Robert L. Bauer

SHRIMP study of zircons from Early Archean rocks in the Minnesota River Valley : Implications for the tectonic history of the Superior Province

Interest in Paleoarchean to early Meso-archean crust in North America has been sparked by the recent identification of ca. 3800–3500 Ma rocks on the northern margin of the Superior craton in the Assean Lake region of northern Manitoba and the Porpoise Cove terrane in northern Quebec. It has long been known that similarly ancient gneisses are exposed on the southern margin of the Superior craton in the Minnesota River Valley and in northern Michigan, but the ages of these rocks have been poorly constrained, because methods applied in the 1960s through late 1970s were inadequate to unravel the complexities of their thermotectonic history. Rocks exposed in the Minnesota River Valley include a complex of mig-matitic granitic gneisses, schistose to gneissic amphibolite, metagabbro, and paragneisses. The best-known units are the Morton Gneiss and the Montevideo Gneiss. The complex of ancient gneisses is intruded by a major younger, weakly deformed granite body, the Sacred Heart granite. Regional geophysical anomalies that extend across the Minnesota River Valley have been interpreted as defining boundaries between distinct blocks containing the various gneissic units. New sensitive high-resolution ion micro-probe (SHRIMP) U-Pb data from complex zircons yielded the following ages: Montevideo Gneiss near Montevideo, 3485 ± 10 Ma, granodiorite intrusion, 3385 ± 8 Ma; Montevideo Gneiss at Granite Falls, 3497 ± 9 Ma, metamorphic event, 3300–3350 Ma, mafic intrusion, 3141 ± 2 Ma, metamorphic overprint (rims), 2606 ± 4 Ma; Morton Gneiss: 3524 ± 9 Ma, granodiorite intrusion, 3370 ± 8 Ma, metamorphic overprints (growth of rims), 3140 ± 2 Ma and 2595 ± 4 Ma; biotite-garnet paragneiss, 2619 ± 20 Ma; and Sacred Heart granite, 2604 ± 4 Ma. Zircons from a cordierite-bearing feldspar-biotite schist overlying the Morton Gneiss yielded well-defined age peaks at 3520, 3480, 3380, and 3140 Ma, showing detrital input from most of the older rock units; 2600 Ma rims on these zircons indicate metamorphism at this time. Zircons from a hypersthene-bearing biotite-garnet paragneiss, overlying the Montevideo Gneiss near Granite Falls, yielded ca. 2600 Ma ages, indicating zircon growth during high-grade metamorphism at this time. Despite some differences in the intensity of the 2600 Ma event between the Morton and Montevideo blocks, both blocks display similar thermochronologic relationships and ages, suggesting that their boundary is not a fundamental suture between two distinct Paleoarchean terranes. Previously obtained zircon age data from the tonalitic gneiss at Watersmeet Dome in northern Michigan indicated formation at ca. 3500 Ma, whereas a granite body near Thayer was dated at 2745 ± 65 Ma and leucogranite dikes are ca. 2600 Ma. Thus, these rocks and those in the Minnesota River Valley were formed in the late Paleoarchean and show a history of igneous activity and metamorphism in the Mesoarchean and Neoarchean. The occurrence of ancient crustal rocks on both the northern and southern margins of the ca. 2900–2700 Superior craton suggests that they are remnants of once more-extensive Paleoarchean crust that existed prior to formation of the Neoarchean Superior craton.

Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota

Correlation of an F 1 recumbent fold and an upright F 2 antiform across the boundary between an Archean migmatite terrane and the adjacent volcanic-plutonic belt indicates a common tectonic evolution for both terranes since the earliest stages of folding. The local boundary between amphibolite facies schists and migmatites of the southern Vermilion Granitic Complex (VGC) and low-grade metagraywackes of the adjacent Vermilion district is marked by nearly vertical dip-slip faulting along the east-trending axial trace of a regional F 2 antiform. The migmatites in the southern VGC have been folded into S-symmetry folds that mark the northern limb of the major antiform. These folds are correlated with F 2 folds of Z symmetry on the southern limb of the fold in adjacent downfaulted rocks of the Vermilion district. Structural facing near the boundary between the two subprovinces is downward on both limbs of the major F 2 structure, which is interpreted to be part of the lower, overturned limb of a large-scale F 1 recumbent fold. A change to upright-facing strata farther south indicates a crossing onto the upper limb of the structure. The F 1 folding is tentatively attributed to gravitational spreading off the southern margin of the rising Lac La Croix Granite of the east-central VGC.

Petrogenesis of Archean lamprophyres in the southern Vermilion Granitic Complex, northeastern Minnesota, with implications for the nature of their mantle source

Petrogenetic modeling of major and trace element and isotopic data is used: 1. to define probable modes of petrogenesis of Archean spessartitic lamprophyric rocks in the southern portion of the Vermilion Granitic Complex (VGC) of northeastern Minnesota, and 2. to place constraints on the nature of the mantle source of these rocks. The lamprophyres range from olto qtz-normative and are associated with cumulate hornblendites and pyroxenites. The silica-rich lamprophyres are shown to be the result of low-pressure fractionation upon emplacement. On the other hand, the composition range of the ol-normative lamprophyres is explained by approximately 40% polybaric fractionation of elinopyroxene + olivine yielding ne-normative liquids. The fractionation explains low Cr, Ni and Sc concentrations compared to primary mantle-derived melts. Modeling of the lamprophyre MgO−FeO compositions using the olivine saturation surface (Hanson and Langmuir 1978) suggests that the 0.42 to 0.55 Mg/(Mg+Fe) ratios of most of the lamprophyres can be explained by the high-pressure fractionation. The model parent melt composition is similar to sanukitoid-type rocks found in Japan and elsewhere in the Superior Province. The lamprophyres have εNd2700 values of +1.4 to +2.0, indicating derivation from a depleted mantle source. Growth curves on an εNd vs. age diagram are consistent with the extraction of the lamprophyres from a depleted source (Sm/Nd>chondrite) just prior to 2700 Ma, the accepted age of the VGC. The lamprophyres have fractionated REE patterns (Ce/Ybn=10–15) that indicate genesis by a) 1% to 3% fusion of a pristine garnet lherzolite or b) ∼10% fusion of an enriched mantle source. However, consideration of the pressure of melting and elemental plots of Al and Ti indicate that garnet was not a residual phase during lamprophyre genesis. Thus, the enrichment of the LREE (80–100 x chondrite), Sr (580–1400 ppm), and Ba (590–1600 ppm) indicate derivation from an enriched mantle. These apparently contradictory chemical characteristics can be reconciled if the source region of the lamprophyres was depleted over a period of time but subsequently enriched just prior to genesis of the lamprophyre magmas. It is suggested that the source of the enriched component may have been fluids derived from dehydration of a subducting ocean crust.

Dye Tracing through Sinks Canyon: Incorporating Advanced Hydrogeology into the University of Missouri's Geology Field Camp

The University of Missouris Branson Geology Field Camp has integrated a series of environmental geology components into its curriculum, including hydrogeology and geophysics. In this paper, we present the results of a dye tracing experiment carried out by undergraduate students as the capstone field experiment of an optional advanced hydrogeology week at the camp. The dye tracing experiment was along the Popo Agie River, which disappears into a karst cave system in Sinks Canyon State Park, Wyoming, and resurfaces about 400 m down the canyon in a large, spring-fed pool, called the “Rise.” At the time of the test, the discharge rate in the river was 4,585 l/s (162 ft3/s). The students used skills developed during the required first week of hydrogeology, including dilution gauging and automated data acquisition, to design and carry out a dye tracing experiment to evaluate flow through the cave system. The leading edge of the Rhodamine WT dye pulse took just over 2 hours and 5 minutes to travel the short distance between the Sinks and the Rise. The peak dye concentration at the Rise was reached 2 hours and 47.5 minutes after the dye addition. The water residence time in the Sinks Canyon cave system was similar to results reported by the U.S. Geological Survey in 1983, indicating that the caves physical flow system has not changed in the last 23 years. Students participating in the advanced hydrogeology week rated the interest and value levels of the dye tracing test high and several of the students presented their results at the Geological Society of America 2006 annual meeting.

Chapter 6.1 Paleoarchean Gneisses in the Minnesota River Valley and Northern Michigan, USA

Publisher Summary This chapter elaborates the Paleoarchean gneisses in the Minnesota river valley and northern Michigan, USA. Meso- to Paleoarchean gneisses occur along the southern margin of the Neoarchean Superior Craton. The most extensive exposure of these rocks is in the Minnesota River Valley (MRV) of southwestern Minnesota, but there are also exposures in northern Michigan. Aeromagnetic mapping of southwestern Minnesota, and detailed gravity and magnetic modeling within the MRV have delineated four crustal blocks in the MRV that are bounded by three east-northeast-trending geophysical anomalies that roughly parallel the Morris fault segment. Most of the work in the MRV has concentrated on exposures in the Montevideo block and the Morton block, which are separated by the Yellow Medicine shear zone. The data indicate that major common tectonothermal events, recognized in the zircon geochronology, occurred in both the Morton and the Montevideo blocks. A mafic intrusion in the granite gneiss at Granite Falls has a well-constrained age of 3140 Ma, indicating another intrusive magmatic event at that time, and zircon overgrowths of this age are also seen in zircons from the Morton Gneiss.

Late Miocene–Quaternary fault evolution and interaction in the southern California Inner Continental Borderland

Changing conditions along plate boundaries are thought to result in the reactivation of preexisting structures. The offshore southern California Borderland has undergone dramatic adjustments as conditions changed from subduction tectonics to transform tectonics, including major Miocene oblique extension, followed by transpressional fault reactivation. However, consensus is still lacking about stratigraphic age models, fault geometry, and slip history for the near-offshore area between southern Los Angeles and San Diego (California, USA). We interpret an extensive data set of seismic reflection, bathymetric, and stratigraphic data from that area to determine the three-dimensional geometry and kinematic evolution of the faults and folds and document how preexisting structures have changed their activity and type of slip through time. The resulting structural representation reveals a moderately landward-dipping San Mateo–Carlsbad fault that converges downward with the steeper, right-lateral Newport-Inglewood fault, forming a fault wedge affected by Quaternary contractional folding. This fault wedge deformed in transtension during late Miocene through Pliocene time. Subsequently, the San Mateo–Carlsbad fault underwent 0.6–1.0 km displacement, spatially varying between reverse right lateral and transtensional right lateral. In contrast, shallow parts of the previously identified gently dipping Oceanside detachment and the faults above it appear to have been inactive since the early Pliocene. These observations, together with new and revised geometric representations of additional steeper faults, and the evidence for a pervasive strike-slip component on these nearshore faults, suggest a need to revise the earthquake hazard estimates for the coastal region.