Jay Namson

Structural transect of the western Transverse Ranges, California: Implications for lithospheric kinematics and seismic risk evaluation

A retrodeformable cross section that integrates surface and subsurface data across the western Transverse Ranges, California, illustrates an actively developing fold and thrust belt that began forming at 2-3 Ma. High-level thrusts are interpreted to root in a mid-crustal detachment at 12-15 km depth, which coincides with the maximum depth of earthquakes. The cross section documents 53 km of convergence above the mid-crustal detachment; dividing this by the time since onset of deformation yields convergence rates of 17.6-26.5 mm/yr. The high-density lithospheric anomaly beneath the Transverse Ranges is related to subduction of lower crust and lithosphere below the mid-crustal detachment to balance the shallow crustal shortening. Thrust ramps coincide with zones of high seismicity in the Transverse Ranges; this suggests that ramp regions have the highest potential for compressive earthquake events: e.g., the recent Whittier Narrows earthquake of October 1, 1987 (M/sub l/ = 5.9), occurred along the eastern part of a ramp zone.

Late Cenozoic Fold and Thrust Belt of the Southern Coast Ranges and Santa Maria Basin, California (1)

Balanced cross sections across the Santa Maria basin and southern Coast Ranges of California show the Pliocene to Quaternary convergence to be a developing basement-involved fold and thrust belt. The fold and thrust belt is seismically active as evidenced by compressive earthquakes, geodetic measurements of present shortening, and folded Quaternary deposits. The Pliocene and Quaternary structure of the southern Coast Ranges is dominated by a series of large anticlinal structures that include the Lompoc-Purisima trend, the Casmalia-Orcutt trend, the Point San Luis anticline, the Santa Lucia Range anticlinorium, and the La Panza Range anticlinorium. The anticlinal trends are interpreted to be fault-bend and fault-propagation folds resulting from thrust ramps off thrust flat and a regional detachment at 11-14 km depth. Most of the thrust faults do not reach the surface (blind thrust). The southwestern range front of the San Rafael Mountains is interpreted to be the result of uplift above a ramp in the Point San Luis thrust. The cross section across the southern Coast Ranges shows 26.8 km of shortening from the edge of the continental margin to the San Andreas fault. Region-wide shortening is interpreted to have begun between 2.0 and 4.0 Ma, which yields an average regional convergence rate of 6.7-13.4 mm/yr. The late Pliocene and Quaternary convergence across the onshore Santa Maria basin is interpreted to be 9.2 km and the average convergence rate is 2.3-4.6 mm/yr. Compressive focal mechanisms and our structural analysis suggest that the ramp parts of the thrust system are the most likely sources for earthquakes. The historic seismic record and length of the anticlinal trends suggest that the thrust ramps probably are capable of generating moderate to large earthquakes (5.0<M[W]<7.5). Major thrust ramps underlie the city of San Luis Obispo and adjacent coastal towns. Cross section restoration reveals early formed hydrocarbon traps and accounts for the abundant hydrocarbon accumulations along the Casmalia-Orcutt anticlinal trend and under the Santa Maria Valley. The cross sections and restorations also illustrate the importance of Miocene and early Pliocene normal faults to oil maturation and trapping. Concealed normal faults along the flanks of the major anticlinal trends and the subthrust area along the north side of the Casmalia-Orcutt anticlinal trend form two relatively untested trap styles.

Field excursion: Petroleum traps and structures along the San Andreas convergent strike-slip plate boundary, California

Fault-related fold models that illustrate the geometry and kinematic development of petroleum traps and structures are frequently used to assist basin exploration and development of structurally complex oil fields. Worldwide, several petroleum-rich provinces are situated in convergent strike-slip settings with adjacent convergent structures that are commonly petroleum traps. Strain studies and modeling of these settings are dominated by the wrench fault model, and examples from the San Andreas fault plate boundary and its trapping influence on adjacent large oil fields in California abound (Wilcox et al., 1973). Use of this model in petroleum exploration and geologic education is problematic and can lead to poor choices and wasted drilling dollars. Here, we show at three field trip stops that the wrench model and its associated flower structures (Harding, 1976) and palm tree structures (Sylvester and Smith, 1976; Sylvester, 1988) fail to explain the oil trapping style and structure of the uppermost crust near the San Andreas fault (Figure 1). Figure 1. Map of southern and southern-central California showing the three field trip stops, petroleum basins, oil fields, the San Andreas fault (SAF), and many of the regional cross section lines constructed by Namson and Davis since 1988. Oil fields are dominantly trapped by young, convergent structures that are the result of late Cenozoic transpression along the SAF plate boundary, which will be demonstrated at the field trip stops. Stop 1 is at the Wheeler Ridge oil field and the convergent San Emigdio Mountains, Stop 2 is along the western big bend segment of the SAF, and Stop 3 is at the Russell Ranch oil field and the convergent Caliente Range. The Neogene basins of southern California are very oil prolific (with a cumulative production of nearly 20 billion bbl of oil and daily production now at 560,000 BOPD). Much of the …