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At least 1,081 records · Page 60Linked to original sources
USGS2013

Clinoform deposition across a boundary between orogenic front and foredeep - an example from the Lower Cretaceous in Arctic Alaska

The Lower Cretaceous Fortress Mountain Formation occupies a spatial and temporal niche between syntectonic deposits at the Brooks Range orogenic front and post-tectonic strata in the Colville foreland basin. The formation includes basin-floor fan, marine-slope and fan-delta facies that define a clinoform depositional profile. Texture and composition of clasts in the formation suggest progressive burial of a tectonic wedge-front that included older turbidites and mélange. These new interpretations, based entirely on outcrop study, suggest that the Fortress Mountain Formation spans the boundary between orogenic wedge and foredeep, with proximal strata onlapping the tectonic wedge-front and distal strata downlapping the floor of the foreland basin. Our reconstruction suggests that clinoform amplitude reflects the structural relief generated by tectonic wedge development and load-induced flexural subsidence of the foreland basin.

Alaska↗
USGS1986

Crustal structure of Yunnan province, People's Republic of China, from seismic refraction profiles

Seismic refraction, profiles in Yunnan Province, southwestern China, define the crustal structure in an area of active tectonics on the southern end of the Himalaya-Burma arc. The crustal thickness ranges from 38 to 46 kilometers, and the relatively low mean crustal velocity indicates a crustal composition compatible with normal continental crust and consisting mainly of meta-sedimentary and silicic intrusive rocks, with little mafic or ultramafic component. This composition suggests a crustal evolution involving sedimentary processes on the flank of the Yangtze platform rather than the accretion of oceanic island arcs, as has been proposed. An anomalously low upper-mantle velocity observed on one profile but not on another at right-angles to it may indicate active tectonic processes in the mantle or seismic anisotropy.

Yunnan Province↗
USGS2000

New structural and stratigraphic insights for northwestern Pakistan from field and Landsat Thematic Mapper data

The remote Waziristan region of northwestern Pakistan includes outcrops of the India-Asia suture zone. The excellent exposure of the Waziristan ophiolite and associated sedimentary lithosomes and their inaccessibility made the use of Landsat Thematic Mapper (TM) data desirable in this study. Landsat TM data were used to create a spectral ratio image of bands 3/4, 5/4, and 7/5, displayed as red, green, and blue, respectively, and a principal component analysis image of bands 4, 5, and 7 (RGB). These images were interpreted in the context of available geologic maps, limited field work, and biostratigraphic, lithostratigraphic, and radiometric data. They were used to create a coherent geologic map of Waziristan and cross section of the area that document five tectonic units in the region and provide a new and more detailed tectonic history for the region. The lowest unit is comprised of Indian shelf sediments that were thrust under the Waziristan ophiolite. The ophiolite has been tectonically shuffled and consists of two separate tectonic units. The top thrust sheet is a nappe comprised of distal Triassic to Lower Cretaceous Neotethyan sediments that were underthrust during the Late Cretaceous by the ophiolite riding on Indian shelf strata. The uppermost unit contains unconformable Tertiary and younger strata. The thrust sheets show that the Waziristan ophiolite was obducted during Late Cretaceous time and imply that the Paleocene and Eocene deformation represents collision of India with the Kabul block and/or Asia.

Geological Society of America Bulletin↗
USGS2002

Upper-mantle origin of the Yellowstone hotspot

Fundamental features of the geology and tectonic setting of the northeast-propagating Yellowstone hotspot are not explained by a simple deep-mantle plume hypothesis and, within that framework, must be attributed to coincidence or be explained by auxiliary hypotheses. These features include the persistence of basaltic magmatism along the hotspot track, the origin of the hotspot during a regional middle Miocene tectonic reorganization, a similar and coeval zone of northwestward magmatic propagation, the occurrence of both zones of magmatic propagation along a first-order tectonic boundary, and control of the hotspot track by preexisting structures. Seismic imaging provides no evidence for, and several contraindications of, a vertically extensive plume-like structure beneath Yellowstone or a broad trailing plume head beneath the eastern Snake River Plain. The high helium isotope ratios observed at Yellowstone and other hotspots are commonly assumed to arise from the lower mantle, but upper-mantle processes can explain the observations. The available evidence thus renders an upper-mantle origin for the Yellowstone system the preferred model; there is no evidence that the system extends deeper than ???200 km, and some evidence that it does not. A model whereby the Yellowstone system reflects feedback between upper-mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep-mantle plume hypothesis.

Geological Society of America Bulletin↗
USGS2005

Reply

In summary, the rift model as presented by Schlische and Withjack (2005) is related only to the Doylestown subbasin - they do not discuss the lithosome relationships throughout the remainder of the basin. But, even in the Doylestown subbasin, their model raises more questions than it answers. They have not demonstrated that syndepositional faulting actually occurred - instead, they only present evidence to support a preconceived fault model, and even some of the evidence they cite is inconsistent with their rift hypothesis. Rift tectonism is not a good m odel for the Birdsboro basin, as seductively simple as such a model may seem. Many aspects of the basin are not explained by rifting. That is why the focus in the original Birdsboro basin paper (Faill, 2003) was only on the sediments, their distributions, and the inferences that could be made therefrom. Discussion of how the basin formed and the overall causative tectonics was eschewed in order to avoid the introduction of preconceived notions (half-graben, rift tectonism). It is hoped that a better understanding of the sediment content and its distribution will help in developing a tectonic model that fully explains the origin, development, and subsequent history of the entire Birdsboro basin.

Geological Society of America Bulletin↗
USGS2008

Track of the Yellowstone hotspot: Young and ongoing geologic processes from the Snake River Plain to the Yellowstone Plateau and Tetons

This field trip highlights various stages in the evolution of the Snake River Plain–Yellowstone Plateau bimodal volcanic province, and associated faulting and uplift, also known as the track of the Yellowstone hotspot. The 16 Ma Yellowstone hotspot track is one of the few places on Earth where time-transgressive processes on continental crust can be observed in the volcanic and tectonic (faulting and uplift) record at the rate and direction predicted by plate motion. Recent interest in young and possible renewed volcanism at Yellowstone along with new discoveries and synthesis of previous studies, i.e., tomographic, deformation, bathymetric, and seismic surveys, provide a framework of evidence of plate motion over a mantle plume. This 3-day trip is organized to present an overview into volcanism and tectonism in this dynamically active region. Field trip stops will include the young basaltic Craters of the Moon, exposures of 12–4 Ma rhyolites and edges of their associated collapsed calderas on the Snake River Plain, and exposures of faults which show an age progression similar to the volcanic fields. An essential stop is Yellowstone National Park, where the last major caldera-forming event occurred 640,000 years ago and now is host to the world's largest concentration of hydrothermal features (>10,000 hot springs and geysers). This trip presents a quick, intensive overview into volcanism and tectonism in this dynamically active region. Field stops are directly linked to conceptual models related to hotspot passage through this volcano-tectonic province. Features that may reflect a tilted thermal mantle plume suggested in recent tomographic studies will be examined. The drive home will pass through Grand Teton National Park, where the Teton Range is currently rising in response to the passage of the North American plate over the Yellowstone hotspot.

Idaho, Wyoming↗
USGS2013

Late Miocene-Pleistocene evolution of a Rio Grande rift subbasin, Sunshine Valley-Costilla Plain, San Luis Basin, New Mexico and Colorado

The Sunshine Valley-Costilla Plain, a structural subbasin of the greater San Luis Basin of the northern Rio Grande rift, is bounded to the north and south by the San Luis Hills and the Red River fault zone, respectively. Surficial mapping, neotectonic investigations, geochronology, and geophysics demonstrate that the structural, volcanic, and geomorphic evolution of the basin involves the intermingling of climatic cycles and spatially and temporally varying tectonic activity of the Rio Grande rift system. Tectonic activity has transferred between range-bounding and intrabasin faults creating relict landforms of higher tectonic-activity rates along the mountain-piedmont junction. Pliocene–Pleistocene average long-term slip rates along the southern Sangre de Cristo fault zone range between 0.1 and 0.2 mm/year with late Pleistocene slip rates approximately half (0.06 mm/year) of the longer Quaternary slip rate. During the late Pleistocene, climatic influences have been dominant over tectonic influences on mountain-front geomorphic processes. Geomorphic evidence suggests that this once-closed subbasin was integrated into the Rio Grande prior to the integration of the once-closed northern San Luis Basin, north of the San Luis Hills, Colorado; however, deep canyon incision, north of the Red River and south of the San Luis Hills, initiated relatively coeval to the integration of the northern San Luis Basin. Long-term projections of slip rates applied to a 1.6 km basin depth defined from geophysical modeling suggests that rifting initiated within this subbasin between 20 and 10 Ma. Geologic mapping and geophysical interpretations reveal a complex network of northwest-, northeast-, and north-south–trending faults. Northwest- and northeast-trending faults show dual polarity and are crosscut by north-south– trending faults. This structural model possibly provides an analog for how some intracontinental rift structures evolve through time.

Colorado, New Mexico↗
USGS2013

Analysis of Neogene deformation between Beaver, Utah and Barstow, California: Suggestions for altering the extensional paradigm

For more than two decades, the paradigm of large-magnitude (~250 km), northwest-directed (~N70°W) Neogene extensional lengthening between the Colorado Plateau and Sierra Nevada at the approximate latitude of Las Vegas has remained largely unchallenged, as has the notion that the strain integrates with coeval strains in adjacent regions and with plate-boundary strain. The paradigm depends on poorly constrained interconnectedness of extreme-case lengthening estimated at scattered localities within the region. Here we evaluate the soundness of the inferred strain interconnectedness over an area reaching 600 km southwest from Beaver, Utah, to Barstow, California, and conclude that lengthening is overestimated in most areas and, even if the estimates are valid, lengthening is not interconnected in a way that allows for published versions of province-wide summations. We summarize Neogene strike slip in 13 areas distributed from central Utah to Lake Mead. In general, left-sense shear and associated structures define a broad zone of translation approximately parallel to the eastern boundary of the Basin and Range against the Colorado Plateau, a zone we refer to as the Hingeline shear zone. Areas of steep-axis rotation (ranging to 2500 km 2 ) record N-S shortening rather than unevenly distributed lengthening. In most cases, the rotational shortening and extension-parallel folds and thrusts are coupled to, or absorb, strike slip, thus providing valuable insight into how the discontinuous strike-slip faults are simply parts of a broad zone of continuous strain. The discontinuous nature of strike slip and the complex mixture of extensional, contractional, and steep-axis rotational structures in the Hingeline shear zone are similar to those in the Walker Lane belt in the west part of the Basin and Range, and, together, the two record southward displacement of the central and northern Basin and Range relative to the adjacent Colorado Plateau. Understanding this province-scale coupling is critical to understanding major NS shortening and westerly tectonic escape in the Lake Mead area. One north-elongate uplift in the Hingeline shear zone is a positive flower structure along a strike-slip fault, and we postulate that most other large uplifts are diapiric, resulting from extension-normal inflow of ductile substrate, rather than second-order isostatic responses to tectonic unloading. We also postulate that large steep-axis rotations, and some small ones as well, result from basal tractions imparted by gradients in southerly directed subjacent ductile flow rather than by shear coupling imparted by laterally variable elongation strains. The shortening strain recorded in the rotations and related structures probably matches or exceeds the magnitude of lengthening, even for the Lake Mead area where we do not question local large (~65 km) west-directed lengthening. We assess the results of extensive recent earth-science research in the Lake Mead area and conclude that previously published models of N-S convergence, westerly tectonic rafting, and N-S occlusion are valid and record unique tectonic escape accommodation for south-directed displacement of the Great Basin sector of the Basin and Range. Genetic ties between the south-directed displacement and plate-interaction forces are elusive, and we suggest the displacement results from body forces inherent in the Basin and Range.

Arizona, California, Nevada, Utah↗
USGS2014

Preface

The tectonic evolution of the Arctic realm, particularly in the Mesozoic, remains a subject of considerable uncertainty. The nature of the crustal entities involved, their origins, and the nature, location, and age of major tectonic boundaries are incompletely studied and understood ( Pease, 2011 ). The largest piece of continental crust of uncertain origin that plays a role in Arctic tectonics is the Arctic Alaska–Chukotka terrane or microplate. The terrane includes northern Alaska, northeasternmost Russia, and the adjacent continental shelves ( Fig. 1 ; Natal'in et al., 1999 ; Miller et al., 2006 ). Because of its size, the origin of this microplate and its movements during the Paleozoic and Mesozoic are critical components of tectonic and paleogeographic models ( Pease, 2011 ).

Alaska↗
USGS2026

Reconstructing ancient sedimentary source-to-sink systems – Examples from southern Laurentia’s Proterozoic accretionary orogens

Provenance analysis is a powerful tool for investigating sediment delivery networks, constraining magmatic histories, and reconstructing the tectonic evolution of orogenic belts and basins. Basin analysis studies increasingly use detrital zircon (DZ) U-Pb forward mixture modeling to enhance provenance interpretations by quantifying the relative contributions of different sources. Forward mixture modeling requires significant a priori knowledge that limits deep-time applications. This challenge is overcome with an inverse mixture modeling approach non-negative matrix factorization to reconstruct the number and age distributions of paleo-source regions of Proterozoic metasedimentary rocks in the southwestern United States. This analysis indicates eight reconstructed end-member distributions representing unique sediment sources: two multi-modal end members characterized by ages older than ca. 1.8 Ga from cratonic Laurentia, five unimodal age distributions between ca. 1.80 Ga and 1.65 Ga consistent with Paleoproterozoic arc magmatic sources, and a ca. 1.6−1.5 Ga end member likely derived from exotic cratons in supercontinent Nuna (Columbia). Sediments deposited between ca. 1.80 Ga and 1.73 Ga yield heterogeneous age distributions suggesting multiple arc-backarc systems and several phases of slab roll back, contraction, and accretionary orogenesis, including input from pre−1.8 Ga Laurentian cratons. Homogenization of DZ signatures during the Yavapai orogeny (ca. 1.72−1.68 Ga) reflect crustal assembly as well as the uplift of Paleoproterozoic arcs in the orogenic hinterland. Detrital zircon age distributions from strata deposited during the Mazatzal orogeny (ca. 1.65−1.60 Ga) suggest the Mazatzal Province is a continental arc constructed on older crust. Mesoproterozoic samples are consistent with multiple basins derived from local recycling and long-distance sediment transport. Collectively, these data record the tectonic transition from the episodic accretion of disparate crustal domains to an increasingly integrated continental margin. These results provide new insights into the Proterozoic tectonic and paleogeographic evolution of the southwestern United States at basin to orogen scales and highlight the power of inverse DZ modeling to extract geologically meaningful quantitative mixture models from sedimentary records alone, offering a powerful tool for deep-time tectonic and basin analysis.

Arizona, California, Colorado, New Mexico, Nevada,↗
USGS2026

Unraveling protracted modification of Archean and Paleoproterozoic crust in central Laurentia, Penokean orogen, with garnet and accessory mineral geochronology and microstructural analysis

Proterozoic metamorphism and deformation of the southern margin of the Superior craton in the Lake Superior region is attributed to the Penokean orogeny (1890−1830 Ma). This model includes a period of crustal inversion in which Archean basement blocks were exhumed through overlying Paleoproterozoic strata, producing the corridor of gneiss domes that parallels the trend of the Penokean orogen across the northern Midcontinent, USA. However, recent geologic mapping and 40 Ar/ 39 Ar geochronology challenge this interpretation, suggesting instead that the gneiss dome structures reflect younger episodes of tectonic activity along the southern margin of Laurentia. In absence of integrated pressure-temperature-time-deformation constraints for these rocks, interpretations are largely limited to their final cooling history, making it difficult to both identify the tectonic forces that shaped the architecture of the Penokean orogenic belt and assess the extent to which later Proterozoic tectonism modified the southern Superior craton. We address this problem with an approach joining thermodynamic modeling, garnet and accessory mineral geochronology, and microstructural analysis for several metamorphic rocks across the gneiss dome corridor. The U-Pb ages of titanite reveal that the Proterozoic geometries of exhumed basement gneiss domes are governed by preexisting Archean structures. Garnet Lu-Hf geochronology constrains the timing of prograde-to-peak metamorphism in the Penokean orogenic belt. Granulite facies metamorphism is related to the final stages of the Penokean orogeny at 1837 Ma and localized in a belt of high-grade rocks near a major Penokean suture. Garnet Lu-Hf ages of samples adjacent to gneiss domes reflect regional metamorphism following the accretionary phase of the Penokean orogeny, between 1825 Ma and 1782 Ma, which we suggest reflects continued crustal thickening related to convergence farther south during this time interval. Combination of garnet microstructures and Sm-Nd ages reflects later exhumation of gneiss domes and buried metasedimentary rocks by ca. 1750 Ma, consistent with previously published 40 Ar/ 39 Ar cooling ages across the region. Reset Lu-Hf and Sm-Nd garnet ages and U-Pb ages of syn-kinematic titanite reflect reactivation of primary Penokean structures during this period of basement uplift. These data document significant modification of the Penokean orogen and the Archean crust of the southern Superior province between 1800 Ma and 1700 Ma. Tectonic activity during this interval coincides with collisional events recognized in western Laurentia, suggesting that the period immediately following the Penokean orogeny may be a broadly important time for crustal growth and modification in proto-North America.

Michigan, Wisconsin↗
USGS2024

Boulders modulate hillslope-channel coupling in the northern Alaska Range

Active orogens balance tectonic rock uplift with erosion, commonly via river incision coupled to landslide denudation of “threshold” hillslopes, but sediment’s role in this feedback is unclear. We report fluvial geometry, and sediment size, prevalence, and mobility across two ≤600-m-tall gneissic northern Alaska Range anticlines that sustain steep landslide-clad hillslopes but differ 10× in late Pleistocene−recent rock uplift rate. Enigmatically, the river steepens and narrows prominently across the fold experiencing slow surface uplift (∼0.5 mm/yr) but remains low-gradient and wide downstream across the anticline undergoing rapid differential rock uplift (∼5 mm/yr). Frequent bedload mobilization across both folds implies fluvial equilibration to sediment transport despite discrepant channel forms and similarly prevalent hillslope-derived boulders. Boulder prevalence correlates significantly with channel slope and width on the slowly uplifting anticline, but weakly on the rapidly uplifting anticline. Strong correlations across the tectonically quiescent anticline may reflect local incision-suppressing boulder aggradation that forces the channel to steepen and narrow, consistent with field observations. Conversely, weak correlations across the rapidly uplifting anticline imply that boulders may modulate expected tectonic channel adjustment by preferentially aggrading to subdue slope, and deflecting frequently mobile bedload to drive lateral erosion that maintains channel width, steepens adjacent hillslopes, and perpetuates hillslope-channel coupling. Hence, hillslope-derived boulders may occupy important roles in regulating feedbacks between river incision and landslide erosion that differ fundamentally at high and low tectonic rates.

Geology↗
USGS2019

Post-12 Ma deformation of the lower Colorado River corridor, southwestern USA: Implications for diffuse transtension and the Bouse Formation

Structural evidence presented here documents that deformation was ongoing within the lower Colorado River corridor (southwestern USA) during and after the latest Miocene Epoch, postdating large-magnitude extension and metamorphic core complex formation. Geometric and kinematic data collected on faults in key geologic units constrain the timing of deformation in relation to the age of the Bouse Formation, a unit that records the first arrival and integration of the Colorado River. North-south–striking extensional, NW-SE–striking oblique dextral, NE-SW–striking oblique sinistral, and east-west–striking contractional faults and related structures are observed to deform pre– (>6 Ma), syn– (6–4.8 Ma), and post–Bouse Formation (<4.8 Ma) strata. Fault displacements are typically at the centimeter to meter scale, and locally exhibit 10-m-scale displacements. Bouse Formation basalt carbonate locally exhibits outcrop-scale (tens of meters) syndepositional dips of 30°–90°, draped over and encrusted upon paleotopography, and has a basin-wide vertical distribution of as much as 500 m. We argue that part of this vertical distribution of Bouse Formation deposits represents syn- and post-Bouse deformation that enhanced north-south–trending depocenters due to combined tectonic and isostatic subsidence in a regional fault kinematic framework of east-west diffuse extension within an overall strain field of dextral transtension. Here we (1) characterize post-detachment tectonism within the corridor, (2) show that diffuse tectonism is cumulatively significant and likely modified original elevations of Bouse Formation outcrops, and (3) demonstrate that this tectonism may have played a role in the integration history of the lower Colorado River. We suggest a model whereby intracontinental transtension took place in a several hundred kilometers-wide area inboard of the San Andreas fault within a diffuse Pacific–North America plate margin since the latest Miocene.

Arizona, California, Nevada↗
USGS1995

Subduction cycles under western North America during the Mesozoic and Cenozoic eras

An extensive review of geologic and tectonic features of western North America suggests that the interaction of oceanic plates with the continent follows a broad cyclical pattern. In a typical cycle, periods of rapid subduction (7–15 cm/yr), andesitic volcanism, and trench-normal contraction are followed by a shift to trench-normal extension, the onset of voluminous silicic volcanism, formation of large calderas, and the creation of major batholiths. Extension becomes pervasive in metamorphic core complexes, and there is a shift to fundamentally basaltic volcanism, formation of flood basalts, widespread rifting, rotation of terranes, and extensive circulation of fluids throughout the plate margin. Strike-slip faulting becomes widespread with the creation of new tectonostratigraphic terranes. A new subduction zone forms and the cycle repeats. Each cycle is 50–80 m.y. long; cycles since the Triassic have ended and begun at approximately 225, 152, 92, 44, and 15 Ma. The youngest two cycles are diachronous, one from Oregon to Alaska, the other from central Mexico to California. The transitions from one cycle to the next cycle are characterized by rapid and pervasive changes termed, in this chapter, “major chaotic tectonic events.” These events appear to be related to the necking or breaking apart of the formerly subducted slab at shallow depth, the resulting delamination of the plate margin, and the onset of a new subduction cycle. These are times of the most rapid apparent and true polar wander of the North American plate, when the plate appears most free to move relative to surrounding plates and relative to the mantle below the asthenosphere. In western North America, magmatism and tectonics during the Jurassic period are quite similar to magmatism and tectonics since mid-Cretaceous time except strike-slip faulting shifted in sense from left lateral to right lateral.

North America, Pacific Ocean↗
USGS2011

Sedimentary response to orogenic exhumation in the northern Rocky Mountain Basin and Range province, Flint Creek basin, west-central Montana

Middle Eocene through Upper Miocene sedimentary and volcanic rocks of the Flint Creek basin in western Montana accumulated during a period of significant paleoclimatic change and extension across the northern Rocky Mountain Basin and Range province. Gravity modelling, borehole data, and geologic mapping from the Flint Creek basin indicate that subsidence was focused along an extensionally reactivated Sevier thrust fault, which accommodated up to 800 m of basin fill while relaying stress between the dextral transtensional Lewis and Clark lineament to the north and the Anaconda core complex to the south. Northwesterly paleocurrent indicators, foliated metamorphic lithics, 64 Ma (40Ar/39Ar) muscovite grains, and 76 Ma (U-Pb) zircons in a ca. 27 Ma arkosic sandstone are consistent with Oligocene exhumation and erosion of the Anaconda core complex. The core complex and volcanic and magmatic rocks in its hangingwall created an important drainage divide during the Paleogene shedding detritus to the NNW and ESE. Following a major period of Early Miocene tectonism and erosion, regional drainage networks were reorganized such that paleoflow in the Flint Creek basin flowed east into an internally drained saline lake system. Renewed tectonism during Middle to Late Miocene time reestablished a west-directed drainage that is recorded by fluvial strata within a Late Miocene paleovalley. These tectonic reorganizations and associated drainage divide explain observed discrepancies in provenance studies across the province. Regional correlation of unconformities and lithofacies mapping in the Flint Creek basin suggest that localized tectonism and relative base level fluctuations controlled lithostratigraphic architecture.

Canadian Journal of Earth Sciences↗
USGS2015

Geological processes and orchid biogeography with applications to southeast central America

This contribution owes its origins to a paper presentation by Dr. Calloway H. Dodson at the Second International Conference on Neotropical Orchidology held in San José, Costa Rica in May of2003 (Dodson 2003). Dr. Dodson outlined some ofthe reasons to suspect that regional geological fac-tors may play important roles in orchid speciation and biogeography and gave examples from the northwestern South America. He also suggested that evolutionary change in orchid might occur over fairly short time periods, perhaps even as short as decades, centuries or millennia (Dodson 2003, SHK lecture notes). These ideas stimulated the author, a professional earth scientist, to begin thinking about how these exciting ideas could begin to be tested in Costa Rica neighboring and Central American countries, an area that has drawn him to return frequently over the last decade. The present contribution is a proposal for integrating geological observations, such as the chronology of arc volcanic activity in Nicaragua,Costa Rica, and Panama, in hypothesis forming and testing of the geographic distribution of orchids (and possibly other biota). I initially focus on comparisons between orchid inventories on the windward slopes of mountainous regions (elevation > 1000 m)with high rainfall (> 1-2 m) in tropical regions, the so-called tropical cloud forests. These regions represent the tropical pre-montane rain forestto lower montane tropical rain forest life zones of Holdridge(1967) and the montane vegetation zone applied to Costa Rica and Panama by Dressler (1993). An important message of this paper is that such tropical mountainous regions are not necessarily static, but may change in elevation over geologic time due to active tectonic deformation and uplift and that the presence of active volcanism in a mountain range may also introduce additional chemical factors, such as volcanic gases, acid rain, and volcanic soils, and also physical factors, such as interruption of gene flow by explosive eruptions and coverage by their air fall products such as ash (tephra), lava flows, and lahars (volcanic mudflows). Thus over a given geological time interval, forests may be slowly increasing in elevation by tectonic uplift or by the accumulation of volcanic products such as steep-sided strato volcanoes (built from both lavas and tephra), or by down-slope accumulations of lava flows or lahars. Mountains may also lose elevation by erosion or by tectonic subsidence. As we shall see, tropical Central America shows an extraordinarily high level of tectonic and volcanic history that has changed its geography and, by implication, climate, life zones,and likely orchid distribution. My working hypo thesis put forward for testing is that orchid adaptations to these changes may have led to the development of new species and endemism in this region.

Lankesteriana↗
USGS2010

Secular variation in economic geology

The temporal pattern of ore deposits on a constantly evolving Earth reflects the complex interplay between the evolving global tectonic regime, episodic mantle plume events, overall changes in global heat flow, atmospheric and oceanic redox states, and even singular impact and glaciation events. Within this framework, a particular ore deposit type will tend to have a time-bound nature. In other words, there are times in Earth history when particular deposit types are absent, times when these deposits are present but scarce, times when they are abundant, and still other times for which we lack sufficient data. Understanding of such secular variation provides a critical first-order tool for exploration targeting, because rocks that have formed or were deformed during a certain time slice may be very permissive for a given deposit type, whereas identification of rocks of less favorable ages would help eliminate large areas during exploration programs. Secular analysis, therefore, is potentially a powerful tool for mineral resource assessment in poorly known terranes, providing a quick filter for favorability of a given deposit type using age of host rocks. Factors bearing on the known age distribution of a particular type of deposit include the following: (1) uneven preservation, (2) data gaps, (3) contingencies of plate motions, and (4) long-term secular changes in the Earth System. The present special issue of Economic Geology is focused on the latter factor, although all of these are interrelated. The selective preservation of certain mineral deposit types and the greater susceptibility for shallowly formed ores in tectonically active environments to be lost to erosion define a pattern that is superimposed on the secular formational trends (e.g., Groves et al., 2005a , b ; Kerrich et al., 2005 ). With improved geochronological methods and the availability of information on important mineral deposits from most parts of the world, data gaps for defining broad temporal distributions of ore types are becoming smaller. It has been increasingly recognized that ore deposit formation is also correlated with plate tectonic setting. Nevertheless, a complex Earth history of supercontinent assembly and breakup has led to the fragmentation of many Paleozoic and Precambrian mineral provinces. The use of plate reconstructions in economic geology, although extremely controversial and conjectural before the late Paleozoic, is critical for defining these provinces prior to the added complications resulting from post-ore plate motions. It is now well established that the temporal patterns of many types of mineral deposits (Fig. 1 1 ) reflect the formation or break-up of supercontinents and the preservation potential of deposits formed during these periods ( Barley and Groves, 1992 ; Titley, 1993 ; Kerrich et al., 2005 ; Goldfarb et al., 2009 ). Approximate time periods for such formation and break-up, respectively, include 2800–2500 and 2450–2100 Ma for Kenorland, 2100–1800 and 1600–1300 Ma for Nuna/Columbia, 1300–1100 and 850–600 Ma for Rodinia, and 600–300 Ma and 200–60 Ma for Gondwanaland-Pangea. A new supercontinent, Amasia, has begun to form during the past 250 m.y., thus overlapping Pangea break-up. Many of the formation-preservation patterns are themselves controlled by progressive cooling of Earth, the change from a mantle-plume buoyancy style to subduction-dominated tectonics, a decreasing buoyancy of the subcontinental lithospheric mantle, and depth of ore formation. In general, orogenic Au, volcanogenic massive sulfide (VMS), epithermal Au-Ag, and porphyry Cu±Au and Mo porphyry deposits form in active margins during periods of supercontinent assembly. Numerous other ore deposit types show an association with supercontinent formation, but develop inland of the active margin. These include many of the MVT Pb-Zn deposits and unconformity-type U deposits. The Tertiary Carlin-type deposits within the deformed shelf sequences along the North American craton margin also appear to have formed during the ongoing growth of Amasia. Those ores associated with periods of supercontinent breakup or attempted breakup are more difficult to define. They probably include diamond, Bushveld-type Ni-Cu-PGE, IOCG, and clastic-dominated Pb-Zn (or SEDEX) deposits in intracontinental areas of failed rifting, and other clastic-dominated Pb-Zn deposits in areas of actual breakup. In all cases, however, these temporal/spatial distributions are ultimately controlled by the secular character of Earth history.

Economic Geology↗
USGS1997

A geologic history of the north-central Appalachians, part 2: The Appalachian basin from the Silurian through the Carboniferous

The north-central Appalachians occupy a critical position within the 3000+ km-long Appalachian orogen, lying southwest of the boundary between the central and northern Appalachians (CNAB). The one-billion-year-long history of tectonic activity in the north-central Appalachians includes the assembly and breakup of a late Proterozoic supercontinent, the creation and evolution of the Appalachian orogen during the Paleozoic, and the Mesozoic transformation of the active orogen into a passive margin during Pangea's disassembly. An important part of the Middle and Late Paleozoic history is the formation and growth of the Appalachian basin, an enormous, elongate continental basin lying cratonward of the active Appalachian internides along Laurentia's eastern margin. The Appalachian basin developed out of the Taconic orogeny, the second of the four orogenies that formed and modified the character of the Appalachian orogen. Prior to the Taconic orogeny, the eastern margin of Laurentia consisted of a broad carbonate shelf facing the Theic Ocean. The Taconic orogeny obduced various Theic components, including microcontinents, magmatic arcs, and accretionary prisms, onto the carbonate shelf. The resulting Taconic highlands formed a topographic barrier between Theia and the craton's interior. Because tectonic activity in the internides continued more or less throughout the remainder of the Paleozoic, the Appalachian basin never had a direct connection with Theia. The coarse-grained molasse from the newly uplifted Taconic highland spread westward over most of the basin during the Early Silurian. Carbonate deposition gradually encroached eastward over the basin as the siliciclastic input from the southeast waned. A resurgence in the earliest Late Silurian, perhaps related to orogenesis in Newfoundland and the Maritimes, expanded the clastic wedge somewhat. Before long, however, carbonate deposition once again dominated most of the north-central basin for the remainder of the Silurian and into the Early Devonian. The Early-to-Middle Devonian Acadian orogeny began introducing siliciclastic material into the eastern part of the Appalachian basin, thereby ending the largely paralic environment that persisted from the Late Silurian. A number of deltas formed around local sediment-input centers during the Middle Devonian. Continued uplift and/or orogenesis must have continued into the Late Devonian because a vast amount of terrigenous sediment was introduced into the Appalachian basin to form the vast Upper Devonian Catskill delta. Although the principal tectonic activity appears to have centered in New England, the large volume of sediment input suggests that orogenesis may have extended southward to the north-central Appalachians. However, direct evidence of actual Acadian deformation and metamorphism is lacking at this latitude. The north-central Appalachian basin underwent a significant change during the Carboniferous. Prior to this time, the shoreline shifted laterally through transgressions and regressions largely as a function of sediment input and regional subsidence. In the Carboniferous, vertical movements in the basin became more important. During the Early Carboniferous, much of the Mauch Chunk delta was eroded in the central and western parts of the basin. At the end of the Early Carboniferous, an unconformity truncated progressively older rocks to the north toward New York. New sediment transport patterns and depositional environments within the basin during the Late Carboniferous probably reflect significant topographic and tectonic changes, not only in the hinterland, but in the basin itself, perhaps in conjunction with climatic changes. Extensive interfluvial swamps on a lower delta plain accumulated thick organic deposits which would become coal. This delta plain graded southwestward into tidal and marine environments. The Alleghany orogeny in the Early Permian interrupted deposition in much of the north-central Appalachian basin and profoundly altered its structure.

American Journal of Science↗