John W. Hillhouse

Magnetic Fabric, Flow Directions, and Source Area of the Lower Miocene Peach Springs Tuff in Arizona, California, and Nevada

We have used anisotropy of magnetic susceptibility (AMS) to define the flow fabric and possible source area of the Peach Springs Tuff, a widespread rhyolitic ash flow tuff in the Mojave Desert and Great Basin of California, Arizona, and Nevada. The tuff is an important stratigraphic marker from the Colorado Plateau to Barstow, California, a distance of 350 km; however, the location of its source caldera is unknown. Dated at 18.5 Ma by 40Ar/39 Ar, the tuff erupted during the early stages of Miocene extension along the lower Colorado River. The thicker accumulations (>100 m) occur at Kingman, Arizona, and in the Piute Mountains, California, on opposite sides of the Colorado River extensional corridor. Our AMS studies produced well-defined magnetic lineations in 30 of 42 sites distributed throughout the tuff. Typical ratios of the principal AMS axes are 1.01 for the magnetic lineation (kmax/kint) and 1.02 for the foliation (kint/kmin); the bulk magnetic susceptibility of the Peach Springs Tuff averages 2.0×10−3 in the SI unit system. The subhorizontal lineations, which presumably parallel the flow directions, form a pattern radiating outward from the approximate center of the outcrop area. Magnetic foliations define an imbrication that generally dips away from the distal margins and toward the center of the outcrop of the tuff. The lineation and imbrication indicate a source region near the southern tip of Nevada. Defining the best intersection of the AMS lineations required restoration of major extension, strike-slip faulting, and associated tectonic rotation in the disrupted tuff. The optimum intersection of magnetic lineations lies in the southern Black Mountains of Arizona on the eastern side of the Colorado River extensional corridor. No caldera structures are known from that area, but the area contains thick sections of the Peach Springs Tuff above a silicic volcanic center. The caldera may be buried under younger deposits in the Mohave Valley of Arizona. Tertiary granite in the Newberry Mountains may represent a deeper level of the Peach Springs Tuff vent that has been exhumed by detachment faulting.

Correlation of upper Cenozoic tephra layers between sediments of the western United States and eastern Pacific Ocean and comparison with biostratigraphic and magnetostratigraphic age data

Five widespread upper Cenozoic tephra layers that are found within continental sediments of the western United States have been correlated with tephra layers in marine sediments in the Humboldt and Ventura basins of coastal California by similarities in major-and trace-element abundances; four of these layers have also been identified in deep-ocean sediments at DSDP sites 34, 36, 173, and 470 in the northeastern Pacific Ocean. These layers, erupted from vents in the Yellowstone National Park area of Wyoming and Idaho (Y), the Cascade Range of the Pacific Northwest (C), and the Long Valley area, California (L), are the Huckleberry Ridge ash bed (2.0 Ma, Y), Rio Dell ash bed (ca. 1.5 Ma, C), Bishop ash bed (0.74 Ma, L), Lava Creek B ash bed (0.62 Ma, Y), and Loleta ash bed (ca. 0.4 Ma, C). The isochronous nature of these beds allows direct comparison of chronologic and climatic data in a variety of depositional environments. For example, the widespread Bishop ash bed is correlated from proximal localities near Bishop in east-central California, where it is interbedded with volcanic and glacial deposits, to lacustrine beds near Tecopa, southeastern California, to deformed on-shore marine strata near Ventura, southwestern California, to deep-ocean sediments at site 470 in the eastern Pacific Ocean west of northern Mexico. The correlations allow us to compare isotopic ages determined for the tephra layers with ages of continental and marine biostratigraphic zones determined by magnetostratigraphy and other numerical age control and also provide iterative checks for available age control. Relative age variations of as much as 0.5 m.y. exist between marine biostratigraphic datums [for example, highest occurrence level of Discoaster brouweri and Calcidiscus tropicus (= C. macintyrei )], as determined from sedimentation rate curves derived from other age control available at each of several sites. These discrepancies may be due to several factors, among which are (1) diachronism of the lowest and highest occurrence levels of marine faunal and floral species with latitude because of ecologic thresholds, (2) upward reworking of older forms in hemipelagic sections adjacent to the tectonically active coast of the western United States and other similar analytical problems in identification of biostratigraphic and magnetostratigraphic datums, (3) dissolution of microfossils or selective diagenesis of some taxa, (4) lack of precision in isotopic age calibration of these datums, (5) errors in isotopic ages of tephra beds, and (6) large variations in sedimentation rates or hiatuses in stratigraphic sections that result in age errors of interpolated datums. Correlation of tephra layers between on-land marine and deep-ocean deposits indicates that some biostratigraphic datums (diatom and calcareous nannofossil) may be truly time transgressive because at some sites, they are found above and, at other sites, below the same tephra layers.

Paleomagnetism and tectonic rotation of the lower Miocene Peach Springs Tuff: Colorado Plateau, Arizona, to Barstow, California

We have determined remanent magnetization directions of the lower Miocene Peach Springs Tuff at 41 localities in western Arizona and southeastern California. An unusual northeast and shallow magnetization direction confirms the proposed geologic correlation of isolated outcrops of the tuff from the Colorado Plateau to Barstow, California, a distance of 350 km. The Peach Springs Tuff was apparently emplaced as a single cooling unit about 18 or 19 Ma and is now exposed in 4 tectonic provinces west of the Plateau, including the Transition Zone, Basin and Range, Colorado River extensional corridor, and central Mojave Desert strike-slip zone. As such, the tuff is an ideal stratigraphic and structural marker for paleomagnetic assessment of regional variations in tectonic rotations about vertical axes. From 4 sites on the stable Colorado Plateau, we have determined a reference direction of remanent magnetization (I = 36.4°, D = 33.0°, α 95 = 3.4°) that we interpret as a representation of the ambient magnetic field at the time of eruption. A steeper direction of magnetization (I = 54.8°, D = 22.5°, α 95 = 2.3°) was observed at Kingman where the tuff is more than 100 m thick, and similar directions were determined at 7 other thick exposures of the Peach Springs Tuff. The steeper component is presumably a later-stage magnetization acquired after prolonged cooling of the ignimbrite. When compared to the Plateau reference direction, tilt-corrected directions from 3 of 6 sites in the central Mojave strike-slip zone show localized rotations up to 13° in the vicinity of strike-slip faults. The other three sites show no significant rotations with respect to the Colorado Plateau. Both clockwise and counterclockwise rotations were measured, and no systematic regional pattern is evident. Our results do not support kinematic models which require consistent rotation of large regions to accommodate the cumulative displacement of major post-middle Miocene strike-slip faults in the central Mojave Desert. Most of our sites in the Transition Zone and Basin and Range province have had no significant rotation, although small counterclockwise rotation in the McCullough and New York Mountains may be related to sinistral shear along en echelon faults southwest of the Lake Mead shear zone. The larger rotations occur in the Colorado River extensional corridor, where 8 of 14 sites show rotations ranging from 37° clockwise to 51° counterclockwise. These rotations occur in allochthonous tilt blocks which have been transported northeastward above the Chemehuevi-Whipple Mountains detachment fault. Upper-plate blocks within 1 km of the exposed detachment unexpectedly show no significant rotation. From this relation, we infer that rotations are accommodated along numerous low-angle faults at higher structural levels above the detachment surface.

Late Cenozoic Arctic Ocean sea ice and terrestrial paleoclimate.

Sea otter remains found in deposits of two marine transgressions (Bigbendian and Fishcreekian) of the Alaskan Arctic Coastal Plain which occurred between 2.4 and 3 Ma suggest that during these two events the southern limit of seasonal sea ice was at least 1600 km farther north than at present in Alaskan waters. Perennial sea ice must have been severely restricted or absent, and winters were warmer than at present during these two sea-level highstands. Paleomagnetic, faunal, and palynological data indicate that the later transgression (Fishcreekian) occurred during the early part of the Matuyama Reversed-Polarity Chron. Amino acid diagenesis in fossil mollusks suggests that since the later transgression the effective diagenetic temperature (EDT) in the deposits has been about −16 °C, which is about 7 °C colder than modern values and slightly colder than the EDT calculated for the past 125 ka. Such a low EDT suggests that permafrost and perennial sea ice have been present nearly continuously since this transgression. Permafrost probably was absent, however, during the earlier (Bigbendian) transgression. Permafrost and extensive perennial sea ice may have been initiated during the late stages of climatic cooling that spanned the Gauss Normal-Matuyama Reversed-Polarity Chron boundary and led into the first major late Cenozoic glaciation of the Northern Hemisphere.

Paleomagnetism, magnetic anisotropy, and mid‐Cretaceous paleolatitude of the Duke Island (Alaska) ultramafic complex

We report paleomagnetic results from layered igneous rocks that imply substantial post mid-Cretaceous poleward motion of the Insular superterrane (western Canadian Cordillera and southeast Alaska) relative to North America. The samples studied are from the stratiform zoned ultramafic body at Duke Island, which intruded rocks of the Alexander terrane at the south end of the southeastern Alaska archipelago at about 110 Ma. Thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the Curie temperature of magnetite. This remanence is likely carried by low-Ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts. The Duke Island intrusion exhibits a well-developed gravitational layering that was deformed during initial cooling (but below 540°C) into folds that plunge moderately to the west-southwest. The characteristic remanence clearly predates this early folding and is therefore primary; the Fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site-mean directions are corrected by restoring the layering to estimated paleohorizontal. All samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion, and we show that it has no significant or systematic effect on the site-mean directions of remanence. At least some of the anisotropy derives from secondary magnetite formed during partial serpentinization. The mean paleomagnetic inclination (56°±10°) corroborates paleomagnetic results from five coeval silicic plutons of the Canadian Coast Plutonic Complex to the south and southeast and implies 3000 km (±1300 km) of poleward transport relative to the North American craton. Between mid-Cretaceous and middle Eocene time, the Insular superterrane and Coast Plutonic Complex shared a common paleolatitude history, with more poleward transport than coeval inboard terranes.

Limits to northward drift of the Paleocene Cantwell Formation, central Alaska

Volcanic rocks of the Paleocene Cantwell Formation in central Alaska apparently originated at a paleolatitude of 83°N (α 95 = 9.7°), as indicated by paleomagnetic results. When compared with the Paleocene pole for the North American craton, the 95% confidence limits of the results suggest that terranes north of the Denali fault have moved no more than 550 km northward relative to the North American craton since Paleocene time.

Paleomagnetism of the Resurrection Peninsula, Alaska: Implications for the tectonics of southern Alaska and the Kula‐Farallon Ridge

The rocks of the Resurrection Peninsula compose an ophiolite (57±1 Ma) within the vast accretionary prism of the Chugach-Prince William terrane. Paleomagnetic data from pillow basalt and sheeted dikes of the ophiolite yield a mean paleolatitude of 54°±7°, which implies 13±9° of poleward displacement. The characteristic component of the remanence resides in a Ti-poor titanomagnetite with a blocking temperature close td 580°C. A positive fold test and the petrology of the magnetic minerals lead us to conclude that the characteristic remanent magnetization (ChRM) in the sheeted dikes originated as a primary thermoremanent magnetization (TRM). We hypothesize that the ChRM of the pillow basalt originated as a chemical remanent magnetization (CRM) acquired soon after formation of the rocks during hydrothermal metamorphism. Structural data indicate polyphase deformation with folding about vertical and horizontal axes. The paleomagnetic data pass a fold test only when we incorporate this interpretation in our structural correction. Geologic relationships lead us to interpret the Resurrection Peninsula ophiolite as a fragment of the extinct Kula-Farallon ridge that accreted to the Chugach-Prince William terrane soon after formation. The paleomagnetic paleolatitude indicates that the ridge intersected the North American continental margin in the vicinity of northern Washington in late Paleocene time. The paleostrike of the sheeted dikes (N26°E±12°) shows that the ridge trended NNE. According to current plate tectonic models, if the Chugach-Prince William terrane moved north with the full coastwise displacement of the Kula plate, the Resurrection Peninsula ophiolite would have arrived at its present position at 45 Ma. This age is perhaps slightly later than that of the terranes that lay immediately inboard (Peninsular and Wrangellia terranes), indicating some relative displacement between those terranes and the Chugach-Prince William terrane. Such displacement, in turn, agrees with studies which imply a Coast plutonicmetamorphic complex provenance for the Chugach-Prince William terrane.

Stratigraphy, paleomagnetism, and anisotropy of magnetic susceptibility of the Miocene Stanislaus Group, central Sierra Nevada and Sweetwater Mountains, California and Nevada

Paleomagnetism and anisotropy of magnetic susceptibility (AMS) reveal pyroclastic flow patterns, stratigraphic correlations, and tectonic rotations in the Miocene Stanislaus Group, an extensive volcanic sequence in the central Sierra Nevada, California, and in the Walker Lane of California and Nevada. The Stanislaus Group (Table Mountain Latite, Eureka Valley Tuff, and the Dardanelles Formation) is a useful stratigraphic marker for understanding the post–9-Ma major faulting of the easternmost Sierra Nevada, uplift of the mountain range, and transtensional tectonics within the central Walker Lane. The Table Mountain Latite has a distinctively shallow reversed-polarity direction (I = −26.1°, D = 163.1°, and α 95 = 2.7°) at sampling sites in the foothills and western slope of the Sierra Nevada. In ascending order, the Eureka Valley Tuff comprises the Tollhouse Flat Member (I = −62.8°, D = 159.9°, α 95 = 2.6°), By-Day Member (I = 52.4°, D = 8.6°, α 95 = 7.2°), and Upper Member (I = 27.9°, D = 358.0°, α 95 = 10.4°). The Dardanelles Formation has normal polarity. From the magnetization directions of the Eureka Valley Tuff in the central Walker Lane north of Mono Lake and in the Anchorite Hills, we infer clockwise, vertical-axis rotations of ∼10° to 26° to be a consequence of dextral shear. The AMS results from 19 sites generally show that the Eureka Valley Tuff flowed outward from its proposed source area, the Little Walker Caldera, although several indicators are transverse to radial flow. AMS-derived flow patterns are consistent with mapped channels in the Sierra Nevada and Walker Lane.

Age, composition, and areal distribution of the Pliocene Lawlor Tuff, and three younger Pliocene tuffs, California and Nevada

The Lawlor Tuff is a widespread dacitic tephra layer produced by Plinian eruptions and ash flows derived from the Sonoma Volcanics, a volcanic area north of San Francisco Bay in the central Coast Ranges of California, USA. The younger, chemically similar Huichica tuff, the tuff of Napa, and the tuff of Monticello Road sequentially overlie the Lawlor Tuff, and were erupted from the same volcanic field. We obtain new laser-fusion and incremental-heating 40 Ar/ 39 Ar isochron and plateau ages of 4.834 ± 0.011, 4.76 ± 0.03, ≤4.70 ± 0.03, and 4.50 ± 0.02 Ma (1 sigma), respectively, for these layers. The ages are concordant with their stratigraphic positions and are significantly older than those determined previously by the K-Ar method on the same tuffs in previous studies. Based on offsets of the ash-flow phase of the Lawlor Tuff by strands of the eastern San Andreas fault system within the northeastern San Francisco Bay area, total offset east of the Rodgers Creek–Healdsburg fault is estimated to be in the range of 36 to 56 km, with corresponding displacement rates between 8.4 and 11.6 mm/yr over the past ∼4.83 Ma. We identify these tuffs by their chemical, petrographic, and magnetic characteristics over a large area in California and western Nevada, and at a number of new localities. They are thus unique chronostratigraphic markers that allow correlation of marine and terrestrial sedimentary and volcanic strata of early Pliocene age for their region of fallout. The tuff of Monticello Road is identified only near its eruptive source.

Accretion of southern Alaska

Abstract Paleomagnetic data from southern Alaska indicate that the Wrangellia and Peninsular terranes collided with central Alaska probably by 65 Ma ago and certainly no later than 55 Ma ago. The accretion of these terranes to the mainland was followed by the arrival of the Ghost Rocks volcanic assemblage at the southern margin of Kodiak Island. Poleward movement of these terranes can be explained by rapid motion of the Kula oceanic plate, mainly from 85 to 43 Ma ago, according to recent reconstructions derived from the hot-spot reference frame. After accretion, much of southwestern Alaska underwent a counterclockwise rotation of about 50 ° as indicated by paleomagnetic poles from volcanic rocks of Late Cretaceous and Early Tertiary age. Compression between North America and Asia during opening of the North Atlantic (68-44 Ma ago) may account for the rotation.