Wet Mountains Research Articles

Aiouea ombrophila (Lauraceae), a new large tree from the wet mountains of the Atlantic Rainforest, Brazil

The study of tree species in the Brazilian Atlantic Rainforest continues to reveal new discoveries. Herbarium collections often harbor specimens that do not fit into known taxa. During our research, we encountered an unidentified Lauraceae specimen, prompting us to conduct fieldwork in the forest to investigate further. This study describes Aiouea ombrophila Brotto & Völtz, a large tree from the rainiest parts of the mountainous slopes of Minas Gerais, Rio de Janeiro and São Paulo states, Brazil, at elevations between 950 and 2000 m. We provide a comprehensive description of the new species, including illustrations, a distribution map, an extinction risk assessment, and a comparative table distinguishing it from other species.

Distribution of mosquito species in various agro-ecological zones of Punjab

BackgroundThe presence and behavior of mosquitoes in various agro-ecological zones of Punjab are influenced by a combination of seasonal and topographical patterns, cropping schemes, and human settlement. These modifications have a significant impact on the occurrence of diseases. Therefore, determining the distribution of these species could help in halting their spread and disease outbreaks. MethodsUnder these prevailing conditions, surveillance studies were carried out at six zones of Punjab, to know the distribution of mosquitoes. Samples were collected in various seasons (once in winter and rainy seasons, and twice in summer), from all possible habitats, having stagnant water. ResultsOut of twenty-four (24) recorded species, three were included in Anophelinae and Culicinae, eleven from tribe Aedini, eight from Culicini and one each from Ficulbiini and Mansoniini. Sampling was done from 91 m (Ahmed Pur East) elevation to 1759 m (Fort Monroe). High and diversified population was recorded during rainy season. The Anopheline mosquitoes were found in rural as well as urban habitats, including rainfed, wet mountains and irrigated plains whereas Aedine species were confined to northern irrigated plains and mostly recorded from Changa Manga National Forest. Culicine species were more diversified and abundant, in all seasons. However, M. chamberlaini was collected from urban (rainfed lands) and rural (northern irrigated plains) settings. C. crassipes was recorded only from rainfed lands. ConclusionTwenty-four mosquito species from the Anophelinae and Culicinae subfamilies were observed in the current investigation. Anophelinae subfamily contained three species. Eleven species of Culicinae belonged to the tribe Aedini, eight to the Culicini, and one each to the Mansoniini and Ficalbiini.

Ediacaran‐Ordovician Magmatism and REE Mineralization in the Wet Mountains, Colorado, USA: Implications for Failed Continental Rifting

AbstractStructures associated with Ediacaran‐Ordovician alkaline magmatism and the timing of rare earth element (REE) mineralization in the Wet Mountains, CO, were analyzed using field, geophysical, and U‐Th‐Pb isotope methods to interpret their tectonic setting in the context of previously proposed rift models. The Wet Mountains are known for thorium and REE mineralization associated with failed rift‐related, Ediacaran‐Ordovician alkaline intrusions and veins. Structural field data indicate that alkaline dikes and mineralized veins are controlled by a system of northwest‐striking, high‐angle faults and tension fractures formed in a 040°‐directed extensional regime. Magnetic and surface expressions of Democrat Creek and McClure Mountain complexes show tectonic elongation toward ∼045°, consistent with NE‐directed extension. Magnetic data also suggest the existence of a fourth, previously unrecognized mafic‐ultramafic complex of inferred Cambrian age with a similar elongated orientation. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS) 208Pb/232Th analysis of low‐uranium zircon from carbonatite dikes and in situ 206Pb/238U LA‐ICP‐MS analysis of monazite in mineralized dikes yielded 465 ± 18 Ma and 489 ± 33 Ma ages, respectively. These ages are consistent with the expected age based on slightly older, cross‐cut syenite dikes and the hypothesized Ordovician end to failed rift‐related magmatism. The Ediacaran‐Ordovician age of alkaline magmatic rocks and the associated northeast‐directed extension direction are similar to those of the along‐strike, Ediacaran‐Cambrian Southern Oklahoma Aulacogen. Therefore, the failed rift system in the Wet Mountains is interpreted to be a northwestern continuation of the Southern Oklahoma Aulacogen with carbonatite magmatism and thorium/REE mineralization representing late intrusive phases.

On the occurrence of amphibolite-facies sapphirine, spinel, phlogopite, anorthite, and corundum in the Wet Mountains, Colorado, USA

On the occurrence of amphibolite-facies sapphirine, spinel, phlogopite, anorthite, and corundum in the Wet Mountains, Colorado, USA

Observed Changes in Crop Yield Associated with Droughts Propagation via Natural and Human-Disturbed Agro-Ecological Zones of Pakistan

Pakistan’s agriculture and food production account for 27% of its overall gross domestic product (GDP). Despite ongoing advances in technology and crop varieties, an imbalance between water availability and demand, combined with robust shifts in drought propagation has negatively affected the agro-ecosystem and environmental conditions. In this study, we examined hydro-meteorological drought propagation and its associated impacts on crop yield across natural and human-disturbed agro-ecological zones (AEZs) in Pakistan. Multisource datasets (i.e., ground observations, reanalysis, and satellites) were used to characterize the most extensive, intense drought episodes from 1981 to 2018 based on the standardized precipitation evaporation index (SPEI), standardized streamflow index (SSFI), standardized surface water storage index (SSWSI), and standardized groundwater storage index (SGWI). The most common and intense drought episodes characterized by SPEI, SSFI, SSWSI, and SGWI were observed in years 1981–1983, 2000–2003, 2005, and 2018. SPEI yielded the maximum number of drought months (90) followed by SSFI (85), SSWSI (75), and SGWI (35). Droughts were frequently longer and had a slower termination rate in the human-disturbed AEZs (e.g., North Irrigated Plain and South Irrigated Plain) compared to natural zones (e.g., Wet Mountains and Northern Dry Mountains). The historical droughts are likely caused by the anomalous large-scale patterns of geopotential height, near-surface air temperature, total precipitation, and prevailing soil moisture conditions. The negative values (<−2) of standardized drought severity index (DSI) observed during the drought episodes (1988, 2000, and 2002) indicated a decline in vegetation growth and yield of major crops such as sugarcane, maize, wheat, cotton, and rice. A large number of low-yield years (SYRI ≤ −1.5) were recorded for sugarcane and maize (10 years), followed by rice (9 years), wheat (8 years), and cotton (6 years). Maximum crop yield reductions relative to the historic mean (1981–2017) were recorded in 1983 (38% for cotton), 1985 (51% for maize), 1999 (15% for wheat), 2000 (29% for cotton), 2001 (37% for rice), 2002 (21% for rice), and 2004 (32% for maize). The percentage yield losses associated with shifts in SSFI and SSWSI were greater than those in SPEI, likely due to longer drought termination duration and a slower termination rate in the human-disturbed AEZs. The study’s findings will assist policymakers to adopt sustainable agricultural and water management practices, and make climate change adaptation plans to mitigate drought impacts in the study region.

Detrital zircon geochronology and provenance of the Middle to Late Jurassic Paradox Basin and Central Colorado trough: Paleogeographic implications for southwestern Laurentia

AbstractMiddle to Upper Jurassic strata in the Paradox Basin and Central Colorado trough (CCT; southwestern United States) record a pronounced change in sediment dispersal from dominantly aeolian deposition with an Appalachian source (Entrada Sandstone) to dominantly fluvial deposition with a source in the Mogollon and/or Sevier orogenic highlands (Salt Wash Member of the Morrison Formation). An enigmatic abundance of Cambrian (ca. 527–519 Ma) grains at this provenance transition in the CCT at Escalante Canyon, Colorado, was recently suggested to reflect a local sediment source from the Ancestral Front Range, despite previous interpretations that local basement uplifts were largely buried by Middle to Late Jurassic time.This study aims to delineate spatial and temporal patterns in provenance of these Jurassic sandstones containing Cambrian grains within the Paradox Basin and CCT using sandstone petrography, detrital zircon U-Pb geochronology, and detrital zircon trace elemental and rare-earth elemental (REE) geochemistry. We report 7887 new U-Pb detrital zircon analyses from 31 sandstone samples collected within seven transects in western Colorado and eastern Utah. Three clusters of zircon ages are consistently present (1.53–1.3 Ga, 1.3–0.9 Ga, and 500–300 Ma) that are interpreted to reflect sources associated with the Appalachian orogen in southeastern Laurentia (mid-continent, Grenville, Appalachian, and peri-Gondwanan terranes). Ca. 540–500 Ma zircon grains are anomalously abundant locally in the uppermost Entrada Sandstone and Wanakah Formation but are either lacking or present in small fractions in the overlying Salt Wash and Tidwell Members of the Morrison Formation. A comparison of zircon REE geochemistry between Cambrian detrital zircon and igneous zircon from potential sources shows that these 540–500 Ma detrital zircon are primarily magmatic. Although variability in both detrital and igneous REE concentrations precludes definitive identification of provenance, several considerations suggest that distal sources from the Cambrian granitic and rhyolitic provinces of the Southern Oklahoma aulacogen is also likely, in addition to a proximal source identified in the McClure Mountain syenite of the Wet Mountains, Colorado. The abundance of Cambrian grains in samples from the central CCT, particularly in the Entrada Sandstone and Wanakah Formation, suggests northwesterly sediment transport within the CCT, with sediment sourced from Ancestral Rocky Mountains uplifts of the southern Wet Mountains and/or Amarillo-Wichita Mountains in southwestern Oklahoma. The lack of Cambrian grains within the Paradox Basin suggests that the Uncompahgre uplift (southwestern Colorado) acted as a barrier to sediment transport from the CCT.

Subgrain Boundaries as Sites of Melt Generation and Pathways for Melt Flow: Implications for Rheology From Combined Chemical and Quantitative Orientation Analysis

AbstractWe report a detailed petrographic, compositional, structural, and crystallographic investigation into the partial melting process. Migmatitic samples that record biotite‐dehydration melting in the Wet Mountains of central Colorado preserve textural evidence for partial melting locations and an incipient former melt network. Detailed analysis of partial melting locations was conducted using a combined energy dispersive X‐ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) approach. Cuspate‐shaped films of former melt, now plagioclase, with low apparent dihedral angles and adjacent to rounded and corroded reactant minerals are interpreted as former locations of partial melting. We identified former melt films infiltrating and melting along quartz grains using optical microscopy and EDS and determined crystallographic misorientation across these transects using EBSD. Forty‐two percent of these transects are associated with subgrain boundaries, and the remaining transects are between adjacent quartz grains with various misorientations. Although previous work has emphasized grain boundaries and triple junctions as sites for melting, the presence of over 40% of the former melt films developed along subgrain boundaries suggests that these are favorable and potentially preferred locations for partial melting. Consequently, melting along subgrain boundaries must be integral to the evolution of partial melt networks and will increase melt network interconnectedness and generate a finer grain size, enhancing permeability and leading to focused melt flow. Given that these factors exert a fundamental control on crustal rheology, the role of melting and migration along subgrain boundaries should be investigated within diverse crustal and mantle lithologies and incorporated into models of partial melting.

The early Paleogene stratigraphic evolution of the Huerfano Basin, Colorado

ABSTRACTSedimentary basins throughout the North American Western Interior contain a record of Late Cretaceous through Eocene deposition related to the Laramide orogeny. The typical stratigraphic progression includes an uppermost Cretaceous fluvio-deltaic geologic formation that is unconformably overlain by an alluvial or paludal Paleocene geologic formation. The Paleocene unit is usually characterized by drab overbank facies, and overlain by an interval of amalgamated fluvial sand bodies. The overlying Eocene geologic units are characterized by red bed overbank facies. These major stratigraphic changes have been variably linked to long-wavelength dynamic subsidence, local uplift, and climatic shifts. Herein, we evaluate the depositional history of the Huerfano Basin of south-central Colorado in this overarching context. Our study presents a detailed lithofacies analysis of the Poison Canyon, Cuchara, and Huerfano Formations integrated with a new bulk (1) organic carbon isotope record, n = 299 measurements (Data Supplement 1A); and (2) magnetic record, n = 247 measurements (Data Supplement 1B). We interpret that the Paleocene Poison Canyon Formation was deposited by a braided or coarse-grained meandering river system with relatively poorly drained floodplains. The Eocene Huerfano Formation was likely deposited by a coarse-grained meandering river system with a comparatively well-drained floodplain. This pattern mirrors other Laramide basins, and is likely related to a regional drying pattern linked to long-term warming during the early Paleogene. Age of the intervening Cuchara Formation is poorly resolved, but is an anomalously thick and coarse-grained fluvial unit, with evidence for extensive reworking of floodplain deposits and a moderate coarsening-upward pattern. The Cuchara Formation is associated with magnetic trends that suggest greater oxidation and weathering, and greater variability in rainfall patterns, as well as a subtle negative shift in carbon isotope values. This pattern indicates a period of widespread progradation within the basin, potentially related to a major Laramide uplift event that affected Colorado’s Wet Mountains, Front Range, and Sangre de Cristo Mountains.

Effects of melt loss, melt retention, and protolith composition on differentiation of anatectic metapelites: A case study of the Wet Mountains, Colorado

Two metasedimentary migmatite samples from the same outcrop in the northern margin of the Oak Creek pluton in the Wet Mountains, Colorado, have distinct mineralogy, textures and major and trace element geochemistry. Sample W6, a garnet-biotite migmatite, has trace element characteristics consistent with a sedimentary protolith, this sample is also rich in silica and lacks sillimanite and cordierite. Sample W6A, a garnet-sillimanite-cordierite migmatite, has strong heavy rare earth element (HREE) enrichment, low silica, and contains abundant aluminum-rich minerals. Both rocks are relatively oxidized, consistent with sedimentary protoliths for both. Based on textural and geochemical evidence, we interpret the differences in mineralogy between the two samples to be the result of differing degrees of melt extraction and variations in protolith composition. Such differences were assessed using isocon analysis and calculated P-T phase diagrams. Melt retention in sample W6 resulted in the absence of sillimanite and cordierite, and back-reaction between minerals and melt in the peak assemblage. By contrast, melt extraction in W6A left garnet in the residuum, and imparted a strong heavy rare earth element enrichment on the sample, allowing preservation of the peak mineral assemblage. Consequently, W6A records peak conditions of 743 °C and ~7.6 kbar, whereas W6 yields lower P-T estimates that may represent the prograde and retrograde metamorphic paths. Our results indicate that melt extraction can be heterogeneous even at outcrop scale, and differing mineralogy may reflect variable proportions of melt loss and local variations in the composition of the protolith. Moreover, at least at small-scale, migmatization of iron-rich protoliths is a plausible mechanism for the production of ferroan, alkaline granitic magmas.

Protracted magmatism and magnetization around the McClure Mountain alkaline igneous complex

AbstractThe McClure Mountain–Iron Mountain igneous complex is an alkalic intrusive center in the northern Wet Mountains of southern Colorado. It was emplaced in early Cambrian time into gneissic/granitic 1.75–1.45 Ga Proterozoic host rocks. Numerous dikes are associated with the complex, primarily along the western side. Although the main intrusive nepheline-syenite body is well dated, the ages of the surrounding dikes are poorly known. Crosscutting relationships and poorly defined K-Ar dates suggest that the dikes are younger than the main intrusion. Paleomagnetic samples were collected from dikes associated with the McClure Mountain igneous complex. Geochronologic samples were also collected from two dikes sampled for their paleomagnetism. We obtained U-Pb zircon ages of 526 ± 8 Ma for a lamprophyric extracomplex dike and 483 ± 2 Ma for a trachytic extracomplex dike. These ages suggest either multistage or protracted dike intrusion around the ca. 524 Ma McClure Mountain complex. Our paleomagnetic data are consistent with previously published results. Dikes of the complex primarily exhibit southeast and shallow paleomagnetic directions, with variable declinations. Results from several baked contact tests indicate that the magnetizations are secondary. A steeply inclined magnetization is pervasive and was acquired over a protracted interval from late Laramide time to the present day.

Distinguishing slow cooling versus multiphase cooling and heating in zircon and apatite (U-Th)/He datasets: The case of the McClure Mountain syenite standard

New zircon, titanite, and apatite (U-Th)/He (ZHe, THe, AHe) data for four samples of the 524 Ma McClure Mountain syenite (MMS) in the Wet Mountains of Colorado constrain the <200 °C thermal history of this 40Ar/39Ar geochronology standard. ZHe dates vary from 420 to 567 Ma, do not correlate with their limited eU range (66–388 ppm), have dispersion attributable to eU zonation, and show no systematic variation between samples. THe dates are 488 to 515 Ma. AHe dates range from 70 to 211 Ma, with some inter-sample variability that could be due to modest post-70 Ma displacements along faults. Our dataset is consistent with recently published (U-Th)/He data for the MMS, although the published ZHe results extend to higher eU and define a negative date-eU correlation not captured by our lower eU zircons. Existing U-Pb and 40Ar/39Ar thermochronologic data, together with the THe and ZHe results, document rapid post-emplacement cooling of the MMS to ≤200 °C by ~500 Ma. A recent (U-Th)/He study inferred that the MMS subsequently underwent simple slow monotonic cooling until present-day. However, geologic relationships are incompatible with such a history, instead requiring that rocks in the vicinity of the MMS were at the surface at ~480 Ma, ~180 Ma, and ~60 Ma, with possible phases of burial and erosion of varying magnitude between these times. Inverse thermal history modeling can simultaneously satisfy the new and previously published ZHe and AHe data while honoring these geologic constraints. The outcomes of this work highlight that date-eU patterns can be generated by protracted cooling as well as by multiphase heating and cooling histories, and underscore the importance of integrating geologic data to help identify the most probable explanation for a (U-Th)/He dataset.

Provenance of Lower Paleogene Strata In the Huerfano Basin: Implications For Uplift of the Wet Mountains, Colorado, U.S.A.

Abstract: Sedimentary basins throughout the western interior of North America preserve a record of Late Cretaceous through latest Eocene sedimentation derived from flanking Laramide uplifts and Sevier hinterland. In northern and western basins, the strata contain a well-documented record of Laramide-style exhumation. However, the tectonic histories of more southerly basins, such as the Huerfano Basin studied herein, are comparatively poorly known despite being key for understanding the spatiotemporal evolution of Laramide tectonism. In this study we address this issue with a comprehensive analysis of provenance for the Poison Canyon, Cuchara, and Huerfano formations in the Huerfano Basin (south-central, Colorado, U.S.A.). As part of our provenance analysis, we characterized gravel clast composition, petrographic compositions of sandstone bodies (N = 31 thin sections), and U–Pb detrital-zircon age spectra (n = 848 age determinations) from fluvial sandstone units in each of the three formations. The results are used to construct a new unroofing and source history for the sediments in the basin that contradicts previous hypotheses. Diagnostic zircon peaks at 516–517 Ma, 1423–1430 Ma, and 1678–1687 Ma show that sediment delivered to the Huerfano Basin did not originate in the San Luis Highlands or the incipient Sangre de Cristo Mountains, but from the Precambrian crystalline core and associated Cambrian plutons of the Wet Mountains, which were exposed by the time Laramide deposition began in the basin. There are no detected provenance shifts upsection, indicating a largely stable or lithologically uniform sediment source from the Paleocene through at least ∼ 51 Ma. This suggests that the major changes recorded in the rock record were more likely related to variations in tectonic and climatic conditions rather than changes in lithology of the rocks being eroded.

The role of subgrain boundaries in partial melting

Evidence for partial melting along subgrain boundaries in quartz and plagioclase is documented for rocks from the Lost Creek Gneiss of the Llano Uplift, central Texas, the Wet Mountains of central Colorado, and the Albany-Fraser Orogen, southwestern Australia. Domains of quartz or plagioclase crystals along subgrain boundaries are preferentially involved in partial melting over unstrained domains of these minerals. Material along subgrain boundaries in quartz and plagioclase has the same morphology as melt pseudomorphs present along grain boundaries and is commonly laterally continuous with this former grain boundary melt, indicating the material along subgrain boundaries can also be categorized as a melt pseudomorph. Subgrain boundaries consist of arrays of dislocations within a crystal lattice, and unlike fractures would not act as conduits for melt migration. Instead, the presence of former melt along subgrain boundaries requires that partial melting occurred in these locations because it is kinetically more favorable for melting reactions to occur there. Preferential melting in high strain locations may be attributed to strain energy, which provides a minor energetic contribution to the reaction and leads to preferential melting in locations with weakened bonds, and/or the presence of small quantities of water associated with dislocations, which may enhance diffusion rates or locally lower the temperature needed for partial melting.

Human Arsenic exposure via dust across the different ecological zones of Pakistan

The present study aims to assess the arsenic (As) levels into dust samples and its implications for human health, of four ecological zones of Pakistan, which included northern frozen mountains (FMZ), lower Himalyian wet mountains (WMZ), alluvial riverine plains (ARZ), and low lying agricultural areas (LLZ). Human nail samples (N=180) of general population were also collected from the similar areas and all the samples were analysed by using ICP-MS. In general the higher levels (p<0.05) in paired dust and human nail samples were observed from ARZ and LLZ than those of other mountainous areas (i.e., WMZ and FMZ), respectively. Current results suggested that elevated As concentrations were associated to both natural, (e.g. geogenic influences) and anthropogenic sources. Linear regression model values indicated that As levels into dust samples were associated with altitude (r2=0.23), soil carbonate carbon density (SCC; r2=0.33), and population density (PD; r2=0.25). The relationship of paired dust and nail samples was also investigated and associations were found for As-nail and soil organic carbon density (SOC; r2=0.49) and SCC (r2=0.19) in each studied zone, evidencing the dust exposure as an important source of arsenic contamination in Pakistan. Risk estimation reflected higher hazard index (HI) values of non-carcinogenic risk (HI>1) for children populations in all areas (except FMZ), and for adults in LLZ (0.74) and ARZ (0.55), suggesting that caution should be paid about the dust exposure. Similarly, carcinogenic risk assessment also highlighted potential threats to the residents of LLZ and ARZ, as in few cases (5–10%) the values exceeded the range of US-EPA threshold limits (10−6–10−4).

The Rocky Mountain Front, southwestern USA

The Rocky Mountain Front (RMF) trends north-south near long 105°W for ∼1500 km from near the U.S.-Mexico border to southern Wyoming. This long, straight, persistent structural boundary originated between 1.4 and 1.1 Ga in the Mesoproterozoic. It cuts the 1.4 Ga Granite-Rhyolite Province and was intruded by the shallow-level alkaline granitic batholith of Pikes Peak (1.09 Ga) in central Colorado. The RMF began as a boundary between thick cratonic lithosphere to the east (modern coordinates) and an orogenic plateau to the west and remains so today. It was reactivated during the 1.1 to 0.6 Ga breakup of the supercontinent Rodinia and during deformation associated with formation of both the Ancestral and Laramide Rocky Mountains. Its persistence as a cratonic boundary is also indicated by emplacement of alkalic igneous rocks, gold-telluride deposits, and other features that point to thick lithosphere, low heat flow, and episodic mantle magmatism from 1.1 Ga to the Neogene. Both rollback of the Farallon flat slab ca. 37 Ma and initiation of the Rio Grande Rift shortly thereafter began near the RMF. Geomorphic expression of the RMF was enhanced during the late Miocene to Holocene (ca. 6–0 Ma) by tectonic uplift and increased monsoonal precipitation that caused differential erosion along the mountain front, exhuming an imposing 0.5–1.2 km escarpment, bordered by hogbacks of Phanerozoic strata and incised by major river canyons. Here we investigate four right-stepping deflections of the RMF that developed during the Laramide orogeny and may reveal timing and structural style. The Sangre de Cristo Range to Wet Mountains and Wet Mountains to Front Range steps are related to reactivation of the eroded stumps of Ancestral Rocky Mountain uplifts. In northern Colorado, the Colorado Mineral Belt (CMB) ends at the RMF; no significant northeast-trending faults cross the Front Range–Denver Basin boundary. However, several features changed from south to north across the CMB. (1) The axis of the Denver Basin was deflected ∼60 km to the northeast. (2) The trend of the RMF changed from north–northwest to north. (3) Structural style of the Front Range–Denver Basin margin changed from northeast-vergent thrusts to northeast-dipping, high-angle reverse faults. (4) Early Laramide uplift north of the CMB was accompanied by southeastward slumping and decollement faulting of upper Cretaceous sedimentary units. (5) The Boulder-Weld coal field developed within the zone of decollement faulting. (6) The huge Wattenberg gas field formed over a paleogeothermal anomaly. (7) Apatite fission track (AFT) cooling ages in the Front Range north of the CMB are almost all associated with Laramide deformation (ca. 80–40 Ma), whereas south of the CMB, AFT ages in the Front Range and Wet Mountains vary widely (ca. 449–30 Ma). Proterozoic rocks still retain pre-Laramide AFT ages in a zone as much 1200 m thick south of the CMB, revealing comparatively modest uplift and erosion. A fourth step is a ∼250 km deflection of the RMF from the Laramie Range to the Black Hills of South Dakota along the southeastern boundary of the Wyoming Archean province. Laramide synorogenic sedimentation occurred mainly in Paleocene and early Eocene time on both sides of the Front Range in Colorado, but the timing and style of basin-margin thrusting differed markedly. Moderate- to high-angle thrusts and reverse faults characterized the east side beginning in the Maastrichtian (ca. 68 Ma). On the west side, low-angle thrusts overrode the Middle Park and South Park basins by 10–15 km beginning in the latest Paleocene–early Eocene. This later contraction correlates temporally with the third major episode of shortening in the Sevier fold and thrust belt, when the Hogsback thrust added ∼21 km of shortening to become the easternmost major thrust in southwest Wyoming and northern Utah. A remarkable attribute of the RMF is that it maintained its position through multiple orogenies and changes in orientation and strength of tectonic stresses. During the Laramide orogeny, the RMF marked a tectonic boundary beyond which major contractional partitioning of the Cordilleran foreland was unable to penetrate. However, the nature of the lithospheric flaw that underlies the RMF is an unanswered question.

Relationship between syndeformational partial melting and crustal-scale magmatism and tectonism across the Wet Mountains, central Colorado

In the Wet Mountains of central Colorado, we document evidence for increasing metamorphic grade and associated higher amounts of partial melting along a transect from northwest to southeast. Field observations of structural orientation and style, qualitative assessment of strain intensity, analysis of metamorphic mineral assemblages, and macroscopic identification of leucosomes and migmatites are complemented by the use of melt microstructures to carefully document the presence and locations of former partial melt and to identify melt-producing reactions. In the northwest Wet Mountains, migmatitic foliation is moderately well developed, and partial melting occurred via granite wet melting and muscovite-dehydration melting, with rare melt pseudomorphs remaining. At Dawson Mountain in the central part of the range, inferred former melt channels are preserved along grain and subgrain boundaries, deformation appears more intense, and anatexis occurred through biotite-dehydration melting. Farthest to the south, the highest intensity of strain is inferred, with very closely spaced foliations, abundant dynamic recrystallization, and local mylonitization occurring in rocks of granitic composition, and partial melting occurring via granite wet melting. Metapelitic rocks experienced biotite-dehydration melting and contain garnet with Mn-rich rims and Mn-poor cores mantled by plagioclase, decussate biotite, and quartz, textures indicating back-reaction between melt and garnet. These textures indicate there was abundant melt within these highest-grade rocks and extensive melt-rock interaction. Throughout the Wet Mountains, deformation is concentrated in areas where melt-producing reactions occurred, and melt appears to be localized along deformation-related features, suggesting a correlation between partial melting and deformation. The northern Wet Mountains contain few plutons, whereas the central and southern portions of the Wet Mountains contain more pervasive dikes and sills and may contain more former melt as a result of both higher metamorphic grade and widespread thermal insulation. The Wet Mountains represent an exhumed section of formerly molten middle crust located at the transition between upper and lower crust and provide insight into processes ongoing at depth in modern orogenic belts. The microstructures indicative of former partial melt, textures associated with melt-rock interaction, and melting reactions we have identified in the Wet Mountains will greatly facilitate the recognition of other such exhumed sections.

Crustal melting, ductile flow, and deformation in mountain belts: Cause and effect relationships

Exhumed sections of migmatites are beautifully exposed in the middle crust of old orogens such as the Proterozoic Wet Mountains of Colorado and young Tertiary–active orogens such as the Himalaya and Karakoram. Migmatites and leucogranites occur both on a regional scale (e.g., Greater Himalayan Sequence) and along more restricted shear zones and strike-slip faults (e.g., Karakoram, Jiale, and Red River faults). Melting and deformation are clearly diachronous across orogenic belts over space and time, yet in general, deformation must precede regional metamorphism and melting in order to thicken the crust and increase pressure and temperature. Some deformation can be synchronous with partial melting and there is almost always post-melting deformation along shear zones or faults. The distinction between pre-, syn-, and post-kinematic granites in three dimensions, in pressure-temperature space, and in time becomes critical. In particular, mapping of macro- and micro-structures combined with precise U-Th-Pb dating of migmatitic leucosomes and granitic rocks in deformation zones can be used to constrain the relative timing of metamorphism, melting, ductile shearing and brittle faulting. In the Himalaya, multiple sill intrusions emanating from a regional migmatite terrane fed growth of leucogranite bodies over a time span of ~30 m.y. and channel fl ow, the southward extrusion of a partially melted section of the mid-crust, occurred along the Himalaya during the Miocene from ca. 24–15 Ma. The Karakoram batholith formed by pre- to post-collisional metamorphism, migmatisation and magmatism over a period lasting at least 65 m.y. Comparisons of the Himalaya and Karakoram migmatite-granite belts with the Proterozoic Wet Mountains in Colorado provide strong evidence for weak middle crust capable of aseismic fl ow, leading to question models of lithospheric rheology that call on strong middle crust.

705. CARPINUS FANGIANA

Carpinus fangiana Hu, from wet mountains of western China, is illustrated and described. Its habitat and associated species are discussed.

Global analysis of reptile elevational diversity

ABSTRACTAim Latitudinal‐ and regional‐scale studies of reptile diversity suggest a predominant temperature effect, unlike many other vertebrate richness patterns which tend to be highly correlated with both temperature and water variables. Here I examine montane gradients in reptile species richness with separate analyses of snakes and lizards from mountains around the world to assess a predominant temperature effect and three additional theories of diversity, including a temperature–water effect, the species–area effect and the mid‐domain effect (MDE).Location Twenty‐five elevational gradients of reptile diversity from temperate, tropical and desert mountains in both hemispheres, spanning 10.3° N to 46.1° N.Methods Elevational gradients in reptile diversity are based on data from the literature. Of the 63 data sets found or compiled, only those with a high, unbiased sampling effort were used in analyses. Twelve predictions and three interactions of diversity theory were tested using nonparametric statistics, linear regressions and multiple regression with the Akaike information criterion (AIC).Results Reptile richness and, individually, snake and lizard richness on mountains followed four distinct patterns: decreasing, low‐elevation plateaus, low‐elevation plateaus with mid‐elevation peaks, and mid‐elevation peaks. Elevational reptile richness was most strongly correlated with temperature. The temperature effect was mediated by precipitation; reptile richness was more strongly tied to temperature on wet gradients than on arid gradients. Area was a secondary factor of importance, whereas the MDE was not strongly associated with reptile diversity on mountains.Main conclusions Reptile diversity patterns on mountains did not follow the predicted temperature–water effect, as all diversity patterns were found on both wet and dry mountains. But the influence of precipitation on the temperature effect most likely reflects reptiles' use of radiant heat sources (sunning opportunities) that are more widespread on arid mountains than wet mountains due to lower humidity, sparser vegetation and less cloud cover across low and intermediate elevations.

Characteristics and implications of ca. 1.4 Ga deformation across a Proterozoic mid-crustal section, Wet Mountains, Colorado, USA

In the Wet Mountains, Colorado, Proterozoic rocks exposed along an oblique north-south tilted section preserve evidence of regional deformation and high temperature metamorphism in the middle and lower crust at ca. 1435–1365 Ma. Deformation of gneisses in the northern Wet Mountains is partitioned within discrete zones of subvertical foliation and northeast-trending folds, a product of northwest-southeast contraction or constriction associated with transcurrent deformation. Gneisses in the north are generally not migmatitic, and granitic intrusions form discrete bodies with distinct contacts. Shear zone foliation is cut by a late syntectonic dike with a U-Pb zircon age of 1430+5/–3 Ma, constraining the age of shear zone deformation in the upper crust. In the central to southern Wet Mountains, gneisses exhibit migmatitic foliation that dips moderately northeast, with dip- to oblique-slip mineral lineation throughout. Granite forms pervasive sills and interconnected sheets with gradational or indistinct contacts. Gneissic granite that yields a U-Pb zircon age of 1435 ± 4 Ma was emplaced into amphibolite gneiss containing 1436 ± 2 Ma metamorphic zircon. Younger, foliated granite sills were emplaced at 1390 ± 10 Ma. Our new results indicate contemporaneous deformation and metamorphism throughout the middle and lower crust at ca. 1.4 Ga. We interpret the zone of migmatitic crust pervaded by granite to represent a weak, low-viscosity, flowing lower crust that controlled the pattern of distributed deformation in the comparatively strong, brittle crust above. Thus, the Wet Mountains may be viewed as a deeply exhumed analog for the mid-crustal, low-viscosity layers that are inferred to exist in modern intracontinental orogenic settings and continental rift provinces.