ON THE EVOLUTION OF THE WESTERN ALPS: PRESSURE CYCLES AND DEFORMATION MODE SWITCHES

 

Marco Beltrando (email: marco.beltrando@anu.edu.au)

Jörg Hermann

Gordon Lister

Roberto Compagnoni

 

Evidence of two burial-exhumation cycles that took place during a single orogeny has been found in rocks belonging to the Piemonte unit of the Western Alps. An early high pressure event, which resulted from tectonic burial down to pressures of 1.5 GPa, is indicated by the presence of relict rutile, garnet and omphacite. This episode was followed by retrogression to greenschist facies conditions at P=0.20-0.35 GPa and T=390-420°C, as indicated by the presence of amphibole of actinolitic/hornblende compositions. Exhumation was accomplished as a result of extensional deformation. Renewed shortening resulted in upright folding of the extensional structures and culminated in the crystallization of barroisitic amphibole, for which conditions of P=0.65-0.80 GPa have been estimated. Renewed generalized extension led to the formation of large-scale recumbent folds and extensional shear planes and resulted in the final exhumation of the study area to upper crustal levels. Existing geochronological data allow only ca. 13-19 Ma for the completion of both burial-exhumation cycles. Therefore, we suggest that the evolution of orogens is characterized by multiple short-lived burial-exhumation cycles related to orogen-scale alternance between shortening and extensional deformation.

Southward extent of the western Idaho shear zone, Owyhee Mountains, Idaho: Constraining along-strike variations in transpressional shear zones

Benford*, Bryn and Tikoff, Basil

University of Wisconsin - Madison

* bryn@geology.wisc.edu, (608) 262-8960

 

            The Late Cretaceous western Idaho shear zone (WISZ) continues south of the western Snake River Plain into the northern Owyhee Mountains.  The shear zone mainly occurs in two blocks of granitic basement rocks that are surrounded by Miocene-aged basalts.  A solid-state fabric exists in the western part of the field area, with a consistent strike of 020, a steep eastern dip, and a down-dip mineral lineation.  Shape preferred orientation (potassium feldspar, quartz, plagioclase, and mafic minerals) and lattice preferred orientation (quartz) analyses show weak fabric.  Anisotropy of magnetic susceptibility analyses indicate weakly developed flattening fabrics with no clear spatial variation.  New strontium analyses, performed at the Boise State University Isotope Geology Laboratory, indicate initial ⁸⁷Sr/⁸⁶Sr ratios exhibit a sharp west-to-east transition from 0.704595 to 0.707899, over a distance of ~30 kilometers. Samples for the strontium analyses are from within the field area and elsewhere in the Owyhee Mountains.

The southern continuation (“Owyhee segment”) has three major distinctions from the main segment of the WISZ: 1) significantly less-developed solid-state fabrics, 2) a trend of 020 rather than north-south, and 3) a ~30-km transition in initial strontium ratios from 0.704 to 0.708 compared to a ~6-km transition near McCall, ID.  A tectonic model explains these differences, assuming a rigid-body collision, transpressional kinematics, and an along-strike change in trend of the shear zone.  A lower finite strain magnitude, a different strain path (larger simple shear and smaller pure shear components), and an increased width for the shear zone in the northern Owyhee Mountains is predicted, relative to the northern segment of the WISZ.  More specifically, for the Owyhee Mountains, the model predicts a local oblique convergence angle between 24º and 39º, 40-69 km of convergent movement, 85-99 km of transcurrent movement, and overall lower finite strain.  For the main segment of the shear zone, the model predicts a local oblique convergence angle of 44º-59º, 65-94 km of convergent movement, and 57-67 km of transcurrent movement.  The differences predicted by the model for the two segments explain the less steep strontium gradient and the weaker fabric documented in the northern Owyhee Mountains.

Constraining the timing and magnitude of extension along the southern section of the Okanagan Valley Fault, southern B.C.

 

Sarah R. Brown, H. Daniel Gibson, & Derek Thorkelson

Department of Earth Sciences,

Simon Fraser University,

8888 University Drive,

Burnaby, BC, V5A 1S6

srbrown@sfu.ca

 

Within the southern Okanagan Valley, medium- to high-grade metamorphic rocks are juxtaposed against nearly pristine volcanic and sedimentary rocks. Previous work has demonstrated that the nature of the boundary between these two disparate packages of rock is predominantly a <1 km-thick ductile shear zone, termed the Okanagan Valley fault (OVF). The shear zone grades abruptly from cataclasite to mylonite in amphibolite-grade gneiss, which, along with younger granitic intrusions, has undergone polyphase deformation and shows evidence of significant flattening. Linear fabric elements and kinematic criteria (e.g., rotated porphyroclasts, boudinaged pegmatite dykes, sheath folds, C/S fabrics) strongly indicate extensional motion through which the hanging-wall has moved to the west.

            Farther to the north, within the central portion of the OVF, recent work has attributed minimal displacement to the OVF and brought into question previous estimates for the magnitude of extensional displacement as determined to the south (~45-90 km) and north (~32 km). The apparent disparity in the estimated magnitude of extension along the trace of the OVF may provide insight into the along-strike variation in displacement along a fault such this, and/or require a revised interpretation of the tectonic nature of this shear zone. To help address this problem, it is imperative to reexamine the southern portion of the OVF as little work has been done in the last 20 years. In order to better understand the nature of the OVF and constrain motion along the southern portion of the shear zone, we intend to conduct a comprehensive thermo-structural analysis. This work will comprise 1:50 000 scale mapping and strain analysis to constrain the dimensions and geometry of the shear zone. This study will be complemented by petrography, geothermobarometry, U-Pb geochronology (e.g., in situ dating of metamorphic and fabric-forming minerals) and thermochronology to constrain the P-T-t path and rate of exhumation related to extension. Lastly, geochemistry of the gneissic rocks within the footwall will be used in an attempt to identify their protolith.

Two-phase shear within a core complex mylonite zone:

The Northern Snake Range Décollement, Nevada

 

Frances J. Cooper*, John P. Platt, Ellen S. Platzman

* fcooper@usc.edu, (213) 821-1094

Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089

 

The relationship between low-angle detachment faults and the mylonitic rocks in their footwalls is unclear. At present two hypotheses exist: (1) Mylonite zones reflect deformation directly related to exhumation along a single detachment fault; or (2) They are a more general feature of the extending middle crust that is “captured” and exhumed by a detachment. Each hypothesis has distinct implications for expected kinematic indicators: (1) A dominant shear sense coupled with slip on a detachment fault; or (2) Varying directions and senses of displacement through time and space.

The Northern Snake Range Décollement (NSRD) in eastern Nevada is a classic Basin and Range low-angle domiform detachment with a well-documented top-east sense of displacement. However, structural observations suggest that the 100 m-thick mylonite zone has experienced both top-east and top-west phases of shear.

Our study focuses on an exposure of the mylonite zone on the eastern side of the Northern Snake Range where rheological contrasts between mylonitized calcareous units and deformed mafic dikes provide good strain markers and kinematic indicators. East-directed shear is indicated by an east-dipping mylonitic fabric, east-vergent folds and east-asymmetric porphyroclasts. Paleomagnetic analyses indicate that the mafic dikes have been rotated top-east, while in high-strain zones they form east-asymmetric boudins. These structures are cut and overprinted by meter-scale west-dipping shear-bands, which in places coincide with east- directed shear zones, producing complicated structures including dikes that have been boudinaged and then folded.

Our observations suggest that the mylonite zone might not be directly related to NSRD slip, but could instead represent a captured feature of the extending middle crust that has accumulated displacement from faults with varying senses of displacement over time.

Numerical modeling of fluvial processes and calibration of bedrock erodabilities in Andean basins.

Víctor H. García and Ernesto O. Cristallini

Laboratorio de Modelado Geológico (LaMoGe), Dept. de Ciencias Geológicas, FCEyN, UBA. Pabellón 2, Ciudad Universitaria, C1428EHA, Ciudad Autónoma de Buenos Aires, Argentina.

Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).

Email: victorg@gl.fcen.uba.ar. Phone number: +5411-4576-3300 ext. 287.

 

A forward numerical model is presented to estimate denudation rates in mountain ranges. The model integrates bedrock erodability, runoff rate, drainage area and local terrain slope to obtain values of fluvial erosion/sedimentation for each point. The algorithm applied is the stream power-law investigated previously by many authors (e.g. Tucker and Singerland, 1996, Whipple and Tucker, 1999). The model is useful for large spatial and long geological time-scales simulations rather than to describe transport processes at small scale. The computational mesh used in the model is a 90-m SRTM DEM of the eastern flank of the Bolivian Central Andes.

For each cell the model calculates the local terrain slope (S), the steepest descent routing, and the water discharge (Q) by multiplying the contributing drainage area with effective runoff rate (R^P). The exponent p used is 0.65 and reflects how diminish the influence of the precipitation in water discharge with its increment. This value was determined using empirical data from Andean denudation rates (Aalto et al., 2006).

The dominant fluvial transport behavior in high mountain landscapes is the detachment-limited bedrock channels (Howard, 1994). In this situation the volume of sediment supplied from upstream cells is small or the thickness of the movable sediment cover on the channel is negligible. The rate of channel incision by bedrock erosion is then determined.

dh = (S^mQⁿ – T_h – Q_s) K_e

dt

where m and n have values of 2/3 and 1/3 respectively. Those values were obtained from previous studies (Howard, 1994). T_h is a critical threshold stress to initiate motion and takes values of 1 m yr^-1 (Tucker and Singerland, 1997). The variable Q_s is the net sediment charge that the stream is carrying from the watershed and K_e (yr^-1) is the bedrock erodability.

Published data of denudation rates measured in Andean mountain basins (Aalto et al., 2006) were used to calibrate bedrock erodabilities by inversion of the stream power-law equation. Denudation rates for other drainage basins can be obtained once all erodabilities are calibrated.

References

Aalto, R. et al., 2006, Journal of Geology, v. 114, p. 85-99.

Howard, A.D., 1994, Water Resources Research, v. 30, p. 739-752.

Tucker, G.E. and Singerland, R., 1996, Basin Research, v. 8, p. 329-350.

Tucker, G.E. and Singerland, R., 1997, Water Resources Research, v. 33, p. 2031-2047.

Whipple, K.X. and Tucker, G.E., 1999, Journal of Geophysical Research, v. 104, p. 17661-17674.

COMMENCEMENT AGE OF NEOTECTONIC REGIME AND SEISMICITY IN SOUTHWEST TURKEY: Akşehir-Simav Fault System

 

Şule Gürboğa (Deveci)*

Ali Koçyiğit

 

* Department of Geological Engineering, Middle East Technical University (sdeveci@metu.edu.tr)

Department of Geological Engineering, Middle East Technical University (akoc@metu.edu.tr)

 

 

                In the area between Karaman in the SE and Sındırgı in the NW, a 10-30 km wide, 500 km long and NW-SE-trending discontinuous oblique-slip normal fault system is exposed. This mega-seismogenic belt is here termed to be the Akşehir-Simav fault system (ASFS). The ASFS is characterized by a series of grabens to horsts and their margin-bounding oblique-slip normal faults to fault zones. Grabens are two major groups: (1) first-order (major) grabens, and (2) second-order (secondary) grabens. Both the major and secondary grabens comprising the ASFS have two graben fills: (a) ancient graben fill deformed (folded and reverse to strike-slip faulted), and (b) modern graben fill overlying on ancient graben fill with an angular unconformity.

In the frame of episodic evolutionary history of the grabens in southwestern Turkey that also contains teh ASFS and related structures three phases succeed each others: (1) phase-I extension in early-middle Miocene, (2) internening short-term contractional phase (last paleotectonic period), and (3) phase-II extension (neotectonic period in Plio-Quaternary period). Therefore, the range of last paleotectonic period is early Miocene - late Pliocene.

The ASFS is a regional seismogenic structure. This is indicated by a series of historical and ground ruptures-forming recent earthquakes sourced from different fault segments comprising the Akşehir and Simav fault zones included in the ASFS. The 94, 1766, 1873, 1876 and 1896 are historical, the 1921, 1944, 1946, 1969, 1970, 2000 and the 2002 are recent earthquakes which caused heavy damage to structures and loss of life. Epicenters of both historical and recent destructive earthquakes are mostly located at or near the intersections between major and secondary graben margin-bounding faults. This situation implies to the interlocking of motion and accumulation of elastic strain energy at these places, i.e. along the normal fault complexities.

            Consequently, the ASFS is an oblique-slip normal fault system with a high seismicity as indicated by both historical and recent devastative earthquakes, and 129 years long seismic gap. Ages and deformation patterns of graben fills, kinematic analyses of slip-plane data of fault arrays strongly indicate that an episodic evolutionary history for grabens, and Plio-Quaternary commencement age of current extensional tectonic regime in the ASFS. This system also contributes to the current NNE-directed crustal extension as much as the E-W- and NE-SW-trending graben-horst systems in Southwest Turkey. This is indicated by the stereographic plots of slip plane data, the fissure-ridge travertines and focal mechanism solutions of recent earthquakes.

Oblique detachment tectonics and melt migration in the Fosdick Mountains, West Antarctica

R. McFadden^1*, C. S. Siddoway², C. Teyssier³, and C. M. Fanning⁴

¹Department of Geology & Geophysics, University of Minnesota-Twin Cities, Minneapolis, MN 55455

²Department of Geology, Colorado College, Colorado Springs, CO 80903, USA

³Institut de Geologie et de Paleontologie, Anthropole, Universite de Lausanne, CH-1015, Lausanne, Switzerland

⁴Research School of Earth Sciences, Australia National University, Canberra, ACT 0200, Australia

*mcfad031@umn.edu, 612-624-8557

 

The Fosdick Mountains form an E-W trending migmatite dome in the northern Ford Ranges of Marie Byrd Land, Antarctica.  Pervasively folded migmatites derived from lower Paleozoic greywacke and middle Paleozoic plutonic rocks constitute the dome.  New field research documents a transition from melt-present to solid-state deformation upon the south flank of the dome, and a mylonitic shear zone mapped for 30 km between Mt. Iphigene and Mt Richardson. Kinematic shear sense is dextral normal oblique, with top-to-the-SW and -WSW transport. 24 km to the south in the Chester Mountains, correlative plutonic rocks are unmetamorphosed and unfoliated, but cut by sparse m-scale dikes of dolerite and two-mica granite and pervasive minor brittle faults that record NNE-SSW stretching.  We here propose that the mylonitic zone represents a crustal-scale detachment fault on the south flank of the Fosdick Mountains, separating dome rocks dominated by melt-present deformation in the middle crust from the brittely deformed, hanging wall block of the Chester Mountains. The structure, here named the South Fosdick detachment fault, forms the south flank of the migmatite dome and was in part responsible for the exhumation of mid-crustal rocks.

Four structurally well defined samples from the South Fosdick detachment zone of the Fosdick Mountains migmatite dome, west Antarctica, yield sensitive, high-resolution ion-microprobe (SHRIMP) U-Pb zircon ages that constrain the time-scale of partial melt crystallization during melt migration in a melt-present shear zone to between ca. 109–102 Ma. (1) Zircons from a biotite granodiorite sigmoidal boudin, that is interpreted to be the protolith for the crystallized anatectic granitic rocks, yields two SHRIMP U-Pb ages of 109.1 ± 0.8 Ma and 105.4 ± 1.0 Ma. (2) A biotite granite from a 10 cm wide leucosome concordant to the shear zone foliation yields an age of 107.4 ± 0.8 Ma. (3) A biotite granite from a discordant dextral normal shear band yields an age of 107.3 ± 0.9 Ma. (4) A biotite granite from a 100 m wide syntectonic granite sheet yields an age of 102.4 ± 0.7 Ma.

The shear zone preserves evidence for melt-enhanced deformation and the analyzed samples all preserve features indicative of crystallization from melt. These samples represent the successive stages of melt migration in a migmatitic shear zone. The U-Pb SHRIMP zircon crystallization data, in conjunction with structural setting and zircon morphologies, indicate that the time required for melt to form, migrate through a network of dilational and shear sites, coalesce, and crystallize was on the order of 7 x 10⁶ yr.

Late-Devonian cooling and Mississippian UPLIFT OF the Chester dome, Southeastern Vermont

Cory K. McWilliams ^a*, Michael J. Kunk ^b, and Robert P. Wintsch ^a

^a Department of Geological Sciences, Indiana University

^b United States Geological Survey, Reston, VA

 

Contrasting ⁴⁰Ar/³⁹Ar ages of muscovite from the Chester dome and its cover rocks in southeastern Vermont suggest that differential uplift of the dome persisted until at least the middle Mississippian.  Cooling ages obtained from a single outcrop containing Proterozoic schists and amphibolites intruded by pegmatites in the interior of the Chester dome are: ~375 Ma (amphibole correlation age), ~335 Ma (muscovite correlation age), ~320 Ma (biotite total fusion age), and ~270 and ~185 Ma (K-feldspar high and low temperature age steps, respectively).  These data define a smooth, continuous late Acadian cooling history.  In contrast, coarse muscovite separates from prograde quartz veins hosted by biotite-grade phyllitic cover rocks of the Silurian-Devonian Waits River Formation along the eastern flank of the dome yield cooling ages between 355 and 364 Ma – 20 to 30 million years older than muscovite from within the core of the Chester dome. Because both core and cover rocks now occur at the same level, uplift of core rocks must have been decoupled from the eastern cover rocks and persisted until at least 335 Ma.

To establish the time of crystallization of retrograde fabrics in the Waits River Formation, muscovite from overprinting cleavages was analyzed. A sample containing two biotite-free cleavages each defined by muscovite and chlorite, and both enveloping relic Acadian garnet porphyroblasts was collected ~6 km east of the core rocks near samples yielding late Devonian cooling ages. The resulting age spectrum climbed from 335 to 350 Ma and is interpreted to reflect a mixture of crystallization-age populations of these two retrograde cleavages with a possible minor relic Acadian muscovite cooling-age component. We hold that these late cleavages probably formed in the Mississippian during uplift of the Chester dome.

Numerical approximations were used to create a one-dimensional finite difference model to simulate tectonic conditions in the Chester dome.  Amphibole-plagioclase and garnet-biotite thermometry, coupled with garnet-muscovite-plagioclase barometry were used to constrain P-T conditions to 600-700 ^oC and 10-14 kbars within the dome.  Geophysical input parameters were used to determine a loading-unloading history consistent with ⁴⁰Ar/³⁹Ar cooling ages obtained within the interior of the dome.  However, P-T paths predicted by this model fail to reach minimum P-T conditions estimated using geotheromobarometry.  Three possible explanations for this tectonic paradox include: 1) a purely conductive one-dimensional heat-flow model may not capture the full scope of geological conditions (e.g. advective (hydrothermal?) heat flow), 2) geological models may omit more complicated tectonic scenarios for the Silurian, or 3) geothermobarometric calculations are exaggerated by inherited Proterozoic garnets in an Acadian matrix..

Our results confirm that the rocks of the Chester dome and its cover reached peak metamorphic conditions during the middle Devonian, Acadian orogeny. However, contrasting muscovite cooling ages and retrograde Mississippian fabrics indicate that uplift of the dome was delayed until the Mississippian, and that the Chester dome may not be entirely an Acadian structure.

Strucutural styles in the Argentinean Andes at 34ºS

 

José F. Mescua^1,2 y Víctor A. Ramos²

1. IANIGLA-CRICYT (CONICET)

2. Laboratorio de Tectónica Andina (UBA)

E-mail: jmescua@lab.cricyt.edu.ar, Phone number: 54-261-524 4238

 

The variations in structural style of the Argentinean Andes in a transect at 34º S are described.  At this latitude, the Argentinean Andes comprise two distinct regions: the Malargüe fold and thrust belt (MFTB) as part of the Cordillera Principal or Main Andes, and the Cordillera Frontal. The MFTB has a mainly thick-skinned structural style. At 34ºS (its northern end) a transition zone to the thin-skinned Aconcagua fold and thrust belt (AFTB) is observed. The Cordillera Frontal is a basement uplift located to the east of the Aconcagua and Malargüe FTBs and its southern culmination is immediately south of 34ºS.

Within the MFTB an inner and outer zone with contrasting structural styles are recognized at this latitude. The inner MFTB, located to the west, is a thick-skinned belt dominated by compressional tectonic inversion of originally extensional faults related to the Gondwana break-up during the Jurassic. The outer MFTB, located to the east, is a thin-skinned belt with several detachment levels within a late Jurassic and Cretaceous sedimentary succesion. East of the outer MFTB the Cordillera Frontal is again thick-skinned, exposing the Paleozoic-early Triassic basement.

Activity periods of the thick- and thin-skinned sectors of the FTB have been constrained by dating of syn- and post-tectonic volcanics within the MFTB (Giambiagi et al., 2005a) and by the relationship between structures and the age of syn-tectonic deposits in the foreland basin (Baldauf, 1997). Both areas show contemporaneous activity between 14 and 7 Ma. Field data from the inner MFTB support this, and favours the hypothesis for contemporaneous deformation in the inner and outer MFTB over the hypothesis of a thin-skinned phase followed by a thick-skinned phase (Kim et al., 2005).  Uplift of the southern end of the Cordillera Frontal is estimated in 9 Ma (Ramos, 2002).

Three stages of structural evolution are proposed. The first stage took place during the early to middle Miocene, and had a mixed strutural style: thick-skinned in the inner MFTB and thin-skinned in the outer MFTB. The contrast in structural style would respond to the presence or absence of previous Mesozoic rift structures; in fact the limit between the inner and outer sectors of the MFTB coincides with the La Manga-Borbollón lineament, proposed by Giambiagi et al. (2005b) as the master fault of the Atuel depocenter of the Jurassic rift. The second stage consisted in the late Miocene uplift of the Cordillera Frontal. The presence of this basement sticking point stopped the migration of the MFTB towards the east and triggered the third deformational stage: an out of sequence thin-skinned event which affected the whole MFTB.  

References

Baldauf, P., 1997. M. S. Thesis, George Washington University (unpublished), 356 p.

Giambiagi, L. B. et al., 2005a. 6º International Symposium on Andean Geodynamics, Extended Abstracts, p. 315-318.

Giambiagi, L. B. et al., 2005b. 16º Congreso Geológico Argentino, Actas v. 2, p. 81-87.

Kim, H. J. et al.,  2005. 16º Congreso Geológico Argentino, Actas v. 2, p. 63-69.

Ramos, V. A., 2002. 15º Congreso Geológico Argentino, Actas v. 3, p. 166-168.

Microstructural Study of Natural Fractures in Cape Roberts Project 3 Core, Western Ross Sea, Antarctica

C. Millan¹, T. Wilson, and T. Paulsen

¹School of Earth Sciences, Ohio State University, Columbus, OH 43210, USA millan.2@osu.edu, (614) 323 9445

This work analyzes microstructures in core recovered offshore from Cape Roberts, in the westernmost Ross Sea of West Antarctica, in order to understand the rifting evolution of the Victoria Land Basin (VLB) and its relationship to the uplift of the Transantarctic Mountain (TAM) rift flank. Textures, fabrics and grain-scale structures observed in microfaults, veins, and clastic dikes are indicative of the deformation mechanisms that produced those structures and the mechanical state of the sediment during deformation. This information provides new constraints on the relative timing of faulting and sedimentation in the VLB along the TAM rift flank boundary.

Microfaults are abundant and display two main types of textures. Some microfaults are characterized by grain-size reduction, poorly sorted angular grains and preferred orientation of clays and/or clast long axes parallel to fault zone walls; these are associated with brittle shear of dewatered and cohesive sediment. Others are "shear zones" where bedding drag, sediment smearing, and no grain-size reduction indicate pre-lithification ductile flow of sediment by sliding of grains due to abundant pore fluid. Veins commonly follow pre-existing fault planes and abundant fibrous calcite perpendicular to vein walls suggests opening-mode origin. Clastic dikes are present throughout the core and also commonly follow fault planes, indicating that injections of liquefied sediment used pre-existing faults as conduits for "dewatering bursts".  

The close association of clastic injections, diagenetic mineralization, and faulting indicates that faulting was synchronous with deposition in the rift basin.  Because the CRP cores were obtained in close proximity to the border fault zone between the VLB and the TAM rift flank, this zone was likely active during rifting and sedimentation in the basin.  Chronological data obtained from the Cape Roberts core constrains the time of rifting in this part of the VLB to early Oligocene (~ 34 Ma) to early Miocene time.  In contrast, apatite fission track data from the TAM front structural boundary onshore reveals a much earlier time for the onset of TAM uplift at ~55 Ma (Fitzgerald, 2002).  There thus appears to be a ~21 m.y. difference between onset of uplift of the rift flank and main-phase rifting in the adjacent VLB.  Further studies designed to obtain ages for faults along the rift flank boundary should aid in understanding this significant time gap.

References

Fitzgerald, P.G., 2002, Royal Society of New Zealand Bulletin, v. 35, p. 435-469.

Exhumation history of the Laramide Ranges using (U-Th)/He thermochronology

 

S. L. Peyton*, P.W. Reiners, P.G. DeCelles and P. Kapp

Department of Geosciences, University of Arizona, Tucson, AZ 85721

*Email: speyton@email.arizona.edu

 

The goal of our project is to document the distribution, timing, rate and amount exhumation of the Rocky Mountains in Wyoming, southern Montana and Utah using low-temperature (U-Th)/He thermochronology. Our study uses (U-Th)/He dating of apatite (AHe) from both petroleum well cuttings and surface samples.  All of the wells in this study penetrate Precambrian basement rocks in the hanging walls of Laramide-age uplifts.  Cuttings have been collected from at least one well through each of the Beartooth, Bighorn, Laramie, Granite, and Medicine Bow ranges, as well as the Uncompahgre uplift.  Apatite separates were obtained for six samples from a well through the Wind River Range.

 

In the Wind River Range, thermal forward modeling of our AHe results together with previously-published apatite fission track (AFT) ages shows that at least two episodes of exhumation are required to match the subsurface data; one episode during the early Eocene and another during the late Miocene.  It is impossible to match the AHe data without including an exhumation event in the Miocene.  Modeling shows removal of approximately 4 km of material in the Eocene, and between 1 and 2 km in the Miocene. 

 

Preliminary results from several samples from the Beartooth Range well show indications of Miocene exhumation, but there are currently insufficient data to recognize a ~50 Ma exhumation event that was documented by previous workers using AFT dating.  Results for additional samples from this well are pending and should help clarify these preliminary data and allow for modeling.  Sample preparation, mineral separations and apatite picking are ongoing for wells and surface samples from other ranges. 

Unraveling Polyphase Metamorphism in Orogens: An Example From the Neoproterozoic Brasília Belt, Brazil

 

Barry L. Reno¹ (reno@geol.umd.edu; 301-405-6964), Michael Brown¹, Philip Piccoli¹ and Rudolph A.J. Trouw²

 

¹Laboratory for Crustal Petrology, Department of Geology, The University of Maryland, College Park, MD 20742, USA

²Departmento de Geologia, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ 21910-900, Brazil

 

The Neoproterozoic Brasília Belt lies between the São Francisco, Amazonas and Paranapanema Cratons in central Brazil.  In the southern portion of this belt, the Andrelândia Nappe Complex records metamorphic conditions from greenschist facies at the base (east) to high-pressure granulite facies at the top (west).  At the top, the Socorro–Guaxupé Nappe represents part of the arc from the upper plate.  Immediately below, the Três Pontas–Varginha (TPV) and Carmo da Cachoeira (CdC) Nappes comprise pelitic sediments metamorphosed during subduction-to-collision orogenesis.

A series of pseudosections was constructed for a range of compositions found in the TPV and CdC Nappes. The peak phase assemblage located in the series of pseudosections constructed for samples from a single outcrop in the TPV nappe constrains maximum P-T conditions to 12-14 kbar and 830-900˚C.  A probability plot of SIMS ²⁰⁷Pb/²⁰⁶Pb dates from zircons contained within leucosome from the same samples yield two ages of zircon growth at ca. 650 Ma and ca. 605 Ma. We interpret the older age to represent the timing of crystallization in melt post subduction-related peak P. Ti-in-zircon thermometry yields temperatures consistent with crystallization from melt from ~950˚C to ~700˚C.   A probability plot of SIMS ²⁰⁷Pb/²⁰⁶Pb dates from zircons associated with Ilm breakdown yields a single age of ca. 658 Ma related close-to-peak, just- peak-T metamorphism, likely associated with detachment of the highest pressure nappe from the subducting plate, and transfer to the overriding plate.

For the CdC Nappe, immediately beneath the TPV Nappe, the peak phase assemblage located in the series of pseudosections constructed for samples from a single outcrop constrains maximum P-T conditions to 11-14 kbar and 820-870˚C.  Microprobe (U-Th)-Pb monazite and SIMS U-Pb monazite dating indicate two ages of growth at ca. 635 Ma and ca. 605 Ma. We interpret the older age to represent the timing of crystallization around peak T. The age of ca. 605 Ma in both nappes is interpreted to represent the timing of emplacement of the SGN on top of the nappe stack.

The cross-cutting Ribeira Belt imparts a sillimanite-grade overprint on the southern part of the Andrelândia Nappe Complex.  Age domains in monazite of ca. 590-530 Ma occur throughout the sillimanite zone and to the north, where younger ages in monazite record the Ribeira Belt overprint imposed during final suturing between the São Francisco and Congo cratons in west Gondwana.  In the northern part of the southern Brasília Belt, monazite ages provide the only petrologic evidence of this cryptic overprinting.

The Zuccale Low-Angle Normal Fault: A Case Study of Post-Collisional Extension in the Northern Apennines of Italy

 

Steven A.F. Smith^*1, Robert E. Holdsworth¹, Cristiano Collettini²

 

¹Reactivation Research Group, Dept. of Earth Sciences, University of Durham, Durham, DH1 3LE, UK

²Dipartimento di Scienze della Terra, Università di Perugia, 06123, Perugia, Italy

*steven.smith@durham.ac.uk, Tel: +44(0)7988777642

 

In the northern Apennines of Italy, an early pulse of Cretaceous-Pliocene collision was closely followed by a later phase of Miocene to recent post-collisional extension. Extension has largely been accommodated along a series of shallowly east-dipping Low-Angle Normal Faults (LANF) which are thought to have developed during eastward-migrating of extension associated with slab roll-back of the underlying Tyrrhenian subduction zone. Active LANF are present beneath the central belt of the northern Apennines, whilst older exhumed examples are found in western Tuscany and on the Tyrrhenian islands. LANF have been investigated over the past 10 years in this area using seismic reflection profiles, high-resolution microseismic surveys, borehole analysis, and detailed field investigations. Integration of these multi-scale datasets has resulted in the northern Apennines becoming a world-class area to study the initiation and evolution of these enigmatic structures.

The present contribution focuses on detailed field-based reconstructions of the evolution of the Zuccale LANF, a crustal-scale structure active between ~13 and 4Ma which is spectacularly exposed on the Island of Elba. The Zuccale LANF has a top-to-the-east displacement of 6-9km and is responsible for exhuming Palaeozoic crystalline schists in its footwall. Deformation within the footwall and hangingwall is entirely brittle, and the fault zone itself is dominated by a striking sequence of phyllonites, foliated cataclasites and fault gouges which can be used to reconstruct the geometric and kinematic history.

Deformation along the Zuccale Fault initially occurred through the development of brittle cataclasites. The formation of these cataclasites increased fault zone permeability and allowed the influx of chemically active fluids, triggering reaction softening and the development of a weak (μ <0.2) and pervasively foliated fault core. The sequence of fault rocks preserved within the fault core is highly heterogeneous, but the distribution of fault rocks is strongly controlled by the interaction between the Zuccale Fault and a suite of higher-angle faults in its footwall. We have used detailed structural analysis to show that the present-day geometry of the footwall structures can be used to place constraints on the evolution, and in particular the dip history, of the main detachment. Our results indicate that the Zuccale Fault could not originally have dipped at angles >20°E, and that it therefore represents a primary LANF.

Subsequent to the development of a foliated fault core, the Zuccale Fault behaved as an efficient structural seal to fluids migrating from depth within the footwall. Build-ups of fluid overpressure at the base of the fault zone resulted in widespread fluidization of pre-existing brittle cataclasites, a newly recognized phenomenon which has important implications for evaluating fault-valve behaviour along LANF. We use a diverse array of outcrop- and grain-scale observations to show that fluid flow leading to fluidization was strongly focussed along high-angle fault and fracture networks in the footwall of the Zuccale Fault. As fluid overpressure increased and reached a critical value, hydrofracturing of the overlying fault core increased permeability and allowed fluids to drain from the footwall to the hangingwall. We relate the fluidized cataclasites to the interseismic period along the Zuccale LANF, whereas the discrete hydrofractures represent co-seismic events and may partially explain the abundant microseismicity observed along active LANF in central Italy.

Polycyclic Deformation and Cementation in the Exhumed Norumbega Fault Zone, Maine-New Hampshire

 

STOESZ, Erin^1*, WINTSCH, R.P.¹, and SCHIEBER, Juergen¹

 

(1) Department of Geological Sciences, Indiana University, 1001 E 10th Str, Bloomington, 47405

 

            Fieldwork, optical and SEM imaging, and microprobe analysis show multiple ductile and brittle reactivations of high-grade fault rocks from the Norumbega fault zone in Maine and New Hampshire.  Mineral replacement reactions occurred at several stages of deformation, and most lead to stronger mineral assemblages and random fabric textures. Ductile deformation under amphibolite facies conditions created a conspicuous foliation in amphibolite and biotite granular schists.  Pegmatites that crosscut these rocks document a high-grade fracturing event into which pegmatitic fluids were intruded.  Ductile fabric in the pegmatites shares the same orientation with the host schists, thus ductile deformation continued with the same kinematics after brittle pegmatite emplacement.  Later localization of strain formed a ~100 meter wide ultramylonite band that cuts these schist and pegmatite protoliths.  Fractures in the feldspar-rich layers within biotite-rich host rocks project as c’ cleavages in phyllosilicate-rich layers and document a later brittle event.  Syntectonic feldspar crystallization is indicated by biotite inclusions within feldspar porphyroblasts in the schists, and extensive embayment of actinolite and biotite grains by feldspars and quartz in the ultramylonite.  This growth disrupted the contiguity of the weaker sheet silicates and resulted in a rock with a stronger texture and mineral assemblage.  This second episode of deformation must have opened conduits through which associated alkaline fluid infiltrated into the fault zone enabling the feldspar replacements.  Thus, the generation of ultramylonite involved brittle and perhaps seismic deformation even though evidence of pervasive fractures has been destroyed by subsequent replacements.  A third brittle deformation involved the localization of pseudotachylite dikes in this strengthened ultramylonite relative to the biotite-bearing schists. This localization further suggests that reaction hardening was a critical control on the localization of seismic, pseudotachylite-producing events.

           

* Corresponding author: estoesz@indiana.edu; 812-857-8687

Preliminary ⁴⁰Ar/³⁹Ar Results and a Tectonic Overview of the Mt. Rogers Area, VA-NC

M. Rebecca Stokes^1*, R. P. Wintsch¹, M. J. Kunk², C. S. Southworth²

¹Dept. of Geological Sciences, Indiana University, Bloomington, IN 47405

²U.S. Geological Survey, 926A National Center, Reston, VA 20192

Argon thermochronology and detailed petrology can be used to constrain the tectonic evolution of complex polymetamorphic rocks. Here, we discuss preliminary results from a transect in the Blue Ridge anticlinorium along the VA-NC state line south of Mount Rogers.  The rocks in the western part of our field area consist primarily of meta-granite of the Grenville basement which has been intruded by post Grenvillian mafic dikes. The cover rocks to the east include Neoproterozoic to early Paleozoic meta-sedimentary rocks of the Ashe and Alligator Back formations. The rocks in our area are cut by NE trending thrust faults including the Fries and Gossan Lead Faults.   

⁴⁰Ar/³⁹Ar cooling ages of amphiboles, muscovites, and potassium feldspars can be used to construct model cooling histories. These cooling histories can be used to estimate the timing of peak metamorphism(s) and the rate(s) of exhumation.  Establishing the time of peak metamorphism allows discrimination between major orogenic events, while the rate of cooling can be used to discriminate between the faster cooling of tectonic exhumation vs. the slower cooling of isostatic rebound of over-thickened crust. 

Metamorphic fabric within and around the bounding ductile fault zones are also being analyzed.  Amphibole and mica lineations within the Ashe Formation trend down dip of the foliation, whereas in the vicinity of the Brevard fault zone (a major tectonic boundary between the Blue Ridge and Inner Piedmont), the lineation turns to a strike parallel orientation, indicating poly-metamorphism within the Ashe Formation.  Using the ⁴⁰Ar/³⁹Ar method, we hope to date the micas defining these different events and place constraints on the timing of movement along the faults. 

Preliminary ⁴⁰Ar/³⁹Ar results of amphibole from a marble within Mesoproterozoic meta-granite west of the Fries fault gave a cooling age of 963 Ma which we interpret as cooling through the 500˚C isotherm following the peak of Grenville metamorphism.  Muscovite from a schist in the eastern cover rocks within the Ashe Formation south of the Fries Fault gave a cooling age of 337 Ma.  These data indicate that the western (Grenvillian) rocks were not metamorphosed to amphibolites-facies conditions since the Neoproterozoic, while the Ashe Formation did not cool from amphibolite-facies conditions until the Mississippian. 

 

 

*Corresponding author. Tel:830-377-7994; Email: mrstokes@indiana.edu

Provenance and Paleogeography of the Mesozoic southern Ordos Basin, North Central China: Implications of U-Pb detrital zircon geochronology

Xiangyang Xie¹, Paul L. Heller, and Kevin, R. Chamberlain

¹Department of Geology and Geophysics, University of Wyoming, 1000 E. University Ave. Laramie, WY 82072, telephone: 307-766-2245, email: xyxie@uwyo.edu

Two deformation belts — the Qinling orogenic belt to the south and the Western Liupanshan thrust belt to the west, including the Qilian-Qaidam terrane — control the evolution of the southern Ordos Basin, China, during early Mesozoic time. U-Pb detrital zircon geochronology is used to identify provenance, reconstruct paleogeography, and document the relative timing of basin margin deformation.

Four samples of the Yanchang Formation, middle to late Triassic age, were collected from the southern Ordos Basin, one from the southern and three from the southwestern basin margin.  In total 447 detrital zircon grains were analyzed. All LA-ICP-MS measurements were conducted at GeoAnalytical Laboratory, Washington State University. Three major age populations — 240–490 Ma, 1.8-2.0 Ga, and 2.2-2.8 Ga — characterize the detrital zircon grains of the Yanchang Formation. The two oldest age groups match ages of basement rocks found in the underlying North China block. However, younger ages can be subdivided into three distinctive groups: 240-300 Ma, 300-400 Ma, and 400-490 Ma. The youngest group matches ages exposed to the west in the Qilian-Qaidam terrane, whereas the older groups indicate a southern, Qinling orogenic belt source area. In all samples detrital zircon from the western source area overwhelms the southern source, regardless of proximity to either basin margin. Thus, the Qinling orogenic belt, even during deformation, was never the dominant source of detrital zircon to the south Ordos Basin. Furthermore, stratigraphic changes in provenance within the Yanchang Formation indicate that: 1) deformation in the Qinling and Western Liupanshan belts began in Middle Triassic time in this area; and 2) the southern source area barely contributed sediment by Late Triassic time. This result is surprising in that most studies suggest that the Qinling orogenic belt was the longer lived and more dominant tectonic feature in this part of China during early Mesozoic time.