Jon Tarasewicz

Carbonate dilation breccias: Examples from the damage zone to the Dent Fault, northwest England

The obliqu-reverse Dent Fault, northwest England, throws Carboniferous limestone units in the footwall against mudston-dominated lower Paleozoic rocks in the hanging wall. The fault zone cuts the kilometer-wide steep limb of a precursory forced monocline. However, individual fault strands comprise centimeter-scale cataclasite cores fringed in the footwall carbonates by damage zones, some meters to tens of meters wide, composed of random-fabric dilation breccias. Breccia texture and microstructure, revealed by stained thin sections and peels, imply rapid coseismic fragmentation and then interseismic resealing by void-filling cements. The cements varied in composition through time from calcite to dolomite and then to ferroan calcite. Pervasive dolomitization of the protolith is common in the breccia zones. A key observation is that each volume of dilation breccia shows only limited refracture. This tendency to singl-phase brecciation suggests that cementation caused reseal-hardening of breccia with respect to intact protolith. Breccia thickness and refracture are greatest at jogs in the Dent Fault, but breccia distribution suggests that damage also accumulated in fault walls and at propagating fault tips. Dilation breccias are a common but poorly documented product of brittle deformation of limestone. Their reseal histories can provide valuable general clues to fault zone evolution.

Transient permeability and reseal hardening in fault zones: evidence from dilation breccia textures

Abstract It is well established that earthquake faulting can create permeability along a fault zone in high competence rocks - by mismatch of the fault walls and by secondary fracture in a surrounding damage zone - and that this permeability is created repeatedly during successive earthquake cycles. Less well proven is that such permeability is transient, being episodically reduced by precipitation of cements in the fracture porosity. The textures of carbonate dilation breccias, formed at around 1.7 km depth on the Dent Fault zone (NW England), lend support to this economically important concept of transient fracture permeability. The key observation is that many breccias reflect only a single episode of brecciation and reseal. A generally applicable explanation of such single-phase breccias is that they were resealed in the interval between major earthquakes, that this reseal made the breccia stronger that the intact rock, and that subsequent brecciation in the same rock volume was inhibited. This reseal-hardening model implies that transient permeability in fault zones may last no longer than the recurrence times of large earthquakes, and that the permeability conduits will change position in the damage zone with time, unless focused at a major fault jog or termination.

Seismic imaging of the shallow crust beneath the Krafla central volcano, NE Iceland

We studied the seismic velocity structure beneath the Krafla central volcano, NE Iceland, by performing 3-D tomographic inversions of 1453 earthquakes recorded by a temporary local seismic network between 2009 and 2012. The seismicity is concentrated primarily around the Leirhnjukur geothermal field near the center of the Krafla caldera. To obtain robust velocity models, we incorporated active seismic data from previous surveys. The Krafla central volcano has a relatively complex velocity structure with higher P wave velocities (V_p) underneath regions of higher topographic relief and two distinct low-V_p anomalies beneath the Leirhnjukur geothermal field. The latter match well with two attenuating bodies inferred from S wave shadows during the Krafla rifting episode of 1974–1985. Within the Leirhnjukur geothermalreservoir, we resolved a shallow (−0.5 to 0.5 km below sea level; bsl) region with low-V_p/V_s values and a deeper (0.5–1.5 km bsl) high-V_p/V_s zone. We interpret the difference in the velocity ratios of the two zones to be caused by higher rock porosities and crack densities in the shallow region and lower porosities and crack densities in the deeper region. A strong low-V_p/V_s anomaly underlies these zones, where a superheated steam zone within felsic rock overlies rhyolitic melt.