Geoscience Colloquia for Fall 09 Semester

Thursday November 12, 2009 4 PM
Dr. Andy Czaja
University of Wisconsin-Madison
Title: “Iron and carbon isotope evidence for ecosystem and environmental diversity in the ~2.7 to 2.5 Ga Hamersley Basin, Western Australia”
Lapham Hall 262

Abstract:

The largest excursion in kerogen δ13C and bulk/mineral δ56Fe values yet measured in the ancient rock record occurs in rocks of ~2.5 to 2.7 Ga age. We report new Fe isotope data, integrated with previously collected C isotope data on the same samples, which document great microbial diversity in the Neoarchean (2.72–2.57 Ga) Hamersley Basin of the Pilbara Craton in Western Australia. Samples of shales, carbonates, and mixed carbonate/shale lithologies from the Mt. Bruce Supergroup were collected from three drill cores recovered from differing regions of the basin; two cores from the center and one from the margin, which were temporally correlated using published high precision U-Pb ages and impact spherule layers. Shallow water clastic rocks deposited in the center of the basin (Tumbiana Formation) record δ13C values for kerogen that indicate C cycling by various anaerobic or aerobic methane pathways, but the restricted range in δ56Fe values indicate little or no Fe redox cycling. Deep water sediments deposited contemporaneously in both parts of the basin (Jeerinah Formation) record slightly positive δ56Fe values in the relatively shallower and suboxic margin, but strongly negative δ56Fe values in the deeper euxinic center of the basin, a pattern consistent with Fe cycling via a basin Fe shuttle, driven by bacterial dissimilatory iron reduction (DIR). δ13C values of kerogens from these units indicate a coupling of microbial Fe cycling to aerobic methanotrophy or anaerobic oxidation of methane. Younger shales, co-deposited with iron formation (Marra Mamba Iron Formation) in the center of the basin, record a shift to near-zero δ56Fe values reflecting an Fe budget dominated by hydrothermal and clastic Fe sources. However, time-equivalent, Fe-rich carbonate/shale lithologies deposited on the margin of the basin (Carawine Dolomite) have δ56Fe values that steadily decrease from near zero to strongly negative values. These relatively Fe-rich carbonates may reflect a carbonate trap of a DIR-driven Fe shuttle, in contrast to the sulfidic trap in the euxinic portion of the Jeerinah Formation. In contrast, younger carbonates formed in the photic zone, but deposited by turbidity currents in relatively deep water in the center of the basin (Wittenoom Formation), have δ56Fe values that decrease with decreasing Fe concentrations, a pattern that indicates Fe cycling by Rayleigh fractionation through precipitation of iron oxides from aqueous ferrous iron.

Refreshments served prior to the colloquium at 3:30 in Lapham 380.


Thursday November 19, 2009 4 PM
Dr. Seth Kruckenberg
University of Wisconsin-Madison
Postdoctoral Fellow
Title"The dynamics of migmatite domes in extending orogens"
Lapham Hall 262

Abstract

Dr. Seth C. KruckenbergThe lower and middle continental crust commonly experiences partial melting during orogeny. The mechanical strength of high-grade metamorphic units undergoing anatexis is drastically decreased as melting takes place. This presumably rapid change in the strength of the upper part of the lithosphere profoundly affects orogenic processes and the partitioning of deformation within the continental lithosphere. Many active and exhumed orogens contain gneiss domes cored by migmatites (the rocks that were once partially molten), suggesting that the processes of partial melting, dome formation, and exhumation are intimately linked during orogenesis.

Kruckenberg1_cropTwo migmatite domes, the Okanogan dome (Washington state) and the Naxos dome (Cyclades, Greece) record the dynamics of migmatite dome formation in extending orogens. Both domes are exposed in cordilleran-style metamorphic core complexes that developed during extension of previously thickened crust and contain a core of anatectic migmatite mantled by metasedimentary rocks exposed below crustal detachments. Structural observations from migmatites in both domes indicate that deformation took place in the presence of melt (e.g., former partial melt collected in dilatancy sites) and the crystallization of dome migmatites was coeval with detachment faulting, dome formation, and upper crustal extension. In the Okanogan dome, both mid to lower crustal rocks (magmatic to solid-state fabrics) and upper crustal units (en echelon graben) record NW-SE extension. A continuous structural section exposed on the western margin of the Okanogan dome documents that structural fabrics acquired in the migmatite domain are systematically reoriented and deformed at higher structural levels at the rheological interface between the flowing partially molten layer and the Okanogan detachment. In the Naxos dome, foliation in the metasedimentary units that mantle the migmatitic core define a simple dome geometry and have consistent lineations (NNE orientation) and kinematics (Top to the N). Migmatitic foliation, defined by thin biotite layers, in the core of the dome defines a complexly folded internal geometry and reveals smaller sub-domal features, likely related to the internal dynamics of the migmatite domain. Application of the anisotropy of magnetic susceptibility (AMS) in the Naxos migmatites, combined with structural analysis, indicates that flow within the migmatite domain is broadly consistent with the pattern of extension recorded in the mantling units but also emphasizes the effect of upwelling of the low-viscosity migmatites during dome formation.

Results from the Okanogan and Naxos domes emphasize the role of partial melting and flow of migmatites in the formation of domes developed during extension of previously thickened crust. Observations in both domes suggest that partial melting and flow within the migmatites was kinematically consistent with regional patterns of extension. Furthermore, these observations suggest that vertical flow of partially molten crust is also an important process in the formation of migmatite domes (and associated subdomes) in extending orogens.

Refreshments served prior to the colloquium at 3:30 in Lapham 380.


Tuesday December 1, 2009 3 PM
Dr. Adolf Yonkee
Weber State University
Title:"Determining the 3-D Kinematic History of the Wyoming Laramide Foreland: Results from Structural and Paleomagnetic Studies of the Chugwater Group"
Lapham Hall 262

Abstract:

chugwater_cropUnderstanding the origins of foreland mountain belts, including formation of diversely oriented arches, mechanisms of basement and cover deformation, and relationships to plate margin stresses, are key issues in tectonics. This presentation integrates results of ongoing structural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic studies of Triassic redbeds of the Chugwater Group exposed in the Laramide foreland of Wyoming. The redbeds carry a near primary remnant magnetization for quantifying vertical-axis rotations, have interpretable AMS fabrics, and contain a thin limestone interval with distinct minor faults. Minor fault sets typically include conjugate contraction faults with dip slip and conjugate subvertical faults with strike slip, which record early layer parallel shortening (LPS) and limited strike-parallel extension. These simple minor fault sets are locally overprinted by more complex fault systems in steep fold limbs. Kinematic analysis of minor fault data yields estimated LPS directions from 050 to 070˚ in structurally simple settings, subperpendicular to regional NW structural trends. In detail, fault data appear to record minor spatial and temporal changes of shortening directions, possibly associated with basement heterogeneity and an evolving stress field. Weak AMS lineations are developed at most sites, providng additional estimates of LPS directions that are consistent with minor fault data. LPS directions estimated from both minor fault and AMS data in steep forelimbs of folds display systematic changes in orientation that correlate with changes in structural trend. However, interpretation of this pattern is ambiguous without paleomagnetic data. By integrating multiple structural and paleomagnetic data sets, limitations of each set are reduced. Initial data sets are used to develop a model that combines vertical-axis rotation of early fabrics, variable wrench shear, and stress/strain refraction along curved folds. Additional studies will test and refine this model, and evaluate links between thick-skin Laramide foreland deformation and thin-skin deformation in the Sevier fold-thrust belt to the west.

Refreshments served prior to the colloquium at 2:30 in Lapham 380.