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Eulachon (Thaleichthys pacificus), a culturally and ecologically important anadromous smelt (Family Osmeridae), ranges from Northern California to the southeast Bering Sea. In recent decades, some populations have experienced declines. Here we use a contig-level genome assembly combined with previously published restriction site-associated DNA sequencing (RADseq)-derived markers to construct an amplicon panel for eulachon. Using this panel, we develop a filtered genetic baseline of 521 variant loci genotyped in 1989 individuals from 14 populations ranging from Northern California through central Alaska. Consistent with prior genetic studies, the strongest separation occurs among three main regions: from Northern California up to and including the Fraser River; north of the Fraser River to southeast Alaska; and within the Gulf of Alaska. Separating the Fraser River from southern US populations and refining additional substructure within the central coast may be possible in mixed-stock analysis; this will be addressed in future work. The amplicon panel outperformed the previous microsatellite panel and thus will be used in future mixed-stock analyses of eulachon to provide new insights for management and conservation of eulachon. L'eulakane (Thaleichthys pacificus), un eperlan (famille des osmerides) anadrome d'importance culturelle et ecologique, est present du nord de la Californie jusqu'au sud-est de la mer de Behring. Au cours des dernieres decennies, certaines populations ont connu des declins. Nous utilisons un assemblage de genome au niveau du contig combine a des marqueurs derives du sequencage de fragments d'ADN associes a un site de restriction (RADseq) deja publies pour constituer un panel d'amplicons pour les eulakanes. A l'aide de ce panel, nous etablissons une base de reference genetique filtree de 521 sites de variants genotypes dans 1989 specimens issus de 14 populations allant du nord de la Californie au centre de l'Alaska. A l'instar de resultats d'etudes genetiques anterieures, la plus forte separation est observee pour trois regions principales, soit du nord de la Californie jusqu'au fleuve Fraser, inclusivement, du nord du fleuve Fraser jusqu'au sud-est de l'Alaska, et dans le golfe d'Alaska. Il pourrait etre possible, dans l'analyse de stocks melanges, de separer les populations du fleuve Fraser de celles du sud des Etats-Unis et d'obtenir une sous-structure plus fine pour le littoral central; cela fera l'objet de travaux futurs. Le panel d'amplicons donne de meilleurs resultats que le groupe de marqueurs microsatellites anterieur et sera donc utilise dans des analyses futures de stocks melanges d'eulakanes afin d'obtenir des renseignements utiles pour la gestion et la conservation de ce poisson. [Traduit par la Redaction], Introduction Eulachon (Thaleichthys pacificus) is a culturally and ecologically important anadromous smelt (Family Osmeridae) distributed in North America from Northern California to the southeast Bering Sea (Hay and McCarter 2000). [...] more...

Abstract: Five species of stalked crinoids collected at depths from 1,693 to 4,13 5 m off the California coast, chiefly on seamounts, are described. Four were photographed in situ and [...]

We examined the dynamics of aboveground forest woody carbon pools--live trees, standing dead trees, and down wood--during the first 6 years following wildfire across a wide range of conditions, which are characteristic of California forest fires. From repeated measurements of the same plots, we estimated change in woody carbon pools as a function of crown fire severity as indicated by a post-fire index, years since fire, pre-fire woody carbon, forest type group (hardwood vs. softwood), elevation, and climate attributes. Our analysis relied on 130 U.S. national forest inventory plots measured before and 1 year after fire, with one additional remeasurement within 6 years after fire. There was no evidence of net change in total wood carbon, defined for this study as the wood in standing trees larger than 12.7 cm diameter at breast height and down wood larger than 7.6 cm in diameter, over the post-fire period in any of the three severity classes. Stands that burned at low severity exhibited considerable shifts from live to standing dead and down wood pools. In stands that burned at moderate severity, live wood decreased significantly whereas no net change was detected in standing dead or down wood. High severity fire burning resulted in movement from standing dead to down wood pools. Our results suggest that the carbon trajectories for stand-replacing fires may not be appropriate for the majority of California's forest area that burned at low to moderate severities. Key words: longitudinal analysis, post-fire dynamics, forest stand recovery, disturbance, Forest Inventory and Analysis. Nous avons etudie la dynamique des reservoirs de carbone ligneux de la partie aerienne des forets (les arbres vivants, les arbres morts sur pied et au sol) au cours des six premieres annees apres un feu pour une large gamme de conditions caracteristiques des feux de foret de la Californie. A partir de mesures repetees des memes placettes, nous avons estime les changements dans les reservoirs de carbone ligneux en fonction de la severite des dommages a la cime des arbres selon un indice apres feu, du nombre d'annees apres le feu, du carbone ligneux avant le feu, du groupe de types forestiers (feuillus ou resineux), de l'altitude et des caracteristiques du climat. Nos analyses ont porte sur 130 placettes de l'inventaire national etats-unien (FIA) mesurees avant et un an apres le feu de meme qu'une autre fois au cours des six premieres annees apres le feu. Il n'y a pas eu d'indices de changement net du carbone ligneux total apres feu, correspondant dans cette etude au bois des arbres sur pied ayant un diametre a hauteur de poitrine plus grand que 12,7 cm et des arbres au sol ayant un diametre superieur a 7,6 cm, dans aucune des trois classes de severite du feu. Les peuplements ayant subi un feu de severite faible ont connu des changements considerables dans les reservoirs de carbone ligneux, passant d'une dominance d'arbres vivants a une dominance d'arbres morts sur pied et au sol. Dans les peuplements brules par un feu de severite moderee, les arbres vivants ont diminue significativement alors qu'aucun changement net n'a ete detecte chez les arbres morts sur pied ou au sol. Le feu de severite elevee a provoque un deplacement de reservoir ligneux des arbres morts sur pied vers les arbres au sol. Nos resultats indiquent que les trajectoires de carbone typiques des feux qui entrainent le remplacement des peuplements peuvent ne pas etre appropriees pour les cas plus communs de peuplements subissant des feux d'intensites faible a moderee. [Traduit par la Redaction] Mots-cles: analyse longitudinale, dynamique apres feu, retablissement des peuplements forestiers, perturbation, programme d'analyse et d'inventaire forestiers (FIA)., Introduction Natural disturbances such as wildfire are increasing in frequency, severity, and extent across forested ecosystems of the western U.S. (Westerling et al. 2006; Miller et al. 2009; Stephens 2005; [...] more...

Biocomplexity contributes to asynchronous population dynamics, buffering stock complexes in temporally variable environments, a phenomenon referred to as a 'portfolio effect'. We previously revealed a weakened but persistent portfolio effect in California's Central Valley fall-run Chinook salmon (Oncorhynchus tshawytscha), despite considerable degradation and loss of habitat. Here, we further explore the timing of changes in variability and synchrony and relate these changes to factors hypothesized to influence variability in adult abundance, including hatchery release practices and environmental variables. We found evidence for increasing synchrony among fall-run populations that coincided temporally with increased off-site hatchery releases into the estuary but not with increased North Pacific environmental variability (measured by North Pacific Gyre Oscillation), nor were common trends well explained by a suite of environmental covariates. Moreover, we did not observe a simultaneous increase in synchrony in the nearby Klamath-Trinity system, where nearly all hatchery releases are on-site. Wavelet analysis revealed that variability in production was higher and at a longer time period later in the time series, consistent with increased environmental forcing and a shift away from dynamics driven by natural spawners. La biocomplexite participe a une dynamique asynchrone des populations, limitant les variations au sein des complexes de stocks dans les milieux variables dans le temps, un phenomene appele << effet portefeuille >>. Nous avons deja fait etat d'un effet portefeuille affaibli, mais persistant chez les saumons quinnats (Oncorhynchus tshawytscha), a montaison automnale de la vallee centrale de Californie, malgre la degradation et la disparition considerables d'habitats. Nous examinons plus en profondeur le moment des modifications de la variabilite et de la synchronie et les relions a des facteurs presumes influencer la variabilite de l'abondance des adultes, dont les pratiques de lacher des ecloseries et des variables environnementales. Nous observons des indices d'une synchronie croissance dans les populations a montaison automnale qui coincide dans le temps avec une augmentation des lachers d'ecloseries hors site dans l'estuaire, mais non avec une variabilite accrue du milieu nord-pacifique (mesuree par l'oscillation du tourbillon nord-pacifique); en outre, un ensemble de covariables environnementales n'explique pas bien des tendances repandues. De plus, nous n'observons pas une augmentation simultanee de la synchronie dans le systeme voisin de Klamath-Trinity, ou presque tous les lachers d'ecloseries se font sur place. L'analyse des ondelettes revele que la variabilite de la production est plus grande et presente une plus longue periode plus tard dans la serie chronologique, ce qui concorde avec un forcage environnemental accru et une dynamique de moins en moins controlee par les geniteurs naturels. [Traduit par la Redaction], Introduction Environmental stochasticity drives variation in ecological dynamics within natural systems (May 1972). In coupled human-natural systems such as fisheries, decoupling this natural environmental variation from human-induced changes to populations [...] more...

Forest communities change in response to shifting climate, changing land use, and species introductions, as well as the interactions of established species. We surveyed the oak (Quercus L. spp.) community and Acorn Woodpecker (Melanerpes formicivorus (Swainson, 1827)) population within 230 ha of oak forest and savanna in central coastal California in 1979 and 2013 to assess demographic changes over a timescale relevant to mature oaks. Overall, percent canopy cover increased, particularly where coast live oak (Quercus agrifolia Nee) and California black oak (Quercus kelloggii Newberry) were most abundant. The density of stems of Q agrifolia increased, whereas the density of stems and basal area of valley oak (Quercus lobata Nee), a species favored by Acorn Woodpeckers, decreased. The number of Acorn Woodpeckers and woodpecker territories increased over the study period, coincident with the increase in percent canopy cover; however, these increases were not related spatially. Instead, increased acorn production associated with broad-scale canopy growth likely more than compensated for the loss of Q lobata. Our findings suggest that forests in this area are becoming denser and savanna is becoming more open, which so far has supported an increase in the Acorn Woodpecker population, despite potential habitat loss if Q lobata continues to decline. Key words: animal territories, community shift, land use change, plant-animal interactions, tree demographics. Les communautes forestieres changent en reponse aux fluctuations climatiques, aux changements dans l'utilisation du territoire et aux introductions d'especes, de meme qu'aux interactions entre especes etablies. Nous avons etudie les communautes de chenes (Quercus L. spp.) et les populations du pic glandivore (Melanerpes formicivorus (Swainson, 1827)) sur 230 ha de chenaie et de savane de la cote centrale californienne en 1979 et en 2013 dans le but d'evaluer les changements demographiques sur une periode de temps appropriee pour des chenes matures. Dans l'ensemble, le pourcentage de recouvrement arborescent a augmente, particulierement la oU le chene de Californie (Quercus agrifolia Nee) et le chene noir de Californie (Quercus kelloggii Newberry) etaient les plus abondants. La densite de tiges de Q agrifolia a augmente, tandis que la densite de tiges et la surface terriere du chene blanc de Californie (Quercus lobata Nee), une espece favorite du pic glandivore, ont diminue. Le nombre d'individus et de territoires du pic glandivore a augmente durant la periode d'etude, coincidant avec l'augmentation du pourcentage de couvert arborescent, mais ces augmentations n'etaient pas correlees spatialement. En fait, la production accrue de glands associee a l'augmentation a grande echelle du couvert arborescent a probablement surcompense la perte de Q lobata. Nos resultats suggerent que les forets de cette region se densifient tandis que les savanes deviennent plus clairsemees, ce qui jusqu'a maintenant a permis un accroissement des populations du pic glandivore, en depit d'un declin potentiel si la population de Q. lobata continue a decroitre. [Traduit par la Redaction] Mots-cles : domaines vitaux, modifications des communautes, changement d'utilisation du territoire, interactions plante-herbivore, demographie des arbres., Devin E. McMahon, Ian S. Pearse, Walter D. Koenig, and Eric L. Walters Introduction Forest and savanna communities can be rapidly altered by species introductions, changing land use patterns, and [...] more...

Timroseite, [Pb.sub.2][Cu.sup.2+.sub.5][([Te.sup.6+][O.sub.6]).sub.2] [(OH).sub.2], and paratimroseite, [Pb.sub.2] [Cu.sup.2+.sub.4][([Te.sup.6+][O.sub.6]).sub.2][([H.sub.2]O).sub.2], are two new tellurates from Otto Mountain near Baker, California. Timroseite is named in honor of Timothy (Tim) P. Rose and paratimroseite is named for its relationship to timroseite. Both new minerals occur on fracture surfaces and in small rugs in brecciated quartz veins. Timroseite is directly associated with acanthite, cerussite, bromine-rich chlorargyrite, chrysocolla, gold, housleyite, iodargyrite, khinite-4O, markcooperite, ottoite, paratimroseite, thorneite, vauquelinite, and wulfenite. Paratimroseite is directly associated with calcite, cerussite, housleyite, khinite-4O, markcooperite, and timroseite. Timroseite is orthorhombic, space group [P2.sub.1]nm, a = 5.2000(2), b = 9.6225(4), c = 11.5340(5) [Angstrom], V = 577.13(4) [[Angstrom].sup.3], and Z = 2. Paratimroseite is orthorhombic, space group [P2.sub.1][2.sub.1][2.sub.1], a = 5.1943(4), b - 9.6198(10), c = 11.6746(11) A, V= 583.35(9) [[Angstrom].sup.3], and Z = 2. Timroseite commonly occurs as olive to lime green, irregular, rounded masses and rarely in crystals as dark olive green, equant rhombs, and diamond-shaped plates in subparallel sheaf-like aggregates. It has a very pale yellowish green streak, dull to adamantine luster, a hardness of about 2 1/2 (Mohs), brittle tenacity, irregular fracture, no cleavage, and a calculated density of 6.981 g/[cm.sup.3]. Paratimroseite occurs as vibrant 'neon' green blades typically intergrown in irregular clusters and as lime green botryoids. It has a very pale green streak, dull to adamantine luster, a hardness of about 3 (Mohs), brittle tenacity, irregular fracture, good {001} cleavage, and a calculated density of 6.556 g/[cm.sup.3]. Timroseite is biaxial (+) with a large 2V, indices of refraction > 2, orientation X = b, Y = a, Z = c and pleochroism: X = greenish yellow, Y = yellowish green, Z = dark green (Z > Y > X). Paratimroseite is biaxial (-) with a large 2 V, indices of refraction > 2, orientation X = c, Y = b, Z = a and pleochroism: X = light green, Y = green, Z = green (Y = Z >> X). Electron microprobe analysis of timroseite provided PbO 35.85, CuO 29.57, Te[O.sub.3] 27.75, Cl 0.04, [H.sub.2]O 1.38 (structure), O[equivalent to]Cl -0.01, total 94.58 wt%; the empirical formula (based on O+Cl = 14) is [Pb.sub.2.07] [Cu.sup.2+.sub.4.80] [Te.sup.6+.sub.2.04][O.sub.12] [(OH).sub.1.98][Cl.sub.0.02]. Electron microprobe analysis of paratimroseite provided PbO 36.11, CuO 26.27, Te[O.sub.3] 29.80, Cl 0.04, [H.sub.2]O 3.01 (structure), O[equivalent to]Cl -0.01, total 95.22 wt%; the empirical formula (based on O+Cl = 14) is [Pb.sub.1.94][Cu.sup.2+.sub.3.96][Te.sup.6+.sub.2.03][O.sub.12] [([H.sub.2]O).sub.1.99][Cl.sub.0.01]. The strongest powder X-ray diffraction lines for timroseite are [[d.sub.obs] in [Angstrom] (hkl) I]: 3.693 (022) 43, 3.578 (112) 44, 3.008 (023) 84, 2.950 (113) 88, 2.732 (130) 100, 1.785 (multiple) 33, 1.475 (332) 36; and for paratimroseite 4.771 (101) 76, 4.463 (021) 32, 3.544 (120) 44, 3.029 (023,122) 100, 2.973 (113) 48, 2.665 (131) 41, 2.469 (114) 40, 2.246 (221) 34. The crystal structures of timroseite ([R.sub.1] = 0.029) and paratimroseite ([R.sub.1] = 0.039) are very closely related. The structures are based upon edge- and corner-sharing sheets of Te and Cu polyhedra parallel to (001) and the sheets in both structures are identical in topology and virtually identical in geometry. In timroseite, the sheets are joined to one another along c by sharing the apical O atoms of Cu octahedra, as well as by sharing edges and comers with an additional Cu[O.sub.5] square pyramid located between the sheets. The sheets in paratimroseite are joined only via Pb-O and H bonds. Keywords: Timroseite, paratimroseite, new mineral, tellurate, crystal structure, Otto Mountain, California DOI: 10.2138/am.2010.3514 more...

Telluroperite, [Pb.sub.3][Te.sup.4+][O.sub.4][Cl.sub.2], is a new tellurite from Otto Mountain near Baker, California. The new mineral occurs on fracture surfaces and in small vugs in brecciated quartz veins in direct association with acanthite, bromine-rich chlorargyrite, caledonite, cerussite, galena, goethite, and linarite. Various other secondary minerals occur in the veins, including six new tellurates, housleyite, markcooperite, paratimroseite, ottoite, thorneite, and timroseite. Telluroperite is orthorhombic, space group Bmmb, a = 5.5649(6), b = 5.5565(6), c = 12.4750(14) [Angstrom], V = 386.37(7) [[Angstrom].sup.3], and Z = 2. The new mineral occurs as rounded square tablets and flakes up to 0.25 mm on edge and 0.02 mm thick. The form {001} is prominent and is probably bounded by {100}, {010}, and {110}. It is bluish-green and transparent, with a pale bluish-green streak and adamantine luster. The mineral is non-fluorescent. Mohs hardness is estimated to be between 2 and 3. The mineral is brittle, with a curved fracture and perfect {001} cleavage. The calculated density based on the empirical formula is 7.323 g/[cm.sup.3]. Telluroperite is biaxial (-), with very small 2V (~10[degrees]). The average index of refraction is 2.219 calculated by the Gladstone-Dale relationship. The optical orientation is X = c and the mineral exhibits moderate bluish-green pleochrosim; absorption: X < Y = Z. Electron microprobe analysis provided PbO 72.70, Te[O.sub.2] 19.26, Cl 9.44, O[equivalent to]Cl -2.31, total 99.27 wt%. The empirical formula (based on O+Cl = 6) is [Pb.sub.2.79][Te.sup.4+.sub.1.03][O.sub.3.72][Cl.sub.2.28]. The six strongest powder X-ray diffraction lines are [[d.sub.obs] in [Angstrom]. (hkl) I]: 3.750 (111) 58, 2.857 (113) 100, 2.78l (020, 200) 43, 2.075 (024, 204) 31, 1.966 (220) 30, and 1.620 (117, 313, 133) 52. The crystal structure ([R.sub.1] = 0.056) is based on the Sillen [X.sub.1] structure-type and consists of a three-dimensional structural topology with lead-oxide halide polyhedra linked to tellurium/lead oxide groups. The mineral is named for the relationship to perite and the dominance of Te (with Pb) in the Bi site of perite. Keywords: Telluroperite, new mineral, tellurite, crystal structure, perite, nadorite, Sillen [X.sub.1], Otto Mountain, California DOI: 10.2138/am.2010.3515 more...

Markcooperite, [Pb.sub.2](U[O.sub.2])[Te.sup.6+][O.sub.6], is a new tellurate from Otto Mountain near Baker, California, named in honor of Mark A. Cooper of the University of Manitoba for his contributions to mineralogy. The new mineral occurs on fracture surfaces and in small vugs in brecciated quartz veins. Markcooperite is directly associated with bromian chlorargyrite, iodargyrite, khinite-4O, wulfenite, and four other new tellurates: housleyite, thorneite, ottoite, and timroseite. Various other secondary minerals occur in the veins, including two other new secondary tellurium minerals: paratimroseite and telluroperite. Markcooperite is monoclinic, space group [P2.sub.1]/c, a = 5.722(2), b = 7.7478(2), c = 7.889(2) [Angstrom], [beta] = 90.833(5)[degrees], V = 349.7(2) [[Angstrom].sup.3], and Z = 2. It occurs as pseudotetragonal prisms to 0.2 mm with the forms {100} and {011} and as botryoidal intergrowths to 0.3 mm in diameter; no twinning was observed. Markcooperite is orange and transparent, with a light orange streak and adamantine luster, and is non-fluorescent. Mohs hardness is estimated at 3. The mineral is brittle, with an irregular fracture and perfect {100} cleavage. The calculated density is 8.496 g/[cm.sup.3] based on the empirical formula. Markcooperite is biaxial (+), with indices of refraction [alpha] = 2.11, [beta] = 2.12, [gamma] = 2.29 calculated using the Gladstone-Dale relationship, measured [alpha]-[beta] birefringence of 0.01 and measured 2 V of 30(5)[degrees]. The optical orientation is X = c, Y = b, Z = a. The mineral is slightly pleochroic in shades of orange, with absorption: X > Y = Z. No dispersion was observed. Electron microprobe analysis provided PbO 50.07, Te[O.sub.3] 22.64, U[O.sub.3] 25.01, C1 0.03, O[equivalent to]Cl -0.01, total 97.74 wt%; the empirical formula (based on O+Cl = 8) is [Pb.sub.2.05][U.sub.0.80][Te.sup.6+.sub.1.18][O.sub.7.99][Cl.sub.0.01]. The strongest powder X-ray diffraction lines are [[d.sub.obs] in [Angstrom] (hkl) I]: 3.235 (120, 102, [bar.1]02) 100, 2.873 (200) 40, 2.985 ([bar.1]21, 112, 121) 37, 2.774 (022) 30, 3.501 (021, 012) 29, 2.220 (221, [bar.2]21, 212) 23, 1.990 (222, [bar.2]22) 21, and 1.715 (320) 22. The crystal structure ([R.sub.1] = 0.052) is based on sheets of comer-sharing uranyl square bipyramids and tellurate octahedra, with Pb atoms between the sheets. Markcooperite is the first compound to show [Te.sup.6+] substitution for [U.sup.6+] within the same crystallographic site. Markcooperite is structurally related to synthetic Pb(U[O.sub.2])[O.sub.2]. Keywords: Markcooperite, new mineral, tellurate, uranyl, crystal structure, Otto Mountain, California DOI: 10.2138/am.2010.3513 more...

Thorneite, [Pb.sub.6]([Te.sup.6+.sub.2][O.sub.10])(C[O.sub.3])[Cl.sub.2] ([H.sub.2]O), is a new tellurate from Otto Mountain near Baker, California, named in honor of Brent Thorne. The new mineral occurs on fracture surfaces and in small vugs in hrecciated quartz veins. Thorneite is directly associated with acanthite, cerussite, gold, hessite, iodargyrite, khinite, wulfenite, and three other new tellurates: housleyite, markcooperite, and ottoite. Various other secondary minerals occur in the veins, including three other new secondary tellurium minerals: paratimroseite, telluroperite, and timroseite. Thomeite is monoclinic, space group C2/c, a = 21.305(1), b = 11.059(1), c = 7.564(1) [Angstrom] [beta] = 101.112(4)[degrees], V = 1748.8(4) [[Angstrom].sup.3], and Z= 4. Crystals are prismatic to bladed with elongation and striations parallel to c and typically occur in parallel and random aggregates. It is yellow and transparent, with pale yellow streak and adamantine luster. Mohs hardness is estimated at 2. The mineral is brittle, with an irregular to splintery fracture and good {100} cleavage. The calculated density is 6.828 g/[cm.sup.3]. Thorneite is biaxial (+), with large 2V, but indices of refraction are too high to he measured. The optic orientation is Y = b, Z [conjunction] a = 29[degrees] in obtuse [beta]. No pleochroism was observed. Electron microprobe analysis provided PbO 73.90, ZnO 0.03, Te[O.sub.3] 20.35, Cl 2.29, [H.sub.2]O 1.28 (structure), C[O.sub.2] 2.29 (structure), O[equivalent to]Cl -0.52, total 99.62 wt%; the empirical formula (based on O+Cl = 16) is ([Pb.sub.5.94][Zn.sub.0.01])([Te.sup.6+.sub.2.08][O.sub.10])([C.sub.1.00] [O.sub.3])[[Cl.sub.1.16][O.sub.0.34][(OH).sub. 0.50]]([H.sub.2]O) The strongest powder X-ray diffraction lines are [[d.sub.obs] in [Angstrom] (hkl) I]: 10.43 (200) 35, 3.733 ([bar.5]11, [bar.2]02, 002) 27, 3.595 ([bar.4]21) 33, 3.351 (112) 66, 3.224 (51l, 131) 100, 3.093 ([bar.2]22, [bar.3]31) 30, 2.900 ([bar.6]21) 44, 2.133 (821, 622, 223, 731, 242) 38. The crystal structure ([R.sub.1] = 0.028) contains edge-sharing octahedral tellurate dimers, [[[Te.sup.6+.sub.2][O.sub.10]].sup.8-] that bond to Pb atoms, which in turn are linked via bonds to Cl atoms, C[O.sub.3] triangles, and [H.sub.2]O molecules. Keywords: Thorneite, new mineral, tellurate, crystal structure, Otto Mountain, California DOI: 10.2138/am.2010.3512 more...

We present a new method for Lu-Hf geochronology using lawsonite, an index mineral of high-pressure, low-temperature metamorphism. The method uses common cation exchange chromatography to purify Lu and Hf from the sample and analysis by muiticollector inductively coupled plasma--mass spectrometry. Lawsonite [sup.176]Lu/[sup.177]Hf ratios are elevated and are similar to other minerals commonly used in Lu-Hf geochronology such as garnet or apatite. We obtain an age of 145.5 [+ or -] 2.4 Ma for lawsonite-blueschist facies metamorphism at Ring Mountain within the Franciscan Complex, California, the lawsonite type locality. The age agrees well with published geochronology and thermochronology for the Franciscan Complex. The new method provides a powerful tool to investigate processes and time scales of blueschist to eclogite facies metamorphism, crust-mantle recycling via subduction, and the geodynamics of convergent margins. doi: 10.1130/G30292A.1 more...

Although earthflows are the dominant erosion mechanism in many mountainous landscapes, estimates of long-term earthflow-driven sediment flux remain elusive because landslide displacement data are typically limited to contemporary time periods. Combining high-resolution topography from airborne LiDAR (light detection and ranging), total station surveying, orthorectified historical aerial photographs, and inventories of meteoric [sup.10]Be in soil pits, we quantified ~150 years of slope movement on a 1.5-km-long earthflow in the Eel River catchment, northern California, United States. Using LiDAR-derived topography, we mapped the upper half of the earthflow into three distinct kinematic zones: an upslope source area, a long narrow transport zone, and a mid-slope compressional zone. From our air photo analysis (1944-2006), average velocities are fastest in the transport zone (1.7 m/a), slowest in the source zone ( more...

The driving forces of microplate transport remain one of the major unknowns in plate tectonics. Our hypothesis postulates that the Baja California microplate is transported along the North America-Pacific plate boundary by partial coupling to the Pacific plate and low coupling to the North America plate. To test this idea, we use numerical modeling to examine the interplate coupling on a multiple-earthquake-cycle time scale along the Baja California-Pacific plate boundary and compare the modeled velocity field with the observed geodetic motion of the Baja California microplate. We find that when the strain can localize along a weak structure surrounding microplate (faults), high interplate coupling, produced by frictional tectonic stresses, can reproduce the observed kinematics of the Baja California microplate as seen from geodetic rigid-plate motions. We also find that the northward motion of Baja California can influence the fault slip partitioning of the major faults in the North America--Pacific plate boundary region north of Baja California. more...

The precise tectonic role of the left-lateral Garlock fault in southern California has been controversial. Three proposed tectonic models yield significantly different predictions for the slip rate, history, orientation, and total bedrock offset as a function of distance along strike. In an effort to test these models, we present the first slip-rate estimate for the western Garlock fault that is constrained by radiocarbon dating. A channel (referred to here as Clark Wash) incised into a Latest Pleistocene alluvial fan has been left-laterally offset at least 66 [+ or -] 6 m and no more than 100 m across the western Garlock fault, indicating a left-lateral slip rate of 7.6 mm/ yr (95% confidence interval of 5.3-10.7 mm/ yr) using dendrochronologically calibrated radiocarbon dates. The timing of aggradational events on the Clark Wash fan corresponds closely to what has been documented elsewhere in the Mojave Desert, suggesting that much of this activity has been climatically controlled. The range-front fault, located a few hundred meters northwest of the Garlock fault, has probably acted primarily as a normal fault, with a Holocene rate of dip-slip of 0.4-0.7 mm/yr. The record of prehistoric earthquakes on the Garlock fault at this site, though quite possibly incomplete, suggests a longer interseismic interval (1200--2700 yr) for the western Garlock fault than for the central Garlock fault. The relatively high slip rate determined here indicates that the western and central segments of the Garlock fault show similar rates of movement that are somewhat faster than rates inferred from geodetic data. The high rate of motion on the western Garlock fault is most consistent with a model in which the western Garlock fault acts as a conjugate shear to the San Andreas fault. Other mechanisms, involving extension north of the Garlock fault and block rotation at the eastern end of the fault may be relevant to the central and eastern sections of the fault, but they cannot explain a high rate of slip on the western Garlock fault. more...

Three-dimensional numerical models are needed to investigate the role of nonvertical strike-slip fault segments on the deformation within restraining bends. Numerical models simulate geologic deformation of two alternative three-dimensional present-day configurations for the San Andreas fault through the restraining bend within the San Gorgonio Pass region (SGPR) in Southern California. Both models produce decreasing strike-slip rates southward along the San Bernardino strand of the San Andreas fault, similar to geologic data. The north-dipping San Andreas fault model better matches the available strike-slip data as well as the geologic uplift data for the southern San Bernardino Mountains than the vertical San Andreas fault model. We conclude that a north-dipping fault configuration is preferred for models of the San Andreas fault in the SGPR. The complexity of the active fault geometry at the SGPR promotes the transfer of strike slip from the San Andreas fault to the nearby but unconnected San Jacinto fault. Slip rates and uplift patterns are sensitive to fault geometry within strike-slip restraining bends. more...

In August 2004, a National Forest fire crew extinguished a 1.2 ha fire in a wilderness area ~40 km northeast of Santa Barbara, California. Examination revealed that the fire originated on a landslide dotted with superheated fumaroles. A 4 m borehole punched near the hottest (262[degrees]C) fumarole had a maximum temperature of 307[degrees]C. Temperatures in this borehole have been decreasing by ~0.1[degrees]C/d, although the cooling rate is higher when the slide is dry. Gas from the fumaroles and boreholes is mostly air with 3-8 vol% carbon dioxide and trace amounts of carbon monoxide, methane, ethane, and propane. The carbon dioxide is [sup.14]C-dead. The ratios of methane to ethane plus propane [[C.sub.1]0/([C.sub.2] + [C.sub.3])] range from 3.6 to 14. Carbon isotope values for the C[O.sub.2] range from -14%o to -23%o [[delta].sup.13]C. [sup.3]He/[sup.4]He values range from 0.96 to 0.97 times that of air. The anomalous heat is interpreted to be due to rapid oxidation of iron sulfide augmented by combustion of carbonaceous matter within the formation. more...

Paleomagnetic and rock magnetic measurements for 27 sites in the tonalitic ca. 108 Ma Alpine and ca. 100 Ma Ramona complexes in the suspect Yuma terrane or western zone of the Peninsular Ranges Batholith near San Diego show that single- and pseudosingle-domain magnetite carries their characteristic remanence. The Alpine and Ramona paleopoles agree with six published paleopoles from Cretaceous igneous rocks in the Yuma terrane, but all eight paleopoles are discordant for North America. Al-in-hornblende geothermobarometric depth determinations at 16 sites, plus published mapping and depth determinations, suggest that the batholith is divided into blocks by orogen-parallel faults or by orthogonal fault sets with large vertical displacements. The paleomagnetic data show that the fault blocks in the terrane underwent, on average, ENE-side-up lilting of ~10[degrees] [+ or -] 3[degrees] about an orogen-parallel axis between ca. 125 Ma and ca. 89 [+ or -] 7 Ma, but they have not been translated northward as a suspect terrane by [greater than or equal to] 500 km, excluding Neogene opening of the Gulf of California. The tilting is attributed to torque from ENE shallow underthrusting of the Farallon plate beneath the WSW overthrusting North American plate, where subsequent extension has caused differential uplift of the fault blocks. Nonsignificant dextral displacement of ~4[degrees] [+ or -] 1[degrees] (440 [+ or -] 110 km) between the eastern and western zones of the batholith suggests that this boundary represents an intra-arc fault. The large paleopole discordance found in younger sedimentary strata of Baja California is attributed mainly to compaction shallowing of the remanence and/or postdeposittonal remagnetization. Keywords: Cretaceous, geobarometry, geothermometry, paleomagnetism, Peninsular Ranges Batholith, tectonics. more...

We use a combination of apatite [sup.4]He/[sup.3]He, (U-Th)/He, and fission-track thermochronology to date slip on the Surprise Valley fault in northeastern California by analyzing a single sample from the Warner Range in the footwall of the fault. This sample, a granitic clast from a conglomerate, yielded a fission-track age of 11.6 [+ or -] 2.8 Ma and a (U-Th)/He age of 3.02 [+ or -] 0.52 Ma. Geologic relationships indicate that this sample was buried to a depth of ~3.3 km prior to exhumation during slip on the Surprise Valley fault. Fission-track age and length data indicate that the sample was fully reset (>120 [degrees]C) prior to exhumation, which began sometime after 14 Ma. A single aliquot of nine apatite grains was step-heated for [sup.4]He/[sup.3]He analysis; modeling of the resulting [sup.4]He distribution indicates that cooling from >80 [degrees]C to -20 [degrees]C occurred between 3 and 1 Ma. Interconsistent time-temperature (t-T) solutions to the combined [sup.4]He/[sup.3]He, (U-Th)/He, and fission-track data require two distinct periods of cooling, consistent with noncontinuous slip on the Surprise Valley fault. Early cooling and fault slip took place between 14 and 8 Ma, followed by more recent fault slip ca. 3 Ma. This timing is consistent with both local geologic relationships and with the regional timing of faulting along the western margin of the Basin and Range Province. These data demonstrate the resolving power of combined fission-track, (U-Th)/He, and [sup.4]He/[sup.3]He thermochronometric data to extract low-temperature t-T information from a single sample close to Earth's surface. Keywords: [sup.4]He/[sup.3]He thermochronology, (U-Th)/He dating, fission-track analysis, Surprise Valley fault. more...

We compare geodetic velocity to geologic fault slip rates to show that tectonic loading was doubled across the eastern California shear zone (ECSZ) during a cluster of major earthquake activity. New slip rates are presented for six dextral faults that compose the ECSZ in the central Mojave Desert. These rates were determined from displaced alluvial fans dated with cosmogenic [sup.10]Be and from a displaced lava flow dated with [sup.40]Ar/[sup.39]Ar. We find that the sum geologic Mojave ECSZ slip rate, [less than or equal to]6.2 [+ or -] 1.9 mm/yr, is only half the present-day geodetically measured velocity of 12 [+ or -] 2 mm/yr. These rates account for cumulative fault slip and geodetic observations that span the 60-km-wide shear zone; therefore this difference cannot be attributed to postseismic relaxation. Redistribution of tectonic loading over the earthquake cycle at a regional scale suggests that earthquake clustering may be enhanced via feedback with weakening of ductile shear zones. Keywords: transient deformation, earthquake clustering, Eastern California shear zone, slip rate, cosmogenic dating. more...

We used high-resolution topography, geomorphic mapping of active surface processes, and geologic mapping to study the topographic and erosional response of small drainage basins to rock uplift along the Dragon's Back pressure ridge along the San Andreas fault in the Carrizo Plain, California. We infer the history of deformation experienced by -40 small drainage basins formed in poorly consolidated sedimentary rocks. A space-for-time substitution directly images the erosional and topographic responses to deformation. Progressive deformation and rock uplift are accompanied by increases in channel steepness and basin relief. As uplift ceases, channel concavity rapidly increases, causing channels to undercut hillslopes--this undercutting promotes the consumption of hillslopes by landsliding. This undercutting also causes basin relief to be greatest after uplift has stopped. This analysis indicates that channels of the Dragon's Back pressure ridge respond to changes in rock uplift rates over thousands of years, whereas hilislope processes may take more than an order of magnitude longer to adjust to changes in rock uplift rates. Our study directly measures changes in erosional processes due to the initiation and cessation of rock uplift, which can typically only be inferred using numerical models, by direct field observations. Keywords: LiDAR data, landscape development, San Andreas fault, hillslope response, channel response. more...

[1] The NNW-trending White-Inyo Range includes intrusive and volcanic rocks on the eastern flank of the Sierran volcano-plutonic arc. The NE-striking, steeply SE-dipping Barcroft reverse fault separates folded, metamorphosed Mesozoic White Mountain Peak mafic and felsic volcanic flows, volcanogenic sedimentary rocks, and minor hypabyssal plugs on the north from folded, well-bedded Neoproterozoic-Cambrian marble and siliciclastic strata on the south. The 163 [+ or -] 2 Ma Barcroft Granodiorite rose along this fault, and thermally recrystallized its wall rocks. However, new SHRIMP-RG ages of magmatic zircons from three White Mountain Peak volcanogenic metasedimentary rocks and a metafelsite document stages of effusion at ~115-120 Ma as well as at ~155-170 Ma. The U-Pb data confirm the interpretation by Hanson et al. (1987) that part of the metasedimentary-metavolcanic pile was laid down after Late Jurassic intrusion of the Barcroft pluton. The Lower Cretaceous, largely volcanogenic metasedimentary section lies beneath a low-angle thrust fault, the upper plate of which includes interlayered Late Jurassic mafic and felsic metavolcanic rocks and the roughly coeval Barcroft pluton. Late Jurassic and Early Cretaceous volcanism in this sector of the Californian continental margin, combined with earlier petrologic, structural, and geochronologic studies, indicates that there was no gap in igneous activity at this latitude of the North American continental margin. Citation: Scherer, H. H., W. G. Ernst, and R. Brooks Hanson (2008), Geologic implications of new zircon U-Pb ages from the White Mountain Peak Metavolcanic Complex, eastern California, Tectonics, 27, TC2002, doi:10.1029/2007TC002141. more...

We performed a multibeam survey of Eel Canyon, offshore northern California. The survey revealed a significant bend in the canyon that appears to be due to the oblique compressional tectonics of the region. A series of steps within a linear depression, ~280 m above the canyon floor, extends from the canyon rim at this bend to the subduction zone and a distinct fan-like topographic rise. We hypothesize that the linear depression is a distributary channel and the steps are cyclicstep bedforms created by turbidity currents. Our interpretation indicates that turbidity currents are able to run up and overspill the 280-m-high canyon wall, resulting in a partial avulsion of the canyon and the construction of a fan lobe that is offset from the canyon mouth. Simple hydraulic calculations show that turbidity currents generated in the canyon head from failure of 2-3 m of material would be capable of partially overflowing the canyon at this bend, assuming steadyuniform flow, full conversion of the failed mass into a turbidity current, and a range of friction coefficients. These estimates are consistent with analyses of sediment cores collected in the head of Eel Canyon, which sug gest that 2-3 m of material fails on decadal time scales. Our calculations show that the overflowing parts of the currents would have large shear velocities (>10 cm/s) and supercritical Froude numbers, consistent with erosion of the distributary channel and formation of cyclic steps by turbidity currents. Keywords: submarine canyons, Eel River, turbidity currents, cyclic steps, channel formation, superelevation, avulsion. more...

Devonian-Middle Jurassic terrane assemblies in the Klamath Mountains and Sierra Nevada Foothills consist chiefly of ophiolite-chert-argillite sequences. Mafic-ultramafic complexes are oceanic, whereas associated fine-grained deep-water terrigenous sediments were derived mainly from adjacent, previously docked Klamath-Sierran terranes. Coeval calc-alkaline arc rocks are volumetrically rare. Geologic and petrochemical relations suggest a rifted are origin for Klamath mafic metavolcanic units inter-layered with distal turbidites in the 170-200 Ma North Fork terrane; detrital zircon U-Pb ages indicate that the clastic debris had a regional eastern Klamath source. The Eastern Hayfork cherty melange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby Eastern Klamath Antelope Mountain Quartzite. The seaward 200 Ma Rattlesnake Creek terrane is an ophiolitic melange with North Fork petrotectonic affinities. The North Fork-Eastern Hayfork-Rattlesnake Creek amalgam correlates with the Calaveras Complex and the outboard Jura-Triassic arc belt in the Sierran Foothills. Geochemical bulk-rock and zircon U-Pb age data support interpretation of the 200 Ma Jura-Triassic arc as an adjacent offshore mafic belt overlying a 300 Ma ophiolitic basement. These oceanic complexes were sutured against the Central Metamorphic Belt-Eastern Klamath-Feather River-Northern Sierra terrane backstop before deposition and deformation of the outboard Upper Jurassic Galice and Mariposa formations. Klamath-Sierran terrane assemblies reflect ~230 m.y. of transpression-transtension involving only minor episodes of subduction, producing ubiquitous ophiolite-chert-argillite lithologies and rare felsic arc rocks. In contrast, the Late Jurassic to largely Cretaceous Klamath-Sierra Nevada quartzo-feldspathic volcanic-plutonic arc attests to massive calc-alkaline magmatism attending a strong eastward component of underflow by the Farallon plate. The coeval Galice-Mariposa formations, followed by the Cretaceous Great Valley forearc and Franciscan trench deposits, are first-cycle felsic debris shed mainly from the Klamath-Sierran arc. These units record ~70 m.y. of rapid sialic crustal growth attending major periods of approximately margin-normal convergence. This profound transition in northern California included Devonian-Middle Jurassic rifting, drifting, and stranding of ophiolite-chert-argillite terranes along an adjacent curvilinear continental margin, then nearly head-on Cretaceous subduction that resulted in massive calc-alkaline igneous activity, the erosion of which generated the felsic Great Valley Group forearc basin and Franciscan Complex trench clastic sedimentary units. Keywords: transpression, transtension, convergence, terrane assembly, northern California, Mesozoic plate tectonics. more...

The Eastern California shear zone is an active, north-northwest-trending zone of intraplate right-lateral shear that absorbs ~25 % of Pacific-North America relative plate motion. The Stateline fault system (SFS), which includes several previously recognized, discontinuously exposed Quaternary structures along the California-Nevada border, is in this paper defined as a continuous, 200-km-long zone of active dextral shear that includes (from south to north) the Mesquite, Pahrump, and Amargosa Valley segments. Recognition of this system expands the known extent of the Eastern California shear zone ~50 km to the east-northeast from its traditionally recognized boundary along the Death Valley fault system. Proximal volcanic and rock avalanche deposits offset across the Mesquite segment of the SFS indicate 30 [+ or -] 4 km of slip on this structure since 13.1 [+ or -] 0.2 Ma. This offset is an order of magnitude larger than previous estimates across this section of the SFS, but it is consistent with larger offsets previously proposed for the central and northern sections. The total offset and averaged slip rate since mid-Miocene time (2.3 [+ or -] 0.35 mm/yr) are similar to those of other major faults across this portion of the Basin and Range, which, from east to west, include the Death Valley, Panamint Valley-Hunter Mountain, and Owens Valley fault systems. However, in contrast to these faults, the average post-mid-Miocene slip rate on the SFS is approximately twice that estimated from present-day geodetic observations and an order of magnitude greater than estimates of average post-mid-Pleistocene slip rates. This discrepancy between long-term, short-term, and geodetically derived slip rates differs from other geologic-geodetic, slip-rate discrepancies in the Eastern California shear zone, where geodetic slip rates are significantly faster than both long-term and short-term geologic slip rates. This suggests that either the slip rate on the SFS has diminished over time, such that the system is an abandoned strand of the relatively young Eastern California shear zone, or that the present-day slip rate represents a transient period of slow slip, such that strands of the shear zone must accommodate a complex spatial and temporal distribution of slip. Keywords: Eastern California shear zone, stateline fault, tectonics, fault offset, fault slip rate, and slip rate discrepancy doi: 10.1130B26138.1 more...

[1] We investigate the relationship between frictional strength and clay mineralogy of natural fault gouge from a low-angle normal fault in Panamint Valley, California. Gouge samples were collected from the fault zone at five locations along a north--south transect of the range-bounding fault system, spanning a variety of bedrock lithologies. Samples were powdered and sheared in the double-direct shear configuration at room temperature and humidity. The coefficient of friction, [mu], was measured at a range of normal stresses from 5 to 150 MPa for all samples. Our results reinforce the intuitive understanding that natural fault gouge zones are inherently heterogeneous. Samples from a single location exhibit dramatic differences in behavior, yet all three were collected within a meter of the fault core. For most of the samples, friction varies from [mu] = 0.6 to [mu] = 0.7, consistent with Byerlee's law. However, samples with greater than 50 wt % total clay content were much weaker ([mu] = 0.2-0.4). Expandable clay content of the samples ranged from 10 to 40 wt %. Frictional weakness did not correlate with expandable clays. Our results indicate that friction decreases with increasing total clay content, rather than with the abundance of expandable clays. The combination of field relations, analytical results, and friction measurements indicates a positive correlation between clay content, fabric intensity, and localization of strain in the fault core. A mechanism based upon foliations enveloping angular elements to reduce friction is suggested for weakening of fault gouge composed of mixed clay and granular material. We provide broad constraints of 1-5 km on the depth of gouge generation and the depth at which fault weakness initiates. We show that slip on the Panamint Valley fault and similar low-angle normal faults is mechanically feasible in the mid-upper crust if the strength of the fault is limited by weak, clay-rich fault gouge. more...

Abstract: We surveyed algal cover, algal biomass, and physical and chemical factors at 14 sites representing a range of land use types in the Malibu Creek watershed in southern California, [...]

Strata interpreted to be eolian are recognized in the Neoproterozoic Big Bear Group in the San Bernardino Mountains of southern California, USA. The strata consist of medium- to large-scale (30 cm to >6 m) cross-stratified quartzite considered to be eolian dune deposits and interstratified thinly laminated quartzite that are problematically interpreted as either eoliantranslatent climbing ripple laminae, or as tidal-flat deposits. High index ripples and adhesion structures considered to be eolian are associated with the thinly laminated and cross-stratified strata. The eolian strata are in a succession that is characterized by flaser bedding, aqueous ripple marks, mudcracks, and interstratified small-scale cross-strata that are suggestive of a tidal environment containing local fluvial deposits. The eolian strata may have formed in a near-shore environment inland of a tidal flat. The Neoproterozoic Big Bear Group is unusual in the western United States and may represent a remnant of strata that were originally more widespread and part of the hypothetical Neoproterozoic supercontinent of Rodinia. The Big Bear Group perhaps is preserved only in blocks that were downdropped along Neoproterozoic extensional faults. The eolian deposits of the Big Bear Group may have been deposited during arid conditions that preceded worldwide glacial events in the late Neoproterozoic. Possibly similar pre-glacial arid events are recognized in northern Mexico, northeast Washington, Australia, and northwest Canada. Keywords: eolian deposits; Neoproterozoic; San Bernardino, Mountains. California, USA more...

Thick eolian deposits are commonly observed beneath desert pavements downwind of dust-emitting playas. These deposits play an important role in piedmont-surface evolution, controlling surface hydrologic conductivity and rates of pedogenesis. To better understand the factors controlling the spatial distribution of eolian deposition, we developed a numerical model that treats deposition from spatially distributed playa sources using analytic point-source solutions for deposition from a Gaussian plume. The model also accounts for complex downwind topography. As a test case, model predictions were compared to eolian deposit thicknesses on Eagle Mountain piedmont, southern Amargosa Valley, California, which receives dust from nearby Franklin Lake playa. The close relationship between the model predictions and mapped thicknesses suggests that eolian transport and deposition can be modeled from basin to regional scales within this framework. These results have important implications for hydrologic, pedogenic, and air-quality problems. Keywords: eolian, playa, Quaternary, numerical model, Gaussian plume. more...

To assess the effects of regional climate change using remote-sensing methods, we conducted integrated geologic, botanical, and field-radiometer baseline studies on the ground in the central White-Inyo Mountains. Vegetation on contrasting geologic substrates in 58, 50 x 50 m sites was studied annually in early July and in late September 1997-2001. We quantified areal abundances of plant species by using field inventories and ground-based values of the normalized-difference vegetation index (NDVI). Distributions and abundances of species are complex functions of microclimate (elevation, slope, and facing direction), seasonality, soil development, bedrock lithologies, and albedo. X-ray diffraction studies demonstrate that the soils consist chiefly of in situ weathering components such as illite, montmorillonite, chlorite, quartz, carbonates, and feldspars. Soils along higher ridges contain modest amounts of wind-blown grains derived from glaciated granodioritic plutons and volcanic roof pendants of the Sierra Nevada. Light-colored limestone and dolomite support only sparse amounts of Sagebrush at any altitude, whereas this drought-tolerant species is widespread on dark argillite, quartzite, phyllite, and granitic rocks. Grasses grow luxuriantly on dark granite and quartzite, but poorly on carbonate bedrock. Trees are numerous on carbonate rocks, but are uncommon on dark granite and quartzite. Higher-albedo rocks and soils retain moisture better than darker substrates and in general support greater vegetative cover and biomass. These studies provided ground control for hyperspectral data collected by ER-2 aircraft along three Airborne Visible and InfraRed Imaging Spectrometer (AVIRIS) flight lines. Because of the ~16 m spatial resolution of the remotely sensed data, only plant communities could be mapped from AVIRIS imagery. On the basis of the dominant species, we identified the following communities: Shadscale/desert scrub; Pinyon-Juniper woodlands; Sagebrush meadows; Mountain Mahogany woodlands; Aspen woodlands; Bristlecone Pine-Limber Pine woodlands; and Alpine fell-fields. When judged by correlation with the 50 x 50 m ground-based inventoried sites, botanical community assignments for the June 2000 flight lines achieved an accuracy of 81%; in contrast, October 1996 assignments were only 65 % correct, reflecting low sun angle and senesced vegetation. Keywords: hyperspectra, microclimate, vegetation, White-Inyo Mountains, remote sensing, geobotany. more...

Mineral (N[H.sup.+.sub.4] + N[O.sup.-.sub.3]) and dissolved organic nitrogen (DON) dynamics were investigated along a soil chronosequence in northern California that ranges in age from about 100 000 to 500 000 BP. Younger soils are slightly acidic and fertile supporting highly productive grasslands and mixed-conifer forests. Older soils are highly acidic and infertile supporting forests of dwarf ( more...

Petrologic models suggest that dehydration and metamorphism of subducting slabs release water that serpentinizes the overlying forearc mantle. To test these models, we use the results of controlled-source seismic surveys and earthquake tomography to map the upper mantle along the Cascadia margin forearc. We find anomalously low upper-mantle velocities and/or weak wide-angle reflections from the top of the upper mantle in a narrow region along the margin, compatible with recent teleseismic studies and indicative of a serpentinized upper mantle. The existence of a hydrated forearc upper-mantle wedge in Cascadia has important geological and geophysical implications. For example, shearing within the upper mantle, inferred from seismic reflectivity and consistent with its serpentinite rheology, may occur during aseismic slow slip events on the megathrust. In addition, progressive dehydration of the hydrated mantle wedge south of the Mendocino triple junction may enhance the effects of a slab gap during the evolution of the California margin. Keywords: serpentinite, mantle, forearc. Moho, Cascadia. more...

The improved resolution of sediment provenance from detrital zircon analysis of Great Valley stratigraphy enables recognition of previously undocumented arc magmatism and the evolution of regional drainage systems within the Cretaceous arc-forearc system related to uplift, magmatism, and structure in the arc. Great Valley detrital zircon age data confirm previous studies that indicate that the locus of the sediment source in the southern Sierra Nevada arc migrated east with the active volcanic front and suggest rapid rates of uplift and unroofing of the southern arc. Sacramento Valley detrital zircon age data indicate a more complex history of drainage in the northern Klamath-Sierran arc than previously documented. Detrital zircon age distributions from the Cache Creek section of the Great Valley Group broaden through time from nearly unimodal age distributions to signatures with multiple age peaks. This transition to more broadly distributed detrital zircon age spectra likely results from a combination of (1) expanding subaerial drainage systems from highly localized to more broadly distributed catchments; (2) changing shelf and submarine-canyon morphology with rising sea level and/or basin subsidence; (3) increased degree of dissection of the Klamath-Sierran arc; and (4) potential drainage capture and redirection within the arc. Sacramento Valley detrital zircon age data also record a pulse of Late Jurassic to Early Cretaceous magmatism in the northwestern Sierra Nevada arc, an age of Cordilleran magmatism and deformation represented by limited exposure in the modern Sierra Nevada. These results offer significant new insights into the evolution of a well-studied arc-forearc system. Keywords: California, forearc basins, Great Valley, provenance, Sierra Nevada, zircon. more...

A group of faults surrounding the Trinity terrane in the core of the Klamath Mountains province of northern California displays a commonality in physical features and in age that sets them apart from other faults in the Klamath Mountains. The faults dip shallowly outward from the central Trinity terrane and feature younger-on-older, detachment-fault geometry. Several exhibit shallowly south-southwest- or south-southeast-plunging slickenside striations. Distinctive muscovite-bearing dacite, muscovite-bearing rhyolite, and plagioclase porphyry dikes are common along these faults, as are epithermal mineralization and associated small, but economic gold and/or copper massive-sulfide deposits. Two of the faults crosscut the Triassic Siskiyou thrust. In addition, the Cecilville fault cuts the Late Jurassic Soap Creek Ridge fault, so extensional faulting postdates the widely developed Nevadan deformational event in the Klamath Mountains. A 126 [+ or -] 3 Ma K-Ar whole-rock age obtained on a muscovitebearing rhyolite dike intruding the fault zone at Menzel Gulch, on the southern margin of the Trinity terrane, gives a minimum age of Early Cretaceous for this fault. Evidence for Tertiary extensional faulting on the La Grange fault, previously reported by Schweickert and Irwin, records a later episode of extension on the southern margin of the Trinity terrane. Keywords: detachment fault, extension tectonics, faults and faulting, Klamath Mountains, massive-sulfide deposits. more...

[1] The Jura-Cretaceous Peninsular Ranges batholith (PRB) of southern and Baja California contains a remarkable example of a crustal transition between juxtaposed oceanic and continental floored arcs. Jura-Cretaceous deformation in the PRB is focused along this lithospheric discontinuity and defines a zone of contractional deformation that stretches at least 800 km along the batholith. The western edge of this zone is associated with a dramatic eastward increase in maximum recorded metamorphic pressures (from 2 to 6 kbar) and temperatures (from 650[degrees]C). Elsewhere in the North American Cordillera, similar lithospheric boundaries are thought to accommodate various amounts of orogen-parallel displacements of outboard tectonic fragments. To test this proposal in the PRB, we examined an unusually well-preserved part of the transition zone in the southern Sierra San Pedro Martir (SSPM). Here a spectacular ~20 km wide, doubly vergent fan structure, which developed over a period >40 m.y. during both Jurassic and Cretaceous magmatism, occurs along the transition zone. This structure consists of moderately inward dipping, outward vergent, fold and thrust belts with widespread mylonitic fabrics that steepen toward the center where both mylonitic and magmatic fabrics occur in syntectonic, vertically sheeted plutons. Mineral lineations change from steeply pitching on the sides of the fan to moderately pitching in the center. A number of constraints on possible displacements, rotation of units, and duration of fabric formation limit the amount of orogen-parallel displacement to a few kilometers across the fan structure in the SSPM. Instead, we propose that the fan structure is a local feature that results from a combination of tectonic wedging caused by a cusp of rigid, pre-Mesozoic continental margin crust at this location and vise tectonics that arose from strong lateral rheological gradients resulting from extensive heating by the sheeted tonalite complex, focused deformation along the lithospheric discontinuity, and subsequent strain softening. We suggest that these mechanical controls on deformation during orthogonal convergence may be common in other orogens where similar fan structures have been attributed to transpression, emphasizing the need for caution in interpreting such features as zones of significant, orogen-parallel displacement. INDEX TERMS: 8020 Structural Geology: Mechanics; 8150 Tectonophysics: Evolution of the Earth: Plate boundary--general (3040); 8102 Tectonophysics: Continental contractional orogenic belts; 9350 Information Related to Geographic Region: North America; 8015 Structural Geology: Local crustal structure; KEYWORDS: transpression, indentor, tectonics, arc, cordillera, Mexico. Citation: Schmidt, K. L., and S. R. Paterson, A doubly vergent fan structure in the Peninsular Ranges batholith: Transpression or local complex flow around a continental margin buttress?, Tectonics, 21(5), 1050, doi: 10.1029/2001TC001353, 2002. Received 22 December 2001; accepted 23 May 2002; published 26 October 2002. more...

Low, stabilized sand dunes cover much of the Cactus and La Posa Plains in western Arizona, near the town of Parker. The provenance of the dunes is not well defined, and it had been hypothesized that they were possibly related to extensive eolian sand deposits in the eastern Mojave Desert of California, on the other side of the Colorado River. Major oxide analyses of bulk sand samples collected from southeastern California and western Arizona show clear chemical differences. The Mojave sands all have Si[O.sub.2] < 79 wt%, [Al.sub.2][O.sub.3] > 10 wt%, and [Na.sub.2]O + [K.sub.2]O > 5 wt%, whereas the Colorado River and Arizona sands have Si[O.sub.2] > 81 wt%, [Al.sub.2][O.sub.3] < 6 wt%, and [Na.sub.2]O + [K.sub.2]O < 4 wt%. These results show that the stabilized dunes near Parker are chemically indistinguishable from Colorado River sands. The chemical differences between the southeastern California sands and the western Arizona sands are supported by X-ray diffraction results that indicate these two sand populations have significant differences in the relative abundance of quartz and feldspar. Our data also indicate a near-source origin for the Mojave sands, whereas the Colorado River and Arizona sands have undergone more prolonged chemical weathering. The major oxide results are consistent with published trace element studies by others that clearly distinguished between mature and immature sands in California and Colorado. The quartz/feldspar ratio may be amenable to measurement by some advanced remote-sensing instruments. Keywords: deserts, eolian, geochemistry, sand, sand dunes, sediment. more...

Determining the history of fossil hydrologic systems that is recorded in vein minerals can be complicated by heterogeneity in the oxygen and carbon isotope compositions at millimeter and smaller scales. Several samples of diagenetic dolomite from two location in the Miocene Monterey Formation, California, have [[delta].sup.18]O and [[delta].sup.13]C values that vary over a few millimeters by ~28 [per thousand] and 6.5 [per thousand], respectively (relative to the Pee Dee belemnite isotope standard). These variations reflect episodic flow and mixing of waters from different sources with distinct diagenetic histories. On the other hand, isotopic conpositions of dolomite in samples with gradual shifts in isotopic compositions and correlated [[delta].sup.18]O and [[delta].sup.13]C values may have been controlled by mass diffusion and thermal conduction between episodic flow events. Primary fluid-inclusion assemblages from samples collected at one location yielded homogenization temperatures of 87.8 [+ or -] 3.6 [degrees]C (2[sigma]). These temperatures were combined with measured [[delta].sup.18]O values very nearly collocated in each dolomite sample to show that [[delta].sup.18][O.sub.water] values ranged between -0.8 [per thousand] and +2.5 [per thousand] (relative to standard mean ocean water). The samples from this Monterey Formation dolomite thus indicate their precipitation from water more enriched in [sup.18]O than late Miocene and younger seawater and must reflect modification of the [[delta].sup.18]O values of the involved waters by mineral-dehydration reactions prior to dolomite precipitation. Samples from the other location contain no primary fluid-inclusion assemblages and few abrupt shifts in isotopic compositions, perhaps reflecting recrystallization. Small-scale variations in isotopic compositions of vein minerals could be interpreted as the result of a changing regional hydrologic pattern, but we think such an interpretation for the Monterey Formation samples would be incorrect. Instead, we infer that these variations provide important information about localized effects on the ancient hydrologic system and that these effects bear on the origins of the paleofluids responsible for the mineral precipitation. Keywords: dolomite, fluid flow, fluid inclusions, Monterey Formation, stable isotopes. more...

Davidson, Guide, Pioneer, Gumdrop, and Rodriguez Seamounts, located at the continental margin offshore California between latitudes 37.5[degrees]N and 34.0[degrees]N, may represent a previously unrecognized type of intraplate oceanic volcanism. Morphologically unlike typical oceanic-island volcanoes or near-ridge seamounts, they are complex, northeast-trending ridges that reflect the ridge-parallel structure of the underlying oceanic crust. [sup.40]Ar/[sup.39]Ar laser fusion ages of mineral separates indicate at least two episodes of volcanism at ca. 16 and ca. 12 Ma, younger by 7-11 m.y. than the underlying ocean crust. Volcanic rocks are predominantly differentiated alkalic basalt, hawaiite, and mugearite. The lack of coherent liquid-lines-of-descent is consistent with small batches of magma forming, fractionally crystallizing, and erupting in isolation from the previous and subsequent batches. The presence of mantle xenoliths suggests that magmas originated in the upper mantle. Xenoliths of alkalic cumulates and xenocrysts of feldspar, amphibole, and titanomagnetite are consistent with fractionation in the upper mantle as well. Sr, Nd, and Pb isotope ratios of some samples are from depleted-mantle sources like those of MORB (mid-oceanic-ridge basalt); others indicate more variably enriched mantle sources. Magmas formed by small-percentage partial melting of variably enriched MORB-source-type mantle, according to the samples' high abundances of incompatible elements such as Nb, Ta, and Th. Coeval middle Miocene basalts in onshore coastal California have a similar isotopic range, but their trace elements show a subduction-related signature. If the coeval volcanic rocks offshore originated in a slab window, as proposed for those onshore, they were not influenced by a slab component. Although a slab-window origin is plausible for the middle Miocene volcanism, later episodes of small, sporadic eruptions on-and offshore probably resulted from decompression melting of mantle rising along existing zones of weakness undergoing extension related to continued movement along transform-fault systems. Keywords: central California, continental margin, geochronology, isotope geochemistry, petrology, submarine volcanoes. more...

The Round Mountain serpentinite melange, tectonically juxtaposed to the Franciscan Complex and to the Coast Range ophiolite, records structural and petrologic evidence for the tectonic processes that once affected late Mesozoic western North America. The melange mainly includes blocks of ultramafic and mafic compositions in a sheared ultramafic matrix. This matrix is thought to be essentially derived from shearing of highly serpentinized dunites, and more rarely harzburgites with or without clinopyroxene. Primary modal mineralogy of the ultramafic components, together with their spinel compositions and major and trace element chemistries, are consistent with both (1) backarc and (2) forearc and arc origins. Blocks of diabase, basaltic lavas, and rare gabbros display chemical characteristics similar to backarc magmas with specific Zr/Y ratios. The medium-(Zr/Y) group may be generated by ~23% of incremental melting from a fertile-mantle source. Rocks related to alkalic magmatism plot in the high-(Zr/Y) group, whereas those influenced by arc processes correspond to the low-(Zr/Y) group. Our data suggest that before the onset of the east-dipping Franciscan subduction, a Middle Jurassic backarc basin slightly affected by alkalic and arc-related magmatic processes (recorded by the first stage of the Coast Range ophiolite) lay west of the ensimatic Smartville island arc that was later involved in the Sierra Nevada orogeny. Subsequent collapse of the backarc basin initiated Franciscan subduction. The Late Jurassic arc and forearc domains (recorded by the second stage of the Coast Range ophiolite) were built on a part of this trapped backarc lithosphere. The Round Mountain serpentinite melange was formed by underthrusting of the upper part of the backarc crust followed by its tectonic accretion to the remaining backarc and the forearc- and arc-related mantle sections of the overriding lithospheric wedge. Keywords: arc, backarc, Coast Range ophiolite, Franciscan Complex, geochemistry, melange. more...

This paper presents analyses of the trace element abundances and isotopic compositions in primitive lavas from the Mt. Shasta region, N California. These data are combined with estimates of pre-eruptive [H.sub.2]O contents and constraints from experimental petrology to develop a model of subduction zone magmatism. These lavas share geochemical characteristics of high-Mg andesites from the Setouchi volcanic belt in SW Japan and Adak-type high-Mg andesites of the western Aleutian arc. Estimates of the pre-eruptive water contents of the Shasta region lavas range from 8 wt % [H.sub.2]O. The pre-eruptive [H.sub.2]O content and an inferred melt of a harzburgitic residue are used to carry out a mass balance for the relative contributions from a mantle-derived melt and slab-derived fluid-rich component. We assume that elements are contributed either from melting of mantle peridotite or from a subduction-related fluid-rich component. Estimated fluid-rich component compositions are characterized by strong light rare earth element (LREE) enrichments ([[La/Gd].sub.N] = 3 to 7) and variable heavy rare earth element (HREE) depletions ([[Dy/ Yb].sub.N] = 1 to 3). Sr and Ba abundances vary by approximately a factor of 2.5 in the fluid compositions calculated for the Mt. Shasta region lavas and large ion lithophile element (LILE) abundances are similar to those calculated by Stolper and Newman (1994) and Eiler et al. (2000). The major elements in the fluid-rich component are [H.sub.2]O (~55-68 wt%), [Na.sub.2]O (~25-33 wt%) and [K.sub.2]O (~5-13 wt%). This composition may be that of a supercritical fluid or a low-degree melt of the slab that has reacted with the overlying mantle wedge. Although the slab beneath Mt. Shasta is inferred to be hot (~600-650 [degrees] C), the calculated fluid-rich components do not resemble a pure slab melt. The calculated isotopic composition of the fluid-rich component is bimodal. One component has [sup.87]Sr/[sup.86]Sr = 0.7028 and [epsilon] Nd = +8, and is most similar to a MORB source. The second component has more radiogenic [sup.87]Sr/[sup.86]Sr = 0.7038 and [epsilon] Nd = + 1 and is most similar to a sediment. These fluid-rich components probably represent a mixture of fluids and melts from the slab (serpentinized mantle, altered basalt, and sediment). more...

Geodetic results from very long baseline interferometry (VLBI), satellite laser ranging (SLR), and the Global Positioning System (GPS) are used to estimate angular velocities between the Sierran microplate, Pacific plate, and North American plate. The Sierra-Pacific pole of rotation lies nearer to the San Andreas fault than does the Pacific-North America pole of rotation and leads to different tectonic implications than if the latter is used. The angular velocities show that the San Andreas fault system and central California Coast Ranges accommodate motion of 39 [+ or -] 2 mm/yr, mainly by strike-slip faulting. (All confidence limits following [+ or -] signs in this paper are 95% confidence limits.) Fault-normal motion is small, is mainly convergent (at rates up to 3.3 [+ or -] 1.0 mm/yr), and varies along the coast, but is divergent (at 2.6 [+ or -] 1.2 mm/yr) across San Pablo Bay and associated topographic lows across which the Sierran and Central Valley watershed drains to the Pacific Ocean. The mountain ranges tend to be larger where the fault-normal convergence rates are larger. The low convergence rate (0.5 [+ or -] 1.8 mm/yr) normal to the San Andreas fault in the Carrizo Plain differs sharply from that previously inferred (8.2 [+ or -] 1.2 mm/yr and 4.9 [+ or -] 1.6 mm/yr) by Feigl et al. (1993). The difference is due to differences between their and our elastic strain accumulation models and between how their and our Pacific plate reference frames are defined. The ranges in most places require a minimum of 4 +2/-1 m.y. of fault-normal convergence at the present rate to attain their present cross-sectional area if erosion is neglected, more if it is not. The amount of convergence previously estimated from a balanced cross section across the Diablo Range in central California requires 10 +8/-3 m.y. of convergence at the present rate. The former is consistent with widely held views about the onset of the Coast Range orogeny, but the latter is not. Both are consistent, however, with the recent plate reconstructions by S. Cande, J. Stock, and colleagues, which indicate that Pacific plate motion relative to North America changed to a more convergent direction, 20 [degrees]-25 [degrees] clockwise of its prior direction, at ca. 8 to 6 Ma and not at 3.5 Ma, as had been previously inferred. The inferred change in direction of plate motion is large compared with the present angle of convergence across the straight and narrow segment of the San Andreas fault of 0.70 [degrees]-4.7 [degrees], from which we infer that the Sierran microplate changed motion relative to North America at the same time (ca. 8 to 6 Ma) as did the Pacific plate. We further infer that the motion accommodated across the Great Basin must also have changed at the same time. We also examine the hypothesis that stable sliding occurs along the San Andreas fault and other northwest-striking strike-slip faults in central California where the fault-normal convergence rate is low or negative, and that these faults are unstable where the fault-normal convergence rate is high. Such a relationship appears to hold in general, but fails in detail. In particular, there are substantial sections of fault with small inferred rates of fault-normal convergence across which the San Andreas fault is locked. Moreover, the creeping section of the San Andreas fault (i.e., the section between Parkfield and the Calaveras junction) is the locus of greater fault-normal convergence (3.2 [+ or -] 1.4 mm/yr) than is the locked part of the fault (0.5 [+ or -] 1.8 mm/yr) south of Parkfield. Thus, this hypothesis is at best a partial explanation for the observed distribution of locked and nonlocked sections of the fault. Keywords: Calaveras fault, Coast ranges, Diablo range, San Andreas fault, San Francisco Bay, Sierra Nevada. more...

Satellite synthetic aperture radar interferometry reveals transient strain accumulation along the Blackwater-Little Lake fault system within the Eastern California shear zone. The surface strain map obtained by averaging eight years (1992-2000) of Earth Resource Satellite (ERS) radar data shows a 120-km-long, 20-km-wide zone of concentrated shear between the southern end of the 1872 Owens Valley earthquake surface break and the northern end of the 1992 Landers earthquake surface break. The observed shear zone is continuous through the Garlock fault, which does not show any evidence of left-lateral slip during the same time period. A dislocation model of the observed shear indicates right-lateral slip at 7 [+ or -] 3 mm/yr on a vertical fault below ~5 km depth, a rate that is two to three times greater than the geologic rates estimated on northwest-trending faults in the eastern Mojave area. This transient slip rate and the absence of resolvable slip on the Garlock fault may be the manifestation of an oscillatory strain pattern between interacting, conjugate fault systems. Keywords: strain transient, synthetic aperture radar interferometry, Eastern California shear zone. more...

Regional mapping indicates that the Middle Jurassic Santa Rita Flat pluton, exposed in the Inyo Range of eastern California, is situated within the core of a south-plunging synform defined by bedding in the surrounding metasedimentary wall rocks, which dip beneath the pluton. However, bedding in rocks preserved above the pluton defines a south-plunging antiform. Anisotropy of magnetic susceptibility analysis revealed that magnetic foliation within the pluton also defines a south-plunging antiform, and the accompanying magnetic lineation plunges subparallel to the axis of this antiform. Our data indicate that the pluton was initially intruded as a sill in the hinge zone of the enveloping synform. Subsequent vertical inflation of the sill resulted in upward doming of the overlying roof and formation of the antiform now observed at the current erosion level in the pluton and preserved roof rocks. Emplacement of the pluton at 164 [+ or -] 1.5 Ma (U-Pb zircon age) overlaps in time with regional thrust faulting and folding ca. 185-148 Ma recognized in the southern Inyo Range. We speculate that space for initial emplacement of the pluton was produced during folding by layer-parallel slip and hinge-zone dilation, producing a saddle reef like structure. Keywords: anisotropy of magnetic susceptibility, phacolith, pluton, saddle reef, White-Inyo. more...

Three-dimensional seismic refraction data reveal the structure of the plate boundaries at the Mendocino triple junction. First arrival traveltimes were inverted to obtain a three-dimensional seismic velocity model. The tomography results indicate a strong lateral velocity contrast in the lower crust. This velocity contrast is beneath the surface trace of the San Andreas fault and tracks along the fault to the triple junction and then along the Mendocino fracture zone. Velocity values appropriate for mafic rocks are more than 5 km shallower on the Pacific plate than east of the San Andreas fault, and 1-3 km shallower than under the Cascadia accretionary prism. The model suggests that the San Andreas fault, which is growing as the triple junction migrates northward, cuts steeply through the entire crust. Pacific oceanic crust does not extend beneath North America at this location. The slabless window south of the triple junction and east of the San Andreas fault must be filled with upwelling asthenosphere. The sharp corner of the Pacific plate migrates northward, displacing the Cascadia accretionary prism and forcing the prism material to move north or east. Keywords: seismic tomography, California, plate boundary, San Andreas fault, Mendocino triple junction. more...

A numerical grid search using the trishear kinematic model can be used to extract both slip and the distance that a fault tip line has propagated during growth of a fault-propagation fold. The propagation distance defines the initial position of the tip line at the onset of slip. In the Santa Fe Springs anticline of the Los Angeles basin, we show that the tip line of the underlying Puente Hills thrust fault initiated at the same position as the 1987 magnitude 6.0 Whittier Narrows earthquake. Keywords: folding, faulting, kinematics, earthquakes. more...

The authors investigate several linear ruptures offshore of the Palos Verdes Peninsula in southern California. They offer the extent of the newly discovered Avalon Knoll fault and estimate likely magnitudes for events along it. more...

Field relationships and new U-Pb geochronology reveal that a significant number of dikes associated with the Independence dike swarm, both within the type locality and elsewhere, are of Cretaceous age. A new U-Pb zircon age for a diorite dike in the type locality of the Independence dike swarm in California confirms that some mafic dikes in the swarm are coeval with previously dated Late Jurassic felsic dikes. Cretaceous dikes in the eastern Sierra Nevada batholith are located near--and may be genetically related to--coeval mafic and felsic plutons. However, the presence of Cretaceous dikes throughout the swarm demonstrates that this is not a local phenomenon, and caution therefore should be exercised when using Independence-like dikes as temporal markers. We have not identified any consistent petrographic or chemical criteria that distinguish Jurassic Independence dikes from Cretaceous dikes. However, Jurassic dikes in the Sierra Nevada commonly contain a sinistral shear fabric that is absent in both dated Cretaceous dikes and undated dikes that intrude dated Cretaceous plutons. Despite intruding at least 60 m.y. apart, the orientations and petrology of the Jurassic and Cretaceous dikes are remarkably similar. Because dikes were intruded over a time period characterized by large changes in the orientation of plate convergence west of the Sierran arc, it is unlikely that the uniform structural orientation of Jurassic and Cretaceous dikes resulted from regional stresses imposed by the subducting plate. Instead, the orientation of Jurassic and Cretaceous dikes (and of many Sierran Cretaceous plutons) may have been controlled by preexisting faults or by the orientation of the continental margin. Keywords: California, Sierra Nevada batholith, dike swarm, geochronology, tectonics. more...

It is generally assumed that individual sets of coplanar and colinear deformation structures form together during events that are of relatively short duration (1-5 m.y.). The record of deformation in a sequence of Late Cretaceous to Holocene sedimentary rocks from the northern Salinian block of California spans at least 30 m.y. and illustrates that this assumption is sometimes incorrect. At different localities, geometrically and kinematically identical contractional structures either predate or postdate local unconformities of varying age within the succession, so that it is possible to define at least four chronologically distinct, but otherwise indistinguishable, deformation episodes. In the absence of the unconformities, the punctuated nature of the deformation would not be apparent, therefore suggesting that subparallel structures may form during successive, distinct deformations spread out over long time periods. In the northern Salinian block, the inferred contractional strain field is approximately normal to the adjacent San Andreas fault and appears to have been consistently oriented in this direction during deformation events recorded over the past 30-45 m.y. The strain pattern is most easily explained by efficient partitioning of transpressional strains into fault-normal shortening and right-lateral faulting during episodic regional deformations. We propose that reactivation of preexisting structural anisotropies controls the observed partitioning of deformation in many transpression zones. more...

Prolific natural hydrocarbon seepage occurs offshore of Coal Oil Point in the Santa Barbara Channel, California. Within the water column above submarine vents, plumes of hydrocarbon gas bubbles act as acoustic scattering targets. Using 3.5 kHz sonar data, seep distribution offshore of Coal Oil Point was mapped for August 1996, July 1995, and July 1973. Comparison of the seep distributions over time reveals more than 50% decrease in the areal extent of seepage, accompanied by declines in seep emission volume, in a 13 km(super 2) area above a producing oil reservoir. Declines in reservoir pressure and depletion of seep hydrocarbon sources associated with oil production are the mechanisms inferred to explain the declines in seep area and emission volume. more...

Warming of surface waters in the California Current since the 1950s has coincided with a significant decline in zooplankton volume. This has been attributed to reduced upwelling of nutrient-rich waters caused by increased thermal stratification across the thermocline. Proxy microfossil evidence preserved in the Santa Barbara Basin suggests that stability increased early in the 1900s, intensified after the early 1940s, and became well established by 1960. Accumulation of upwelled radiolarians in the basin has steadily declined since 1900, while oxygen isotopes in surface-dwelling planktonic foraminifera reflect increasing surface temperatures. Comparison of the (delta)(super 18)O records between surface and thermocline-dwelling planktonic foraminifera reveals that the temperature difference between surface and thermocline water has increased during the twentieth century. Instrumental records of surface and thermocline temperatures, monitored since 1950, support these results. This evidence suggests that relaxation of North Pacific anticyclonic gyre circulation deepened isopycnics, causing onshore movement of warmer, less saline waters and reduced upwelling of cool, nutrient-rich waters. more...

The Hosgri fault is located immediately offshore of south-central California and is part of the transform boundary between the Pacific and North American plates. This fault terminates to the southeast into east-trending folds and reverse-separation faults of the western Transverse Ranges. Our new structure-contour maps of deformed horizons show a spatial relationship between faulting and folding consistent with right-lateral slip. Restoration of these digital maps quantifies post-Miocene right-lateral slip across the southern Hosgri fault to be 3.5 km. This slip is absorbed by folding, thrust overlap, and clockwise vertical-axis rotation of elongate blocks between strands of the fault. The restored part of a block located to the east has rotated 8 degree clockwise. Extrapolating this restored rotation to the 50 km block length produces an estimate of 7 km of dextral shear for a total, including the 3.5 km fault slip, of 10.5 km of post-Miocene displacement. Our three-dimensional approach precludes interpretations for reverse slip on the Hosgri fault, and is not consistent with models for more than 80 km of late Cenozoic right-lateral fault slip. more...

A strong oxygen-minimum zone (OMZ) currently exists along the California margin because of a combination of high surface-water productivity and poor intermediate-water ventilation. However, the strength of this OMZ may have been sensitive to late Quaternary ocean-circulation and productivity changes along the margin. Although sediment-lamination strength has been used to trace ocean-oxygenation changes in the past, oxygen levels on the open margin are not sufficiently low for laminations to form. In these regions, benthic foraminifera are highly sensitive monitors of OMZ strength, and their fossil assemblages can be used to reconstruct past fluctuations. Benthic foraminiferal assemblages from Ocean Drilling Program Site 1017, off Point Conception, exhibit major and rapid faunal oscillations in response to late Quaternary millennial-scale climate change (Dansgaard-Oeschger cycles) on the open central California margin. These faunal oscillations can be correlated to and are apparently synchronous with those reported from Santa Barbara Basin. Together they represent major fluctuations in the strength of the OMZ which were intimately associated with global climate change-weakening, perhaps disappearing, during cool periods and strengthening during warm periods. These rapid, major OMZ strength fluctuations were apparently widespread on the Northeast Pacific margin and must have influenced the evolution of margin biota and altered biogeochemical cycles with potential feedbacks to global climate change. more...