Browsing by Subject "Geological time"

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Proterozoic tectonic evolution of southern Laurentia: new constraints from field studies and geochronology in southern Colorado and northern New Mexico, U.S.A.
(2005) Jones, James V.; Connelly, James N.
New field studies and geochronology from southern Colorado and northern New Mexico constrain the Proterozoic growth and modification of southern Laurentia. The Sangre de Cristo Mountains of southern Colorado preserve evidence for three episodes of Proterozoic magmatism, deformation, and metamorphism. Early deformation produced penetrative, NW-striking fabrics and occurred in an arc setting between 1750 – 1730 Ma. Post-Yavapai granitoid magmatism occurred at 1695±2 Ma and 1682±3 Ma and was contemporaneous with regional quartzite deposition. Deformation at 1637±6 Ma produced localized NE-striking, subvertical fabrics with dextral shear sense. Granitic magmatism at 1434±2 Ma and 1407±6 Ma was accompanied by NW – SE shortening between 1420 – 1412 Ma that produced subvertical, NE-striking fabrics. Thick sequences of quartz arenite were deposited across the region between the Yavapai and Mazatzal orogenies (ca. 1.70 Ga and 1.65 Ga). New geochronology reveals that deposition occurred on exhumed, Yavapai-aged basement (1706±5 Ma and 1698±4 Ma) with detritus dominated by Paleoproterozoic sources only slightly older than the quartzites themselves. Regional quartzite sedimentation was contemporaneous with nearly continuous magmatism in the region at deeper crustal levels. The first-cycle, synorogenic character of quartzites contrasts with their extreme compositional maturity, requiring perhaps anomalous environmental influences that enhanced chemical weathering during deposition. New geochronology and structural studies from the Wet Mountains, Colorado, reveal contrasting structural styles during widespread Mesoproterozoic A-type granitic magmatism. At shallower crustal levels, strongly localized deformation at 1430+5/-3 Ma produced subvertical fabrics throughout the N-striking Five Points shear zone. At deeper crustal levels, penetrative deformation accompanying granitic magmatism at 1435±5 Ma and 1390±10 Ma produced moderately- to shallowly-dipping fabrics. Regionally consistent fabric orientations and kinematics are interpreted to represent an intracontinental response to convergent tectonism, and contrasting, yet coeval, styles of deformation require a structural discontinuity in the middle crust between ca. 1430 – 1360 Ma. Weak, flowing lower crust is consistent with models for intraplate orogenesis and the development of orogenic plateaus, and the southern Wet Mountains might represent an exhumed analog for mid-crustal, low-viscosity layers inferred beneath modern intracontinental orogenic systems such as Tibet and the Altiplano.
Sequence stratigraphy, petrography, and geochronology of the Chilga rift basin sediments, northwest Ethiopia
(2002-05) Feseha, Mulugeta Yebyo; Kocurek, Gary; Kappelman, John W.
This dissertation presents results of an integrated approach including geochronology, paleomagnetism, petrography, x-ray diffraction analysis, depositional environment and sequence stratigraphic interpretation of Oligoceneage continental rift sediments. It provides a convincing analysis of the sedimentary and structural processes involved in the evolution of an Oligocene rift basin and offers inferences for both modern and ancient rift basins. The Chilga rift basin is located in northwestern Ethiopia, between N 12º 25' to 12º 39' latitude and E 37º 03' to 37º 11' longitude. It is a continental rift basin with isolated outcrops of sedimentary and volcanic basin fill separated by faults related to rift processes. Sediments filling the Chilga basin are rich in volcanic ashes, lignite beds, silty sandstones, silty claystones, and vertebrate and floral fossil rich sandstones. Continental rift basins, such as the Chilga basin that contain outcrops which can be correlated with marker beds and that can be dated with a suitable dating method provide a unique opportunity in understanding the tectonic and stratigraphic evolution of rift basins. Correlation and dating of ash beds allows the tectonic and stratigraphic evolution of the Chilga basin to be determined. Radioisotopic and paleomagnetic age dating techniques were used to constrain the age of the basin filling sediments and the underlying basaltic rocks. Results indicate that the Chilga sediments were deposited between 27 – 28.5 Ma. To interpret the depositional environment, the fine-grained Chilga sediments were analyzed using petrographic and x-ray diffraction analysis. Results of the analyses indicate that the Chilga sediments are composed of framework grains of mainly unaltered plagioclase, potassium feldspars, and volcanic rock fragments. The matrix is commonly enriched in kaolinite and illitemontmorillonite and the cement is mainly composed of authigenic siderite and some iron oxide coatings. Interpretations of these characteristics suggest that the Chilga sediments were deposited in alluvial-lacustrine environment with a nearby fine-grained sediment source and indicate a reducing diagenetic environment. A sequence stratigraphic interpretation of the Chilga basin sediments is inferred from the vertical and lateral stacking pattern of stratigraphic units. Key units that were used for the sequence stratigraphic interpretation of the Chilga basin were siderite-cemented silty claystones, which indicate a reducing condition and an increase in lake water depth, and channel incisions and lignite beds, which indicate lake shallowing. The Chilga sedimentary basin records three 4th order sedimentary cycles formed by the rise and fall of the Chilga Lake throughout the course of basin evolution. The identification of normal faults that cut the oldest Chilga sediments, tectonic growth faults, and fault sets that cut the entire Chilga sediment succession indicate that the Chilga basin was tectonically active both during and after sediment deposition. Regional correlation of the isolated Chilga outcrops using ash beds combined with structural interpretations indicate that the Chilga basin started with two sub-basins bounded by a paleo-high. Later, regional tectonic subsidence eliminated the marginal paleo-highs and caused the enlargement of the Chilga basin. Later stages of uplifting, faulting, and erosion have resulted in the formation of the existing, isolated Chilga outcrops.
Structural, metamorphic and geochronologic constraints on the origin of the Condrey Mountain schist, north central Klamath Mountains, northern California
(1985-08) Helper, Mark Alan; Maxwell, J. C. (John C.)
The Condrey Mountain Schist (CMS) occupies a window through Late Triassic amphibolite facies melange in the north central Klamath Mountains in northern California and southwest Oregon. The schists owe their present level of exposure to a large structural dome centered on the Condrey Mountain Window. Transitional blueschist-greenschist facies assemblages are widespread in mafic schists in the structurally lowest levels of the window; structurally higher CMS near the window margins contains medium- to high-pressure greenschist facies parageneses. An ⁴⁰Ar/³⁹Ar crossite age indicates a late Middle Jurassic age of metamorphism. All subunits of the CMS contain evidence of progressive, polyphase deformational and metamorphic histories. The styles and geometries of minor structures in the central part of the window suggest that early folding and transposition was the result of noncoaxial deformation, and that rotational strains were replaced by irrotational flattening strains with time. Rotational strains were accompanied by the development of epidote-crossite assemblages and the growth of deerite in meta-ironstones; irrotational flattening strains were accompanied and followed by the growth of albite, actinolite, spessartine, and the Ba-silicate, cymrite. Pressure-temperature estimates, the relative ages of mineral growth and deformation, and strain geometries are consistent with, but not restricted to, a subduction zone environment. High shear strains may reflect descent and burial, whereas flattening and late, static mineral growth occur during uplift. Pressure-temperature estimates for the overlying CMS greenschists suggest temperatures similar to those in the central part of the window, but at slightly lower pressures. Thrusting of the overlying amphibolites at 150-156 Ma occurred while the amphibolites were above about 500°C. Stretching lineations indicate a movement vector of about N45W. Comparisons of the sequence and timing of metamorphic and structural events, radiometric ages, and movement directions during thrusting indicate the CMS does not represent an inlier of Klamath Western Jurassic Belt flysch but is instead an older, isolated thrust plate. Similarities with the age of metamorphism and plutonism in the overlying amphibolites suggest the two plates may be remnants of the same Middle Jurassic paired metamorphic belt.
Volcanic stratigraphy, structural geology, and K-Ar geochronology of the Durango area, Durango, Mexico
(1973) Keizer, Richard Paul, 1947-; McDowell, Fred W. (Fred Wallace), 1939-
Detailed mapping in the Sierra Madre Occidental, near Durango City, Mexico, has documented more than 700 m of rhyolitic volcanic rock (Río Chico Formation) overlain by basaltic lava-flow rock (Metates Formation). The Río Chico Formation consists of nine persistent ash-flow sheets, each covering several hundred square kilometers, intercalated with minor ash-flow sheets, basaltic and rhyolitic lava-flow rock, and rhyolitic air-fall tuff. Rhyolitic plugs intrude the Río Chico Formation in several locations. The rhyolitic volcanic rock overlies an older sequence of andesitic volcanic rock. Two of the older ignimbrites (the Aguila and Santuario Members) were erupted from the Chupaderos Caldera centered about 8 km north of Durango City. Faults peripheral to the caldera are located about 10 km south and west of Durango City. Rhyolitic to dacitic flow domes south and west of Durango City mark the outer margin of the caldera. They were emplaced during an interval of erosion and caldera collapse which followed eruption of the Aguila ignimbrites. Later eruption of the Santuario ignimbrites buried much of the irregular post-collapse topography. A younger sequence of ignimbrites (Tunal Member) thickens considerably toward a probable source south of the mapped area. Still younger ignimbrites originated from an eruptive center(s) outside the map area, possibly to the west. The nearly horizontal ignimbrites are broken by north- and northwest-trending normal faults with displacements in the 200 m range. Initial faulting occurred during eruption of the ignimbrites, possibly because of collapse. Major episodes of block faulting also occurred before and after eruption of the rocks in the Metates Formation. K-Ar dates on the volcanic sequence indicate 3 distinct eruptive events. A tentative K-Ar date on a plagioclase separate from one sample of the basal andesite sequence is 46.4 m.y. Nineteen K-Ar ages on feldspar and biotite of the Río Chico Formation indicate that rhyolitic volcanism was in progress 29 to 31 m.y. ago. Three K-Ar dates on amphibole and feldspar from the Metates Formation indicate that basaltic volcanism, and concurrent block faulting, was in progress about 12 m.y. ago