Upcoming Talks and Other Things
November 10-11, 2016, National Academy of Sciences Building
2101 Constitution Ave NW Washington DC 20418
City Club Earthquake Forum, Kells Pub Portland, November 1, 5:30 pm
New Yorker Festival, Manhattan, October 3, School of Visual Arts, Theatre 1, 10 am.
NWEA Workshop, Hood River Inn, October 2.
Oregon Coast Economic Summit, August 27, Grand Ronde.
The Really Big One: A Public Forum On Earthquake Hazards
and Preparedness in the PNW, University of Oregon, Eugene, August 6, 7 PM. 156 Straub Hall.
Preliminary study of existing lake sedimentary records suggests a record of great earthquakes.
New core and high resolution reflection data illuminate thesouthern Cascadia paleoseismic record.
Seismically generated turbidites in Effingham Inlet, western Vancouver Island.
Chris Goldfinger, Mark Legg, Marc Kamerling, Randall Milstein, Craig Nicholson, Jason D. Chaytor, Robert S. Yeats, Gary G. Huftile
Deformation Rates based on Low-Stand Shorelines
We use submerged paleoshorelines as strain markers to investigate recent vertical
tectonic movement at the intersection of the offshore Santa Cruz-Catalina Ridge with the
southern boundary of the Western Transverse Ranges, within the California Continental
Borderland. Past submerged shoreline positions were identified using high-resolution multibeam
bathymetry, side-scan sonar, seismic reflection profiles, submersible observations and the
presence of intertidal and subtidal invertebrate fossils. Numerous AMS 14C ages of shells from
these paleoshorelines were found to be between ~ 27,000 years (RC) and 11,500 years before
present (BP) years, indicative of shoreline colonization during and following the Last Glacial
Maximum (LGM) and establish these paleoshorelines as useable datum for measuring vertical
change since this time. Removal of the non-tectonic component of vertical change using an icevolume
equivalent eustatic sea level compilation indicates between 20 m and 45 m of uplift of
the eastern portion of the northern Channel Islands block since the LGM lowstand, resulting in
an uplift rate of 1.44 ± 0.46 mm/yr over the last 23 ka. This rate closely matches published sliprates
for the Channel Islands thrust that underlies the northern Channel Islands platform. Results
from post-LGM shoreline features on Pilgrim Banks are somewhat more ambiguous. Preliminary
analysis of seismic reflection profiles coupled to the multibeam bathymetry, which shows
extensive upper-crustal fold-thrust style deformation, further illustrates the transpressional
interaction of the Borderland and Western Transverse Ranges blocks where the Santa Cruz-
Catalina Ridge and northern Channel Islands intersect.
Figure 1. Gray-shaded multibeam compilation map of the California Continental Borderland
offshore of southern California. The primary focus area of this study, the Santa Cruz-Catalina
Ridge and the Northern Channel Islands platform are enclosed by the dashed polygon. The
contour interval for bathymetry is 250 m. The traces of onshore and offshore faults from
Jennings (1994) are marked in red, with the major faults and fault zones labeled. Abbreviations
are: A-DF – Anacapa-Dume fault, CF – Chino fault, EF – Elsinore fault, MCF – Malibu Coast
fault, NIFZ – Newport-Inglewood fault zone, PVF – Palos Verdes fault, RCF – Rose Canyon
fault, RF – Raymond fault, SC-CRFZ – Santa Cruz-Catalina Ridge fault zone, SCIF – Santa
Cruz Island fault, SGF – San Gabriel fault, SJcF – San Jacinto fault, SMF – Santa Monica fault,
SMdF – Sierra Madre fault, SRIF – Santa Rosa Island fault, WF – Whittier fault. (b) Tectonic
setting of the California Continental Borderland, which extends from Point Arguello to Cedros
Island. The various post-Oligocene tectonic terranes that comprise the southern California
Borderland, i.e., Inner and Outer Borderland, Southern Borderland Rift, and Northern Channel
Islands/Western Transverse Ranges (WTR) blocks are indicated by the thick dashed lines. The
thinner dashed lines and arrows indicate the location of the lithostratigraphic terranes of Vedder
(1987); PAB – Patton Accretionary Complex, NFB – Nicolas Forearc Belt, and CSB – Catalina
Schist Belt. Structure modified from Legg (1991).
Figure 2. (a) Simplified map of the major onshore and nearshore fault systems of Los Angles
basin and vicinity, showing the style of fault interactions at the boundary between the leftoblique
and south-vergent thrusts that dominate the Transverse Ranges and right-lateral and
right-oblique Peninsular Ranges oriented blocks. (b) Simplified map of the major offshore rightoblique
fault systems of the northern California Continental Borderland and left-oblique and
south-vergent thrusts that dominate the Western Transverse Ranges in the vicinity of the two
blocks. Faults mapped as part of this study are not shown. Abbreviations are as for Figure 1,
plus: CIT - Channel Islands thrust, CuF - Cucamonga fault, FFZ - Ferrelo fault zone, HF -
Hollywood fault, MCT - Mid-Channel thrust, PHT - Puente Hills thrust, ORF - Oak Ridge fault,
ReCF - Redondo Canyon fault, RMT - Red Mountain thrust, SC-CRFZ - Santa Cruz-Catalina
Ridge fault zone, SCF - San Cayetano fault, SJF - San Jose fault, and SPBFZ - San Pedro basin
Figure 3. (a) Composite illustration of erosional features commonly found on rocky shorelines
which, when submerged or uplifted, can be used as indicators of previous sea level position. (b)
Common styles of shoreline platform morphology that result in uplifted and submerged terraces.
HT - high tide, LT - low tide. Modified from Bird (2000).
Figure 4. (a) High-resolution multibeam bathymetry of part of the northern Channel Islands
(NCI) platform, focused on the southern and western side of Santa Cruz Island and southern side
of Anacapa Island, compiled from data collected in 2003 and 2004 with addition bathymetry
around Anacapa Island from Dartnell et al. (2005) and Gull Island from Kvitek et al. (2004).
The approximate locations of the Santa Cruz Island (SCIF), Santa Rosa Island (SRIF), and Santa
Cruz-Catalina Ridge (SC-CRFZ) faults are shown. Contour interval is 100 m for bathymetry,
250 m for topography. Boxes show the location of Figure 10. (b) High-resolution multibeam
bathymetry of the crest of the southern SC-CR, with Pilgrim Banks at the top of the map. The
bathymetric high south of Pilgrim Banks is commonly referred to as Kidney Bank or Hidden
Reef. Red lines show faults of the SC-CRFZ. Box shows location of Figure 11. Contour
interval is 100 m.
Figure 5. Composite schematic diagram of paleoshoreline features observed during submersible
dives on the northern Channel Islands platform and Pilgrim Banks. Examples of these features
can be seen in photographs taken from the submersible: (a) large, well-preserved Mytilus
californicus shells on bench, Pilgrim Banks. (b) notched/under-cut rock outcrop on south side of
Santa Cruz Island. (c) rounded cobbles/boulders on probable paleoshoreline, between Santa Cruz
and Anacapa Islands, southern NCI platform.
We have used submerged LGM and younger paleoshorelines preserved around the
Northern Channel Islands and Pilgrim Banks atop the Santa Cruz-Catalina Ridge to determine
the vertical strain history at the intersection of two opposing structural trends in southern
California. Slope breaks and terrace which we believe to represent LGM and younger
paleoshorelines on the eastern NCI platform indicate as much as 1.44 ± 0.46 mm/yr Late
Pleistocene to Recent uplift of the islands above the blind Channel Islands thrust, a result that
compares favorably with previous estimates of slip on the fault. As for Pilgrim Banks atop the
SC-CR, we currently favor an interpretation that has the ridge undergoing no vertical tectonic
motion or accumulation of strain in the period since the LGM, with the northward tilt of the ridge
possibly a result of crustal shortening or pre-existing morphology of the ridge. These results are
likely to change as new information becomes available, especially on sea level position during
and before the LGM, the response of the solid Earth to the melting of the Wisconsinan sheet, and
as our ability to more accurately constrain submerged paleoshorelines features improves.
Using both the result from the paleoshoreline analysis, and from preliminary analysis of
seismic reflection profiles and bathymetry data, we find that although there appears to be a
significant component of underthrusting occurring at the intersection of the Western Transverse
Ranges and Borderland provinces, along the Channel Islands thrust interface, it may only
represents a small fraction of the total required to accommodate northward motion of the
Borderland block. It appears that much of the required contractional motion may be distributed
into upper-crustal shortening and left-lateral block motion above an upper-crustal detachments,
both at the intersection and prior to it, along the length of the major right-lateral fault systems.
Much of this motion is partitioned into both bending fault-termination and fold-and-thrust beltstyle
deformation on the western side of the Santa Cruz-Catalina Ridge, with the development of
large anticlines and listric thrust faults on the northeastern flank of the Santa Cruz basin, south of
the southern margin of the Western Transverse Ranges. Although our results indicate some uplift
of the NCI platform, they do not allow us to determine if this uplift is partitioned, with the region
to the west of the SC-CR (i.e., Santa Cruz Island) uplifting at a rate different to that on the
eastern side of the ridge (i.e., Anacapa Island).
The agreement of the rate of uplift of the eastern NCI platform found through the analysis
of paleoshorelines, with previous estimates of fault slip on the Channel Islands thrust, provides
positive validation of the use of submerged paleoshorelines as an additional method of extracting
the Holocene vertical tectonic component of deformation. That said, currently the technique
provides results that can be greatly influenced by uncertainties inherent in underwater
geophysical and sampling methods and therefore will require additional improvements and wider
application to fully realize its potential.
Enigmatic Crater Structures of the Borderlands
Digital mosaics of swath and conventional bathymetry data reveal large, distinct
near-circular crater structures in the inner Continental Borderland
offshore of southern California. Two have maximum crater diameters
that exceed 30 km, and a third has a crater diameter of about 12 km.
All three features exhibit the morphology of large complex
craters; raised outer rim, ring moat and central uplift, however their exact origin remains a mystery. Preliminary analyses of
available seismic, gravity and magnetic data over these structures
reveal both similarities and distinct differences in geometry,
structure, and geophysical signature to known impact sites. All three
crater structures, however, occur within the Catalina terrane, a highly
extended volcanic and metamorphic province floored by Catalina Schist
basement. A likely alternative origin may thus involve explosive
volcanism, caldera collapse and resurgent magmatism, and/or possibly
plutonism and schist remobilization, associated with the Catalina
terrane. No single model for crater formation, whether impact, caldera
or pluton, fully accounts for all of the present observations regarding
the morphology, internal structure, and known geology of these
near-circular features. Timing of crater formation postdates the
initial rifting and rotation of the western Transverse Ranges, and
appears to predate major right-slip along the San Clemente and San
Diego Trough fault systems, or about 18 to 16 Ma. Regardless of their
origin, these complex craters represent some of the largest structures
of their kind in western North America and provide a unique opportunity
to better understand the development of unusual crater structures in a
Figure 1. Perspective view of Catalina "crater" from the Northwest looking toward San
Diego. Bathymetry is partial coverage multibeam data, with
sparse sounding data in the foreground. Central uplift and "moat"
are clearly visible.
Figure 2. Map view of the southern Califirnia borderlands bathymetry in the vicinity of the San Diego Trough and San Diego. The Catalina "crater" feature is at center, and has been deformed on its east and west sides by the San Diego Trough Fault and San Clemente Fault respectively.
Figure 3. Manson Crater showing typical imact features that are at least superficailly similar to those of the Catalina "Crater".
Figure 4. (A) Gravity model for the Catalina strucure and (B) interpreted single-channel seismic time section USGS-952A across Catalina Crater. Note general asymmetry and the presence of deposits of unknown age and origin (light purple) imaged as part of the deeper moat fill (B, left side). Moat strata are folded, suggesting some structural relief is related to later compressive overprint. Depth of moat fill is inferred assuming a minimum sediment velocity of 1500 m/s.
Stewart  recently proposed a set of simple criteria to classify the origin of buried circular structures in terrestrial sedimentary basins where direct geologic sampling or evaluation are not yet available. Using these criteria, and given the size, circular shape, crater form, central uplift, and depth-to-diameter ratio of these offshore structures, Catalina Crater, Emery Knoll, and Navy Crater would each qualify as either large igneous resurgent caldera or impact craters. Owing to the known regional volcanism of similar age [Luyendyk et al, 1998; Weigand et al., 2002] and obvious alignment within the Inner Borderland Rift, we prefer a volcanic origin for these structures, although this line of reasoning has been known to be previously misleading [c.f., Hoyt, 1987; French, 1990]. The present lack of recognized extensive pyroclastic or ejecta deposits associated with these structures we attribute to their unusual marine setting, their significant age (>15 Ma), and the subsequent erosion and rafting away of lighter volcanic materials. Future more detailed geologic investigations and tectonic reconstructions may find more extensive deposits of this type than are currently mapped.
If these offshore structures are volcanic in origin, they represent the largest previously undiscovered caldera complex in western North America. These may be the elusive source of early-to-mid-Miocene silicic volcanic and breccia deposits found on adjacent islands and onshore regions, although the amounts of silicic deposits identified to date are orders of magnitude less than expected. If any of these structures represents an impact site, it would be the first of its kind to be discovered in the eastern Pacific, and the first to be recognized to occur in recently exhumed, at the time of impact, ductile schist basement. If these structures result from mid-crustal exhumation, plutonic intrusion, and schist remobilization, then they represent some of the largest structures of their kind, and would illuminate how such large, circular features develop in an oblique shear environment and evolve on such a large regional scale.
With the available data, we cannot conclusively exclude any of the three hypotheses for the formation of these large, offshore crater structures. More importantly, no single model for creation of such large, circular, complex crater structures on the Earth can adequately explain all the current observations for these features. In particular, the conspicuous absence at present of diagnostic signatures, such as shocked minerals or large ignimbrite deposits, that would be expected if these structures were of impact or volcanic origin, make accurate interpretation of their origins difficult. In any case, the existence of such large offshore near-circular structures implies that care must be taken in inferring origin based solely on crater morphology [e.g., Underhill, 2004] and that models for creation of such large, circular features may need to be modified. Regardless of their origin, the presence of such unusual features offshore of southern California suggests that the regionally extensive San Onofre Breccia, previously interpreted to represent a near-fault tectonic breccia associated with Inner Borderland rifting, may be—at least in part—an explosion breccia associated with caldera formation or impact. Catastrophic origins, either caldera or impact related, for these features also would likely create other important geologic markers that may exist, as yet unrecognized, within the regional stratigraphic record. In particular, unconformities in deep water Miocene stratigraphic sequences may represent seafloor erosion from tsunami generated by these events. Identification and analysis of such markers would help to accurately constrain the timing, and catastrophic origin of the crater features.
Legg, M., Nicholson, C., Goldfinger, C., Milstein, R., Kamerling, M., 2004 Large Enigmatic Crater Structures Offshore Southern California: Geophysical Journal International, v.159, n.2, p.803-815.
Legg, M.R., Goldfinger, C., Kamerling, M.J., Chaytor, J.D., 2007, Morphology, structure and evolution of California Continental Borderland restraining bends, in Cunningham, W. D. & Mann, P. (eds) Tectonics of Strike-Slip Restraining and Releasing Bends, Geological
Society, London, Special Publications, 290, 143–168.
We thank Chris Sorlien, John Crowell, Roy Shlemon, Jim Ashby, two anonymous reviewers and Richard Grieve for constructive comments on various drafts of this paper, Jon Childs and Ray Sliter (US Geological Survey) for the seismic data we reprocessed along USGS-120, and Vicki Langenheim (USGS) for the isostatic residual gravity used in Figure 9a. Research supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award number Legg, 01HQGR0017 and Goldfinger, 01HQGR0018. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.