CHAPTER 5
GENERAL CONCLUSIONS
5.1. Evolutionary History
Terrace treads of San Juan Creek and Bell Canyon have been mapped and correlated into 12 discrete levels representing former longitudinal stream valley profiles. The pronounced continuity of the terrace correlations indicates that formative influences reflect major regional activities such as climactic regimes and tectonic uplift, rather than minor localized depositional events resulting from sentiment loads, gradients, or stream velocities exceeding localized threshold factors that inhibit or permit deposition (Chapter 2.4).
Merritts et al. (1994) has shown that river valleys can have terraces that result from 2 entirely different regional depositional systems, including depositional controlling factors normal to upstream expansion of river systems creating sequential strath terraces (uplift may or may not be occurring), and glacio-eustatic depositional controls on downstream areas of a tectonically uplifting coastline creating thick alluvial fill terraces. Terraces throughout San Juan Creek and Bell Canyon consist of thick alluvial fill material, and so represent downstream terraces controlled by glacio-eustatic formative influences (Figure 5.1).
These downstream alluvial filled terraces can be further subdivided into estuarine thalassostatic sediment in the lower reach of the stream where glacio-eustatic fluctuation continually infills and incises the thalweg, and an upstream aggradational wedge accumulating as a result of gradient adjustment to the fluctuating base level. Terraces throughout this study area consist of poorly sorted (well graded) fill material representing deposition in the aggradational wedge reach of the San Juan Creek drainage basin. Original deposition of the terrace material probably results during the onset of interglacial cycles (transition from MIS 2 to MIS 1 for example) when sea level rises. Rise in sea level from 15 ka to 8 ka represents 1.6 cm of estuarine sedimentation and clastic wedge aggradation per year (Garrison, 2007). The filling of canyon valleys creates an unusually wide alluvial floodplain just as southwestern North America oscillates to a dryer climate. Valley widening and down cutting are probably associated with the onset of glacial stadial and interstadial climates cycles (MIS 3 to MIS 2 for example) when sea level (base level) begins to drop and southwestern North America becomes much wetter (Owen et al., 2003; Clark and Bartlein, 1995) allowing for increased stream power and down cutting (Figure 5.2).
In the case of San Juan Creek and Bell Canyon, valley widening is also associated with over steepening of the outside cut banks along broad curvatures in the river pattern preserving terraces along the inside banks of the curves (Section 2.3.2).
Both the ancient and modern thalwegs will consist of estuarine (thalassostatic sediment; downstream and stratigraphically lower) and a fluvial thalweg aggradational wedge (fill terraces; upstream and stratigraphically higher) all resulting from synchronously depositing environments imposed by glacio-eustatic sea level fluctuation. Estuarine thalweg material (thalassostatic) will only fill the incised fluvial valley to the highest point of sea level, after which aggradational fluvial sediment will continue filling the valley until a new gradient is established (Figure 5.1).
5.2. Dating and Uplift Rate Calculations
The correlation of fluvial terraces with marine terraces is possible because marine and fluvial terraces are depositionally and chronologically equivalent at the debauching mouth of San Juan Creek. Additionally, the correlation is feasible once fluvial terraces are equilibrated to sea level by subtracting the terrace elevation from the modern stream gradient profile at the location where the terrace projects perpendicularly onto the profile. Once equilibrated to sea level, an average elevation for each terrace level can be calculated and the terrace levels can be compared with marine terraces. Former longitudinal stream profiles T11c, T10, T8, T4, and T2 established throughout San Juan Creek and Bell Canyon in this study have been correlated with dated marine terraces 1, 2, 3, 4a, and 4b, respectively (Barrie et al., 1992; see Table 5.1 for ages).
Former longitudinal stream profiles that have been equilibrated to sea level (as indicated above) can be dated by comparison with the glacio-eustatic sea level fluctuation curve. The San Juan Creek fluvial terrace levels correlate with 9 sea level highstands ranging in age from 85 to 460 ka, showing that in addition to the 5 marine terraces mapped by Barrie et al. (1992), at least 5 more unmapped marine terraces could be present on this stretch of southern California coastline. Plotting these data on the glacio-eustatic sea level fluctuation curve indicates an uplift rate of 0.29 m/ka for the San Juan Creek study area. This uplift rate is very similar to uplift rates established by other researchers from Newport Beach to Dana Point with the exception of Taylor et al. (2006). The uplift rate established for this current study is the southernmost uplift rate above approximately 0.25 m/ka before the sudden change at San Onofre to 0.09 m/ka, suggesting that the Cristianitos fault, the Mission Viejo fault, and the Bell Canyon Lineament may well be part of a structural divide separating distinct tectonic regimes with significantly different rates of motion.
If uplift rates are similar throughout broad seismotectonic zones of southern California, and if they are distinctly separated by tectonic features such as the Bell Canyon Lineament, then this zonation may imply that west of the San Andreas fault there may exist large seismotectonic blocks within which uplift rates (and perhaps seismic rates?) move with relatively coordinated speeds and motions. Defining such blocks and their boundaries based on the rates of motion within them would be extremely helpful in understanding the remaining 10 mm/yr of Pacific/North American Plate boundary motion taken up by deformation west of the San Andreas fault.
5.3. Graben Gradient Changes, Slip Rates, and Recurrence Intervals
5.3.1. Direction of Uplift Rate Gradient
Displacement of terrace levels along San Juan Creek and in Bell Canyon indicate that the western side of the Mission Viejo fault has been downthrown relative to the eastern side. Offset terrace couples on the Cristianitos fault indicate that the east side of this fault has been downthrown relative to the western side creating a graben between the 2 faults. These 2 faults systems appear to cut across the Bell Canyon and Gobernadora Canyon Lineaments, indicating that the faults and the graben they form postdate the lineaments (Chapter 4.4).
By assessing the amount of tilt occurring during the uplift time of each former longitudinal stream profile, it is possible to see the trend of tilting through time. Analysis indicates that the amount of uplift rate gradient tilt decreases with increasing age. Plotting this trend indicates that uplift rate gradient tilting begins ~315 ka (Chapter 4.3), just after the initial activity on the Mission Viejo fault commenced (see below for fault activity analysis). Analyzing the direction of tilt indicates that before 125 ka, tilting oscillations were oriented within the NE/SW compass quadrants, but after this time, oscillations were oriented within the NW/SE compass quadrants (levels T10 and T11). This switch in tilting directions may signal a possible change in the stresses controlling seismic and tectonic activity within the downdropped graben between these 2 faults. Specifically, this orientation may indicate that before 125 ka (or 195 ka as indicated by the graben analysis), the northern portion of the Mission Viejo fault and the southern portion of the Cristianitos fault were more active, orienting gradient tilting in these 2 directions. After 195 ka, activity may have shifted to the northern Cristianitos fault and the southern Mission Viejo fault.
5.3.2. Slip Rates and Recurrence Interval
Assessing the amount of displacement of former valley profiles over the discrete time intervals of their offset suggests that activity on the Mission Viejo fault is still present, and that the slip rate is increasing. Plotting the offset of terrace levels through time (the slip rate intervals between various terrace levels) indicates that the slip on the Mission Viejo fault began ~345 ka and has attained a current rate of 0.057 m/ka (Chapter 4.4). Using this rate of slip and a maximum displacement based on the entire length of the fault, the maximum recurrence interval is calculated to be 19.5 ka. As discussed above, if its fault ruptured in smaller segments, then the recurrence interval will be more frequent. In comparison, rates of motion (slip rates and recurrence intervals) on other faults in southern California are well known.
The San Andreas fault at Wallace Creek on the Carrizo Plain has an average slip rate of 35 mm/yr for the last 13,000 years (Sieh and Wallace, 1987), and at Cajon Creek near Cajon Pass the average slip rate is 24 mm/yr (Weldon, 1987). The recurrence interval on the San Andreas fault varies greatly, ranging from <20 years (at Parkfield only) to over 300 years within the Salton Trough (Fialko, 2006; Petersen and Wesnousky, 1994). Major faults southwest of the San Andreas included the Banning (as opposed to the Mission Creek fault), San Jacinto, and Elsinore faults. The San Gorgonio Pass (Banning) fault zone may have a slip rate of 6-13 mm/yr (Sieh and Wallace, 1987), but other authors indicate that the total amount of Holocene right slip on this fault zone remains unclear (Dillon and Ehlig, 1993). The recurrence interval for this fault is also uncertain. The San Jacinto fault motion may range from 10-15 mm/yr (Dillon and Ehlig, 1993) to 20-25 mm/yr (Dorsey et al., 2001), and the interval between surface ruptures ranges between 100 and 300 years, per segment (Petersen and Wesnousky, 1994). Establishment of rates of motion for the Elsinore fault are hampered by the lack of distinctive units within the Tertiary rocks precluding calculations of a slip rate from a distinctive piercing point. Estimates indicate a possible 10 km of total horizontal movement coupled with 1 km of vertical movement since Late Pleistocene suggesting a slip rate of ~5 mm/yr (Hull, 1990; Dillon and Ehlig, 1993). The interval between major ruptures may be ~250 years (Petersen and Wesnousky, 1994). Closer to the study area to the south (San Diego, La Jolla, and Linda Vista), the Rose Canyon fault zone has a length of ~30 km and a minimum slip rate of 1.1 mm/yr. The recurrence interval of this fault remains uninvestigated. To the north (from Costa Mesa and Newport Beach to Culver City), the Newport-Inglewood fault zone has a length of ~75 km and a slip rates of 0.6 mm/yr. The recurrence interval of this fault is also unknown (Jennings, 1994; Petersen and Wesnousky, 1994).
Although several terrace couples are offset by the Cristianitos fault, only 1 couple is closely spaced enough to assess fault displacement. The offset between these 2 terraces yields a slip rate of 0.03 m/ka and a recurrence interval of 52.3 ka. Given the dearth of data, these numbers for the Cristianitos fault can be considered only as rough estimates. However, if the slip rate on the Cristianitos fault is increasing as seems to be the case on the Mission Viejo fault, then the current slip rate may be larger than this calculation estimates. This will cause the recurrence interval to be more frequent, as will also the rupturing of the fault in smaller segments as discussed above. Shlemon (1992) states that on the coast adjacent to San Onofre, an intensive study using a variety of dating techniques has indicated that no movement has occurred at this location for at least 125 ka. A recurrence interval of ~52 ka seems incongruous with the fact that the southern Cristianitos fault has not ruptured for the last 125 ka. There are 3 possible reasons for this incongruency.
First of all, the recurrence interval calculated for the Cristianitos fault is based only on 1 set of offset terraces. This lack of data provides no statistical corroboration for the recurrence interval, so the interval calculation may be anomalous. The second possible reason for the incongruency between the calculated recurrence interval and the unruptured portion of the southern Cristianitos fault may lie in the fact that recurrence intervals are statistical averages, and therefore, may contain intrinsic variability in the periodicity of seismic events. Since the ~52 ka recurrence interval is only a statistical estimate, it could represent the correct interval even though a 72 ka hiatus has occurred between predicted fault ruptures. The recent earthquake on the San Andreas fault near Parkfield, California is an example of this variant behavior. This segment of the fault ruptures with unusual regularity in timing and magnitude, providing a recurrence interval of 22 years, based on historic seismic events in 1857, 1881, 1901, 1922, 1934, and 1966. If this 22-year periodicity were an accurate indicator of seismic events, the next earthquake should have occurred in 1988, but instead it occurred in 2004, representing a deviation time of almost a full recurrence interval (Bakun et al., 2005).
Another example of seismic event variation from recurrence interval periodicity is represented by the Mission Creek fault of the San Andreas fault zone. The average recurrence interval in this area of the Coachella Valley is ~215 years, with the last major events occurring ~326 years ago. This represents a hiatus of over one half the periodicity of the recurrence interval (Fumal et al., 2002). Sykes and Menke (2006) suggest that intrinsic variability in recurrence intervals may be small for very active faults where deformation is relatively simple, but for multibranching faults, and faults in uplifted terrains near subduction zones, recurrence intervals may have larger variability and more irregular earthquake recurrence. This description seems applicable to the segmented, branching, and complexly graben-associated Mission Viejo and Cristianitos faults transecting uplifted terraces on a tectonically active coastline. The third possible reasons for variability between the Cristianitos fault ~52 ka recurrence interval and the 125 ka unbroken southern end has been established in this study by the graben tilting analysis (uplift rate gradients, Chapter 4.3). As Figure 5.3 shows, uplift of the graben block between the Cristianitos and Mission Viejo faults has not occurred at a consistent rate and orientation in comparison with the blocks around it.
. Differences in the uplift orientation of the central block may be the result of uneven fault activity on the north and south portions of both of the Cristianitos and Mission Viejo faults that surround it. Figure 5.3 shows that 2 corners of the block must remain stationary to achieve the tilting (via uplifted or downdropping) of the graben block. For example, in order to lift/drop NE or SW corners of the graben block, the NW and SE corners must remain stationary (locked) to allow for the pivot of the block. Therefore, between 315 and 125 ka the south Mission Viejo fault and the north Cristianitos fault may have been largely inactive having a lower to nil slip rate. From 125 ka (or 195 ka as indicated by the graben analysis) to recent the north Mission Viejo fault and the south Cristianitos fault may have remained inactive. This analysis shows that the northern portion of the Cristianitos fault may be experiencing most of its recent activity (125 ka to present), while the southern portion has remained locked during the last 125 ka.
Ultimately, more data and analysis are needed to establish the validity of the recurrence interval estimated for the Cristianitos fault. This type of follow on study would include trenching at several locations combined with sample dating analysis to more accurately define rupture timing and recurrent intervals. Considering the slip rate and recurrence interval established for the Mission Viejo fault, this current study may indicate that a more accurate determination of fault rupture, fault displacement per seismic episode, and seismic slip rate is warranted.
In summary, seismic activity along the Cristianitos and Mission Viejo faults seems to represent normal faults on the dextral shear couple, the interaction of which has caused a graben to form between these 2 faults over the last ~345 ka. Regional uplift rates and seismic slip rates are in agreement with the rates of motion established by most other researchers in the area. Establishing the age of the most recent seismic events on these 2 faults (northern Cristianitos fault and the southern Mission Viejo fault) by means of a more in-depth follow on study would allow an assessment of current seismic hazards based on the recurrence intervals calculated above. Given the rapid urban development within this area, an establishment of seismic risk and hazard parameters for this area may be requisite.
APPENDIX
GIS MAP COORDINATE DETAILS AND AERIAL PHOTOGRAPH FLIGHT LINES
6.1. Map Coordinate Details
Correlation of terraces into levels was accomplished by the construction of 27 cross-sections which were digitized from a topographic base map, compiled on Microsoft Excel, placed as GIF illustrations onto the Adobe Illustrator, resized to match scales, and correlated using the transparency function. Field mapping of geomorphic terrace surfaces was accomplished by site visitation between September, 2005 and May, 2006. All map data was compiled on Transverse Mercator projection NAD27 (projection data listed in Table 6.1). An air photographic analysis of the drainage basin was conducted using aerial photographs from the Whittier College aerial photograph collection (flight lines as noted in Table 6.2). Digitized data indicating the terrace location, elevation, and number can be seen in Table 6.3.
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