Thrust-Induced Collapse of Mountains— An Example from the “Big Bend” Region of the San Andreas Fault, Western Transverse Ranges, California By Karl S. Kellogg Scientific Investigations Report 2004–5206 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey Charles G. Groat, Director U.S. Geological Survey, Reston, Virginia: 2004 For sale by U.S. Geological Survey, Information Services Box 25286, Denver Federal Center Denver, CO 80225 For more information about the USGS and its products: Telephone: 1-888-ASK-USGS World Wide Web: http://www.usgs.gov/ Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. iii Contents Abstract ……………………………………………………………………………………… 1 Introduction …………………………………………………………………………………… 1 Geology of the Mount Pinos and Frazier Mountain Region …………………………………… 3 Fracturing of Crystalline Rocks in the Hanging Wall of Thrusts ……………………………… 5 Worldwide Examples of Gravitational Collapse ……………………………………………… 6 A Spreading Model for Mount Pinos and Frazier Mountain ………………………………… 6 Conclusions …………………………………………………………………………………… 8 Acknowledgments …………………………………………………………………………… 8 References …………………………………………………………………………………… 8 Illustrations 1. Regional geologic map of the western Transverse Ranges of southern California …………………………………………………………………………… 2 2. Simplified geologic map of the Mount Pinos-Frazier Mountain region …………… 2 3. View looking southeast across the San Andreas rift valley toward Frazier Mountain …………………………………………………………………… 3 4. View to the northwest of Mount Pinos, the rift valley (Cuddy Valley) of the San Andreas fault, and the trace of the Lockwood Valley fault ……………… 3 5. View to the northeast of Frazier Mountain, showing the trace of the Frazier Mountain thrust ……………………………………………………… 4 6. Photograph, facing north, of hills underlain by Pleistocene landslide deposits ………………………………………………………………… 4 7. Photograph showing typical roadcut along Mount Pinos Road that exposes the highly fractured, cataclastically deformed, and argillically altered biotite gneiss ……………………………………………… 4 8. Equal-area, lower-hemisphere, stereographic projection of (A) poles to fractures measured along Mount Pinos Road and (B) poles to foliation measured throughout the Mount Pinos Massif ……………… 5 9. Three models for gravitational spreading of mountains after (A) Beck (1968), (B) Mahr (1977), and (C) Kellogg (2001b) …………………………… 7 10. Proposed models for gravitational spreading of the Mount Pinos and Frazier Mountain massifs ……………………………………………………… 7 11. View toward Liebre Mountain facing southeast from Interstate Highway 5 ……… 8 Thrust-Induced Collapse of Mountains—An Example from the “Big Bend” Region of the San Andreas Fault, Western Transverse Ranges, California By Karl S. Kellogg Abstract Introduction Mount Pinos and Frazier Mountain are two prominent The “Big Bend” region of the San Andreas fault (SAF), in the mountains just south of the San Andreas fault in the western western Transverse Ranges of southern California (fig. 1), is a zone Transverse Ranges of southern California, a region that has of relatively large transpression due to the left-stepping, right-slip undergone rapid Quaternary contraction and uplift. Both fault geometry (Davis, 1983). This transpression has produced mountains are underlain, at least in part, by thrusts that place widespread thrusting and folding, both north and south of the SAF, that has significantly increased crustal thickness (for example, granitic and gneissic rocks over sedimentary rocks as young as Namson and Davis, 1988). South of the SAF, the locus of Pliocene. Broad profiles and nearly flat summits of each mountain contractional tectonism has apparently migrated south during the have previously been interpreted as relicts of a raised erosion Quaternary to the Santa Barbara Channel (Rockwell, 1983), where surface. However, several features bring this interpretation folding and thrusting are currently active. In a roughly mirror-image into question. First, lag or stream gravels do not mantle the scenario, thrusting north of the SAF has migrated northward with summit surfaces. Second, extensive landslide deposits, mostly time, with a large component of shortening accommodated along pre-Holocene and deeply incised, mantle the flanks of both the south-dipping Pleito-Wheeler Ridge fault system (Davis, 1983). mountains. Third, a pervasive fracture and crushed-rock network In concert with thrusting, the western Transverse Ranges have pervades the crystalline rocks underlying both mountains. The been uplifted rapidly during the Quaternary, with rates of uplift orientation of the fractures, prominent in roadcuts on Mount estimated at 4–8 millimeters per year (Yeats, 1983). This rapid– Pinos, is essentially random. “Hill-and-saddle” morphology uplift has been accompanied by rapid erosion and dissection, as shown by thick Quaternary basin-fill and mountain-flanking deposits characterizes ridges radiating from the summits, especially on adjacent to most of the mountain uplifts (see Dibblee, 1982). Mount Pinos; outcrops are sparse on the hills and are nonexis- The author of this paper proposes a model in which tent in the saddles, suggesting fractures are concentrated in the hanging-wall rocks were weakened by widespread fracturing saddles. Latest movement on the thrusts underlying the two that resulted from movement along low- to moderate-angle mountain massifs is probably early Quaternary, during which thrusts that placed brittle, crystalline rocks of Cretaceous and the mountains were uplifted to considerably higher (although early Proterozoic age over structurally weak, mostly fluvial unknown) elevations than at present. A model proposes that sedimentary rocks of the Miocene Caliente and Pliocene during thrusting, ground accelerations in the hanging wall, Quatal Formations. Uplift and erosion during and subsequent particularly near thrust tips, were high enough to pervasively to thrusting and the resulting steepening of the mountain flanks, fracture the hanging-wall rocks, thereby weakening them and possibly driven by ongoing seismic activity, promoted episodic producing essentially an assemblage of loose blocks. Movement gravitational collapse of the mountains. over flexures in the fault surface accentuated fracturing. The The proposed model is speculative for several reasons, mostly due to the difficulty of constraining the ages of either thrusting or lowered shear stresses necessary for failure, coupled with deep the inferred associated gravitational collapse. Despite these dissection and ongoing seismic activity, reduced gravitational difficulties (which will be discussed further), one objective of potential by spreading the mountain massifs, triggering flanking this paper is to provide a testable alternative to the conventional landslides and producing broad, flat-topped mountains. This paradigm for peneplained mountaintop surfaces (see Dibblee, study developed from mapping in the western Transverse Ranges 1982) in active tectonic environments. This study evolved from as part of the U.S. Geological Surveyʼs Southern California mapping in the region as part of the U.S. Geological Surveyʼs Areal Mapping Project (SCAMP). Southern California Areal Mapping Project (SCAMP). 2 Thrust-Induced Collapse of Mountains—Transverse Ranges, California Figure 1. Regional geologic San Joaquin Valley 0 10 20 30 KM Quaternary alluvium map of the western Trans- verse Ranges of Southern Cuy Pliocene to ama PWRF lt Oligocene Val fau California, showing the Big ley k loc ar Eocene to Bend region of the San Mount Pinos G Area Mojave Upper Cret. Andreas fault and the location oAfREA OF FIGURE 2 Desert of Mount Pinos and Frazier fig. 2 Basement complex Mountain, modified from Jen- lt Big Pine fau Ridge S nings and Strand (1969) and Lieb A P Frazier Basin re M N ine oun AN Dibblee (1982). Faults show- Mo S tain D unta Mountain a R in fau n E ing teeth are thrust or reverse lt G A a S b faults, with the teeth in the r ie F Santa Ynez fault l AU hanging wall. PRWF is the fa L u T Santa Barbara lt Pleito-Wheeler Ridge fault. San Gabriel C Mountains A SA L N I TA F C B O H A San F R A R Ventura erna N BA ndo N NE R Valley I L A A Oxnard MAP AREA Santa Monica 119 15' W 34 119 W 52' PINE 30" MOUNTAIN CLUB San Emigdio Mountain N F i g . 1 S 3 San Andreas a Abel w m fault system Mountain ill CUDDY VALLEY thrust trhust 7 Cerro Noroeste Mount Pinos San Emigdio Mesa Road t Mount Pinos ul fa Frazier y Mountain lle a LOCKWOOD VALLEY V d oo 4 ckw Lo 6 Frazier 34 Mountain 45' thrust N Landslide Tertiary sedimentary Fracture, fault, scarp 5 deposits rocks Strike-slip fault Thrust fault Alluvial deposits Pelona Schist 0 1 2 3 4 5 KM (mostly fans) Road Basement 6 Photograph site; figure number rocks (arrow shows direction of photograph) Figure 2. Simplified geologic map of the Mount Pinos-Cerro Noroeste region. The camera position for photograph site 4 is about 2 kilometers southwest of position shown. Modified from Kellogg (1999, 2003) and Kellogg and Miggins (2002). The Lockwood Valley fault was formerly known as the eastern end of the Big Pine fault (Minor and Kellogg, 1997). F i g . 2 Geology of the Mount Pinos and Frazier Mountain Region 3 Geology of the Mount Pinos both mountain massifs are