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The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern (1). Multi-year droughts during the instrumental period2 and decadal-length droughts of the past two millennia (1, 3) were shorter and climatically different from the future permanent, 'dust-bowl-like' megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming (4). So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies (5) to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant [C.sub.4] plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ~2°C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4-6°C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase., The hydroclimatology of the southwestern US shows significant natural variability including major historical droughts (1). Models of climate response to anthropogenic warming predict future dustbowl-like conditions that will last much [...]

A composite magnetostratigraphy based on the magnetic polarity data from four sections of the uppermost Triassic and lower most Jurassic Moenave Formation, Utah and Arizona, USA, can be correlated to the marine successions at Saint Audrie's Bay (UK), Oyuklu, Turkey, and the Southern Alps, Italy, and to the nonmarine sections in Morocco, northern Africa, and the Newark Basin, eastern North America, all deposited across the Triassic-Jurassic boundary. Our proposed correlation provides a stratigraphic framework to tie Triassic-Jurassic sedimentation in the American Southwest to the marine UK, Turkey, and Italy sections, and to the Pangea rift history, including extrusive igneous rocks, preserved in Morocco and in the Newark Basin. The Moenave polarity record is characterized by mostly normal polarity, as is consistent with other polarity records across the Triassic-Jurassic boundary, and is interrupted by at least two well-defined reverse-polarity magnetozones. On the basis of available paleontologic information, we interpret the oldest welldefined, reverse-polarity magnetozone, M2r of the Moenave Formation, to correlate with SA5n.2r or SA5n.3r of the Saint Audrie's Bay record, H- of the Oyuklu record, BIT5n.Ir of the Italcementi Quarry record, the oldest reverse magnetozone in sedimentary rocks in Morocco, and with reverse magnetozone E23r of the Newark Basin. The youngest reverse magnetozone of the Moenave Formation, M3r, is correlated to the latest Triassic magnetozones SA5n.5r of the Saint Audrie's Bay record, J- of the Oyuklu record, and with the interval of reverse polarity in the 'intermediate unit' of the Morocco record. Magnetostratigraphic correlations and marine biostratigraphic information support placement of the Triassic-Jurassic boundary in the middle to upper Whitmore Point Member of the Moenave Formation, the Lias Group of the Saint Audrie's Bay section, the chert-rich limestone of the Oyuklu section, above the Zu Limestone in Italy, and in the central Atlantic magmatic province extrusive zone in the Morocco and the Newark records. doi: 10.1130/B30136.1

In the Teton River Canyon, eastern Idaho, the ca. 2.06 Ma, 130-m-thick Huckleberry Ridge Tuff exhibits large-scale rheomorphic fold geometries defined by eutaxitic fabrics parallel to both the primary internal zonation and the basal contact with older strata. Paleomagnetic data from a large-amplitude (>150 m), northwest-trending, overturned fold near the failed Teton Dam indicate folding above maximum magnetization unblocking temperatures (>580[degrees]C). The in situ characteristic remanent magnetization (ChRM) direction is indistinguishable from previous studies of undeformed Huckleberry Ridge Tuff, and a fold test is negative (k minimized at 100% unfolding). Anisotropy of magnetic susceptibility data reveal magnetic foliation planes that dip northeast, roughly parallel to the axial surface of the fold. Because deformed and undeformed Huckleberry Ridge Tuff exposures preserve the same anomalous ChRM direction, large-scale rheomorphic structures in the tuff must have formed rapidly at high temperatures shortly after development of compaction fabrics. Post-welding, high-temperature deformation is consistent with field evidence indicating rapid, plastic secondary deformation of much of the tuff prior to devitrification. doi: 10.1130/G30492.1

The Silver Peak extensional complex, located in the Silver Peak Range of west-central Nevada, is a displacement-transfer system linking the Furnace Creek-Fish Lake Valley fault system and transcurrent faults of the central Walker Lane. Late Neogene, northwest-directed motion of an upper plate, composed of lower Paleozoic sedimentary rocks and late Tertiary volcanic and volcaniclastic strata, exhumed a lower-plate assemblage of metamorphic tectonites with Proterozoic and Mesozoic protoliths. Paleomagnetic investigation of Miocene-Pliocene pyroclastic and sedimentary rocks of the upper plate and Miocene mafic dikes in the lower plate reveals modest horizontal-axis tilting (northwest-side-up) and vertical-axis rotation (clockwise) within the extensional complex. Eight to ten samples from each of 123 sites were demagnetized; 95 sites yielded interpretable results. Dual-polarity results from one population of mafic dikes in the lower-plate assemblage indicate moderate, northwest-side-up tilting (declination D = 329[degrees], inclination I = 37[degrees], [[alpha].sub.95] = 4.3[degrees] number N = 30 sites; in situ) ([[alpha].sub.95] = the confidence limit for the calculated mean direction expressed as an angular radius from the calculated mean direction). Some dikes yield exclusively normal-polarity results that are interpreted to indicate modest clockwise vertical-axis rotation (D = 021[degrees], I = 57[degrees], [[alpha].sub.95] = 4.3[degrees], N = 19 sites; in situ) concurrent with uplift of the lower-plate rocks, and nine sites yield magnetization directions that are north-directed with positive inclinations of moderate steepness, similar to an expected Miocene field. Late Miocene pyroclastic rocks in the upper plate yield normal-polarity magnetizations suggestive of moderate, clockwise, vertical-axis rotation (D = 032[degrees], I = 53[degrees], [[alpha].sub.95] = 8.8[degrees], N = 10 sites). The apparent clockwise rotation is unlikely to result from incomplete sampling of the geomagnetic field, because the overall dispersion of the VGP (virtual geomagnetic pole) positions is high for the latitude of the site location. Middle Miocene sedimentary rocks probably were remagnetized shortly after deposition. Of eight [sup.40]Ar/[sup.39]Ar determinations from mafic dikes in the lower plate, five groundmass concentrates yield saddle-shaped age spectra, and one separate provided a plateau date of low confidence. Isochron analysis reveals that all six groundmass concentrates contain excess Ar. If rapid cooling and Ar retention below ~250[degrees] C are assumed, the preferred age estimate for mafic intrusions is provided by isochron dates and suggests emplacement between 12 and 10.5 Ma. The [sup.40]Ar/ [sup.39]Ar age-spectrum data are consistent with existing fission-track cooling and K-Ar isotopic age information from lower-plate granitic rocks and indicate rapid cooling of the lower-plate assemblage from well above 300 [degrees] C to 100[degrees] C between 13 and 5 Ma. Rapid cooling may explain the overall distribution of paleomagnetic results from lower-plate intrusions such that the earliest acquired magnetizations reflect both northwest-side-up tilt and clockwise rotation and the younger magnetizations reflect northwest-side-up tilt. Overall, the paleomagnetic data from the Silver Peak extensional complex are interpreted to suggest that vertical-axis rotation of crustal-scale blocks, associated with displacement transfer in the central Walker Lane, may play an integral part in accommodating strain within a continental displacement-transfer system. Keywords: ash-flow tuffs, extensional tectonics, geochronology, metamorphic core complexes, paleomagnetism, Walker Lane.

The southern Sangre de Cristo Mountains, New Mexico, contain evidence of glacial activity from the late Pleistocene to late Holocene. Sediment cores recovered from an alpine bog (3100 m) trapped behind a Pinedale age moraine, ~2 km downvalley from a high-elevation cirque, reached glacial-age debris and recovered ~6 m of lake clays overlain by gyttja. Accelerator mass spectrometry, dating, sedimentology, variations in magnetic properties, and organic carbon data reveal six distinct periods of glacial and/or periglacial activity. These include a late Pleistocene Pinedale glacial termination just before 12 120 [sup.14]C yr B.P., a Younger Dryas chron cirque glaciation, an early Neoglacial periglacial event (ca. 4900 [sup.14]C yr B.P.), a late Holocene cirque glaciation (3700 [sup.14]C yr B.P.), as well as late Holocene periglacial events at 2800 [sup.14]C yr B.P. and the Little Ice Age (ca. 120 14C yr B.P.). Cold events in the middle to late Holocene correlate with subtle ice-rafting events in the North Atlantic and records of cold events in North America and Europe and were probably hemispheric in extent. Keywords: climate change, glacial geology, Holocene, New Mexico, Younger Dryas.

The Chuar Group, approximately 5,250 feet thick, offers a readily-dated, accurate record of tectonic, environmental, and biological changes that began the Phanerozoic Eon. This group was deposited during the extension of the north-tending Butte fault system and contains many cyanobacteria, algae, and heterotrophic protists.

A study of extension, volcanism, and sedimentation in the middle Eocene Panther Creek half graben in central Idaho shows that it formed rapidly during an episode of voluminous volcanism. The east-southeast-tilted Panther Creek half graben developed across the northeast edge of the largest cauldron complex of the Challis volcanic field and along the northeast-trending Trans-Challis fault zone. Two normal fault systems bound the east side of the half graben. One fault system strikes northeast, parallel to the Trans-Challis fault zone, and the other strikes north to northwest. The geometry of the basin-fill deposits shows that movement on these two normal fault systems was synchronous and that both faults controlled the development of the Panther Creek half graben. Strikes of the synextension volcanic and sedimentary rocks are similar throughout the half graben, whereas dips decrease incrementally upsection from as much as 60 [degrees] to less than 10 [degrees]. Previous K-Ar dates and a new 4 [degrees] Ar/39Ar plateau date from the youngest widespread tuff in the basin suggest that most of basin formation spanned 3 m.y. between about 47.7 Ma and 44.5 Ma. As much as 6.5 km of volcanic and sedimentary rocks were deposited during that time. Although rates of extension and subsidence were very high, intense volcanic activity continually filled the basin with ash-flow tuffs, outpacing subsidence and sedimentation, until the end of basin development. After the abrupt end of Challis volcanism, locally derived pebble to boulder conglomerate and massive, reworked ash accumulated in the half graben. These sedimentary rocks make up a small part of the basin fill in the Panther Creek half graben and were derived mainly from Proterozoic metasedimentary rocks uplifted in the footwall of the basin. The east-southeast tilt of the sedimentary rocks, their provenance and coarse grain size, and the presence of a gravity slide block derived from tilted volcanic rocks in the hanging wall attest to continued tectonism during conglomerate deposition. Provenance data from the sedimentary rocks imply that the highland in the footwall of the Panther Creek half graben was never thickly blanketed by synextension volcanic rocks, despite intense volcanic activity. Analysis of the Panther Creek half graben and other intra-arc rift basins supports previous interpretations that relative rates of volcanism and subsidence control the proportion of volcanic rocks deposited in intra-arc rifts.

At present, few paleomagnetic data exist to test tectonic models that link northern Mexico with the evolution of the southern Cordillera of North America. We present new paleomagnetic data for the Antimonio terrane in northwest Mexico. A moderate-inclination, dual-polarity yet secondary magnetization in Triassic-Jurassic strata of the Antimonio Formation and a primary remanence in Cretaceous igneous rocks (combined result: D = 338.9 [degrees], I = 57.4 [degrees]; [[Alpha].sub.95] = 5.6 [degrees], k = 34.8; N = 20 sites) are interpreted to indicate stability of the Antimonio terrane with respect to the craton since the Late Cretaceous. A more important result, however, is a shallow-inclination, dual-polarity magnetization in Antimonio Formation strata that fails a fold test (D = 186.7 [degrees], I = -28.4 [degrees]; [[Alpha].sub.95] = 7-6 [degrees], k = 28.2; N = 14 sites). We interpret the shallow-inclination magnetization as a secondary remanence acquired during accretion of the Antimonio terrane to the North American margin in the Middle or Late Jurassic, or during deformation within an essentially autochthonous Jurassic continental arc.

Paleomagnetic results from Early Proterozoic metabasite sills and Middle Proterozoic diabase dikes from the southern Highland Mountains of southwestern Montana give well-defined, dual-polarity magnetizations that are statistically identical to those from a small Late Cretaceous pluton that cuts the dikes. The concordance of paleomagnetic directions from rocks of three widely separated ages indicates that the Proterozoic rocks were remagnetized, probably during Late Cretaceous time. Paleomagnetic, rock magnetic, and petrographic observations from the metabasite and diabase samples indicate that remanence is carried primarily by low-Ti magnetite. Combining virtual geomagnetic poles from metabasite sills, diabase dikes, and the Late Cretaceous pluton, we obtain a paleomagnetic pole at 85.5 [degrees] N, 310.7 [degrees] E (K = 19.9, [A.sub.95] = 9.1 [degrees], N = 14 sites) that is similar to a reference pole from the 74 Ma Adel Mountain Volcanics of western Montana. Biotite and hornblende 40Ar/39Ar isotopic dates from host basement gneiss and a hornblende from a remagnetized metabasite sill yield ages of ca. 1800 Ma; these dates probably record cooling of the southern Highland Mountains following high-grade metamorphism at 1.9-1.8 Ga. The gneiss and metabasite age spectra show virtually no evidence of disturbance, indicating that the basement rocks were never heated to temperatures sufficient to cause even partial resetting of their argon systems. Thus, the overprint magnetization of the Highland Mountains rocks is not a thermoremanent magnetization acquired during conductive cooling of nearby Late Cretaceous plutons. Remagnetization of the metabasite sills and diabase dikes was probably caused by localized thermochemical and thermoviscous effects during circulation of Late Cretaceous hydrothermal fluids related to epithermal mineralization. The absence of significant disturbance to the 40Ar/39Ar age spectrum from the remagnetized metabasite hornblende indicates that some secondary magnetizations may go unrecognized and undated, even if 40Ar/39Ar dating is applied.

Reliable ages of proximal conglomerates in the Cordilleran foreland basin that are associated with emplacement and erosion of major thrust sheets are essential for reconstructing the kinematic history of the Sevier fold-thrust belt. Although these conglomerates have been dated by palynology, their absolute ages have been difficult to determine because of their coarse-grained texture and a lack of marine interbeds and tuffaceous deposits. We collected sets of oriented samples from outcrops in northeastern Utah, USA, to construct an overall magnetic polarity stratigraphy that can be correlated to the geomagnetic polarity time scale (GPTS). We sampled fine-grained, hematitic interbeds in the Upper Cretaceous Echo Canyon Conglomerate and Weber Canyon Conglomerate. Common paleomagnetic and rock magnetic analyses were conducted, and several rock magnetic results indicated that the dominant magnetic carriers in these weakly magnetized rocks are hematite and very subordinate magnetite/titanomagnetite/maghemite and goethite. Demagnetization results show that hematitic, fine-grained sandstone to siltstone intervals carry a geologically stable magnetization with directions and polarity consistent with the Late Cretaceous geomagnetic field. A small percentage of samples carry a low laboratory unblocking temperature secondary overprint residing primarily in goethite. Magnetic polarity results indicate that the Echo Canyon Conglomerate is exclusively of normal polarity and that the younger Weber Canyon Conglomerate is of normal polarity in its lowermost part, reverse polarity in the middle, and normal polarity in the upper part of the sequence. The new data indicate that these coarse-grained strata were most likely deposited over the time span of the magnetic polarity Chron (C) 34n to C33r interval and the younger C33r to C33n interval; the former interval includes the Santonian-Campanian stage boundary (ca. 83.4 Ma/83.1 Ma). Palynological data suggest that these rocks span Coniacian-Santonian time (ca. 89--84 Ma); thus, the most parsimonious correlation of the normal polarity magnetozone of the Echo Canyon Conglomerate is with the youngest part of C34n Superchron, which is of ca. 30 Ma duration (ca. 115 Ma to 83.4 Ma/83.1 Ma). The normal polarity magnetozone of the lower part of the younger Weber Canyon Conglomerate likely correlates to the youngest part of C34n, whereas the reverse and normal magnetozone from the middle and upper parts of the Weber Canyon Conglomerate likely correlate to C33r and C33n, respectively. We infer that the Santonian-Campanian boundary resides in the lower Weber Canyon Conglomerate, which implies that deposition of the unit started prior to the C34n/C33r boundary age (ca. 83.4 Ma/83.1 Ma) and continued through the C33r and C33n chrons. Sediment provenance data and growth structures tie the Echo Canyon and Weber Canyon Conglomerates to emplacement of the Crawford thrust sheet. Based on the magnetic polarity data, as constrained by the biostratigraphic age estimates from these synorogenic deposits, we hypothesize that the principal displacement along the Crawford thrust started during the Coniacian (>C34n/C33r boundary age) and continued into the middle Campanian (

The characteristic magnetization of the non-hematitic sandstones of the upper Carnian-lower Norian Dockum Group is an 'early' magnetization acquired at the time of or after deposition. But the characteristic magnetization of the red bed sites has been analysed as secondary magnetization of earliest Jurassic age. The observation of conflicting magnetostratigraphies support the secondary origin of this magnetization. The general position of the J-1 cusp shown by poles in the Piedmont province, the Newark basin and the Colorado plateau is confirmed and the magnitude of rotation of Colorado is less than 5 degrees.

Paleomagnetic data are used to test controversial aspects of Cordilleran metamorphic core complexes, including the original dip of extensional structures, origin of the mylonitic front, and applicability of rolling-hinge models. We obtained paleomagnetic data (115 sites, 82 accepted for analysis) from the weakly deformed interior of a synkinematic, footwall intrusive suite and Proterozoic footwall rocks of the South Mountains metamorphic core complex, central Arizona. These rocks yield dual polarity, high unblocking temperature, and high to moderate coercivity magnetizations. Positive baked contact tests indicate that footwall rocks possess primary thermoremanent magnetizations (TRMs) or high-temperature thermochemical remanent magnetizations (TCRMs) acquired early in their cooling history and during ductile and brittle extensional deformation of structurally higher rocks. This is consistent with thermochronologic data indicating rapid synkinematic cooling from crystallization through the range of laboratory unblocking temperatures for the magnetic mineralogy of these rocks (between about 22 and 17.5 Ma). Paleomagnetic data are considered as two populations based on the structural asymmetry of the South Mountains metamorphic core complex: (1) a front side characterized by northeast-dipping ([approximately]10 [degrees]) mylonitic fabrics and brittle extensional structures, and (2) a back-dipping side characterized by rollover of the mylonitic zone to form a southwest- or back-dipping ([approximately]15 [degrees]) mylonitic front. Comparison of paleomagnetic data from these two sides suggests that the back-dipping mylonitic front was synkinematically tilted about 10 [degrees] down-to-the-southwest. The data support a folded shear zone hypothesis for origin of the mylonitic front and the interpretation that footwall rocks possess primary, Miocene-age TRMs or TCRMs. A second regional fold test involved data from sites on both flanks of the topographically prominent northeast-trending mountain range-scale antiform. The negative result from this fold test demonstrates that this structure formed early in the extensional history and prior to magnetization acquisition by the plutons. We obtained a well-grouped footwall grand mean from 62 front-side and 20 back-dipping site means (N = 82, D = 1.0 [degree] , I = 51.7 [degrees], k = 41.8, [[Alpha].sub.95] = 2.5 [degrees]). We calculated this grand mean with the assumption that front-side sites have remained structurally untilted, whereas back-dipping side sites require removal of 10 [degrees] of southwest dip. This grand mean is statistically indistinguishable (95% confidence level) from time-averaged Miocene expected directions. We thus conclude that the current gentle dip of front-side mylonites and detachment faults is original. Therefore, both ductile and brittle extensional deformations of the South Mountains metamorphic core complex were accommodated along low-angle structures (dip of [less than or equal to] 15 [degrees]). Our interpretation refutes the widespread applicability of models that predict metamorphic core complexes to represent tilted crustal blocks originally bounded by moderate-angle normal faults and does not support rolling-hinge models of metamorphic core complex evolution that require a moderate-angle ramp.

Paleomagnetic data have been obtained from Miocene intrusions, including crosscutting mafic and felsic dikes, and Proterozoic crystalline rocks to evaluate deformation during Miocene and younger extension and unroofing of the Black Mountains, Death Valley, California. Synrift intrusions contain a well-defined and, at the site level, well-grouped magnetization, interpreted to be of dual polarity, the in situ direction of which is discordant in declination and inclination with an expected late Cenozoic reference direction. The oldest of these intrusions (11.6 + or - 0.2 Ma) gives a magnetite-dominated remanent magnetization of high coercivity and high laboratory unblocking temperatures (about 550 to 580 degrees C) that is interpreted to predate unroofing from mid-crustal depths. In situ site mean directions of this magnetization are directed toward the west and west-northwest with moderate to shallow positive (down) and negative (up) inclinations. The variation in direction of magnetization, particularly inclination, with site locality around the turtleback structures along the western flank of the Black Mountains, is interpreted to result from folding of the intrusion after remanence acquisition. Younger intrusions (less than or equal to 8.7 Ma) generally give magnetizations with inclinations similar to expected Miocene values. Two populations of in situ site means are identified: one with southwest declination and negative inclination; the other with northward declination and positive inclination. A preferred interpretation for footwall deformation involves, from oldest to youngest: (1) southwest-side down tilting of the entire range block of some 20 degrees to 40 degrees to possibly 70 degrees and, at least locally, folding of the crystalline rocks, on a trend parallel to the Death Valley turtlebacks, between 11.6 and 8.7 Ma; (2) progressive east-to-west footwall unroofing between 8.7 and ca. 6.5 Ma; and (3) clockwise rotation (50 degrees to 80 degrees) of much of the Black Mountains after the core detached from stable terrane to the west. We interpret late rotation of the Black Mountains as oroflexure related to right-lateral shear along the Death Valley fault zone.