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USGS2005

Geochemistry of sulfate minerals in high- and low-temperature environments: A tribute to Robert O. Rye

This special issue is a tribute to Robert O. Rye, known as "Bob" to most, in light of his highly productive and ongoing career. Almost all of the papers in this issue are derived from topical sessions on sulfate minerals in hydrothermal and low-temperature environments. The sessions, held at the 2000 Geological Society of America meeting in Reno, NV, and sponsored jointly by the Mineralogical Society of America, the Society of Economic Geologists and the Geochemical Society, included 29 oral and 9 poster presentations. The sessions complemented a short course on the crystallography, geochemistry and environmental significance of sulfate minerals, sponsored by the Mineralogical Society of America and the Geochemical Society, which immediately preceded the meeting (Alpers et al., 2000). The 23 papers in this special issue typify the breadth of Bob Rye’s research in this important area of geochemistry.

Chemical Geology↗

USGS2007

The impact of floods and storms on the acoustic reflectivity of the inner continental shelf: A modeling assessment

Flood deposition and storm reworking of sediments on the inner shelf can change the mixture of grain sizes on the seabed and thus its porosity, bulk density, bulk compressional velocity and reflectivity. Whether these changes are significant enough to be detectable by repeat sub-bottom sonar surveys, however, is uncertain. Here the question is addressed through numerical modeling. Episodic flooding of a large versus small river over the course of a century are modeled with HYDROTREND using the drainage basin characteristics of the Po and Pescara Rivers (respectively). A similarly long stochastic record of storms offshore of both rivers is simulated from the statistics of a long-term mooring recording of waves in the western Adriatic Sea. These time series are then input to the stratigraphic model SEDFLUX2D , which simulates flood deposition and storm reworking on the inner shelf beyond the river mouths. Finally, annual changes in seabed reflectivity across these shelf regions are computed from bulk densities output by SEDFLUX2D and compressional sound speeds computed from mean seafloor grain size using the analytical model of Buckingham [1997. Theory of acoustic attenuation, dispersion, and pulse propagation in unconsolidated granular materials including marine sediments. Journal of the Acoustical Society of America 102, 2579–2596; 1998. Theory of compressional and shear waves in fluidlike marine sediments. Journal of the Acoustical Society of America 103, 288–299; 2000. Wave propagation, stress relaxation, and grain-tograin shearing in saturated, unconsolidated marine sediments. Journal of the Acoustical Society of America 108, 2796–2815]. The modeling predicts reflectivities that change from <12 dB for sands on the innermost shelf to >9 dB for muds farther offshore, values that agree with reflectivity measurements for these sediment types. On local scales of ∼100 m, however, maximum changes in reflectivity are <0.5 dB. So are most annual changes in reflectivity over all water depths modeled (i.e., 0–35 m). Given that signal differences need to be ⩾2–3 dB to be resolved, the results suggest that grain-size induced changes in reflectivity caused by floods and storms will rarely be detectable by most current sub-bottom sonars.

Continental Shelf Research↗

USGS2023

Climate-induced shifts in grassland bird nesting phenology have implications for grassland management

Grasslands are among the most impacted ecosystems globally. In the midcontinent of North America, a > 80% loss of grasslands has made their conservation a major priority for resource managers. Grassland ecosystems evolved under periodic disturbances; consequently, grassland management often involves regular actions such as grazing, haying, or burning to maintain ecosystem integrity. The timing of such practices has direct implications on grassland ecology, agricultural economics, and survival and fecundity of grassland nesting birds (hereafter grassland birds). We conducted a meta-analysis on the nesting phenology of grassland birds throughout North America, focusing on nest-survival literature. We constructed a well-fitting model to predict median date of expected nest departure (hereafter fledge date) for grassland birds across the midcontinent. Predictions from our model demonstrate considerable spatial variation in median nesting phenology that is predictable using a spatially explicit spring phenology index. Median fledge dates were 8 or 13 days earlier in years of extreme weather conditions (dry or wet, respectively) than in years of average conditions. Species that generally nest in taller vegetation tended to have later median nest phenologies than those using shorter vegetation. Our results incorporate the most rigorous information available in the literature on nesting phenology of 36 grassland bird species and improve information available to managers about nesting phenology of grassland birds in the midcontinent of North America. Our predictions approximate the day when one-half of the nesting efforts would be complete for a given area within the midcontinent grasslands and can inform management and conservation decisions about the timing of management actions in grassland ecosystems.

Global Ecology and Conservation↗

USGS2008

Paleomagnetism of Proterozoic mafic dikes from the Tobacco Root Mountains, southwest Montana

Paleomagnetic data from Proterozoic mafic dikes in southwestern Montana provides evidence for two distinct episodes of subparallel dike emplacement at ca. 1450 and 780 Ma. Published geochemical data from dikes in the southern Tobacco Root Mountains has identified three distinct compositional groups, termed groups A, B, and C. Geochronological data from the group A dikes yielded a Sm-Nd age of 1448 ?? 49 Ma. Emplacement of these dikes is thought to reflect mafic magmatism associated with extension accompanying development of the adjacent Mesoproterozoic Belt Basin. Paleomagnetic results from these dikes and a group C dike yield antipodal magnetizations with a group-mean direction of D = 225.0??, I = 61.8?? (k = 27.9, ??95 = 7.7??, N = 14 independent means/24 sites). The average paleomagnetic pole (8.7??N, 216.1??E, A95 = 10.3??) is considered to be primary on the basis of positive baked contact tests and similarity to poles of ca. 1.45-1.4 Ga from intrusions elsewhere in North America, but is discordant with respect to poles from age equivalent sedimentary rocks of the Meosoproterozoic Belt Supergroup. 40Ar/39Ar dates from geochemical group B dikes are consistent with published U-Pb dates that demonstrate dike emplacement at 780 Ma as part of the regional Gunbarrel magmatic event. Hornblende concentrates from the group B dikes yield 40Ar/39Ar apparent ages of 778-772 Ma, whereas biotite from a baked contact zone yielded a plateau date of 788 Ma. Paleomagnetic results from the group B dikes yield a mean direction of D = 301.5??, I = -17.1?? (k = 65.7, ??95 = 4.0??, N = 12 independent means/23 sites) with a paleomagnetic pole at 14.6??N, 127.0??E (A95 = 3.2??). The combination of geochronologic data, results of a baked contact test, and spatial agreement of the paleomagnetic poles with poles of similar age elsewhere in North America indicates that this is also a primary magnetization associated with dike emplacement. Paleomagnetic data from some of the Tobacco Root Mountains dikes provide evidence that they were partially to completely remagnetized during latest Cretaceous to early Tertiary time, perhaps due to thermal affects associated with emplacement of the Late Cretaceous Tobacco Root Batholith. The overall agreement of paleomagnetic poles from the Proterozoic dikes with those of age equivalent rocks elsewhere in North America and agreement of the secondary magnetization with expected directions for the latest Cretaceous/early Tertiary indicate that the rocks of the Tobacco Root Mountains have not experienced significant tilting or vertical axis rotation since the Mesoproterozoic. The new paleomagnetic poles from this study thus provide key data for refining Meso- and Neoproterozoic parts of the North American APW path. ?? 2008 Elsevier B.V. All rights reserved.

Precambrian Research↗

USGS2018

Geochemistry and mineralogy of late Quaternary loess in the upper Mississippi River valley, USA: Provenance and correlation with Laurentide Ice Sheet history

The midcontinent of North America contains some of the thickest and most extensive last-glacial loess deposits in the world, known as Peoria Loess. Peoria Loess of the upper Mississippi River valley region is thought to have had temporally varying glaciogenic sources resulting from inputs of sediment to the Mississippi River from different lobes of the Laurentide Ice Sheet. Here, we explore a new method of determining loess provenance using K/Rb and K/Ba values (in K-feldspars and micas) in loess from a number of different regions in North America. Results indicate that K/Rb and K/Ba values can distinguish loess originating from diverse geologic terrains in North America. Further, different loess bodies that are known to have had the same source sediments (using other criteria) have similar K/Rb and K/Ba values. We also studied three thick loess sections in the upper Mississippi River valley region. At each site, the primary composition of the loess changed over the course of the last glacial period, and K/Rb and K/Ba values parallel changes in carbonate mineral content and clay mineralogy. We thus confirm conclusions of earlier investigators that loess composition changed as a result of the shifting dominance of different lobes of the Laurentide Ice Sheet and the changing course of the Mississippi River. We conclude that K/Rb and K/Ba values are effective, robust, and rapid indicators of loess provenance that can be applied to many regions of the world.

Quaternary Science Reviews↗

USGS2023

Regional variability in peatland burning at mid-to high-latitudes during the Holocene

Northern peatlands store globally-important amounts of carbon in the form of partly decomposed plant detritus. Drying associated with climate and land-use change may lead to increased fire frequency and severity in peatlands and the rapid loss of carbon to the atmosphere. However, our understanding of the patterns and drivers of peatland burning on an appropriate decadal to millennial timescale relies heavily on individual site-based reconstructions. For the first time, we synthesise peatland macrocharcoal records from across North America, Europe, and Patagonia to reveal regional variation in peatland burning during the Holocene. We used an existing database of proximal sedimentary charcoal to represent regional burning trends in the wider landscape for each region. Long-term trends in peatland burning appear to be largely climate driven, with human activities likely having an increasing influence in the late Holocene. Warmer conditions during the Holocene Thermal Maximum (∼9–6 cal. ka BP) were associated with greater peatland burning in North America's Atlantic coast, southern Scandinavia and the Baltics, and Patagonia. Since the Little Ice Age, peatland burning has declined across North America and in some areas of Europe. This decline is mirrored by a decrease in wider landscape burning in some, but not all sub-regions, linked to fire-suppression policies, and landscape fragmentation caused by agricultural expansion. Peatlands demonstrate lower susceptibility to burning than the wider landscape in several instances, probably because of autogenic processes that maintain high levels of near-surface wetness even during drought. Nonetheless, widespread drying and degradation of peatlands, particularly in Europe, has likely increased their vulnerability to burning in recent centuries. Consequently, peatland restoration efforts are important to mitigate the risk of peatland fire under a changing climate. Finally, we make recommendations for future research to improve our understanding of the controls on peatland fires.

Quaternary Science Reviews↗

USGS2019

Natural hazards and mineral commodity supply: Quantifying risk of earthquake disruption to South American copper supply

Mineral resources, and their mining and enrichment operations, are not equally distributed across Earth. The concentration of mineral supply in certain regions, owing to the geology or geography of the mineral resource, raises the level of risk related to supply disruption. Where mineral production coincides with areas prone to natural hazards, supply may be especially at risk. However, the level of risk that natural hazards pose to mineral supply has yet to be quantified on a global or regional scale. Using copper in South America as a case study, this paper offers methods for quantifying (i) the coincidence of mineral production and seismic hazards, and (ii) the Expected Annual Disruption (EAD) of the mineral supply from earthquakes. The first of these methods indicates that, of the 101 copper producing facilities in South America considered, 76 are located within an area of high seismic hazard, taken here as the area with>85% chance of exceeding Modified Mercalli Intensity VI earthquake shaking in 50 years. Collectively, the 76 facilities comprise 82%, 87%, and 91% of the 2015 South American mine production, smelter capacity, and refinery capacity, respectively. For each of the 101 facilities, the second method calculates the EAD using a full earthquake shaking hazard forecast at the location, the annualized copper production of the facility, and models of the vulnerability of that production to shaking. The EADs are summed by country, here within South America, as a demonstration of how supply risk could eventually be quantified globally. Consideration of two illustrative vulnerability models shows that future work is needed to determine percentages of disruption to mineral production for different levels of earthquake shaking. Ultimately, the methods presented herein could be applied to other mineral commodities and/or adapted for other natural hazards, and the resulting EADs could be summed. Results from these methods could be used to focus more detailed risk assessments where the risk is highest.

Resources Policy↗

USGS2018

Slip history of the La Cruz fault: Development of a late Miocene transformin response to increased rift obliquity in the northern Gulf of California

The Gulf of California rift has accommodated oblique divergence of the Pacific and North America plates in north-western México since Miocene time. Due to its infancy, its rifted margins preserve a rare onshore record of early continental break-up processes and an opportunity to investigate the role of rift obliquity in strain localization. We map rift-related structures and syn-tectonic basins on southern Isla Tiburón, a proximal onshore exposure of the rifted North America margin. We integrate analysis and geochronology of syn-tectonic sedimentary basins and mapping of crosscutting relationships to characterize the style and timing of fault activity. On southern Isla Tiburón, an early phase of extension initiated between~19–17 Ma and ~12.2Ma. Subsequently, these normal faults and related basins were cut by the La Cruz strike-slip fault and buried by deposits of the La Cruz basin, an elongate, fault-controlled trough coextensive with the La Cruz fault. Crosscutting relationships show that the NW-striking La Cruz fault accrued 5 ± 2 km of dextral slip ~8–4 Ma. The La Cruz fault and parallel Tiburón transform were kinematically linked to detachment faulting that accommodated latest Miocene to Pliocene oblique opening of the offshore Upper Tiburón pull-apart basin. The onset of strike-slip faulting on Isla Tiburón was synchronous with the ~8–6 Ma onset of transform faulting and basin formation along >1000 km of the reconstructed Pacific-NorthAmerica plate boundary. This transition coincides with the commencement of a clockwise azimuthal shift in Pacific-North America relative plate motion that increased the obliquity of the Gulf of California rift and formed the Gulf of California shear zone. The record from the proto-Gulf of California illustrates how highly oblique rift geometries, where transform faults are kinematically linked to pull-apart basins, enhance the ability of continental lithosphere to rupture and, ultimately, hasten the formation of new oceanic rift basins.

Gulf of California↗

USGS2023

Unzipping supercontinent Pangea: Geologic, potential field data, and buried structures, and a case for sequential Atlantic opening

Amalgamation of Pangea culminated with zippered N-to-S closing of the Theic ocean during the Alleghanian orogeny. Transpressional-rotational collision produced widespread dextral faulting throughout the eastern Appalachian hinterland, and thrust faulting in the western hinterland and foreland. The partially buried southern Appalachian Eastern Piedmont fault system is a product of late Paleozoic transpressional dextral faulting. Eastern Piedmont fault system faults are cut by two 200–190 Ma Central Atlantic Magmatic Province (CAMP) diabase dike sets (∼NW and N S), which postdate initial rifting, producing the Late Triassic-Early Jurassic basins. Sinistral reactivation of suitably oriented Paleozoic faults (e.g., Towaliga) that offset Jurassic dikes implies faulting is coeval with CAMP dike emplacement, or occurred very soon after 200–190 Ma dike emplacement. A large NE-striking sinistral fault (Estill fault) offsets magnetic and gravity highs in the Brunswick (Charleston) terrane in South Carolina and Georgia. This fault is at least 185 km long, with ca. 75–80 km maximum of sinistral separation. The Estill fault geometry agrees with the stress field associated with Central Atlantic Magmatic Province dike emplacement. We conclude that movement along the Estill and other sinistral faults represents Early Jurassic displacement that is well documented in eastern North America prior to Atlantic opening. The early Mesozoic post-failed rifting (basin formation) preceded dike emplacement and pre-spreading sinistral faulting. We recognize four stages in the Late Triassic–Jurassic tectonics and kinematics of eastern North America based on field, crosscutting relationships, and available geophysical data: (1) initial W-to-E failed transtensional rifting with formation of the S-to-N-younging Triassic-Jurassic basins; (2) CAMP diabase dikes intruded 200–190 Ma; (3) sinistral movement of large blocks of crust in the North Atlantic region including southeastern North America; and (4) spreading and opening of the Atlantic Ocean 195–170 Ma. These stages mark the reversal of Alleghanian rotational dextral transpressional (zippered) collision forming Pangea supercontinent and ‘unzipping’ the supercontinent prior to Atlantic opening.

Alabama, Georgia↗

USGS2010

Limited evidence of trans-hemispheric movement of avian influenza viruses among contemporary North American shorebird isolates

Migratory routes of gulls, terns, and shorebirds (Charadriiformes) are known to cross hemispheric boundaries and intersect with outbreak areas of highly pathogenic avian influenza (HPAI). Prior assessments of low pathogenic avian influenza (LPAI) among species of this taxonomic order found some evidence for trans-hemispheric movement of virus genes. To specifically clarify the role of shorebird species in the trans-hemispheric movement of influenza viruses, assess the temporal variation of Eurasian lineages observed previously among North American shorebirds, and evaluate the necessity for continued sampling of these birds for HPAI in North America, we conducted a phylogenetic analysis of >700 contemporary sequences isolated between 2000 and 2008. Evidence for trans-hemispheric reassortment among North American shorebird LPAI gene segments was lower (0.88%) than previous assessments and occurred only among eastern North American isolates. Furthermore, half of the reassortment events occurred in just two isolates. Unique phylogenetic placement of these samples suggests secondary infection and or involvement of other migratory species, such as gulls. Eurasian lineages observed in North American shorebirds before 2000 were not detected among contemporary samples, suggesting temporal variation of LPAI lineages. Results suggest that additional bird migration ecology and virus phylogenetics research is needed to determine the exact mechanisms by which shorebirds in eastern North America become infected with LPAI that contain Eurasian lineage genes. Because of the low prevalence of avian influenza in non-eastern North America sites, thousands more shorebirds will need to be sampled to sufficiently examine genetic diversity and trans-hemispheric exchange of LPAI viruses in these areas. Alternatively, other avian taxa with higher virus prevalence could serve as surrogates to shorebirds for optimizing regional surveillance programs for HPAI through the LPAI phylogenetic approach.

Virus Research↗

USGS2017

Unprecedented last-glacial mass accumulation rates determined by luminescence dating of loess from western Nebraska

A high-resolution chronology for Peoria (last glacial period) Loess from three sites in Nebraska, midcontinental North America, is determined by applying optically stimulated luminescence (OSL) dating to 35-50 ??m quartz. At Bignell Hill, Nebraska, an OSL age of 25,000 yr near the contact of Peoria Loess with the underlying Gilman Canyon Formation shows that dust accumulation occurred early during the last glacial maximum (LGM), whereas at Devil's Den and Eustis, Nebraska, basal OSL ages are significantly younger (18,000 and 21,000 yr, respectively). At all three localities, dust accumulation ended at some time after 14,000 yr ago. Mass accumulation rates (MARs) for western Nebraska, calculated using the OSL ages, are extremely high from 18,000 to 14,000 yr-much higher than those calculated for any other pre-Holocene location worldwide. These unprecedented MARs coincide with the timing of a mismatch between paleoenvironmental evidence from central North America, and the paleoclimate simulations from atmospheric global circulation models (AGCMs). We infer that the high atmospheric dust loading implied by these MARs may have played an important role, through radiative forcing, in maintaining a colder-than-present climate over central North America for several thousand years after summer insolation exceeded present-day values. ?? 2003 Elsevier Science (USA). All rights reserved.

Quaternary Research↗

USGS2002

Early Holocene change in atmospheric circulation in the Northern Great Plains: An upstream view of the 8.2 ka cold event

Elk Lake, in northwestern Minnesota, contains numerous proxy records of climatic and environmental change contained in varved sediments with annual resolution for the last 10,000 years. These proxies show that about 8200 calendar years ago (8.2 cal. ka; 7300 radiocarbon years) Elk Lake went from a well-stratified lake that was wind-protected in a boreal forest to a well-mixed lake in open prairie savanna receiving northwesterly wind-blown dust, probably from the dry floor of Lake Agassiz. This change in climate marks the initiation of the widely recognized mid-Holocene “altithermal” in central North America. The coincidence of this change with the so-called 8.2 cal. ka cold event, recognized in ice-core and other records from the circum-North Atlantic, and thought by some to be caused by catastrophic discharge of freshwater from proglacial lakes Agassiz and Ojibway, suggests that the two “events” might be related. Our interpretation of the Elk Lake proxy records, and of other records from less accurately dated sites, suggests that change in climate over North America was the result of a fundamental change in atmospheric circulation in response to marked changes in the relative proportions of land, water, and, especially, glacial ice in North America during the early Holocene. This change in circulation probably post-dates the final drainage of proglacial lakes along the southern margin of the Laurentide ice sheet, and may have produced a minor perturbation in climate over Greenland that resulted in a brief cold pulse detected in ice cores.

Minnesota↗

USGS2001

Crustal deformation rates in Central and Eastern U.S. inferred from GPS

Analysis of continuous GPS observations between 1996 and 2000 at 62 stations distributed throughout the central and eastern United States suggests that the area is generally stable. Seven of the 62 stations show anomalous velocities, but there is reason to suspect their monument stability. Assuming the remaining 55 stations are stable with respect to interior North America, we have found the North America-ITRF97 Euler vector (-1.88 o ± 1.04 o N, 77.67 o ± 0.39 o W, 0.201 o ± 0.004 o Myr -1 ) that minimizes the RMS station velocity. Referred to fixed North America, all of these velocities are less than 3.2 mm yr-1. Motion of several stations suggests the Mississippi embayment may be moving southward away from the rest of the continent at a rate of 1.7±0.9 mm yr -1 . The motion of the embayment produces a large gradient in velocity which, in turn, implies the highest seismic moment accumulation rate that we found. Although the highest rate is only marginally significant, the fact that it occurs near New Madrid, where earthquake risk is thought to be high, argues that the anomaly may be real. Nevertheless, the identification of the anomaly remains tentative.

Geophysical Research Letters↗

USGS2010

The North American upper mantle: Density, composition, and evolution

The upper mantle of North America has been well studied using various seismic methods. Here we investigate the density structure of the North American (NA) upper mantle based on the integrative use of the gravity field and seismic data. The basis of our study is the removal of the gravitational effect of the crust to determine the mantle gravity anomalies. The effect of the crust is removed in three steps by subtracting the gravitational contributions of (1) topography and bathymetry, (2) low-density sedimentary accumulations, and (3) the three-dimensional density structure of the crystalline crust as determined by seismic observations. Information regarding sedimentary accumulations, including thickness and density, are taken from published maps and summaries of borehole measurements of densities; the seismic structure of the crust is based on a recent compilation, with layer densities estimated from P-wave velocities. The resultant mantle gravity anomaly map shows a pronounced negative anomaly (−50 to −400 mGal) beneath western North America and the adjacent oceanic region and positive anomalies (+50 to +350 mGal) east of the NA Cordillera. This pattern reflects the well-known division of North America into the stable eastern region and the tectonically active western region. The close correlation of large-scale features of the mantle anomaly map with those of the topographic map indicates that a significant amount of the topographic uplift in western NA is due to buoyancy in the hot upper mantle, a conclusion supported by previous investigations. To separate the contributions of mantle temperature anomalies from mantle compositional anomalies, we apply an additional correction to the mantle anomaly map for the thermal structure of the uppermost mantle. The thermal model is based on the conversion of seismic shear-wave velocities to temperature and is consistent with mantle temperatures that are independently estimated from heat flow and heat production data. The thermally corrected mantle density map reveals density anomalies that are chiefly due to compositional variations. These compositional density anomalies cause gravitational anomalies that reach ~250 mGal. A pronounced negative anomaly (−50 to −200 mGal) is found over the Canadian shield, which is consistent with chemical depletion and a corresponding low density of the lithospheric mantle, also referred to as the mantle tectosphere. The strongest positive anomaly is coincident with the Gulf of Mexico and indicates a positive density anomaly in the upper mantle, possibly an eclogite layer that has caused subsidence in the Gulf. Two linear positive anomalies are also seen south of 40°N: one with a NE-SW trend in the eastern United States, roughly coincident with the Grenville-Appalachians, and a second with a NW-SE trend beneath the states of Texas, New Mexico, and Colorado. These anomalies are interpreted as being due to (1) the presence of remnants of an oceanic slab in the upper mantle beneath the Grenville-Appalachian suture and (2) mantle thickening caused by a period of shallow, flat subduction during the Laramie orogeny, respectively. Based on these geophysical results, the evolution of the NA upper mantle is depicted in a series of maps and cartoons that display the primary processes that have formed and modified the NA crust and lithospheric upper mantle.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Late Cretaceous paleomagnetism of the Tucson Mountains: Implications for vertical axis rotations in south central Arizona

The Tucson Mountains of southern Arizona are the site of an Upper Cretaceous caldera from which the rhyolitic Cat Mountain Tuff was erupted at about 72 Ma. Two magnetic units within the Cat Mountain Tuff are distinguished by paleomagnetic data in both the northern and southern Tucson Mountains. The resurgent Amole pluton (≃72 Ma) in the northern Tucson Mountains was emplaced soon after eruption of the Cat Mountain Tuff but cooled and was magnetized after northeastward tilting (50°–85°) of the adjacent caldera-fill sequence. Petrologic and paleomagnetic data indicate that the lower magnetic unit of the Cat Mountain Tuff caps the Silver Bell Mountains to the northwest. A previous paleomagnetic investigation ( N = 34) indicates that the Silver Bell Mountains have been rotated clockwise 30°±16° (95% confidence level) about a vertical axis relative to cratonic North America. A similar paleomagnetic study of Upper Cretaceous volcanic, volcaniclastic, and intrusive units in the Tucson Mountains ( N = 26) indicates that these rocks have been rotated 7°±14° clockwise relative to stable North America. A direct comparison of paleomagnetic directions for the lower unit of the Cat Mountain Tuff shows a 17°±10° clockwise rotation between the Silver Bell Mountains and the Tucson Mountains which supports the relative accuracy of the absolute rotations determined for these two mountain ranges. Preliminary paleomagnetic directions for middle Tertiary units from the Silver Bell and Tucson Mountains are consistent with clockwise rotation having occurred prior to deposition of these rocks. Clockwise rotation of crustal blocks in southern Arizona likely was associated with strike-slip movement on major northwest trending faults in the region, and this movement may have been related to oblique subduction of oceanic plates along the western continental margin in Late Cretaceous and early Tertiary time. The available paleomagnetic data indicate that rocks in southern Arizona have not remained unrotated with respect to North America since Late Cretaceous time and that vertical axis rotations may have played an important role in the region during Laramide deformation.

Journal of Geophysical Research↗

USGS1996

The Amazon-Laurentian connection as viewed from the Middle Proterozoic rocks in the central Andes, western Bolivia and northern Chile

Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Perú form the Arequipa-Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171±20 Ma and 1158±12 Ma; U-Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U-Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite- and granulite-facies metamorphic mineral assemblages (∼1080 Ma metamorphism; U-Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Belén Schist in northern Chile generally have present-day low 206 Pb/ 204 Pb (<18.0), elevated 207 Pb/ 204 Pb (>15.57), and elevated 208 Pb/ 204 Pb (37.2 to 50.7) indicative of high time-averaged Th/U values. The Middle Proterozoic rocks in general have higher present-day 206 Pb/ 204 Pb values than those of the Early Proterozoic rocks of the Arequipa massif ( 206 Pb/ 204 Pb between 16.1 and 17.1) but lower than rocks of the southern Arequipa-Antofalla craton ( 206 Pb/ 204 Pb> 18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa-Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa-Antofalla craton overlap those of the Amazon craton, thereby supporting a link between these cratons and seemingly precluding part of the Arequipa-Antofalla craton from being a detached fragment of another craton such as eastern Laurentia, which has been characterized by a different U/Pb history. Pb isotopic compositions for the Arequipa-Antofalla craton are, furthermore, distinct from those of the Proterozoic basement in the Precordillera terrane, western Argentina, indicating a Pb isotopic and presumably a tectonic boundary between them. The Pb isotopic compositions for the Precordillera basement are similar to those of eastern Laurentia, and support other data indicating that these rocks are a detached fragment of North America. Finally, the distinct Pb isotopic evolution history of the Arequipa-Antofalla craton and eastern Laurentia require minor modification to tectonic models linking eastern North America-Scotland to the oroclinal bend in western South America.

central Andes↗

USGS2012

Late Cretaceous paleomagnetism and clockwise rotation of the Silver Bell Mountains, south central Arizona

Late Cretaceous ash flow volcanism in the Silver Bell Mountains of southern Arizona (32.3°N, 248.5°E) was associated with caldera formation and porphyry copper mineralization. Oriented samples from 34 sites in volcanic, volcaniclastic, and intrusive units related to this episode of igneous activity (73–69 Ma) yield a mean paleomagnetic direction of I = 61.2°, D = 24.0°, α 95 = 7.6°. Primary remanent magnetizations are indicated by the presence of both normal and reversed polarities and by the significantly improved grouping (95% confidence level) of site-mean directions for a widespread tuff unit after structural correction. A comparison of this direction with a direction at Silver Bell calculated from contemporaneous units for stable North America ( I = 58.8°, D = 342.3°, α 95 = 7.7°) indicates a Late Cretaceous paleolatitude anomaly of −2.4°±7.6° (not significant at the 95% confidence level) and a declination anomaly of 41.7°±14.3° for the Silver Bell Mountains. Previously determined paleomagnetic data for southeastern Arizona suggest that this apparent clockwise rotation in the Silver Bell Mountains is a local phenomenon. Although preliminary, the average paleomagnetic direction for Oligocene and lower Miocene rocks in the Silver Bell area ( I = 43.8°, D = 357.3°, α 95 = 16.5°) is similar to that calculated for stable North America ( I = 50.2°, D = 352.2°, α 95 = 3.9°), implying that the observed rotation in the Silver Bell Mountains occurred before 26 Ma and was most likely associated with dextral strike-slip movement along the Ragged Top and related WNW trending faults bounding the Silver Bell Mountain block. These data, in conjunction with plate reconstructions and other paleomagnetic data from southwestern North America, imply that WNW trending strike-slip faults may have played an important role during Late Cretaceous to early Tertiary deformation in southern Arizona.

Journal of Geophysical Research Solid Earth↗

USGS2010

A kinematic model of southern California

We propose a kinematic model for southern California based on late Quaternary slip rates and orientations of major faults in the region. Internally consistent motions are determined assuming that these faults bound rigid blocks. Relative to North America, most of California west of the San Andreas fault is moving parallel to the San Andreas fault through the Transverse Ranges and not parallel to the motion of the Pacific plate. This is accomplished by counterclockwise rotation of California south of the San Andreas fault and by the westward movement of central California north of the Gar lock fault. The velocities of the blocks are calculated along several paths in southern California that begin in the Mojave Desert and end off the California coast. A path that crosses the western Transverse Ranges accumulates the accepted relative North America-Pacific plate velocity, whereas paths to the north and south result in a significant missing component of motion. This implies the existence of a zone of active deformation in southern California that is interpreted to include the western Transverse Ranges and northwest trending, predominately strike-slip faults close to the coast both north and south of the Transverse Ranges. Strain on this system accounts for about a third of the total North America-Pacific plate motion.

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