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NSF National Science Foundation (NSF) [EAR 0958976, 0958723, 0958915, 0959107, 0958859, EAR-1338583]; Deutsche ForschungsgemeinschaftGerman Research Foundation (DFG); Lamont-Climate Center; Ann and Gordon Getty Foundation

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Ultramafic pseudotachylytes have been regarded as earthquake fossils formed at mantle depths (i.e., >30 km). Here we show that pseudotachylytes hosted by ultramafic rocks from three localities have distinct magnetic properties. Fresh host-peridotites contain only small amounts of coarse-grained magnetite. In contrast, the ultramafic pseudotachylytes contain variable amounts of significantly finer magnetite that formed coseismically through melting. Among each locality, magnetite abundance in the pseudotachylytes ranges over several orders of magnitude (4~2000 ppm), and magnetic grain size varies considerably (from single domain to multidomain). Because the host-peridotites are compositionally similar, the pseudotachylyte magnetic properties are interpreted to primarily reflect the physical and cooling conditions prevailing during seismic slip. Further, the examination of laboratory-produced ultramafic pseudotachylytes shows that quenching does not produce superfine magnetite. We hypothesize that the magnetic properties of ultramafic pseudotachylytes are controlled by fO2 and in consequence vary systematically with depth of formation. Therefore these properties can be used to assess if the ruptures producing the earthquakes that these pseudotachylytes represent nucleated at actual mantle depths or at shallow depths during exhumation of mantle rocks.

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We present the magnetic properties of the Noachian Martian breccia NWA 7034. Among the 25 unpaired Martian meteorites studied to date, NWA 7034 has a unique magnetic mineralogy. It contains about 15 wt % of iron oxides as magnetite that has experienced cation substitution and partial alteration to maghemite, with about a quarter of the oxides being pure maghemite. It also contains oxyhydroxides in the form of superparamagnetic goethite. The presence of maghemite and goethite makes NWA 7034 the most oxidized Martian meteorite. The overall magnetic assemblage is partly linked to near-surface hydrothermal alteration. The high concentration of magnetic phases with high laboratory unblocking temperatures makes NWA 7034 a plausible analogue source lithology for the strong magnetization of the Martian Noachian crust. Near-surface hydrothermal alteration can enhance the remanence of Martian rocks and account for local, high magnetic anomalies of shallow source.

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Cordilleran metamorphic core complexes (MCCs) and their associated structures (detachment faults and mylonites) are the products of large-magnitude Cenozoic extension. Paleomagnetic data from the Harquahala, Harcuvar, Buckskin, and Whipple MCCs of western Arizona and southeastern California (∼1900 samples; 191 of 278 sites accepted for inclusion in group and grand means) allow us to estimate the magnitude of footwall tilting associated with extension. All MCC crystalline rocks sampled are in the footwalls of associated brittle detachment faults. Rocks sampled include synkinematic, Miocene intrusions (dikes and plutons), Late Cretaceous granites, and Proterozoic host rocks. These rocks (with different levels of success at the site level) yield dual polarity, high unblocking temperature, and high to moderate coercivity magnetizations. Demagnetization behavior reveals that those sites with well-grouped magnetizations and high laboratory unblocking temperatures and/or high to moderate coercivities have characteristic remanent magnetizations carried by either magnetite, with minor hematite, or by hematite, with minor magnetite. Results of baked contact tests show that these footwall rocks possess thermoremanent magnetizations or high-temperature thermochemical remanent magnetizations acquired early in their cooling history, either before or during extension. Our overall interpretation of the paleomagnetic data is that during Cenozoic extension, MCC footwalls tilted (flattened) only a modest amount (

The tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens. The classic central Cordilleran foreland fold-and-thrust belt in the United States (Late Jurassic to early Tertiary Sevier belt) and the more internal zones to the west (central Nevada thrust belt) provide data critical to understanding the developmere of internal and external parts of orogens. The Garden Valley thrust system, part of the central Nevada thnkst belt, crops out in south-central Nevada within a region generally considered to be the hinterland of the Jurassic to Eocene Sevier thrust belt. The thrust system consists of at least four principal thrust plates composed of strata as young as Pennsylvanian in age that are unconformably overlain by rocks as old as Oligocene, suggesting that contraction occurred between those times. New U/Pb dates on intrusions that postdate contraction, combined with new paleomagnetic data showing significant tilting of one area prior to intrusion, suggest that regionally these thrusts were active before -85-100 Ma. The thrust faults are characterized by long, relatively steeply dipping ramps and associated folds that are broad and open to close, upright and overturned. Although now fragmented by Cenozoic crustal extension, individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike. We correlate the structurally lowest thrust of the Garden Valley thrust system, the Golden Gate-Mount Irish thrust, southward with the Gass Peak thrust of southern Nevada. This correlation carries the following regional implications. At least some of the slip across Jurassic to mid-Cretaceous foreland thrusts in southern Nevada continues northward along the central Nevada thrust belt rather than noaheastward into Utah. This continuation is consistent with age relations, which indicate that thrusts in the type Sevier belt in central Utah are synchronous with or younger than the youngest thrusts in southern Nevada. This in turn implies that geometrically similar Sevier belt thrusts in Utah must die out southward before they reach Nevada, that slip along the southem Nevada thrusts is partitioned

The Colorado River extensional corridor (CREC) accommodated up to 100% crustal extension between ∼23 and 12 Ma. The southernmost Sacramento Mountains core complex lies within this region of extreme extension and exposes a footwall of Proterozoic, Mesozoic, and Miocene crystalline rocks as well as Miocene volcanic and sedimentary rocks in the hanging wall to the regionally developed Chemehuevi-Sacramento detachment fault (CSDF) system. New structural, U-Pb-zircon, Ar-Ar, and fission track geochronologic and paleomagnetic studies detail the episodic character of both magmatic and tectonic extension in this region. Extension in this part of the CREC was initiated with tectonic slip along a detachment fault system at a depth between 10 and 15 km. Magmatic extension at these crustal levels began at ∼20–19 Ma and directly account for 5–18 km of extension (10–20% of total extension) in the southern Sacramento Mountains. Three discrete magmatic episodes record rotation of the least principal stress direction, in the horizontal plane, from 55° to 15° over the following ∼3 Myr. The three intrusions bear brittle and semibrittle fabrics and show no crystal-plastic fabric development. The final 3–4 Myr of stretching were dominated by amagmatic or tectonic extension along a detachment fault system, with extension directions rotating back toward 75°. The data are consistent with extremely rapid cooling and uplift of Miocene footwall rocks; the ∼19 Ma Sacram suite was emplaced at a mean pressure of ∼3.0 kbars and uplifted rapidly to a level in the crust where brittle deformation was manifested by movement on the detachment fault at ∼16 Ma. By ∼14 Ma the footwall was exposed at the surface, with detritus shed off and deposited in adjacent hanging wall basins.

Two ancient magnetizations have been isolated in rocks of the Caborca terrane, northwest Mexico. The characteristic magnetizations of Neoproterozoic and Paleozoic miogeoclinal shelf-strata, arc-derived Lower Jurassic marine strata, and Jurassic volcanic and volcaniclastic rocks are of dual polarity and east-northeast declination (or south-south-west) and shallow inclination. Magnetizations in Neoproterozoic and Paleozoic miogeoclinal strata are interpreted as secondary (J*) and to be of similar age to those observed in Lower and Middle Jurassic rocks. Remanence acquisition is bracketed between about 190 and 160 Ma. The overall mean (D=15.0°, I=8.5°; n=38 sites; six localities; k=19.1, α95=5.5°) suggests a moderate to large clockwise rotation of 12 to 50° (depending on reference direction assumed) of the Caborca terrane, and rocks of the Sonoran segment of the Cordilleran volcanic arc, with respect to the North America craton. When compared with expected inclinations, observed values are not anomalously steep, arguing against statistically significant southward latitudinal displacement of the Caborca block after remanence acquisition. Late Cretaceous intrusions yield primary, dual-polarity steep inclination “K” magnetizations (D=341.4°, I=52.3°; n=10 sites; five localities; k=38.3, α95=7.9°) and have locally remagnetized Neoproterozoic and Jurassic strata. When present, secondary (K*) magnetizations in Neoproterozoic strata are of higher coercivity and higher unblocking temperature than the characteristic (J*) magnetization. Importantly, the regional internal consistency of data for Late Cretaceous intrusions suggests that effects of Tertiary tilt or rotation about a vertical axis over the broad region sampled (∼5000 km2) are not substantial. Late Cretaceous primary (K) magnetizations and secondary (K*) magnetizations yield a combined mean of D=348.1°, I=50.7° (N=10 localities; 47 sites; k=53.5, α95=6.7°), indicating at most small (

Small, yet systematic, differences between paleomagnetic poles derived from strata on the Colorado plateau and paleopoles determined from rocks on the North America craton have been interpreted to support the hypothesis of modest post-Late Cretaceous clockwise rotation of the plateau, as a quasi-rigid body, with respect to the craton. Using an iterative search for the best fit Euler pole and rotation angle, comparison of the best quality Late Carboniferous through Late Jurassic paleomagnetic poles from the Colorado plateau and the North America craton gives a cumulative rotation estimate (based on a rotation pole at 34°N, 105°W) of 7.4°±3.8° (95% confidence limits). A similar comparison using subsets of the cratonic database from localities in (1) northeast North America and (2) the craton platform interior give larger (8.8°±3.6°) and smaller (5.1°±3.8°) estimates, respectively, reflecting the fact that poles from localities in northeast North America, in particular those from Triassic rift basins, indicate a larger rotation (as concluded in direct pole to pole comparisons). The Euler pole, as determined by the paleomagnetic data only, can lie anywhere within a relatively large area that encompasses locations in the western United States previously proposed from geological observations. Paleomagnetic data and geologic observations, together or independently, do not support the hypothesis of a large Colorado plateau rotation (of 11° to 15°). If geologically reasonable, previous estimates of significant (>∼20 km) dextral slip along the eastern margin of the plateau require a position for the Euler pole east of the 105°W meridian.

The Electra Lake Gabbro is a small 1.435 Ga pluton that intrudes 1.7 to 1.6 Ga gneisses and schists of the Needle Mountains in southwestern Colorado. Paleomagnetic samples were collected from the main phases of the gabbro, diabase dikes, granite, and alaskite dikes that cut the gabbro and from a partially melted zone in gneiss along the southern margin of the pluton. Gabbro, diabase, and some melt zone samples have a single-polarity characteristic magnetization of northeast declination (D) and moderate negative inclination (I). Demagnetization behavior and rock magnetic characteristics indicate that the remanence is carried by nearly pure magnetite. After correction for the minor west dip of overlying Paleozoic strata, we obtain a mean direction of D = 32.1°, I = −41.9° ( k: = 94, α95 = 3.3°, N = 21 sites) and a paleomagnetic pole at 21.1°S, 221.1°E, (K = 89, A95 = 3.4°). This pole is similar to poles from the Middle Proterozoic Belt Supergroup but is located at a higher southerly latitude than poles from other 1.47–1.44 Ga plutons from North America, most of which plot at equatorial latitudes. The reason for this discrepancy is not clear but may result from a combination of factors, including unrecognized tilting of the gabbro, the failure of this relatively small pluton to fully average paleosecular variation, and uncertainties in the overall reliability of other 1.5–1.4 Ga poles of the North American apparent polar wander path.

The origins of multiple magnetizations of the Archean Stillwater Complex have been investigated through scanning electron microscopy and scanning transmission electron microscopy observations of mineralogical relations, using representative samples from nine sites in mafic Banded series rocks. On the basis of directional grouping and demagnetization behavior, three magnetizations (here labeled A, B, and C) have been recognized. The natural remanent magnetizaton (NRM) is lypically dominated by only one of these magnetizations and multicomponent behavior in individual specimens is rare. The A remanence resides in magnetic grains of high median destructive fields and high, discrete laboratory unblocking temperatures and is inferred to be a primary thermoremanent magnetization, of circa 2.71 Ga age. The B and C magnetizations, of lower median destructive fields and more distributed unblocking temperatures, are inferred to be secondary and related to alteration, including serpentinization, involving limited, moderate to low-temperature fluid interaction, perhaps in response to thermotectonic events (e.g., mafic dike emplacement). Samples with NRM dominated by the A magnetization contain titanium-free magnetite needles (width

We report paleomagnetic data for 26 accepted sites collected in two sections of flat-lying strata of the upper Carnian-lower Norian (∼225 Ma) Dockum Group, northwest Texas. Six additional sites in coarse-grained conglomeratic sandstones gave no usable results. The total assemblage of 26 VGPs is streaked along the Late Triassic - earliest Jurassic track of the North America apparent polar wander path and their mean is inconsistent with the accepted upper Carnian-lower Norian reference pole. In detail, 12 sites in grayish white (nonhematitic) sandstones have weak magnetizations (less than about 1 mA/m) carried by magnetite or maghemite that give a paleopole at 56.4°N–96.3°E (N=12 dual-polarity VGPs; K=44.2; A95=6.6°) in close agreement with other results for upper Carnian-lower Norian rocks in North America. The 14 remaining sites in tan and redcolored (hematitic) sandstones, siltstones, and claystones give high unblocking temperature characteristic magnetizations carried by hematite, with paleopoles at 59.0°N–53.8°E (normal; N=7 VGPs, K=62.4, A95=7.6°) and 59.3°N–77.8°E (reverse; N=7 VGPs, K=204.2, A95=4.2°). These poles fall along the younger track of poles and near the J-I cusp of the North American APWP as defined by (unrotated) poles derived from Colorado plateau rocks. We suggest that the characteristic magnetization of the non-hematitic sandstones is an “early” magnetization, acquired during or soon after deposition. However, the characteristic magnetization of the red bed sites is interpreted as a secondary magnetization, for which we infer an earliest Jurassic age. The secondary origin for this magnetization is supported by the observation of conflicting magnetostratigraphies. The extreme westward position of the poles derived from red bed sites, particularly those with normal polarity, confirms the general position of the J-I cusp indicated by poles in the Piedmont province, the Newark basin, and the Colorado plateau; it also suggests that the magnitude of rotation of the Colorado plateau is no greater than about 5°. We compile an apparent polar wander path for North America, including Colorado plateau data, which suggests a fast rate of apparent polar wander throughout the Triassic period (about 0.8°/m.y.) with a gradual increase that preceded the opening of the Atlantic.

Mariscal Mountain anticline is a major Laramide asymmetric fold located along the frontal margin of the Cordilleran orogenic belt in the Trans-Pecos region of southwest Texas. Exposed within the limbs and nose of the anticline are concordant mafic and felsic intrusions whose emplacement, based largely on their arcuate outcrop geometry, has been interpreted to predate folding. Paleomagnetic fold tests on samples from large (≥ 30 m thick) and thin (≤ l m thick) gabbro sills are negative at the 99% confidence level. Baked sandstones adjacent to the thick sill give sample directions identical to those of the gabbro, whereas sample directions from a site well removed from thermal effects of the gabbro are scattered. Results of the contact tests are consistent with a thermoremanent origin for the gabbro sill magnetization. The negative fold test indicates that sill intrusion at Mariscal Mountain anticline postdated Laramide folding. Two 40Ar/39Ar analyses of plagioclase from the gabbro give discordant age spectra but yield an inverse correlation age of 37.0 ± 1.3 Ma (2σ). This date is similar to those from rocks of similar composition elsewhere in Trans-Pecos Texas, but significantly younger than whole rock and pyroxene K-Ar dates of Late Cretaceous and early Eocene age commonly cited for the Mariscal Mountain gabbro. The paleomagnetic and geochronologic data indicate that Laramide deformation had clearly ceased by late Eocene/early Oligocene time, consistent with absence of contractional deformation in middle Eocene to lower Oligocene volcanic and sedimentary rocks elsewhere in the region. Although paleomagnetic data from the gabbro sills may not adequately average secular variation, the in situ group mean direction (D = 348.6°, I = 50.9°, k = 65.1, α95 = 4.3°, n = 18 sites) is indistinguishable from expected mid-Tertiary reference directions. This indicates no evidence of significant vertical axis rotation in the Mariscal Mountain area. Analysis of paleomagnetic data from studies elsewhere in the Big Bend region indicates that significant problems exist with most data sets used to argue for the existence of local vertical axis rotations. Thus we conclude that existing paleomagnetic evidence regarding the timing and magnitude of rotations associated with Basin and Range extension is equivocal.

[1] We investigate the petrographic, geochemical and magnetic properties of fault pseudotachylytes formed by frictional melting in granitic rocks from Southern California, the Italian Alps and Kyushyu, Japan. The main magnetic remanence carriers are mixtures of grain sizes of fine grained magnetite. These ferrimagnetic grains record a stable, multicomponent magnetization that consists of one or more of the following: coseismic thermal remanent magnetization, coseismic lightning isothermal remanent magnetization and post-seismic chemical remanent magnetization. Fault pseudotachylytes from the three localities display contrasting magnetic properties, which suggests that oxygen fugacity and host rock composition ultimately control the magnetic assemblage.

We integrated geophysical and geological methods to evalute the structural evolution of the active Teton normal fault, Wyoming, and its role in the development of the dramatic topography of Teton Range and Jackson Hole. Comparison of variations in surface offsets with the topographic expression of the Teton range crest and drainage divide, and the overall structure of the range, suggests that the effects ofpostglacial faulting cannot be discriminated from the influence of pre-extensional structures and differential; erosion on the footwall topography. In contrast, the effects of multiple scarp-forming normal faulting earthquakes are expressed by the anomalous drainage pattern and westward tilt of the hanging wall, Jackson Hole, toward the Teton fault. Kinematic boundary element fault models suggest that the westward tilt of the valley floor is the product of 110-125 m of displacement on a 45 deg-75 deg E dipping Teton fault in the past 25,000-75,000 years. Comparisons with historic normal faulting earthquake displacements imply that this range of displacement corresponds to 10-50, M greater than 7 scarp-forming earthquakes. A total throw of 2.5 to 3.5 km across the Teton fault is suggested by inverse ray-tracing and forward gravity models. These models also suggest that Laramide age structures have been offset across the Teton fault and obscure its geophysical signature but also continue to influence the structural and topographic expression of the footwall and hanging wall blocks. Paleomagnetic analyses of the approximately 2.0 Ma Huckelberry Ridge Tuff suggest that the overall westward tilt of the Teton Range is a result ofabout 10 deg of west side down tilt across the Teton fault since tuff emplacement. This suggests that much if not all of the throw across the Teton fault has accumulated in the past 2 m.y. Complex demagnetization and rock magnetic behavior and local emplacement of the Huckleberry Ridge Tuff on preexisting topogrpahy preclude determination of the amount or variations in throw along strike of the Teton fault from the paleomagnetic data.

We present the results of a combined paleomagnetic and 40Ar/39Ar geochronologic investigation of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite in the southern Laramie Range of Wyoming and Colorado. Anorthosites and monzosyenites of the Laramie anorthosite complex yield a well-defined characteristic magnetization of northeast declination (D) and moderate negative inclination (I), although antipodal normal and reverse polarity magnetizations are present at three sites. A grand mean direction from 29 of 35 sites in the complex is D = 44.6°, I = −48.7° (k = 77.4, α95 = 3.1°). Alternating field (AF) and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low-Ti titanomagnetite of single or pseudo-single domain character that occurs as elongate to rod-shaped inclusions in plagioclase and potassium feldspar. The Sherman Granite contains a dual polarity magnetization that is less well defined than that of the Laramie anorthosite complex but similar in declination and inclination (D = 53.1°, I= −48.1°, k = 46.5, α95 = 7.6°, n = 8/14 sites); rock magnetic data indicate the primary carrier of remanence in Sherman Granite is magnetite. The 40Ar/39Ar geochronologic data from Sherman Granite hornblende, biotite, and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 Ma. Microcline data indicate that the Laramie anorthosite complex and Sherman Granite have probably not been thermally remagnetized. Paleomagnetic poles from the Laramie anorthosite complex and Sherman Granite are indistinguishable at the 95% confidence level, and individual virtual geomagnetic poles (VGPs) from both units are combined to provide a mean pole at 215.0°E, 6.7°S (K = 46.9, A95 = 3.5°, N = 37 VGPs). The location of this pole is similar to paleomagnetic poles derived from 1480 Ma to 1450 Ma intrusions elsewhere in North America, but it plots significantly north of those from Middle Proterozoic sedimentary strata of the Belt Supergroup and Sibley Group. In addition to the characteristic magnetization, samples from some sites in Sherman Granite contain a remanence of southeast declination and shallow negative inclination (D = 154.9°, I = −16.0°, k = 90.6, α95 = 9.7°, n = 4 sites). This secondary magnetization was probably acquired during late Paleozoic time.

Ethics education is an increasingly important component of the pre-professional training of geoscientists. Geoethics encompasses the values and professional standards required of geoscientists to work responsibly in any geoscience profession and in service to society. Funding agencies (e.g., the National Science Foundation, the National Institutes of Health) require training of graduate students in the responsible conduct of research; employers are increasingly expecting their workers to have basic training in ethics; and the public demands the highest standards of ethical conduct by scientists. However, there is currently no formal course of instruction in ethics in the geoscience curriculum, and few faculty members have the experience, resources, and sometimes willingness required to teach ethics as a component of their geoscience courses.

Paleomagnetic data from volcanic and crystalline rocks elucidate the evolution of a major Miocene accommodation zone in the northern Colorado River extensional corridor. The accommodation zone is a 10-km-wide belt of intermeshing conjugate normal faults that facilitates reversals in the dominant tilt direction of fault blocks and dip direction of major normal fault systems. Tilt-corrected means (e.g., N = 28 sites, D = 353.4°, I = 61.3°, α95 = 6.7°, k = 17.4) from Miocene volcanic strata overlap expected Miocene directions at the 95% confidence level. These data and geologic relations suggest that at exposed structural levels the accommodation zone did not facilitate distributed strike-slip displacement between opposing tilt block domains. Vertical axis rotations are probably negligible in most of the corridor, as the Miocene structural grain generally mimics that in the zone. Discrepancies between characteristic remanent magnetizations (ChRM) in crystalline rocks and expected directions are therefore attributed to rotations about horizontal (i.e., tilting or flexing) axes. ChRMs from Cretaceous and Miocene intrusions suggest 50°–90° of tilting of large crystalline terranes on either side of the accommodation zone. The magnitude of tilting inferred from the paleomagnetic data is similar to that of Tertiary strata in nearby fault blocks, implying that these crystalline terranes are parts of highly tilted fault blocks rather than flexed lower plate rocks. Major low-angle normal faults that bound highly tilted parts of these crystalline terranes probably nucleated at steep dips and were rotated to gentle dips by block tilting. Paleomagnetic data indicative of negligible tilting (e.g., Miocene intrusions, northern Black Mountains crystalline terrane, N = 13 sites, D = 359°, I = 55°, α95 = 9°, k = 24) and geologic relations imply that lower plate rocks may surface in both the east and west tilted domains 35–50 km away from the zone. The trend toward shallower structural levels, with respect to Miocene extension, and tapering of highly extended terrane toward the accommodation zone imply that the magnitude of upper crustal extension decreases toward the zone. Temporal patterns of major extension, especially an apparently continuous northward younging across both the east and west tilted domains, further suggest that the accommodation zone served as a long-lived rupture barrier between conjugate normal fault systems rather than as a short-term boundary between opposing systems that propagated toward and converged at the zone.

In central New Mexico, red sedimentary rocks unconformably overlying Permian carbonates of the San Andres Formation have been correlated with the Early-Middle Triassic Moenkopi and Late Triassic Chinle Formations of the Colorado Plateau. Paleomagnetic samples from Triassic sections exposed on basement cored uplifts along both the east and west side of the Rio Grande rift near Albuquerque yield, upon thermal and chemical demagnetization, well-defined, high unblocking temperature, dual-polarity magnetizations carried by hematite. The characteristic magnetization is interpreted as an early acquired chemical remanent magnetization based on a positive intraformational microconglomerate test and bedding-parallel magnetization polarity zonation. The Moenkopi and lowermost Chinle formations produced paleomagnetic poles respectively at 57.6°N-100.3 °E (N=36 sites, K=74.1, A95=2.8°) and 60.8°N-88.9°E (N=17 sites, K=130.3, A95= 3.1°). These data plus previously published and additional results from the underlying Permian strata suggest that portions of central New Mexico have experienced a small clockwise rotation (i.e., less than 10°) similar to that of the Colorado Plateau with respect to the North American craton. The paleomagnetic directions of the Chinle Formation and related strata in eastern New Mexico document about 12° (great circle distance) of rapid apparent polar wander during mid-Carnian to late Norian times along a track which contains other cratonic poles of similar age. We present a preliminary magnetic polarity time scale for the Triassic that incorporates the present New Mexico data and previously published data, mostly form continental red bed sequences. This magnetic polarity scale provides a basic framework which can be tested with future data from Triassic sections where additional biostratigraphic control exists.

Paleomagnetic investigations have been carried out on latest Cambrian to Early Ordovician alkaline intrusions, their contact intermediate to mafic composition metaigneous rocks, and overlying lower Paleozoic and early Tertiary strata in the Florida Mountains, southwestern New Mexico. A well-defined, predominantly normal polarity magnetization of high median destructive induction and high, usually discrete laboratory unblocking temperatures is isolated from sites in metaigneous rocks in contact with syenites exposed on the northwestern flank of this late Cenozoic uplift. Combinations of stable endpoint and planar data are used in data analysis at the site level (D = 262°, I = −30°, α95 = 10°, k = 12; N = 22 site means). In addition to the presence of reverse polarity magnetizations at two sites, the dispersion of site mean directions in these rocks (VGP angular standard deviation of 19.6°) suggests that considerable variation of the geomagnetic field has been averaged. After correction for a combination of early Tertiary (Laramide) deformation and late Tertiary Basin-Range block tilting, this magnetization gives a paleomagnetic pole at 6°S, 169°E and is interpreted to have been acquired soon after intrusion of alkaline plutons at 503±10 Ma, based on published U-Pb zircon isotopic age determinations. A lack of well-defined, coherent magnetizations in the plutonic rocks may reflect the complex subsolidus alteration history of these rocks, as interpreted with Rb-Sr isotopic data. Hematitic sandstones of the earliest Ordovician Bliss Formation, which directly overlie the crystalline rocks in portions of the range, give dual-polarity magnetizations whose directions suggest that these rocks have probably been remagnetized; this remagnetization possibly occurred during deformation in late Cretaceous/Tertiary time. Ordovician carbonate rocks of the El Paso and Montoya formations give only a Holocene magnetization. Paleocene to earliest Eocene(?) hematite-cemented clastic rocks of the Lobo Formation possess dual-polarity magnetizations. The origin of their magnetization is difficult to assess with the available data. In total, however, paleomagnetic data from the Bliss and Lobo formations argue against any significant vertical axis rotation of the area since Laramide deformation ending in Early Tertiary time. The intermediate to mafic composition metaigneous rocks from the Florida Mountains provide a latest Cambrian-Early Ordovician paleomagnetic pole that compares favorably with other North American paleopoles satisfying several acceptance criteria. The Florida Mountains data further support recent constructions of apparent polar wander paths for North America and thus the hypothesis that significant counterclockwise rotation of North America occurred during the Ordovician.

We collected paleomagnetic data from 137 sites in middle Eocene volcanic rocks of the northeastern Basin and Range province in order to characterize the three-dimensional kinematics of continental extension in the Lost River and Lemhi ranges, Idaho, and to test previous block rotation models. Statistically significant flattening of directions at three of 10 localities reflects either insufficient averaging of paleosecular variation or erroneous tilt corrections. Five of the seven remaining locality means are statistically indistinguishable from the middle Eocene reference direction. This indicates that vertical axis rotation did not accompany post-50 Ma normal faulting. Discordant declinations from two localities in the northern Lost River Range indicate 30°–40° of counterclockwise rotation of a large domain of NNW striking volcanic rocks. Inclination flattening at one of the two rotated localities is moderate but statistically significant (F±ΔF=9.9°±6.5°). In total, the paleomagnetic data conflict with earlier predictions of uniform clockwise rotation in response to right lateral transtension adjacent to the eastern Snake River Plain. There is a close correspondence between counterclockwise rotated localities and domains of NNW striking volcanic rocks and between unrotated localities and domains of north striking volcanic rocks. New mapping shows that counterclockwise rotated localities are all in the hanging wall of a major west dipping low-angle normal fault, the Donkey fault (named for the first time). This fault loses displacement southwards, was active in Eocene-Oligocene time and is probably a tilted high angle fault. The spatial distribution of rotated and unrotated localities is best explained by counterclockwise rotation of the hanging wall of the Donkey fault during opening of a southward tapering half graben. According to this hanging wall rotation model the footwall of the Donkey fault remained fixed relative to its rotating hanging wall. Our proposed kinematic model is generally applicable to extending continental lithosphere and provides a mechanism for coeval crustal thinning and vertical axis rotation. Thus vertical axis rotation may occur in the absence of strike-slip faults in extensional orogens.

Paleomagnetic data obtained from Oligocene to lower Miocene igneous rocks and middle Miocene basaltic rocks at fifteen localities from a region surrounding Dixie Valley in west central Nevada indicate that parts of the area experienced counterclockwise vertical-axis rotation, and these data provide constraints on the extent and timing of rotation. Counterclockwise vertical-axis rotation probably exceeding 30° is indicated for Oligocene to lower Miocene rocks in the central part of the study area. Calculated rotations increase with increasing age of the Oligocene to lower Miocene rocks, indicating that vertical-axis rotation was concurrent with ash-flow deposition (about 23–33 Ma). Paleomagnetic data indicate that middle Miocene basaltic rocks (about 10–17 Ma) postdate significant counterclockwise vertical-axis rotation, although this interpretation is complicated because the rocks were erupted episodically in pulses that apparently were short lived with respect to geomagnetic secular variation. Counterclockwise rotation was related to a deformation event that predated development of the present basin and range physiography of the area. The total amount of crustal rotation in Tertiary time in the area is poorly known. Rotation estimates calculated from time-averaged mean directions that incorporate data from a broad range of the Oligocene to lower Miocene units probably underestimate total Tertiary rotation because the mean directions include data from units that postdate much of the rotation. For example, a rotation estimate for a composite sequence from the central part of the study area is −23° ± 15°, whereas estimates from older and younger halves of the sequence are −37° ± 21° and −11° ± 16°, respectively. Paleomagnetic data indicate that Oligocene to lower Miocene rocks at some localities in the northern and southern parts of the study area (e.g., the Golconda Canyon locality) probably did not experience significant Tertiary counterclockwise rotation. Although the paleomagnetic data alone are insufficient to fully define the geographic area subjected to Tertiary counterclockwise rotation, the data suggest that a discrete subregion was affected rather than the entire Basin and Range province.

[1] Processes responsible for the acquisition of ancient yet secondary magnetizations are important facets of the geologic history of rocks and, when the age of such magnetizations can be estimated with confidence, provide useful information on the ancient geomagnetic field. In west central New Mexico near Mesita, on the Colorado Plateau, hematitic sandstone and siltstone beds of the Middle Jurassic (Callovian) Summerville Formation and overlying Bluff Sandstone are host to numerous large (up to 100 m2 in map area) pipe-like sandstone bodies. The pipes are as strongly cemented by hematite (colors range from 10R 6/6 to 10R 3/4) as the host strata; paleomagnetic data from them and their host strata are interpreted to indicate that these rocks have been remagnetized, probably in association with sandstone pipe formation. Reverse polarity magnetizations isolated in both alternating field and thermal demagnetization from pipes are well grouped and are similar to, and not statistically distinct from, those in adjacent host strata. The grand-mean direction for 16 sites (7 sites in sandstone pipes and 9 in host strata), corrected for slight (5°) west-northwest tilt of the strata, is D = 163.0°, I = −44.3° (α95 = 2.7°, k = 169). This direction yields a pole position of 72.8°N, 135.7°E (dp = 2.1°, dm = 3.4°). Assuming a modest (i.e., ∼5°) clockwise rotation of the Colorado Plateau, the pole lies at 68.7°N, 143.8°E. Median destructive fields for the remanence in pipes and host strata are typically 40–50 mT; over 90% of the remanence is “unblocked” or removed during changes in the magnetic mineralogy by temperatures of ∼400–450°C. Isothermal remanent magnetization (IRM) acquisition data, and thermal demagnetization of “saturation” IRM, however, demonstrate that the dominant magnetic phase is of high coercivity and relatively high (above 600°C) laboratory unblocking temperatures in both sandstone pipes and host strata, yet it does not appear to contribute significantly to the characteristic remanent magnetization. The similarity in demagnetization properties between pipes and adjacent host strata, the absence of a well-defined high unblocking temperature remanence that is more typical of hematite-cemented detrital strata, and the essentially uniform reverse polarity of the remanence are all interpreted to indicate that pipes and host strata contain secondary, yet early acquired magnetizations and that magnetization acquisition continued after pipe injection. We propose that acquisition of the secondary magnetization took place in the presence of alkaline, high pH brines formed by the dissolution of the underlying gypsum-dominated Lower Jurassic Todilto Formation strata and therefore the remanence is early in age. On the basis of a comparison with Summerville and Morrison (Middle and Late Jurassic) paleomagnetic poles from rocks on the Colorado Plateau, we interpret the secondary remanence in Summerville strata and sandstone pipes near Laguna to be latest Middle to Late Jurassic in age. If realistic, this interpretation further emphasizes the importance of fluid-rock interaction in the acquisition of secondary magnetizations.

[1] Twenty-eight selected sites (individual beds) in the Moenave Formation at the Echo Cliffs, northern Arizona, strata give a Hettangian paleomagnetic pole at 63.7°N, 59.7°E (dp = 2.6°, dm = 5.1°). The Wingate Sandstone and Rock Point Formation at Comb Ridge, southeast Utah, provide a Rhaetian paleopole at 57.4°N, 56.6°E (N = 16 sites; dp = 3.4, dm = 6.5). High unblocking temperatures (>600°C), high coercivity, and data analyses indicate that the characteristic magnetization is primarily a chemical remanence residing in hematite. The Hettangian and Rhaetian poles are statistically indistinguishable (at 95% confidence), they resemble existing data for the Glen Canyon Group, and they provide further validation to the J1 cusp of the North American apparent pole wander path (APWP). The red siltstone and upper members of the Chinle Group, on the south flank of the Uinta Mountains, northern Utah, define a Rhaetian pole at 51.6°N, 70.9°E (N = 20 sites; dp = 3.5°, dm = 6.9°). The Gartra and upper members of the Chinle Group in the north flank of the Uinta Mountains, give paleopoles at 52.0°N, 100.3°E (N = 6 sites; dp = 5.4°, dm = 10.5°) and 50.9°N, 50.1°E (N = 5 sites; dp = 8.8°, dm = 17.5°), respectively. These data indicate no significant rotation of the Uinta Mountains with respect to the craton. In total, data for the plateau and its bordering region of Cenozoic uplifts support estimates of small rotation of the plateau and provide evidence against the hypothesis of a Late Triassic standstill of the North American APWP. Our magnetostratigraphic results are consistent with lithographic and biostratigraphic data that place the Triassic-Jurassic boundary within the Dinosaur Canyon Member of the Moenave Formation, not at a regional hiatus.

In February 1982, the first issue of Tectonics was published. In the editorial policy statement for the journal, founding editors John Dewey, Paul Tapponier, and Clark Burchfiel wrote, “The central theme of Tectonics is the mechanical and thermal evolution of the lithospheric crust and mantle and the way that this is reflected in cratons, basins and mountains from the broad regional scale to the fine scale.” The editors further stated, “We expect that papers on these and related topics would emanate from a wide variety of earth science disciplines ranging from physical modeling to geological field observation.” Finally, with the confidence from this incredible team of editors, they noted, “We are aiming for a very rapid review process, allowing a maximum of about 1 month between submission and notification to the author of acceptance or rejection.”

[1] Many professional societies in the geosciences, including AGU, have made it their practice to accept for presentation almost every abstract submitted for professional meetings, including meetings of national and international stature. Unfortunately, it is becoming clear that this generous policy can have serious and unintended consequences. As some readers will know, a tiny minority of AGU members are concerned with furthering a religiously motivated creationist agenda and (in all good faith, presumably) completely misrepresent science to that end. For most scientists this is a trivial matter. We know that an AGU abstract, for example, is just an abstract. It does not have the status of a peer-reviewed scientific contribution, although the research reported will often provide the basis for one. We know that radioactive decay rates depend on basic physical constants and the laws of quantum mechanics. We also know, from the fact that we can understand the structure of the oldest rocks or the spectra of the most distant galaxies, that these things have not changed for billions of years. If an abstract makes claims to the contrary, we might attribute this to human error, or to instrumental limitations, or even to some real and interesting confounding phenomenon, such as the presence of bacterial contamination in an unexpected location. In any case, we are unlikely to be misled, and we might even learn something. So, seemingly, no great harm is done.

A workshop was convened in New Mexico to plan for the Colorado Plateau Coring Project (CPCP) and identify the target site for initial coring. The giant continental and near-shore to shallow marine epicontinental basins of the American Southwest are particularly well exposed on the Colorado Plateau and its environs and contain a rich record of early Mesozoic (∼251–145 million years ago) strata. This time period was punctuated by two major mass extinctions and is notable for the evolutionary appearance of the modern biota and its apparent dramatic climate changes. Classic studies of these basins, their strata, and their fossils have made this sequence instrumental in framing the context for the early Mesozoic world. Ambiguities in temporal resolution, uncertainties in global correlations with other early Mesozoic strata, and major doubts about latitudinal position still hamper testing of competing climatic, biotic, and tectonic models for the evolution of western Pangea.

An increasing number of geoscience departments are recognizing the need to assume a larger role in facets of public education. The role of geoscience departments in the preparation and professional development of K-12 science teachers is particularly critical and meaningful, and it merges with other fundamental missions within universities. The importance of geoscience content for teachers and the general public is evident in the numerous Earth science—related public policy issues gaining ever increasing attention, including global climate change, energy, and water resources. Also, Earth science is not only included in science content education standards, but it is also interesting to students and provides an important gateway to foster interest in science in general.

This may be the most straightforward book review imaginable to write. Just buy this book and use it! You will not regret it. Verlyn Klinkenborg's 23 August 2005 editorial in the New York Times (“Grasping the depth of time as a first step in understanding evolution”) serves as a most timely way begin a review of A Geologic Time Scale 2004 (GTS2004). Klinkenborg writes, “One of the most powerful limits to the human imagination is our inability to grasp, in a truly intuitive way the depths of terrestrial and cosmological time.”

As the authors state in their preface to Environmental Magnetism: Principles and Applications of Enviromagnetics, this is a quite new science. It began from disparate beginnings involving interdisciplinary approaches to studying young sediment records to tease out a history of past but geologically recent global change. Today the subject has blossomed into an important, vibrant field that envelops many aspects of the geosciences and beyond. It is a true delight to review this book, and not just because Subir Banerjee has done such an outstanding job of providing in the forward, as he states, “a welcoming invitation for the reader to the contents of the book and its special character.” It is indeed of special character, written by two highly accomplished scholars who have devoted a considerable amount of their research attention to linking the magnetic record of geologic materials to Earth processes. The first book on this subject, Environmental Magnetism by R.Thompson and F. Oldfield,was published in 1986 and provided the community with a compilation of several applications oriented toward environmental studies using rock and paleomagnetic approaches. Evans and Heller's volume is quite different.

There is no question that extensional tectonics and related processes have played a major role in the geologic evolution of the British Isles, and in the formation and trapping of immense “super-giant” fields of hydrocarbon resources in the North Sea. The latter have been of considerable importance to the economy of the United Kingdom. This two-part volume is the second in a new series of publications by The Geological Society, “Key Issues in the Earth Sciences.” It is a compilation of 35 well-known papers on extensional tectonics published over the past 2 decades.

An AGU Chapman Conference, “Time scales of the Geomagnetic Field,” was held recently on the 70th birthday of Neil D. Opdyke, professor at the University of Florida, as a tribute to his outstanding contributions to the Earth sciences over more than 4 decades. Seventy-five participants, a quarter of whom came from outside North America, registered for the meeting and took part in the scientific sessions. The meeting was organized into three topics, each of which occupied a day of lectures and posters.

Since the first lodestone was used as a compass, the subject of rock magnetism has fascinated a broad range of scientists. In the late 1840s and early 1850s, A. Delesse and M. Melloni showed that Earth materials were capable of faithfully recording the ambient geomagnetic field, and in the early 1900s, P. David and B. Brunhes first recognized magnetizations anti-parallel to the local field.

On 1 July 2002, officers of the AGU Geomagnetism and Paleomagnetism (GP) Section and the GP Section Executive Committee began a new 2-year term. The officers, listed here, take this opportunity to address the GP membership about the state of the section and where it is headed.

Models for Neogene crustal deformation in the central Death Valley extended terrane, southeastern California, differ markedly in their estimates of upper crustal extension versus shear translations. Documentation of vertical axis rotations of range-scale crustal blocks (or parts thereof) is critical when attempting to reconstruct this highly extended region. To better define the magnitude, aerial extent, and timing of vertical axis rotation that could mark shear translation of the crust in this area, paleomagnetic data were obtained from Tertiary igneous and remagnetized Paleozoic carbonate rocks along a roughly east-west traverse parallel to about 36°N latitude. Sites were established in ∼7 to 5 Ma volcanic sequences (Greenwater Canyon and Brown's Peak) and the ∼10 Ma Chocolate Sundae Mountain granite in the Greenwater Range, ∼8.5 to 7.5 Ma and 5 to 4 Ma basalts on the east flank of the Black Mountains, the 10.6 Ma Little Chief stock and upper Miocene(?) basalts in the eastern Panamint Mountains, and Paleozoic Pogonip Group carbonate strata in the north central Panamint Mountains. At the site level, most materials yield readily interpretable paleomagnetic data. Group mean directions, after appropriate structural corrections, suggest no major vertical axis rotation of the Greenwater Range (e.g., D = 359°, I = 46°, α_(95) = 8.0°, N = 12 (7 normal (N), 5 reversed (R) polarity sites)), little post-5 Ma rotation of the eastern Black Mountains (e.g., D = 006°, I = 61°, α_(95) = 4.0°, N = 9 N, 6 R sites), and no significant post-10 Ma rotation of the Panamint Range (e.g., D = 181°, I = −51°, α_(95) = 6.5°, N = 9 R sites). In situ data from the Greenwater Canyon volcanic rocks, Chocolate Sundae Mountain granite, Funeral Peak basalt rocks, the Little Chief stock, and Paleozoic carbonate rocks (remagnetized) are consistent with moderate south east-side-down tilting of the separate range blocks during northwest directed extension. The paleomagnetic data reported here suggest that the Panamints shared none of the 7 Ma to recent clockwise rotation of the Black Mountains crystalline core, as proposed in recent models for transtensional development of the central Death Valley extended terrane.

A microanalytical technique, employing a cryogenic magnetometer and progressive alternating field demagnetization, is described and used to investigate the natural remanent magnetization of individual components (groundmass, crystals, and lithic and pumice fragments) in seven ash-flow tuff lithologies and to determine if subblocking temperature flow-or compaction-related movements took place in the units on scales ranging from the volume of the individual microspecimen (1–2 mm in diameter) to that of a standard paleomagnetic core specimen. Closely grouped directions from microspecimens of vitrophyre (α95′s = 4.9° - 12.4° ) and close correspondence of these directions with their site mean directions determined from standard samples suggest that the magnetic components of these units have not been disturbed by movements at low temperatures. Microspecimens from devitrified portions of the cooling units are of significantly lower coercivity than vitrophyre material and yield directions either more scattered about their site mean (α95 greater than 15° ) or uniformly displaced in one direction (up to 35°) away from the mean. Devitrification may be as significant a contributor to the scatter of microspecimen directions as are postblocking temperature randomizing or systematic microtectonic movements. Vitrophyres near the base and extremities of ash-flow tuff cooling units are probably the most accurate indicators of the paleofield in each cooling unit.

Clinker, rock baked and fused by in situ burning of underlying coal, posesses a thermoremanent and/or thermochemical magnetization that resides in magnetite, hematite, and goethite. Data from individual sites imply that clinker may provide a high resolution recording of the geomagnetic field (e.g., within-site virtual geomagnetic pole angular standard deviations are usually less than 10°) , but this is dependent on its thermal history and the time over which magnetic phases formed. Data from 17 sites in the Powder River Basin (13 normal polarity, 4 reverse) yield a mean of Decl. = 348°, Incl. = 62°, α95 = 5°, k = 42 and a corresponding pole position of 81° lat., 158° long. with a virtual geomagnetic pole angular standard deviation of 17.3°. The mechanism of clinker formation, the wide geographic distribution, and the range of available fission-track ages suggest that clinker may contain a nearly complete geomagnetic field record for much of the Quaternary.