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Introduction. Turbidites: geological formations that have their origins in turbidity currents deposits, that deposit from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean. .

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Introduction

Introduction

  • Turbidites:geological formations that have their origins in turbidity currents deposits, that deposit from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.

  • Sediments are transported and deposited bydensity flow, not by tractional or frictional flow.

  • Bouma sequence:from conglomerates at the bottom to shales on the top

Idealised sequence of sedimentary textures and structures

in a classical turbidite, or Bouma sequence (Bouma, 1962).


Introduction1

Introduction

  • Interest of the off-fault paleoseismology

    • GPS → high degree of certainty, in few years, of the crustal strain accumulation.. But just for a portion of a cycle..

    • Earthquake records → not long enough

    • Onshore paleoseismology → erosion, urban area..

    • Off-fault paleoseismology

  • Interest of marine turbidite records

    • Have to prove they are earthquake-triggered

    • Marine records: more continuous, extend further back in time, more precise in time (datable foraminifera)

  • Method used

    • 74 piston, gravity cores from channel/canyon systems draining Northern California

    • Mapping channels with multibeam sonar (bathymetry, channel morphology, sedimentation patterns

    • Sampled all major channel systems between Mendocino and north of Monterey Bay

  • Results

    • Good agreement with shorter land record

    • Opportunity to investigate long tem earthquake behaviour of North San Andreas Fault


Introduction

Piston core

removed

from corer

Piston corer

Split piston

core being

subsampled.

http://oceanworld.tamu.edu/students/forams/forams_piston_coring.htm


Introduction

  • 4 segments of SAF:

    • Santa Cruz Mountains

    • Peninsula

    • North Coast

    • Offshore

  • Several onshore paleoseismic sites:

    • Vedanta: max slip rate in late Holocene 24 +/- 3mm/yr and 210 +/- 40 years

    • Fort Ross: ~230 yr

    • South of the Golden Gate: 17 mm/yr


How to identify earthquake triggered turbidites

How to identify earthquake-triggered turbidites

  • Possible causes of turbidites:

    • Storm or tsunami wave loading

    • Sediment loading

    • Storm discharges

    • Earthquakes

  • Seismically triggered turbidites are different:

    • Wide area extent

    • Multiple coarse fraction pulses

    • Variable provenance

    • Greater depositional volume

  • Use a temporal and spatial pattern of event correlation over 320 km of coastline


  • Synchronous triggering and correlative deposition of turbidites

    Synchronous triggering and correlative deposition of turbidites

    • Regional stratigraphic datum missing

    • Correlations depend on stratigraphic correlations of other datums and radiocarbon ages

    • The Confluence Test:

      • If one canyon contains n turbidites and a second canyon also shows n turbidites, and if these n events have been independently triggered, the channel below the confluence should contain at least 2n instead of only n.

    • 8 major confluences

    • 3 heavy minerals


    Event fingerprinting

    Event “fingerprinting”

    • All cores are scanned, collecting P-wave velocity, gamma-ray density, magnetic susceptibility data, imaged with X-radio and grain size analyzed


    Event fingerprint

    Event “fingerprint”

    • First, these data were used to correlate stratigraphy between coresat a single site

    • Found that it was possible to correlate unique physical property signatures ofindividual turbiditesfrom different sites within the same channel

    • Even possible to correlate turbiditesbetween different channels(some of which never met)

    • The turbidite “fingerprint” = basis of long-distance correlations


    Event fingerprint1

    Event “fingerprint”

    Evolution of a single event down channel

    over a distance of 74 km


    Radiocarbon analysis

    Radiocarbon analysis

    • Extraction ofplanktic foraminiferafrom the hemipelagic sediment below each turbidite

    • Bioturbation and basal erosion do not biase 14C ages

    • Method:

      • Determine hemipelagic thickness

      • Estimate the degree of basal erosion

      • Observe that differential erosion is most likely source of variability at any site

      • Conversion of hemipelagic thickness to time (using average of sedimentation rate)


    Results

    Both have 22 events

    Less dated turbidites

    Low foram abundance

    Results

    Upper section

    poorly preserved


    Results confluence and mineralogy

    Results: confluence and mineralogy

    • Good correlationbetween these cores suggests that input mixing at each confluence has little effect on the stratigraphy of the turbidites

    • Synchronous triggering is theonly viable explanation

    • Non-synchronous triggering should produce an amalgamated record that increases in complexity below each confluence, with only partial correlations for the synchronous events

    • Strict test of synchroneity


    Results stratigraphic correlation

    Results: stratigraphic correlation

    Regional correlation of turbidite stratigraphy spanning the Holocene


    Results stratigraphic correlation1

    Results: stratigraphic correlation

    Noyo canyon is cut by the NSAF and as an epicentral distance of zero → explains thicker turbidite records


    Time series

    Time series

    • -The youngest 15 events have a mean repeat time of ~200 yr +/ 60 yr

    • ~95 yr: minimum interval

    • ~270 yr: maximum value

    • Values consistent with previous paleoseismic data onshore

    • Same total number of events onshore and offshore = land and marine record the same events


    Discussion

    Discussion

    • Good correspondencewith land paleoseismic dates (individual matching, total number of events)

    • Offshore turbidites as paleoseismic indicators for the NSAF

    • Mean recurrence interval coherent with onshore

    • Epicentral distanceis the controlling factor for turbidite size

    • Turbidites correlate across channels where the mineralogy is different, the physiography is different the sediment sources are different and the underlying geology is different too

    • Minimum magnitude and triggering distancefrom the earthquake hypocenter : at least M7.4

    • But observations of turbidites of small events may also be a function of the resolutions of the observations

    • Majority of repeat time intervalsbetween 150 and 250 yr


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