COASTAL DEFORMATION AND TECTONIC EVOLUTION OFFSHORE SAN ONOFRE August 2013
Coastal Deformation and Tectonic Evolution Offshore San Onofre Mark R. Legg Legg Geophysical, Inc. The true sign of intelligence is not knowledge but imagination. Albert Einstein
The California Continental Borderland is an Organized Train Wreck Charlie Hollister, 1998 Translations and Vertical-Axis Rotations Extension and Contraction
Fundamental Observations Heterogeneous Reality Earth does NOT consist of uniform layers over a homogeneous half-space Faults are neither Planar nor Discrete discontinuities Pre-Existing Geological Structure re controls subsequent ent deformation Fault Systems are Complex Broad Deformation Zone 10 km to 100 km wide at the surface Major Southern California NW-Trending Right-Slip Faults are Right-Stepping En Echelon Systems g pp g y Complexity is Dynamic multiple structures are activated during large earthquakes Active Strain Partitioning over Geological and Earthquake Time- Scales Model the System - NOT Just the Faults
Transpressional Fault Systems of the Inner Borderland d Rift Coastal Fault System Newport-Inglewood Rose Canyon Fault Zone (Right-Slip) San Mateo and Carlsbad Fault Zones (Right Oblique-Reverse Slip) Oceanside Detachment Blind Thrust San Clemente Fault System Santa Cruz-Catalina Ridge Catalina Fault Zone San Clemente Fault Zone San Diego Trough San Pedro Basin Fault Zone Palos Verdes Coronado Bank Fault System Palos Verdes THUMS Huntington Beach Fault Zone Coronado Bank Estero Agua Blanca Fault Zone
Eastern Boundary of the Inner Borderland Rift is the Distal Breakaway of the Western Transverse Ranges Three (3) Right-Stepping En Echelon Sections comprise the WTR Breakaway: LA Basin, South Coast, Coronado Bank South Coast Offshore Zone of Deformation exists along the Central Section (SC) of the WTR/IB Breakaway WTR/IB Breakaway lies in the Hanging Wall of the Inner Borderland Detachment (Subduction Megathrust?) Major Tectonic Elements of the California Continental Borderland LA SC CO Coastal Fault System = SC
Working Hypotheses Repetition of Structural Style Represents the Active Process Fault Wedge Tips, Continental Collision and Indentor Tectonics San Clemente Fault System San Andreas Fault System (SoCal) Vertical Deformation as Yo-Yo Tectonics Catalina and San Onofre Horizontal Deformation as Telescoping (Rifts and Collisions) Hanging g Wall Erosion Roll-Over Collapse and Block Faulting Cristianitos Fault Coronado Bank Fault Zone 4D Tectonic Evolution of Rifting and Collision Spatial Variation in 3D and Temporal Variation Rheology Varies in 4D Strength changes with Time (Magmatism) Evolution of Low-Angle and High-Angle Faulting in Oblique Rifts Subsequent Basin Inversion and Transpression
Repeated Fault Patterns Eastern California Shear Zone Transverse Ranges presents major obstacle to NW transport of Baja California Microplate Northward Convergence at WTR SONGS 13 July 1986 Ms=5.8 NE-SW Convergence due to Big Bend in San Andreas New Faults Bypass Major Restraining Bends 14 Dec 2012 Mw=6.3
Low-Angle Faulting Multiple Levels of Detachment Exist Thirtymile Bank Detachment extends beneath Coronado Bank Detachment (Oceanside Blind Thrust?) Low-Angle Plane of Seismicity represents Seismogenic detachment beneath Coronado Bank Vertical-Axis Rotation of major crustal blocks (WTR) is facilitated by mid- or deep crustal detachments Original Detachment from Inner Borderland Rift was Dismembered by High-Angle Faulting San Diego Trough fault offsets Thirtymile Bank detachment Newport-Inglewood (South Coast) fault offsets Oceanside Blind Thrust Uncertain regarding geometry of deeper detachment at the coast (Thirtymile Bank/Santa Catalina beneath NIFZ-Rose Canyon?)
High-Angle Faults Offset Detachment Legg, Nicholson, and Sorlien, 1998
Tectonic Evolution of San Diego Trough PHASE 1 Oblique Rifting Strain Partitioning Oblique-slip on E-dipping 30MB detachment Right-slip in hanging wall on vertical wrench faults Roll-Over forms in hanging wall block (Coronado Bank) PHASE 2 Lateral Translation Right-slip on vertical fault cuts through detachment when extension ceases Coastal Fault System Transpression may result in Basin Inversion and Reactivation of low-angle faults
Borderland Tectonic Models and the Formation of Restraining and Releasing Bends Wrench Fault Tectonics Left-Stepping Fault Pattern for Right-Slip ~280 km Right-Slip Borderland NW-Trending Faults (D. Howell, 1974) Oblique-Rifting Gulf of California Right-Stepping Transform Faults Link Basins California Continental Borderland (R. Yeats, 1968) Basin and Range Continental Extension Yo-Yo Tectonics Vertical Deformation as Yo-Yo Tectonics (A. Junger, 1976?) Porpoising along Sinuous Strike-Slip Fault Zones (J. Crowell, 1962) Horizontal Deformation as Telescoping (Rifts and Collisions) Basin Formation (Oblique Rifting) and Inversion (Transpression) due to changing Relative Plate Motion Vectors (Legg et al. 2007)
Transpressional Fault Wedges SJH Capistrano Embayment Crespi Knoll
Location of Seismic Reflection Profiles Offshore SONGS Capistrano Embayment Lasuen Knoll Zone of Extensional Faulting (QT- Inversion?)
Landslide Analog of Transpressional Fault Systems Laminated Beach Sands in Dextral Transpression San Onofre State Beach (1998)
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Landslide Analog of Transpressional Fault Systems Map View of Dextral Transpression San Onofre State Beach (1998)
Landslide Analog of Transpressional Fault Systems Laminated Beach Sands Folded and Faulted along North Edge of Slide Block
Transpressional Fault Rupture Patterns El Asnam Earthquake 10 Oct 1980 M7.3 Right Oblique- Reverse Faulting Left Oblique- Reverse Faulting
W5-82sc Line 3-218 Spatial Nyquist 30 Hz Newport-Inglewood F.Z. Interbed (Peg-Leg) Multiples?
July 13, 1986 Offshore Oceanside Ms=5.8 Legg, Kohler, Shintaku & Weeraratne, 2013 (in revision)
Large Transpressional Earthquake Santa Catalina Mw=7.6 1981 * 1986 * Legg, Borrero, & Synolakis, 2004
Paleoseismic Event(s) - Carlsbad Buried Scarp warps lower paleosol (ca 40ka) Light-colored sands reworked from sand blows bury fault scarp at colluvial wedge Fault has 8-inches vertical separation, but slickensides are sub-horizontal (strike-slip) Younger faulting continues through upper paleosol to unknown level Liquefied sand injected into overlying cohesive deposits during ancient earthquake
Coastal Fault System - Earthquake Potential Large complex fault ruptures are likely Complex faulting along boundaries of crustal blocks remnant from Miocene rifting Variable rheology of geological units within and adjacent to active blocks control pattern of deformation including magnitude of slip Deep blind thrust or detachment surfaces may be seismogenic Multiple levels of detachment surfaces exist due to the complex tectonic evolution Vertical and high-angle crustal faults may merge with or truncate lowangle shear zones remnant from Mesozoic subduction and Neogene rifting Depth of detachment relative to brittle-ductile transition will determine potential for seismic versus aseismic slip Paleoseismic evidence of large Holocene earthquakes exists Liquefaction, faults, fractures and fissures, and tsunami deposits are recognized in the South Coast Region
Location of Seismic Reflection Profiles Offshore SONGS
Coastal Fault System Vertical Profile Newport-Inglewood fault zone is sub-vertical to 3.5 sec twtt (~5.6 km*) Oceanside Blind Thrust dips gently east; poorly imaged below 2.5 sec twtt (~3.4 km*) Oblique-Reverse faults (San Mateo) have moderate dip above OBT Tilted Block Oceanside Detachment *Velocity-Depth Conversion based on Tolmachoff (1993) Reprocessed Chevron 2-D Seismic
Coastal Fault System Vertical Profile Newport-Inglewood fault zone is sub-vertical to 2.5 sec twtt (~3.4 km) Oceanside Blind Thrust dips gently east; poorly imaged below 2.5 sec twtt (~3.4 km) Oblique-Reverse faults (San Mateo) have moderate dip above OBT Tilted Block Oceanside Detachment Reprocessed Chevron 2-D Seismic
Coastal Fault System Vertical Profile Newport-Inglewood fault zone is sub-vertical to 4.5 sec twtt (~8.7 km) Oceanside Blind Thrust dips gently east; poorly imaged below 2.5 sec twtt (~3.4 km) OBT appears offset by NIRC, but image is confused by Multiples(?) Tilted Block Oceanside Detachment Reprocessed Chevron 2-D Seismic
Location of Major Fault Zones Offshore SONGS Lasuen Knoll Santa Catalina Island