Whitney M. Behr

Uncertainties in slip-rate estimates for the Mission Creek strand of the southern San Andreas fault at Biskra Palms Oasis, southern California

This study focuses on uncertainties in estimates of the geologic slip rate along the Mission Creek strand of the southern San Andreas fault where it offsets an alluvial fan (T2) at Biskra Palms Oasis in southern California. We provide new estimates of the amount of fault offset of the T2 fan based on trench excavations and new cosmogenic 10Be age determinations from the tops of 12 boulders on the fan surface. We present three alternative fan offset models: a minimum, a maximum, and a preferred offset of 660 m, 980 m, and 770 m, respectively. We assign an age of between 45 and 54 ka to the T2 fan from the 10Be data, which is significantly older than previously reported but is consistent with both the degree of soil development associated with this surface, and with ages from U-series geochronology on pedogenic carbonate from T2, described in a companion paper by Fletcher et al. (this volume). These new constraints suggest a range of slip rates between ∼12 and 22 mm/yr with a preferred estimate of ∼14–17 mm/yr for the Mission Creek strand of the southern San Andreas fault. Previous studies suggested that the geologic and geodetic slip-rate estimates at Biskra Palms differed. We find, however, that considerable uncertainty affects both the geologic and geodetic slip-rate estimates, such that if a real discrepancy between these rates exists for the southern San Andreas fault at Biskra Palms, it cannot be demonstrated with available data.

Lithospheric shear zones as constant stress experiments

Below the seismogenic zone, plate boundaries are defi ned by lithospheric ductile shear zones. These localize strain, and are the main reason that convective motions in the Earth’s interior are expressed at the surface as plate tectonics. We argue here that lithospheric shear zones operate at approximately constant stress, equal to the yield strength of the surrounding rocks. This places constraints on the bulk strength of the lithosphere, and allows us to calculate the cumulative width of the shear zones as a function of depth. The concept applies most clearly to strike-slip shear zones, but is also applicable to thrust- and normal-sense shear zones, although these evolve with time due to temperature changes associated with burial or exhumation. If shear zones operate at constant stress, this affects their microstructural evolution, and hence their rheology. Decreases in grain size due to dynamic recrystallization may cause a switch in the dominant deformation mechanism to grain-size‐sensitive creep, leading to weakening and strain localization. A common argument, based on a constant strain rate approach, has been that such transitions will be inhibited by grain growth. In a constant stress shear zone, however, dynamic recrystallization continues to maintain the low grain size even after the switch has occurred. Grain growth is inhibited under these conditions, and hence the switch is permanent as long as the boundary conditions remain the same. Grain-size reduction in shear zones is therefore a critical factor in maintaining plate tectonic processes.

Metamorphic core complexes: windows into the mechanics and rheology of the crust

Metamorphic core complexes are products of normal-fault displacements sufficient to exhume rocks from below the brittle–ductile transition. These faults (detachments) may initiate within the brittle crust at steep angles, but they sole into the ductile middle crust, and during displacement rotate to gentler dips due to hanging-wall extension. The exhumed footwall commonly adopts an arched or domed geometry owing to flexural isostatic readjustment, and may be overlain by strongly extended upper crustal rocks that slipped on gently dipping, low-friction shallow segments of the detachment. Metamorphic rocks exhumed beneath the detachment record progressively increasing flow stress, strain localization and strain-rate with decreasing temperature, providing a window into physical conditions and deformational processes in the mid-crust. The metamorphic and deformational history of the footwall rocks may reflect tectonic processes that predate formation of the detachment fault, in addition to those accompanying exhumation. These processes may include diapiric emplacement of gneiss domes, or exhumation in a subduction channel, and may not be directly related to formation of the core complex. Factors favouring core complex formation are high gravitational potential energy of the extending crust, weak rheology and a change in the tectonic boundary conditions such as a cessation or slowing of plate convergence.

Brittle faults are weak, yet the ductile middle crust is strong: Implications for lithospheric mechanics

A global compilation of shear stress magnitude from mylonites developed along major fault zones suggests that maximum shear 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., Byerlees 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 of 1–50 MPa. The sharp contrast between static shear stresses estimated on the seismogenic parts of major faults and those estimated from steady-state microstructures in 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.

230Th/U dating of a late Pleistocene alluvial fan along the southern San Andreas fault

U-series dating of pedogenic carbonate-clast coatings provides a reliable, precise minimum age of 45.1 ± 0.6 ka (2σ) for the T2 geomorphic surface of the Biskra Palms alluvial fan, Coachella Valley, California. Concordant ages for multiple subsamples from individual carbonate coatings provide evidence that the 238 U- 234 U- 230 Th system has remained closed since carbonate formation. The U-series minimum age is used to assess previously published 10 Be exposure ages of cobbles and boulders. All but one cobble age and some boulder 10 Be ages are younger than the U-series minimum age, indicating that surface cobbles and some boulders were partially shielded after deposition of the fan and have been subsequently exhumed by erosion of fine-grained matrix to expose them on the present fan surface. A comparison of U-series and 10 Be ages indicates that the interval between final alluvial deposition on the T2 fan surface and accumulation of dateable carbonate is not well resolved at Biskra Palms; however, the “time lag” inherent to dating via U-series on pedogenic carbonate can be no larger than ∼10 k.y., the uncertainty of the 10 Be-derived age of the T2 fan surface. Dating of the T2 fan surface via U-series on pedogenic carbonate (minimum age, 45.1 ± 0.6 ka) and 10 Be on boulder-top samples using forward modeling (preferred age, 50 ± 5 ka) provides broadly consistent constraints on the age of the fan surface and helps to elucidate its postdepositional development.

Calibrating Ti concentrations in quartz for SIMS determinations using NIST silicate glasses and application to the TitaniQ geothermobarometer

Abstract The recently developed titanium-in-quartz (TitaniQ) geothermobarometer of Wark and Watson (2006) and Thomas et al. (2010) has the potential to be applied to a wide range of igneous and metamorphic rocks. For Ti concentrations > -10 ppm, the concentrations can be measured using an electron microprobe, but lower concentrations are below detection limits and require techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or secondary ion mass spectrometry (SIMS). SIMS is ideal for this purpose as it maximizes lateral and depth resolution. We used SIMS to analyze synthetic quartz crystals characterized for Ti concentration by electron probe (18-813 ppm Ti) and compare this calibration to the commonly available NIST 61X glasses using both high mass resolution (HMR) and conventional energy filtering (CEF) techniques. We used a primary beam of 16O- ions and detected positive secondary ions. During HMR sessions, the mass spectrometer was operated at a mass resolving power (M/ΔM) of ~2000 to separate molecular ions from elemental Ti peaks. For CEF analyses, the instrument detected secondary ions sputtered from the sample with excess kinetic energies of 75 ± 20 eV. Titanian quartz measurements reveal general homogeneity and a linear increase in Ti+/Si+ ion ratios with increasing Ti concentrations. Background signals represent 0-1 ppm. The slopes of the calibration curves for the titanian quartz crystals are -70% of the curves constructed using NIST glasses, indicating a much higher ion yield for Ti from the glasses compared to the simple oxide of silicon. We demonstrate, however, that a simple correction factor allows NIST glasses to be used to quantitatively determine the Ti concentrations of quartz (with 3.8% error using HMR, and 8.2% error using CEF) until homogenous, well-characterized samples of SiO2 become generally available.

Deformation in the mantle wedge associated with Laramide flat‐slab subduction

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 is 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 to 444 MPa, and calculated effective viscosities using a wet olivine flow law are on the order of 1019−1023 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.

Early Miocene subduction in the western Mediterranean: Constraints from Rb‐Sr multimineral isochron geochronology

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 multimineral isochron 87Rb/86Sr 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 multimineral isochrons were obtained for one eclogite and two schists, yielding ages of 20.1 ± 1.1, 16.0 ± 0.3, and 13.3 ± 1.3 Ma, respectively. Despite that the other two eclogite samples appeared to preserve prograde mineral assemblages, low 87Rb/86Sr 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 40Ar/39Ar 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.

Holocene geologic slip rate for the Banning strand of the southern San Andreas Fault, southern California

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% confidence interval (CI)) and 6.4 + 3.7/−2.1 ka (95% CI) using U-series dating of pedogenic carbonate clast coatings and 10Be 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% CI) to 4.9 + 1.0/−0.9 mm/yr (median, 95% CI). 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 10Be exposure dating demonstrates the importance of using multiple geochronometers when estimating the depositional age of alluvial landforms.

Dehydration-induced rheological heterogeneity and the deep tremor source in warm subduction zones

We present observations from an exhumed subduction complex that resembles the environment of modern deep episodic tremor and slow slip (ETS). We focus on the Cycladic Blueschist Unit on Syros Island in Greece. Syros metabasites consist of blueschists and eclogites that record prograde deformation, with peak metamorphism of 1200–1600 MPa and 450–550 °C. Field observations reveal that coexistence of blueschist and eclogite sets up an important rheological contrast: blueschists show distributed viscous dislocation creep, whereas eclogites dispersed within the blueschist matrix show brittle shear fractures and veins. These observations are consistent with the inferred prominent role of high fluid pressures from geophysical studies, but are inconsistent with models of deep ETS that invoke changes in rate-and-state friction parameters along a narrow fault. Instead, we suggest deep ETS may be controlled by coupled brittle-viscous deformation in partially eclogitized oceanic crust embedded within high-fluid-pressure patches along the plate interface.