The Anatolia (Eurasia), Arabia, and Africa tectonic plates intersect in southeast Turkey, near th... more The Anatolia (Eurasia), Arabia, and Africa tectonic plates intersect in southeast Turkey, near the Gulf of İskenderun, forming a tectonically active and unstable triple junction (the A3 triple junction). The plate boundaries are marked by broad zones of major, dominantly left-lateral transform faults including the East Anatolian fault zone (the Anatolia-Arabia boundary) and the Dead Sea fault zone (the Arabia-Africa boundary). Quaternary basalts occur locally within these “leaky” transform fault zones (similar to those observed within oceanic transform faults), providing evidence that mantle melting, basalt genesis, and eruption are linked to crustal deformation and faulting that extends into the upper mantle. We investigated samples of alkaline basalt (including basanite) from the Toprakkale and Karasu volcanic fields within a broad zone of transtension associated with these plate-boundary faults near the İskenderun and Amik Basins, respectively. Toprakkale basalts and basanites ha...
Surficial deposits in the Denver West quadrangle record alluvial, mass-movement, glacial, and eol... more Surficial deposits in the Denver West quadrangle record alluvial, mass-movement, glacial, and eolian processes in the central part of the Front Range and the western margin of the Colorado Piedmont during the Quaternary and late Neogene. Many of the surficial deposits are poorly exposed. Deposits that are of limited extent (less than about 200 m wide) were not mapped, including (1) fill material in urban areas, (2) mine-and-mill waste and dredge tailings in mining areas, (3) thin mass-movement deposits above present treeline (such as block fields and block streams), and (4) thin sheetwash deposits that locally mantle gently sloping map units. Descriptions of surficial units on this map are based chiefly on information in maps and reports by
Resumen Trabajos como el presentado en este artículo, que incluyen estudios básicos de cartografí... more Resumen Trabajos como el presentado en este artículo, que incluyen estudios básicos de cartografía, microtectónica, petrografía, geoquímica y geocronología de rocas de arco, permitirán iniciar una revisión geológica individual y colectiva de los arcos volcánicos que afloran en gran parte del territorio de México. La combinación de este tipo de esfuerzos permitirá proponer modelos de evolución geológica más precisos y menos especulativos para este importante proceso de generación de corteza mediante la acreción de arcos volcánicos (p. ej. terreno Guerrero). La geoquímica y petrología de las rocas metavolcaniclásticas (filitas), metalavas y metaignimbritas de Taxco y Taxco Viejo son muy similares y, en conjunto, permiten proponer un origen equivalente para su formación que, seguramente, está asociado a un ambiente de formación de tipo arco volcánico. En este estudio se describe por primera vez que la Roca Verde Taxco Viejo es, en realidad, una metaignimbrita. La deformación estudiada (macro-y microtectónica) de las rocas metavolcánicas de Taxco y Taxco Viejo es muy similar independientemente de la naturaleza del protolito. Estas rocas presentan planos de foliación muy bien desarrollados en conjunto que buzan tanto al E-NE como hacia el W-SW, lo cual sugiere la existencia de más de un evento de deformación regional. En este estudio proponemos que las edades 206 Pb/ 238 U obtenidas a partir del método SHRIMP-RG en cristales únicos de circón, para las muestras de Taxco (137.1 ± 0.9 Ma) y Taxco Viejo (135.6 ± 1.4 Ma), son más confiables que las obtenidas mediante la técnica convencional ID-TIMS (poblaciones multigrano), y representan las mejores estimaciones de la edad de cristalización para ambos protolitos volcánicos. Por último, destacamos que estas edades U-Pb resultan iguales, dentro de los límites de error asignados a cada muestra, por lo que sugerimos existe una conexión temporal entre el vulcanismo cretácico temprano presente en Taxco y en Taxco Viejo. Esta edad cretácica temprana para los arcos volcánicos metamorfoseados, que se discute a detalle en este trabajo, fue inicialmente presentada en una convención científica en 2004, y desde entonces significó terminar con la especulación clásica de posibles edades precámbrica y paleozoica para estas rocas conocidas formalmente como Esquisto Taxco y Roca Verde Taxco Viejo. Este avance sustancial del conocimiento permitió dar mayor certeza a la hipótesis de que estas rocas de arco son de edad cretácica y/o jurásica como lo sugerían las relaciones estratigráficas observadas en los ensambles vulcanosedimentarios con control fosilífero de lo que hoy se conoce como terreno Guerrero. Esta edad cretácica de los arcos-acompañada de la hipótesis, cada vez más influyente, de la posible evolución geológica alóctona de estos arcos volcánicos, que propondría la posterior acreción de los mismos a una zona continental (p. ej. Oaxaquia) en el Cretácico Tardío-presenta una clara alternativa a las ideas fijistas/autóctonas clásicas que sugirieron un basamento antiguo para el terreno Guerrero que implicaría un origen continental para este arco volcánico hoy representado por las rocas de Taxco y Taxco Viejo.
Descriptions of surficial units along the eastern flank of the Front Range on this map are based ... more Descriptions of surficial units along the eastern flank of the Front Range on this map are based on those of Scott (1963a), Trimble and Machette (1979), and Kellogg and others (2008). Within South Park, distribution and description of surficial units were derived from reports by Stark and others (1949), Widmann and others (2005), Kirkham and others (2006), and the authors' new mapping and field observations. Formal names for fluvial and pediment deposits are those established by Scott (1960, 1963a) in the Colorado Piedmont. South Park glacial, glaciofluvial, and alluvial deposits are based on inferred age relationships among geomorphic position in the landscape, soil development, and reports by Widmann and others (2005, 2006), and Kirkham and others (2006). Surficial units were mapped primarily by interpretation of 1:40,000-scale, blackand-white aerial photographs, USGS black-and-white digital ortho-photoquads (1-meter resolution), Landsat Thematic Mapper images, and field analyses of deposits and landforms. Age assignments for surficial deposits within the map area are based chiefly on the relative degree of modification of their original surface morphology, relative heights above present stream channels, and degree of soil development and clast weathering. Soil-horizon designations are based on those of the Soil Survey Staff (1999) and Birkeland (1999). Some of the surficial deposits contain pedogenic calcium carbonate. Stages of pedogenic calcium carbonate morphology (referred to as stages I through IV) in Bk and K soil horizons follow the classification from Gile and others (1966) and Machette (1985). Grain sizes within unit descriptions follow those of the Soil Survey Staff (1951) and are as follows: sand (0.05-2 mm), silt (0.002-0.05 mm), and clay (<0.002 mm). Grain sizes are limited to field estimates. The term "clasts" is defined as granules and larger particles (>2 mm in diameter, visible without magnification), and the term "matrix" refers to sand and finer particles (≤ 2 mm in diameter). Many of the surficial deposits are poorly exposed. Deposits that are of limited extent (commonly less than about 50 m wide) were not mapped, including 1) fill material in developed areas, 2) mine-and-mill waste and dredge tailings in mining areas, 3) thin mass-movement deposits above treeline (such as block fields and block streams), and 4) thin sheetwash deposits that locally mantle gently sloping map units. In addition to the relative age of deposits based on erosional, geomorphic, and weathering characteristics, we have inferred ages for some of the surficial units based on the marine oxygen isotope records of Lisiecki and Raymo (2005). These records reflect global ice volumes (for example, Shackleton and Opdyke, 1973, 1976). However, ice volume changes on continents may supercede oxygen isotope variation in marine cores by 500-3,000 years (Mix and Ruddiman, 1984).
New high-resolution chemostratigraphy, in combination with updated biostratigraphy, refines the c... more New high-resolution chemostratigraphy, in combination with updated biostratigraphy, refines the chronostratigraphic resolution of the nearly 2 km thick Ordovician section below Anticosti Island. A total of 1414 horizons, spaced at 1.0–1.5 m intervals, were sampled from the New Associated Consolidated Paper (NACP) drill core and analyzed for major and trace elemental composition by portable X-ray fluorescence analyzer (pXRF). Select micrite and calcareous shale powders were then analyzed for δ13Ccarb (number of samples, N = 364) and 87Sr/86Sr (N = 25). Our results indicate a Floian to early Darriwilian (F3–Dw1) age for the Romaine Formation, a middle Darriwilian to Sandbian (Dw2–Sa1) age for the Mingan Formation, a lower to mid-Katian (Ka1–Ka2) age for the Macasty Formation, an upper Katian (Ka3) age for the lower Vauréal Formation, an uppermost Katian (Ka4) age for the upper Vauréal, and a Hirnantian (H1) age for the Ellis Bay Formation. This integrated chemostratigraphic and biostr...
The lunar Pb isotopic evolution is explored based on an investigation of the U-Th-Pb systematics ... more The lunar Pb isotopic evolution is explored based on an investigation of the U-Th-Pb systematics of the primitive Apollo 16 anorthositic breccias 76075 and 62237. The isotope data for acid-leached residues from 67075 lie within or on a triangle formed by plagioclase, opaques, and a pyroxen-olivine mixture, and the isotope data for 62237 lie within or on a triangle formed by plagioclase, olivine, and pyroxene. The results suggest that the moon had high first-stage values for the Pb evolution equations from the onset.
U-Pb isotopic systematics from analyses of single zircons identify at least two provenance ages, ... more U-Pb isotopic systematics from analyses of single zircons identify at least two provenance ages, approximately 575 Ma and approximately 330 Ma, for zircons from the impact layer of the K-T boundary, Raton Basin, Colorado. These data are a preliminary confirmation of results reported from the same layer. The zircon provenance ages provide a unique signature for identification of the source crater since igneous rocks of these ages (or sedimentary rocks derived from them) must characterize part of the impact stratigraphy.
U-Pb data from lunar meteorites Yamato (Y)-86032 and Y-793274 confirm that they are of lunar orig... more U-Pb data from lunar meteorites Yamato (Y)-86032 and Y-793274 confirm that they are of lunar origin, but also indicate that they are not from the same source region on the Moon and experienced different events while residing as lunar regolith. The Pb of both clast and matrix from Y-86032 is the least ra diogenic among lunar meteorites thus far analyzed, similar to that of lunar an orthosite 60025, and indicates derivation from a low-23 �Uf 04 Pb (tt) source, assum ing initial lunar Pb compositions were essentially primitive in nature. Whereas the data from Y-86032 plot slightly above the geochron on a 2 onpb;20 4 pb vs. 20 7 Pb/ 204 Pb diagram, data from 60025 and Y-82192 (considered a pare of Y-86032) plot precisely on the geochron. Very small amounts of U, Th, and Pb were leached from Y-86032 using O.lN HBr; the Pb isotopic compositions of the leaches are essentially modern terrestrial values, indicating that this rock probably did not reside long at the lunar surface and was not exposed to volatile-rich gases expelled during impact event (s). In contrast, large amounts of the three elements (up to five times those of Y-86032) were removed from Y-793274 during leaching, and the Pb isotopic com positions of these leaches are very radiogenic and similar to those of mare basalt and some lunar soils, indicating that this rock was subjected to lunar impact related, volatile-rich gases containing radiogenic 200 Pb-rich Pb, very similar to typical lunar soils and some breccias containing significant amounts of mare derived material. This is the first occurrence of leaches that are more 205 Pb-rich than their corresponding residues and indicate that the meteorite may have origi nated from near a mare region. To compare, the Pb from leaches of Y-791197 clasts (N. NAKAMURA et al.; Mem. Natl Inst. Polar Res., Spec. Issue, 41, 106, 1986a) was 200 Pb-poor, indicating the meteorite may have originated from the lunar highlands. The Pb in residues of Y-793274 is more radiogenic than that of Y-86032 and Y-82192, similar to that in ALHA81005 and lunar anorthositic gabbros 78155 and 15418, and indicate a lunar source region with high-p (> 100 and possibly as high as 300). Corrected U-Pb data indicate a formation age of-4.4 Ga and a disturbance to the system at "'4.0 Ga.
We have undertaken U-Th-Pb isotopic studies on lherzolitic shergot tite, Yamato-793605. Four mine... more We have undertaken U-Th-Pb isotopic studies on lherzolitic shergot tite, Yamato-793605. Four mineral separates (olivine, plagioclase, and two phases of pyroxene) and a whole-rock sample were leached with dilute acid in order to remove secondary Pb contamination. However, preferential leaching of U and Th over Pb occurred. The Pb isotopic data points of five residue fractions scattered, and a calculated Pb-Pb age does not clearly indicate either crystallization from a melt or a later disturbance to the U-Pb system, in contrast with other shergottites. The chord through U-Pb data points for pyroxene (PX 1) leaches and residue inter sects concordia at 4439 ± 9 Ma and 212 ± 62 Ma, suggesting a young disturbance event resulting in Pb loss. The later disturbance event partially reset the U-Pb sys tem. The estimated 238U/2°4Pb (= µ) value from the whole-rock residue for the source of Y-793605 is about 5, suggesting that Y-793605 was derived from a volatile-rich source environment and confirms a low-µ source for shergottites compared to vol canic rocks of the Earth.
The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives, 2005
We utilize new geological mapping, conventional isotope dilution-thermal ionization mass spectrom... more We utilize new geological mapping, conventional isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses, and whole-rock radiogenic isotope characteristics to distinguish two contrasting Proterozoic basement complexes in the international border region southeast of Yuma, Arizona. Strategically located near the truncated southwest margin of Laurentia, these Proterozoic exposures are separated by a northwest-striking Late Cretaceous batholith. Although both complexes contain strongly deformed Paleoproterozoic granitoids (augen gneisses) intruded into fi ne-grained host rocks, our work demonstrates marked differences in age, host rock composition, and structure between the two areas. The Western Complex reveals a >5-km-thick tilted section of fi nely banded felsic, intermediate, and mafi c orthogneiss interspersed with tabular intrusive bodies of medium-grained leucocratic biotite granite (1696 ± 11 Ma; deepest level), mediumgrained hornblende-biotite granodiorite (1722 ± 12 Ma), and coarse-grained porphyritic biotite granite (1725 ± 19 Ma; shallowest level). Penetrative ductile deformation has converted the granites to augen gneisses and caused isoclinal folding and transposition of primary contacts. Exposed in a belt of northwest-trending folds, these rocks preserve southwest-vergent shear fabric annealed during amphibolite facies
Geological Society of America Special Papers, 1989
The Colorado Proterozoic province is separated from Archean rocks of the Wyoming province by a ma... more The Colorado Proterozoic province is separated from Archean rocks of the Wyoming province by a major structural boundary, the Cheyenne belt. Proterozoic rocks south of the Cheyenne belt are exposed in the Sierra Madre, Medicine Bow Mountains, and Laramie Range of southern Wyoming. They consist of metavolcanic units, metagraywacke, pelitic schist and gneiss, amphibolite, and felsic to mafic intrusive rocks that locally resemble rocks of central Colorado. North of the Cheyenne belt, Archean granite and gneiss of the Wyoming craton are overlain by a Late Archean and Early Proterozoic supracrustal sequence that contains quartzite, metadolomite, phyllite, and subordinate metavolcanic rocks. The eugeoclinal character of the metamorphic rocks south of the Cheyenne belt contrasts sharply with the dominantly siliciclastic supracrustal rocks north of the Cheyenne belt. Although specific sequences south of the belt have not yet been correlated between the Sierra Madre, Medicine Bow Mountains, and Laramie Range, similarities in age, lithology, and major element chemistry suggest that they are part of a single geologic terrane. Macroscopic structure and microscopic kinematic indicators within the Cheyenne belt suggest that accretion of the Proterozoic rocks of northern Colorado to the Archean Wyoming craton was accomplished primarily by large-scale thrustiig. Following accretion of individual thrust blocks, the boundary zone was steepened by folding and reactivated locally during a period of strike-slip movement. Presence of similar lithologies and shear zones south of the Cheyenne belt suggests that the southern margin of the Wyoming craton may have been a long-lived zone of crustal accretion. ~STRODUCTION ary has since been recognized in the Sierra Madre to the west and may be present in the Richeau Hills to the east, across a total. A major structural boundary between the Archean Wyo-distance of over 200 km (Fig. 1). In the Laramie Range, the ml nL! craton to the north and the Proterozoic rocks of northern Middle Proterozoic Laramie Anorthosite and the Sherman Gran-Colorado t~ the south was first recognized in the Medicine Bow ite occupy the projected position of the boundary (Fig. I), but its ''ounbins in southeastern Wyoming by Houston and McCallum location is reasonably well constrained by Nd isotopic data (Geist 1. Originally named the Mullen Creek-Nash Fork shear and others, 1987). In recognition of its regional extent, structural a for exposures in the Medicine Bow Mountains, the bound-complexity, diverse character, and tectonic importance, the
The histories of the pre-Mesozoic landmasses in southern Mé xico and their connections with Laure... more The histories of the pre-Mesozoic landmasses in southern Mé xico and their connections with Laurentia, Gondwana, and among themselves are crucial for the understanding of the Late Paleozoic assembly of Pangea. The Permian igneous and metamorphic rocks from the Chiapas massif as part of the southern Maya block, Mé xico, were dated by U-Pb zircon geochronology employing the SHRIMP (sensitive high resolution ion microprobe) facility at Stanford University. The Chiapas massif is composed of deformed granitoids and orthogneisses with inliers of metasedimentary rocks. SHRIMP data from an anatectic orthogneiss demonstrate that the Chiapas massif was part of a Permian (~272 Ma) active continental margin established on the Pacific margin of Gondwana after the Ouachita orogeny. Latest Permian (252-254 Ma) medium-to high-grade metamorphism and deformation affected the entire Chiapas massif, resulting in anatexis and intrusion of syntectonic granitoids. This unique orogenic event is interpreted as the result of compression due to flat subduction and accretionary tectonics. SHRIMP data of zircon cores from a metapelite from the NE Chiapas massif yielded a single Grenvillian source for sediments. The majority of the zircon cores from a para-amphibolite from the SE part of the massif yielded either 1.0-1.2 or 1.4-1.5 Ga sources, indicating provenance from South American Sunsá s and Rondonian-San Ignacio provinces.
We present geochronologic and paleomagnetic data from a north-trending quartz diorite intrusion t... more We present geochronologic and paleomagnetic data from a north-trending quartz diorite intrusion that cuts Archean metasedimentary and metaigneous rocks of the South Pass Greenstone Belt of the Wyoming craton. The quartz diorite was previously thought to be either Archean or Early Proterozoic (?) in age and is cut by north and northeast-trending Proterozoic diabase dikes of uncertain age, for which we also report paleomagnetic data. New U-Pb analyses of baddeleyite and zircon from the quartz diorite yield a concordia upper intercept age of 2170 F 8 Ma (95% confidence). An 40 Ar/ 39 Ar amphibole date from the same sample yields a similar apparent age of about 2124 F 30 Ma (2r), thus confirming that the intrusion is Early Proterozoic in age and that it has probably not been thermally disturbed since emplacement. A magmatic event at ca. 2.17 Ga has not previously been documented in the Wyoming craton. The quartz diorite and one of the crosscutting diabase dikes yield essentially identical, well-defined characteristic remanent magnetizations. Results from eight sites in the quartz diorite yield an in situ mean direction of north declination and moderate to steep positive inclination (Dec. = 355j, Inc. = 65j, k = 145, a 95 = 5j) with a paleomagnetic pole at 84jN, 215jE (dm = 6j, dp = 7j). Data from other diabase dike sites are inconsistent with the quartz diorite results, but the importance of these results is uncertain because the age of the dikes is not well known. Interpretation of the quartz diorite remanent magnetization is problematic. The in situ direction is similar to expected directions for magnetizations of Late Cretaceous/early Tertiary age. However, there is no compelling evidence to suggest that these rocks were remagnetized during the late Mesozoic or Cenozoic. Assuming this magnetization to be primary, then the in situ paleomagnetic pole is strongly discordant with poles of 2167, 2214, and 2217 Ma from the Canadian Shield, and is consistent with proposed separation of the Wyoming Craton and Laurentia prior to about 1.8 Ga. Correcting the quartz diorite pole for the possible effects of Laramide-age tilting of the Wind River Range, based on the attitude of nearby overlying Cambrian Flathead Sandstone (dip = 20j, N20jE), gives a tilt corrected pole of 75jN, 58jE (dm = 4j, dp = 6j), which is also discordant with respect to time-equivalent poles from the Superior Province. Reconstruction of the Superior and Wyoming Province using a rotation similar to that proposed by Roscoe and Card [Can. J. Earth Sci. 46(1993)2475] is problematic, but reconstruction of the Superior and Wyoming Provinces based on restoring them to their correct paleolatitude and orientation using a closest approach fit indicates that the two cratons could have been adjacent at about 2.17 Ga prior to rifting at about 2.15 Ga. The paleomagnetic data presented are consistent with the hypothesis that the
The Anatolia (Eurasia), Arabia, and Africa tectonic plates intersect in southeast Turkey, near th... more The Anatolia (Eurasia), Arabia, and Africa tectonic plates intersect in southeast Turkey, near the Gulf of İskenderun, forming a tectonically active and unstable triple junction (the A3 triple junction). The plate boundaries are marked by broad zones of major, dominantly left-lateral transform faults including the East Anatolian fault zone (the Anatolia-Arabia boundary) and the Dead Sea fault zone (the Arabia-Africa boundary). Quaternary basalts occur locally within these “leaky” transform fault zones (similar to those observed within oceanic transform faults), providing evidence that mantle melting, basalt genesis, and eruption are linked to crustal deformation and faulting that extends into the upper mantle. We investigated samples of alkaline basalt (including basanite) from the Toprakkale and Karasu volcanic fields within a broad zone of transtension associated with these plate-boundary faults near the İskenderun and Amik Basins, respectively. Toprakkale basalts and basanites ha...
Surficial deposits in the Denver West quadrangle record alluvial, mass-movement, glacial, and eol... more Surficial deposits in the Denver West quadrangle record alluvial, mass-movement, glacial, and eolian processes in the central part of the Front Range and the western margin of the Colorado Piedmont during the Quaternary and late Neogene. Many of the surficial deposits are poorly exposed. Deposits that are of limited extent (less than about 200 m wide) were not mapped, including (1) fill material in urban areas, (2) mine-and-mill waste and dredge tailings in mining areas, (3) thin mass-movement deposits above present treeline (such as block fields and block streams), and (4) thin sheetwash deposits that locally mantle gently sloping map units. Descriptions of surficial units on this map are based chiefly on information in maps and reports by
Resumen Trabajos como el presentado en este artículo, que incluyen estudios básicos de cartografí... more Resumen Trabajos como el presentado en este artículo, que incluyen estudios básicos de cartografía, microtectónica, petrografía, geoquímica y geocronología de rocas de arco, permitirán iniciar una revisión geológica individual y colectiva de los arcos volcánicos que afloran en gran parte del territorio de México. La combinación de este tipo de esfuerzos permitirá proponer modelos de evolución geológica más precisos y menos especulativos para este importante proceso de generación de corteza mediante la acreción de arcos volcánicos (p. ej. terreno Guerrero). La geoquímica y petrología de las rocas metavolcaniclásticas (filitas), metalavas y metaignimbritas de Taxco y Taxco Viejo son muy similares y, en conjunto, permiten proponer un origen equivalente para su formación que, seguramente, está asociado a un ambiente de formación de tipo arco volcánico. En este estudio se describe por primera vez que la Roca Verde Taxco Viejo es, en realidad, una metaignimbrita. La deformación estudiada (macro-y microtectónica) de las rocas metavolcánicas de Taxco y Taxco Viejo es muy similar independientemente de la naturaleza del protolito. Estas rocas presentan planos de foliación muy bien desarrollados en conjunto que buzan tanto al E-NE como hacia el W-SW, lo cual sugiere la existencia de más de un evento de deformación regional. En este estudio proponemos que las edades 206 Pb/ 238 U obtenidas a partir del método SHRIMP-RG en cristales únicos de circón, para las muestras de Taxco (137.1 ± 0.9 Ma) y Taxco Viejo (135.6 ± 1.4 Ma), son más confiables que las obtenidas mediante la técnica convencional ID-TIMS (poblaciones multigrano), y representan las mejores estimaciones de la edad de cristalización para ambos protolitos volcánicos. Por último, destacamos que estas edades U-Pb resultan iguales, dentro de los límites de error asignados a cada muestra, por lo que sugerimos existe una conexión temporal entre el vulcanismo cretácico temprano presente en Taxco y en Taxco Viejo. Esta edad cretácica temprana para los arcos volcánicos metamorfoseados, que se discute a detalle en este trabajo, fue inicialmente presentada en una convención científica en 2004, y desde entonces significó terminar con la especulación clásica de posibles edades precámbrica y paleozoica para estas rocas conocidas formalmente como Esquisto Taxco y Roca Verde Taxco Viejo. Este avance sustancial del conocimiento permitió dar mayor certeza a la hipótesis de que estas rocas de arco son de edad cretácica y/o jurásica como lo sugerían las relaciones estratigráficas observadas en los ensambles vulcanosedimentarios con control fosilífero de lo que hoy se conoce como terreno Guerrero. Esta edad cretácica de los arcos-acompañada de la hipótesis, cada vez más influyente, de la posible evolución geológica alóctona de estos arcos volcánicos, que propondría la posterior acreción de los mismos a una zona continental (p. ej. Oaxaquia) en el Cretácico Tardío-presenta una clara alternativa a las ideas fijistas/autóctonas clásicas que sugirieron un basamento antiguo para el terreno Guerrero que implicaría un origen continental para este arco volcánico hoy representado por las rocas de Taxco y Taxco Viejo.
Descriptions of surficial units along the eastern flank of the Front Range on this map are based ... more Descriptions of surficial units along the eastern flank of the Front Range on this map are based on those of Scott (1963a), Trimble and Machette (1979), and Kellogg and others (2008). Within South Park, distribution and description of surficial units were derived from reports by Stark and others (1949), Widmann and others (2005), Kirkham and others (2006), and the authors' new mapping and field observations. Formal names for fluvial and pediment deposits are those established by Scott (1960, 1963a) in the Colorado Piedmont. South Park glacial, glaciofluvial, and alluvial deposits are based on inferred age relationships among geomorphic position in the landscape, soil development, and reports by Widmann and others (2005, 2006), and Kirkham and others (2006). Surficial units were mapped primarily by interpretation of 1:40,000-scale, blackand-white aerial photographs, USGS black-and-white digital ortho-photoquads (1-meter resolution), Landsat Thematic Mapper images, and field analyses of deposits and landforms. Age assignments for surficial deposits within the map area are based chiefly on the relative degree of modification of their original surface morphology, relative heights above present stream channels, and degree of soil development and clast weathering. Soil-horizon designations are based on those of the Soil Survey Staff (1999) and Birkeland (1999). Some of the surficial deposits contain pedogenic calcium carbonate. Stages of pedogenic calcium carbonate morphology (referred to as stages I through IV) in Bk and K soil horizons follow the classification from Gile and others (1966) and Machette (1985). Grain sizes within unit descriptions follow those of the Soil Survey Staff (1951) and are as follows: sand (0.05-2 mm), silt (0.002-0.05 mm), and clay (<0.002 mm). Grain sizes are limited to field estimates. The term "clasts" is defined as granules and larger particles (>2 mm in diameter, visible without magnification), and the term "matrix" refers to sand and finer particles (≤ 2 mm in diameter). Many of the surficial deposits are poorly exposed. Deposits that are of limited extent (commonly less than about 50 m wide) were not mapped, including 1) fill material in developed areas, 2) mine-and-mill waste and dredge tailings in mining areas, 3) thin mass-movement deposits above treeline (such as block fields and block streams), and 4) thin sheetwash deposits that locally mantle gently sloping map units. In addition to the relative age of deposits based on erosional, geomorphic, and weathering characteristics, we have inferred ages for some of the surficial units based on the marine oxygen isotope records of Lisiecki and Raymo (2005). These records reflect global ice volumes (for example, Shackleton and Opdyke, 1973, 1976). However, ice volume changes on continents may supercede oxygen isotope variation in marine cores by 500-3,000 years (Mix and Ruddiman, 1984).
New high-resolution chemostratigraphy, in combination with updated biostratigraphy, refines the c... more New high-resolution chemostratigraphy, in combination with updated biostratigraphy, refines the chronostratigraphic resolution of the nearly 2 km thick Ordovician section below Anticosti Island. A total of 1414 horizons, spaced at 1.0–1.5 m intervals, were sampled from the New Associated Consolidated Paper (NACP) drill core and analyzed for major and trace elemental composition by portable X-ray fluorescence analyzer (pXRF). Select micrite and calcareous shale powders were then analyzed for δ13Ccarb (number of samples, N = 364) and 87Sr/86Sr (N = 25). Our results indicate a Floian to early Darriwilian (F3–Dw1) age for the Romaine Formation, a middle Darriwilian to Sandbian (Dw2–Sa1) age for the Mingan Formation, a lower to mid-Katian (Ka1–Ka2) age for the Macasty Formation, an upper Katian (Ka3) age for the lower Vauréal Formation, an uppermost Katian (Ka4) age for the upper Vauréal, and a Hirnantian (H1) age for the Ellis Bay Formation. This integrated chemostratigraphic and biostr...
The lunar Pb isotopic evolution is explored based on an investigation of the U-Th-Pb systematics ... more The lunar Pb isotopic evolution is explored based on an investigation of the U-Th-Pb systematics of the primitive Apollo 16 anorthositic breccias 76075 and 62237. The isotope data for acid-leached residues from 67075 lie within or on a triangle formed by plagioclase, opaques, and a pyroxen-olivine mixture, and the isotope data for 62237 lie within or on a triangle formed by plagioclase, olivine, and pyroxene. The results suggest that the moon had high first-stage values for the Pb evolution equations from the onset.
U-Pb isotopic systematics from analyses of single zircons identify at least two provenance ages, ... more U-Pb isotopic systematics from analyses of single zircons identify at least two provenance ages, approximately 575 Ma and approximately 330 Ma, for zircons from the impact layer of the K-T boundary, Raton Basin, Colorado. These data are a preliminary confirmation of results reported from the same layer. The zircon provenance ages provide a unique signature for identification of the source crater since igneous rocks of these ages (or sedimentary rocks derived from them) must characterize part of the impact stratigraphy.
U-Pb data from lunar meteorites Yamato (Y)-86032 and Y-793274 confirm that they are of lunar orig... more U-Pb data from lunar meteorites Yamato (Y)-86032 and Y-793274 confirm that they are of lunar origin, but also indicate that they are not from the same source region on the Moon and experienced different events while residing as lunar regolith. The Pb of both clast and matrix from Y-86032 is the least ra diogenic among lunar meteorites thus far analyzed, similar to that of lunar an orthosite 60025, and indicates derivation from a low-23 �Uf 04 Pb (tt) source, assum ing initial lunar Pb compositions were essentially primitive in nature. Whereas the data from Y-86032 plot slightly above the geochron on a 2 onpb;20 4 pb vs. 20 7 Pb/ 204 Pb diagram, data from 60025 and Y-82192 (considered a pare of Y-86032) plot precisely on the geochron. Very small amounts of U, Th, and Pb were leached from Y-86032 using O.lN HBr; the Pb isotopic compositions of the leaches are essentially modern terrestrial values, indicating that this rock probably did not reside long at the lunar surface and was not exposed to volatile-rich gases expelled during impact event (s). In contrast, large amounts of the three elements (up to five times those of Y-86032) were removed from Y-793274 during leaching, and the Pb isotopic com positions of these leaches are very radiogenic and similar to those of mare basalt and some lunar soils, indicating that this rock was subjected to lunar impact related, volatile-rich gases containing radiogenic 200 Pb-rich Pb, very similar to typical lunar soils and some breccias containing significant amounts of mare derived material. This is the first occurrence of leaches that are more 205 Pb-rich than their corresponding residues and indicate that the meteorite may have origi nated from near a mare region. To compare, the Pb from leaches of Y-791197 clasts (N. NAKAMURA et al.; Mem. Natl Inst. Polar Res., Spec. Issue, 41, 106, 1986a) was 200 Pb-poor, indicating the meteorite may have originated from the lunar highlands. The Pb in residues of Y-793274 is more radiogenic than that of Y-86032 and Y-82192, similar to that in ALHA81005 and lunar anorthositic gabbros 78155 and 15418, and indicate a lunar source region with high-p (> 100 and possibly as high as 300). Corrected U-Pb data indicate a formation age of-4.4 Ga and a disturbance to the system at "'4.0 Ga.
We have undertaken U-Th-Pb isotopic studies on lherzolitic shergot tite, Yamato-793605. Four mine... more We have undertaken U-Th-Pb isotopic studies on lherzolitic shergot tite, Yamato-793605. Four mineral separates (olivine, plagioclase, and two phases of pyroxene) and a whole-rock sample were leached with dilute acid in order to remove secondary Pb contamination. However, preferential leaching of U and Th over Pb occurred. The Pb isotopic data points of five residue fractions scattered, and a calculated Pb-Pb age does not clearly indicate either crystallization from a melt or a later disturbance to the U-Pb system, in contrast with other shergottites. The chord through U-Pb data points for pyroxene (PX 1) leaches and residue inter sects concordia at 4439 ± 9 Ma and 212 ± 62 Ma, suggesting a young disturbance event resulting in Pb loss. The later disturbance event partially reset the U-Pb sys tem. The estimated 238U/2°4Pb (= µ) value from the whole-rock residue for the source of Y-793605 is about 5, suggesting that Y-793605 was derived from a volatile-rich source environment and confirms a low-µ source for shergottites compared to vol canic rocks of the Earth.
The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives, 2005
We utilize new geological mapping, conventional isotope dilution-thermal ionization mass spectrom... more We utilize new geological mapping, conventional isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses, and whole-rock radiogenic isotope characteristics to distinguish two contrasting Proterozoic basement complexes in the international border region southeast of Yuma, Arizona. Strategically located near the truncated southwest margin of Laurentia, these Proterozoic exposures are separated by a northwest-striking Late Cretaceous batholith. Although both complexes contain strongly deformed Paleoproterozoic granitoids (augen gneisses) intruded into fi ne-grained host rocks, our work demonstrates marked differences in age, host rock composition, and structure between the two areas. The Western Complex reveals a >5-km-thick tilted section of fi nely banded felsic, intermediate, and mafi c orthogneiss interspersed with tabular intrusive bodies of medium-grained leucocratic biotite granite (1696 ± 11 Ma; deepest level), mediumgrained hornblende-biotite granodiorite (1722 ± 12 Ma), and coarse-grained porphyritic biotite granite (1725 ± 19 Ma; shallowest level). Penetrative ductile deformation has converted the granites to augen gneisses and caused isoclinal folding and transposition of primary contacts. Exposed in a belt of northwest-trending folds, these rocks preserve southwest-vergent shear fabric annealed during amphibolite facies
Geological Society of America Special Papers, 1989
The Colorado Proterozoic province is separated from Archean rocks of the Wyoming province by a ma... more The Colorado Proterozoic province is separated from Archean rocks of the Wyoming province by a major structural boundary, the Cheyenne belt. Proterozoic rocks south of the Cheyenne belt are exposed in the Sierra Madre, Medicine Bow Mountains, and Laramie Range of southern Wyoming. They consist of metavolcanic units, metagraywacke, pelitic schist and gneiss, amphibolite, and felsic to mafic intrusive rocks that locally resemble rocks of central Colorado. North of the Cheyenne belt, Archean granite and gneiss of the Wyoming craton are overlain by a Late Archean and Early Proterozoic supracrustal sequence that contains quartzite, metadolomite, phyllite, and subordinate metavolcanic rocks. The eugeoclinal character of the metamorphic rocks south of the Cheyenne belt contrasts sharply with the dominantly siliciclastic supracrustal rocks north of the Cheyenne belt. Although specific sequences south of the belt have not yet been correlated between the Sierra Madre, Medicine Bow Mountains, and Laramie Range, similarities in age, lithology, and major element chemistry suggest that they are part of a single geologic terrane. Macroscopic structure and microscopic kinematic indicators within the Cheyenne belt suggest that accretion of the Proterozoic rocks of northern Colorado to the Archean Wyoming craton was accomplished primarily by large-scale thrustiig. Following accretion of individual thrust blocks, the boundary zone was steepened by folding and reactivated locally during a period of strike-slip movement. Presence of similar lithologies and shear zones south of the Cheyenne belt suggests that the southern margin of the Wyoming craton may have been a long-lived zone of crustal accretion. ~STRODUCTION ary has since been recognized in the Sierra Madre to the west and may be present in the Richeau Hills to the east, across a total. A major structural boundary between the Archean Wyo-distance of over 200 km (Fig. 1). In the Laramie Range, the ml nL! craton to the north and the Proterozoic rocks of northern Middle Proterozoic Laramie Anorthosite and the Sherman Gran-Colorado t~ the south was first recognized in the Medicine Bow ite occupy the projected position of the boundary (Fig. I), but its ''ounbins in southeastern Wyoming by Houston and McCallum location is reasonably well constrained by Nd isotopic data (Geist 1. Originally named the Mullen Creek-Nash Fork shear and others, 1987). In recognition of its regional extent, structural a for exposures in the Medicine Bow Mountains, the bound-complexity, diverse character, and tectonic importance, the
The histories of the pre-Mesozoic landmasses in southern Mé xico and their connections with Laure... more The histories of the pre-Mesozoic landmasses in southern Mé xico and their connections with Laurentia, Gondwana, and among themselves are crucial for the understanding of the Late Paleozoic assembly of Pangea. The Permian igneous and metamorphic rocks from the Chiapas massif as part of the southern Maya block, Mé xico, were dated by U-Pb zircon geochronology employing the SHRIMP (sensitive high resolution ion microprobe) facility at Stanford University. The Chiapas massif is composed of deformed granitoids and orthogneisses with inliers of metasedimentary rocks. SHRIMP data from an anatectic orthogneiss demonstrate that the Chiapas massif was part of a Permian (~272 Ma) active continental margin established on the Pacific margin of Gondwana after the Ouachita orogeny. Latest Permian (252-254 Ma) medium-to high-grade metamorphism and deformation affected the entire Chiapas massif, resulting in anatexis and intrusion of syntectonic granitoids. This unique orogenic event is interpreted as the result of compression due to flat subduction and accretionary tectonics. SHRIMP data of zircon cores from a metapelite from the NE Chiapas massif yielded a single Grenvillian source for sediments. The majority of the zircon cores from a para-amphibolite from the SE part of the massif yielded either 1.0-1.2 or 1.4-1.5 Ga sources, indicating provenance from South American Sunsá s and Rondonian-San Ignacio provinces.
We present geochronologic and paleomagnetic data from a north-trending quartz diorite intrusion t... more We present geochronologic and paleomagnetic data from a north-trending quartz diorite intrusion that cuts Archean metasedimentary and metaigneous rocks of the South Pass Greenstone Belt of the Wyoming craton. The quartz diorite was previously thought to be either Archean or Early Proterozoic (?) in age and is cut by north and northeast-trending Proterozoic diabase dikes of uncertain age, for which we also report paleomagnetic data. New U-Pb analyses of baddeleyite and zircon from the quartz diorite yield a concordia upper intercept age of 2170 F 8 Ma (95% confidence). An 40 Ar/ 39 Ar amphibole date from the same sample yields a similar apparent age of about 2124 F 30 Ma (2r), thus confirming that the intrusion is Early Proterozoic in age and that it has probably not been thermally disturbed since emplacement. A magmatic event at ca. 2.17 Ga has not previously been documented in the Wyoming craton. The quartz diorite and one of the crosscutting diabase dikes yield essentially identical, well-defined characteristic remanent magnetizations. Results from eight sites in the quartz diorite yield an in situ mean direction of north declination and moderate to steep positive inclination (Dec. = 355j, Inc. = 65j, k = 145, a 95 = 5j) with a paleomagnetic pole at 84jN, 215jE (dm = 6j, dp = 7j). Data from other diabase dike sites are inconsistent with the quartz diorite results, but the importance of these results is uncertain because the age of the dikes is not well known. Interpretation of the quartz diorite remanent magnetization is problematic. The in situ direction is similar to expected directions for magnetizations of Late Cretaceous/early Tertiary age. However, there is no compelling evidence to suggest that these rocks were remagnetized during the late Mesozoic or Cenozoic. Assuming this magnetization to be primary, then the in situ paleomagnetic pole is strongly discordant with poles of 2167, 2214, and 2217 Ma from the Canadian Shield, and is consistent with proposed separation of the Wyoming Craton and Laurentia prior to about 1.8 Ga. Correcting the quartz diorite pole for the possible effects of Laramide-age tilting of the Wind River Range, based on the attitude of nearby overlying Cambrian Flathead Sandstone (dip = 20j, N20jE), gives a tilt corrected pole of 75jN, 58jE (dm = 4j, dp = 6j), which is also discordant with respect to time-equivalent poles from the Superior Province. Reconstruction of the Superior and Wyoming Province using a rotation similar to that proposed by Roscoe and Card [Can. J. Earth Sci. 46(1993)2475] is problematic, but reconstruction of the Superior and Wyoming Provinces based on restoring them to their correct paleolatitude and orientation using a closest approach fit indicates that the two cratons could have been adjacent at about 2.17 Ga prior to rifting at about 2.15 Ga. The paleomagnetic data presented are consistent with the hypothesis that the
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