Nicolas Barth

Scale dependence of oblique plate-boundary partitioning: New insights from LiDAR, central Alpine fault, New Zealand

We combine recently acquired airborne light detection and ranging (LiDAR) data along a portion of the Alpine fault with previous work to define the ways in which the plate-boundary structures partition at three different scales from 6 to 10 0 m. At the first order ( 6 –10 4 m), the Alpine fault is a remarkably straight and unpartitioned structure controlled by inherited and active weakening processes at depth. At the second order (10 4 –10 3 m), motion is serially partitioned in the upper ∼1–2 km onto oblique-thrust and strike-slip fault segments that arise at the scale of major river valleys due to stress perturbations from hanging-wall topographic variations and river incision destabilization of the hanging-wall critical wedge, concepts proposed by previous workers. The resolution of the LiDAR data refines second-order mapping and reveals for the first time that at a third order (10 3 –10 0 m), the fault is parallel-partitioned into asymmetric positive flower structures, or fault wedges, in the hanging wall. These fault wedges are bounded by dextral-normal and dextral-thrust faults rooted at shallow depths (

Extreme hydrothermal conditions at an active plate-bounding fault

Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre. At temperatures above 300–450 degrees Celsius, usually found at depths greater than 10–15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional–mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.

Strain within the ultrahigh-pressure Western Gneiss region of Norway recorded by quartz CPOs

Abstract Electron back-scatter diffraction (EBSD) was used to measure the crystal preferred orientations (CPOs) from 101 samples across the ultrahigh-pressure Western Gneiss region of Norway to assess slip systems, sense of shear, CPO strength, and strain geometry. The CPOs suggest a dominance of prism ⟨a⟩ slip, with lesser amounts of prism [c] slip and basal ⟨a⟩ slip; there are few Type I and Type II girdles. The major structural feature in the study area – the high-strain, top-W, normal-sense Nordfjord–Sogn Detachment Zone – is characterized by asymmetric and strong CPOs; an eastern domain with strong asymmetric CPOs shows top-E shear. Strain throughout the study area was characterized by a mix of plane strain and constriction with no evidence of flattening. Adjacent gneiss and quartzite/vein samples have similar CPOs.

The Cascade rock avalanche: implications of a very large Alpine Fault-triggered failure, New Zealand

Catastrophic deep-seated rock slope failures (RSFs; e.g., rock avalanches) can be particularly useful proxies for fault rupture and strong ground motion, and currently represent an underappreciated hazard of earthquakes in New Zealand. This study presents observations of the previously undescribed Cascade rock avalanche (CRA), a c. 0.75 km3 single-event, long-runout, catastrophic failure interpreted to have been coseismically triggered by a large to great earthquake c. 660 AD on the Alpine Fault. Despite its size and remarkable preservation, the CRA deposit has been previously identified as a terminal moraine and fault-damaged outcrop, highlighting the common misinterpretation of similar rock avalanche deposits. Comparisons are drawn between the CRA and other Alpine Fault-attributed rock avalanches, such as the better-studied c. 860 AD Round Top rock avalanche, to re-assess coseismic rock avalanche hazard. Structural relationships indicate the rock mass comprising the CRA may have formerly been a portion of a larger (c. 3 km3) RSF, before its catastrophic collapse on a deep-seated gravitational collapse structure (sackung). Sackungen and RSFs are common throughout the Southern Alps and other mountainous regions worldwide; in many cases, they should be considered potential precursors to catastrophic failure events. Two masses of rock in the Cascade River Valley show precursory signs of potential catastrophic failures of up to c. 2 km3; a similar mass may threaten the town of Franz Josef.

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

ABSTRACT 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.

Evaluation of coseismic landslide hazard on the proposed Haast-Hollyford Highway, South Island, New Zealand.

ABSTRACT Coseismic landsliding presents a major hazard to infrastructure in mountains during large earthquakes. This is particularly true for road networks, as historically coseismic landsliding has resulted in road losses larger than those due to ground shaking. Assessing the exposure of current and planned highway links to coseismic landsliding for future earthquake scenarios is therefore vital for disaster risk reduction. This study presents a method to evaluate the exposure of critical infrastructure to landsliding from scenario earthquakes from an underlying quantitative landslide hazard assessment. The method is applied to a proposed new highway link in South Island, New Zealand, for a scenario Alpine Fault earthquake and compared to the current network. Exposure (the likelihood of a network being affected by one or more landslides) is evaluated from a regional-scale coseismic landslide hazard model and assessed on a relative basis from 0 to 1. The results show that the proposed Haast-Hollyford Highway (HHH) would be highly exposed to coseismic landsliding with at least 30–40 km likely to be badly affected (the Simonin Pass route being the worse affected of the two routes). In the current South Island State Highway network, the HHH would be the link most exposed to landsliding and would increase the total network exposure by 50–70% despite increasing the total road length by just 3%. The present work is intended to provide an effective method to assess coseismic landslide hazard of infrastructure in mountains with seismic hazard, and potentially identify mitigation options and critical network segments.

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

Fault 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 faults 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.

Detection Thresholds of Archaeological Features in Airborne Lidar Data from Central Yucatán

Abstract In this article we evaluate ∼48km2 of airborne lidar data collected at a target density of 15 laser shots/m in central Yucatán, Mexico. This area covers parts of the sites of Chichén Itzá and Yaxuná, a kilometer-wide transect between these two sites, and a transect along the first few kilometers of Sacbé 1 from Yaxuná to Cobá. The results of our ground validation and mapping demonstrate that not all sizable archaeological features can be detected in the lidar images due to: (1) the slightly rolling topography interspersed with 1-6 m-high bedrock hummocks, which morphologically mimic house mounds, further complicated by the presence of low foundations; (2) the complex forest structure in central Yucatán, which has particularly dense near-ground understory resulting in a high number of mixed-signal ground and low vegetation returns which reduces the fidelity and accuracy of the bare-earth digital elevation models; and (3) the predominance of low archaeological features difficult to discern from the textural noise of the near-ground vegetation. In this article we explore different visualization techniques to increase the identification of cultural features, but we conclude that, in this portion of the Maya region, lidar should be used as a complement to traditional on-the-ground survey techniques.

Links between orogenic and placer gold on the Old Man Range, Central Otago, New Zealand

Gold-bearing normal faults of inferred Late Cretaceous age cut both Caples Terrane and Torlesse Terrane schists on the Old Man Range in Central Otago. Mineralised fault rocks are dominated by brittle-formed breccias and fault gouges, with minor hydrothermal silicification and quartz vein formation, and scattered pyrite and arsenopyrite. Gold occurs replacing silicates, encapsulated in sulphide minerals, and as free particles in vein quartz. The orientations of mineralised faults were controlled by pre-existing joints in the host rocks, with west strike in Torlesse schists and northwest strike in Caples schists. A major component of placer gold on the slopes of the Old Man Range has been derived from nearby mineralised faults, and detrital gold particles are equant and angular rather than flaky. Gold in a Late Pleistocene Clutha River channel is flaky and has been transported from outside the area, but there is little or no evidence of externally derived detrital gold along the course of a Middle Pleistocene Clutha River channel.

Lidar reveals uniform Alpine fault offsets and bimodal plate boundary rupture behavior, New Zealand: COMMENT

De Pascale et al. (2014) present four dextral offsets (fluvial terrace risers and channels) identified from an unnamed creek 800 m southwest of Gaunt Creek (one of the best-studied outcrops on the Alpine fault). They interpret these offsets to record three cumulative single-event displacements associated with the last three major surface-rupturing earthquakes on the central Alpine fault and use these data to draw conclusions about the seismogenic behavior of the Alpine fault overall. Herein I argue (1) their apparent dextral offsets occur on a steep dextral-normal fault subsidiary to the main dextral-thrust trace of the Alpine fault at this location, (2) that the subsidiary fault in question is kinematically incapable of producing single-event displacements on the order of 6–8 m, and thus (3) that their discussions and conclusions regarding more complex or bimodal behavior on the Alpine fault are unfounded in light of their data. The nature of complex near-surface fault segmentation on the Alpine fault in the Gaunt Creek–Darnley Creek area has been well established by previous workers. Serially partitioned dextral-thrust and linking strike-slip slip surfaces segment the main Alpine fault plane at a map scale of 1–10 km and to a depth of ~1–2 km (e.g., Norris and Cooper, 2007, and references therein). Superimposed on this serial partitioning are parallel partitioned dextral-thrust and dextral-normal faults defining 100–500-m-wide fault wedges in the near surface (Barth et al., 2012). All of the measurements by De Pascale et al. are clearly made on a subsidiary dextral-normal fault (Southern Alps-side-down motion) and not the main dextral-thrust Alpine fault plane at this location (Fig. 1; cf. their figure 3). This particular subsidiary fault had previously been observed in the field and three-dimensionally modeled by Easterbrook (2010). Barth et al. (2012) used the same 2010 lidar dataset as the study in question to show that these southeast-facing, dextral-normal sense fault scarps are ubiquitous along the length of the 34-km-long central Alpine fault lidar swath, and when tied to outcrop observations (e.g., Norris and Cooper, 2007; Easterbrook, 2010), indicate the presence of parallel partitioning between a dominant (but poorly preserved) outboard dextral-thrust and a subsidiary steep dextral-normal fault (better preserved because the fault plane is steeper and its scarp is inboard of the rangefront). Structural contouring of the Alpine fault in this region by Norris et al. (2012) indicates the fault trace in question is subsidiary to the main Alpine fault plane and likely terminates into it at an intersection angle greater than 50 and at a shallow depth of ~100 m. Barth et al. (2012, their figure 9A) used a simple vector summation of fault plane/slickenline measurements and a plate boundary vector of 251 to show that the (frontal) basal dextral-thrust plane is expected to accommodate greater than three times the displacement of the corresponding fault wedge-bounding dextral-normal fault. A single-event dextral displacement of 7 m on one of these subsidiary dextral-normal faults would kinematically require there to be greater than three times more displacement on the corresponding dextral-thrust trace, a value wholly unrealistic with known Alpine fault behavior. With no direct evidence for the timing of their apparent dextral offsets, De Pascale et al. compile measured offset data along the Alpine fault and near-fault/on-fault paleoseismic proxies to fit their measurements to. Note that all of their compiled offsets occur in locations where the surface geometry of the Alpine fault is considerably less complex than the area in question and strike-slip motion is more dominant overall. Their catalog of paleoseismic events relies on how they chose to define their 20–220 yr uncertainty range for events, yet no explanation of the necessary interpretations of the various proxies and how they were selectively weighted was presented in the paper, or as a supplement in the GSA Data Repository, so readers are left unknowing.