Mapping seafloor faults in

1
Mapping seafloor faults in Southern
California R.D. Francis, M.R. Legg, and many
student participants
2
Palos Verdes fault (PVF), Coronado Bank fault
zone (CBF), Agua Blanca fault (ABF) system major
structure of the Inner borderland

• Pull-apart history of the Inner Borderland and
  uplift of the Palos Verdes Peninsula
• Recent offset (harbor area McNeilan et al.,
  1996)
• Earthquake potential
• Wilmington and Beta oil fields

Inner
Borderland
Focus of project Use of high-resolution seismic
reflection methods to image the PVF and
associated young sediments that were deposited
during faulting- to determine fault history.
3
Mapping the Ocean Floor

• Bathymetry measurement of ocean depths and the
  charting of the topography of the ocean floor
• Echo sounder (sonar) reflects sound from ocean
  floor depth (high frequency)

4
seafloor
5
Seismic reflection (low frequency sound
penetrates the seafloor)
6
Seismic reflection
7
Multibeam Sonar

• Employs an array of sound sources and listening
  devices (high frequency)
• Obtains a profile of a narrow strip of seafloor
• Map made from many parallel strips

8
CSULB study of the Palos Verdes fault
Multibeam image (USGS Gardner and Dartnell,
2000)
9
Methods Single and multi-channel seismic with a
1 kilojoule sparker resolution of a few
meters Correlation between seismic profiles
and USGS multibeam bathymetry
10
Extent of USGS multibeam bathymetric data
1,400 km of seismic tracklines - designed to
study the PVF- 10 years of data
11
Fault traces from seismic reflection profiles
12
PVF
seafloor
east
west
Seismic reflection image of the Palos Verdes
fault (PVF)
13
slumps
14
3D imaging of slumps
153a
154
15
Seafloor rises west of the PVF- indicate active
faulting
outcrops
69I
Map of seafloor rises (C.I. 1 m)
16
Cabrillo fault- Does it exist? Is it a
megaslide?
Graduate student John Schwiebert
Cabrillo fault (?)
17
Cabrillo fault at the shoreline
Woodring et al., 1946
18
Woodring et al., 1946
19
Modern photo of same general area Where is the
fault?
20
Only one house from 1910 to 1940 second window
was boarded over
21
The blue house is the 1910 house therefore the
two photos were taken from nearly the same
vantage point.
22
Resistant ledges in tide zone near the 1910 house
1910 house
23
east
Dolomitic, resistant beds dip 35 to 40o east
visible only at extremely low tides
south
24
Ledges
Discordant dips
25
Reefs in aerial photos
26
?
reefs
?
27
Nearshore side-scan sonar images
- gap without sonar data
Next slide
Side scan sonar data
28
sand
reef
29
3.5 kHz echo sounder profile 2E-7
Kelp anchored to rocky bottom
30
Cabrillo Beach
Extent of rocky bottom west of the CF
No kelp
Point Fermin landslide
Cabrillo fault
Kelp growing on outcrops
Kelp
31
Nearshore map of the CF is on trend with the
offshore portion mapped with multibeam bathymetry
(herring bone pattern) and seismic reflection
profiles
32

• Cabrillo fault
• Herring-bone pattern of outcrops
• Large vertical separation fault in seismic
• Lines up with nearshore mapping

99
102
33

• Cabrillo fault Conclusions
• CF is not a megaslide scarp, but a tectonic fault
• CF is associated with the PVF
• CF is associated with the seafloor rises,
  suggesting recent activity

34
How old is the Palos Verdes fault and what is its
history?
San Gabriel Canyon
35
80a

• History of the Palos Verdes fault
• Delmontian transtension- before 3 m.y.b.p.
• Repettian transpression- after 3 m.y.b.p.

7
Palos Verdes anticlinorium
36
PVF Palos Verdes fault, CF Cabrillo fault
Proposed geologic map showing continuation of
formations offshore Palos Verdes anticlinorium
37
San Gabriel Canyon
Interpretation Palos Verdes Tectonic Block
38
?
Palos Verdes Tectonic Block

• Bounded on the east by the PVF
• Continuous Palos Verdes Anticlinorium
• Bounded on west by San Pedro Basin
• Young- most of the uplift in the last 600,000
  years

39
San Pedro Basin Study
40
San Pedro Basin and the San Pedro Basin fault A
subsiding depocenter between two uplifted
blocks, Palos Verdes and Santa Catalina Island
San Pedro Basin fault
41
Complex tectonics of San Pedro Basin more data
are needed
San Pedro Basin fault
transpression
transtension