I would also like to thank the friendly atmosphere in the laboratory maintained by the faculty, staff and students. I would also like to thank my faithful badminton partner, Yar Hsu, for all the training and entertainment.
List of Tables
Introduction
Generally, the deep Moho is observed in the relatively undeformed block of the Western Peninsula Range (WPR), the western Sierra Nevada (SN), and the San Bernardino Mountains. The shallowest Moho is observed in the present extent of the Salton Trough and the southern extent of the Death Valley Fault Zone in the Mojave Desert.
Seismic Refraction Evidence for Steep Faults Cutting Highly Attenuated
Continental Basement in the Central Transverse Ranges, California
Abstract
Introduction
The first overthrust involved the latest Cretaceous–Early Paleogene subduction of the shallow segment of the Farallon Plate [Saleeby 2003]. It provides a large-scale, high-resolution seismic data set in the Transverse Ranges section of the modern plate margin.
Analysis of the Refraction Data
The unusual low-speed segment (the second 4 km/s segment away from the bulkhead) is observed to the left of the fault. The resulting model and comparison of the synthetic first arrivals to data are shown in Figure 2.8.
Correlation of Seismic and Geologic Structure
The peak points of the regression segments in the synthetic data are almost aligned, which is consistent with the real data. So we deduce that the dip angle of the Pelona fault is about 40◦−60◦ south.
Conclusions
A 2-D velocity structure was therefore constructed as a preliminary model from the near-missing part of the data. The Pelona fault was a major active south-dipping normal fault during the late Oligocene-early Miocene formation of the Soledad Basin.
Acknowlegements
Between the shale and the Cenozoic strata lie the strongly decomposed, intensely cracked and differently hydrated remnants of the upper "granite". These low velocities reflect the highly degraded structural and textural state of the upper plate basement rocks in response to multiple extensional and subsequent compressional deformations.
Regional Mapping of Crustal Structure in Southern California from Receiver
Functions
Introduction
All this indicates that further research into the crustal structure beneath the San Gabriel Mountains is needed to resolve the controversy. The RF method has the advantage that it provides a very good local estimate of the crustal structure beneath a station.
Data and Analysis
Selection of the water level and Gaussian parameters for each record is determined by the pre-signal noise levels. Since estimates of the Moho depth depend only weakly on the mean crustal P velocity, it is assumed to be 6.3 km/s for all stations for simplicity and consistency. For groups that fail these criteria, but have prominent and consistent Pms phases, such as the SW station group of VCS (Figure 3.4), the average pile traces are included in the cross-sections of the corresponding provinces (such as Figure 3.7).
Individual traces of the RFs are shown at the top, sorted by beam parameter, and arrival times for the three phases Pms, PpPms and PsPms are marked by the dashed lines.
Results
- Southern Sierra Nevada and Walker Lane Belt
- Peninsula Range and the Salton Sea
- Mojave Desert and San Bernardino Mountains
- San Gabriel Mountains
The shallowest Moho is observed at the northern tip of the Salton Sea at about 128 km along the profile. The blue crosses are the surface projection of the Moho bore points for different sets of stations. For the LARSE I profile, only the first step associated with shallowing is observed from the northern extent of the root of the San Bernardino Mountains to the western Mojave Desert.
There is a fairly well-resolved image of the transition from the 3-layer structure to the 2-layer structure along the LARSE I profile (Figure 3.14 L).
Discussion
Complexities in the subsurface structure appear to correlate very well with the surface geology of the basement terrain map [Dibblee 1982b] for the San Gabriel Mountains (Figure 3.17). This study confirms the results of a previous study by Zhu [2000] that the San Andreas Fault, as well as the San Gabriel Fault and some other major faults in the San Gabriel Mountains. Examples for the first case include large differences in RF between stations along the LARSE I profile, between different back-azimuth groups for TA2, VCS, and other SCSN stations in the San Gabriel Mountains (Figure 3.14).
A common problem for all profiles is lateral variations in the Vp/Vs ratio.
Conclusions
Large lateral variations beneath the San Gabriel Mountains (SGM) are inferred from several types of evidence: (1) The existence of local deep Moho beneath some station groups in the western SGM and its absence beneath other stations as well as beneath the eastern SGM and Liebre- The sawmill mountains. This study also indicates a broad spatial distribution of the bright spot beneath the San Gabriel Mountains with a SE-SW dip direction in the southern part of the western San Gabriel Mountains. Crustal structure is complicated in the eastern Mojave Desert, especially beneath stations LDF and DAN.
Large lateral variations in crustal structure also exist beneath the Sierra Nevada and the nearby Walker Lane area.
Acknowledgements
Deep Moho of 35–39 km is observed along a sliver in the western Peninsular Ranges and the sharp transition from the deep Moho here to shallow Moho of 19–22 km to the east near the Salton Trough suggests that the crust of the western Peninsular Ranges are of continental origin and that the movement of the western Island Ranges away from the main body of the North American plate most likely involves the entire crust. A deep Moho of 36–39 is observed in the western Sierra Nevada and two stations in the Death Valley fault zone, while flat shallow Moho of 28–31 km is observed beneath the Coso geothermal area and the southernmost Walker Lane area. The existence of a deep Moho beneath the Walker Lane area suggests that the Moho is probably not as flat as previous reflection studies suggest beneath the Basin and Range.
A Notch Structure on the Moho beneath the Eastern San Gabriel Mountains
- Introduction
- Data
- Synthetic Models
- Discussion
- Conclusions
- Conclusion
Note the large difference in the Pms arrival time for the different aft azimuth groups. Note the similarity of the synthetic RFs for the notch structure to the data in Figure 4.6. The synthetic RFs are plotted in the same order as the models in Figure 4.8, with the synthetics for the notcha structure shown at the top left, etc.
This indicates a sharp contrast in the physical properties of the crust between these two blocks.
Appendix A
Effect of Faults on First Arrivals
However, the intersection of shot #108 should give a reasonable estimate of the layer thickness for the hanging wall side. The important points in the model are the layer penetration points (U for the hanging wall side and K for the footwall side), which are closely related to the dip direction of the fault. For the vertical fault described above (Figure 2.5), the kink point A of F1 is slightly to the hanging wall side of the fault, and the kink point X of F2 is ishtanθ away from the fault, with a distance between them in the range htanθ < D < 2htanθ.
If the fault is a thrust fault (Figure A.1b), the fault point A F1 is located well (at least htanθ) on the side of the hanging wall of the fault.
Appendix B
Receiver Functions for Individual Stations in the San Gabriel Mountains
Note the late Pms arrivals for NW RFs with back azimuth less than 320◦ , which indicate the existence of a deep Moho. Note the absence of the Pms phase for a portion of the NW RFs with backazimuth in the range of 310◦ to 318◦ , which is probably due to entanglement in the deep structure along the northern branch of the San Gabriel Rift (Figure 3.17). This RF group varies systematically with the radius parameter, which is shown in Figure 4.3 and is explained by the existence of a level in the Moho.
Note the large difference in the Pms arrivals for the radial components for events from the NW and those from the SE and SW.
Appendix C
Receiver Functions for Stations across the Salton Trough
Asymmetric Extension of the Salton Trough
Continental crustal changes, however, occurred at the edge of the block, particularly near the Salton Valley, where a shallow Moho of 25–28 km is observed. The asymmetric extension of the Salton Trough can be inferred by comparing the lateral variations of the Pms arrival times for the three western profiles with those in the eastern profile. For the 3 profiles in the west, the transition from the shallow Moho at ∼22–25 km below the Salton Valley to the deeper Moho of 33–37 km below the western peninsular range is quite sharp within a lateral distance of ≤40 km (as eg as along the central profile).
Stations, such as IRM and BLY, that are about 100 km from the axis of the Salton Trough still have a shallow Moho of 26–28 km.
RFs for Station JCS and Nearby Stations
For the central profile, the apparent level structure in the Moho at the horizontal distance of ~90 km is due to the difference in Pms arrival in the direction perpendicular to the projected BB0 line. The Moho drilling points for the eastern RF groups are located in the central block and are relatively closer to the Salton Valley where the shallow Moho is observed. RFs for another DPP station, at ∼33 km west of station JCS, show a similar deeper Moho of 34–37 km (Figure C.4) and thus the existence of deeper Moho for the western RF sets of JCS in the relatively intact western Peninsula Ranges is confirmed.
Note the low amplitude in the tangential components for the SW-W BAZ group and relatively larger amplitude for the E-SE-S and NW groups.
Appendix D
Anomalous Receiver Functions for
Stations in the Mojave Desert and Sierra Nevada
Mojave Desert Stations
Note the large difference in the Pms arrivals for stations from NW, SW-W and those from SE-S and the prominent negative pulse to Moho Pms for the SE-S group stations. The characteristic feature of this station is that a mid-crustal low-velocity zone and a relatively flat mid-crustal interface are observed, which are absent in other stations in the Mojave Desert, indicating the complexity of the crustal structure beneath this station. Interesting thing about the stacking of this station is that, for SE RF group Figure D.6, it has a later Pms arrival time, but a shallower Moho of ~27 km is inferred from the grid search due to the high Vp/Vs ratio ( 1.84) chosen for this group, while a slightly deeper Moho of 30 km is inferred for the NW group due to the lower Vp/Vs value (1.70) (Figure D.5).
The PsPms multiples for the NW group are not well defined and there is more than one local maximum contour, so finding the grid for this group is probably not problematic, even though the PpPms multiple is very prominent.
Sierra Nevada Stations
Note the early Pms arrival times and the relatively small Vp/Vs ratio for this group. Note the relatively late Pms arrival times and the large Vp/Vs ratio chosen for this group. A negative pulse at ~1.5–2 sec. is generally observed at the radial component of this station (Figure D.8), amplitude variation of this phase on the radial and tangential components indicates that it descends to the SW.
Note the early Pms arrivals at ~3.9-4.1 sec. on the radial components and the relatively small amplitudes on the tangential components.
Appendix E
Moho Depth and Vp/Vs Ratio for the Regional RF Study
The size of the circle represents the value of the Vp/Vs ratio, which is directly proportional to its difference by 1.75, with Vp/Vs ratios less than 1.75 shaded.
Bibliography
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Troxel, Structure of the Central Death-Valley Pull-Apart Basin and Proximity from Cocorp Profiles in the Southern Great-Basin, Geol.