Mica-controlled anisotropy within mid-to-upper crustal mylonites: an EBSD study of mica fabrics in the Alpine Fault Zone, New Zealand

Dempsey, Edward D.; Prior, Dave; Mariani, Elisabetta; Toy, Virginia; Tatham, Daniel J.

doi:10.1144/sp360.3

Cited by 36 publications

(44 citation statements)

References 29 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In particular, single crystal velocity properties of muscovite and biotite micas can reach >50% [e.g., Ward et al, 2012]. The anisotropy of rock aggregates containing mica and quartz minerals is more complex, with greater anisotropy obtained for aggregates containing >20% mica [Dempsey et al, 2011;Ward et al, 2012;Erdman et al, 2013]. Assuming that our results represent anisotropy due to the alignment of mica-rich foliations in schist packages, the orientation of the slow axis is consistent with their emplacement during flat slab subduction [Porter et al, 2011].…”

Section: Rock-induced Anisotropysupporting

confidence: 59%

Layered crustal anisotropy around the San Andreas Fault near Parkfield, California

Audet

2015

JGR Solid Earth

View full text Add to dashboard Cite

The rheology of the Earth's crust controls the long-term and short-term strength and stability of plate boundary faults and depends on the architecture and physical properties of crustal materials. In this paper we examine the seismic structure and anisotropy of the crust around the San Andreas Fault (SAF) near Parkfield, California, using teleseismic receiver functions. These data indicate that the crust is characterized by spatially variable and strongly anisotropic upper and middle crustal layers, with a Moho at ∼35 km depth. The upper layer is ∼5-10 km thick and is characterized by strong (≥30%) anisotropy with a slow axis of hexagonal symmetry, where the plane of fast velocity has a strike parallel to that of the SAF and a dip of ∼40 ∘ . We interpret this layer as pervasive fluid-filled microcracks within the brittle deformation regime. The ∼10-15 km thick midcrustal layer is also characterized by a weak axis of hexagonal symmetry with ≥20% anisotropy, but the dip direction of the plane of fast velocity is reversed. The midcrustal anisotropic layer is more prominent to the northeast of the San Andreas Fault. We interpret the mid crustal anisotropic layer as fossilized fabric within fluid-rich foliated mica schists. When combined with various other geophysical observations, our results suggest that fault creep behavior around Parkfield is favored by intrinsically weak and overpressured crustal fabric.

show abstract

Section: Rock-induced Anisotropysupporting

confidence: 59%

Layered crustal anisotropy around the San Andreas Fault near Parkfield, California

Audet

2015

JGR Solid Earth

View full text Add to dashboard Cite

show abstract

“…Schists and gneisses, which are dominant in the exhumed middle continental crust, consist of flat, sheet-like minerals such as biotite, muscovite, chlorite, sericite, amphibole, talc, and graphite, interleaved with quartz and feldspar ( Figure 2) and have been considered as typical examples of TI materials with minimum velocities for waves propagating perpendicular to the foliation [e.g., Brocher and Christensen, 1990;Burlini and Fountain, 1993;Cholach and Schmitt, 2006;Dempsey et al, 2011;Godfrey et al, 2000;Ji et al, 2002;Naus-Thijssen et al, 2011a;Shapiro et al, 2004]. Phyllites are restricted to those schists with very fine phyllosilicates grains (Figure 2e), which usually result from mylonitization of schists.…”

Section: Samplesmentioning

confidence: 99%

“…The interpretation of seismic data generally uses an assumption that the rock formations interrogated by seismic waves have a simple transverse isotropy (TI) or hexagonal symmetry [e.g., Bostock and Christensen , ; Christensen and Okaya , ; Levin and Park , ; Okaya and Christensen , ; Porter et al , ]. In a TI material, P wave velocities are virtually the same in all radial directions perpendicular to the axis of symmetry, along which the seismic velocity can be either higher (e.g., laminated anorthosite [ Ji et al , ]) or lower (e.g., mica schist [ Brownlee et al , ; Dempsey et al , ; Erdman et al , ; Naus‐Thijssen et al , ; Ward et al , ; Wenk et al , ]) than the velocity normal to the axis. Shear wave splitting is maximum and null for propagation perpendicular and parallel to the axis of symmetry, respectively.…”

Section: Introductionmentioning

confidence: 99%

Magnitude and symmetry of seismic anisotropy in mica‐ and amphibole‐bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast Tibetan Plateau

Shao

Michibayashi

et al. 2015

JGR Solid Earth

View full text Add to dashboard Cite

We calibrated the magnitude and symmetry of seismic anisotropy for 132 mica‐ or amphibole‐bearing metamorphic rocks to constrain their departures from transverse isotropy (TI) which is usually assumed in the interpretation of seismic data. The average bulk Vp anisotropy at 600 MPa for the chlorite schists, mica schists, phyllites, sillimanite‐mica schists, and amphibole schists examined is 12.0%, 12.8%, 12.8%, 17.0%, and 12.9%, respectively. Most of the schists show Vp anisotropy in the foliation plane which averages 2.4% for phyllites, 3.3% for mica schists, 4.1% for chlorite schists, 6.8% for sillimanite‐mica schists, and 5.2% for amphibole schists. This departure from TI is due to the presence of amphibole, sillimanite, and quartz. Amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation, forming orthorhombic anisotropy with Vp(X) > Vp(Y) > Vp(Z). Effects of quartz are complicated, depending on its volume fraction and prevailing slip system. Most of the mica‐ or amphibole‐bearing schists and mylonites are approximately transversely isotropic in terms of S wave velocities and splitting although their P wave properties may display orthorhombic symmetry. The results provide insight for the interpretation of seismic data from the southeast Tibetan Plateau. The N‐S to NW‐SE polarized crustal anisotropy in the Sibumasu and Indochina blocks is caused by subvertically foliated mica‐ and amphibole‐bearing rocks deformed by predominantly compressional folding and subordinate strike‐slip shear. These blocks have been rotated clockwise 70–90° around the east Himalayan Syntaxis, without finite eastward or southeastward extrusion, in responding to progressive indentation of India into Asia.

show abstract

“…In addition, rocks formed within fault shear zones develop strong intrinsic elastic anisotropy from the alignment of platy minerals (such as mica) and foliation (Christensen, 1965;Jones & Nur, 1982;Shea & Kronenberg, 1993). The combined effects of seismic anisotropy and mechanical damage result in nonunique solutions when attempting to constrain the fault structure and physical properties from seismic waves (Dempsey et al, 2011;Gulley et al, 2017;Kelly et al, 2017).…”

Section: Introductionmentioning

confidence: 99%

Constraining Microfractures in Foliated Alpine Fault Rocks With Laser Ultrasonics

Simpson

Adam

Wijk

et al. 2020

Geophysical Research Letters

View full text Add to dashboard Cite

Quantifying the amount and alignment of microfractures is important to understand the geomechanics, fluid flow, and seismic imaging of fault zones. At the Alpine Fault, New Zealand, the preferred alignment of minerals, foliation, and fractures results in elastic wave anisotropy. We have designed a unique laser-ultrasonic laboratory setup to study Alpine Fault rock samples at upper crustal conditions. Combined with differential effective medium modeling, we distinguish microfracture porosity and orientation from mineral alignment, as a function of distance to the principal slip zone (PSZ). Nearest to the PSZ, the cataclasite has the lowest P wave anisotropy with the most (randomly oriented) fractures. Next, the ultramylonite exhibits the greatest P wave anisotropy (∼45%) with 40% of its fractures aligned with foliation. Further from the PSZ, P wave anisotropy is 14-19% on average, due to 20-30% of the fractures being oriented in the same direction as mineral alignment. Plain Language SummaryThe movement on faults causes fractures and alignment of minerals in the adjacent rocks. This causes seismic waves to travel fastest parallel to these features, a phenomenon known as anisotropy. Quantifying the amount and orientation of the fractures that contribute to this anisotropy is important for understanding fault zones and the processes which could influence the nature of earthquakes. However, this requires a method that can separate the effects of the fractures from those of the minerals. We have designed a system that uses laser ultrasonics to measure the speed of waves through rocks under pressure at spatially dense locations. We combine these measurements with numerical modeling, allowing us to accurately quantify the amount and orientation of fractures in rocks from the Alpine Fault, New Zealand. We find that the total amount of fractures increases toward the fault. Additionally, the amount of fractures aligned with the fault plane (up to 40%) increases toward the fault until the fractures become almost completely randomly oriented for the cataclasite in the core of the fault.

show abstract

Mica-controlled anisotropy within mid-to-upper crustal mylonites: an EBSD study of mica fabrics in the Alpine Fault Zone, New Zealand

Cited by 36 publications

References 29 publications

Layered crustal anisotropy around the San Andreas Fault near Parkfield, California

Layered crustal anisotropy around the San Andreas Fault near Parkfield, California

Magnitude and symmetry of seismic anisotropy in mica‐ and amphibole‐bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast Tibetan Plateau

Constraining Microfractures in Foliated Alpine Fault Rocks With Laser Ultrasonics

Contact Info

Product

Resources

About