Styles of underplating in the Marin Headlands Terrane, Franciscan Complex, California

This is a pre-copy-editing, author-produced PDF of an article accepted for publication in The Geological Society of America Special Papers following peer review. The definitive publisher-authenticated version: "Regalla, C., Rowe, C., Harrichhausen, N., Tarling, M. and Singh, J., 2018. Styles of underplating in the Marin Headlands Terrane, Franciscan Complex, California. GSA Special Publications no. 534" is available online at: http://rock.geosociety.org/Store/detail.aspx?id=spe534.Geophysical images and structural cross-sections of accretionary wedges are usually aligned orthogonal to the subduction trench axis. These sections often reveal underplated duplexes of subducted oceanic sediment and igneous crust that record trench-normal shortening and wedge thickening facilitated by down-stepping of the décollement. However, this approach may under-recognize trench-parallel strain and the effects of faulting associated with flexure of the downgoing plate. New mapping of a recently exposed transect across a portion of the Marin Headlands terrane, California, USA documents evidence for structural complexity over short spatio-temporal scales within an underplated system. We document the geometry, kinematics, vergence and internal architecture of faults and folds along ~2.5 km of section, and identify six previously unmapped intra-formational imbricate thrusts and thirteen high-angle faults that accommodate shortening and flattening of the underthrust section. Thrust faults occur within nearly every lithology without clear preference for any stratigraphic horizon, and fold vergence varies between imbricate sheets by ~10-40°. In our map area, imbricate bounding thrusts have relatively narrow damage zones (≤5-10 m), sharp, discrete fault cores, and lack veining, in contrast to the wide, highly-veined fault zones previously documented in the Marin Headlands terrane. The spacing of imbricate thrusts combined with paleo-convergence rates indicates relatively rapid generation of new fault surfaces on ~10-100 ka timescales, a process which may contribute to strain hardening and locking within the seismogenic zone. The structural and kinematic complexity documented in the Marin Headlands are an example of the short spatial and temporal scales of heterogeneity that may characterize regions of active underplating. Such features are smaller than the typical spatial resolution of geophysical data from active subduction thrusts, and may not be readily resolved, thus highlighting the need for cross-comparison of geophysical data with field analogues when evaluating the kinematic and mechanical processes of underplating

Dynamic earthquake rupture preserved in a creeping serpentinite shear zone

Laboratory experiments on serpentinite suggest that extreme dynamic weakening at earthquake slip rates is accompanied by amorphisation, dehydration and possible melting. However, hypotheses arising from experiments remain untested in nature, because earthquake ruptures have not previously been recognised in serpentinite shear zones. Here we document the progressive formation of high-temperature reaction products that formed by coseismic amorphisation and dehydration in a plate boundary-scale serpentinite shear zone. The highest-temperature products are aggregates of nanocrystalline olivine and enstatite, indicating minimum peak coseismic temperatures of ca. 925 ± 60 °C. Modelling suggests that frictional heating during earthquakes of magnitude 2.7–4 can satisfy the petrological constraints on the coseismic temperature profile, assuming that coseismic fluid storage capacity and permeability are increased by the development of reaction-enhanced porosity. Our results indicate that earthquake ruptures can propagate through serpentinite shear zones, and that the signatures of transient frictional heating can be preserved in the fault rock record

Deformation processes, textural evolution and weakening in retrograde serpentinites

Serpentinites play a key role in controlling fault rheology in a wide range of geodynamic settings, from oceanic and continental rift zones to subduction zones. In this paper, we provide a summary of the most common deformation mechanisms and frictional strengths of serpentine minerals and serpentinites. We focus on deformation mechanisms in retrograde serpentinites, which show a progressive evolution from undeformed mesh and bastite pseudomorphic textures to foliated, ribbon-like textures formed by lizardite with strong crystallographic and shape preferred orientations. We also discuss the possible mechanical significance of anastomosing slickenfibre veins containing ultraweak fibrous serpentines or relatively strong splintery antigorite. Our review and new observations indicate that pressure solution and frictional sliding are the most important deformation mechanisms in retrograde serpentinite, and that they are frictionally weak (μ~0.3). The mineralogical and microstructural evolution of retrograde serpentinites during shearing suggests that a further reduction of the friction coefficient to μ of 0.15 or less may occur during deformation, resulting in a sort of continuous feedback weakening mechanism

The internal structure and composition of a plate-boundary-scale serpentinite shear zone: the Livingstone Fault, New Zealand

Abstract. Deciphering the internal structure and composition of large serpentinite-dominated shear zones will lead to an improved understanding of the rheology of the lithosphere in a range of tectonic settings. The Livingstone Fault in New Zealand is a terrane-bounding structure that separates the basal portions (peridotite; serpentinised peridotite; metagabbros) of the Dun Mountain Ophiolite Belt from the quartzofeldspathic schists of the Caples and Aspiring Terrane. Field and microstructural observations from 11 localities along a strike length of ca. 140 km show that the Livingstone Fault is a steeply dipping, serpentinite-dominated shear zone tens of metres to several hundred metres wide. The bulk shear zone has a pervasive scaly fabric that wraps around fractured and faulted pods of massive serpentinite, rodingite and partially metasomatised quartzofeldspathic schist up to a few tens of metres long. S–C fabrics and lineations in the shear zone consistently indicate a steep east-side-up shear sense, with significant local dispersion in kinematics where the shear zone fabrics wrap around pods. The scaly fabric is dominated (>98 % vol) by fine-grained (≪10 µm) fibrous chrysotile and lizardite–polygonal serpentine, but infrequent (<1 % vol) lenticular relicts of antigorite are also preserved. Dissolution seams and foliation surfaces enriched in magnetite, as well as the widespread growth of fibrous chrysotile in veins and around porphyroclasts, suggest that bulk shear zone deformation involved pressure–solution. Syn-kinematic metasomatic reactions occurred along all boundaries between serpentinite, schist and rodingite, forming multigenerational networks of nephritic tremolite veins that are interpreted to have caused reaction hardening within metasomatised portions of the shear zone. We propose a conceptual model for plate-boundary-scale serpentinite shear zones which involves bulk-distributed deformation by pressure–solution creep, accompanied by a range of physical (e.g. faulting in pods and wall rocks; smearing of magnetite along fault surfaces) or chemical (e.g. metasomatism) processes that result in localised brittle deformation within creeping shear zone segments

Crystallographic orientation mapping of lizardite serpentinite by Raman spectroscopy

The serpentine mineral lizardite displays strong Raman anisotropy in the OH-stretching region, resulting in significant wavenumber shifts (up to ca. 14.5 cm−1) that depend on the orientation of the impinging excitation laser relative to the crystallographic axes. We quantified the relationship between crystallographic orientation and Raman wavenumber using well-characterised samples of Monte Fico lizardite by applying Raman spectroscopy and electron backscatter diffraction (EBSD) mapping on thin sections of polycrystalline samples and grain mounts of selected single crystals, as well as by a spindle stage Raman study of an oriented cylinder drilled from a single crystal. We demonstrate that the main band in the OH-stretching region undergoes a systematic shift that depends on the inclination of the c-axis of the lizardite crystal. The data are used to derive an empirical relationship between the position of this main band and the c-axis inclination of a measured lizardite crystal: y=14.5cos 4 (0.013x+0.02)+(3670±1), where y is the inclination of the c-axis with respect to the normal vector (in degrees), and x is the main band position (wavenumber in cm −1) in the OH-stretching region. This new method provides a simple and cost-effective technique for measuring and quantifying the crystallographic orientation of lizardite-bearing serpentinite fault rocks, which can be difficult to achieve using EBSD alone. In addition to the samples used to determine the above empirical relationship, we demonstrate the applicability of the technique by mapping the orientations of lizardite in a more complex sample of deformed serpentinite from Elba Island, Italy.publishedVersio

Magma plumbing systems: a geophysical perspective

Over the last few decades, significant advances in using geophysical techniques to image the structure of magma plumbing systems have enabled the identification of zones of melt accumulation, crystal mush development, and magma migration. Combining advanced geophysical observations with petrological and geochemical data has arguably revolutionised our understanding of, and afforded exciting new insights into, the development of entire magma plumbing systems. However, divisions between the scales and physical settings over which these geophysical, petrological, and geochemical methods are applied still remain. To characterise some of these differences and promote the benefits of further integration between these methodologies, we provide a review of geophysical techniques and discuss how they can be utilised to provide a structural context for and place physical limits on the chemical evolution of magma plumbing systems. For example, we examine how Interferometric Synthetic Aperture Radar (InSAR), coupled with Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) data, and seismicity may be used to track magma migration in near real-time. We also discuss how seismic imaging, gravimetry and electromagnetic data can identify contemporary melt zones, magma reservoirs and/or crystal mushes. These techniques complement seismic reflection data and rock magnetic analyses that delimit the structure and emplacement of ancient magma plumbing systems. For each of these techniques, with the addition of full-waveform inversion (FWI), the use of Unmanned Aerial Vehicles (UAVs) and the integration of geophysics with numerical modelling, we discuss potential future directions. We show that approaching problems concerning magma plumbing systems from an integrated petrological, geochemical, and geophysical perspective will undoubtedly yield important scientific advances, providing exciting future opportunities for the volcanological community

El defensor de Córdoba : diario católico: Año XV Número 4299 - 1913 octubre 22

Copia digital. Madrid : Ministerio de Cultura. Subdirección General de Coordinación Bibliotecaria, 200

Distinguishing the Raman spectrum of polygonal serpentine

Positively identifying serpentine mineral types is important for a wide range of disciplines in the Earth sciences and health sciences. Although Raman spectroscopy has been widely applied as a tool to distinguish four of the main serpentine minerals (i.e., antigorite, lizardite, chrysotile, and polygonal serpentine), some uncertainty remains as to whether all four varieties have unique Raman spectra. In this paper, submicron Raman spectroscopy mapping was performed directly on electron-transparent regions of serpentine samples that were unambiguously identified by transmission electron microscopy (TEM). The increased spatial resolution of the Raman mapping technique (~370 nm), combined with the detailed characterization provided by TEM, indicates that polygonal serpentine has the same Raman spectrum as lizardite and therefore cannot be spectrally distinguished from lizardite. Furthermore, the Raman spectral profile that has previously been reported as unique to polygonal serpentine is likely to represent a mixture of chrysotile and polygonal serpentine/lizardite. To positively discriminate between lizardite and polygonal serpentine requires TEM investigation