Annual Report, 1997

One of the most structurally complex sections of the San Andreas fault system occurs where the San Andreas fault bisects the central Transverse Ranges in southern California (Figure 1, 1b). In this vicinity, which corresponds to the densely populated Inland Empire, several major deformational systems converge upon the San Andreas fault zone, including the convergent structures of the Los Angeles Basin on the west, the San Jacinto fault on the southeast, and the Eastern California shear zone on the northeast. The San Andreas fault zone is also inherently complex within the central Transverse Ranges, with numerous subparallel fault strands, a major restraining bend in San Gorgonio Pass, and a transpressive nature that contrasts with the transrotational regime in the Imperial Valley the southeast. Limits to our understanding of the hazards posed by this complex architecture of active deformation include deficiencies in the characterization of individual seismogenic structures as well as an inability to explain how these structures interact over single seismic cycles or during single seismic events. Thus there is a need to place new constraints on the kinematics and subsurface geometry of potentially active structures in the area, as well as to develop an understanding of how the structural elements interact by constraining the long term tectonic evolution of the San Andreas fault system in the vicinity of the central Transverse Ranges.

In the past year we have learned more about the architecture and behavior of this stretch of the San Andreas fault system, by continuing to study the uplift and tectonic evolution of the San Bernardino Mountains. As a result of transpression along the San Andreas fault zone in the past few million years, a component of plate motion has been accommodated by oblique reverse-slip and motion on thrust faults, from which high topography has been built. In keeping with SCEC goals, we have been working to characterize individual structures that have uplifted the San Bernardino Mountains. In addition, because these structures are produced by transpression, they probably interact with the San Andreas fault zone at depth and have played a sign)ficant role in its evolution. By studying the uplift tectonics, we have learned about the development of the transpressive strike-slip system and formed a greater understanding of the basic kinematic and mechanical interaction of structures.

We are now entering the final phase of this project. We spent roughly three-fourths of last year using radiogenic helium thermochronometry to study the uplift tectonics of the San Bernardino Mountains, and spent the remainder of our time on a detailed geomorphic study of a weathered granite surface atop the range. Our results are described below.

Radiogenic helium thermochronometry

We have placed new constraints on the timing and magnitude of uplift of crystalline blocks within the San Bernardino Mountains using radiogenic helium thermochronometry (in collaboration with Ken Farley, Caltech). We summarized our initial observations in last year's progress report, but have since completed the study, which will appear in Tectonics in early spring (1998). In the meantime, a preprint may be viewed at Caltech's Seismolab website (web-service.gps.caltech.edu/ preprints/).

Radiogenic helium dating offers information on the low-temperature cooling history of rocks, and is thus valuable for studying the most recent history of uplift from shallow depths to the surface. This thermochronometer is based on the thermally activated diffusion of 4He in granitic apatite, produced by radioactive decay of uranium and thorium. In apatite, helium is completely diffused at temperatures >100oC, partly diffused (and

partly retained) between 100oC and 40oC, and completely retained below 40oC (over geologic timescales). The method thus captures the timing of uplift through the upper few kilometers of crust for regions with typical geothermal gradients. In addition, isochrons constructed from helium ages can constrain the relative vertical motions between crystalline blocks or the post-cooling deformation within a block. This is possible, however, only if isochrons were relatively flat (due to small lateral variation in geothermal gradient and uniform cooling rate) across a region prior to relative vertical motions. We used this method in the San Bernardino Mountains because the timing and relative magnitude of uplift between crystalline blocks were not known. Uplift within the range initiated between 2 and 3 million years ago, based on stratigraphic arguments (May and Repenning, 1982; Meisling and Weldon, 1989), but prior to our work resolution of the timing for individual blocks and the post-2 Ma history of uplift was poor. The uplift magnitude of the main part of the range, the Big Bear plateau (Figure 1), was loosely constrained by the plausible correlation of deeply weathered granite surfaces atop it and on the floor of the surrounding Mojave Desert (Dibblee, 1975). However, the uplift magnitude of blocks south of the Big Bear plateau were unconstrained due to the lack of similar correlated surfaces. These blocks, the San Gorgonio, Wilson Creek, Yucaipa Ridge, and Morongo blocks, are higher and steeper than the Big Bear plateau, which suggests they have experienced greater uplift. However, the magnitude of uplift was not known because the amount of erosion from their tops was unconstrained. Thus our strategy was to date samples from granitic rock in each of the major blocks and compare their uplift timing and magnitude.

We determined 14 helium ages from the San Bernardino Mountains in order to meet the objectives of this study (Figure 1, 2). For detailed description of method and data, we refer to our upcoming article (Spotila et al., in press). A helium age represents the calculated time necessary to produce a measured amount of 4He, given measured amounts of U and Th. To constrain the possible thermal histories that could have produced a measured age, it is necessary to numerically model the rates of helium production and diffusion and the use suites of helium ages as well as other thermochronologic data. Once the thermal history is constrained, useful geologic information on the history of uplift through the upper few km of crust can be found.

Helium ages from the Big Bear block range from 20 to 64 Ma. These dates predate uplift and reflect a slow cooling history since the Late Cretaceous (Figure 2). Thermal histories that numerically reproduce these ages and K/Ar ages from nearby rocks atop the plateau (Miller and Morton, 1980) indicate that the block was cooling so slowly that it could have been stationary within the crust for a long period during the Tertiary and exhumed only several km since the Cretaceous. This is consistent with the proposed age of the weathered surface atop the plateau (Oberlander, 1972), which requires essentially no erosion since ~10 Ma. Isochrons constructed from these ages show a tilt similar to that shown by the weathered granite surface across the plateau. This suggests the weathered surface formed parallel to the crustal level defined by the helium system. This is consistent with the hypothesis that the block was exposed to deep weathering in the late Miocene and subsequently uplifted along the thrusts, because it does not require post-uplift beveling to explain the shape of the plateau and shows no internal deformation during uplift.

Helium ages from the San Gorgonio block are similar to those from the Big Bear block (~56-14 Ma) and also predate the recent uplift of the San Bernardino Mountains. The geometry of isochrons constructed from these ages suggests that the weathered granitic surface atop the block was originally contiguous with that atop the Big Bear plateau. Thus we conclude that the San Gorgonio block has experienced roughly 1.5 km more uplift than the Big Bear block and that this uplift is well represented by the topographic form of the block. The structure of the block appears to be a northward plunging, gentle antiform, based on the orientations of geologic datums and tilted helium isochrons (Figure 3) (Spotila et al., in press). Such a structure is better explained by uplift due to local complexities associated with the high-angle faults that bound it, than by uplift along the northern thrust.

The helium ages from the Wilson Creek and Yucaipa Ridge blocks within the San Andreas fault zone are astonishingly young (0.7-1.6 Ma) and indicate recent, rapid cooling. These ages suggest thermal histories that require ~3-4 km of uplift in the past few million years. This magnitude of uplift indicates that the Yucaipa Ridge block would have stood at least 1 km above the present topography of the San Gorgonio block, if no erosion had occurred (Figure 4). Such uplift would have proceeded at a minimum rate of ~1.5 mm/yr, but could well have been shorter lived and therefore more rapid (>10 mm/yr). This high rate and magnitude of uplift is confined to these crustal slices within the San Andreas fault zone, and implies local uplift associated with strike-slip faulting as opposed to displacement on the North Frontal thrust system.

These results show that the range consists of several tectonic blocks that have risen by different mechanisms associated with transpression. Helium ages are consistent with uplift of the Big Bear block along the North Frontal thrust system and support the hypothesis that the present plateau topography roughly represents structural displacement. The greater magnitude of uplift represented by the smaller southern blocks, however, is not the result of motion along the northern thrust. The additional uplift of these southern blocks resulted from more local structures. The San Gorgonio block is bound by poorly characterized high angle faults on the north and the Mill Creek fault on the south. The Yucaipa Ridge-Wilson Creek blocks are bound by the Mill Creek fault on the north and the Mission Creek fault on the south. Oblique reverse motion due to geometric complexities along these steeply dipping strike-slip faults are required to uplift these blocks

We propose several possible mechanisms for uplift of southern blocks along the strike-slip faults that bound them (Figure 5). Of these possibilities, we favor oblique slip or slip partitioning along dipping strands of the San Andreas fault zone (Figure 5a), which would be a long-term example of the style of rupture in the Loma Prieta earthquake, or reverse motion within a left-step or bend in the San Andreas fault zone (San Gorgonio Pass) (Figure 5b). It is not yet possible to discriminate between these mechanisms, however, because the uplift timing of all blocks and the histories of motion along the strikeslip faults are not adequately constrained. Regardless of the mechanism, the uplift has probably accommodated at least several kilometers of right-slip, a sign)ficant component of San Andreas motion. It is not clear if this uplift within the San Andreas fault system occurs during strike-slip earthquakes or separate, purely reverse events. Likewise, these results suggest that the Mill Creek fault may still be active, in contrast to previous hypotheses, and that the high angle faults that bound the San Gorgonio block on the north represent potential seismic structures. Lastly, the results argue that the North Frontal thrust system is primaril responsible only for uplift of the Big Bear block, and may not continue further south under the range.

Deep granitic weathering atop the Big Bear plateau

We have continued to study the origins of weathering atop the Big Bear plateau, in an effort to determine if the weathering surface represents a structural datum that constrains the magnitude and geometry of vertical displacement of the plateau. If so, the distribution of uplift can be used to learn about the subsurface geometry of the North Frontal thrust system. So far, we have expanded our 1996 results by examining the weathered surface in the field, including soil profiles and laboratory analysis of soil characteristics. Our geomorphic study of the surface is in the early stages, and our plan for future work is described in our 1998 SCEC proposal.

References:

Dibblee, T.W., 1975. Late Quaternary uplift of the San Bernardino Mountains on the San Andreas and
related faults, in Crowell, J.C., ea., San Andreas fault in southern California, California Division of
Mines and Geology Special Report, 118, 127-135.
May, S.R. and Repenning, C.A., 1982. New evidence for the age of the Old Woman Sandstone, Mojave

Desert, California, in Sadler, P.M., and Kooser, M.A., eds., Late Cenozoic stratigraphy and structure of the San Bernardino Mountains, Geol. Soc. Amer. Cordilleran Section Meeting Guidebook, 6, 93-96,

1982.

Meisling, K.E. and Weldon, R.J., 1989. Late Cenozoic tectonics of the northwestern San Bernardino Mountains, southern California, Geol. Soc. Amer. Bull., 101, 106-128.

Miller, F.K. and Morton, D.M., 1980. Potassium-argon geochronology of the eastern Transverse Ranges and southern Mojave Desert, southern California, U.S. Geol. Survey Professional Paper, 1152, 30p.

Oberlander, T.M., 1975. Morphogenesis of granitic boulder slopes in the Mojave Desert, CA, Journal of Geology, 80, 1-20.

Spotila, J.A., Farley, K.A., and Sich, K., (in press). Uplift history of the San Bernardino Mountains along the San Andreas fault, California, constrained by radiogenic helium thermochronometry, Tectonics.