonly described in models based on exhumed faults. We propose that the hydrogeologically active damage zone of the Alpine Fault and other large active faults in areas of high topographic relief can be subdivided into an inner zone in which damage is controlled principally by earthquake rupture processes and an outer zone in which damage reflects coseismic shaking, strain accumulation and release on interseismic timescales, and inherited fracturing related to exhumation

Seismic anisotropy and its impact on imaging the shallow Alpine Fault: an experimental and modeling perspective

The transpressional Alpine Fault in New Zealand has created a thick shear zone with associated highly anisotropic rocks. Low seismic velocity zones (LVZ) and high seismic reflectivity are recorded in the Alpine Fault Zone, but no study has explored the underlying physical rock parameters of the shallow crust that control these observations. Protomylonites are the volumetrically dominant lithology of the fault zone. Here we combine experimental measurements of P‐wave speeds with numerical models of elastic wave anisotropy of protomylonite samples to explore how the fault zone can be seismically imaged. Numerical models that account for the porosity‐free real samples' fabric elastic tensors from EBSD are calculated by MTEX and a finite element model (FEM), while microfractures are modeled with differential effective media (DEM) theory. At effective pressures representative of the Alpine Fault brittle zone, experimental wave speeds are lower than those predicted by MTEX/FEM. A possible DEM model suggests that a combination of random and aligned microfractures with aspect ratios increasing with pressure can explain the experimental wave speeds for pressures <70 MPa. Such micro‐porosity in the form of foliation‐ and mica basal plane‐ parallel microfractures and grain boundaries is validated with synchrotron X‐ray microtomography and TEM images. Finally, by modeling anisotropy of seismic reflection coefficients with angle of incidence, we demonstrate that the high reflectivity and LVZ observed at the Alpine Fault can only be explained if this microporosity is accounted for throughout the brittle fault zone, even at depths of 7‐10 km

Micro- And nano-porosity of the active Alpine Fault zone, New Zealand

Porosity reduction in rocks from a fault core can cause elevated pore fluid pressures and consequently influence the recurrence time of earthquakes. We investigated the porosity distribution in the New Zealand's Alpine Fault core in samples recovered during the first phase of the Deep Fault Drilling Project (DFDP-1B) by using two-dimensional nanoscale and three-dimensional microscale imaging. Synchrotron X-ray microtomography-derived analyses of open pore spaces show total microscale porosities in the range of 0.1 %–0.24 %. These pores have mainly non-spherical, elongated, flat shapes and show subtle bipolar orientation. Scanning and transmission electron microscopy reveal the samples' microstructural organization, where nanoscale pores ornament grain boundaries of the gouge material, especially clay minerals. Our data imply that (i) the porosity of the fault core is very small and not connected; (ii) the distribution of clay minerals controls the shape and orientation of the associated pores; (iii) porosity was reduced due to pressure solution processes; and (iv) mineral precipitation in fluid-filled pores can affect the mechanical behavior of the Alpine Fault by decreasing the already critically low total porosity of the fault core, causing elevated pore fluid pressures and/or introducing weak mineral phases, and thus lowering the overall fault frictional strength. We conclude that the current state of very low porosity in the Alpine Fault core is likely to play a key role in the initiation of the next fault rupture

Policy and the Impact on Placement, Involvement, and Progress in General Education: Critical Issues that Require Rectification

Students with significant disabilities continue to be among the most segregated in schools. In this article, we argue that the principles of least restrictive environment and involvement and progress in the general curriculum have been interpreted in ways that perpetuate segregation, rather than increasing students’ access to meaningful curriculum in inclusive educational contexts. We examine this issue from three broad perspectives: federal policy related to least restrictive environment, interpretations of policies related to involvement and progress in the general curriculum, and the implementation of policies related to assessment of grade-level standards. We discuss implications of each of these issues for providing and increasing involvement and progress in general education contexts and content

Evolution of microporosity and permeability of quartzofeldspathic rocks during changes in crustal conditions and tectonite fabric

Most crustal rocks contain some microporosity that can host fluids and allow them to permeate, which in turn impacts their physical properties and rheology. Using electron microscopic methods we have examined the nature of microporosity in quartzofeldspathic rocks recovered from surface outcrops and boreholes that are representative of those actively deforming at up to 35km depth beneath New Zealand’s Southern Alps. In these exhumed samples the main types of microporosity are: (i) dilatant grain boundaries, (ii) grain boundary dislocation etch pits, (iii) intragranular fluid inclusions of various types, (iv) dilatant sites on phyllosilicate basal planes. We observe changes in the distribution and nature of porosity with proximity to the mylonitic shear zone down-dip of the active Alpine Fault, and these may be related to changes in the nature of the tectonite fabric. Our measurements of anisotropy of experimentally measured electrical conductivity and elastic wave propagation, and its change with increasing confining pressure (Pconf), provide insights into the relationship of microfracture porosity and mineral orientation. For example, electrical and elastic wave anisotropy (ρk/ρ⊥ and vpk/vp⊥) is high but decreases rapidly with increasing Pconf in samples with the strongest foliations, comprising foliation domains of quartz+feldspar with planar and through going phyllosilicate microlithons. Conversely, most weakly foliated samples where the same phases are well-mixed are less anisotropic and display less change in electrical and elastic wave anisotropy with increasing Pconf. This suggests linked type (iv) pore spaces parallel to foliation, which can host saline fluids, are preferentially closed with increasing Pconf. These types of changes are likely to only be significant at low Pconf, i.e., brittle conditions. At greater depth, in the creeping part of the shear zone, changes in the geometric arrangement of microporosity are more likely to result from differential thermal expansion and/or fluid-rock interactions. The TESA toolbox (https://umaine.edu/mecheng/vel/software/tesa_toolbox/) allows us to predict how thermal contraction may yield anisotropic grain boundary porosity for real microstructures. We can then validate these predictions by relating evidence of limited fluid-rock reactions to equilibrium phase diagrams for real bulk compositions