Geological Society of America Special Paper 365 2002

Middle Miocene reconstruction of the central and eastern San Gabriel Mountains, southern California, with implications for evolution of the San Gabriel fault and Los Angeles basin

Jonathan A. Nourse* Department of Geological Sciences, California State Polytechnic University, Pomona, California 91768, USA

ABSTRACT

Geological mapping of basement rocks in the central and eastern San Gabriel Mountains reveals a consistent range-scale tectonostratigraphic sequence dismem- bered into diamond-shaped blocks by late Cenozoic strike-slip faults. These faults and contemporaneous reverse faults record brittle response of the San Gabriel block to stresses that accompanied development of the transform plate boundary in south- western California. Sequential restoration of northwest-striking right-lateral faults and northeast-striking left-lateral faults is accomplished in map view by matching distinct Proterozoic and Mesozoic crystalline sheets that occur at similar structural levels. This procedure removes 16 km of Quaternary dextral slip on the Scotland–San Jacinto–Glen Helen fault system, variable (1–3 km) of Pliocene-Quaternary sinistral slip on the Stoddard Canyon, San Antonio Canyon, Sunset Ridge, San Dimas Canyon– Weber, and Pine Mountain faults, and 22 km of late Miocene dextral displacement on the north branch San Gabriel–Icehouse Canyon–Lytle Creek fault system. The model also incorporates 15 km of late Miocene dextral displacement on the Sawpit Canyon–Clamshell fault, feeding slip from the south branch San Gabriel fault onto the eastern segment of the north branch. Effects of late Cenozoic shortening within the San Gabriel block are not restored. The deepest exposed portion of the tectonostratigraphy is the Pelona Schist, a polydeformed assemblage of metagraywacke, metabasalt, and metachert, structurally overlain by greenschist-facies mylonite derived from Paleoproterozoic gneiss and Me- sozoic plutons. Especially conspicuous upper-plate marker units include Paleoproter- ozoic porphyritic granite augen gneiss, the Late Triassic Mount Lowe intrusion, and Late Cretaceous quartz diorite or granodiorite. The remarkable along-strike conti- nuity of these crystalline sheets provides multiple piercing lines that project across the San Andreas fault into the Chocolate Mountains. Palinspastic reconstruction incorporating 160 km restoration of the San Gabriel block along the San Andreas fault yields a middle Miocene paleogeography containing several provocative features: (1) continuity of the Vincent thrust and Mount Lowe intrusion between the San Gabriel Mountains and Chocolate Mountains (2) reassem- bly of the late Oligocene Telegraph Peak–Mount Barrow granite and associated rhyolite-dacite intrusions, and (3) demarcation of a broad zone of transtension within

*E-mail: [email protected]

Nourse, J.A., 2002, Middle Miocene reconstruction of the central and eastern San Gabriel Mountains, southern California, with implications for evolution of the San Gabriel fault and Los Angeles basin, in Barth, A., ed., Contributions to Crustal Evolution of the Southwestern United States: Boulder, Colorado, Geological Society of America Special Paper 365, p. 161–185. 161 162 J.A. Nourse

a right step between the early-mid Miocene Clemens Well fault and an inferred bor- derland transform fault. The transtensional zone contains a middle Miocene mafic intermediate dike swarm emplaced along conjugate shear fractures in San Gabriel basement rocks. This zone is bounded on the east by the ancestral San Antonio Can- yon fault, the south end of which facilitated opening of the Los Angeles pull-apart basin.

INTRODUCTION is related to the San Andreas and the Sierra Madre–Cucamonga fault systems that form its north and south boundaries, respec- The San Gabriel Mountains (Figs. 1–2) contain a compli- tively. Despite proximity to the Los Angeles metropolitan area, cated assemblage of crystalline rocks disrupted by a network the range has yet to be completely mapped at 1:24,000 or even of brittle faults associated with late Cenozoic development of 1:50,000 scale. In addition to geologic complexity, other obsta- the Pacific-North American transform plate boundary. Prote- cles include rugged relief (locally Ͼ2500 m), the inherently rozoic and Mesozoic rocks of the San Gabriel block have long unstable nature of slopes that have lost cohesion because of been utilized as piercing points for reconstructions of the south- brittle cataclasis, and persistence of chaparral and poison oak ern San Andreas fault system (Crowell, 1952, 1962; Hill and at lower elevations. Fortunately, a network of highways, fire Dibblee, 1953; Ehlig, 1968, 1975; Powell, 1993; Matti and roads, and fire breaks provides good exposures that can be cor- Morton, 1993; Dillon and Ehlig, 1993). All of these tectonic related with accessible outcrops in canyon bottoms. Figure 2A models position the San Gabriel Mountains somewhere near shows Perry Ehlig’s 1975 compilation of bedrock geology in the present-day Salton Sea during middle Miocene time, east the San Gabriel block. Field and geochronological studies bear- of the Frazier Mountain block and west of the Orocopia and/or ing on rock unit correlations are summarized below. Chocolate Mountains (compare Figures 1A–C). However, Mio- The earliest descriptions of San Gabriel Mountains base- cene configuration of the San Gabriel block and its geometric ment rocks are detailed by Arnold and Strong (1905) and Her- relationship to the future Los Angeles basin remains contro- shey (1912). Hershey proposed an Archean (pre-Belt Series) versial, largely due to differing interpretations of displacement age for the schist of Sierra Pelona, but it is unclear whether the on various branches of the San Gabriel fault (Figs. 1B and 1C, larger body in the eastern San Gabriel Mountains was mapped. see also Powell, 1993). This paper examines details of the San Subsequently, Pelona Schist was reported to be truncated on the Gabriel fault system within the San Gabriel Mountains, and northeast by the San Andreas fault (Noble, 1926). However, the presents a middle Miocene palinspastic reconstruction of the km) of strike-slip displacement on םlarge magnitude (200 region. New constraints on Miocene activity of the San Gabriel the San Andreas fault was not postulated until much later (Hill fault and associated faults provide fresh insight into the kine- and Dibblee, 1953; Crowell, 1962). Recognition of Pelona-type matic development of the Los Angeles basin. schist in the Orocopia/Chocolate Mountains played an impor- Reconstruction of the San Gabriel Mountains basement re- tant role in these interpretations. In the meantime, mapping in quires correlation of distinct metamorphic and plutonic sheets the central San Gabriel Mountains (Miller, 1934) delineated a between multiple fault blocks. Field relationships described in this paper include mapping by Perry Ehlig (1958, 1975) north Mesozoic(?) batholith (Mount Wilson diorite) intruded into of the San Gabriel fault. Ehlig’s work is integrated with recent banded gneiss and augen gneiss of presumed Precambrian age. mapping (Nourse et al., 1998a, 1998b; Nourse, unpub.) and Perry Ehlig devoted much of his career to mapping the other studies (Morton, 1973; Dibblee, 1982; Jacobsen, 1983; eastern and central San Gabriel Mountains high country. His May and Walker, 1989; Morton and Matti, 1991; Norum, 1997) significant contributions include detailed descriptions of to define a coherent tectonostratigraphy throughout the San Ga- sheared rocks along the Vincent thrust (Ehlig, 1958, 1968, briel Mountains block east of Mount Wilson (Fig. 3). Unique 1975, 1981) and definition of petrologic zones within the Mount offsets of this tectonostratigraphic sequence and crosscutting Lowe intrusive complex (Ehlig, 1981; Barth and Ehlig, 1988). relationships involving early Miocene and middle Miocene in- Other noteworthy publications proposed cross-fault correla- trusive rocks constrain the kinematics of late Cenozoic strike- tions of the Pelona Schist, Mount Lowe intrusion, and middle slip faulting in the region. Tertiary conglomerate to localities northeast of the San Andreas fault in the Orocopia/Chocolate Mountains region (Ehlig, 1975; PREVIOUS WORK IN THE SAN GABRIEL Dillon and Ehlig, 1993). Of particular interest to this paper is MOUNTAINS BLOCK evidence for 22 km of dextral displacement on the north branch of the San Gabriel fault and speculation for early right-lateral The San Gabriel Mountains block (Fig. 2A) preserves a movements on the Sawpit Canyon fault (Ehlig, 1981; Dibblee, protracted history of tectonism, only the latest phase of which 1982; Dillon and Ehlig, 1993). Implications for evolution of the San Gabriel fault and Los Angeles basin 163

A growing body of geochronological data has aided cor- plate, composed of Proterozoic gneiss intruded by tabular Me- relation of basement units within the complexly faulted San sozoic plutons, is separated from Pelona Schist by a greenschist Gabriel Mountains block. Preliminary U/Pb zircon work by Sil- facies mylonite zone of variable thickness, derived mainly from ver et al. (1963) established a Mesoproterozoic age for em- upper-plate protoliths (Nourse, 1991; Nourse and Litzenburg, placement of the anorthosite body in the western San Gabriel 1998), although the uppermost part of the Pelona Schist is also Mountains. Later U/Pb study (Silver, 1971) reported Paleopro- intensely sheared (Ehlig, 1981; Jacobson, 1983). terozoic ages for banded gneiss into which porphyritic granites New mapping from the area between Mount Wilson and (now augen gneisses) were intruded. A Triassic age was also Middle Fork Lytle Creek is presented in Figure 3A. This work determined for bodies of “Parker Mountain-Lowe granodiorite” provides details not included on previously published maps (Silver, 1971), and Carter and Silver (1971) reported a Creta- (Jennings and Strand, 1969; Ehlig, 1981; Dibblee, 1982, 1998; ceous U/Pb age for a granite body in the Josephine Mountain Bortugno and Spittler, 1986; May and Walker, 1989). For ex- intrusion. Silver’s work has been refined and augmented by ample: (1) the distribution of Proterozoic units is more clearly additional isotopic, geochemical, and geochronological studies. defined, with new subdivisions (2) additional Mesozoic units For example, Rb/Sr, trace element, and U/Pb analyses have are distinguished within the Proterozoic complex (3) several demonstrated the primitive petrologic character of the Mt Lowe metaplutonic bodies of probable Triassic age are recognized intrusion (Joseph et al., 1982) and verified a Late Triassic age (4) greenschist facies mylonite associated with the Vincent Ma (Barth et al., 1990). Conventional U/Pb zircon thrust is extended to several isolated regions south of the San 2 ע of 218 analyses coupled with ion microprobe analyses have yielded a Gabriel fault (5) foliation orientations define several new folds -Ma for the anorthosite- in the basement complex, and (6) patterns in the mapped tec 3.5 ע better constrained age of 1191 jotonite-syenite complex (Barth et al., 1995a; 2001). Other tonostratigraphy provide multiple piercing lines across the work by Barth et al. (1995b) documented a strong degree of north branch San Gabriel fault system and across several crustal contamination in tonalite and granite of the Josephine younger northeast-striking left-lateral faults. Mountain intrusion, which yielded discordant zircons with in- .Ma 3 ע terpreted lower intercept ages of 76–81 Ma and 72 Pelona Schist (map unit KTps) Similarly, tonalite and granite intruded into the metasedimen- tary strata of Ontario Ridge in the eastern San Gabriel Moun- tains yielded discordant zircons with interpreted lower intercept At the deepest level of the tectonostratigraphy lies Pelona -Ma, respectively (May and Schist, composed of interlayered greenschist facies metagray 8 ע ages of ca. 85 Ma and 78 Walker, 1989). U/Pb zircon work on Proterozoic gneisses in the wacke (“grayschist”), metabasalt (“greenschist”), and meta- study area is limited to one analysis reported from Glendora chert. Primary sedimentary and volcanic structures are gener- Ridge (Ehlig, 1981). ally transposed. Within the upper few hundred meters of the Petrologic studies from the past two decades include de- schist penetrative foliation is oriented parallel to the Vincent tailed petrographic and geochemical data that provide the basis thrust and upper-plate mylonitic fabric. This foliation defines for correlating undated portions of plutonic and gneissic rock map-scale open folds with subhorizontal west- or northwest- units of the study area. The palinspastic reconstruction pre- trending axes (Ehlig, 1958; Jacobson, 1983). sented here also aligns map-scale basement folds and homo- The Pelona Schist is believed by many to represent part of clinally dipping sections. Foliation orientations from peripheral a marine basin that was subducted beneath North American regions are documented by Ehlig (1958), Morton (1973), Dib- basement during latest Cretaceous or Paleocene time (Haxel blee (1982), Jacobsen (1983) May and Walker (1989), and Mor- and Dillon, 1978; Ehlig, 1981; Jacobson et al., 2000). Ion probe ton and Matti (1991). analyses of individual detrital zircons in metagraywacke indi- cate sediment derivation from Paleoproterozoic, Triassic, Ju- rassic, and Middle to Late Cretaceous sources (Jacobson et al., DEFINITION AND CORRELATION OF 2000). Debate continues about the duration of sedimentation in LITHOSTRATIGRAPHIC UNITS the Pelona basin, the tectonic setting of this basin, and the po- larity of subsequent subduction (Ehlig, 1968; Haxel and Dillon, The east-central San Gabriel Mountains reveal a folded 1978; Nourse and Litzenburg, 1998; Jacobson et al., 2000; Sil- stack of crystalline sheets constituting the lower and upper ver and Nourse, 2001). plates of the latest Cretaceous-Paleocene Vincent thrust (Figs. The contact between Pelona Schist and overlying mylonite 2 and 3). Pioneering work by Perry Ehlig (1958, 1968) char- provides a structural marker useful for deducing sense and mag- acterized this gently to moderately south-dipping thrust contact nitude of displacement on late Cenozoic faults. Internal strati- along a strip of high country extending from Mount San An- graphic markers, such as thick layers of greenschist abundant tonio to Mount Baden-Powell. The lower plate of the Vincent at higher structural levels, facilitate interpretation of displace- thrust system is composed of metamorphosed deep marine sed- ments on the San Jacinto and San Antonio Canyon fault systems imentary and mafic volcanic rocks (Pelona Schist). The upper in the northeastern San Gabriel Mountains (Norum, 1997). Figure 1. Caption on facing page. Figure 1. Simplified geologic maps of southern California illustrating: (A) Present-day location of the San Gabriel Mountains, configuration of major strike-slip faults, and distribution of Pelona–Orocopia–Chocolate Mountains schist bodies (shown in squiggle pattern). Modified from Figure 1 in Dillon and Ehlig (1993). (B) Widely cited middle Miocene palinspastic reconstruction showing restoration of the San Gabriel Mountains along major dextral strike-slip faults. Based on Ehlig (1981) and Dillon and Ehlig (1993); modified from Figure 5 in Matti and Morton (1993). FM—Frazier Mountain, LG—Lowe Granodiorite, LM—Liebre Mountain, SCM—southern Chocolate Mountains. Fault abbreviations as follows: SGF—San Gabriel fault, NSGF—north branch San Gabriel fault, SSGF—south branch San Gabriel fault, PBF—Punchbowl fault, SAF—San Andreas fault, SFF—San Francisquito fault, FF—Fenner fault, CWF—Clemens Well fault. This model restores 240 km on the San Andreas fault, 22 km on the north branch San Gabriel fault, and 38 km on the south branch San Gabriel fault. (C) Alternative middle Miocene paleogeology showing San Gabriel Mountains restored along major strike-slip faults (modified from Powell, 1993, Figure 11). Fault abbreviations as follows: CF—Canton fault, NSGF—north branch San Gabriel fault, SSGF—south branch San Gabriel fault, VCF—Vasquez Creek fault. This model restores 162 km on the San Andreas fault, 22 km on the north branch San Gabriel fault, 5 km on the Vasquez Creek fault (a.k.a. south branch San Gabriel fault), and 15 km on the Canton fault. 166 J.A. Nourse

Figure 2. Caption on facing page.

Vincent thrust mylonite (map unit KTmy) grayschist (Jacobson, 1990). Unfoliated late Oligocene Tele- graph Peak granite (map unit Tgr; May and Walker, 1989) and This unit (Figs. 4A and 4B) consists of mylonitized upper- associated rhyolite porphyry dikes and sills also intrude the plate granite, granodiorite, quartz diorite, diorite, gneiss, or am- schist and mylonite. phibolite, retrograded and sheared during movement on the Vincent thrust. The Vincent thrust mylonite varies in thickness Paleoproterozoic banded gneiss (map unit PCgn) from 10 m to 1000 m. A characteristic green color results from chlorite and epidote alteration of hornblende- and biotite-rich protoliths. Gneiss characterized by centimeter to millimeter scale The mylonite exhibits a penetrative foliation generally ori- banding and isoclinal folding constitutes the oldest upper-plate ented parallel to the thrust contact with Pelona Schist. The rock unit. This unit and associated intrusive sheets of augen thickest exposures occur in the East Fork San Gabriel Canyon gneiss, collectively referred to as “San Gabriel gneiss” (Miller, between Mount San Antonio and Mount Baden-Powell. Other 1934; May and Walker, 1989), form the framework into which exposures form east-west elongate antiformal windows directly Mesozoic plutons were intruded. On Glendora Ridge the San m xenoliths ם south of the north branch San Gabriel fault. Gabriel gneiss occurs as isolated 1 m to 100 Mylonitization occurred in Late Cretaceous or Paleocene within diorite and granodiorite. time. The older age limit is constrained by analyses of upper- The banded gneiss ranges in composition from felsite to plate Mount Waterman batholith, dikes of which are progres- amphibolite. Persistent amphibolite facies crystalloblastic fab- sively transposed into the Vincent thrust mylonite. Granodiorite ric and local mylonitization has obscured original textures such of the Mount Waterman batholith sampled 3 km west of Mount that protolith interpretation is difficult. Abrupt compositional Baden-Powell yielded a U/Pb zircon age of ca. 74 Ma (Silver and textural variations along with commonly observed sheared and Nourse, 2001), whereas a hornblende 40Ar/39Ar date of 71 intrusive contacts suggest that a heterogeneous accumulation of -Ma is reported from another part of the batholith (Grove fine-grained layered protoliths was intruded prior to metamor 2 ע and Lovera, 1996). The minimum age of mylonitization is con- phism and deformation by dikes or sills of gabbroic to granitic -Ma on muscovite composition. Subordinate quartzite and metapelite (e.g., garnet 0.4 ע strained by 40Ar/39Ar dates of 55.3 -Ma on muscovite from Pelona biotite gneiss) indicate a minor sedimentary component. Pri 0.6 ע from mylonite and 60.8 Figure 2. (A) Perry Ehlig’s geologic map of the San Gabriel Mountains basement complex, showing study area (Fig. 3B) and locations of faults and geographic features mentioned in the text. Modified from Ehlig (1975). (B) Index map of the central and eastern San Gabriel Mountains, showing geographic features mentioned in text and sources of geologic data compiled on Figures 3A and 3B.

Figure 3. Geologic maps of the central and eastern San Gabriel Mountains. Geographic locations are shown in italics: CR—Cogswell Reservoir, CP—Cucamonga Peak, MR— Morris Reservoir, MBP—Mount Baden Powell, MSA—Mount San Antonio, MW—Mount Wilson, OP—Ontario Peak, PM—Pine Mountain, SAD—San Antonio Dam, SDR— San Dimas Reservoir, SGD—San Gabriel Dam, SGR—San Gabriel Reservoir, and TP—Telegraph Peak. Fault labels as follows: CF—Cucamonga fault, DF—Duarte fault, GHF— Glen Helen fault, ICF—Icehouse Canyon fault, LF—Lytle Creek fault, MFLCF—Middle Fork Lytle Creek fault, NSGF—north branch San Gabriel fault, PMF—Pine Mountain fault, PF—Punchbowl fault, RHF—Raymond Hill fault, SCCF—Sawpit Canyon-Clamshell fault, SAF—San Andreas fault, SACF—San Antonio Canyon fault, SDCF—San Dimas Canyon fault, SJF—San Jacinto fault, SJoF—San Jose fault, SMF—Sierra Madre fault, SSGF—South Branch San Gabriel fault, SF—Scotland fault, SCF—Stoddard Canyon fault, SRF—Sunset Ridge fault, VCF—Vasquez Creek fault, VF—Verdugo fault, WF—Weber fault. A: Proposed subdivision of rock units and definition of faults in the central and eastern San Gabriel Mountains. See text for detailed rock descriptions. New mapping by this author is included from the Mount Baldy quadrangle, Glendora quadrangle, north 1/2 of Azusa quadrangle, northeast 14 of Mount Wilson quadrangle, east 1/2 of Crystal Lake quadrangle, south 14 of San Antonio quadrangle, south 1/3 of Telegraph Peak quadrangle, and northwest 14 of Cucamonga Peak quadrangle. Geologic data outside of these areas is compiled from Ehlig (1958), Jennings and Strand (1969), Morton (1973), Dibblee (1998), and Bortugno and Spittler (1986). B: Present-day geologic map of the southern and east-central San Gabriel Mountains used as template for palinspastic reconstructions of Figures 5–8. Peripheral faults and rock units not shown on Figure 3A are compiled from Jennings and Strand (1969), Dibblee (1998), and Bortugno and Spittler (1986). 170 J.A. Nourse

Figure 4. Photographs showing rock textures and field relationships in the central and eastern San Gabriel Mountains. A: Mylonite of the Vincent thrust (KTmy) from Vincent Narrows area, showing boudinaged Late Cretaceous(?) granite in sheared quartz diorite host. View to the S25W. Hammer is 45 cm long. B: Close-up view of KTmy unit from Vincent Narrows, showing rotation of K-feldspar porphyroclast in sheared granodiorite. View to the S25W. Hammer handle is 4 cm wide. Implications for evolution of the San Gabriel fault and Los Angeles basin 171 mary contacts are generally transposed into a penetrative foli- 1989). Early Miocene and Pliocene movements on the San An- ation that is axial planar to common mesoscale isoclinal folds. tonio Canyon fault (see below) have sinistrally offset this unit A similar structurally inferior position relative to the 8–10 km to the region west of San Antonio Dam. Possibly Mount Lowe intrusion suggests that the banded gneiss is an correlative strata occur west of Pasadena (Powell, 1993). along-strike continuation of the Mendenhall gneiss of the west- The north-dipping metasedimentary section of Ontario ern San Gabriel Mountains. However, the latter unit is higher Ridge is separated from structurally deeper Cretaceous Cuca- grade (commonly granulite facies), and contains a much lower monga granulite (map unit Kcg; May and Walker, 1989; Morton proportion of interlayered augen gneiss. Felsic components of and Matti, 1991) by “black-belt mylonite” (map unit Kmy; Hsu, the Mendenhall gneiss have yielded discordant Mesoprotero- 1955). May and Walker interpreted the black-belt mylonite as zoic U/Pb zircon ages from conventional isotope dilution meth- being derived from metasedimentary rocks, Cretaceous granu- ods (Silver et al., 1963; Barth et al., 1995a). Recent ion probe lite, and Cretaceous tonalite. Due to intense deformation (Fig. spot analyses (Barth et al., 2001) show that Paleoproterozoic 4E) and amphibolite facies metamorphism, protolith age of the zircons from felsic portions of the Mendenhall gneiss were iso- metasedimentary sequence is uncertain. 3.5 ע topically disturbed during intrusion of the nearby 1191 Ma anorthosite-jotunite-syenite complex. Triassic Mount Lowe intrusion (map units TRqm, TRqmzd, and TRdi) Paleoproterozoic granite augen gneiss (map unit PCagn) A distinctive foliated Triassic plutonic complex intrudes A medium to coarse-grained biotite granite augen gneiss Paleoproterozoic framework rocks in the upper plate of the Vin- with flattened or isoclinally folded K-feldspar augen occurs as cent thrust. Originally named “Lowe granodiorite” (Miller, concordant sheets or irregular plutons intruded into the banded 1934) for exposures on Mount Lowe, later studies (Ehlig, 1981; gneiss or as xenoliths within Mesozoic intrusions. Most com- Barth and Ehlig, 1988; Barth et al., 1990) showed this pluton mon is a dark brown weathering biotite-rich variety with 0.5– (map unit TRqmzd) to be compositionally zoned from a biotite 2 cm long K-feldspar augen (Fig. 4C). Coarser-grained, leu- quartz monzonite or granodiorite interior to a hornblende quartz cocratic varieties contain K-feldspar augen as long as 8 cm monzodiorite margin. The central portion of the pluton is char- composing Ͼ60% of the rock volume (Fig. 4D). Biotite, quartz, acterized by ovoid K-feldspar phenocrysts (“pigeon eggs”) as and feldspar are pervasively recrystallized in both varieties, large as 10–12 cm. The marginal facies (“dalmationite”) dis- with concomitant development of axial planar foliation. Augen plays conspicuous 0.5–1.5 cm hornblende phenocrysts (Fig. gneiss shares axial planar foliation with the banded gneiss and 4F). displays isoclinal folds of similar orientation. The Mount Lowe intrusion forms a “tail” whose southwest Late Paleoproterozoic U/Pb zircon ages are reported by contact with Paleoproterozoic banded gneiss constitutes a pierc- Silver (1971) from augen gneisses of the San Gabriel Moun- ing point on the north branch of the San Gabriel fault (Ehlig, tains and Soledad basin, as well as augen gneiss exposures west 1981; Dibblee, 1982, Fig. 2A). Recent mapping (Fig. 3) extends of the San Gabriel fault at Frazier Mountain and east of the San several thin sheets of quartz monzodiorite 25 km farther east to Andreas fault in the Orocopia and Chuckwalla Mountains. San Antonio Canyon. The eastern Mount Lowe intrusion is Medium-grained augen gneiss of Glendora ridge yielded a intimately associated with fine-grained biotite-hornblende dio- Ma (T. Davis, 1978, personal rite, hornblende-biotite quartz diorite, and minor coarse-grained 20 ע 207Pb/206Pb age of 1670 commun., in Ehlig, 1981). An augen gneiss layer within the gabbro (map unit TRdi). This diorite contains abundant xeno- Mendenhall gneiss contains zircons with oscillatory-zoned liths of banded gneiss and augen gneiss and is locally intruded Ma, by quartz monzodiorite (Fig. 4G). Foliated, medium-grained 40 ע cores indicating an upper intercept U/Pb age of 1679 whereas the rims and additional homogeneous zircons yield porphyritic quartz monzonite (map unit TRqm) intrudes diorite -Ma (Barth in the walls of Cogswell and San Gabriel Reservoirs. The di 22 ע near concordant U/Pb ages that cluster at 1172 et al. 2001). Considering the strong degree of recrystallization orite is interpreted to be an early phase of the Mount Lowe and foliation development in the augen gneisses, the generally intrusion based on similarities to intrusive relationships de- discordant Proterozoic zircons probably record an intense scribed northwest of Mount Baden Powell (Cox et al., 1983). thermal-deformational overprint of Late Paleoproterozoic gran- (Ma Jurassic(?) granodiorite (map unit Jgd 3.5 ע ite that preceded later thermal effects of the 1191 anorthosite intrusion (Barth et al., 1995a; 2001). Foliated porphyritic biotite granodiorite with distinct lav- Pre-Cretaceous metasedimentary strata (map unit ms) ender K-feldspar phenocrysts occurs at the high levels of the synform that defines the structure of Sunset Ridge. This pluton Complexly folded masses of quartzite, marble, pelitic forms concordant sheets intruded into Proterozoic and Triassic gneiss, phyllite, and graphitic schist form the wall rocks of Late rocks. Dikes of Late Cretaceous(?) leucocratic granite and peg- Cretaceous intrusions on Ontario Ridge (May and Walker, matite crosscut all of these units, but share a protomylonitic 172 J.A. Nourse

Figure 4. C: Paleoproterozoic biotite granite augen gneiss (PCagn) from lower West Fork San Gabriel Canyon. Hammer handle is 4 cm wide. D: Coarse-grained Paleoproterozoic augen gneiss (PCagn) of Glendora Ridge, showing isoclinally folded augen. Pencil is 16 cm long. Implications for evolution of the San Gabriel fault and Los Angeles basin 173

Figure 4. E: Folds in boulder of quartzite–calc-silicate gneiss, a common component of the ms unit south of Icehouse Canyon. Daypack is 40 cm wide. F: Xenolith of Triassic Mount Lowe hornblende quartz monzodiorite (TRqmzd) within Late Cretaceous quartz diorite (Kgr) host of Mount San Antonio. Narrow part of hammer is 3 cm wide. 174 J.A. Nourse

Figure 4. G: Xenolith of Triassic quartz monzodiorite in Late Cretaceous quartz diorite of Mount San Antonio. Note also the xenolith of Triassic diorite (TRdi) surrounded by quartz monzodiorite. Pencil is 16 cm long. H: Middle Miocene basalt dike (arrows) offset by west-northwest striking dextral fault of the San Gabriel system. View to the southwest. Dike is 50 cm wide. Implications for evolution of the San Gabriel fault and Los Angeles basin 175 fabric with Jurassic(?) granodiorite probably related to the Vin- Middle Miocene mafic-intermediate dike swarm cent thrust (Nourse and Litzenburg, 1998). The granodiorite resembles dated Jurassic plutons of the San Gabriel Mountains, Conspicuous swarms of basalt, basaltic andesite, and an- Ma granodiorite of Pleasant View Ridge desite dikes, not mappable at the scale of Figure 3, intrude both 3 ע including the 164 northwest of Mount Baden-Powell (U/Pb zircon age reported plates of the Vincent thrust system. These dikes are generally in Barth, 1989; see also Cox et al., 1983). steeply dipping, and occur in northeast and northwest-striking domains that record emplacement along conjugate strike-slip Late Cretaceous quartz diorite-tonalite-granodiorite-granite shear fractures (Nourse et al., 1998b; Nourse, 1999). The dikes (map unit Kgr) tend to be most concentrated within two kilometers of the San Gabriel fault (Perry Ehlig, 1996, personal commun.). A middle Miocene age is inferred (Nourse et al., 1998b) because the The pre-Cretaceous upper-plate stratigraphy of the Vincent dikes: (a) intrude the Telegraph Peak granite (b) are overprinted thrust is “stitched together” by several intrusive sheets of lo- and displaced by various strands of the late Miocene San Ga- cally porphyritic, medium-grained biotite-hornblende quartz di- briel fault (Fig. 4H), and (c) are compositionally and geochem- orite or tonalite and hornblende-biotite granodiorite. These sills ically similar to the nearby 15–16 Ma Glendora volcanic rocks extend eastward from the “Wilson diorite” (Miller, 1934) to the (Shelton, 1955; Nourse et al., 1998b). summits of Mount San Antonio, Ontario Peak, and Cucamonga Peak. Quartz diorite and granodiorite are intruded by irregular LATE CENOZOIC STRIKE-SLIP FAULTS: bodies of leucocratic biotite granite and pegmatite generally too DISPLACEMENT AND AGE CONSTRAINTS small to map on Figure 3. Quartz diorite and granodiorite of the eastern San Gabriel Mountains share a weak to moderate Figures 3A and 3B illustrate numerous strike-slip offsets foliation with granite dikes (Nourse, 1991; Nourse and Litz- of the crystalline stratigraphy described above. A network of enburg, 1998). Quartz diorite south of Cogswell Reservoir and west- or northwest-striking dextral faults and northeast-striking on Mount San Antonio (Fig. 4F) contains abundant xenoliths sinistral faults subdivides the east-central San Gabriel Moun- of Mount Lowe quartz monzodiorite. These xenoliths define tains into diamond-shaped blocks of various sizes. Synthetic part of the Mount Lowe “tail” utilized to constrain dextral dis- faults with minor slip, spaced at 1–50 m intervals, commonly placements on the San Gabriel fault. penetrate the interiors of these blocks. The overall effect at out- Granite from the Josephine Mountain intrusion yielded a crop scale is to produce a pervasive brittle overprint that tends Late Cretaceous U/Pb zircon age (Carter and Silver, 1971). Two to obscure older structures and fabrics. In one view, the San samples of foliated tonalite from Ontario Ridge preserve dis- Gabriel Mountains represent an example of “map-scale cata- ,Ma and ca. 85 Ma 3 ע cordant U/Pb crystallization ages of 88 clasis” (S. Richard, 2000, personal commun.). Indeed, it seems Ma age 8 ע while crosscutting granite yielded a discordant 78 amazing that the gross pre-Miocene structure and basement (May and Walker, 1989). Work farther west (Barth et al., 1995b) stratigraphy of this region has preserved its character through- indicates that the Late Cretaceous Josephine Mountain intrusion out its extensive late Cenozoic history of brittle deformation. is calc-alkaline and has assimilated a significant fraction of The San Gabriel block is bounded on the northeast by the country rock, as recorded by inherited Paleoproterozoic com- active dextral San Andreas fault and on the south by the active ponents in zircon arrays. Cucamonga-Sierra Madre thrust; therefore the eastern San Ga- briel Mountains are currently experiencing a state of transpres- Late Oligocene Telegraph Peak granite (map unit Tgr) sion. Late Cenozoic reverse faulting has accompanied strike- slip faulting within the range; these shortening effects are Both plates of the Vincent thrust system are intruded by manifested by gaps that appear between fault blocks in the pal- unfoliated, medium-grained, leucocratic, porphyritic biotite inspastic map reconstructions. The following section docu- granite. Exposed on Telegraph Peak, this pluton has injected ments timing and displacement constraints for the major strike- rhyolite or rhyodacite porphyry sills and dikes of similar major slip faults identified on Figure 3B and restored on Figures 5– and trace element character (Nourse et al., 1998b) into base- 8. These faults are described from youngest movements to ment rocks to the west and south. A larger pluton of similar oldest, in the same sequence that the reconstruction is presented composition and texture (Mount Barrow granite) occurs on the later. north side of the San Andreas fault in the northern Chocolate Mountains (Miller and Morton, 1977). May and Walker (1989) Quaternary dextral faults Ma U/Pb zircon age for Telegraph Peak granite 1 ע report a 26 in the easternmost San Gabriel Mountains. K/Ar ages of 19 Ma San Andreas fault (SAF). The San Gabriel block is truncated and 14 Ma (Miller and Morton, 1977) probably reflect thermal on the northeast by the active Mojave Desert strand of the San disturbances related to widespread emplacement of middle Andreas fault (Powell, 1993; Fig. 1A). This fault has been ac- Miocene dikes (see also Hsu et al., 1963). cumulating dextral slip for the past 5 million years. Estimates 176 J.A. Nourse

Figure 5. Early Quaternary (ca 1.5–1.0 Ma) palinspastic reconstruction of the central and eastern San Gabriel Mountains, showing 16 km restoration along the dextral San Jacinto fault system. Rock unit patterns and fault labels same as in Figure 3B. Fault blocks have been restored along faults shown with heavy dots. Solid-line faults were active at earlier times. of total displacement vary from 240 km (Ehlig, 1981; Dillon 3 and 5). All three strands of the San Jacinto fault system are and Ehlig, 1993) to 160 km (Powell, 1993; Matti and Morton, interpreted to merge in the upper North Fork Lytle Creek drain- 1993), however, all models (Figs. 1B–1C) restore the San Ga- age, with their combined slip feeding onto the Punchbowl fault briel Mountains to a position in the present-day Salton Trough (PF). Ongoing field studies are testing this hypothesis, which near the Orocopia and Chocolate Mountains. Other faults de- contradicts the views of Morton (1975) and Matti and Morton scribed below record dismemberment of the San Gabriel block (1993). prior to and during its northwestward translation along the San Andreas fault. Pliocene-Quaternary sinistral faults San Jacinto fault system (SJF, SF, and GHF). Pelona Schist and Telegraph Peak granite in the northeastern San Ga- These northeast-striking faults are described in order of briel Mountains are sliced by three strands of the Quaternary geographic position, from east to west. Timing of displacement San Jacinto fault system mapped on Figure 3 as the Scotland, is constrained as follows: (a) all of the faults considered cause San Jacinto, and Glen Helen faults. Microseismicity indicates left-lateral displacements or deflections of the north branch of that one or more of these faults is still active (Cramer and Har- the San Gabriel fault, which ceased moving in earliest Pliocene rington, 1987). All three fault strands appear to dextrally offset time (ca. 5 Ma; see also Crowell, 1952), and (b) some of these the northeast ends of the San Antonio Canyon and Weber faults faults are truncated by known active dextral faults such as the (Norum, 1997). Independent constraints on displacement are San Jacinto and San Andreas faults. Basement separations are derived from matching the following piercing lines: (a) a sharp consistent with sinistral offsets of the San Gabriel fault, but in northwestern intrusive contact of the Telegraph Peak granite one case excessive separation indicates a period of ancestral against Pelona Schist (b) segments of a northeast-striking faults movement (pre–12 Ma) that predates San Gabriel fault activity. that coincide with projected traces of the San Antonio Canyon Stoddard Canyon fault (SCF). This northeast-striking fault and Weber fault, and (c) internal stratigraphic markers structure follows the axis of Stoddard Canyon and crosses On- (metabasalt and metachert) and foliation domains within the tario Ridge west of the saddle between Cucamonga Peak and Pelona Schist. These cross-fault ties yield consistent dextral Bighorn Peak. It displaces the Icehouse Canyon fault sinistrally displacements of 3.5 km, 7 km, and 5 km on the Scotland, San ϳ1.5 km to a position coinciding to the Middle Fork Lytle Jacinto, and Glen Helen faults, respectively (compare Figures Creek. A steeply north-dipping contact between Cretaceous(?) Implications for evolution of the San Gabriel fault and Los Angeles basin 177

Figure 6. Early Pliocene (ca. 5 Ma) palinspastic reconstruction of the central and eastern San Gabriel Mountains, showing variable magnitudes of restoration along northeasterly sinistral faults. Rock unit patterns and fault labels same as in Figure 3B. Fault blocks have been restored along faults shown with heavy dots. Solid-line faults were active at earlier times. granodiorite and metasedimentary rocks on Ontario Ridge is Miocene movement on the San Gabriel-Icehouse Canyon fault separated a similar distance. Sinistral displacement postdates (Nourse et al., 1994). A north-dipping section on Ontario Ridge late Miocene dextral slip on the Icehouse Canyon fault. The consisting of (from bottom to top) quartzite, pelitic gneiss, mar- northeast end of the Stoddard Canyon fault appears to be trun- ble, granite, and tonalite is offset left laterally 8–10 km to a cated by the Scotland fault. To the southwest, the fault merges position west of San Antonio Dam. Five to seven km of this with and contributes slip to the San Antonio Canyon fault. displacement accumulated before the Vincent thrust and Pelona San Antonio Canyon fault (SACF). This fault, named for Schist were translated into the area by the San Gabriel–Icehouse its location in San Antonio Canyon, records two stages of sin- Canyon fault system (compare Figures 3B, 6, and 8). Early istral movement (Nourse et al., 1994). Pliocene and younger Miocene movement on this “ancestral” San Antonio Canyon displacement of ϳ3 km is estimated from offset of the north fault is demonstrated by subhorizontal fault striae that crosscut branch San Gabriel fault (and corresponding west-trending val- rhyolite dikes of the Telegraph Peak granite. These faults are ley) to Icehouse Canyon. This displacement value is consistent intruded by middle Miocene mafic dikes (Nourse et al., 1998b, with apparent left-lateral separation of: (a) the Vincent thrust, Nourse, 1999). and (b) a map pattern of open folds (antiform-synform-anti- The northeast segment of the San Antonio Canyon fault form) developed in the Pelona Schist, Vincent thrust, and upper- appears to be inactive because it is truncated and dextrally dis- plate mylonite on opposite sides of the San Antonio Canyon placed by three strands of the active San Jacinto fault system. fault (Jones, 1993). A minor splay of the San Antonio Canyon However, Holocene activity on the southwest segment is sug- fault transects the northwest part of Ontario Ridge causing gested by focal mechanisms of micro earthquakes (Cramer and 1 about ⁄4 km left-lateral offset of a Cretaceous granodiorite- Harrington, 1987), and possible kinematic linkage (via a right metasedimentary gneiss contact and minor deflection of the Ice- step) to the San Jose fault, which generated left-lateral oblique house Canyon fault. reverse earthquakes in 1988 and 1990 (Hauksson and Jones, South of Icehouse Canyon, the San Antonio Canyon fault 1991). preserves evidence for sinistral displacement predating late Sunset Ridge fault (SRF). A northeast-striking fault zone 178 J.A. Nourse

Figure 7. Late Miocene (ca. 9 Ma) palinspastic reconstruction of the central and eastern San Gabriel Mountains, showing 22 km restoration along the North Fork San Gabriel–Icehouse Canyon–Middle Fork Lytle Creek fault system. Rock unit patterns and fault labels same as in Figure 3B. Fault blocks have been restored along faults shown with heavy dots. Solid-line faults were active at earlier times. transecting the northwest side of Sunset Ridge is associated prominent north-northeast–striking faults with subhorizontal with ϳ0.8 km sinistral separation of an elongate augen gneiss striae marks the displacement zone, which is aligned with the screen in Jurassic(?) granodiorite. Synthetic faults within this axis of lower San Dimas Canyon. One of these faults juxtaposes zone show minor displacements of northwest-striking middle quartz diorite against banded gneiss in the east wall of San Miocene mafic dikes. The Sunset Ridge fault appears to cut Dimas Reservoir. across the San Gabriel fault and disrupt basement stratigraphy Along strike to the north-northeast, the San Dimas Canyon to the northeast. It joins the San Antonio Canyon fault north of fault is poorly exposed, but appears to connect a series of sad- Icehouse Canyon, contributing sinistral displacement to the dles and jogs in minor streams. Farther northeast, the San Ga- northeast segment of this fault. briel fault and an antiformal window of Vincent thrust mylonite San Dimas Canyon–Weber fault (SDCF and WF). A pro- are sinistrally offset a minor amount (0.5 km), where the San nounced jog in the mountain front northeast of Glendora cor- Dimas fault appears to join the Weber fault, mapped by Ehlig responds to an apparent left-lateral offset of the unconformity (1975). The gently dipping Vincent thrust is apparently dis- between Glendora volcanic rocks (Shelton, 1955) and crystal- placed 4 km by the Weber fault. Discrepancies in magnitude of line basement. This offset coincides with 1.5 km sinistral sep- apparent displacement on the San Dimas Canyon-Weber fault aration of a moderately north-dipping basement section con- system probably reflect a transition from pure strike-slip motion sisting of (from deeper to shallower): (a) Paleoproterozoic in the southwest to a significant component of dip-slip (north- augen gneiss and banded gneiss intruded by granite (b) Creta- west side up) in the northeast. ceous(?) granodiorite, and (c) banded gneiss. A system of Pine Mountain fault system (PMF). A series of northeast- Implications for evolution of the San Gabriel fault and Los Angeles basin 179

Figure 8. Early late Miocene (ca. 12 Ma) palinspastic reconstruction of the central and eastern San Gabriel Mountains, showing proposed 15 km restoration along the Sawpit Canyon-Clamshell fault system. Rock unit patterns and fault labels same as in Figure 3B. Fault blocks have been restored along faults shown with heavy dots. Solid-line faults were active at earlier times. Arrows indicate residual basement offset inherited from early Miocene sinistral movement on the ancestral San Antonio Canyon fault. striking faults with subhorizontal striae penetrate Pine Moun- into north and south branches (Fig. 2A). The south branch ap- tain. The fault shown on Figure 3 records ϳ1 km sinistral offset pears to merge with and is probably overprinted by the Sierra of a valley marking the main trace of the San Gabriel fault. It Madre thrust. Displacement on the north branch in the central juxtaposes augen gneiss (on the southeast) with quartz diorite San Gabriel Mountains is well constrained, but much debate and Vincent thrust mylonite. Farther west, additional left-lateral continues about: (a) where to transfer slip on the south branch displacement appears to be distributed along a system of north- beyond the point where it “disappears,” and (b) how to balance easterly faults that coincide with sharp jogs in the West Fork total San Gabriel fault displacement with other late Miocene San Gabriel River. These minor displacements (generally Ͻ0.5 dextral faults to the southeast or possible late Miocene trans- km) were not incorporated into the palinspastic reconstruction. tensional basin formation within the northeastern Los Angeles basin (compare Wright, 1991, Matti and Morton, 1993, Powell, Late Miocene dextral faults 1993, Weldon et al., 1993, Kenney and Weldon, 1998). The classic piercing point on the north branch San Gabriel North branch San Gabriel–Icehouse Canyon–Middle Fork fault (Ehlig, 1981; Dibblee, 1982) is a northeast-dipping contact Lytle Creek fault system (NSGF, ICF, and MFLCF). The San between Proterozoic gneiss and the structurally overlying “tail” Gabriel fault records a complicated and controversial displace- of the Mount Lowe intrusion (Figs. 2A and 3B). This contact, ment pattern (Ehlig, 1975; Powell, 1993; Weldon et al., 1993; exposed west of Mount Wilson, is offset 22 km dextrally from Matti and Morton, 1993). Mapping by John Crowell (1952) a position 2 km east of Cogswell Reservoir. Additional mapping documented ϳ45 km late Miocene dextral displacement on the of the basement stratigraphy (Fig. 3A) confirms this value and “type” San Gabriel fault between the western San Gabriel provides new piercing lines that support a comparable amount Mountains and its northwestern terminus with the San Andreas of displacement between Pine Mountain and Mount San An- fault (Fig. 1). Northwest of Pasadena the San Gabriel fault splits tonio. 180 J.A. Nourse

The Mount Lowe “tail” and adjacent crystalline strata, ex- Clamshell fault. A different mismatch occurs when 22 km posed continuously from the south side of Mount Wilson into dextral displacement is restored on the north branch San the West Fork San Gabriel Canyon, provide an important cross- Gabriel–Icehouse Canyon–Middle Fork Lytle Creek fault sys- fault tie between the West Fork exposures and those on the tem (Fig. 7). The fault block immediately north of Icehouse south flank of Mount San Antonio. Specifically, the Pine Moun- Canyon and Middle Fork Lytle Creek, composed mainly of tain area reveals a moderately northwest-dipping section com- mylonitized leucogranite, granodiorite, and quartz diorite, con- posed of (from deepest to shallowest): (a) Vincent thrust my- trasts markedly with Triassic(?) diorite, Paleoproterozoic augen lonite (b) Paleoproterozoic gneiss (c) Cretaceous quartz diorite gneiss and banded gneiss, and Triassic quartz monzodiorite ex- (d) Triassic quartz monzodiorite (e) Triassic(?) diorite, and (f) posed to the south. Triassic(?) quartz monzonite. A strikingly similar southwest- One mechanism for reconciling these discrepancies applies dipping succession is exposed between the summit of Mount 15 km of late Miocene dextral displacement to the Sawpit– San Antonio and the San Gabriel fault, ϳ20–25 km to the east. Clamshell fault, adding this slip to the eastern segment of the Dextral displacement on the north branch San Gabriel fault was north branch San Gabriel fault, which records 22 km of younger apparently localized near the hinge of a regional west-plunging displacement. The resulting reconstruction (Fig. 8) juxtaposes synform (see also Figure 7). north-dipping rock units currently exposed northeast of Mon- East of San Antonio Canyon, the north branch San Gabriel rovia with north-dipping mylonite exposed north of the Ice- fault follows the axis of Icehouse Canyon, where the 50-m- house Canyon and Middle Fork Lytle Creek faults. Comparison thick Icehouse Canyon fault zone is intermittently exposed. of rock units and structures between these two areas is a focus Other faults of uncertain importance but similar orientation of ongoing research (Nourse, unpub. mapping). transect ridges ϳ1 km north of Icehouse Canyon. East of the left-lateral offset caused by the Stoddard Canyon fault, the Ice- MIDDLE MIOCENE TO RECENT PALINSPASTIC house Canyon fault and its northerly branches follow east- RECONSTRUCTION trending Middle Fork Lytle Creek Canyon to the point where they are truncated by the Scotland fault. The Icehouse Canyon- Palinspastic reconstruction is accomplished in four steps Middle Fork Lytle Creek fault system records at least 22 km (Figs. 5–8) that restore the sequence of fault displacements de- dextral displacement, however, analysis of another fault of the scribed above. The present-day geologic map (Fig. 3B) pro- San Gabriel system (described below) suggests the possibility vides the main template utilized in reconstruction. The proce- for 37 km total displacement. dure assumes pure strike-slip translations of fault blocks. Sawpit Canyon–Clamshell fault system (SCCF). The Resulting gaps in the map view reconstruction reflect compo- Sawpit Canyon–Clamshell fault system of Morton (1973) is an nents of reverse slip along nonvertical structures. These trans- east-northeast striking crush zone as much as 300 m wide. Al- pressional features are manifestations of: (a) the pronounced though some workers consider this fault to be a left-lateral left jog that formed early during evolution of the late Miocene structure continuous with the active Raymond Hill fault, Ho- San Gabriel fault, and/or (b) Quaternary reactivation of certain locene thrust activity is indicated by seismic studies that cor- faults during development of the Sierra Madre–Cucamonga relate it with the 1991 M 5.8 Sierra Madre earthquake (Hauks- thrust. son, 1994). Interestingly, the Sawpit–Clamshell fault may also First, ϳ16 km of Quaternary dextral displacement is re- record late Miocene dextral displacement (Dibblee, 1982; Dil- stored on the San Jacinto fault system, with slip partitioned into lon and Ehlig, 1993), an interpretation supported by analyses 3.5 km, 7 km, and 5 km on the Scotland, San Jacinto, and Glen of small-scale brittle fault structures along certain segments (J. Helen faults, respectively. Figure 5 shows the Telegraph Peak Evans, 1994, personal commun.). In the model proposed in this granite reassembled in the resulting early Quaternary paleo- paper, the Sawpit–Clamshell fault represents an eastward con- geography. Second, minor Pliocene-Quaternary sinistral dis- tinuation of the cryptic south branch San Gabriel fault, whose placement is restored on multiple northeasterly faults that dis- curved trace was sliced off and abandoned by the straighter, rupt the San Gabriel fault (Fig. 6). The model assumes younger north branch San Gabriel fault. Effectively, a transpres- displacement values of 1.5 km, 3 km, 0.8 km, 1.5 km, and 1 km sional left jog in the early San Gabriel fault was straightened for the Stoddard, San Antonio, Sunset Ridge, San Dimas- out by transfer of slip to a mechanically more efficient north Weber, and Pine Mountain faults, respectively. This procedure branch. aligns discontinuous segments of the San Gabriel fault. Third, A significant mismatch of basement stratigraphy across the 22 km of dextral displacement is restored on the north branch Sawpit Canyon–Clamshell fault cannot be resolved by left- San Gabriel fault, aligning the Mount Lowe “tail” in the west, lateral or thrust restorations. Specifically, the northwest-dipping and bringing the Pine Mountain section adjacent to similar homoclinal section of Pine Mountain (described earlier) con- strata on Mount San Antonio (Fig. 7). This displacement prob- trasts with multiple folds in a section of medium-grained augen ably occurred between 9 Ma and 5 Ma. Lastly, 15 km of dextral gneiss, Triassic quartz monzodiorite, banded gneiss, and Tri- displacement is restored on the Sawpit Canyon-Clamshell fault, assic(?) diorite exposed southeast of the Sawpit Canyon– contributing additional slip to the eastern segment of the north Implications for evolution of the San Gabriel fault and Los Angeles basin 181 branch San Gabriel fault. Figure 8 shows the paleogeography and eastern Mount Lowe intrusion between the eastern San at ca. 12 Ma, just prior to significant right-lateral movements Gabriel Mountains, Banning block, and southern Chocolate on the south branch San Gabriel fault. Mountains. A variety of isolated structural features and paleodeposi- DISCUSSION tional patterns are explained by the geometric complexities that accompanied San Gabriel fault evolution. The early San Gabriel Implications for San Gabriel fault evolution fault shown in Figures 8 and 9 exhibits several bends or jogs. The overall transpressional character of the San Gabriel fault The palinspastic reconstruction proposed above resolves a was recognized by Weldon et al. (1993), who attribute forma- number of issues raised by Powell and Weldon (1992), Matti tion of the Squaw Peak-Liebre Mountain thrusts, uplift of the and Morton (1993), and Powell (1993), regarding late Miocene proto-Transverse Ranges, and accumulation of several late kinematic development of the San Gabriel fault system. In par- Miocene basins (including the Ridge Basin; Fig. 9) to compo- ticular, it provides a new mechanism for transferring 20 km of nents of vertical displacement near restraining bends in the San dextral displacement on the south branch San Gabriel fault to Gabriel fault. Evidence supporting this transpressional model regions farther southeast. This new reconstruction is most simi- is preserved in the central and eastern San Gabriel Mountains, lar to one of the general models proposed by Powell and Wel- where late Miocene(?) reverse slip occurred along a segment don (1992; see their Figure 8C, p. 458), in which slip on both of the proposed south branch San Gabriel fault that displays branches of the San Gabriel fault merges eastward with a south- a pronounced left jog (Figs. 8–9). Specifically, the Sawpit– ern strand of the early San Andreas fault. This strand was prob- Clamshell fault and the Icehouse Canyon fault both preserve an ably the Banning fault, whose possible connections to the San early system of north-dipping reverse faults cut by northeasterly Gabriel fault system are developed by Matti and Morton (1993). left-lateral faults of inferred Pliocene age. Presence of Vincent For the most part, I concur with the Matti and Morton model, thrust mylonite in both hanging walls indicates that deep levels including their hypothesis that the south branch San Gabriel of the San Gabriel basement were uplifted on the north side. fault is older than the north branch. However, our interpreta- These transpressional effects probably diminished after ca. 9 tions differ in that dextral displacement on the south branch San Ma, when transfer of slip to the straighter north branch San Gabriel fault is shunted onto the Banning fault via the Sawpit Gabriel fault (Matti and Morton, 1993) caused the restraining Canyon–Clamshell–Icehouse Canyon–Middle Fork Lytle Creek bend in the south branch to be abandoned. fault system rather than the Stoddard Canyon–Middle Fork Lytle Similarly, anorthosite boulders in displaced conglomerate Creek fault system. Previously described evidence for minor sin- of the Frazier Mountain block (Fig. 9) suggest that an uplifted istral slip on the Stoddard Canyon fault seems at odds with the anorthosite source in the western San Gabriel Mountains shed 22 km dextral displacement utilized by Matti and Morton (1993). debris southwestward across the San Gabriel fault during Displacement discrepancies between the northwestern por- Mohnian time (Crowell, 1952). In this case, however, emer- tion of the San Gabriel fault and the north branch of the San gence of the western San Gabriel Mountains and deposition of Gabriel fault are discussed in detail by Powell (1993). I agree the anorthosite-clast conglomerate may be explained by local with Powell’s 42 km value for total displacement on the north- vertical movements driven by transtensional stress (note the west end of the San Gabriel fault, and concur that 22 km is a apparent right jog in reconstructed San Gabriel fault in this good number for displacement on the north branch. The 20 km region; Fig. 9). difference could most easily be partitioned onto other structures such as the Vasquez Creek fault and the Canton fault (Fig. 9). Paleogeographic implications for development Powell (1993) argued that the Vasquez Creek fault can only of the Los Angeles basin account for 5 km of the slip discrepancy, but his more recent work (R. Powell, 2001, personal commun.) indicates that larger The proposed middle Miocene tectonic reconstruction displacements are possible. The 1993 Powell model assumes (Figs. 8 and 9) illuminates several intriguing paleogeographic 5 km and 15 km displacement on the Vasquez Creek and Canton features that bear on early development of the Los Angeles faults, respectively, and cryptically absorbs this slip farther basin. The San Gabriel Mountains formed the northern bound- southeast via oblique extension in the Los Angeles basin and ary of this late Miocene–Pliocene marine depocenter. Espe- in the San Gabriel–San Bernardino Valleys (see Powell, 1993, cially interesting are the restored positions and geometry of the: Figure 14B, p. 39). To reconcile fault relations in the central (a) late Oligocene Telegraph Peak granite (b) ancestral (early and eastern San Gabriel Mountains I prefer to transfer 15 km Miocene) San Antonio Canyon fault, and (c) middle Miocene displacement on the Vasquez Creek fault onto the Sawpit Can- mafic-intermediate dike swarm and associated Glendora vol- yon–Clamshell system. In this perspective, only 5 km of late canic rocks. Miocene dextral displacement is absorbed by extension or Fault slices of the Telegraph Peak granite restore near the transtension in the Los Angeles basin (Fig. 9). The new recon- Mount Barrow granite of the Chocolate Mountains (Powell, struction offers closer continuity of the Vincent thrust system 1993) forming an ovoid pluton (Fig. 9). Rhyolite-dacite sills 182 J.A. Nourse

Figure 9. Proposed middle Miocene (ca. 18–13 Ma) paleogeography of southern California, showing the San Gabriel and Frazier Mountain blocks restored adjacent to the Orocopia Mountains and Chocolate Mountains. Elements of the palinspastic base are modified from Figure 2 in Crowell (1952), Figures 14a and 14b in Powell (1993) and Figures 7b and 7c in Matti and Morton (1993). Fault abbreviations as in Figure 1C and Figure 3B. The model hypothesizes genetic relations between middle Miocene faults, igneous intrusions, volcanic rocks, and future development of the Los Angeles and Ventura pull-apart basins. Note the broad region of transtension in step-over zone between the Clemens Well and offshore borderland faults. In this model, north-northeast sinistral or oblique-normal faults such as the ancestral San Antonio Canyon fault, Raymond Hill fault, and Santa Monica/Malibu Coast fault connect a right step between the Clemens Well and Offshore Borderland faults. The middle Miocene Glendora and Conejo volcanic rocks are extruded near these boundary structures. Northeast and northwest-striking mafic-intermediate dikes are emplaced into conjugate shear fractures developed within the eastern San Gabriel Mountains basement. emanating from the pluton intrude both plates of the Vincent Another interesting feature of the middle Miocene recon- thrust, whereas less common steep-dipping dikes strike south- struction is a residual 6 km sinistral offset of basement stratig- southwest and west-northwest (Nourse et al., 1998b). Dikes on raphy across the north-northeast striking ancestral San Antonio Glendora Ridge probably connected the Mount Barrow–Tele- Canyon fault (Fig. 8). As described earlier, displacement oc- graph Peak granite with the Mountain Meadows dacite of the curred during early Miocene time. Interestingly, this fault zone Pomona Valley. This isolated dacite exposure, which underlies marks the eastern limit of a mid Miocene dike swarm, and its the Glendora volcanic rocks and Puente Formation and overlies southward projection into the Pomona Valley forms the eastern Late Cretaceous(?) quartz diorite (Shelton, 1955), has yielded boundary of the middle Miocene Glendora volcanic rocks. As a biotite 40Ar/39Ar date of ca. 27 Ma (G. Hazelton, 1998, per- shown on Figure 9, the ancestral San Antonio Canyon fault sonal commun.). Hence, basement underlying the northeastern does not connect with the sinistral Raymond Hill-Malibu Coast Los Angeles basin was adjacent to the eastern San Gabriel fault as implied by Matti and Morton (1993). Instead, the San Mountains during late Oligocene time. Antonio, Raymond Hill, and Malibu Coast faults appear to be Implications for evolution of the San Gabriel fault and Los Angeles basin 183 separate conjugate faults associated with a right step between resolves issues regarding disposition of the south branch San the early Miocene Clemens Well fault and a dextral transform Gabriel fault and its relationship to early strands of the southern fault located in the California borderland (see also Powell and San Andreas fault system. Also implied are genetic relation- Weldon, 1992, Figure 7, p. 455). The transtensional geometry ships between Los Angeles basin development, the ancestral implied by this reconstructed fault configuration strongly sup- (early Miocene) San Antonio Canyon fault, and the middle ports a pull-apart origin for the Los Angeles and Ventura basins. Miocene mafic-intermediate igneous rocks. A third intriguing feature of the middle Miocene recon- struction is distribution of the middle Miocene mafic-interme- ACKNOWLEDGMENTS diate dike swarm and its presumed surface expression, the Glen- dora volcanic rocks (Nourse et al., 1998b). The dike swarm was Field work in the San Gabriel Mountains between 1994 and intruded along a preexisting (early Miocene) network of north- 2001 was funded by the USGS Southern California Aerial Map- east- and northwest-striking conjugate shear fractures (Nourse, ping Project and by California Division of Mines and Geology. 1999) developed throughout much of the San Gabriel Moun- I am grateful to numerous Cal Poly Pomona undergraduate stu- tains west of San Antonio Canyon. These dikes occur within a dents who contributed to the mapping: Ruben Acosta, Erin right step between the Clemens Well fault and an inferred bor- Stahl, Matt Chuang, Brent Norum, Garrett Hazelton, Rogan derland fault (Fig. 9). The model proposes that the middle Mio- Jones, Melissa Pratt, Scott Gibson, Gianfilipe Pedrotti, Shaun cene Glendora volcanic rocks and middle Miocene Conejo- Wilkins, Jeff De Land, Seth Brodie, Terry Watkins, Jennifer Zuma volcanic rocks (Weigand, 1982) mark the bottoms of Beal, and David Hankins. Scholarships from the Thomas W. separate pull-apart basins located west of the ancestral San An- Dibblee, Jr. Foundation supported the fieldwork of three stu- tonio and Santa Monica–Malibu Coast faults respectively. dents. Dan McDermitt and Bryan Kriens assisted with several Hence, a right-stepping Clemens Well fault system was the excursions to remote areas. Discussions with Perry Ehlig, John driving mechanism for the early-mid Miocene extension re- Crowell, Bob Powell, Doug Morton, Garrett Hazelton, Ray corded by basal strata of the Los Angeles and Ventura basins. Weldon, Lee Silver, and Jim Evans illuminated various aspects The main phase of Los Angeles basin subsidence coincided of San Gabriel Mountains geology and Los Angeles basin evo- with late Miocene–Pliocene movements on the San Gabriel lution. Judith Chester and Doug Morton provided constructive fault (Wright, 1991; Ingersoll and Rumelhardt, 1999), during comments on an early version of this paper. Detailed technical which minor splays such as the Canton–Verdugo–Whittier– reviews by Joan Fryxell and Andy Barth greatly improved the Elsinore fault and offshore borderland faults formed northwest- clarity of the final manuscript. striking boundaries of active pull-aparts. Complicating this sce- nario is coincident clockwise rotation of the western Transverse Ranges (Nicholson et al., 1994). 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