Paul W. Layer

Recent investigations of the 0–5 Ma geomagnetic field recorded by lava flows

We present a synthesis of 0–5 Ma paleomagnetic directional data collected from 17 different locations under the collaborative Time Averaged geomagnetic Field Initiative (TAFI). When combined with regional compilations from the northwest United States, the southwest United States, Japan, New Zealand, Hawaii, Mexico, South Pacific, and the Indian Ocean, a data set of over 2000 sites with high quality, stable polarity, and declination and inclination measurements is obtained. This is a more than sevenfold increase over similar quality data in the existing Paleosecular Variation of Recent Lavas (PSVRL) data set, and has greatly improved spatial sampling. The new data set spans 78°S to 53°N, and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time-averaged field (TAF) and paleosecular variation (PSV) for the Brunhes and Matuyama chrons, and for the 0–5 Ma interval combined. The Brunhes and Matuyama chrons exhibit different TAF geometries, notably smaller departures from a geocentric axial dipole field during the Brunhes, consistent with higher dipole strength observed from paleointensity data. Geographical variations in PSV are also different for the Brunhes and Matuyama. Given the high quality of our data set, polarity asymmetries in PSV and the TAF cannot be attributed to viscous overprints, but suggest different underlying field behavior, perhaps related to the influence of long-lived core-mantle boundary conditions on core flow. PSV, as measured by dispersion of virtual geomagnetic poles, shows less latitudinal variation than predicted by current statistical PSV models, or by previous data sets. In particular, the Brunhes data reported here are compatible with a wide range of models, from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude. Discriminating among such models could be helped by increased numbers of low-latitude data and new high northern latitude sites. Tests with other data sets, and with simulations, indicate that some of the latitudinal signature previously observed in VGP dispersion can be attributed to the inclusion of low-quality, insufficiently cleaned data with too few samples per site. Our Matuyama data show a stronger dependence of dispersion on latitude than the Brunhes data. The TAF is examined using the variation of inclination anomaly with latitude. Best fit two-parameter models have axial quadrupole contributions of 2–4% of the axial dipole term, and axial octupole contributions of 1–5%. Approximately 2% of the octupole signature is likely the result of bias incurred by averaging unit vectors.

Miocene high-pressure metamorphism in the Cyclades and Crete, Aegean Sea, Greece: Evidence for large-magnitude displacement on the Cretan detachment

The Cyclades in the backarc region of the present Hellenic subduction zone are known for widespread Late Cretaceous to Eocene high-pressure metamorphism in the Cycladic blueschist unit. We report 40 Ar/ 39 Ar and Rb/Sr phengite ages of 24–21 Ma for high- pressure metamorphism (8–10 kbar, 350–400 °C) in the lowest tectonic unit in the Cyclades, the Basal unit, which structurally underlies the Cycladic blueschist unit. The Basal unit is correlated with the Tripolitza unit of the External Hellenides in the forearc region of the Hellenic subduction zone. The Tripolitza unit is unmetamorphosed on Crete, where it is separated from the underlying high-pressure (8–10 kbar, 300–400 °C) Plattenkalk and Phyllite-Quartzite units by the extensional Cretan detachment. The age for high- pressure metamorphism in the latter units is similar to our age for the Basal unit in the Cyclades. Because pressure-temperature conditions in the Plattenkalk and Phyllite- Quartzite units on Crete and the Basal unit in the Cyclades are also similar, they must have been in close proximity in the early Miocene Hellenic subduction zone. A palinspastic reconstruction suggests a subsequent displacement of >100 km on the Cretan detachment. This is one of the greatest displacement magnitudes ever reported from detachment faults. Because of this large offset, the Cretan detachment was an efficient agent for exhuming high-pressure rocks.

Crust Formation and Plate Motion in the Early Archean

Mounting evidence for voluminous continental crust formation in the early Archean involving intracrustal melting and selective preservation of granitoid rocks suggests that initial crust formation crust formation and growth were predominantly by magmatic underplating in plumegenerated Iceland-type settings. Collision of these early islands to give rise to larger blocks is suggested by extensive horizontal shortening in both supracrustal and granitoid assemblages. Preservation of early Archean high-grade gneisses that were once at depths of 20 to 30 kilometers implies that these blocks developed thick, subcrustal roots despite high mantle heat flow. Rigid continental plates must have existed since at least 3.5 billion years ago, and greenstone belts (composed of mixed metavolcanic and metasedimentary sequences intruded by granitoid plutons) probably developed on or near these microcontinents. Paleomagnetic data with good age control from at least one ancient craton suggest that plate motion was at normal minimum average velocities of about 17 millimeters per year with respect to the poles during the period 3.5 billion to 2.4 billion years ago. If this is true on a global scale, Archean plate motion was not faster than in later geologic times.

Geochronological constraints on the evolution of the southern Dom Feliciano Belt (Uruguay)

Abstract: New U–Pb sensitive high-resolution ion microprobe, K–Ar and Ar–Ar data from the southernmost Dom Feliciano Belt allow the identification of four major events. Orthogneisses from the Punta del Este Terrane indicate a magmatic episode at c. 770 Ma and high-grade metamorphism at 641 ± 17 Ma. Granitoid emplacement at 627 ± 23 Ma was roughly coeval with peak metamorphism. Volcaniclastic rocks of the Las Ventanas Formation dated at 573 ± 11 Ma can be correlated with the peripheral foreland basin (571 ± 8 Ma). Transpression and coeval high-K calc-alkaline magmatism is recorded in the Maldonado granite dated at 564 ± 7 Ma. The following events are postulated: (1) magmatism at 850–750 Ma related to rifting; (2) metamorphism and granite emplacement at 650–600 Ma; (3) molasse sequences and foreland basins at c. 573 Ma; (4) late magmatism at 580–560 Ma associated with transpression. The data indicate that (1) the Punta del Este Terrane could be a portion of the Coastal Terrane of the Kaoko Belt, (2) granitoid emplacement at 650–600 Ma in the Punta del Este and Nico Pérez terranes favours westward subduction, and (3) widespread post-collisional synkinematic magmatism occurred in the Dom Feliciano and Kaoko belts between 580 and 550 Ma. Supplementary material: Analytical methods and data are available at http://www.geolsoc.org.uk/SUP18369.

Eocene volcanism above a depleted mantle slab window in southern Alaska

The Caribou Creek volcanic fi eld lies along the continent-side edge of forearc basin rocks in south-central Alaska and consists of over 1000 m of shallow-dipping basalt and andesite lavas with minor mafi c pyroclastic deposits. Dacite and rhyolite lavas along with shallow intrusions form dome complexes with associated pyroclastic deposits that overlie and crosscut the basalt and intermediate lavas. The basalts are tholeiitic and strongly depleted in the light rare earth elements (La/ Yb = 0.18–1.5), with concentrations of high fi eld strength elements (e.g., Zr, Hf, Ti, Y) similar to mid-ocean-ridge basalt and with variable enrichment in fl uid-mobile elements (e.g., Cs, Ba, and Pb). Intermediate and felsic rocks show enrichment in the rare earth elements and fl uid-mobile elements plus Rb and K, but retain low La/Yb ratios (0.48–3.6). A few andesite and dacite samples are strongly depleted in the heavy rare earth elements and are geochemically similar to adakites (e.g., Sr/Y up to 52). Ten 40 Ar/ 39 Ar ages for the Caribou Creek volcanic rocks range from 49.4 ± 2.2 to 35.6 ± 0.2 Ma. An adakite-like tuff beneath the other volcanic rocks yields an age of 59.0 ± 0.4 Ma. Caribou Creek basalts were derived from mid-ocean-ridge–like depleted mantle that was emplaced beneath the southern margin of Alaska through a slab window following spreading ridge subduction. Caribou Creek volcanism was coeval with oblique subduction, oroclinal bending, and right-lateral strike-slip faulting in south-central Alaska, all of which could have induced crustal extension to allow adiabatic melting of the depleted mantle reservoir to form basaltic magmas. The basalts then evolved by fractional crystallization with moderate to high degrees of assimilation of Jurassic arc basement rocks to form the intermediate and felsic magmas. Enrichment of the basaltic parent magmas in fl uid-mobile elements occurred by contamination from the Jurassic arc rocks and/or by contamination with metasomatic mantle remnant from preceding subduction. High heat fl ow through the slab window induced partial melting of garnet-bearing mafi c parts of the Jurassic arc basement to form the adakite-like rocks. The Caribou Creek volcanic rocks demonstrate that slab windows can directly infl uence magmatism inboard of accretionary prism and forearc basin settings given a suitable deformation regime (e.g., crustal extension) and that the infl uence of a slab window on continental margin magmatism can be long-lived (>20 m.y.).

Tectonic setting of the plutonic belts of Yakutia, northeast Russia, based on 40Ar/39Ar geochronology and trace element geochemistry

Samples from plutonic belts and individual intrusions in the Sakha Republic (Yakutia), northeastern Russia, were analyzed using 40Ar/39Ar geochronology and trace element geochemistry. Four phases of activity were identified: a subduction event along the southwest margin of the Kolyma-Omolon superterrane with apparent ages of ca. 160–140 Ma, a superposed collisional event dated at 143–138 Ma, a subduction event along the northern margin of the superterrane dated at 130–123 Ma with a slightly older to contemporaneous extensional event to the west; and intrusions associated with east-west extension between 106 and 92 Ma. These phases are interpreted to represent (1) subduction under and (2) accretion to Asia of Kolyma-Omolon followed by local extension, (3) subduction along the northern edge of this superterrane, and (4) regional extension following closure of the South Anyui suture and/or the start of subduction associated with the Okhotsk-Chukotka volcanic belt.

Mesozoic sedimentary-basin development on the allochthonous Wrangellia composite terrane, Wrangell Mountains basin, Alaska: A long-term record of terrane migration and arc construction

The ∼7000 m of Upper Triassic–Upper Cretaceous strata of the Wrangell Mountains basin depositionally overlie the allochthonous Wrangellia composite terrane in south-central Alaska. New sedimentologic, compositional, and geochronologic data from these strata provide a record of the migration of the terrane from an intraoceanic setting at low paleolatitudes (∼12°N) to its present position along the continental margin of southern Alaska (∼61°N). We recognize several distinct stages of basin development: (1) Upper Triassic–Lower Jurassic carbonate strata represent an intraoceanic carbonate platform built on a remnant volcanic arc at low paleolatitudes (∼12°N). (2) During the Middle to Late Jurassic, a subduction zone formed along the southern margin of the Wrangellia composite terrane, prompting development of an intraoceanic arc and backarc basin on top of the terrane. (3) A narrow thrust belt and retroarc foreland basin formed along the inboard margin of the arc during the latest Jurassic. The foreland basin and arc were subsequently folded, uplifted, and eroded during the latest Jurassic–Early Cretaceous as recorded by an angular unconformity and isotopic ages from clasts in conglomerate. Regional deformation of the foreland-basin strata, shortening and uplift of the Wrangellia composite terrane, and uplift and erosion of the Middle to Late Jurassic arc are interpreted to represent the initial collision between the terrane and the continental margin of western North America. (4) Following regional deformation, a new volcanic arc was constructed inboard (northward) of the Wrangell Mountains basin. Upper Lower to Upper Cretaceous siliciclastic strata were deposited by shallow- to deep-marine deposystems in a continental-margin forearc basin. Distributions of lithofacies types and formation-thickness changes across intrabasinal normal faults document synextensional deposition. (5) The final stage of basin development was characterized by shortening and coarse-grained sedimentation along a fault system that separated the trenchward (southern) margin of the forearc basin from the subduction complex. (6) The Wrangell Mountains basin arrived at its current position by northward translation along orogen-parallel strike-slip fault systems. Comparison of the sedimentary record of the Wrangell Mountains basin, located on the outboard margin of the Wrangellia composite terrane, with the sedimentary record of the Nutzotin basin, located along the inboard margin of the terrane, demonstrates distinct changes in the locations of depocenters, the timing of deformation, and the composition of sediment. Similar stratigraphic and structural variations characterize outboard and inboard segments of the Wrangellia composite terrane in southeastern Alaska and coastal British Columbia.

Spatial variations in focused exhumation along a continental-scale strike-slip fault: The Denali fault of the eastern Alaska Range

40 Ar/ 39 Ar, apatite fission-track, and apatite (U-Th)/He thermochronological techniques were used to determine the Neogene exhumation history of the topographically asymmetric eastern Alaska Range. Exhumation cooling ages range from ∼33 Ma to ∼18 Ma for 40 Ar/ 39 Ar biotite, ∼18 Ma to ∼6 Ma for K-feldspar minimum closure ages, and ∼15 Ma to ∼1 Ma for apatite fission-track ages, and apatite (U-Th)/He cooling ages range from ∼4 Ma to ∼1 Ma. There has been at least ∼11 km of exhumation adjacent to the north side of Denali fault during the Neogene inferred from biotite 40 Ar/ 39 Ar thermochronology. Variations in exhumation history along and across the strike of the fault are influenced by both far-field effects and local structural irregularities. We infer deformation and rapid exhumation have been occurring in the eastern Alaska Range since at least ∼22 Ma most likely related to the continued collision of the Yakutat microplate with the North American plate. The Nenana Mountain region is the late Pleistocene to Holocene (∼past 1 Ma) primary locus of tectonically driven exhumation in the eastern Alaska Range, possibly related to variations in fault geometry. During the Pliocene, a marked increase in climatic instability and related global cooling is temporally correlated with an increase in exhumation rates in the eastern Alaska Range north of the Denali fault system.

Phenocrysts versus xenocrysts in the youngest Toba Tuff: Implications for the petrogenesis of 2800 km3 of magma

The petrogenesis of the 2800 km 3 of magma erupted as the youngest Toba Tuff has been investigated using experimental petrology and 40 Ar/ 39 Ar dating of biotite, sanidine, hornblende, and plagioclase from the tuff. We find that hornblende does not crystallize experimentally from the magma at temperatures and pressures indicated by the natural mineral assemblage. Hornblende is also not in isotopic equilibrium with biotite and sanidine, both of which grew experimentally. Hornblende thus appears xenocrystic, despite being a major phase in the tuff. Some plagioclase is also xenocrystic, on the basis of Ar isotopes, but others are probably phenocrystic, because plagioclase grew experimentally. Crystal clots of hornblende + plagioclase observed in the tuff suggest that the xenocrysts came from a common source, which was at least 1.5 Ma (the oldest hornblende 40 Ar/ 39 Ar age). Our results suggest that the Toba Tuff magma resided at nearly water-saturated pressures of 100–150 MPa and that xenocrysts were entrained as recently as 10 yr before the eruption. The ubiquitous presence of hornblende in the tuff indicates that entrainment occurred throughout the 2800 km 3 of magma.

Arc-rift transition volcanism in the Puertecitos Volcanic Province, northeastern Baja California, Mexico

The Neogene Puertecitos Volcanic Province of northeastern Baja California records a transition from arc-related volcanic activity to rift volcanism associated with opening of the Gulf of California. The eastern Puertecitos Volcanic Province is divided into three volcanic sequences based on mapping, petrology, and ^(40)Ar/^(39)Ar geochronology. The lowest sequence comprises early to middle Miocene (20–16 Ma) arc-related andesitic lava flows, volcanic necks, and proximal pyroclastic and epiclastic deposits up to 400 m in thickness, with minor basaltic lava flows. Following the initiation of crustal extension in the region (11–6 Ma), synrift volcanism produced two rhyolitic sequences that discordantly overlie the arc-related rocks. The older synrift sequence (6.4–5.8 Ma) is composed of rhyolite domes and a series of pyroclastic flows up to 300 m thick. The upper sequence (3.2–2.7 Ma) consists of ash-flow tuffs and pumice-lapilli pyroclastic flows, collectively up to 200 m thick. Minor andesite eruptions followed each episode of silicic synrift volcanism. Synvolcanic faults produced topographic relief that controlled deposition of the pyroclastic flows and caused gentler dips upsection. Rhyolite domes are aligned parallel to the predominant north-northwest to north-northeast fault pattern. All three volcanic sequences are calc-alkaline. However, the synrift andesite is characterized by lower K_(2)O, lower incompatible element concentrations, and less fractionation of light rare earth elements than the arc-related basalt and andesite. This suggests that the primary melts were more primitive for synrift andesite than for the arc-related rocks.