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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

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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.

  • 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).


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


Piston core

removed

from corer

Piston corer

Split piston

core being

subsampled.

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


  • 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

  • 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

    • 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”

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


    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 “fingerprint”

    Evolution of a single event down channel

    over a distance of 74 km


    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)


    Both have 22 events

    Less dated turbidites

    Low foram abundance

    Results

    Upper section

    poorly preserved


    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

    Regional correlation of turbidite stratigraphy spanning the Holocene


    Results: stratigraphic correlation

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


    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

    • 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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