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The Chicxulub impact event at ca. 66 Ma left in its wake the only complex crater on Earth with a preserved peak ring, characterized by a well-developed magnetic anomaly low. To date, little is known about its magnetic properties. The joint Integrated Ocean Drilling Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364 drill core M0077A revealed that the peak ring consists of uplifted and strongly deformed granitoid basement rocks overlain by a 130-m-thick impact melt and suevite layer. Pre- and postimpact hydrothermal systems affected this basement with maximum temperatures up to 450 °C. We used microscopy, mineral chemistry, temperature-dependent magnetic susceptibility, and hysteresis properties to characterize the magnetic mineralogy of pre-, syn-, and postimpact rocks. Compared to its amount of pure, stoichiometric shocked magnetite, the granitoid basement shows low magnetic susceptibility, which is in line with earlier experimental studies indicating that shock reduces magnetic susceptibility. Cation-substituted magnetite with varying compositions in the melt rocks carries a higher induced and remanent magnetization compared to the basement. In the granitoid basement, magnetite was partially oxidized to hematite by a pre-impact hydrothermal event, but at lithological contacts with high-temperature impact melt rock, this hematite was locally retransformed back to magnetite. Elsewhere in the granitoid basement, the temperature reached in the hydrothermal system was too low for hematite retransformation. It was also too low to anneal all the lattice defects in the shocked magnetite, which likely occurs above 540 °C. The presence of shocked magnetite in the granitoid basement well explains the magnetic anomaly low due to its unusually low induced magnetization. [ABSTRACT FROM AUTHOR]

The timing of the onset of magnetochron M0r and its duration are disputed, reflecting both a limited set of radioisotopic dates and uncertain magnetostratigraphic correlations. We present a chronostratigraphic framework for a reversed polarity interval based on two chronometers (40Ar/39Ar, U-Pb) and newly published paleomagnetic data from the Qingshan Group, Jiaolai Basin, China. Bayesian modeling of U-Pb zircon and 40Ar/39Ar sanidine dates suggests a minimum duration of 540 ± 37 k.y. (95% credible interval) for the reversed polarity interval. These findings are compatible with an astrochronologic age model for Msequence seafloor magnetic anomalies, indicating that the reversely magnetized sediments correspond to magnetochron M0r rather than the shorter chron "M-1r." Integration of U-Pb and 40Ar/39Ar ages constrains the onset of M0r to 120.29 ± 0.09 Ma, which is 1 m.y. younger than that inferred in the current geologic time scale (GTS 2020). This finding also implies that the Cretaceous normal superchron (CNS) began at 119.70 ± 0.12 Ma and that the average seafloor spreading rate during the CNS was 3.5% higher than that inferred from GTS 2020. It also suggests that oceanic anoxic event 1a began at 119.40 ± 0.12 Ma, thereby providing an updated chronologic basis for exploring the primary trigger of this carbon cycle perturbation. [ABSTRACT FROM AUTHOR]

Oceanic core complexes (OCCs) and detachment faults play a key role in crustal accretion at slow and ultraslow spreading centers. We investigated the effect of different magma supply at three OCCs of the Southwest Indian Ridge using high-resolution deep-sea bathy- metric and magnetic data. The average equivalent thickness of extrusive basalt deduced from the magnetic anomalies, a proxy for magma supply, decreases from west to east, from the Yuhuang (49.25°E) to Longqi (49.65°E) to Junhui (51.75°E) OCCs. Conversely, serpentinite outcrops become more abundant, the domal OCC morphology flattens as the footwall rotation (measured by the magnetization vector inclination) increases, and hydrothermal evidence becomes sparse. Combined with results from the amagmatic easternmost Southwest Indian Ridge, our study shows that the magma supply controls the character and evolution of the OCCs and detachment faults on the Southwest Indian Ridge. [ABSTRACT FROM AUTHOR]

The crust beneath transform faults at slow-spreading ridges has been considered to be thin, comprising a thin mafic layer overlying serpentinized peridotite. Using wide-angle seismic data, we report the presence of a Moho at ~6 km depth and a low-velocity anomaly extending down to 9 km beneath the 20-km-wide Romanche transform valley floor in the equatorial Atlantic Ocean. The low crustal velocities above the Moho could be due to either highly serpentinized mantle peridotite or fractured mafic rocks. The existence of clear Moho reflections and the occurrence of a large crustal-depth rupture during the 2016 magnitude 7.1 earthquake suggest that the crust likely consists of fractured mafic material. Furthermore, the presence of low velocities below the Moho advocates for extensive serpentinization of the mantle, indicating that the Moho reflection is unlikely to be produced by a serpentinization front. The crust to the north of the transform fault likely consists of mafic material, but that in the south appears to be more amagmatic, possibly containing serpentinized peridotite. Our results imply that the transform fault structure is complex and highly heterogeneous, and thus would have significant influence on earthquake rupture and alteration processes. [ABSTRACT FROM AUTHOR]

The high strength of the Tarim Basin (northwestern China) lithosphere, widely regarded as a Precambrian craton, is evidenced by its resistance to Cenozoic deformation in the Himalayan-Tibetan orogen. However, Neoproterozoic suturing and early Paleozoic shortening within the Tarim Basin suggest that its rigidity is a relatively recent phenomenon with unknown cause. We reprocessed high-resolution magnetic data that show a 300-400-km-diameter radial pattern of linear anomalies emanating from a central region characterized by mixed positive-negative anomalies. We suggest that this pattern was generated by the previously hypothesized Permian (ca. 300-270 Ma) plume beneath the Tarim Basin. Constrained by published geochemical and geochronological data from plume-related igneous rocks, we propose that the ~30 m.y. Permian plume activity resulted in a more viscous, depleted, thicker, dehydrated, and low-density mantle lithosphere. The resulting stronger lithosphere deflected strain from the Cenozoic India-Asia convergence around Tarim Basin, including Pamir overthrusting to the northwest and Altyn Tagh left-slip displacement to the northeast, thus shaping the geometry of the Himalayan-Tibetan orogen. [ABSTRACT FROM AUTHOR]

Catalina Basin, located within the southern California Inner Continental Borderland (ICB), United States, is traversed by two active submerged fault systems that are part of the broader North America-Pacific plate boundary: the San Clemente fault (along with a prominent splay, the Kimki fault) and the Catalina fault. Previous studies have suggested that the San Clemente fault (SCF) may be accommodating up to half of the ~8 mm/yr right-lateral slip distributed across the ICB between San Clemente Island and the mainland coast, and that the Catalina fault (CF) acts as a significant restraining bend in the larger transform system. Here, we provide new high-resolution geophysical constraints on the seabed morphology, deformation history, and kinematics of the active faults in and on the margins of Catalina Basin. We significantly revise SCF mapping and describe a discrete releasing bend that corresponds with lows in gravity and magnetic anomalies, as well as a connection between the SCF and the Santa Cruz fault to the north. Subsurface seismic-reflection data show evidence for a vertical SCF with significant lateral offsets, while the CF exhibits lesser cumulative deformation with a vertical component indicated by folding adjacent to the CF. Geodetic data are consistent with SCF right-lateral slip rates as high as ~3.6 mm/yr and transpressional convergence of <1.5 mm/yr accommodated along the CF. The Quaternary strands of the SCF and CF consistently cut across Miocene and Pliocene structures, suggesting generation of basin and ridge morphology in a previous tectonic environment that has been overprinted by Quaternary transpression. Some inherited crustal fabrics, especially thinned crust and localized, relatively hard crustal blocks, appear to have had a strong influence on the geometry of the main trace of the SCF, whereas inherited faults and other structures (e.g., the Catalina Ridge) appear to have minimal influence on the geometry of active faults in the ICB. [ABSTRACT FROM AUTHOR]

The Gales Creek fault (GCF) is a 60-km-long, northwest-striking dextral fault system (west of Portland, Oregon) that accommodates northward motion and uplift of the Oregon Coast Range. New geologic mapping and geophysical models confirm inferred offsets from earlier geophysical surveys and document ~12 km of right-lateral offset of a basement high in Eocene Siletz River Volcanics since ca. 35 Ma and ~8.8 km of right-lateral separation of Miocene Columbia River Basalt at Newberg, Oregon, since 15 Ma (~0.62 ± 0.12 mm/yr, average long-term rate). Relative uplift of Eocene Coast Range basalt basement west of the fault zone is at least 5 km based on depth to basement under the Tualatin Basin from a recent inversion of gravity data. West of the city of Forest Grove, the fault consists of two subparallel strands ~7 km apart. The westernmost, Parsons Creek strand, forms a linear valley southward to Henry Hagg Lake, where it continues southward to Newberg as a series of en echelon strands forming both extensional and compressive step-overs. Compressive step-overs in the GCF occur at intersections with ESE-striking sinistral faults crossing the Coast Range, suggesting the GCF is the eastern boundary of an R′ Riedel shear domain that could accommodate up to half of the ~45° of post- 40 Ma clockwise rotation of the Coast Range documented by paleomagnetic studies. Gravity and magnetic anomalies suggest the western strands of the GCF extend southward beneath Newberg into the Northern Willamette Valley, where colinear magnetic anomalies have been correlated with the Mount Angel fault, the proposed source of the 1993 M 5.7 Scotts Mills earthquake. The potential-field data and water-well data also indicate the eastern, Gales Creek strand of the fault may link to the NNW-striking Canby fault through the E-W Beaverton fault to form a 30-km-wide compressive step-over along the south side of the Tualatin Basin. LiDAR data reveal right-lateral stream offsets of as much as 1.5 km, shutter ridges, and other youthful geomorphic features for 60 km along the geophysical and geologic trace of the GCF north of Newberg, Oregon. Paleoseismic trenches document Eocene bedrock thrust over 250 ka surficial deposits along a reverse splay of the fault system near Yamhill, Oregon, and Holocene motion has been recently documented on the GCF along Scoggins Creek and Parsons Creek. The GCF could produce earthquakes in excess of Mw 7, if the entire 60 km segment ruptured in one earthquake. The apparent subsurface links of the GCF to other faults in the Northern Willamette Valley suggest that other faults in the system may also be active. [ABSTRACT FROM AUTHOR]

Rifting may lead ultimately to continental breakup, but the identification and characterization of the resulting crustal distribution remains challenging. Also, spatial and temporal changes in breakup magmatism may affect the geophysical character of the newly formed oceanic crust, resulting in contrasting interpretations of crustal composition and distribution. In the Eastern Black Sea Basin (EBSB), the evolution from rifting to breakup has been long debated, with several interpretations for the distribution of stretched continental and oceanic crust. We interpret basement morphological variations from long-offset seismic reflection profiles, highlighting a northwest-to-southeast transition from faulted and tilted continental blocks to a rough and then smoother basement. We model magnetic anomalies to further constrain the various basement domains, and infer the presence of a weakly magnetized, stretched continental crust in the northwest, and a 0.4-3.8 A/m layer coinciding with the smooth basement in the central and southeastern area. We conclude that the EBSB oceanic crust extends farther to the northwest than was suggested previously from an abrupt change in crustal thickness and lower-crustal velocity. The apparent discrepancy between these different types of geophysical evidence may result from changes in magma supply during breakup, affecting the thickness and velocity structure of the resulting oceanic crust. [ABSTRACT FROM AUTHOR]

Early magnetic studies of the Japan Trench showed that seafloor spreading magnetic anomalies progressively fade away and disappear during subduction, reflecting the increasing distance to magnetized sources and the removal of their remanent magnetization with alteration and increasing temperature. An improved magnetic anomaly map derived from both scalar and vector magnetic anomaly data, coupled with a better knowledge of the slab geometry in one hand, of the magnetic structure of the oceanic crust on the other hand, allow us to constrain the thermal structure of the subducting slab. We, for the first time, identify two steps in the anomaly disappearance: first the magnetization of extrusive basalt is rapidly erased between 9 and 12 km, where titanomagnetite reaches its blocking temperature between 150 °C and 350 °C, then the magnetization of deeper crustal layers slowly decreases down to ~20 km, reflecting the progressive slab heating toward the Curie temperature of magnetite, 580 °C. The resulting slab temperatures are higher than predicted by most thermal models. Recent observations and models suggest rejuvenated hydrothermal activity triggered by lithospheric flexure before subduction that may significantly heat up the subducting oceanic crust through thermal blanketing and possibly serpentinization, with consequences on the depth of the seismogenic zone. [ABSTRACT FROM AUTHOR]

The authors discuss a report that examined changes in the significance of the age and direction trends of the eastern Australian shield volcanoes. Topics discussed include errors in calculation of synthetic apparent polar wander path (APWP), conversion of APWPs to plate motion, and lack of significant longitudinal motion in the Australian APWP.

Continental arc systems often show evidence of large-scale migration both toward and away from the incoming plate. In oceanic arc systems, however, while slab roll-back and the associated processes of backarc spreading and arc migration toward the incoming plate are commonplace, arc migration away from the incoming plate is rarely observed. We present a new compilation of marine magnetic anomaly and seismic data in order to propose a new tectonic model for the eastern Caribbean region that includes arc migration in both directions. We synthesized new evidence to show two phases of backarc spreading and eastward arc migration toward the incoming Atlantic. A third and final phase of arc migration to the west subdivided the earlier backarc basin on either side of the present-day Lesser Antilles arc. This is the first example of regional multidirectional arc migration in an intra-oceanic setting, and it has implications for along-arc structural and geochemical variations. The back and forth arc migrations were probably due to the constraints imposed by the neighboring American plates on this isolated subduction system, rather than variations in subducting slab buoyancy. [ABSTRACT FROM AUTHOR]

We used integrated analyses and forward and inverse modeling of potential field data to interpret the enigmatic Brunswick magnetic anomaly (BMA), a prominent negative magnetic lineation that is developed within the ocean-continent transition of the Southeast Georgia Embayment. From magnetic profiles extracted from gridded data, residual magnetic maps, and simultaneous 2.5-dimensional forward and 3-dimensional Euler inverse modeling of the gravity and magnetic fields, we suggest that the source of the BMA is a series of semicontinuous to discrete, late-stage rift-related mafic intrusions of Mesozoic age. This interpretation is supported by the observation that the BMA is independent of the East Coast magnetic anomaly, implying a predrift source, and that the amplitude and frequency of the anomaly change nearshore across a fracture zone that divides the offshore BMA, where continental breakup occurred, from the onshore BMA. Modeling demonstrates for the first time that a Mesozoic rift-related mafic body can explain the anomaly onshore. The lack of a BMA equivalent on the West African margin may indicate that Atlantic rifting began with a single lithospheric dislocation. [ABSTRACT FROM AUTHOR]

The early--middle Eocene (ca. 56-41 Ma) is recorded in the pelagic Scaglia Rossa and Variegata Formations of the Umbria-Marche Basin (central Italy). Geochemical and magnetostratigraphic alignment between the Bottaccione section (Gubbio, central Italy) and the Smirra core (Cagli, central Italy) allows us to generate a continuous and wellpreserved new record that, combined with previously published data from the same area, creates a continuous high-resolution record from the Paleocene-Eocene thermal maximum (ca. 56 Ma) to the lower part of chron C21n. Comparison with carbon isotope records from Ocean Drilling Program Sites 1258 and 1263 reveals a satisfactory match, providing further evidence of the global significance of the long-term trend and superposed perturbations captured by the d13C records. The identification of astronomically forced geochemical cycles allows us to develop a 405 k.y. tuned age model, thereby extending the astrochronology from ca. 56.0 to ca. 47.5 Ma. Marine magnetic anomaly profiles from major oceanic basins characterized by high seafloor spreading rates were used to independently test the astronomical polarity time scale associated with our tuning, as well as other polarity time scales. Our age model suggests the existence of periods of relatively constant seafloor spreading rates separated by rapid changes, while the other time scales generate more gradual variations and also include large and short-term deviations in spreading rates that occur simultaneously in different oceanic basins, implying errors in polarity reversal ages. The Umbria- Marche age model further contributes to the closure of the middle Eocene gap in the astronomical time scale. [ABSTRACT FROM AUTHOR]

A correction is presented to the article "Phanerozoic cratonization by plume welding."

The article presents the author's reply to a critique on his manuscript on oceanic isostasy as rift-to-drift transition trigger. Topics discussed include the paradox of the variety of magmatic and tectonic structures, and studies on the implications of ease and difficulty in transitioning. Also noted is the beneficial further exploration of the nuances of ocean participation in the rift to drift transition.

Understanding the relative motion between the Pacific plate and its neighboring plates in the Paleogene has important consequences for deciphering the relationship between absolute and relative plate motions in the Pacific Ocean basin, the history of circum-Pacific subduction, and the cause of the Hawaiian-Emperor bend (HEB). We quantitatively model the Farallon/ Vancouver-Pacific-Antarctic seafloor spreading history from 67 to 33 Ma based on a comprehensive synthesis of magnetic anomaly and fracture identifications. We find a well-constrained increase from 75 ± 5 mm/yr to 101 ± 5 mm/yr in Pacific-Farallon full spreading rates between 57.6 Ma and 55.9 Ma, followed by a stepwise increase to 182 ± 2 mm/yr from 49.7 to 40.1 Ma. The increases in Pacific-Farallon spreading rates are not accompanied by any statistically significant change in spreading direction. The 57.6-55.9 Ma surge of Pacific-Farallon spreading reflects an eastward acceleration in Farallon plate motion, as it precedes west Pacific subduction initiation and is not associated with any significant change in Pacific-Antarctic spreading. We interpret the increase in Pacific-Farallon spreading rates after ca. 50 Ma as a consequence of further acceleration in Farallon plate motion. We find no indication of a major change in Pacific plate absolute motion at this time. Our model suggests that changes in relative motion direction between the Pacific and Farallon and Pacific and Antarctic plates were insignificant around the formation time of the HEB (ca. 47.5 Ma), and the bend is largely a consequence of Hawaiian hotspot motion, which ceased rapid motion after 47 Ma. [ABSTRACT FROM AUTHOR]

In Chinese loess, the Brunhes-Matuyama (B-M) geomagnetic reversal occurs ~25 k.y. prior to the age found in marine sediments. This offset has been attributed by some to post-depositional magnetic overprinting of loess, while others have argued it is due to errors in the loess time scale. Here we solve this long-standing debate by exploiting a new method to extract reproducible records of geomagnetic field intensity from loess with 10Be-a proxy for global average geomagnetic field intensity-and using it to show that a pronounced minimum in field intensity (a requirement for dipole field reversal) is recorded in two separate loess records at ca. 780 ± 3 kyr B.P. This timing is synchronous with the B-M reversal timing seen in marine records, verifying the standard loess time scale as correct, but it is ~25 k.y. younger than the age (depth) of the magnetic polarity reversal recorded in these same Chinese loess sediments, demonstrating that loess magnetic overprinting has occurred. [ABSTRACT FROM AUTHOR]

The northern Nevada rift is a prominent mafic dike swarm and magnetic anomaly in north-central Nevada inferred to record the Middle Miocene (16.5-15.0 Ma) extension direction in the northern Basin and Range province in the western United States. From the 245°-250° rift direction, Basin and Range extension is inferred to have shifted 45° clockwise to a modern direction of 290°-300° during the late Miocene. The region surrounding the northern Nevada rift was actively extending while the rift formed, and these domains are all characterized by extension oriented 280°-300°. This direction is distinctly different from the rift direction and nearly identical to the modern Basin and Range direction. Although the rate, structural style, and distribution of Basin and Range extension appear to have undergone a significant change in the late Miocene (ca. 10 Ma), the overall spreading direction does not. Middle Miocene extension was directed perpendicular to the axis of the thickest crust formed during Mesozoic shortening and this orientation may reflect gravitational collapse of this thick crust. Orientation of northern Nevada rift dikes may reflect a short-lived regional stress field related to the onset of Yellowstone hotspot volcanism. [ABSTRACT FROM AUTHOR]

We use geophysical data to examine the structural framework of the northern San Francisco Bay region, an area that hosts the northward continuation of the East Bay fault system. Although this fault system has accommodated ∼175 km of right-lateral offset since 12 Ma, how this offset is partitioned north of the bay is controversial and important for understanding where and how strain is accommodated along this stretch of the broader San Andreas transform margin. Using gravity and magnetic data, we map these faults, many of which influenced basin formation and volcanism. Continuity of magnetic anomalies in certain areas, such as Napa and Sonoma Valleys, the region north of Napa Valley, and the region south of the Santa Rosa Plain, preclude significant (>10 km) offset. Much of the slip is partitioned around Sonoma and Napa Valleys and onto the Carneros, Rodgers Creek, and Green Valley faults. The absence of correlative magnetic anomalies across the Hayward-Rodgers Creek-Maacama fault system suggests that this system reactivated older basement structures, which appear to influence seismicity patterns in the region. [ABSTRACT FROM AUTHOR]

The Cocos-Nazca spreading center is one of the few examples of the formation of a spreading center by splitting of oceanic lithosphere. It was created when the Farallon plate broke up in the early Miocene following the collision of the Pacific-Farallon spreading center with the North American continent. Much of the ancient Farallon plate corresponding to the area of opening is lost to subduction beneath Central America and South America, but new data from the conjugate area on the Pacific plate allow the first detailed reconstruction of the break-up process. The opening began after chron 7 (25 Ma) at a location of focused crustal extension caused by overlapping spreading centers that had evolved in response to a slight reorientation of a Pacific-Farallon ridge segment. Beginning at chron 6B (22.7 Ma), eastward progressing seafloor spreading started along an axis that most likely migrated toward the region of weak lithosphere created by the Galapagos hotspot. By chron 6 (19.5 Ma), plate splitting from the spreading center to the trench was complete, allowing the fully detached Cocos and Nazca plates to move independently. This kinematic change resulted in a significant ridge jump of the newly established Pacific-Nazca spreading center, a change in plate motion direction of the Nazca plate by 20° clockwise, and a large increase in Pacific-Cocos plate velocity in the middle Miocene. [ABSTRACT FROM AUTHOR]

Variations in rock magnetic properties are responsible for the many linear, short-wavelength, low-amplitude magnetic anomalies that are spatially associated with faults that cut Neogene basin sediments in the Rio Grande rift, including the San Ysidro normal fault, which is well exposed in the northern part of the Albuquerque Basin. Magnetic-susceptibility measurements from 310 sites distributed through a 1200-m-thick composite section of rift-filling sediments of the Santa Fe Group and prerift Eocene and Cretaceous sedimentary rocks document large variations of magnetic properties juxtaposed by the San Ysidro fault. Mean volume magnetic susceptibilities generally increase upsection through eight map units: from 1.7 to 2.2E-4 in the prerift Eocene and Cretaceous rocks to 9.9E-4-1.2E-3 in three members of the Miocene Zia Formation of the Santa Fe Group to 1.5E-3-3.5E-3 in three members of the Miocene-Pleistocene Arroyo Ojito Formation of the Santa Fe Group. Rock magnetic measurements and petrography indicate that the amount of detrital magnetite and its variable oxidation to maghemite and hematite within the Santa Fe Group sediments are the predominant controls of their magnetic property variations. Magnetic susceptibility increases progressively with sediment grain size within the members of the Arroyo Ojito Formation (deposited in fluvial environments) but within members of the Zia Formation (deposited in mostly eolian environments) reaches highest values in fine to medium sands. Partial oxidation of detrital magnetite is spatially associated with calcite cementation in the Santa Fe Group. Both oxidation and cementation probably reflect past flow of groundwater through permeable zones. Magnetic models for geologic cross sections that incorporate mean magnetic susceptibilities for the different stratigraphic units mimic the aero-magnetic profiles across the San Ysidro fault and demonstrate that the stratigraphic level of dominant magnetic contrast changes with different exposure levels into the fault. These data indicate that tectonic juxtaposition of primary variations of magnetic properties of strata across the fault is the source of the associated magnetic anomaly. This study indicates that magnetic anomalies over faults and folds can be generated by sediments (1) deposited within tectonic basins having volcanic or basement source areas rich in magnetite, (2) having depositional environments with sufficient but varying energy to transport dense magnetic minerals and cause stratigraphic changes of magnetic properties, and (3) having magnetic minerals preserved owing to their youth or nonreactive geochemical environments. [ABSTRACT FROM AUTHOR]

Changes in plate motion are thought to be recorded in the trend of fracture zones, even though fracture zones provide no information about the spreading rate. Using newly compiled published and unpublished magnetic data from the Southwest Indian Ridge, we calculated finite rotation poles for A13, A8, and A6, from which we determined a 50% decrease in spreading rate from slow to ultraslow at ca. 24 Ma not accompanied by a signifi cant change in spreading direction. This spreading rate decrease is concurrent with changes in plate motions at only two of the four adjoining plate boundaries. Finally, we discuss the possible relationships of this event with other absolute or relative plate motion events that occurred at ca. 24 Ma at the global scale. [ABSTRACT FROM AUTHOR]

We consider the origin and character of a prominent large-scale geophysical feature in north-central Nevada that is coincident with the western margin of the northern Nevada rift-a mid-Miocene rift that includes mafic dike swarms and associated volcanic rocks expressed by a NNW-striking magnetic anomaly. The geophysical feature also correlates with mid-Miocene epithermal gold deposits and is coincident with the central part of the Battle Mountain-Eureka mineral trend. The Reese River Valley, a 2-km-deep Cenozoic basin, is located along the western margin of this feature and is inferred from the inversion of gravity data to be influenced by, and perhaps in part structurally controlled by, the geophysical feature. Geophysical modeling indicates that the source of the geophysical anomaly must extend to mid-crustal depths, perhaps reflecting a transition from Paleozoic crust in the southwest to Precambrian crust in the northeast, the presence of felsic intrusive rocks in the middle crust, or the edge of mid- to subcrustal mafic intrusions related to late Tertiary magmatic underplating associated with hotspot magmatism. These cases offer very different possibilities for the age, depth, and origin of the source of the geophysical anomaly, and they present distinct implications for crustal evolution in the northern Great Basin. For example, if the anomaly is due to a pre-Cenozoic basement structure, then its coincidence with the mid- Miocene northern Nevada rift suggests that the trend of the rift was guided by the preexisting crustal structure. On the other hand, if the anomaly is related to Tertiary mafic intrusions, then the western limit of this magmatism may have been influenced by hotspot fracturing of the crust. [ABSTRACT FROM AUTHOR]

Analysis of aeromagnetic and gravity data reveals new details of the structure, igneous geology, and temporal evolution of the prominent, enigmatic ca. 32 Ma Pine Canyon caldera and the Chisos Mountains (Big Bend National Park, Texas). The main caldera-filling Pine Canyon Rhyolite, the oldest member of the South Rim Formation, is reversely magnetized, allowing it to be used as a key marker bed for determining caldera fill thickness. Modeling of gravity and magnetic anomalies indicates that the Pine Canyon Rhyolite is probably thicker in the northeastern part of the caldera. Lineaments in the magnetic data suggest the presence of buried faults beneath the caldera that may have led to increased downdrop in the northeast versus the southwest, allowing a thicker section of caldera fill to accumulate there. The Pine Canyon caldera has been interpreted as a downsag caldera because it lacks surficial faulting, so these inferred faults are the first mapped features there that could be responsible for caldera collapse. The caldera boundary correlates well with the margins of a gravity low. General features of the caldera match well with basic models of downsag calderas, meaning that the Pine Canyon caldera may be a classic example of downsagging, of which few well-described examples exist, in terms of a geophysical signature. The source of a long-wavelength magnetic high over the Chisos Mountains is interpreted as a previously unknown broad intrusion, the long axis of which trends parallel to a major crustal boundary related to the Ouachita orogeny or an even earlier Precambrian margin. This feature represents the largest intrusion (28–34 km diameter, 1–4 km thick, 700–3000 km3 in volume) in an area where relatively small laccoliths are ubiquitous. The intrusion most likely represents a long-lived (>1 m.y.) reservoir replenished by small batches of magma of varying composition, as reflected in the variation of eruptive products from the Pine Canyon and Sierra Quemada calderas. The intrusion may represent the easternmost occurrence of voluminous Tertiary magmatism in the southwestern United States. [ABSTRACT FROM AUTHOR]

The northern Cordilleran fold-and-thrust belt in Canada and Alaska is at the boundary between the broad continental margin mobile belt and the stable North American craton. The fold-and-thrust belt is marked by several significant changes in geometry: cratonward extensions in the central Yukon Territory and northeastern Alaska are separated by marginward re-entrants. These geometric features of the Cordilleran mobile belt are controlled by relations between lithospheric strength and compressional tectonic forces developed along the continental margin. Regional magnetic anomalies indicate deep thermal and compositional characteristics that contribute to variations in crustal strength. Our detailed analysis of one such anomaly, the North Slope deep magnetic high, helps to explain the geometry of the fold-and-thrust front in northern Alaska. This large magnetic anomaly is inferred to reflect voluminous mafic magmatism in an old (Devonian?) extensional domain. The presence of massive amounts of mafic material in the lower crust implies geochemical depletion of the underlying upper mantle, which serves to strengthen the lithosphere against thermal erosion by upper mantle convection. We infer that deep-source magnetic highs are an important indicator of strong lower crust and upper mantle. This stronger lithosphere forms buttresses that play an important role in the structural development of the northern Cordilleran fold-and-thrust belt. [ABSTRACT FROM AUTHOR]

The White River altered area, Washington, and the Goldfield mining district, Nevada, are nearly contemporaneous Tertiary (ca. 20 Ma) calc-alkaline igneous centers with large exposures of shallow (<1 km depth) magmatic-hydrothermal, acid-sulfate alteration. Goldfield is the largest known high-sulfidation gold deposit in North America. At White River, silica is the only commodity exploited to date, but, based on its similarities with Goldfield, White River may have potential for concealed precious and/or base metal deposits at shallow depth. Both areas are products of the ancestral Cascade arc. Goldfield lies within the Great Basin physiographic province in an area of middle Miocene and younger Basin and Range and Walker Lane faulting, whereas White River is largely unaffected by young faults. However, west-northwest-striking magnetic anomalies at White River do correspond with mapped faults synchronous with magmatism, and other linear anomalies may reflect contemporaneous concealed faults. The White River altered area lies immediately south of the west-northwest-striking White River fault zone and north of a postulated fault with similar orientation. Structural data from the White River altered area indicate that alteration developed synchronously with an anomalous stress field conducive to left-lateral, strike-slip displacement on west-northwest- striking faults. Thus, the White River alteration may have developed in a transient transtensional region between the two strike-slip faults, analogous to models proposed for Goldfield and other mineral deposits in transverse deformational zones. Gravity and magnetic anomalies provide evidence for a pluton beneath the White River altered area that may have provided heat and fluids to overlying volcanic rocks. East-to eastnortheast-striking extensional faults and/or fracture zones in the step-over region, also expressed in magnetic anomalies, may have tapped this intrusion and provided vertical and lateral transport of fluids to now silicified areas. By analogy to Goldfield, geophysical anomalies at the White River altered area may serve as proxies for geologic mapping in identifying faults, fractures, and intrusions relevant to hydrothermal alteration and ore formation in areas of poor exposure. [ABSTRACT FROM AUTHOR]

Understanding the marine magnetic anomaly record is critical for constructing realistic geodynamo models of global geomagnetic field, polarity reversal mechanisms, and longterm geomagnetic field behavior. One of the least understood portions of the marine magnetic anomaly record is also the oldest part of the record, the Jurassic quiet zone (JQZ), where anomalies become weak and difficult to correlate. The reason for the existence of the JQZ is unclear. It has been suggested that the JQZ is a true polarity superchron, similar to the Cretaceous normal superchron. Continental magnetostratigraphic studies have suggested that the JQZ is a period of rapid polarity reversal, of low field intensity, or both. We show results of a deep-tow survey of Pacific Jurassic crust that confirms the existence of magnetic anomalies within the JQZ. We tie Ocean Drilling Program Hole 801C (167.4 Ma) into the record and show that seafloor-spreading magnetic anomalies are present around the hole and extend to 170 Ma crust. We find a rise in reversal rate with increasing age with reversal rates over 10 rev/m.y. at 160 Ma and at 167 Ma. Anomaly amplitudes decrease in the record from 155 Ma until 162 Ma, where low-amplitude anomalies are difficult to correlate. Prior to 167 Ma, anomalies regain amplitude and remain strong until the end of our record at 170 Ma. The JQZ thus appears to be a combination of low-amplitude magnetic anomalies combined with rapid field fluctuations, which could be due to either intensity or polarity changes. [ABSTRACT FROM AUTHOR]

Spectral analysis of both shipborne and airborne magnetic maps of the southern Tyrrhenian Sea reveals seven subparallel positive-negative magnetic anomaly stripes over the flat-lying deep floor of the Marsili oceanic basin. This represents the first evidence of oceanic magnetic anomalies in the Tyrrhenian Sea. The central positive stripe is along the Marsili seamount, a superinflated spreading ridge located at the basin axis. The stratigraphy of Ocean Drilling Program Site 650 and K/Ar ages from the Marsili seamount suggest that the Marsili Basin opened at the remarkable full-spreading rate of ∼19 cm/yr between ca. 1.6 and 2.1 Ma about the Olduvai subchron. This is the highest spreading rate ever documented, including that observed at the Cocos-Pacific plate boundary. Renewed but slow spreading during the Brunhes chron (after 0.78 Ma), coupled with huge magmatic inflation, gave rise to the Marsili volcano. Our new data and interpretation show that backarc spreading of the Tyrrhenian Sea was episodic, with sudden rapid pulses punctuating relatively long periods of tectonic quiescence. [ABSTRACT FROM AUTHOR]

We report on extensive off-axis bathymetry, gravity, and magnetic data that provide a 26-m.y.-long record of axial tectonic and magmatic processes over a 660-km-long and melt-poor portion of the ultraslow Southwest Indian Ridge. We describe a new type of seafloor (the smooth seafloor) that forms at minimal ridge melt supply, with little or no axial volcanism. We propose possible mechanisms leading to this avolcanic or nearly avolcanic mode of spreading, in contradiction with the traditional view of mid-ocean ridges as primarily volcanic systems. We also show evidence for large-offset asymmetric normal faults and detachments at the ridge axis, with asymmetry persisting in some cases for tens of millions of years. [ABSTRACT FROM AUTHOR]

Understanding demagnetization by hypervelocity impacts is crucial for the interpretation of planetary magnetic anomalies and remanent magnetization in meteorites. We describe an innovative approach for investigating the effects of impacts on the remanent magnetization of geologic materials. It consists of the combination of pulsed laser impacts and Superconducting Quantum Interference Device (SQUID) microscopy. Laser impacts are nondestructive, create shocks with peak pressures as high as several hundred GPa, and allow well-calibrated modeling of shock wave propagation within the impacted samples. High-resolution SQUID microscopy quantitatively maps the magnetic field of room-temperature samples with an unprecedented spatial resolution of ∼100 μm. We present shock modeling and magnetic field data obtained for two laser impacts on a magnetite-bearing basalt sample. Magnetic measurements show a demagnetized area at the impact locations. We also show that high-resolution magnetic measurements combined with impact modeling provide a continuous relation between the demagnetization intensity and the peak pressure undergone by the sample. This promising technique will allow for the investigation of the demagnetization behavior of a variety of geological materials upon impacts, with implications for our understanding of the magnetization of extraterrestrial materials and of terrestrial impact structures. [ABSTRACT FROM AUTHOR]

Direct fossil evidence for early microbial life on Earth is rare. Microbiological data indicate that sulfate and iron reduction are both among the earliest forms of microbial respiration, and direct evidence for the early origin of sulfate reduction comes from sulfur isotopic anomalies in ancient sediments. Fe isotope geochemistry potentially provides a new way of identifying microbial iron reduction early in Earth's history. We present Fe isotopic data for sedimentary pyrite from the 2.7 Ga Belingwe sedimentary basin in Zimbabwe. Isotopically light Fe and a remarkable covariation between Fe and S isotopes provide strong evidence for coexisting Fe and S reduction. Our results are consistent with an early origin for sulfate reduction and provide direct geochemical evidence for the antiquity of bacterial Fe reduction. The covariation of Fe and S isotopes may provide a useful new tracer of microbial evolution early in Earth history. [ABSTRACT FROM AUTHOR]

Continental mantle in subduction zones is hydrated by release of water from the underlying oceanic plate. Magnetite is a significant byproduct of mantle hydration, and forearc mantle, cooled by subduction, should contribute to long-wavelength magnetic anomalies above subduction zones. We test this hypothesis with a quantitative model of the Cascadia convergent margin, based on gravity and acromagnetic anomalies and constrained by seismic velocities, and find that hydrated mantle explains an important disparity in potential-field anomalies of Cascadia. A comparison with aeromagnetic data, thermal models, and earthquakes of Cascadia, Japan, and southern Alaska suggests that magnetic mantle may be common in forearc settings and thus magnetic anomalies may be useful in mapping hydrated mantle in convergent margins worldwide. [ABSTRACT FROM AUTHOR]

Biotic changes in nannofossils and radiolarians associated with the Valanginian δ13C anomaly are documented at Ocean Drilling Program Hole 1149B in the Pacific Ocean: they are coeval and similar to those previously documented in the Tethys, suggesting a global perturbation of marine ecosystems. A marked increase in abundance of Diazomatolithus, absence of nannoconids, and a Pantanellium peak characterize the Valanginian δ13C excursion. Such changes are interpreted as being due to global enhanced fertility and a biocalcification crisis under conditions of excess CO2. The occurrence of organic C-rich black shales in the Southern Alps and in the Pacific in the interval corresponding to the δ13C excursion suggests a Valanginian oceanic anoxic event (OAE). Volcanism of the Paranà-Etendeka large igneous province (ca. 132 Ma) was presumably responsible for an increase of CO2, triggering a climate change and accelerated hydrological cycling, possibly causing an indirect fertilization of the oceans. Widespread nutrification via introduction of biolimiting metals at spreading ridges could have significantly increased during the Gondwana breakup and simultaneous tectonic events in three separate oceans. There is no paleontological or δ18O evidence of warming during the Valanginian OAE. On the contrary, both nannofossils and oxygen isotopes record a cooling event at the climax of the δ13C excursion. Weathering of basalts and burial of organic C-rich black shales were presumably responsible for CO2 drawdown and establishment of reversed greenhouse conditions. Keywords: Valanginian; oceanic anoxic event; planktonic communities; carbon isotopes. [ABSTRACT FROM AUTHOR]

Examines the location of the Nubia-Somalia boundary along the Southwest Indian Ridge. Analysis of locations of the old edge magnetic anomaly; Proposal of the concept of transform faults; Improvement of spatial resolution using magnetic profiles from many cruises.

Presents bathymetric and magnetic anomaly data from the Phoenix Ridge, Antarctica that show that extinction of all three remaining segments occurred at the time of magnetic chron C2A, synchronous with a ridge-trench collision south of the Hero Fracture Zone. Slowdown of the spreading rates at the time of chron C4 resulting from the extinction of the West Scotia Ridge.

Explores dike intrusion as a possible cause of linear Martian magnetic anomalies. Details on the dike intrusion model; Description of magnetic lineations in the southern hemisphere of Mars; Significance of upper bound in Martian crustal magnetization; Temperature requirements for the production of Martian crust.

Suggests the occurrence of a widespread melt beneath the Tibetan crust based on magnetic anomaly data. Presence of Curie isotherm in the upper crust across the Tibetan plateau; Achievement of granite temperature melt temperature in the upper crust.

Discusses the implications for impact-related magnetic signatures of magnetic anomaly near the center of the Vredefort structure in South Africa. Magnetic profiles across the basement; Thermal metamorphism in the Vredefort basement; Cause of magnetic anomaly.

Presents the magnetic petrology of a suite of deep-crustal rocks from the Brien intrusion within the Proterozoic continental collision zone between the Grenville and Superior provinces of the Canadian Shield. Geology; Petrography; Geochemistry; Magnetic properties; Implications for magnetic anomalies of continental collision zones.

The article focuses on the potential of mercury (Hg) as indicator of massive volcanism in the records of marine sedimentary. Topics discussed include the role of large igneous provinces (LIPs) in sustaining biotic and environmental crises, the implication of Hg anomalies in the LIP activity, and the role of Hg isotopes as tracers of deposition pathways to marine sediments.