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    Andrews_F&M_2010v2.pptx Andrews_F&M_2010v2.pptx Presentation Transcript

    • Graham Andrews – Geological Survey of Canada (Vancouver) April 6th 2010 Grey‟s Landing ignimbrite, ID WHEN IS A LAVA FLOW NOT A LAVA FLOW? UNRAVELING ANCIENT SUPER-ERUPTIONS USING FIELD OBSERVATIONS AND STRUCTURAL MAPPING
    • OUTLINE 1. About me 2. Introduction to supervolcanoes 3. High-grade ignimbrites 4. Structural analysis 5. Model of syn-depositional flow
    • A LITTLE ABOUT ME…  Geology at High School  Uni. of Leicester, UK  U/G – specialized in structure, tectonics, ig. pet. & volcanology  PhD –  ductile deformation of rhyolite tuffs & lavas  SRP super-eruptions folded Silurian turbidites, N.I.
    • A LITTLE ABOUT ME…  PDF at UBC & GSC  BC Neogene regional geology and tectonics  Calderas, extension, and mineralization in NV and BC  Eocene MCCs  Volcanic dams  Pseudotachylites  Rheology Holocene subglacial lava flow, experiments
    • SUPERVOLCANOES • BTIP (Paleocene) • Yellowstone, WY • SRP (Miocene) • Mid-Continent Rift •Campi Flegri (mid-Proterozoic) • Long Valley, CA • Aso • Valles, NM • Tenerife •Santorini • Bijli (Proterozoic) • Sierra Madre Occidental • Taal (Oligocene) • Karoo • Toba • Whitsunday (Jurassic) (Cretaceous) • Altiplano-Puna (Miocene) • Etendeka • Yardea Dacite • Chon Aike (Cretaceous) (mid-Proterozoic) • Taupo (Jurassic) modern ancient associated with mature extensional continental arcs, continental rifts, and hot-spots.
    • SUPER-ERUPTIONS - INTRO Superb, but not super…  Devastating rhyolite eruptions >10 km3  Every 100 – 1000 years  Global impact  Nearly made Homo sapiens extinct (Toba, Indonesia 70 ka)  Largest volcanic features and deposits Montserrat, January 2010
    • NOVA Special impression of a super-eruption at Yellowstone
    • SUPER-ERUPTIONS – ASH DISPERSAL Ashfall Fossil Beds State Park, NE Teleoceras, U- Haul SRP Nebraska Miocene SRP distal ash fall deposits F&M are >5 m thick in Nebraska (1600 km
    • SUPER-ERUPTIONS - PRODUCTS ignimbrite rhyolite lava flows Big Obsidian Flow, Newberry, OR • Effusive eruptions – lavas ooze out and flow • Explosive eruptions – hot ash and pumice avalanches away from volcano deposit Mazama Tuff, Crater Lake, OR
    • SO WHAT ARE IGNIMBRITES?  An ignimbrite is the deposit of a pyroclastic flow. Manam, Papua New Guinea, 1996
    • SO WHAT ARE IGNIMBRITES?  Ignimbrites are typically composed of „juvenile‟ ash and pumice lapilli, plus variable amounts of „accidental‟ lithic lapilli. pumice lapilli lithic lapillus Fasnia ignimbrite –
    • IGNIMBRITE WELDING GRADE non-welded moderate high-grade coalescence Temp < 750 ºC > 1000 ºC „typical‟ igs SRP igs “fluffy” pumices Fasnia ignimbrite –
    • IGNIMBRITE WELDING GRADE non-welded moderate high-grade coalescence Temp < 750 ºC > 1000 ºC „typical‟ igs SRP igs fiamme - flattened plastic compaction <40% pumices “pure shear” un-named ignimbrite – South
    • IGNIMBRITE WELDING GRADE non-welded moderate high-grade coalescence Temp < 750 ºC > 1000 ºC „typical‟ igs SRP igs very flattened and >10:1 stretching ratio stretched fiamme – “simple shear” no pumice Facies: rheomorphic Mogan D rheomorphism  “eye ignimbrite – Gran structure” - sheath fold Canaria
    • IGNIMBRITE WELDING GRADE non-welded moderate high-grade coalescence Temp < 750 ºC > 1000 ºC „typical‟ igs SRP igs „flow-banding‟ – >100:1 stretching ratio indistinguishabl “simple shear” e from a lava Facies: ‘lava-like’ & rheomorphic Grey‟s Landing rheomorphism  ignimbrite – Idaho isoclinal flow fold
    • WHAT IS RHEOMORPHISM?  Rheomorphism is the plastic deformation of a welded ignimbrite during emplacement, as a result of ductile flow  Rheomorphism requires low viscosity tuff, therefore:  high temperature (>900 °C)  high dissolved H2O, F or Cl  high Al or Na + K composition  a combination of the 3
    • Mogan „D‟ - Gran Understanding rheomorphism is Canaria all about structural geology  rheomorphic ignimbrites are ductile shear zones!!! L1 & F 1 transport direction L1 & F 1 stretched vesicle L = stretching lineation ‘rodding’ lineation – L F = fold hinge
    • WHAT IS RHEOMORPHISM?  Rheomorphic flow:  may be syn- and / or post-depositional,  syn-depositional rheomorphism is strongly simple-shear, producing lineations and sheath folds  like a ductile shear zone in the crust,  post-depositional rheomorphism is moderately pure-shear, producing buckle-style folds  like a lava flow or a glacier.  TheSCALE of folding is dictated by the SCALE of the layer being deformed.
    • CASE STUDY – GREY’S LANDING IG., IDAHO Yellowstone hot- spot track [Andrews & Branney, in press. - GSA Bull. ]
    • Grey‟s Landing Ignimbrite [Andrews et al., 2008 [Andrews & Branney, - Bull. Volc.] in press. - GSA Bull. ]
    • contorted domain flat domain [Andrews et al., 2008 [Andrews & Branney, - Bull. Volc.] in press. - GSA Bull. ]
    • [Andrews & Branney, in press. - GSA Bull. ] flat domain folds
    • [Andrews & Branney, in press. - GSA Bull. ] flat domain folds
    • [Andrews & Branney, in press. - GSA Bull. ]
    • simple shear flow [Andrews & Branney, in press. - GSA Bull. ]
    • contorted domain flat domain [Andrews et al., 2008 [Andrews & Branney, - Bull. Volc.] in press. - GSA Bull. ]
    • refolded F1 “flat” folds contorted domain folds - small
    • contorted domain folds - large
    • pure shear flow [Andrews & Branney, in press. - GSA Bull. ]
    • syn-depositional rheomorphism [Andrews & Branney, in press. - GSA Bull. ]
    • syn-depositional rheomorphism [Andrews & Branney, in press. - GSA Bull. ]
    • aerosol can analogy aerosol of paint particles
    • aerosol can analogy aerosol of paint particles coalesced flow of paint
    • T1 [Andrews PhD]
    • T2 [Andrews PhD]
    • T3 [Andrews PhD]
    • T4 [Andrews PhD]
    • T5 – deposition ceased [Andrews PhD]
    • post-depositional rheomorphism horizontal shortening – gravity- driven contorted domain contorted domain flat domain flat domain [Andrews PhD]
    • SUMMARY  Supervolcanoes are huge, complex systems requiring study by volcanologists, structural geologists, petrologists, geochemists, geophysicists, etc.  lots of research opportunities.  Small & medium-scale features (volcanic and structural) reveal how rheomorphic ignimbrites form  kinematics recreate the flow,  The same approach works for lavas, glaciers, plutons, mudflows, mylonite zones, etc.
    • Thank you – questions?
    • FUTURE RESEARCH & PROJECTS  Remote-sensing mapping of the SRP & Boise areas   GIS, satellite images, airphotos, existing geology data  Mapping, textural description, and measurements of welded ignimbrites and lavas (ID, NV, OR, Spain)   SEM, petrography, fieldwork, XRD  Paleo-elevations in the SRP (very long term!)   fieldwork, stratigraphy, thermochronology, O, palynology, geophysics,  Pleistocene basaltic volcanism in BC   Ar/Ar dating, fieldwork, stratigraphy  Origin of multi-rimmed basalt pillows (BC & Idaho)   SEM, XRD, petrography
    • D2 folding of the upper ‘free’ surface
    • L1 is independent of the underlying slope  therefore D1 is not gravity controlled. F2 is perpendicular to the dip direction of the slope, L2 is parallel  therefore D2 is slope dependent. D2 is probably gravity-driven.
    • [Andrews & Branney, in press. - GSA Bull. ]
    • super-eruptions produced huge ‘flood rhyolites’ Jarbidge Canyon, ID-NV
    • super-eruptions produced huge ‘flood rhyolites’ Q - were the eruptions explosive (ignimbrites) or effusive (lavas)? Jarbidge Canyon, ID-NV
    • super-eruptions produced huge ‘flood rhyolites’ Q - were the eruptions explosive (ignimbrites) or effusive (lavas)? A – both. But how to tell them apart? Jarbidge Canyon, ID-NV