The Franciscan Complex (California, USA) – The model case for return-flow in a subduction channel put to the test

Abstract. Return flow in a deep subduction channel (DSC) has been proposed to explain rapid exhumation of high pressure–low temperature metamorphic rocks, entirely based on the fossil rock record. Supported by thermo-mechanical models, the DSC is envisioned as a thin layer on top of the subducted plate reaching down to minimum depths of about 150 km. We perform numerical simulations of high-frequency seismic wave propagation (1–5 Hz) to explore potential seismological evidence for the in situ existence of a DSC. Motivated by field observations, for modeling purposes we assume a simple block-in-matrix (BIM) structure with eclogitic blocks floating in a serpentinite matrix. Homogenization calculations for BIM structures demonstrate that effective seismic velocities in such composites are lower than in the surrounding oceanic crust and mantle, with nearly constant values along the entire length of the DSC. Synthetic seismograms for receivers at the surface computed for intermediate depth earthquakes in the subducted oceanic crust for models with and without DSC turn out to be markedly influenced by its presence or absence. While for both models P and S waveforms are dominated by delayed high-amplitude guided waves, models with DSC exhibit a very different pattern of seismic arrivals compared to models without DSC. The main reason for the difference is the greater length and width of the low-velocity channel when a DSC is present. Seismic velocity heterogeneity within the DSC or oceanic crust is of minor importance. The characteristic patterns allow for definition of typical signatures by which models with and without DSC may be discriminated. The signatures stably recur in slightly modified form for earthquakes at different depths inside subducted oceanic crust. Available seismological data from intermediate depth earthquakes recorded in the forearc of the Hellenic subduction zone exhibit similar multi-arrival waveforms as observed in the synthetic seismograms for models with DSC. According to our results, observation of intermediate depth earthquakes along a profile across the forearc may allow to test the hypothesis of a DSC and to identify situations where such processes could be active today.

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Structural and rheological evolution of the Laramide subduction channel in southern California

10.5194/se-2016-147 ◽

2016 ◽

Author(s):

Haoran Xia ◽

John P. Platt

Keyword(s):

Shear Stress ◽

Strain Rate ◽

Southern California ◽

Microstructural Analysis ◽

Carbonaceous Material ◽

Dislocation Creep ◽

Return Flow ◽

Pressure Solution ◽

Subduction Channel ◽

San Gabriel Mountains

Abstract. The Pelona schist in the San Gabriel Mountains, southern California, formed in the Laramide subduction channel, exhibits multiple phases of deformation/metamorphism and provides valuable insights into the rheological properties of the subduction channel. Petrological and microstructural analysis indicates that the Pelona schist has undergone three major deformational/metamorphic events. Subduction of volcanic and sedimentary protoliths during D1 was recorded by aligned mineral inclusions in albite and epidote porphyroblasts. Metamorphic temperature and pressure at the end of subduction yielded by Raman spectroscopy of carbonaceous material and Si-in-muscovite barometry were 518.9 ± 19.6 °C and 10.5 ± 0.4 kbar, respectively. During D1 the dominant deformation mechanism was quartz pressure solution, and the estimated shear stress and strain rate were less than 10 MPa and 5.8×10−13 s−1. D2, the first stage exhumation of the Pelona schist along the upper section of the subduction channel during return flow, was recorded by retrogressive metamorphism, isoclinal folding, and a pervasive schistosity that wraps around earlier porphyroblasts. Metagreywacke was deformed mainly by quartz pressure solution and metachert was deformed dominantly by dislocation creep during D2. The shear stress in metagreywacke was less than 10 MPa and that in metachert was between 8.6 and 13.3 MPa, resulting in a strain rate of 1.4×10−13 to 5.5×10−13 s−1. A topography driven model is proposed as the main driving force of D2 exhumation. D3 records normal-sense movement on the Vincent fault, which separates the schist from overlying arc and continental basement. This resulted in the second stage of exhumation, creating a major synform and associated mylonitic fabric in the upper section of the Pelona schist. Conditions at the beginning of D3 were 390°C and 5.8 kbar given by the TitaniQ thermometer and phengite barometer. The deformation was dominated by quartz dislocation creep with a strain rate of ~5.1 ×10−13 s−1 at a shear stress of ~22 MPa.

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Forced Return Flow Deep in the Subduction Channel, Syros, Greece

Tectonics ◽

10.1029/2019tc005768 ◽

2020 ◽

Vol 39 (1) ◽

Cited By ~ 4

Author(s):

Uwe Ring ◽

Hermes Pantazides ◽

Johannes Glodny ◽

Alasdair Skelton

Keyword(s):

Return Flow ◽

Subduction Channel ◽

Forced Return

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Seismic visibility of a deep subduction channel: insights from numerical simulation of high-frequency seismic waves emitted from intermediate depth earthquakes

Solid Earth Discussions ◽

10.5194/sed-5-1461-2013 ◽

2013 ◽

Vol 5 (2) ◽

pp. 1461-1509

Author(s):

W. Friederich ◽

L. Lambrecht ◽

B. Stöckhert ◽

S. Wassmann ◽

C. Moos

Keyword(s):

Oceanic Crust ◽

High Frequency ◽

Seismic Waves ◽

High Amplitude ◽

Synthetic Seismograms ◽

Return Flow ◽

Seismic Velocities ◽

Subduction Channel ◽

Intermediate Depth ◽

Subducted Oceanic Crust

Abstract. Return flow in a deep subduction channel (DSC) has been proposed to explain rapid exhumation of high pressure-low temperature metamorphic rocks, entirely based on the fossil rock record. Supported by thermo-mechanical models, the DSC is envisioned as a thin layer on top of the subducted plate reaching down to minimum depths of about 150 km. We perform numerical simulations of high-frequency seismic wave propagation (1 to 6 Hz) to explore potential seismological evidence for the in-situ existence of a DSC. Motivated by field observations, for modeling purposes we assume a simple block-in-matrix structure with eclogitic blocks floating in a serpentinite matrix. Homogenization calculations for block-in-matrix structures demonstrate that effective seismic velocities in such composites are lower than in the surrounding oceanic crust and mantle, with nearly constant values along the entire length of the DSC. Synthetic seismograms for receivers at the surface computed for intermediate depth earthquakes in the subducted oceanic crust for models with and without DSC turn out to be markedly influenced by its presence or absence. In models with channel, P and S waveforms are dominated by delayed high-amplitude guided waves emanating from the waveguide formed by oceanic crust and DSC. Simulated patterns allow for definition of typical signatures and discrimination between models with and without DSC. These signatures stably recur in slightly modified form for earthquakes at different depths inside subducted oceanic crust. Comparison with available seismological data from intermediate depth earthquakes recorded in the forearc of the Hellenic subduction zone reveal similar multi-arrival patterns as observed in the synthetic seismograms for models with DSC. According to our results, observation of intermediate depth earthquakes along a profile across the forearc may allow to test the hypothesis of a DSC and to identify situations where such processes could be active today.

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Eventlike exhumation of high-grade blocks in the young Franciscan subduction zone

10.5194/egusphere-egu2020-4218 ◽

2020 ◽

Author(s):

Daniel Rutte ◽

Joshua Garber ◽

Andrew Kylander-Clark ◽

Paul Renne

Keyword(s):

Subduction Zone ◽

Sierra Nevada ◽

Subduction Zones ◽

Amphibolite Facies ◽

Return Flow ◽

High Grade ◽

Metamorphic Overprint ◽

Franciscan Complex ◽

Metamorphic History ◽

State Process

The metamorphic history of exhumed high-grade rocks provides invaluable insight into the thermomechanical processes of subduction zones. While subduction in most orogens has been terminated by continent collision entailing variably strong overprint of related units, the Franciscan Complex of California allows studying a >150 Myr long subduction history that started at ~175 Ma and ended by transformation into a transform plate boundary (San Andreas fault) without significant metamorphic overprint. The highest grade metamorphic rocks of the Franciscan Complex of California are found as blocks in serpentinite and shale matrix m&#233;langes. They include amphibolites, eclogites, blueschists, and blueschist facies metasediments. These Franciscan m&#233;langes inspired the subduction channel return-flow model, but other processes e.g., buoyancy-driven serpentinite diapirism have been argued to be concordant with our current understanding of their metamorphic history, too.We investigate a suite of metabasite blocks from serpentinite and shale matrix m&#233;langes of the Califonia Coast Ranges. Our new dataset consists of U-Pb dates of metamorphic zircon and 40Ar/39Ar dates of calcic amphibole and white mica. Combined with published geochronology, particularly prograde Lu-Hf garnet ages from the same blocks, we can reconstruct the timing and time scales of prograde and retrograde metamorphism of individual blocks. We find: (i) Exhumation from the eclogite-amphibolite facies occurred only in a short episode at 165&#173;&#8211;160 Ma with an apparent southward younging trend. (ii) Exhumation of the blocks was uniform and fast in the eclogite-amphibolite facies with rates of 2&#8211;8 km/Myr. In the blueschist facies exhumation of the blocks was less uniform and slowed by an order of magnitude. (iii) The age of amphibole in a metasomatic reaction zone indicates that at least one amphibolite was enclosed in a serpentinite matrix by ~155 Ma. Considering the entire subduction zone system, the high-grade exhumation temporally correlates with a significant pulse of magmatism in the respective magmatic arc (Sierra Nevada) and termination of forearc spreading (Coast Range Ophiolite).Our findings do not support a steady-state process that is continuously exhuming high-grade rocks. Instead the subduction zone system changed with an eventlike character resulting in exhumation of high-grade rocks enclosed in serpentinite.

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Structural and rheological evolution of the Laramide subduction channel in southern California

Solid Earth ◽

10.5194/se-8-379-2017 ◽

2017 ◽

Vol 8 (2) ◽

pp. 379-403 ◽

Cited By ~ 8

Author(s):

Haoran Xia ◽

John P. Platt

Keyword(s):

Shear Stress ◽

Strain Rate ◽

Southern California ◽

Microstructural Analysis ◽

Carbonaceous Material ◽

Dislocation Creep ◽

Return Flow ◽

Pressure Solution ◽

Subduction Channel ◽

San Gabriel Mountains

Abstract. The Pelona Schist in the San Gabriel Mountains, southern California, formed in the Laramide subduction channel, exhibits multiple phases of deformation/metamorphism and provides valuable insights into the rheological properties of the subduction channel. Petrological and microstructural analysis indicates that the Pelona Schist has undergone three major deformational/metamorphic events. Subduction of volcanic and sedimentary protoliths during D1 was recorded by aligned mineral inclusions in albite and epidote porphyroblasts. Metamorphic temperature and pressure at the end of subduction yielded by Raman spectroscopy of carbonaceous material and phengite barometry were 519 ± 20 °C and 10.5 ± 0.4 kbar, respectively. During D1 the dominant deformation mechanism was quartz pressure solution, and the estimated shear stress at the end of D1 was less than 10 MPa. D2, the first stage exhumation of the Pelona Schist along the upper section of the subduction channel during return flow, was recorded by retrogressive metamorphism, isoclinal folding, and a pervasive schistosity that wraps around earlier porphyroblasts. Metagreywacke was deformed mainly by quartz pressure solution and metachert was deformed dominantly by dislocation creep during D2. The shear stress in metagreywacke was less than 10 MPa and that in metachert was between 8.3 + 2.7/− 1.5 and 12.9 + .9/− 2.3 MPa, resulting in a strain rate of 1.4  ×  10−13 to 5.5  ×  10−13 s−1. A topography driven model is proposed as the main driving force of D2 exhumation. D3 records normal-sense movement on the Vincent Fault, which separates the schist from overlying arc and continental basement. This resulted in the second stage of exhumation, creating a major synform and associated mylonitic fabric in the upper section of the Pelona Schist. Conditions at the beginning of D3 were 390 ± 13 °C and 5.8 ± 0.8 kbar given by the TitaniQ thermometer and phengite geobarometer. The deformation was dominated by quartz dislocation creep with a strain rate of 4.5 ± 1.2  ×  10−13 s−1 at a shear stress of 20.1 + 7.3/− 4.0 MPa.

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High tolerance of progressive elastic compression in peripheral arterial disease

VASA ◽

10.1024/0301-1526/a000799 ◽

2019 ◽

Vol 48 (5) ◽

pp. 413-417 ◽

Cited By ~ 2

Author(s):

Serge Couzan ◽

Jean-François Pouget ◽

Claire Le Hello ◽

Céline Chapelle ◽

Silvy Laporte ◽

...

Keyword(s):

Venous Insufficiency ◽

Ankle Brachial Index ◽

Arterial Disease ◽

Return Flow ◽

High Tolerance ◽

Compression Stockings ◽

Short Term ◽

Poor Tolerance ◽

The Mean ◽

Peripheral Arterial

Summary. Background: Theoretically progressive compression stockings, which produce a higher compression at the calf than at the ankle level, improve venous return flow without exacerbating peripheral arterial insufficiency (PAD). We aimed to evaluate the short-term tolerance of elastic progressive compression stockings on peripheral arterial vascularisation in patients with symptomatic PAD and associated mild venous insufficiency. Patients and methods: Monocentric, prospective, open pilot study of 18 patients (acceptability study, 6 x 6 plan) evaluating the short-term tolerance of progressive compression stockings (18 ± 2 mmHg at calf and 8 ± 2 mmHg at ankle level) in patients with PAD (ankle brachial index ABI > 0.60 < 0.75) and chronic venous insufficiency (C1s–C4 stages of the CEAP classification). Day 15 tolerance was evaluated by a composite primary criteria comprising: no decrease > 15 % of ABI on each side, no decrease > 15 % of toe brachial index (TBI) on each side and no decrease > 25 % of the number of active plantar flexions performed while standing. Results: The proportion of men was 77.8 %, mean age was 77.3 ± 7.5 years and no patient were diabetic. At inclusion, the mean low ABI was 0.60 ± 0.04 and the mean high ABI was 0.77 ± 0.18. The mean low TBI was 0.32 ± 0.09 and the mean high TBI 0.46 ± 0.15. The mean number of active standing plantar flexions was 33.0 ± 5.0. The majority of the patients were classified in CEAP C2s and C3 classes (class 2: 16.7 %, class C2s: 27.8 %, class C3: 44.4 %, class C4: 5.6 % and class C4s: 5.6 %). Poor tolerance occurred in no patient. By day 30, no patient had worsening of their arterial and venous symptoms. No adverse events occurred during the study. Conclusions: These results suggest a high tolerance of progressive elastic stockings (18 ± 2 mmHg at calf and 8 ± 2 mmHg at ankle level) in symptomatic PAD.

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Estimate of Block Medium Structure Parameters: A Model Case-Study of Seismic Sounding of a Brick Wall

Физико-технические проблемы разработки полезных ископаемых ◽

10.15372/ftprpi20200402 ◽

2020 ◽

Keyword(s):

Structure Parameters ◽

Brick Wall ◽

Block Medium ◽

Model Case ◽

Seismic Sounding

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Global return flow ratio

10.29104/wins.l.0111.2018 ◽

2017 ◽

Cited By ~ 1

Author(s):

Chloé Meyer

Keyword(s):

Drinking Water ◽

Water Source ◽

Return Flow ◽

Drinking Water Source ◽

Chemical Contamination ◽

Flow Ratio ◽

Piped Water ◽

Dug Wells

Population using an improved drinking water source (piped water into dwellings, yards or plots; public taps or standpipes; boreholes or tubewells; protected dug wells; or protected springs and rainwater) that is located on premises and available when needed and which is free of faecal and priority chemical contamination. Basin Pollution Quality Waste

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Clinical Implications of the Center of Balance for a Veteran with Traumatic Brain Injury

Vision Development & Rehabilitation ◽

10.31707/vdr2015.1.4.p280 ◽

2015 ◽

pp. 280-289

Keyword(s):

Traumatic Brain Injury ◽

Case Report ◽

Brain Injury ◽

Benign Paroxysmal Positional Vertigo ◽

Medical Center ◽

Treatment Plan ◽

Successful Outcome ◽

Model Case ◽

Positional Vertigo ◽

Paroxysmal Positional Vertigo

Background: It is known that traumatic brain injury (TBI), even of the mild variety, can cause diffuse multisystem neurological damage. Coordination of sensory input from the visual, vestibular and somatosensory pathways is important to obtain proper balance and stabilization in the visual environment. This coordination of systems is potentially disrupted in TBI leading to visual symptoms and complaints of dizziness and imbalance. The Center of Balance (COB) at the Northport Veterans Affairs Medical Center (VAMC) is an interprofessional clinic specifically designed for patients with such complaints. An evaluation entails examination by an optometrist, audiologist and physical therapist and is concluded with a comprehensive rehabilitative treatment plan. The clinical construct will be described and a case report will be presented to demonstrate this unique model. Case Report: A combat veteran with a history of a gunshot wound to the skull, blunt force head trauma and exposure to multiple explosions presented with complaints of difficulty reading and recent onset dizziness. After thorough evaluation in the COB, the patient was diagnosed with and treated for severe oculomotor dysfunction and benign paroxysmal positional vertigo. Conclusion: Vision therapy was able to provide a successful outcome via improvement of oculomotor efficiency and control. Physical therapy intervention was able to address the benign paroxysmal positional vertigo. The specific evaluation and management as pertains to the aforementioned diagnoses, as well as the importance of an interprofessional rehabilitative approach, will be outlined.

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