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.

The incorporation of increasingly multidisciplinary aspects of geoscience curricula into a traditional geology field camp requires compromises. Among these, decisions about projects to reduce or eliminate and course prerequisites are two of the most challenging. Over the past 10 yr, the University of Missouri's geology field camp has completed a two-stage plan to expand our projects in hydrology and geophysics while maintaining traditional aspects of our course and our standard prerequisites. The first stage added projects in surface and groundwater hydrology, seismic refraction, and surficial mapping during the fifth week of our six-week course, replacing an existing mapping project. The second stage added advanced project options that students can select to complete during the last week of the course. Advanced projects in hydrology and geophysics were added as alternatives to the existing hard-rock structural analysis project that had been the sixth-week project for all students. This staged addition has allowed us to: (1) integrate these projects into a curriculum that maintains a strong emphasis on historical bedrock geology, geologic mapping, and three-dimensional visualization; and (2) accommodate differences in the coursework that students have completed prior to beginning the field camp. Rather than requiring students to have prerequisite courses in hydrogeology or geophysics in order to select these advanced project options, we include sufficient instruction during the fifth and sixth weeks that builds upon previous projects to provide the required background. To set up the context for our expanded hydrology and geophysics projects, this paper briefly describes our traditional field projects and our instructional philosophies. We describe the expanded projects that have been implemented during the fifth and sixth weeks of our course, project objectives, and the ways that these projects reinforce lessons learned during traditional field projects. We present the results of student surveys that have been used to evaluate the success of these efforts, and we discuss the personnel and equipment expenses required.

The University of Missouri's 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 cave's 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.

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.

Hyporheic exchange increases the potential for solute retention in streams by slowing downstream transport and increasing solute contact with the substrate. Hyporheic exchange may be a major mechanism to remove nutrients in semi-arid watersheds, where livestock have damaged stream riparian zones and contributed nutrients to stream channels. Debris dams, such as beaver dams and anthropogenic log dams, may increase hyporheic interactions by slowing stream water velocity, increasing flow complexity and diverting water to the subsurface.Here, we report the results of chloride tracer injection experiments done to evaluate hyporheic interaction along a 320 m reach of Red Canyon Creek, a second order stream in the semi-arid Wind River Range of Wyoming. The study site is part of a rangeland watershed managed by The Nature Conservancy of Wyoming, and used as a hydrologic field site by the University of Missouri Branson Geologic Field Station. The creek reach we investigated has debris dams and tight meanders that hypothetically should enhance hyporheic interaction. Breakthrough curves of chloride measured during the field experiment were modelled with OTIS-P, a one-dimensional, surface-water, solute-transport model from which we extracted the storage exchange rate alpha and cross-sectional area of the storage zone A(s) for hyporheic exchange. Along gaining reaches of the stream reach, short-term hyporheic interactions associated with debris dams were comparable to those associated with severe meanders. In contrast, along the non-gaining reach, stream water was diverted to the subsurface by debris dams and captured by large-scale near-stream flow paths. Overall, hyporheic exchange rates along Red Canyon Creek during snowmelt recession equal or exceed exchange rates observed during baseflow at other streams. Copyright (c) 2005 John Wiley & Sons, Ltd.

Evidence from shear zone geometries, finite strains, and the kinematics of shear indicate that well-developed ductile to brittle-ductile shear zones concentrated in the boundary region of the Quetico and Wawa subprovinces are a result of regional dextral transpression. However, the local concentration of shear in the subprovince boundary region is interpreted as a response to local partitioning of shear and shortening strains rather than a response to local suturing or accretion along this segment of the subprovince boundary. The shear zones are concentrated primarily in metasedimentary rocks that are interlayered with steeply dipping basaltic flows in the northern Wawa subprovince near the sub-province boundary. This association suggests that shear strains developed during the transpression were concentrated in the subprovince boundary region as a result of local rheological contrasts between steeply dipping mafic flows and associated metasedimentary rock, not in response to accretion along the subprovince boundary.

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 ε Nd 2700 values of +1.4 to +2.0, indicating derivation from a depleted mantle source. Growth curves on an ε Nd 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.

An analysis of the deformation along the boundary between the Vermilion Granitic Complex (VGC) and the Vermilion district indicates that the two terranes have seen a similar deformation history since the earliest stages of folding in the area. Despite this common history, variations in structural style occur between the two terranes, such as the relative development of D sub 1 fabrics and D sub 2 shear zones, and these can be attributed to differences in the crustal levels of the two terranes during the deformation. Similarly, the local development of F sub 3 folds in the VGC, but not in the Vermilion district, is interpreted to be a result of later-D sub 2 pluton emplacement which was not significant at the level of exposure of ther Vermilion district.

Analysis of folding in the southwestern Vermilion granitic complex suggests that F2 folds, which formed in a regional north–south compressional regime, were locally reoriented during pluton emplacement and were subsequently refolded by F3 folds generated by continuing north–south compression. An east–west-trending F2 antiform, crossing the boundary between the southern Vermilion granitic complex and the adjacent Vermilion district, was intruded by a tonalite pluton during the later stages of F2 folding. Either emplacement of the pluton or emplacement of the nearby Lac La Croix batholith is believed to have reoriented the F2 fold so that, subsequently, the pluton and the F2 antiform were refolded by a doubly plunging east–west-trending F3 fold, now cored by the pluton.A similar relationship occurs between F2 and F3 folds north of the Vermilion fault in the main portion of the complex. Here, F3 conical folds, not cored by plutons, refold F2 folds about northwest-trending vertex axes. Analysis of the refolding of minor F2 folds and L2 lineations in this area suggests they were nearly coaxial with the F3 folds prior to the F3 folding.

Research Article| September 01, 1985 Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota Robert L. Bauer Robert L. Bauer 1Department of Geology, University of Missouri, Columbia, Missouri 65211 Search for other works by this author on: GSW Google Scholar Author and Article Information Robert L. Bauer 1Department of Geology, University of Missouri, Columbia, Missouri 65211 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1985) 13 (9): 657–660. https://doi.org/10.1130/0091-7613(1985)13<657:COERAY>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert L. Bauer; Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota. Geology 1985;; 13 (9): 657–660. doi: https://doi.org/10.1130/0091-7613(1985)13<657:COERAY>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Correlation of an F1 recumbent fold and an upright F2 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 F2 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 F2 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 F2 structure, which is interpreted to be part of the lower, overturned limb of a large-scale F1 recumbent fold. A change to upright-facing strata farther south indicates a crossing onto the upper limb of the structure. The F1 folding is tentatively attributed to gravitational spreading off the southern margin of the rising Lac La Croix Granite of the east-central VGC. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Amphibolite-grade Archean migmatites in the southern Vermilion Granitic Complex with well-defined paleosome-melanosome and melanosome-leucosome boundaries and with exceptionally wide melanosomes (on the order of centimeters) were studied to elucidate granite-forming processes during high-grade metamorphism. Metagreywacke paleosomes containing 50% plag, 28% qtz, 20% biot and minor hbld, and apat, have (Ce/Yb)/sub N/ = 13.5 to 21 with 650-960 ppm Ba, 42-110 ppm Rb, and 982-1159 ppm Sr. Melanosomes containing 45% plag, 35% biot, 20% hbld and minor qtz and apat, have (Ce/Yb)/sub N/ = 6.8 to 9.3 and have 950-1750 ppm Ba, 41-194 ppm Rb, and 1020-1926 ppm Sr. Leucosomes containing 82% plag, 13% qtz, 5% biot and minor hbld and apat, have overall depleted REE patterns with positive Eu anomalies and 460-750 ppm Ba, 41-43 ppm Rb, and 1876-2106 ppm Sr, suggesting cumulate plagioclase. Mass balance calculations preclude formation of the melanosome from mixing the paleosomes and leucosomes. However, major and trace element modeling suggest that the leucosome formed by in situ partial melting followed by fractional crystallization and filter pressing which resulted in the removal of the residual liquid. Model REE patterns for the melt drive off by this process are REE enriched with a negative Eu anomaly. Suchmore » patterns which have been found in some low Sr granites are difficult to produce by simple belting models. Partial melting under conditions of tectonic stress may thus provide an explanation for such granites.« less

Alkalic gabbros and tonalites comprise a significant portion of the Archaean crust in the Vermilion Granitic Complex of NE Minnesota. The origin of these and associated rocks has been modeled using major and trace element approaches. Samples of the alkalic gabbro collected from three different intrusions have similar major element, REE, and transition metal concentrations. The REE patterns of these rocks can be modeled as the result of 1% to 3% melting of an undepleted garnet herzolite mantle with REE concentrations three times that of chondrites. However, their Al/sub 2/O/sub 3//CaO ratios of 2 - 3, Sr content of 900-1400 ppm and Ba of 100 - 1600 ppm suggest that the source may have been an enriched, metasomatized mantle. The hornblendites associated with these alkalic rocks have REE patterns which are consistent with crystallization and accumulation from the gabbroic magma. Major and trace element modeling suggest that the granitic dikes which are common throughout the area may be residual liquids formed by 60% crystallization of plagioclase, biotite, hornblende and apatite from the nearby tonalites such as the Burntside of Wakemup Bay plutons. Porphyritic hornblende monzonites composed of centimeter sized hornblende crystals floating in a granitic matrix occur locally. The fieldmore » relations as well as the major and trace element data are consistent with the formation of these monzonitic rocks by mixing of the granite with partially consolidated hornblendite. These results suggest a complex interaction between alkalic gabbros and tonalites involving fractionation and mixing during the development of the Archaean crust of NE Minnesota.« less