Deformation of Mid-Crustal Sheeted Granitoids: An example from the eastern Transverse Ranges, southern California.

Brown, Kenneth (kenlbrow@indiana.edu (317)-274-7484), Barth, Andrew P.

 

      Detailed studies of North American Cordilleran sheeted plutons (Miller and Paterson, 2001; Mahan et al., 2003; Manduca et al., 1993) reveal that many have predominantly moderately dipping to sub-vertical geometries, indicating construction within a fundamentally vertical boundary zone in mid-crustal terrains in the Cordillera.  In contrast, the Bighorn sheeted complex of the eastern Transverse Ranges in southern California preserves fabrics that indicate a gently-dipping geometry.

      The eastern Transverse Ranges is a well-exposed terrain that extends >100km across strike of the North American Cordillera in southern California.  Preliminary barometry shows that the eastern Transverse Ranges constitutes a nearly continuous, tilted cross-section of the Mesozoic arc. Whereas the central part of this tilted section is dominated by comparatively homogeneous Mesozoic plutons that intrude Proterozoic basement, the western part is dominated by sheeted plutons of Late Jurassic and Late Cretaceous age.  Preliminary field work shows that a transition from foliated mid-crustal sheets to non-foliated plutons exists, suggesting an important lithospheric boundary in the Mesozoic arc of southern California. This study will examine the timing and development of magmatic and solid-state fabrics along transects across this transitional zone, to better understand the emplacement mechanisms of mid-crustal sheeted plutons and to better constrain the tectonic evolution of the eastern Transverse Ranges.

       Preliminary microscopic investigations reveal that deformation within individual sheets is variable; no correlation to composition has yet been made.  Although these sheeted plutons were originally described as foliated metamorphic rocks, microscope investigations also reveal well preserved magmatic textures and fabrics defined by euhedral to subhedral zoned plagioclase and hornblende. Deformation within individual sheets is primarily observed as overprinting solid-state fabrics.

References:

Mahan et al., 2003, Geological Society of America Bulletin. v.115, p.1570-1582.

Manduca et al., 1993, Geological Society of America Bulletin. v.105, p.749-765.

Miller and Paterson, 2001, Geological Society of America Bulletin, v.113, p.1423-1442.

Strain partitioning in an anatectic domain

Christopher Gerbi, Norwich University Northfield VT 05663; cgerbi@norwich.edu

 

Rock strength diminishes markedly during melting, reducing viscosity contrasts between different pre-melting lithologies.  Even with this reduced strength contrast, deformation can strongly partition in an anatectic domain.  The Chain Lakes massif, western Maine, probably formed as a forearc basin to the Early Ordovician Notre Dame arc system and originally contained a sedimentary-volcanic sequence of pelitic to semi-pelitic strata and minor felsic and mafic volcanic rocks.  The entire massif, which covers approximately 900 km², partially melted ca. 468 Ma, perhaps due to a ridge subduction event.  On the order of 40% of the protolith melted.  The massif comprises three distinct facies, each exhibiting micro- and macrostructural evidence of anatexis.  Bulk compositions of the three facies are similar.  The structurally highest unit, the McKenney Stream Facies, has no layering and could be considered a “dirty granite”, with 0-5 cm equant lithic fragments distributed homogeneously.  The Sarampus Falls facies, the middle unit, exhibits wispy banding, defined by biotite-sillimanite schlieren, and 0-30 cm elliptical lithic fragments.  The Twin Bridges facies, the structurally lowest unit, is strongly banded and has fewer lithic fragments.  The fragments in all units are interpreted to have formed by dissagregation of the original stratigraphy. 

Evidence for deformation increases markedly down section, with the compositional banding in the Twin Bridges facies implying high strain.  Fibrolite within the schlieren are isoclinally folded and the fabric is consistently oriented over a wide area.  In contrast, the McKenney Stream facies is isotropic.  Although precise melt percentages are difficult to estimate, the Twin Bridges unit appears to contain the highest proportion of restitic minerals and appears to have experienced the highest strain.  Given the similar quartzofeldspathic make-up of the different units, the reason for a high degree of strain partitioning is uncertain.

Deformation-induced polymorphic transformation: experimental deformation of kyanite, andalusite, and sillimanite

 

Eric T. Goergen*, Donna L. Whitney, Mark E. Zimmerman, and Take Hiraga

Department of Geology and Geophysics, University of Minnesota, Minneapolis MN  55455 USA

*email: goer0074@umn.edu

 

The proper interpretation of metamorphic and deformation microstructures is  critical to our understanding of the metamorphic and strain histories experienced by a rock. Another critical aspect is being able to place these interpretations into an appropriate reference frame (generally P-T) to relate our observations to tectonic processes. However, this is often precluded by our weak understanding of the relationships between phase equilibria and strain phenomena, and how these processes are related in pressure-temperature space. Quartz and plagioclase fabrics are valuable in determining the basic state of strain in a sample, but used on their own are not reliable indicators of P-T conditions or, more importantly, of P-T path. 

The 3 Al₂SiO₅ polymorphs (andalusite, kyanite, sillimanite) are common in metapelitic rocks and impure quartzite, and although not rheologically important, are very useful for determining metamorphic conditions. Owing to their sluggish reaction kinetics, 2-3 polymorphs may coexist metastably, allowing interpretation of part of the pressure-temperature path if the sequence of crystallization can be determined. To characterize the mechanical properties and relative strengths of the Al₂SiO₅ polymorphs (andalusite, kyanite, sillimanite), and to determine the effect of ductile deformation on polymorphic transformation, we experimentally deformed fine-grained aggregates of each polymorph in shear and torsion using Paterson gas medium devices. In both types of experiments, we simultaneously deformed a stack of three hot-pressed discs (one each of andalusite, kyanite, and sillimanite), and we also deformed a disc of each polymorph individually in torsion. Shear experiment conditions were 1000°C at 300 MPa confining pressure (sillimanite stability field) for 4 hours and 150% shear strain. Torsion conditions were 1250°C at 300 MPa and a constant shear strain rate of 2 x 10^-4 s^-1 to 400% shear strain; shear stress varied from a maximum of 200 MPa for the first 200% shear strain to an average of 100 MPa during the last 200%. All experiments resulted in development of strong crystallographic preferred orientation but only a slight shape preferred orientation. All three polymorphs developed an alignment of the (001) axis parallel with or slightly oblique to the shear plane; in the latter case, the c-axes track the maximum principal axis of finite strain. 

Initial EBSD results for the torsion experiment suggested that most of the grains did not transform, however, TEM investigations of both the kyanite and andalusite individual torsion experiments show wide spread transformation to sillimanite at the submicron scale. In both the kyanite and andalusite experiments antithetic shear bands developed consisting of sillimanite (var. fibrolite). Polymorphic transformation to sillimanite was also observed in both the kyanite and andalusite experiments as 1-10 nm-scale sillimanite crystals surrounding clasts of the original polymorph. In all cases the sillimanite was aligned sub-parallel with the shear plane. No transformation was observed at any scale as a result of hot-pressing the samples.

Quartz rheology – multidisciplinary analyses of a fossil brittle-ductile shear array in the Southern Alps, New Zealand

Susanne Grigull, Susanne.Grigull@vuw.ac.nz, +64-4-463-5233 ext. 8375

School of Geography, Environment and Earth Sciences, Victoria University of Wellington

Ever wondered how little we know about quartz? – My project aims to improve our understanding of the rheological behaviour of natural quartz under brittle-ductile deformation conditions. Situated in the central Southern Alps, New Zealand, the study area covers a 1-2 km wide shear array, which strikes parallel to the SE-dipping Alpine Fault and lies 5-7 km structurally above it. The dextral-reverse Alpine Fault is the major plate boundary structure on the South Island of New Zealand, absorbing ~60-70% of the oblique convergence between the Pacific and Australian plates in the central Southern Alps. The shear array is interpreted to be part of an exhumed zone of dextral-oblique (up to the NW) backshearing due to a deep embrittlement episode that affected the mid-lower crust of the Pacific Plate during its westward translation (and eastward tilting) onto the Alpine Fault’s crystalline footwall ramp during the past 3-4 Myr (Little 2004). Here closely spaced near-vertical shears brittlely offset biotite-zone, quartzofeldspathic wall rocks (Alpine schist) but ductilely or brittlely-ductilely offset cross-cutting older quartz veins embedded in that schist. Previous work (Wightman 2005) has established well-constrained P-T-t conditions that were operative during the activity of the shears: the embedded quartz veins were deformed to finite shear strains of 5-15 at depths of >20 km, temperatures of 450-500°C and fluctuating fluid pressures of ≥310 MPa. As quartz is unable to deform ductilely at temperatures below 300°C, the timeframe for deforming and exhuming the quartz veins from 21 km depth to the 300°C-isotherm at ~10 km depth under the Southern Alps can be estimated. Assuming an Alpine Fault dip of 45° and a late-Quaternary dip-slip rate on the Alpine Fault  of ~10 mm/yr (Norris & Cooper 2001), the maximum available time is 1.5 Myr requiring minimum strain-rates of 1 x 10^-13 s^-1 and 3 x 10^-13 s^-1 for finite shear strains of 5 and 15 respectively.

By developing numerical computer models and restricting the models to the above mentioned P-T-t conditions, I plan to simulate the observed patterns and deformation conditions affecting these neotectonically sheared quartz veins. I intend to test under what conditions, if any, any existing experimentally derived flow laws for quartz can be applied to these naturally deformed rocks. The geological features that I plan to simulate consist mainly of shape (distribution of curvature) in the sheared veins, their geometrical relationship to the brittle fault tips in the quartzofeldspathic schist, and their finite shear strain as well as scaling relationships between thicknesses of the sheared vein and the ratio of ductile and brittle offset to the zone of ductile shearing.

Here, I summarise the results of previous work and focus on geological field observations and their implications for the understanding of the quartz vein deformation and the propagation behaviour of the shears. I address the following questions: How does the thickness of a quartz vein control its ductility? How does the orientation of a quartz vein with respect to a shear influence its curvature? How do the shears interact with the quartz veins? Besides these questions I will present a list of open questions and a brief overview of future steps in the project.

 

Little, T.A., 2004, Tectonics, v. 23 (TC2013), 24 p.

Norris, R.J. & Cooper, A.F., 2001, Journal of Structural Geology, v. 23, p. 507-520.

Wightman, R.H., 2005, unpublished PhD thesis, Victoria University of Wellington.

The East Derby Fault: Evidence for Late Paleozoic Dextral Motion within the Connecticut Valley Synclinorium

Martha Lynne Growdon* and Bob Wintsch, 1001 East 10^th Street, Bloomington IN, 47405

* mgrowdon@indiana.edu, 812-339-9206

 

Preliminary work shows that the East Derby fault zone (EDFZ) is broader than indicated by Rodgers (1985), and has a protracted late Palezoic history.  This northeast trending fault cuts kyanite grade rocks and juxtaposes the Orange-Milford Belt, on the east, against Connecticut Valley Synclinorium (CVS) rocks, on the west.  The core of the fault zone is defined by greenschist facies phyllites and phyllonites, but our investigations show the zone to be much wider.  In fact, stratigraphic units mapped between Rodgers’ fault zone and the porphyritic Harrison Gneiss are orthoschists derived from, and cutting, this gneiss. Zoned, 1-5cm long, K-feldspars with Carlsbad twinning show development of marginal microcline twinning, significant grain size reduction and local to near complete replacement by myrmekite.  Magmatic biotite recrystallizes into biotite-rich folia separated by quartz and plagioclase domains and ribbons.  Albite twinned plagioclase recrystallizes to finer grained, zoned and untwinned metamorphic plagioclase.  Closer to the mapped EDFZ, K-feldspar grain-size reduction becomes more complete and the developing biotite schistocity is joined by equal proportions of muscovite, forming centimeter long folia; muscovite fish and kinked grains become common.  Finally biotite nearly disappears and is replaced by muscovite with minor chlorite.  These textures show continued grain size reduction and transition from biotite schist to chlorite phyllite.  Thus deformation occurred during waning metamorphic conditions from middle amphibolite to lower greenschist facies.

Regional geochronology (Lanzirotti, 1996; Moecher et. al., 1997) defines the metamorphic cooling history.  Metamorphism peaked in the middle Devonian Acadian Orogeny.  Cooling through the Argon closure temperature of amphibole (~500°C) and muscovite (~350°C) occurred ~ 340 and 300 Ma respectively, showing that the margin of the Harrison Gneiss was being strongly deformed throughout the Carboniferous.  Kinematic indicators in biotite-rich orthoschists and greenschist facies phyllites suggest right lateral motion persisted during most of this retrogression and very possibly into the Permian.  Thus the EDFZ may appreciably translate the Orange-Milford Belt southwestward relative to other CVS rocks to the west.

PALEOCENE-EOCENE FLOW OF PARTIALLY MOLTEN CRUST AND COUPLED EXTENSION IN THE OKANOGAN DOME, WASHINGTON

Seth C. Kruckenberg^a,*,Christian Teyssier^a, Donna Whitney^a, Mark Fanning^b, and W. James Dunlap^b   *kruc0030@umn.edu

^aDepartment of Geology & Geophysics, University of Minnesota, Minneapolis, MN, 55455

^bResearch School of Earth Sciences, Australian National University, Canberra, ACT 0200

 

The Okanogan gneiss dome (southern Omineca Belt, Washington state) is located in the hinterland of the North American Cordillera and is part of a chain of metamorphic core complexes containing gneiss domes exhumed during Eocene extension of thickened crust.  Structural analysis of migmatites, new U-Pb SHRIMP analyses of zircon, monazite, and titanite, and Ar-Ar analyses of biotite from migmatites exposed in the footwall of the Okanogan detachment document the timing and duration of crustal melting and exhumation of the Okanogan dome. 

A unique ~3 km thick structural section is preserved in the footwall of the Okanogan detachment that exposes migmatites at the deepest structural level.  Gneissic foliation in footwall rocks defines two broad subdomes (10-15 km across) elongated NW parallel to lineation and separated by broad synforms.  Three distinct structural zones with similar foliation and lineation orientation are observed beneath the Okanogan detachment.  From bottom to top these domains are a migmatite zone dominated by diatexite, a ~0.5 km thick transition zone, and a ~1.5 km thick detachment mylonite zone.

In the migmatite domain, isoclinal fold axes are oriented dominantly NW, parallel to lineation, and cascade down from the axis of an elongate subdome, emphasizing a component of vertical flow within the migmatite unit.  Structural observations, such as leucosome in inter-boudin regions and melt-filled shear bands, indicate that deformation in the migmatites took place in the presence of melt.  U-Pb SHRIMP analyses of zircon and monazite from successive generations of leucosome support this interpretation and yield Paleocene-Eocene ²⁰⁶Pb/²³⁸U crystallization ages from ca. 51-60 Ma.  One sample of folded granitic leucosome contained titanite with a ²⁰⁶Pb/²³⁸U age of ca. 47 Ma.  Structurally overlying the migmatite domain, over a few hundred meters of structural thickness, rocks recognized as the migmatite unit are deformed in the solid state and show a dramatic change in fold orientation and vergence direction.  Hinges trend N-NE, fold the migmatitic lineation, and verge uniformly to the W-NW.  Open asymmetric folds in the vicinity of the migmatite unit tighten progressively upward into isoclinal folds at the top of the transition zone that grade upward into the ~1.5 km thick mylonite zone.  The nearly continuous section from the migmatite domain to the detachment zone records Ar-Ar biotite ages of ca. 48 Ma with consistent deformation in all units.

These data suggest that rocks of the Okanogan dome underwent partial melting (migmatization) for at least 8 m.y. which was in part coeval with upper crustal extension and ductile flow of the mid-crust. Leucosome crystallization had ceased by ca. 51 Ma followed by rapid cooling of footwall rocks through ~300 ºC by ca. 47 Ma. These data are similar to crystallization ages in migmatites from other domes in the northern Cordillera hinterland, suggesting that crustal anatexis was laterally widespread over much of the mid-crust during Paleocene to Eocene time and nearly coeval with extension and exhumation of orogenic middle crust.

Syn-tectonic garnet microstructures in an upper-greenschist facies high strain zone, central Massachusetts: affects of strain, chemical reaction, and pre-existing chemical/physical heterogeneities

Matthew A. Massey* and David P. Moecher, University of Kentucky

*mamass1@uky.edu, (859) 257-6538

 

Multiple textural generations of Grt from an upper-greenschist facies high strain zone in the Bronson Hill terrane of south-central Massachusetts were examined petrographically and by elemental X-ray mapping on the electron microprobe.  The N-S-striking shear zone dips moderately to steeply to the west, and strong Qtz+Grt+Ms+Chl lineations plunge moderately to the SW; S-C-C’ fabrics, asymmetric folds, and porphyroclasts (delta- and sigma-type) consistently indicate sinistral movement.  Microscopically, the mylonitic foliation consists of layers of fine- to medium-grained Ms+Chl+Qtz+Ilm and layers of fine-grained Grt.  Low-strain domains of randomly-oriented Ms+Chl+Ctd pseudomorph relict St porphyroblasts; locally, the Ms+Chl+Ctd pseudomorph assemblage is aligned parallel to foliation.  Grt is manifest as two distinct end members, although both are often in association: (1) euhedral to subhedral, coarse-grained porphyroclasts ~1 cm in diameter; (2) euhedral to subhedral, fine-grained (~100 µm) Grt layers, which partially define the foliation and lineation.  All type 1 Grt display variably resorbed margins, and sigma-type asymmetric strain shadows. Type 2 Grt can be further classified into: A) fine-grained Grt layers, locally displaying asymmetric folds; B) aggregates of fine-grained Grt up to ~1.5 cm in diameter, often exhibiting delta- and sigma-type asymmetry; C) anhedral coarser-grained Grt most often coincident with low-strain domains surrounding many asymmetric type 2-B aggregate microstructures.  Elemental x-ray mapping reveals multiple zoning with respect to Ca and Mn, and complete homogenization of Mg.  Ca zoning in type 1 Grt microstructures exhibits: (1) euhedral Ca-enriched core, though often patchy; (2) thin, euhedral Ca-poor overgrowth; (3) a thicker Ca-enriched overgrowth with irregular/scalloped outer margin; (4) an irregular/scalloped Ca-depleted rim of variable thickness.  Detailed Ca mapping of patchy cores show highly fractured Ca-enriched cores subsequently healed by more Ca-depleted compositions correlating with zone 2 overgrowth.  Ca zones 1, 2, and 3 coincide with a Mn-enriched core with decreased Mn compositions toward the margin; a Mn-depleted rim is coincident with Ca zone 4.  Individual Grt composing type 2-A and 2-B microstructures display Ca-enriched cores, Ca-depleted rims, and complete homogenization of Mn and Mg.  Type 2-C Grt associated with low-strain regions asymmetric aggregates show concentric zoning consisting of anhedral Ca-depleted cores with few areas of Ca-enrichment, a thin Ca-enriched overgrowth, a thin Ca-depleted rim, and homogenization of Mg and Mn throughout.  Where type 1 Grt is in contact with type 2 microstructures, Mn-enriched cores of show Mn depletion in areas closest to type 2 microstructures; Chl is often in association with type 1-2 interfaces.  Textural and chemical observations suggest that type 1 fractured Grt cores represented physical and chemical heterogeneities that were taken advantage of by syn-tectonic grain-size reduction and/or chemical reaction to produce more stable type 2 Grt, most likely in conjunction with St retrogression.  High-strain coupled with nucleation produced type 2-A and 2-B microstructures; low-strain domains provided areas for continued syn-tectonic Grt growth by coalescence.  The role of physical grain-size reduction and recycling of type 1 Grt vs. chemical reaction and neocrystallization in production of type 2 Grt is the aim of future study including EBSD analysis.

Transpressional deformation and strain partitioning: Examples from the South Desert of Capitol Reef National Park, Utah

Pellowski, Christopher J., Department of Geology and Geological Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701, (605) 394-2461, Christopher.Pellowski@gold.sdsmt.edu

 

The northern boundary of Capitol Reef National Park near Torrey, Utah is located within the Utah transition zone, which exhibits overlapping tectonic styles. Mesoscopic structures in this transition zone record the interplay and overprinting of Sevier, Laramide, and Basin and Range tectonic styles.

 

Jurassic units within the South Desert region of Capitol Reef National Park were examined to determine styles of tectonic deformation.  Using field evidence along with descriptive, kinematic, and dynamic analyses, as well as gravity and magnetic data, it was determined that a principal displacement zone (PDZ) exhibiting right-handed strike-slip occurs in that area.

 

Stereographic analyses of cataclastic and noncataclastic deformation bands and joints in the Entrada Sandstone and the Salt Wash Member of the Morrison Formation, as well as folds in the Carmel Formation, suggest a mechanism involving an unrecognized “forcing structure,” such as a strike-slip basement fault that does not exhibit significant vertical displacement nor surface offset.  This implies that the basement rock has been faulted and possibly affected the structural character of the overlying sedimentary rocks.

 

Orientation of principal stress directions within the South Desert has changed over time from Jurassic to Tertiary.  These changes in principal stress directions between the San Rafael Swell and Miners Mountain were recorded in the deformation bands of the Salt Wash Member of the Morrison Formation, indicating a partitioning of strain along the eastern front of the Sevier fold-and-thrust belt.  The principal stress directions changed from SW-NE during Laramide, to E-W during Sevier, and finally to N-S during Basin-and-Range.

The distribution and relative importance of extensional faults and fractures:

A field study in Southern England

Martin Putz & David Sanderson

Dept. of Earth Science and Engineering, Imperial College London

Contact: martin.putz@imperial.ac.uk  Tel: +44 (0)207 5941359

Extension in the upper crust is largely accommodated by slip and opening on fractures, producing faults, joints and veins. This brittle deformation occurs over a wide range of scales from grain-scale processes up to major fault zones with kilometre displacements.

It is commonly assumed that populations of fractures follow fractal scaling relationships. Innumerable studies dealing with scaling laws support this assumption, but are mostly based on data covering only about two orders of magnitude of displacement, or combine data from different sources (different study areas, tectonic settings, lithologies, strains, etc).

In this study we take a different approach and directly measure extension over >6 orders of magnitude instead of predicting size-distributions from restricted data-sets. From these measurements we address a number of important questions regarding the deformation:

1.   What is the relative importance of faults and veins in accommodating the total strain in a region?

2.   What are the spatial relationships between localized and distributed deformation?

3.   Is strain accommodated in a scale invariant or scale dependent manner across the structures in an extensional region?

As a suitable study area we chose a locality in SW England providing long continuous cliff sections and a high resolution seismic data set. The well defined stratigraphy allows accurate determination of fault-displacements in the field and high quality outcrops can be sampled with a resolution of down to 0.1mm displacements.

We find that fractures in the region are responsible for producing an extension of about 7% which is accommodated by both slip on faults and opening of extensional fractures to produce veins. Power-law scaling of frequency of both veins and faults is found, but these do not share a single power-law relationship, showing a major change in scaling at the transition between extensional (Mode I) and shear (Mode II/III) deformation.

Analysis of cumulative heave (aperture) data by applying a simple non-parametric method (Kuiper’s Test) shows that extension is not homogenously distributed and that its heterogeneity is scale dependent. The low end of the scale range (0.1mm to 10mm aperture) appears to accommodate a low but continuous extension (»0.1%) whilst thick veins and pull-apart structures occur mainly in deformation zones around faults and show the highest heterogeneity (»0.5% extension). A similar trend can be observed for faults where the smaller faults (1m to 30m) accommodate a continuously distributed extension (»1%) whilst the heterogeneity increases with structure size as most of the extension is localized on the largest faults (»5% extension).

We conclude that for our study area and probably for most regions with similar layered rocks (interbedded mudstones and carbonates with bed thicknesses between 0.1m and 2m) it is not possible to extrapolate extensions and fracture frequency from one scale to the other, without also considering their spatial heterogeneity.

An experimental cell for high-T “wet” deformation of volcanic materials

 

 

G. Robert¹, J.K. Russell¹, and D. Giordano²

 

 

¹Volcanology & Petrology Laboratory, University of British Columbia, Vancouver, Canada (grobert@eos.ubc.ca)

²Dipartimiento di Scienze Geologiche, Università di Roma Tre, Roma, Italy

 

The Volcanology-Deformation-Rig (VDR) was developed for exploring the high-T rheological properties of volcanic materials [1]. The VDR is designed to perform high-T, low-load (< 1136 kg) deformation experiments at constant load, or displacement rate, or at controlled load rates. The rig is ideal for determining the rheological response of volcanic products within a wide range of natural conditions: T up to 1000°C, applied stresses up to 150 MPa, and strain rates between 10^-6 and 10^-2 s^-1. The resulting data provide a powerful means of developing constitutive equations governing the multiphase (liquids ± vesicles ± solids) rheology of volcanic material during flow and deformation [2].

However, many seminal issues in volcanology involve the behaviour of the volatile phase during flow and deformation and its effect on magma rheology and volcanic behaviour. We have, therefore, designed and built a high-T resistant, sealed fluid pressure cell. The cell gives us the capacity to run high-T deformation experiments at controlled H₂O pressures that simulate nature (0-150 MPa). The apparatus has been calibrated against the viscosity of NIST SRM 717a standard glass at different temperatures.

Fluid pressure in the cell is either allowed to vary throughout the experiment, or fixed for the duration of the experiment. We plan to use the hydrothermal cell for high-T experiments on natural volcanic materials (e.g., cores of obsidian, pumice, or sintered ash) to elucidate the rheology of multiphase volcanic products and to study feedback mechanisms between porosity and permeability evolution.

[1] Quane S. et al., 2004. American Mineralogist (Letters) v. 89, p. 873-877. 

[2] Quane S. and Russell J.K., 2005. Journal of Volcanology & Geothermal Research v. 142, p. 67-87.

Effect of Plate Geometry and Buoyancy of Weak Lithosphere

on Strain Evolution in Analogue Vice Models

Kerstin Schemmann^1,*, David Boutelier², Alexander R. Cruden², Onno Oncken¹

 

¹ GeoForschungsZentrum Potsdam, Germany

² Department of Geology, University of Toronto, Canada

 

^*Corresponding author: kschem@gfz-potsdam.de, phone +49 331 288 1370 (K. Schemmann)

 

Hot and therefore weak lithosphere is characteristic of tectonic settings where thickened or juvenile crust is associated with high surface heat flow. Such large, hot orogens are present in the geological record since the Archean and are presently associated with high orogenic plateaux (Tibet, Altiplano). Previous “vice models” examined the effect of orogen-parallel flow and the rheological properties of weak lithosphere as is compressed between two stronger blocks (Ellis et al. 1998; Cruden et al. 2006). We build on these studies by analyzing the effect of vice geometry in combination with different degrees of buoyancy of the weak lower crust in scaled analogue models.

Both the weak orogen and the strong vices consist of an upper crust of sand and ductile lower crust and mantle lithosphere layers composed of mixtures of silicone polymers and plasticene. The model lithosphere floats isostatically on an asthenosphere of water. Rheological properties are kept similar in every experiment; only the density of the weak lower crust is changed (buoyant, non-buoyant, non-buoyant with an additional low-viscosity layer). Both straight and curved vice geometries are compared.

A digital stereoscopic camera system is used for particle image velocimetry (PIV), from which the complete particle displacement field can be calculated for each deformation increment in the experiments. This provides high spatial and temporal resolution of the strain evolution in the upper crust, as well as the development of topographic relief. Structures and characteristics of the lower crust are examined after the experimental run when the upper crust is removed. Deformation of the lower crust and mantle lithosphere can also be studied in cross sections that are cut perpendicular to the orogen.

Resulting structures are comparable in all model runs, including thrust faults of variable vergence cropping out at the surface, or conjugate systems of reverse faults bounding pop-ups, and wide synclines surrounded on both sides by steep anticlines within the lower crust (similar to pop-down structures described by Cagnard et al. 2006). Strain localization along such structures is more pronounced in experiments with buoyant crust, in which additional thickening of the mantle lithosphere occurs. In experiments with non-buoyant ductile crust, strain is primarily accumulated by homogenous thickening of the lower crust and upper crustal features develop only after half of the bulk shortening has occurred.

The geometry of the vices is expressed as a recurrent pattern in the initial evolution of topography, but not in the overall fault pattern. However, the general structural outcome and nature of strain localization is clearly different when comparing straight vs. curved vice shapes in experiments with similar buoyancy.

 

Cagnard, F. et al., 2006, Tectonophysics, v. 421, p. 145-160.

Cruden, A.R. et al., 2006, Geological Society of London Special Publication, v. 253, p. 79-104.

Ellis, S. et al., 1998, Canadian Journal of Earth Sciences, v. 35, p. 1323-1346.

CONTINENTAL BASIN DEVELOPMENT IN A SUBDUCTION SETTING, NE SOUTH ISLAND, NEW ZEALAND

*Pragnyadipta Sen1 (psen2@uiuc.edu), Mary S. Hubbard1, Jarg Pettinga2, and Jack Oviatt1

1Kansas State University, Dept. of Geology, Manhattan, KS 66506,

2Univ. of Canterbury, Dept. of Geological Sciences Christchurch, New Zealand

 

                                                                        Abstract

 

The northeastern South Island of New Zealand, inland from the terminating Hikurangi trench, consists of high mountains to the north and actively forming medium-small scale basins to the south. Deformation in the high mountains corresponds with the northern end of the transpressional Alpine fault system. Basin development to the south is likely the result of interplay between this continental fault system and the terminating subduction offshore. To better understand the relationship between basin formation and this transition zone setting, we mapped the structures of the eastern Blythe River basin. On the eastern margin of the basin the SE-dipping John Brown Fault has a reverse sense and an overturned anticline in the hanging wall. To the west, within the basin, Quaternary deposits are disrupted by southeasterly dipping reverse faults. The west flank of the basin is defined by the back limb of a thrust-propagated asymmetric anticlinal fold. The south margin of the basin is defined by the Stonyhurst fault zone, which includes a series of right stepping en-echelon fault scarps in Quaternary deposits and a fault within the Tertiary deposits. The northern boundary of the basin is marked by an inferred fault, the Blythe River fault with Lower Tertiary units to the south and Quaternary units to the north. The arrangement of basin bounding structures is consistent with several interpretations. One possibility is that the northern and southern bounding faults represent a left step-over of a dextral system. Another possibility is that the region is the product of NW-directed upper crustal contraction forming the east and west basin margins. The northern and southern basin bounding structures would be strike-slip transfer faults and could represent reactivated inherited basement faults such as those prevalent east of the plate boundary deformation zone, across the north Chatham Rise.

 

 

 

* Present address: University of Illinois at Urbana-Champaign. Email: psen2@uiuc.edu

   Telephone Number: 1-785-317-0776.

 

Structural significance of L tectonites in the eastern-central Laramie Mountains, Wyoming

Walter A. Sullivan

email: wasulliv@uwyo.edu

 

The formation of L tectonites is little understood and scarcely studied, however, it is likely an important part of plastic deformation in the crust. To improve our understanding of this strain phenomenon, I present a detailed case study of a km-wide domain of L tectonites developed in and around the ~2.05 Ga Boy Scout Camp Granodiorite (BSCG) in the Laramie Mountains, Wyoming. Detailed mapping and structural analyses allow for the reconstruction of the structural setting of the domain of apparent constrictional strain. Elongation lineations in and around the BSCG, including the L tectonites, are south-southwest-trending and moderately plunging. In compositionally heterogeneous rocks (Archean banded gneiss and gneissic granite), hinge lines of minor folds are subparallel with the elongation lineation. The regional fold axes defined by poles to compositional banding and foliation measured from these rocks lies in the center of the lineation measurements from all the rock types in the area. Poles to foliation in the compositionally homogeneous BSCG and metamorphosed diabase dikes cluster in the northwest quadrant and define the axial surface of the regional folds.

These data show that the pervasive elongation lineations in and around the BSCG developed parallel with the local fold hinge lines and regional axes of folds with axial surfaces that strike-east-northeast and dip moderately to the southeast. Map-scale folds in this area verge towards the northwest. Incorporation of 1) the constraints imposed by the shape fabric orientation data, 2) the constraints imposed by the orientation of the local and regional fold axes and 3) the constraints developed from map patterns and observations such as the geometry and vergence of the map-scale folds shows that the domain of L tectonites in and around the BSCG developed in the hinge zone of a large northwest-vergent synform during bulk constrictional deformation. Regional Paleoproterozoic strain within this area is quite heterogeneous (Allard, 2003; Resor and Snoke, 2005) and strain dies out to the east and west of the BSCG. Also, constraints from metamorphic petrology (Patel et al., 1999) show that the moderate plunge of lineations in this area is not a result of rotation of fabrics after deformation. The pervasive linear fabric in and around the BSCG must then have developed in response to oblique extrusion of material parallel with the axis of folding between two relatively rigid crustal blocks. Correlation with other deformation fabrics in the central Laramie Mountains indicates that this structure is probably coeval with northwest-directed contractional deformation during the 1.78-1.74 Ga Medicine Bow orogeny. In conclusion, the domain of L tectonites in and around the BSCG appears to represent folding coupled with oblique extrusion of material from the deforming zone between two relatively rigid blocks during regional contractional deformation.

 

Allard, S.T., 2003, Ph.D. thesis, University of Wyoming, Laramie.

Resor, P.G., and Snoke, A.W., 2005, in Bruhn, D., and Burlini, L. eds., Geological. Society of London Special. Publication 245, p. 81–107.

Patel, S.C. et al., 1999, Journal of Metamorphic Geology, v. 17, p. 243–258.

Strength of SAFOD fault gouge under hydrothermal conditions: Experimental constraints and stress modeling of the San Andreas fault

Sheryl Tembe¹, Teng-fong Wong¹ and David Lockner²

¹Department of Geosciences, SUNY Stony Brook, (stembe@ic.sunysb.edu)

²US Geological Survey, Menlo Park, CA

 

The San Andreas Fault Observatory at Depth (SAFOD) scientific borehole is designed to directly monitor an active fault zone at seismogenic depth in order to answer the many fundamental questions on the physical and chemical processes operative within the San Andreas and other major plate-bounding faults. One of the major goals of the project is the determination of the insitu state of stress in and adjacent to the fault. The results are critical to resolving the San Andreas stress-heat flow paradox, which implies a coseismic frictional strength of 0.1-0.2 for the upper 15 km of the crust, well below the friction coefficient observed for most crustal rocks.

We selected the weakest SAFOD material collected thus far for further investigation. Frictional sliding experiments were conducted on clay rich (up to 60% illite) SAFOD fault gouge from the southwest strand of the SAF (from 3067 m measured depth) at elevated temperatures and pressures comparable to those under seismogenic conditions. In most experiments steady sliding behavior was observed and an overall trend for the frictional strength to increase considerably with temperature, from 0.4 at 100°C up to 0.75 at 440°C. At a given temperature the friction coefficient fluctuated slightly with effective normal stress in a somewhat unsystematic manner.

If the friction coefficients represent lower bounds for clay-rich SAF gouge materials, then the resolved shear stress cannot be as low as the values of ~0.1-0.2 implied by the absence of heat flow anomaly and fault normal compression, unless the fault zone is under an anomalously high pore pressure. Lower bounds on the pore pressure excess inside the fault gouge zone that needs to be invoked to satisfy all these constraints can be estimated on the basis of our hydrothermal data. Our analysis for a transitional stress regime is modified from a conceptual model introduced by Rice (1992) to allow for a range of friction coefficients for the country rock and fault zone. The country rock surrounding the fault zone is assumed to be under hydrostatic pore pressure. Since the frictional strength of the SAFOD core at 3067m MD is appreciably lower than what is predicted from Byerlee’s law (0.6 or 0.85), the pore pressure excess required for fault normal compression is predicted to be considerably less than that predicted using the original Rice’s (1992) model assuming Byerlee friction but are still comparable to the vertical overburden stress. Our preliminary data in conjunction with the analysis provide useful constraints on the stresses and pore pressure associated with some of these processes and underscore the necessity to pursue comprehensive friction measurements under hydrothermal conditions, as well as permeability and poromechanical properties of SAFOD cores.

 

Rice, J.R., (1992), Fault stress states, pore pressure distributions, and the weakness of the San Andreas fault, in Fault mechanics and Transport Properties of Rocks, ed. B. Evans, and T.-f. Wong, 475-504, Academic Press, San Diego.

Proposed In situ ⁴⁰Ar/³⁹Ar Analyses of Mylonites from the Catalina Metamorphic Core Complex, AZ

Terrien, J.J., and Baldwin, S.L., Syracuse University, 204 Heroy Geology Laboratory, Syracuse NY, 13244, jjterrie@syr.edu

Metamorphic core complexes are common features of extended terranes such as the Basin and Range province. The Catalina metamorphic core complex (MCC) located in southeastern Arizona is considered the type example of MCCs.  The footwall of the Catalina MCC is intruded by three magmatic suites: the Leatherwood, Wilderness and Catalina suites, which are thought to have been emplaced from ~70-27 Ma (Keith et al., 1980).  The Wilderness suite consists of seven peraluminous granitic sills that have inflated the crust by ~4 km and are thus assumed to have played a large role in the thermal history of the Catalina MCC (Force, 1997).  The Wilderness suite ranges from mylonitic to undeformed.  Constraining the timing of deformational events in the Catalina MCC is critical to assessing the timing of MCC formation and evolution.

Previous U-Pb studies of zircon from the Wilderness suite indicated a zircon crystallization age of ~50 Ma (Keith et al., 1980).  Cathodoluminescent imagining reveals complex zoning of the zircons. U-Pb Laser Ablation ICPMS analyses of the complexly zoned Wilderness suite zircons indicate an outer rim with a U-Pb age of ~38 Ma, an inner rim with a U-Pb age of ~55 Ma and a Precambrian core.  ⁴⁰Ar/³⁹Ar step heat experiments on potassium feldspars from the deformed and undeformed Wilderness suite sills yield age gradients ranging from ~30-25 Ma.  These data can be interpreted and modeled using Lovera’s multi-diffusion domain models to represent either slow cooling or a reheating event.  Apatite and zircon fission track data indicate that the Catalina MCC experienced a period of rapid cooling from 30 to 25 Ma that was followed by a period of slow cooling (Fayon et al., 2001).  Unpublished U-Th-He apatite thermochronology indicates ages ranging from ~28-14 Ma.  (Carter and Gleadow, personal communication).  Mylonitiziation is predicted to be older than ~14 Ma the youngest U-Th-He age in apatite; U-Th-He in apatite has a closure temperature of ~70^oC. 

Microstructural observations from the mylonitic Wilderness suite sills show both brittle and ductile deformation structures and evidence of recrystallization.  Recrystallized minerals include muscovite that occurs as elongate grains or ribbons wrapping around porphyroclasts and recrystallized potassium feldspar riming porphyroclasts.  The Wilderness suite sills also contain undeformed biotite, which occurs as inclusions in potassium feldspar and is interpreted to be magmatic in origin.  In situ ⁴⁰Ar/³⁹Ar analyses are required to distinguish multiple generations of a mineral within a sample.  In situ analyses on synkinematic mica and recrystallized rims on potassium feldspar porphyroclasts from an oriented thick section will allow direct dating of deformation to decipher the complex history that is unobtainable using conventional ⁴⁰Ar/³⁹Ar analysis.

In situ ⁴⁰Ar/³⁹Ar analyses of both the magmatic biotite and the recrystallized biotite and muscovite will potentially allow for a comparison between the crystallization/cooling of magmatic biotite and the recrystallized micas which reflect the timing of ductile deformation.  A potential problem is the inability to resolve the subtle differences in age between the magmatic and recrystallized micas within the Wilderness suite sills.  Therefore, recrystallized micas and potassium feldspar from the Precambrian host rock will also be analyzed; mid-Tertiary ages are likely related to MCC development rather than Precambrian magmatic cooling.   

References:

Fayon, A.K., Peacock, S. M., Stump, E., and Reynolds, S.J., 2000, Fission track analysis of the footwall of the Catalina detachment fault, Arizona: Tectonic denudation, magmatism, and erosion, Journal of geophysical Research, v. 105, no. B5, p.11047-11062.

 

Force, E.R., 1997, Geology and Mineral Resources of the Santa Catalina Mountains, Southeastern Arizona, A Cross-Sectional Approach: University of Arizona Press. 135p.

 

Keith, S.B., Reynolds, S.J., Damon, P.E., Shafiqullah, M., Livingston, D.E., Pushkar, P.D., 1980, Evidence for multiple intrusion and deformation within the Santa Catalin-Rincon-Tortolita crystalline complex, southeastern Arizona. In Crittenden, M.D., Coney, P.J., Davis, G.H., ed., Cordilleran Metamorphic Core Complexes: The Geological Society of America, Inc. Memoir 153, p.217-283.

MECHANICAL EVOLUTION OF THE ALPINE FAULT ZONE

 

V.G. Toy¹, R.J. Norris¹, R.H. Sibson¹ and D.J. Prior²

¹Geology Department, University of Otago, PO Box 56, Dunedin 9054, New Zealand. 

Ph: +64 3 4799088; virginia@geology.co.nz

²School of Earth and Ocean Sciences, University of Liverpool, U.K.

 

The principal component of the Pacific-Australian plate boundary through the South Island of New Zealand is the 480 km long dextral reverse Alpine fault.  Plate motions have been stable for the last 5 Myr, meaning the boundary conditions of deformation are well known.  The fault has behaved seismically in the past, generating a ~M8 earthquake approximately every 300 years.  We aim to provide insight into the plastic behaviour of the fault zone below seismogenic depths and the relationship of the strain accumulated there to the initiation of seismic failures at, and above, the brittle-plastic transition.

Mylonites exposed in the fault hanging wall are generally S>L tectonites.  Where lineations are present they have orientations that are approximately 35° oblique to the plate motion vector.  These orientations are in part controlled by lithological heterogeneity and incomplete removal of a remnant fabric from the parent Alpine Schist; however, some do form in the oblique orientation.  It is possible to model formation of lineations in these orientations using ratios of flattening to simple shear similar to those indicated by deformed markers (Norris & Cooper, 2003, J. Struct. Geol. 25; 2141-2157).  Unfortunately, it is difficult to reconcile such simple models with field observations and the boundary conditions of the fault zone, indicating more complicated models are needed.

The aseismic behaviour of the fault zone is dominated by a quartz rheology.  Crystallographic preferred orientation studies were carried out on quartz-rich samples from two transects across the mylonite zone.  There is a transition in c-axis patterns from crossed to single girdles or Y-axis maxima as the fault is approached from the SE, corresponding to an increase in the degree of dynamic recrystallisation and removal of pre-existing fabrics. The form and asymmetry of the CPO fabrics suggest that deformation is dominantly simple shear.  We propose that the transition in fabric type occurs because deformation at amphibolite facies conditions, in the mylonites and ultramylonites only, is intense enough to form a highly oriented fabric which cannot be reactivated by normal, low temperature basal<a> slip at shallower depths.  By implication, strain is intensely localised in a narrow zone at all crustal levels, including in the amphibolite facies.

Pseudotachylytes provide unequivocal evidence of seismic failure within the fault zone but their total thickness is too small for frictional melting to have occurred during all seismic failures.  In the hanging-wall mylonites they are localised along the foliation, and commonly associated with lithological heterogeneity, either due to primary strength contrasts such as between quartzofeldspathic and metabasic layers, or associated with retrogression only in certain layers which also have a smaller recrystallised grain size.  We suggest these represent small failures that occurred to relieve stress heterogeneities in the inhomogeneous hanging wall.  Pseudotachylytes within the fault core/gouge zone and footwall granitoids were formed at shallower crustal levels in hydrated assemblages that were drained on seismic timescales due to the high permeability of the fault gouge. 

In all outcrops, veins precipitated from fluids of varying compositions alternate with friction melt and further ductile deformation.  We are currently carrying out fluid inclusion work to elucidate the fluid conditions during formation of all these structures.

We studied the influence of solute impurity (Mg) on grain-growth and the high-temperature plasticity of calcite aggregates using the Paterson gas-medium apparatus. Synthetic marbles were produced by hot isostatic pressing (HIPing) mixtures of calcite and dolomite powders for different intervals (2 to 30 hrs) at 850ºC and 300 MPa confining pressure. The HIP treatment resulted in homogeneous calcite aggregates with Mg content ranging from 0.5 to 17 mol%. The grains size varied from 5 to 27 μm. We performed creep tests on samples after HIP at differential stresses from ~ 20 to 170 MPa using constant strain rate and stress-stepping methods. Both back-scattered electron images and chemical analysis suggested that the dolomite phase was completely dissolved, and that the Mg distribution is homogeneous throughout the samples. A bimodal distribution of large protoblasts and small recrystallized neoblasts coexisted in some deformed samples (ε ~ 0.25).

Grain sizes increase with HIP time and decrease with increasing Mg content (> 3.0 mol%). The growth exponent varies from 2.7 to 3.5 for samples containing 0.3 to 17.0 mol% Mg. The deformation data showed three different regions: 1. at stresses below 40 MPa, the material deforms by diffusion creep, and the strength decreases with increasing Mg content due to the difference in grain size. 2. At about 40 MPa, the deformation mechanism changes from diffusion to dislocation creep; this transition stress is larger for samples with higher Mg content and smaller grain size. 3. At stress above 80 MPa, the stress exponent n is greater than 3, indicating an increasing contribution of dislocation creep. The strength of calcite increases with the Mg content in the dislocation creep regime.