ALBERT

Description: Laramide crustal deformation in the Rocky Mountains of the west-central United States is often considered to relate to a narrow segment of shallow subduction of the Farallon slab, but there is no consensus as to how deformation along the slab–mantle lithosphere interface was accommodated. Here we investigate deformation in mantle rocks associated with hydration and shear above the flat slab at its contact with the base of the North American plate. The rocks we focus on are deformed, hydrated, ultramafic inclusions hosted within diatremes of the Navajo Volcanic Field in the central Colorado Plateau that erupted during the waning stages of the Laramide orogeny. We document a range of deformation textures, including granular peridotites, porphyroclastic peridotites, mylonites, and cataclasites, which we interpret to reflect different proximities to a slab–mantle-interface shear zone. Mineral assemblages and chemistries constrain deformation to hydrous conditions in the temperature range ∼550-750°C. Despite the presence of hydrous phyllosilicates in modal percentages of up to 30%, deformation was dominated by dislocation creep in olivine. The mylonites exhibit an uncommon lattice preferred orientation (LPO) in olivine, known as B-type LPO in which the a-axes are aligned perpendicular to the flow direction. The low temperature, hydrated setting in which these fabrics formed are consistent with laboratory experiments that indicate B-type LPOs form under conditions of high stress and high water contents; furthermore, the mantle wedge context of these LPOs is consistent with observations of trench-parallel anisotropy in the mantle wedge above many modern subduction zones. Differential stress magnitudes in the mylonitic rocks estimated using paleopiezometry range from 290 and 444 MPa, and calculated effective viscosities using a wet olivine flow law are on the order of 10 19 -10 23 Pa s. The high stress magnitudes, high effective viscosities and high strains recorded in these rocks are consistent with models that invoke significant basal shear tractions as contributing to Laramide uplift and contraction in the continental interior. This article is protected by copyright. All rights reserved.

Description: Abstract We analyze peridotites from a wide range of tectonic settings to investigate relationships between olivine crystallographic preferred orientation (CPO) and deformation conditions in naturally deformed rocks. These samples preserve the five olivine CPO types (A through E‐type) that rock deformation experiments have suggested are controlled by water content, temperature, stress magnitude, and pressure. The naturally deformed specimens newly investigated here (65 samples) and compiled from an extensive literature review (445 samples) reveal that these factors may matter less than deformation history and/or geometry. Some trends support those predicted by experimentally determined parametric dependence, but several observations disagree — namely that all CPO types are able to form at very low water contents and stresses, and that there is no clear relationship between water content and CPO type. This implies that at the low stresses typical of deformation in the mantle, CPO type more commonly varies as a function of strain geometry. Because olivine CPO is primarily responsible for seismic anisotropy in the upper mantle, the results of this study have several implications. These include (1) the many olivine CPO types recorded in samples from individual localities may explain some of the complex seismic anisotropy patterns observed in the continental mantle, and (2) B‐type CPO – where olivine's “fast axes" align perpendicular to flow direction – occurs under many more conditions than traditionally thought. This study highlights the need for more experiments, and the difficulty in using olivine CPO in naturally‐deformed peridotites to infer deformation conditions.

Description: A global compilation of shear stress magnitude from mylonites developed along major fault zones suggests that maximum stresses between 80 and 120 MPa are reached at temperatures between 300 and 350 °C on normal, thrust, and strike-slip faults. These shear stresses are consistent with estimates of brittle rock strengths based on sliding friction (e.g., Byerlee's law), and with in-situ measurements of crustal stress measured in boreholes. This confirms previous suggestions that in some areas at least, the continental crust is stressed close to failure down to the brittle-ductile transition. Many major active faults in all tectonic regimes are considered to be relatively weak, however; peak static shear stresses for brittle faults estimated by a variety of techniques lie in the range 1-50 MPa. The sharp contrast between static shear stresses estimated on the seismogenic parts of major faults and those estimated from ductile rocks immediately below the seismogenic zone suggests that there is an abrupt downward termination, probably controlled by temperature, of the weakening processes that govern fault behavior in the upper crust. These data also imply that seismogenic parts of major fault zones contribute little to lithospheric strength, and are unlikely to have much influence on either the slip rate or the location of the faults. Conversely, ductile middle crust immediately below the brittle-ductile transition deforms at high stresses, and forms a significant load-bearing element within the lithosphere.

Description: Abstract Removal of mantle lithosphere by Rayleigh‐Taylor (R‐T) instabilities is invoked to explain the formation of high plateaus and mountain ranges. Here we report geochemical and microstructural observations from mantle xenoliths from Lunar Crater volcanic field, central Nevada, which we interpret to directly sample a R‐T instability beneath the Basin and Range. The xenoliths comprise a suite of mylonitic and granular peridotites with fertile and refractory major and trace element compositions, suggesting a mantle lithospheric origin. Temperatures calculated using several geothermometers are 1,200–1,300 °C, in contrast to xenoliths from other localities in the Basin and Range (typically ~1,000 °C). High Lunar Crater temperatures suggest the xenoliths originate from the base of the mantle lithosphere. The mylonitic peridotites exhibit olivine deformation microstructures characteristic of deformation in the dislocation creep regime; orthopyroxenes experienced brittle deformation. Recrystallized grain sizes (~80 μm) suggest the mylonites deformed at ~50 MPa. Recrystallized olivines demonstrate isochemical deformation, suggesting the ~50‐MPa differential stresses were achieved at ~1,200 °C, implying strain rates of 2 × 10−9 to 4 × 10−7/s, and corresponding to effective viscosities of 8.9 × 1013 to 1.4 × 1016 Pa/s. The most plausible mechanism for producing such conditions in the deep lithosphere beneath central Nevada is extreme strain localization within an actively deforming R‐T instability. The most highly strained samples have relatively low effective viscosities, suggesting that strain is preferentially partitioned into weaker rocks within the deforming lithosphere. This study highlights the importance of strain localization and associated weakening as mechanisms for facilitating lithospheric R‐T instabilities.

Description: The Betic Cordillera of southern Spain is a complex orogen formed in the context of convergence between Africa and Iberia from the Mesozoic to the present. The internal zone of the orogen includes three tectonic complexes, two of which have been subducted to high pressure conditions, then exhumed back to the surface during subsequent extension. Subduction in the structurally lower complex, known as the Nevado-Filabride Complex (NFC), has been a topic of debate for several years due to conflicting geochronological data. Here we use multi-mineral isochron 87 Rb/ 86 Sr dating on carefully selected mineral samples from high pressure metamorphic rocks in the NFC to better constrain the timing of high pressure metamorphism and subduction in the region. Out of five samples analyzed, statistically valid multi-mineral isochrons were obtained for one eclogite and two schists, yielding ages of 20.1+/-1.1 Ma, 16.0+/-0.3 Ma, and 13.3+/-1.3 Ma, respectively. Despite that the other two eclogite samples appeared to preserve prograde mineral assemblages, low 87 Rb/ 86 Sr ratios in white mica precluded precise age calculations. These new ages are in close agreement with previously published Lu-Hf ages on garnet and U-Pb ages on metamorphic zircon overgrowths for the same rocks, but are substantially younger than published data from the 40 Ar/ 39 Ar technique. Combined with recently published tomographic images of slab structure beneath the Alboran Sea, the new ages support a tectonic model in which subduction occurred both prior to the Miocene and during the early to mid-Miocene, but that it was punctuated in time by a pulse of extensional exhumation in the early Miocene associated with lithospheric delamination and/or slab tearing. This article is protected by copyright. All rights reserved.

Description: Marine species with planktonic larvae often have high spatial and temporal variation in recruitment that leads to subsequent variation in the ecology of benthic adults. Using a combination of published and unpublished data, we compared the population structure of the salt marsh snail, Littoraria irrorata , between the South Atlantic Bight and the Gulf Coast of the United States to infer geographic differences in recruitment and to test the hypothesis that the Deepwater Horizon oil spill led to widespread recruitment failure of L. irrorata in Louisiana in 2010. Size-frequency distributions in both ecoregions were bimodal, with troughs in the distributions consistent with a transition from sub-adults to adults at ~13 mm in shell length as reported in the literature; however, adult snails reached larger sizes in the Gulf Coast. The ratio of sub-adults to adults was 1.5–2 times greater in the South Atlantic Bight than the Gulf Coast, consistent with higher recruitment rates in the South Atlantic Bight. Higher recruitment rates in the South Atlantic Bight could contribute to higher snail densities and reduced adult growth in this region. The ratio of sub-adults to adults in Louisiana was lower in 2011 than in previous years, and began to recover in 2012–2014, consistent with widespread recruitment failure in 2010, when large expanses of spilled oil were present in coastal waters. Our results reveal an important difference in the ecology of a key salt marsh invertebrate between the two ecoregions, and also suggest that the Deepwater Horizon oil spill may have caused widespread recruitment failure in this species and perhaps others with similar planktonic larval stages.

Description: Northwest directed slip from the southern San Andreas Fault is transferred to the Mission Creek, Banning, and Garnet Hill fault strands in the northwestern Coachella Valley. How slip is partitioned between these three faults is critical to southern California seismic hazard estimates, but is poorly understood. In this paper, we report the first slip rate measured for the Banning fault strand. We constrain the depositional age of an alluvial fan offset 25 ± 5 m from its source by the Banning strand to between 5.1 ± 0.4 ka (95% C.I.) and 6.4 +3.7/-2.1 ka (95% C.I.) using U-series dating of pedogenic carbonate clast-coatings and 10 Be cosmogenic nuclide exposure dating of surface clasts. We calculate a Holocene geologic slip rate for the Banning strand of 3.9 +2.3/-1.6 mm/yr (median, 95% C.I.) to 4.9 +1.0/-0.9 mm/yr (median, 95% C.I.). This rate represents only 25-35% of the total slip accommodated by this section of the southern San Andreas Fault, suggesting a model in which slip is less concentrated on the Banning strand than previously thought. In rejecting the possibility that the Banning strand is the dominant structure, our results highlight an even greater need for slip rate and paleoseismic measurements along faults in the northwestern Coachella Valley in order to test the validity of current earthquake hazard models. In addition, our comparison of ages measured with U-series and 10 Be exposure dating demonstrates the importance of using multiple geochronometers when estimating the depositional age of alluvial landforms.