Diapositive 1

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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 by
  density 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).
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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

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Piston core removed from corer
Piston corer
Split piston core being subsampled.
http//oceanworld.tamu.edu/students/forams/forams_
piston_coring.htm
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• 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

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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

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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

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Event fingerprinting

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

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Event fingerprint

• First, these data were used to correlate
  stratigraphy between cores at a single site
• Found that it was possible to correlate unique
  physical property signatures of individual
  turbidites from different sites within the same
  channel
• Even possible to correlate turbidites between
  different channels (some of which never met)
• The turbidite fingerprint basis of
  long-distance correlations

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Event fingerprint
Evolution of a single event down channel over a
distance of 74 km
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Radiocarbon analysis

• Extraction of planktic foraminifera from 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)

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Results
Both have 22 events
Less dated turbidites Low foram abundance
Upper section poorly preserved
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Results confluence and mineralogy

• Good correlation between these cores suggests
  that input mixing at each confluence has little
  effect on the stratigraphy of the turbidites
• Synchronous triggering is the only 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

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Results stratigraphic correlation
Regional correlation of turbidite stratigraphy
spanning the Holocene
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Results stratigraphic correlation
Noyo canyon is cut by the NSAF and as an
epicentral distance of zero ? explains thicker
turbidite records
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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

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Discussion

• Good correspondence with land paleoseismic dates
  (individual matching, total number of events)
• Offshore turbidites as paleoseismic indicators
  for the NSAF
• Mean recurrence interval coherent with onshore
• Epicentral distance is 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 distance from
  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 intervals between 150 and
  250 yr