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GUIDEBOOKTO'FIELD TRIP #10 94thAnnual Meeting, Cordilleran Section of the GeologicalSociety of America

TGNEOUSAND TECTONI9 RESPONSE OFTHE EASTERN TO NEOGENEEXTENSION AND ROTATION OFTHE BLOCK

JonathanA. Nourse,Peter W. Weigand,and Ganett B. Hazelton

@*', u'rP @P "..;.* #u"t I '.{. April 3-'10, 1998 '.f' Field Trip Program . i-,-:, CordilleranSection GSA

Editor RichardJ. Beftl

Publishedby State UniversityLong Beach, Departrnentof GeologicalScierrees, April 1, 1998 IGNEOUS AND TECTONIC RESPONSEOF TIIE EASTERNSAN GABRIEL MOUNTAINS TO NEOGENE BXTENSIONAND ROTATION OF TIIE TRANSVERSERANGBS BLOCK

Jonathan A. Nourse, Department of Geological Sciences, California State PolytechnicUniversity, Pomona, CA 91768;[email protected]

Peter W. Weigand, Department of Geological Sciences, California State University, Northridge, CA 91344-8266;[email protected]

Garrett B. Hazelton, Department of Earth and SpaceSciences, University of California, ,CA 90095; [email protected]

INTRODUCTION rocks constituting the upper and lower plates of the Vincent thrust system (Nourse, 1991; Hazelton and Neogeneigneous rocks of the Pomona- Nourse,1994). Stops3, 4, 5, and 6 will focus on Glendoraarea and the eastemSan Gabriel relationshipsof thesedikes to premagmatic, Mountains (Fig. 1) offer a unique opportunityto synmagmatic,and postmagmatic faults. study processesof magmatismassociated with early Throughout this excursion we will argue that development of the dextral transform plate the Neogeneintrusions of the eastern San Gabriel boundaryin southwesternCalifornia. Distinct Mountainsfed magmato volcanic centersin the magmatic pulses affected this region during Late Pomona-Glendoraarea (Weigand and others, 1994). Oligoceneand Middle Miocene time, prior to 315 Theseigneous rocks were not associatedwith km northwesttranslation along the SanAndreas fault subduction. Instead,they record two pulsesof system. Tectonic models imply magma magmaemplaced into a continentalbasement emplacementwithin extensionalor transtensional weakenedby normal faults and conjugate shear environments. Magmatism precededand overlapped fractures. Dike orientationspreserve the complex in spaceand time with initial structuraldevelopment stressgeometries associated with brittle failure of the of the Los Angelesbasin and clockwiserotation of basement. the westernTransverse Ranges block (Hornafiusand Correlation of the Pomona-Glendora others,1986; Nicholson and others,1994). volcanicstrata to intrusivefeeders in the San Gabriel Sedimentarystrata that overlie the volcanicrocks Mountainsolaces new constraintson locationsof record rapid late Middle Miocene to Early Pliocene youngerstrike-slip faults inferred to transectthe subsidenceassociated with fillins of the Los northeasternLos Angelesbasin. In particular,our Angelesbasin (Yerkes, 1965; Wright, l99l: data suggeststhat the enigmatic south branchof the Rumelhartand Ingersoll, 1997). Post-magmatic SanGabriel fault (Crowell,1975 Ehlig, 1981; faults record the block rotationsand translations Dibblee, 1982) is locatedsouth of the San JoseHills. which continue throughoutthe region today. This trip beginsin the PomonaValley (Stops STATIGRAPIIY AND FIELD SEQUENCE l arrdZ), where Quaternaryanticlines (Yerkes, 1965; Wright, 1991)reveal Mesozoic basementoverlain Volcanic and Sedimentary Strata by Late OligoceneMountain MeadowsDacite, in of the Pomona-Glendora Area turn overlain by Middle Miocene Glendora Volcanics (Shelton,1955). A synclineat Volcanic rocks in Pomona and Glendora PuddingstoneReservoir (Stop l) preservesa (Fig. 1) were extrudedacross azene that would later younger sedimentarysection consisting of Middle mark the northeastmargin of the Los Angelesbasin. Miocene conglomerateand sandstoneof the Igneousactivity predatedthe accelerationof basin- TopangaFormation and Upper Miocene-Lower wide subsidencerecorded by marine sedimentation Plioceneshale and siltstoneof the PuenteFormation betweenlate Middle Miocene and Early Pliocene (Mull, 1934). We will then drive into the easternSan time (Wright, 1991; Rumelhart and Ingersoll, 1997). Gabriel Mountainsto view Late Olieoceneand Key age relationshipsare summaized in Fig. 2. Middle Miocene dike swarmsintrud'ed into basement Stratigraphiccontrol throughout this presently

I 0-1 Figure 1. Distribution of Oligoceneand Miocene volcanicand intrusiverocks and faults in the easternSan Gabriel Mountains and PomonaValley area(modified from Bortugno and Spittler, 1986). Circled numbers denotefield trip stopsdescribed in text. Fault abbreviations:(SAC) San Antonio Canyon,(SJo) San Jose,(IC) IcehouseCanyon, (MLC) Middle Fork Lytle Creek,(SJa) San Jacinto, (SMFZ) SierraMadre. urbanized area was acquired from both surface The stratigraphicallyhigher Glendora geologicalmapping and well data (Mull, 1934; Volcanicsare exposedbetween the foothills of the Shelton,1955). easternSan Gabriel Mountains and the northern The largestoutcrop of Mountain Meadows PuenteHills (Fig. 1). This heterogeneousassemblage Dacite is poorly exposedin a roadcutbetween of flows, flow breccias,volcaniclastic conglomerates, Mountain Meadows golf courseand the Los Angeles and tuffs exhibits a compositionalrange biween County Fairgrounds(see highlightsen route to Stop olivine basaltand rhyolite (Mull, 1934; Shelton, 3). Broaderareal extent is demonstratedby its 1955). Andesitebreccia is the most volumetrically presencein severalwells that penetratethe San Jose abundantlithology. Andesite and dacite sampled Hills (Fig. 1). This 50m- to 90m-thick tabularbody from southeastof PuddingstoneReservoir yielded restson Mesozoic(?)quartz diorite or granite wholerock tr?Ar datesof 19.6t1.1 Ma and 18.2t1.1 o0Ar/3eAr basement,and is overlain by the Middle Miocene Ma and plagioclase datesof 16.3+1Ma ooAr/3eAr GlendoraVolcanics (Shelto;, 1955). and 15.9t0.3 Ma, respectively. Two samplesof analyseson three aliquots of biotite separatedfrom volcanic rock from ElephantHill (Stop 2) yielded the Mountain Meadows Dacite yield an ageof 27.5- wholerock lTAr agesof 18.8+1.2Ma and 13.2X0.4 28.0Ma. Ma, respectively(L. Herber, pers. comm., 1994).

10-2 Upper layersof the GlendoraVolcanics are locally interstratifiedwith sandstoneand conglomeratebeds of the TopangaFormation that contain Luisian and Relizian foraminifera (Shelton, 1955). In the PuddingstoneLake area,the Topanga Formation is overlain by lower Mohnian marine siltstone, shale,sandstone of the PuenteFormation. MIOOLE Tongues of conglomeratethat interfinger with these xroc€IE grd marine beds record the episodic uplift of San pos3iblt oldct Gabriel Mountains basementsources during Late Miocene and Pliocene time (Woodford and others, 1946; seealso Rumelhartand Ingersoll, 1997). Prc - Ocftbo Yolconas Oligocene-MioceneIntrusions in the Eastern San Gabriel Mountains 14 6 U o Two igneousrock suitesintrude crystalline Fo U basementof the easternSan Gabriel Mountains (Fig. c o 1). The oldestgroup is composedof Telegraph E Peak graniteporphyry (Miller and Morton, 1977) G and associatedrhyolite-quartz latite porphyry sills I u and dikes (Fig 3). Geochemicalsignatures of the granite and rhyolite porphyry are identical (see *Ar/"Ar below) and datesfrom two rhyolite dikes indicatea Late Oligoceneemplacement age (26.30+0.04Ma and 25.7+0.2Ma), consistentwith a Figure 2. Generalized stratigraphiccoluml for the UlPb date (25.6X1 Ma) on the Telegraph Peak Pomona-Glendoraarea (taken from Shelton.1955). porphyry (May and Walker, 1989). The felsic rocks are sharply intruded by a dike swarrn that ranges

Polesto Rhyolite Porphyry DikesMeasured Polesto RhyolitePorphyry DikesMeasured Southof the San Gabriel Fault-- N = 22 North of the SanGabriel Fault-- N = 35 Figure 3. Lower hemisphereequal area sterenetsshowing poles to Late Oligocenerhyolite porphyry dikes intrudedinto upper and lower platesof the Vincent thrust systemin the easternSan Gibriel Moirniains (Stereonetplotting program by R. Allmendinger).

I 0-3 in compositionfrom olivine basaltto hornblende- below) suggestsmore specific correlation of this biotite andesite (Hazelton and Nourse, 1994). The suite with the Middle Miocene GlendoraVolcanics. youngerdikes are not yet dated,but field ^ The Telegraph Peak granite porphyry is a observationsdemonstrate a geneticassociation with a 50km' sill-shapedpluton intruded approximately systemof normal and oblique-slip faults that coincident with the structural horizon marked by the predatedonset of significant dextral slip on the San Vincent thrust. Rhyolite sills with lesscommon Gabrielfault system(ca. 12 Ma; Crowell,1975). connectingdikes radiate to the west of Telegraph Their age is thus bracketedbetween Late Oligocene Peak into both the lower plate Pelona Schist and Late Miocene. Geochemicalcomparison (see

Poles to Mafic-lntermediate Dikes Poles to Mafic-lntermediate Dikes Measured South of the San Gabnel Fault Measured North of the San Gabriel Fault N:282 N=304

C.l. = Z.Oo/J1o/oarca C.l. = 2.Oo/ol1o/oarca

1% Arsa Contour Diagram of Poles to 1Vo Are-n Contour Diagram of Poles to lntermediate-Mafic Dikes Measured Mafic-lntermediate Dikes Measured South of the San Gabrid Fault North of the San Gabrid Fault Figure 4. Lower hemisphereequal areasterenets showing poles to Middle Miocene mafic-intermediatedikes measuredin the easternSan Gabriel Mountainsbasement terrane (plotting programby R. Allmendinger).

I 0-4 (Stops3 and 5). The felsic intrusionsappear to be are demonstratedby changesin orientationof the broadly folded with their foliated country rocks younger dike swarm between adjacent fault blocks along east-westtrends. Rhyolite dikes generally (Hazelton and Nourse, 1994.) strike north or northeast(Fig. 3). Structural Abundant slickensidedsurfaces, offset correctionof this data set (i.e., unfolding of the host marker units, and other brittle kinematic indicators rocks) resultsin subverticaldips and more tightly constrain the post-magmatichistory of fault clustered poles. movement. The San Gabriel fault. active between 12 The younger mafic-intermediatedikes dip Ma and 5 Ma (Crowell, 1975), has been considered steeplyand exhibit predominantlynorthwest and less to be an early branch of the common northeaststrikes (Fig. a). Within a given system. Northwesterly to westerly splays of the San fault block, lighter colored andesitedikes and darker Gabriel fault system display consistentdextral offsets basalticdikes sharecommon orientations. of the mafic-intermediatedikes (Fig. 5), providing a Throughoutthe study areathe andesiticdikes are younger constraint on their age of emplacement. In consistentlyyounger. the Mt. Baldy area,both groupsof Neogene The mafic-intermediatedike swarm is most intrusions occur in three fault blocks separatedby concentratedwithin 1 to 2 km of the north branch two major strandsof the San Gabriel fault (Fig. 1). of the San Gabrielfault (P. Ehlig, pers.comm., Stops 3, 5, and 6 will view rhyolite and intermediate- 1994; Fig. 1). Structuralobservations support a mafic dikes that were originally separatedby a geneticconnection between dike intrusion and the distance of 22km. These dikes are also displaced 3 early phasesof movementon the San Gabrielfault. km sinistrally by post-5 Ma movement on the San Specifically,these dikes reflect an extensionalor Antonio Canyon fault (Nourseand others, 1994). transtensionalstress regime that prevailednear the San Gabriel fault prior to 22km right-lateral displacement(Ehlig, 1981;Dibblee, 1982). Stop 3 shows examplesof the younger dikes intrudedalong normal faults and/orshear fractures. Stop 5 shows similar dikes disruptedby post-magmaticbrittle dextral faults within the San Gabriel fault zone. The overall distributionand structuralseometrv of the mafic-intermediatedike swarm rec"ordsthi tectonic responseof the easternSan Gabriel Mountains basementto a regionalMiddle Miocene stress regime associatedwith major modificationsof the southernCalifornia borderland (e.g., initial opening of the Los Angelesbasin and northwesttranslation and clockwiserotation of the westernTransverse Ranges).

Post-Magmatic Fault Displacementsand Block Rotations

Field relationshipsobserved within the Mt. Baldy and Glendora7.5 minute quadrangles (Nourse and others,1998) consistentlvdemonstrate that both Neogeneintrusive suites predate lateral displacementsand rotationsrelated to conjugate faulting within the San Andreasfault system(sensu lato). The easternSan Gabriel Mountains basement is broken into diamond-shapedblocks (Nourseand others, 1998;Fig. 1) boundedby northwest-striking dextral faults (SanGabriel, San Jacinto,and San Andreasfaults) and northeast-strikinesinistral faults (San Antonio Canyon,San Dimas Cinyon, and Figure 5. Photographof northeast-strikingbasalt Sawpit Canyonfaults). Prominentvalleys are eroded dike (see arrows) dextrally displaced by a along thesefault traces,and associatedfracture sets subvertical west-northweststriking fault (heavy line) control drainagepatterns throughout the range. Dike is 50 cm wide. Oblique view is lookins down Differential vertical-axisrotations caused by into Robb's Gulch near San Gabriel Dam. interactionsbetween the dextral and sinistralfaults (Photographtaken by J. Nourse, 1997).

l0-s GEOCIIEMISTRY

6 As introduced above, five suites of Cenozoic igneousrocks which crop out in the Pomona- al Glendora area and easternSan Gabriel Mountains Fo fall into two age-compositionalcategories: (1) an + older assemblegeconsisting of the extrusive rs* z4 Mountain MeadowsDacite, the intrusiveTelegraph Peak granite porphyry pluton, and associatedfelsic sills and dikes, and (2) a youngerassemblege 2 consistingof the extrusiveGlendora Volcanics and the intrusivemafic-intermediate dikes. Genetic relationshipsbetween suites within eachof theseage- 0 compositionalassembleges have been hypothysized (Weigandand others 1994; Roberts 1995). Along with publishedanalyses, new geochemicalanalyses are plotted below in order to illustrate the classificationof samplesfrom thesefive suitesand to further assesspossible petrogenetic relationships betweensuites within eachof the two assembleses. Older analysescan be found in Shelton(1955t Ehlig (1958),Higgins (1976),Miller and Morton (19-17),and Roberts(1995). The total alkalis vs silica plot (Fig. 6,{) assignsrock namesto samplesaccording to the classificationscheme of Le Bas and others(1986). One of the two MoUntainMeadows Dacite samples is compositionallyclassified as rhyolite accordingto this scheme,as are all the samplesof felsic sills and dikes (11 analyses).Telegraph Peak pluton samples (6 analyses)fall into this samefield, which is compositionallyidentical to granite. Basedon petrographiccriteria and l4 chemicalanalyses, Shelton (1955) reportedthat the GlendoraVolcanics .- 0F o were polygeneticand composedof a rangerock types that varied from basaltthrough rhyolite. Additional geochemicaldata (47 analyses)further High-K confirm this range,although samples of true rhyolite {( appearrare. Basedon the entire geochemicaldata set,andesite and dacitedominate the volcanic tr Medium-K section. Samplesof mafic-intermediatedikes (17 o -6 analyses)range in compositionfrom basaltto :< * andesite,confirming petrographicobservations tr f -o*FHr'fr E (Hazelton,1993). All five of theseigneous suites o; -t tr Low-K belong to the calc-alkalinemagma sJries (Fig. 68) and to the low- and medium-K2Oseries (Fig. 6C). Harker diagrams(not shown), in which m4-oroxides are plottedvs SiO2,show that the individual igneous si02 suiteswithin eachof the two age-defined Figure 6. Classification diagramsfor the five assemblegesare similar to each igneoussuites of the Glendora-Baldyarea. other with respectto theseoxides. Symbology for this and following figures: solid A plot of the rare-earthelements (REE) diamond=TelegraphPeak pluton, open plus=felsic showsthat samplesfrom the older, felsic suitesare dikes, solid circle=MountainMeadows Dacite, open similar to eachother, particularly samples of the box=GlendoraVolcanics, star=mafic-intermediate TelegraphPeak intrusion and the felsic sills and dikes. 6A. Rock nomenclaturediaeram. 68. AFM dikes (Figs.7A and 78). This is not surprising,since diagram of Irvine and Baragar091q. 6C. K2O field evidenceshows many of thesesills and dikes seriesdiagram of Gill (1981).

I 0-6 La Ce Pl Tm

La Ce Pr Nd PmSmEu Gd Tb Dy Ho Er frn \rU Uu

Hf Ti Tb YT

Figure 7. Trace-elementdiagrams for the older lgneousassemblage. Symbol,ogy as in Figure 6. 7A. Rare-earthelement (REE) piot of the-felsic dikes normalized to chondritii meteorites (Nakamura, 1974). 78. plot REE of samplesfrom Figure 8. Trace-elementdiagrams for the younger the TelegraphPeak pluton anri Mountain^Meadows igneousassemblage. Symboloey as in Fisure 6.- Dacite. 7C. Spider diagram of the felsic dikes 8A. REE plot of the mafic-inteimediated"ftes. normalizedwith the scheme of Thompsonand 88. REE plot of samplesfrom the Glendora gth"Ir(1984). 7D. Spider diagram of samplesfrom Volcanics. 8C. Spider the diagramof the mafic- TelegraphPeak pluton and-Mountain Meadows rntermediaredikes. Dacite, 8D. Spider diagram of samples from the GlendoraVolcanics.

I 0-7 emenating from the pluton (Morton and Matti, IMPLICATIONS FOR LOCATION OF TI{E 1991). The felsic dike sampledepleted in the REE SOUTH BRANCH SAN GABRTEL FAULT (Fig. 78) showspetrographic evidence of alteration (Roberts,1995). The single sampleof Mounrain Previouslypublished tectonic models Meadows Dacite for which trace-elementabundances (Crowell, 1975;Dibblee,1982; Powell, 1993)require are available has similar abundancesof the lieht REE 45km to 60km total dextral slip on the Late comparedto samplesof the pluton and felsiclills Miocene-PlioceneSan Gabriel fault, with 22km slip but has somewhathigher abundancesof the heavy distributed on the north branch within the San REE (Fig. 7A). Spiderdiagram patterns (Figs. 7C GabrielMountains block (Ehlig, 1981;Fig. 1). By and 7D) of samplesfrom the older, felsic units are default, a cryptic south branch San Gabriel fault is strikingly similar to eachother. Similar to what the invoked to accountfor the remaining slip. REE plot showed, the altered dike samplehas an Presumably,this fault runs south of the San Gabriel anomalous pattern, and the Mountain Meadows Mountains, either along the Sierra Madre fault zone Dacite sample differs from the Telegraph Peak and or farther southwithin the . The felsic dike sampleswith respectto the heavy REE; datapresented in this papersuggest a direct tie otherwiseit is compositionallyvery similar. betweenthe volcanic rocks of Glendora-Pomona Rare-earthelement plots (Figs. 8A and 88) area and the intrusive rocks of the easternSan of the younger GlendoraVolcanics and mafic- Gabriel Mountains.Our proposedcorrelation intermediatedikes are also similar to eachother. prohibitsthe existenceof a major dextral strike-slip Spider diagramsof samplesfrom theseyounger fault along the south margin of the San Gabriel suites(pigs. 8C and 8D) show somewhatless Mountains. We argue that the south branch San regularity for thesesamples than did the REE plots. Gabriel fault must be locatedsouth of the Glendora Many of the dike sampleshave elevatedabundances Volcanicsoutcrops at ElephantHill (Fig. 1, Stop 2). of Rb and K comparedto Glendorasamples, and 'I severaldike samplesdon't exhibit a Nb-Ta trough, 0.5132 whereasall of the Glendorasamples do. Roberts A l( (1995) suggestedthat the mafic-intermediatedikes MORB could be further subdividedinto severalgroups )k based 0 5128 on trace-elementcriteria. Nevertheless,the ! GlendoraVolcanics and mafic-intermediatedikes z exhibit broad similarity with respectto these E petrogeneticallysensitive elements, and oz severalof the 0.5124 andesiticdikes are compositionallyidentical to +t+ Glendora andesites. Available isotooicdata are summarizedin Figure 9. Differencesare evidentbetween the two 0.5120 tl agelcompositionalassemblages as well as within each 0.700 o.702 0.7M 0.706 0.708 0.710 assemblege.Samples from the older TelegraphPeak 12 pluton and felsic dike suiteshave moderatelvhieh 8tsr/86sr valuesand moderatelylow t+:*Ort+4*0" 0 values,suggestive of a continentalcrust origin. U Samplesfrom the youngerGlendora Volcanics and + mafic-intermediatedike suiteshave lower 8tsr/tus. )k ta:p67ra+116 valuesand higher valuesand are closein =o MoRB f-\ E magnitudeto primitive mantlefrom which mid- * oceanicbasalts (MORB) arederived. We believethat parentalmagmas to thesetwo suiteslikely originated in the mantle. Samplesfrom the two agel compositionalgroups overlap with respectto oxygen isotopevalues. At this time, the isotopicdata are 0 0.700 o.702 0.7u 0.706 0.708 0.710 insufficient to make more definite petrological ttsrfs, interpretations. FigqTg9. Symbologyas in Figure6. '*'Nd/'*Nd.Isotopicplots. o'Sr/ooSr. 9A,. vs Data from MORB derivedfrom depletedasthenosphere fall within the encircledarea. Lines representestimates of bulk 18O 87sr/tusr. Earth values. 98. uJ Area of MORB samplesagain indicated.

I 0-8 FIELD TRIP GUIDE AND ROAD LOG Stop 1A: Glendora Volcanics Overlain by Topanga Formation Directions to Stop 1A: WaIk through fence onto the peninsula Drive easton I-10. Exit onto I-210 westI 57 protruding eastward into PuddingstoneReservoir. Freeway north (to Pasadena).Take the first exit (Via Theseoutcrops include rhyolite, dacite, and andesite Verde Drive). Zero your odometerhere. flows and breccias that typify the uppermost Mile 0: Turn left onto Via Verde and approach stratigraphiclevels of the GlendoraVolcanics (see Bonelli RegionalPark. also Brem, 1981). This heterogeneousvolcanic Mile 0.1:Take the first left onto RagingWaters assemblageformsthe northwesilimb of a syncline Drive. Drive through southeast-dippingsection of cored by conglomerateand sandstoneof the upper PuenteFormation, then the stratigraphicallydeeper Middle Miocene TopangaFormation and marine Topanga Formation. siltstones,shales, and sandstonesof the Upper Mile 0.8: Angle right at south end of Puddingstone Miocene-LowerPliocene Puente Formation (Fig. Dam. Drive onto dam. 11). Basalt flows and rhyolitic water-lain(?)tuff are Mile 0.9: Turn left and immediatelypull off on exposeda few hundredmeters to the west. Andesite right to park. breccias and agglomeratesand basalt flows composingstratigraphically lower levels of the -'1. GlendoraVolcanics are exposedin the hills north of i' :" Tr" ;- -:Y-l- - -.|'t'a^--R PuddingstoneReservoir (Mull, 1934; Brem, i981).

? .4"2 Gently southeast-dippingconglomerate and sandstonebeds of the upper Middle Miocene Topanga Formation (Shelton, 1955) unconformably overlie the GlendoraVolcanics near the south =-@{:ffiM abutmentof PuddingstoneDam (Fig. 11). Conglomerateclasts are predominantlyderived from the GlendoraVolcanics. Intercalatedcoarse. cream ;r colored feldspathicsandstone beds are common. Stop 18: Stratigraphically Higher j ,:b-ii{.. j;i,,f/ Puente Formation t,.o*if" sf(-' * ."r'--'--:-ra ! )'i ""' i;i,..I ""^Li;;* ,6;----' #is -i":illj*.,...-^:t"fft,'"-*' Farther south along Drive, the I du*.*p,*,&4."-*fi srr |ftm TopangaFormation is overlain by moderately i ic'--'fty' ITTR'* southeast-dipping,light brown shaleand siltstoneof " {-g,iff.{t",i" l=' the Upper Miocene PuenteFormation (Mull, 1934). ;\ These basinal marine stiata are interstratified with 5m- to 2Om-thickconglomerate lenses generally composedof volcanic detritus. One distinct conglomeratehorizon is accessiblealong a paved road that anglesoff to the right 0.4 miles south of the dam (Fig. 10). Abundantbasement clasts, probably derived from the southeasternSan Gabriel Mountains,include biotite granite,quartzite, and mylonite similar to the "Black Belt mylonite" (Alf, 1948;Hsu and Edwards, 1963). Deep levels of the easternSan Gabriel Mountains basementwere apparentlyexposed and sheddingdebris into the Los ! ls: Angeles basin during depositionof the Puente _Jl Formation (see also Rumelhartand Ingersoll, 1997). t A similar stratigraphicsectionjincluding mollusc-bearingsandstone beds, forms the southeast flank of the syncline.Good exposuresoccur along the southernshoreline of PuddingstoneReservoir, accessiblethrough Bonelli Regio-nalPark (free of Figure 10. Road map showinglocation of field trip chargeon winter weekdays). stopsin circled numbers.

10-9 ofareanearStop'l.,Rockynits:Tv=intermedietetofelsicvolcanicbreccias, flo?s, and agglom"rut'*, fi=finely laminatedlacustrine(?) tuff or rhyolite,.Tb=basalt,Tcg=-c-6nt1.merate.and ,*drion", ff=siltstone, shale,uttd -ino. sandstoneof ihe PuenteFormation. (Mapping by J. Nourseand R. Acosta,1998).

Highlights en route to StoP 2: flank the valley to the west (righ0. Take Temple Avenue exit. Retrace route to I-210, stopping on the right Mrle 4.'7: Turn left onto Temple Avenue and cross to view outcropsof PuenteFormation. Cross the 57 Freeway.Elephant Hill (underlainby overpass. GlendoraVolcanics) is to the left. Mile 2.0: Turn right onto I-210 east/ 57 Freeway Mile 5.1: Tum left onto Mission Boulevard just south.Get in left hand lanes. Mile 5.8: Pull off to the left and park near gate Mile 3.2: Cross I-10 Freeway.The SanJose Hills, past housing develoPment. underlainby the Topangaand PuenteFormations,

t0-10 Stop 2: Interior View of a Mid-Miocene lqlqk Belt "mylonite(All 194g;Hsu and Edwards, Volcano at Elephant Hill 1963). Mile 20.0: Intersectionwith Shinn Road. Excavations Continue adjacentto the housing straightahead. Mt Baldy Road approximately developmentprovide a crosssection view t-hroueha follows the traceof the left-lateraiSan Antonio volcano that probably typified the Glendora Canyon fault (Nourseand others., 1994). Volcanics. Intermediate San and felsic flows and Antonio Creek incisesthe steepcanyon to brecciaswith prominent the right. flow foliation dip radially Pleistocenestream terraces exposed in away-from road cuts a central_felsiteplug. Rhyodaiite breciia reflecr youthful uplift of the area. sampledfrom near the center of the-volcanoand Mile 21.3: Evey Fault (a splay of the San Anronio daciteflow rock sampled from outer layersyielded fault) cuts Mesozoicquarttdiorite on the left. whole-rockIUAr ages of 18.8+1.2Ma hnd iZ.Z+O.q Mlle 21.9: Enter first of two tunnels.Roadcuts \4a, resp^ectively(I,. Herber,pers. comm., lgg4). qpoy" tunnelsexpose predominantly southwest- Of specialinterest are hypabyssaland dipping San Gabriel gneiss(here a mixture of mid- plutonic clastscontained in som-e'ofihe flo* Proterozoicand Mesozoicorthogneisses; see Nourse breccias.Biotite dacite po^ryhyryclasts bear a strong and others,1998). resemblenceto the Late OligoceneMounatain Mlle 73.2: SanGabriel gneiss structurally overlies MeadowsDacite. Also present are fragmentsof foliated Mesozoicquarti diorite along a 2m+hick leucocraticbiotite granite and quartz d]orite that mylonrte zone. resemblebasement exposed pomona in ihe area.The Mlle 23.4: Pull off and park on the right just before readeris referred to Brem (1981) for additionalfield Hogback saddle. descriptionsand discussionof the ElephantHill area. Stop 3: Felsic and Mafic Dikes Intrude San Highlights en route to Stop 3: Gabriel BasementlSynmagmatic and Postmagmatic Faults Continuedown Mission Boulevard. Mesozoic quartzdiorite basementunderlies This stop introducesLate Olieocene GlendoraVolcanics and along the lower slopesof the hill Middle Mioceneintrusive units that r6present to the south. possiblefeeders to Mountain Meadowj Dacite Mile 6.1: and Turn left onto HumaneWav. GlendoraVolcanics of the Pomona-Glendora Turn right onto Valtey Yil. 9.9' Boulevard. region. Crosscuttingfield relationsdemonstrate an Mile 7.0: Turn left onto Fairplei Drive. Glendora rntruslvesequence that will be viewed againat Stops Volcanicscrop out in the hilli straishtahead. 5 and 6. Thesetwo intrusiveunits braclet Mile 7.8: Turn-right onto I-10 east"bound(Fairplex movementson two families of brittle l)nve crosses faults that I-10 and traversesthe Glendora predatedmajor dextral displacement Volcanics. on the nearby To view the one known surfaceexposure north branchSan Gabriel fault. of Mountain Meadows Dacite, continue 1.2 miies up The roadcuton the west side of the Mt. Fairplexto its intersection with McKinley Avenue. Baldy Road displaysa moderatelynorthwest-dipping Good outcrops ^ occur on the southwestcorner of this dike of.rhyolite or rhyodaciteporphyry " rntersectton). sharply- intrudedinro southwest-dipping,weti-fbliatea^ Mlle 12.7: Exit 1-10 at Monte Vista Avenue.Turn Mesozoicquartz diorite. Sieeplv left. Procede dippine. north up Monte Vista Avenue. subhorizontallystriated, sticteniideO Basementexposures iauii surfaces of the easternSan Gabriel with north and west-northweststrikes pervade Mountainsare straight the ahead. rhyolite and its quartzdiorite host. Brittle Mile 14.1to 14.7: kinematic llluvial fan depositsderived indicatorson theseconjugate fault from San Antonio surfacesrecord Canyon are expbsedin quarry to sinsistraland dextralsheai senses, the left. respectively.A secondfamily of faults marked bv w6aktv striated Mile 15.7: CrossBaseline Avenue. hematitesurfaces and white breccia goue" Mile 17.5: Intersection o. ,ones with Mount Baldv Road. cuts the strike-slipfaults described Turn right abovi. these and drive along the approximite trace of steepto. moderately dipping, the.Cucamonga/_Sierra northwest-striking Midre ,everse fault (Crook raurrsclrsplay conststent normal offsets and others,1987). of both the rhyolite dike and pegmatitesills in quartz Mile 18.8: San Antonio the diorite. flood control/groundwater Four dark-gray,northwest-strikins rechargedam is basaltic to the right; quartzite-andpelitic andesiteor andesitedikes intrude gn-eis1in the footwall sharply-acrossthe of the Si-enaMadre fiult to the rhyolite and/orquartz diorite. Three left. The steepslopes oi thesedikes acrossthe canyonto the right appearto have intrudedinto the younger are underlainby the Cucamongagrinulite family of terrane normal faults,as displayedby offset riarker units on (May and Walker, 1989)which dils beneaththe

t 0-ll oppositesides of the dikes. This relationshipis Stop 5: Felsic and Mafic Intrusions Displaced by partially obscuredby a youngerfault overprint. Splays of the San Gabriel and San Antonio Canyon Faults (possible lunch break) Highlights en route to Stop 4: Immediatelyupstream from the Buckhorn Continueup Mt. Baldy Road,passing Mt. Restaurantare exposuresof southwest-dipping Baldy School on the right. mylonite and gneiss(Nourse and others, 1998) Mlle 24.9: Tum sharply left onto Glendora Ridge intrudedby rhyolite porphyry dikes and sills, in turn Road. Climb through exposuresof Cow Canyon intruded by a denseswarm of mafic and landslide,derived from Ontario Ridge acrossSan intermediatedikes. Various textures and Antonio Canyon to the left. compositionsof brown, dark gray, and light gray Mile 25.8: Pull off on right into parking areaat basaltor andesitedikes show chilled margins. These Cow Canyon saddle(Elevation 4527). outcropslie within the north branchof the San Gabriel fault zone, north of the major strand and Stop 4: View of San Gabriel Fault Zone south of a minor strand. Seventy mafic and (possible lunch break) intermediatedikes measuredin this areacluster into The valley to the west is controlledby at least two orientations:N45W/85NE and N80W60NE. three strandscomposing the north branchof the Commonly, oppositemargins of thesedikes exhibit San Gabriel fault zone. This fault recorded 22 km modestoffsets that suggestintrusion along shear of dextral slip, probably between12 Ma and 5 Ma fractures. (theseage constraintsare from the northwestend of The youngerdike swarm is disruptedby the San Gabrielfault nearRidge Basin; seeCrowell, brinle faults with distinct orientations and styles. An 1975). To the east, the SanGabriel fault is early northwesterlyfault set tendsto slice through truncatedby the San Antonio Canyon fault and the interiors of mafic dikes roughly parallel to their displacedsinistrally 3 km to lcehouseCanyon margins. Mesoscopickinematic featureson (Nourseand others, 1994). The intersectionof the subhorizontally striated fault surfacesindicate two faults is buried by the Cow Canyonlandslide, dextral shearsense. Younger,northeast-striking, which forms this geomorphicallypeculiar saddle. northwest-dippingfaults that cut the dikes at high Throughoutthe easternSan Gabriel anglesdisplay prominentleft-lateral and oblique- Mountains, the younger mafic-intermediatedike sinistralreverse offsets. In accordancewith map swarm is most concentratednear the San Gabriel scalerelationships discussed at Stop 4, the northwest fault zone (Fig. 1). Thesesteeply dipping dikes fault set is part of the San Gabriel fault system and predatethe main phaseof dextral displacementon the northeastfault set is related to the San Antonio the San Gabriel fault and are generally intruded Canyon fault. A third family of reverse faults along northwestor northeasttrends (Fig, 4). The marked by thick breccia zones and poorly striated dike orientationsare consistentwith existenceof a fault surfacesis oriented roughly parallel to transtensionalor pure dextral strike-slipstress southwest-dippingfoliation in the host gneisses. regime during intrusion. Commonly observed One of thesetruncates a set of five distinct mafic- mismatchesof markers acrossdike walls are intermediatedikes and recordssignificant offset. suggestiveof intrusion into conjugateshear Boulderswithin the flood plain of San fractures. Antonio Creek offer an opporrunityto examine diverseplutonic and metamorphiclithologies Hightights en route to Stop 5: composingthe local basement.Of particularinterest to this trip is a medium-grained,unfoliated, Retraceroute down GlendoraRidge Road. leucocraticbiotite granite porphyry derived from Mtle 26.7: Turn left onto Mt. Baldy Road and enter TelegraphPeak in the headwatersof IcehouseCreek. Mt. Baldy Village. As describedearlier, the TelegraphPeak pluton is Mile 27.0: Mt. Baldy Visitor Centeron the left (a the sourceof the early generation(Late Oligocene) good place for a restroombreak). rhyolite porphyry sills and dikes. Mile 27.3: Park on left about 150 m upstreamfrom the bridge. The canyonwall acrossSan Antonio Highlights enroute to Stop 6: Creekto the west displaysa wide angleview of the relationsat StoD5. Continueup the Mt. Baldy Road. To the right are Ontario Ridge and Sugarloaf Peak, both underlainby Cretaceoustonalite and granite intruded into metasedimentaryrocks (May and Walker, 1989). To the left, the shoulderof is underlainby mylonites and gneisses

l0-12 composingthe upper plate of the Vincent thrust Kinematic features here indicate Dure left-lateral (Nourse,1998). displacement. Mile 28.1: Cross IcehouseCreek (usually dry here). Mile 28.5: Intersectionwith IcehouseCanyon Drive; ACKNOWLEDGMENTS continueleft. The road climbs steeply(many switchbacks)through a landslidederived from Mt. Geologicalmapping in the San Gabriel San Antonio. Mountainsby J. Nourse was funded through the Mile 30.8: SnowcrestInn to the left. USGS Areal Mapping Project Mile 31.2: Turn left onto San Antonio Falls Road. and by a grant from California Division of Mines (locked gate).Drive or hike up this dirt fire road. and Geology. Field assistancefrom Ruben Acosta, Mile 31.6: Park on left. Erin Stahl, Matthew Chuang, Rogan Jones,Brent Norum, and Melissa Pratt is greatly appreciated. Stop 6: Felsic Sills and Mafic Dikes Intruded into L. Herber contributed unpublished IUAr dates from PelonaSchist; Younger Transpression the Elephant Hill area. P. Weigand thanks the Along the San Antonio Canyon Fault Collegeof Scienceand Mathematicsand the ResearchGrants committee, both at CSU Northridge, Walk up the road througha west dipping for helping to fund much of the geochemicaland sectionof PelonaSchist that composesthe footwall dating work reported here, and the Student Projects of the Vincent thrust. Greenschist,grayschist, and Committee of the University Corporation for an quartzitederived from mafic igneousrocks, awardto G. Robertsfor geochemicalanalyses. G. graywacke/mudstone, and chert, respectively,are Robertswas responsiblefor the beginning strongly shearedand transposed.These oceanic geochemicalwork. G. Brem aided in collecting rocks were overthrustby San Gabrielcontinental somekey samples. G. Hazeltonwould like to thank basementduring late Mesozoic-EarlyCenozoic time M. Harrisonfor use of the argon massspectrometer (Ehlig, 1981). Along this stretchof the road,the facilitiesat UCLA and M. Grove for his assistance metamorphic fabrics are intruded by several rhyolite with the 40{rl3eAranalyses of the Mountain porphyry dikes and one mafic dike. MeadowsDacite. This study is an outgrowthof From the first switchback(mile 31.8),a view Hazelton'sMS thesiscompleted at SDSU. of San Antonio Falls revealsuniformly dipping PelonaSchist intrudedby two prominentrhyolite REFBRENCES CITED porphyry sills, in turn cut by thinnermafic- intermediatedikes. Severalnortheast-striking faults Alf, R. M., 1948,A mylonite belt in the southeastern disrupt the continuity of the rhyolite sills. Apparent San Gabriel Mountains,California: left-lateral and/or reverse displacementsare GeologicalSociety of America Bulletin, v. compatablewith more accessibleexposures observed 59, p. 1101-1120. fartherup San Antonio Falls Road. Follow a nrurow Brem.G. F.. 1981.Middle Miocenevolcanic rocks trail to the baseof the falls. A somewhattreacherous of the ElephantHill-Puddingstone Reservoir scrambleup the talus slopesouth of the falls will area,in Woyski, M. S., ed., Tour and field afford up-closeexamination of the intrusive guide to petroleum researchand production relationships. facilities, the geology of the easternPuente The road climbs another3.5 miles to the ski Hills, northeasternLos Angelesbasin, and areafacilities, during which it crossesthe San the coastalgeomorphology of southwestern Antonio Canyonfault four times as it bendsfrom a OrangeCounty, Califomia: Natonal N30E to N80E strike. Brittle strain near this fault Associationof Geology TeachersFar appearsto be partitioned into reverseand sinistral WesternSection, p. 75-84. componentsto accomodatelocal transpression.For Bortugno,E. J. and Spittler,T. E., 1986, example,near Manker Creek crossing(mile 32.6 to Geologic Map of the 1:250,000San 32.9, about300m northwestof the San Antonio BernardinoQuadrangle, California: fault), rhyolite porphyry sills in PelonaSchist are California Division of Mines and Geology. offset by a family of reverse faults. Differential Crook,R., Jr., Allen, C. R., Kamb,8., Payne,C. M., movementsbetween reverse faults have rotated schist and Protor, R. J., 1987,Quaternary geology foliations to steepsouthwest dips and resultedin and seismic hazard of the Siena Madre and moderatenortheast dips for two mafic-intermediate associatedfaults. western San Gabriel dikes" One mafic dike exposed120 m up Manker Mountains,in Morton, D. M., and Yerkes,R. Creek displayssignificant sinistral offsets. Farther F., eds.,Recent reverse faulting in the up the road (mile 34.4), the main traceof the San TransverseRanges, Californial p . 27-63. Antonio fault is well exposedbeneath the ski lift. Crowell, J. C., I975, The San Gabriel fault and Ridge basin,southern California, in Crowell, J. C.,

10-'l3 ed., San Andreasfault in southernCalifornia: May, D..J.,and Walker, N. W., 1989,Late Cretaceous Califomia Division of Mines and Geology juxtaposition of metamorphicterranes in the SpecialReport 118, p. 208-218. southeasternSan Gabriel Mountains, Dibblee,T. W., 1982,Geology of the SanGabriel California: GeologicalSociety of Amenca Mountains,southern California, in Fife, D. Bulletin,v. 101,p. 1246-1267. L., and Minch, J. A., eds.,Geology and Miller, F. K., and Morton, D. M., 1977,Comparison mineral wealth of the Califomia Transverse of granitic intrusionsin the Pelonaand Ranges;Mason Hill Volume: SantaAna, CA, OrocopiaSchists, southern Qalifornia: U. S. South CoastGeological Society Annual GeologicalSurvey Journal of Research,v. 5, Symposiumand GuidebookNumber 10, no. 5, p.643-649. p. r31-r47. Morton,D. M., amd Matti, J. C., 1991,Geolocic map Ehlig, P. L., 1981,Origin and tectonichistory of the of the Devore 7.5 minute quadrangle,San basementterrane of the San Gabriel Bernardino County, California: United States Mountains,central Transverse Ranges, in GeologicalSurvey Open File Map OF90- Emst, W. G., ed.,The geotectonic 695. developmentof califomia: Englewood Mull, B. H.,1934, Miocenevolcanics of the SanJose Cliffs, NJ, Prentice-Hall,p. 254-283. Hills [M.S. thesis]:Berkeley, University of Gill, J. B., 1981,Orogenic andesites and plate California,95 p. tectonics: Berlin, Springer,389 P. Nakamura,N., 1974,Determination of REE, Ba, Fe, Hazelton,G. 8., 1993,A palinspasticreconstruction Mg, Na, and K in carbonaceousand ordinary of the easternSan Gabriel Mountainsbased chondrites: Geochimicaet Cosmochimica on the occurrenceof a middle Cenozoicdike Acta,v. 38, p.757-775. swarn [M.S. thesis]:,California Nicholson,C., Sorlien,C. C., Atwater,T., Crowell,J. StateUniversity, 158 p. C., and Luyendyk, 8. P., 1994,Microplate Hazelton,G. 8., andNourse, I. A.,1994, Constraints capture, rotation of the western Transverse on the direction of Miocene extensionand Ranges,and initiation of the San Andreas degreeof crustaltilting in the easternSan systemas a low anglefault system:Geology, GabrielMountains, southern California: v.22, n. 6, p. 491-a95. GeologicalSociety of America Abstracts Nourse,J. A., 1991,Upper plate tectonostratigraphy with Programs,v. 26, n. 2, p. 58. and structure of the Vincent thrust, eastern Higgins, R. E., 1976,Major-element chemistry of the and centralSan Gabriel Mountains, Cenozoicvolcanic rocks in the Los Angeles California: GeologicalSociety of America basinand vicinity, in Howell,D. G., ed., Abstractswith Programs,v. 23, n.7, p. A480. Aspectsof the geologichistory and the Nourse,J. A., Hazelton,G. 8., andJones, R. K., 1994, California continental borderland: Evidencefor two phasesof late Cenozoic AmericanAssociation of Petroleum sinistraldisplacement on the San Antonio GeologistsMiscellaneous Publication 24, p. Canyon fault, easternSan Gabriel Mountains, 216-227. California, GeologicalSociety of America Hornafius,J. S., Luyendyk,B. P., Terres,R. P., and Abstractsw/ Programs, v. 26, n- 2, p. 77. Kamerling,M. J., 1986,Timing and extent Nourse,J. A., Acosta,R. G., Stahl,E. R. andChuang, of Neogenetectonic rotation in the western M. C., 1998,Geologic Map of Los Angeles TransvereseRanges, California: Geological County portionsof the Mt. Baldy and Societyof AmericaBulletin, v. 97, p. 1416- Glendora7.5 minute quadrangles: 1481. GeologicalSociety of America Abstracts Hsu, K. J., andEdwards, G., 1963,Age of the with Programs,v. 30, n.5, p 56. intrusiverocks of the southeasternSan Powell, R. E., 1993,Balanced palinspastic Gabriel Mountains:Geological Society of reconstructionof pre-lateCenozoic AmericaBulletin, v.74, p. 507-5I2. palegeology,southern California: geologic Irvine, T. N., and Barager,W. R. A., 1971,A guideto and kinematic constraintson evolution of the the chemicalclassification of the common SanAndreas fault system,in Powell, R. 8., volcanicrocks: CanadianJoumal of Earth Weldon, R. J., andMAtti, J. C., eds.,The San Sciences,v.8, p. 523-548. Andreasfault system:displacement, Le Bas.M. J.. Le Maitre, R. W., Streckeisen,A., and palinspasticreconstruction, and geologic Zanettin,8., 1986,A chemicalclassification evolution: GeologicalSociety of America of volcanicrocks basedon the total alkali Memoir 178,p. f -i06. silica diagram: Journalof Petrology, v. 2'7, Roberts,G., 1995,Geochemical study of Miocene o. 745-750. intrusiveand volcanic rocks in the eastern San Gabriel Mountains,California [Senior

10-.|4 Thesisl:Northridge, California State University,24 p. Rumelhart.P. E. and Ingersoll,R. V., 1997, Provenanceof upper Miocene Modelo Formationand subsidenceanalysis of the Los Angelesbasin, southern California: implicationsfor paleotectonicand paleogeographicreconstructions: Geological Societyof AmericaBulletin, v. 109,n.'7, p. 885-899. Shelton,J. S., 1955,Glendora Volcanic rocks, Los Angelesbasin, California: GeologicalSociety of America Bulletin, v. 66, p. 45-90. Thompson,R. N., Morrison,M. A., Hendry,G. L., a nd Parks,S. J., 1984,An assessmentof the relativeroles of crust and mantle in magma genesis: An elementalapproach: PhilosophicalTransactions of the Royal Societyof London,Series A, v. 310,p. 549- 590. Weigand,P. W., 1982,Middle Cenozoicvolcanism of the westernTransverse Ranges, in Fife, D. L., and Minch, J. A., eds.,Geology and Mineral Wealth of the CaliforniaTransverse Ranges, MasonHill Volume: SantaAna, CA, South CoastGeological Society Annual Symposiumand GuidebookNumber 10, p. 170-188. Weigand,P. W., Nourse,J. A., andHazelton, G. B., 1994,Miocene intrusive rocks and Glendora Volcanicsin the easternSan Gabriel Mountains,California: calc-alkalinerocks in an extensionalsetting, Geological Society of America Annual Abstractswith programs, v. 26. n. 1, p. A-293. Wright, T. L,7991, Structuralgeology and tectonic evolutionof the Los Angelesbasin, California,in Biddle, K. T., ed., Active margin basins:American Associationof Petr6leumGeologists Memoir 52, p. 35- 134. Woodford,A. O., Moran,T. G., and Shelton,J. S., 1946,Miocene conglomeratesof Puenteand SanJose Hills, California:American Associationof PetroleumGeologists Bulletin, v. 30, no. 4, p. 574-560. Yerkes,R. F., McCulloh,T. H., Schoellhamer,J. E., and Vedder,J. G., 1965,Geology of the Los Angelesbasin California- an introduction: United StatesGeological Survey Professional Paper420-4., p. 57.

'l 0-15 Field Trip Program, 94th Annual Meeting of the Cordilleran Section, GeologicalSociety of America, April 3-10, 1998, CSU Long BeachnCA.

1. Anatomy of a Craton Margin Hinge Zone: Sequence Stratigraphy of Upper Neoproterozoic - Basal Cambrian Succession, Eastern , California. Leaders:John Cooper,California State Univ. Fullerton;Chris Fedo,George Washington University. 2. Keeping Ahead of the Bulldozers - Orange County Geology. Leaders:Debra Brooks, Santiago Canyon College; Elizabeth Simmons, FREY Environmental,Inc. 3. San Antonio Canyon: Cultural Geography, Geology, Late Cenozoic Geology, Geomorphology, and Environmental Geology of an Alpine Valley in southern California. Leader:Larry Herber,Cal Poly Pomona. 4. The lndependence Dikes and Malic Rocks of the Eastern Sierra. Leaders:Allen Glaznerand BrianCarl, University of NorthCarolina, Chapel Hill. 5. Aureole Deformation Associated with Inflation of the Concordant Eureka Valley-Joshua Flat-Beer Creek Composite Pluton, Central White-lnyo Range, Leaders:Sven Morganand RickLaw, Virginia Tech. 6. Late Miocene-PleistoceneDetachment Faulting In The Northern Gulf Of California and its Role in Evolution of the Pacific-North American Plate Boundary. Leaders:Gary Axen, UCLA;ArturoMartin-Barajas and John Fletcher,CICESE. 7. Geology and Marine Geophysics of Catalina lsland and the California Continental Borderland. Leaders:R. Dan Francis,California State University Long Beach; Robert G. Bohannon,U.S. G.S. 8. Old Oil Fields and a New Life: A Visit to the Los Angeles Basin. Leaders:Don Clarke,City of Long Beach;George Otott, Thums Long Beach Company. 9. The Saint Francis Dam Story: No Place for Poor Geology - Not by a Dam Site! Leader:Jack Green,California State University Long Beach. 10. lgneous and Tectonic Response of the Eastern San Gabriel Mountains to Neogene Extension and Rotation of the Transverse Ranges. Leaders:Jon Nourse,Cal PolyPomona; Peter Weigand, CSU Northridge;Garrett Hazelton, UCLA. 11. Active Folding And Buried Reverse Faulting, Santa Barbara Fold Belt, California. Leaders:Ed Keller,Larry Gunola, Molly Trecker, and Ross Hartleb,UC Santa Barbara 12. THUMS Oil lslands and Oil Tour of Historical Long Beach. Leaders:Don Clarke,City of Long Beach;George Otott, Thums Long Beach Company. 13. Late Cretaceous Denudation History of the Peninsular Ranges as Recorded in Upper Cretaceous - Paleocene Sedimentary Rocks, Northern . Leaders:Pat Abbott,Dave Kimbrough, and ChuckHezig, SDSU;Marty Grove, UCLA.

For a limited time, copies of these outstanding guidebooks can be purchased from: Departmentof GeologicalSciences CaliforniaState University, Long Beach 1250 BellflowerBoulevard LongBeach, CA 90840-3902USA Phone:562-985-5850 Fax 562-985-8638