Age and Tectonic Significance of Volcanic 

Rocks in the Northern Los Angeles Basin, 

California

By Thane H. McCulloh, Robert J. Fleck, Rodger E. Denison, Larry A. Beyer, 

and Richard G. Stanley

Volcanic rocks in the eastern Santa Monica Mountains dated at 17.4 Ma by argon isotopes and by 

strontium isotope ratios of interbedded fossil carbonates appear to be an early expression of 

deep crustal magmatism accompanying the earliest extensional tectonism associated with 

rifting in the Los Angeles Basin

Professional Paper 1669

 

U.S. Department of the Interior

U.S. Geological Survey 
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply endorsement 

by the U.S. Government.

United States Government Printing Office: 2002

For additional copies please contact:

USGS Information Services

Box 25286

Denver, CO 80225

This report and any updates to it are available at

http://geopubs.wr.usgs.gov/prof-paper/pp1669/

Additional USGS publications can be found at

http://geology.usgs.gov/products.html 

For more information about the USGS and its products:

Telephone: 1–888–ASK–USGS (1-888-275-8747)

World Wide Web: http://www.usgs.gov/

Text edited by Peter H. Stauffer

Layout and design by Stephen L. Scott 

Manuscript approved for publication, September 19, 2002

FRONT COVER

Mapped outcrops and known and inferred subsurface occurrences of Miocene volcanic rocks (orange colors), principal faults, and important 

drill holes (dots) draped on a shaded relief map of the study area. Geology and drill holes adapted from figure 1, p. 3.

(Shaded relief base is digital elevation model, using 30-meter cell size, from the U.S. Geological Survey National Elevation Dataset (NED). 

Topography has vertical exaggeration of x2 and sun illumination from azimuth 315° and elevation 45°. For more information on the NED, 

see http://gisdata.usgs.gov/ned/.) 

Cataloging-in-Publication data is on file with the Library of Congress
CONTENTS

Abstract--------------------------------------------------------------------------------------------- 1

Introduction---------------------------------------------------------------------------------------- 1

Topanga Formation of the eastern Santa Monica Mountains-------------------------------- 2

Ages of Topanga volcanics in the eastern Santa Monica Mountains----------------------- 5

 Basalt Ar-Ar isotopic ages-------------------------------------------------------------------- 5

 Fossil carbonate strontium isotope ages----------------------------------------------------- 5

Age of the oldest Glendora Volcanics----------------------------------------------------------- 5

Other Los Angeles Basin volcanic rocks------------------------------------------------------- 8

Discussion, conclusions, and recommendations----------------------------------------------- 9

 Tectonic implications-------------------------------------------------------------------------- 9

 Stratigraphic and age relationships---------------------------------------------------------- 11

 Ar and Sr isotopic dating---------------------------------------------------------------------- 11

 Extent of 17.4-Ma volcanics of Topanga Formation (Hoots, 1931)--------------------- 11

 Age trends of volcanism in southern California------------------------------------------- 12

Acknowledgments--------------------------------------------------------------------------------- 12

References------------------------------------------------------------------------------------------ 12

Appendix. Techniques and interpretation of 40Ar/39 Ar and 87Sr/86Sr dating--------------- 16

 40Ar/39Ar analytical techniques--------------------------------------------------------------- 16

 Interpretation of 40Ar/39Ar incremental-heating experiments---------------------------- 17

 40Ar/39Ar results from the eastern Santa Monica Mountains------------------------- 17

 40Ar/39Ar results from the Glendora Volcanics----------------------------------------- 19

 87Sr/86Sr dating method and techniques----------------------------------------------------- 23

 

III

FIGURES

1. Map showing outcrops and subsurface occurrence of Miocene volcanic rocks within the Los Angeles Basin

 and around its perimeter-------------------------------------------------------------------------------------------------------------- 3

2. Geologic map of the eastern Santa Monica Mountains showing Topanga Formation (Hoots,1931), volcanic

 outcrops, and other features---------------------------------------------------------------------------------------------------------- 4

3. Plot of strontium isotope results from table 2 showing ages derived from curve of 87Sr/86Sr variation in 

 seawater for part of the Miocene----------------------------------------------------------------------------------------------------- 8

4. Chronological chart of Miocene volcanic rocks in and around the Los Angeles Basin-------------------------------------- 10

5. Diagrams of the 40Ar/39Ar age spectrum and Cl/K ratio from incremental heating of basalt samples A, B, and

 C from the Topanga Formation (Hoots, 1931) in the eastern Santa Monica Mountains------------------------------------- 20

6. Diagrams of the 40Ar/39Ar age spectrum, Cl/K ratio, and inverse isochron from incremental heating of 

 Glendora Volcanics samples GV-9 and GV-12------------------------------------------------------------------------------------ 21

7. Diagrams of the 40Ar/39Ar age spectrum, Cl/K ratio, and inverse isochron from incremental heating of 

 Glendora Volcanics samples GV-17 and GV-18----------------------------------------------------------------------------------- 22

TABLES

 1. Locations and 40Ar/39Ar ages of volcanic rock samples from the the Topanga Formation (Hoots, 1931)

in the eastern Santa Monica Mountains and from the Glendora Volcanics in the San Jose Hills, California-------------- 6

 2. Locations, analytical results, and derived 87Sr/86Sr ages of fossil carbonate samples from the Topanga 

Formation (Hoots, 1931) of the eastern Santa Monica Mountains and the Topanga Formation type area, California--- 7

 3. Incremental heating 40Ar/39Ar ages of volcanic rocks of the Topanga Formation (Hoots, 1931) from the 

eastern Santa Monica Mountains and Glendora Volcanics, California---------------------------------------------------------- 18

 
1

Age and Tectonic Significance of Volcanic Rocks in 

the Northern Los Angeles Basin, California

By Thane H. McCulloh1, Robert J. Fleck2, Rodger E. Denison3, Larry A. 

Beyer2, and Richard G. Stanley2

17136 Aberdeen Avenue, Dallas, Texas 75230

2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 

94025

3Department of Geosciences, University of Texas at Dallas, P.O. Box 830688, 

Richardson, Texas 75083-0688

Abstract

Volcanic rocks, mostly basalts and some andesites, are 

interbedded with middle Miocene strata and are overlain by 

younger rocks throughout the greater part of the Los Angeles Basin, California. Roughly correlative flows, previously 

dated radiometrically (or paleontologically) at about 16.4 to 

10.7 Ma, crop out in five separate regions around the basin 

perimeter. Los Angeles Basin volcanic rocks have special 

meaning because they offer clues to tectonomagmatic events 

associated with onset of clockwise transrotation of the western Transverse Ranges region and to the timing and locus of 

the initial basin opening.

Whole-rock 40Ar/ 39Ar dating of near-tholeiitic olivine 

basalts of the Topanga Formation (Hoots, 1931) from three 

sites in the easternmost Santa Monica Mountains, combined 

with 87Sr/86Sr dating of fossil carbonates from interstratified 

marine beds at nine sites, establish a new age of 17.4 Ma for 

these oldest known Topanga-age volcanics of the Los Angeles Basin. We also record three new 40Ar/39Ar ages (15.3 

Ma) from andesitic flows of the lower Glendora Volcanics 

at the northeast edge of the basin, 70 km east of the Santa 

Monica Mountains. A whole-rock determination of 17.2±0.5 

Ma for nearby altered olivine basalt in the unfossiliferous 

Glendora volcanic sequence is questionable because of a 

complex 40Ar/39Ar age spectrum suggestive of 39Ar recoil, 

but it may indicate an older volcanic unit in this eastern area. 

We hypothesize that the 17.4-Ma volcanics in the eastern Santa Monica Mountains are an early expression of 

deep crustal magmatism accompanying the earliest extensional tectonism associated with rifting. The extremely thick 

younger volcanic pile in the western and central parts of the 

range may suggest that this early igneous activity in the eastern area was premonitory. Paleomagnetic declination data 

are needed to determine the pre-transrotational orientation of 

the eastern Santa Monica Mountains volcanic sequence. The 

new age determinations do not yield unequivocal support for 

either of two proposed explanations of possible age trends of 

Miocene volcanic rocks in southern California but underscore 

the need for further work. 

Introduction

Deformed middle Tertiary extrusive volcanic rocks with 

locally associated or separate shallow intrusives are a widespread though volumetrically minor constituent of the Cenozoic stratal sequences in parts of coastal southern California 

and the California Continental Borderland. Exposures of such 

volcanic rocks extend for more than 400 km along the coast 

from Point Arguello southeast through the southern part of 

the western Transverse Ranges Province to the Los Angeles 

Basin and Continental Borderland and into coastal Baja California. Tectonically important because of constraints they 

impose on the nature and timing of major middle Tertiary 

crustal rearrangements, these rocks represent multiple separate volcanic fields that were fed from numerous independent 

eruptive or intrusive centers, many of them now buried or 

submerged (Shelton, 1955; Crouch, 1981, p. 204-205; Vedder, 

1987; Legg, 1991, p. 294-295).

The ages of previously dated volcanic rocks in this 

region range between extremes of 4.36±0.8 Ma for alkalic 

basalt at Northeast Bank (Hawkins and others, 1971) and 

19.1± 0.7 Ma for shallow hypabyssal granodioritic intrusives 

on Santa Catalina Island (Forman, 1970, table 1; Vedder 

and others, 1979) and 18.5±1.0 Ma for a basalt flow on San 

Miguel Island (Luyendyk and others, 1998). However, the 

great majority of dated rocks fall in a narrow range of 16-

15 Ma (with noteworthy outliers at 17.7 Ma and 11.0 Ma) 

(Turner, 1970; Turner and Campbell, 1979; Dickinson, 1997, 

table 2; Nourse and others, 1998; Luyendyk and others, 

1998). 

Hypotheses or explanations advanced to account for the 

nature, timing, and locations of middle Tertiary volcanism 

in coastal southern California, including the Los Angeles 

Basin, appeal to ridge-trench interactions associated more or 

less closely with the partial subduction of the Monterey and 

Arguello microplates beneath North America and their subsequent capture by the Pacific plate (Atwater, 1989; Severinghaus and Atwater, 1990; Nicholson and others, 1994; Atwater 

and Stock, 1998, p. 392-395). Results reported here do not 

support an appealing hypothesis (Luyendyk and others, 1998, 

fig. 3), based on published data from the coastal region south 

Introduction
2 Age and Tectonic Significance of Volcanic Rocks in the Northern Los Angeles Basin, California 

of the southern Coast Ranges, that suggests a systematic trend 

from oldest in the west (originally south, prior to transrotation) to youngest in the east (originally north).

Middle Tertiary volcanic rocks are mostly flows, some 

subaerial but many submarine. Hypabyssal intrusives and 

tuffs, breccias, and other pyroclastic or volcanogenic sediments occur but are subordinate. Rhyolites and dacites are 

widespread but relatively rare and tend to be among the 

younger units. The most abundant rocks are basaltic andesites 

and alkalic basalts in a calc-alkaline magma series that borders on tholeiitic basalt (Shelton, 1955; Eaton, 1958; Crowe 

and others, 1976; Higgins, 1976; Weigand, 1982; Johnson 

and O’Neil, 1984, tables 1 and 2; Weigand and Savage, 1993; 

Weigand and others, 1998).

Miocene lavas and fragmental volcanic rocks intercalated 

with mostly marine clastic strata, together with local hypabyssal intrusives, crop out discontinuously around the perimeter 

of the Los Angeles Basin (Hoots, 1931; Woodring and others, 

1946; Shelton, 1955; Yerkes, 1957; Vedder and others, 1957, 

Dibblee, 1982; Conrad and Ehlig, 1983). Correlative volcanic units occur in drill holes within the basin beneath variable thicknesses of marine clastic strata (White, 1952; Eaton, 

1958; Yerkes and others, 1965, West and Redin, 1990; West 

and Redin, 1991; Wright, 1991, fig. 5; Blake, 1991, McCulloh 

and others, 2001). Figure 1 shows the distribution of both 

outcrops and drill-hole occurrences compiled from available 

sources, together with our best estimates of present and likely 

original distributional limits. Locations of critical drill holes 

that provide evidence for the areas where volcanics are absent 

are not shown because they are published elsewhere (Eaton, 

1958; Yerkes and others, 1965; Wright, 1991, figs. 4, 5). We 

draw attention especially to the absence of evidence for volcanic rocks in a large subsurface region west of long. 118°W, 

north of lat. 34°N, and south of the Santa Monica-HollywoodRaymond Fault Zone. Possible meanings for this absence 

will be discussed below. Outlined on figure 1 is an area in the 

easternmost Santa Monica Mountains where samples, whose 

isotopic ages are reported here, were collected—basalts (three 

localities) and marine carbonate fossils from beds above, 

within, and beneath submarine flows (nine localities). Also 

shown are outcrops of Glendora Volcanics in the northeastern 

San Jose Hills. Isotopic ages of four samples from the basal 

and lower flows of this sequence are also reported here.

Topanga Formation of the Eastern Santa 

Monica Mountains

The Topanga Formation is a Miocene mixed sedimentary 

and volcanic unit in the Los Angeles region first defined by 

Hoots (1931). The thickness of the formation (Hoots, 1931) 

in the easternmost Santa Monica Mountains probably exceeds 

1,650 m. Its base is an erosional unconformity. In the western part of this area, marine basal Topanga sandstones rest 

on uppermost Paleocene conglomerate and sandstone (Colburn and Novak, 1989, figs. 3, 8) that, in turn, overlie coarse 

unfossiliferous Upper Cretaceous clastic strata (fig. 2). East 

of Cahuenga Pass and the northwest-plunging syncline that 

parallels the pass, either basalt or nonmarine sandstone rests 

directly on Lower Cretaceous granodiorite or quartz diorite 

(Hoots, 1931; Miller and Morton, 1980), all pre-Miocene 

strata and some older Miocene units present in the western 

and central Santa Monica Mountains having been overlapped 

(Dibblee, 1989, fig. 7).

The Topanga Formation (Hoots, 1931) is divisible, both 

east and west of Cahuenga Pass, into three parts: (1) A basal 

marine conglomeratic sandstone that ranges in thickness 

from zero in the east to 100 m in the west, (2) a dominantly 

basaltic middle unit of multiple submarine lava flows and 

tuffs as much as about 670 m thick, and (3) an upper unit of 

sedimentary breccia, conglomerate, sandstone, and siltstone 

that probably exceeds 1,000 m in thickness and locally contains marine fossils. Fossiliferous calcareous sandstones, 

calcarenites, and coquinoid limestones are interbedded with 

the basalt flows and also occur beneath and above the flows. 

The locations of basalt samples used for Ar-Ar dating and 

of fossils analyzed for 87Sr/86Sr are shown on figure 2. The 

stratigraphically highest of these (locality b) is identical to 

UCLA locality 2303, which yielded a fauna identified as 

“Middle Miocene...Temblor” on the basis mainly of the presence of Lyropecten crassicardo and Turritella ocoyana (W. P. 

Popenoe in Neuerburg, 1953, p. 23) and is younger than any 

of the nearby lavas. The stratigraphically lowest site (locality f) underlies all of the volcanic units and is virtually at the 

basal unconformity.

A hiatus and angular erosional unconformity separates 

the Topanga Formation from the overlying deep marine 

Miocene Modelo Formation (Hoots, 1931). Deposited on 

a submarine fan complex, the Modelo consists of shale and 

sandstone, clastic detritus derived from erosion of mountainous terrain presently to the north of the Los Angeles Basin 

(Sullwold, 1960; Redin, 1991; Dibblee, 1991). The area of 

Modelo Formation mapped at the easternmost end of the Hollywood Hills as resting unconformably on Topanga Formation and granodiorite (Hoots, 1931, plate 16; Dibblee, 1989, 

1991) was considered part of the “Topanga Formation” by 

Lamar (1970). Foraminiferal faunas from several localities in that area belong in the lowermost Mohnian benthic 

foraminiferal stage (Lamar, 1970, p. 19) and therefore fit 

better both faunally and lithologically with the Modelo (or 

Puente) Formation (Blake, 1991). We endorse the assignment 

of these rocks to the Modelo Formation, which reinforces our 

judgement of the prominence of the pre-Modelo erosional 

break and angular unconformity. 

The great thickness and stratigraphic complexities of 

the sequence that Kew (1923) originally defined faunally and 

named “Topanga Formation” led later workers to subdivide 

his “Formation” into three formations (Durell, 1954) and 

subordinate members and to elevate the name Topanga to 

group rank (Yerkes and Campbell, 1979, p. E13-E24, fig. 3). 

The site of the well-known Topanga Canyon molluscan fauna 

(Arnold, 1907, p. 525-526; Kew, 1923, p. 48), now in the type 

section of the lowest formational unit of the Topanga Group 

in the central Santa Monica Mountains, the Topanga Canyon 
3

Formation (Yerkes and Campbell, 1979), is located about 22 

km west of our map area (fig.2). Further elaborations of the 

subdivisions of the Topanga Group emphasize their facies 

equivalency (for example, Fritsche, 1993, fig. 4). The base of 

the very thick Conejo Volcanics (middle formational unit of 

the Topanga Group in the central Santa Monica Mountains) is 

no older than upper Relizian (Yerkes and Campbell, 1979, p. 

E21), or about 16 Ma according to the most recent pertinent 

chronostratigraphic framework (Barron and Isaacs, 2001, 

fig. 22.1). This foraminifer-based age essentially agrees with 

the 15.9±0.8-Ma K-Ar age of plagioclase from basalt “150 ft 

above base Conejo Volcanics” (Turner and Campbell, 1979, 

p. E21, table 1), with recalculation using current decay constants. We are not yet prepared to say that the stratigraphic 

nomenclature developed in the western and central Santa 

Monica Mountains is applicable directly to rocks of the eastTopanga Formation of the eastern Santa Monica Mountains

Figure 1.—Outcrops and subsurface occurrences of Miocene volcanic rocks located within the Los Angeles 

Basin and around its perimeter. Locations of drill holes (without volcanics) drilled to pre-volcanic sequences 

that help to define the limits of the subsurface areas of volcanic rocks are published elsewhere (Wright, 1991, 

figs. 4, 5). Fault abbreviations: BC, Benedict Canyon; C, Chino; EM, East Montebello; H, Hollywood; LC, Las 

Cienegas; N-I, Newport-Inglewood; PV, Palos Verdes; R, Raymond; SM, Santa Monica; SM-D, Sierra MadreDuarte; SV, San Vicente; W, Whittier. Five oil fields mentioned in the text are labeled: DOF, Dominguez; HBOF, 

Huntington Beach; INOF, Inglewood; LCOF, Las Cienegas; LBAOF, Long Beach Airport. Inset frame shows area 

of figure 2. Locations of new Ar-Ar dated Glendora Volcanicsamples are also shown.

�����

�����

�������

������

�

�

��

�

�

� � � �

�

� � �

����������

� ��

�������� �������

����

�����

������� ����

������������������������������������� ��������

���������

��

������

���������

���

�������

�����

��������������������������������������������������������

������������������

� � � � � �

�

��

��

�

��

����

��

������

�

�

�

���

�����

� � �

� � � �

�

�

�

�

�

�

�

�

�

����

����

��

������������������������������

�������������

�

�

�

�

�

�

�

� �

��������

����������������������

�������

����������

���������������������

����������

�������������

���������

�

�

���

�������

������

��������������

��

�����������

��������������������

���������������������

�����������������������������

������������������������������

�

���������������������

��������������������������������������������

�����������������������������������������������

������������

���������������������������������������

�������������������������������������������������

����
4 Age and Tectonic Significance of Volcanic Rocks in the Northern Los Angeles Basin, California 

a

b

c

d e B

f

g

C

h

i 53

50

77

55

65

58

58

65

65

53

33

72

47

64

47

37 33

40

89

44 70

70

30

20

20

34

22 24

46

20

54

35

25

30

25

35

32

27

37

53

50 25

33

50

47

60

63

57

45

50

52

40

60

40

35

82

26

30

60

���

CAHUENGA PASS

L

O S LOS

A N G E L E S R I V E R

EXPLANATION

34�

07.5'N

118�17.5' 118�20' 118�22.5'W

0 1 2

KILOMETERS

T. 1 N.

T. 1 S.

R. 14 W. R. 13 W.

R. 15 W. R. 14 W.

A

A'

A

? ? ?

?

? ? ?

?

?

? ?

NO VERTICAL EXAGGERATION

A

A'

TEEF

0

2000

4000

SRETE M

0

10001500

500

? ?

?

BC

HL

H

enecoi M

40 Strike of strata with dip amount

Fold axis (dashed where projected)

Upper Cretaceous &Paleocene clastic rocksJurassic metamorphicsand Cretaceous granitoidplutons

Modelo FormationTopanga Fm. coarseclastic strata, basalt(dark), & limestone

Quaternarydeposits

Sites of dated samples

Ar/ Ar 40 39

Sr/ Sr 87 86 Fossil carbonate

Basalt

(

( )

)

Figure 2.—Geologic map of the eastern Santa Monica Mountains showing Topanga Formation (Hoots, 1931) volcanic outcrops, associated older and younger sedimentary strata, metamorphic and plutonic basement rocks, localities of samples of dated basalts and fossil carbonates, and location of vertical section A-A’ (shown in inset in upper left-hand corner). Important input came from Hoots (1931), Neuerburg (1953), and Dibblee (1991). Fault labels and symbols as in figure 1. Additional fault abbreviation: HL, “Hollister” (Neuerburg, 1953). See tables 1 and 2 for details of samples.
5

ernmost part of the range, and therefore we prefer to adhere 

to the original assignments (Hoots, 1931).

Ages of Topanga Volcanics in the 

Eastern Santa Monica Mountains

Fossils in strata underlying and interbedded with volcanics of the Topanga Formation (Hoots, 1931) in the eastern Santa Monica Mountains are largely littoral or shallow 

neritic forms that generally are only crudely diagnostic of 

age. An occurrence of middle Relizian and younger (16.4 

Ma and younger) Lyropecten crassicardo in one assemblage 

from the north slope of Mount Hollywood (UCLA locality 

2302 reported in Neuerburg, 1953, table 3) is clouded by the 

associated occurrence of the early Miocene “Lyropecten cf. 

miguelensis” (see Vedder, 1973, fig. 9). Other fossil assemblages are either not precisely age diagnostic or from beds 

younger than any nearby lavas. Recognizing the importance 

of better constrained ages for the volcanics and related sedimentary strata, we have turned to nonpaleontologic methods.

Basalt Ar-Ar Isotopic Ages

Samples of medium- to fine-grained basalt were collected 

from outcrops of the middle part of the Topanga Formation 

(Hoots, 1931) in the eastern Santa Monica Mountains west of 

the Cahuenga Pass syncline, spanning a distance along strike of 

about 4 km. After petrographic evaluation for presence of volcanic glass or alteration products, three of these were selected 

for age determination by the 40Ar/39Ar incremental-heating 

method. These three samples are all from flows roughly at the 

same stratigraphic horizon in the middle part of the volcanic 

sequence; the easternmost sample is from near the syncline 

axis (fig. 2). Analytical data, procedures, and interpretations 

of age spectra are presented in the appendix. 40Ar/39Ar age 

spectra obtained from all three samples indicate the presence 

of excess 40Ar in the rocks, requiring identification of increments minimally affected by this interference (see appendix, 

table 3, fig. 5). Ages of all three samples are the same within 

analytical uncertainties and have a weighted mean age of 

17.38±0.08 Ma (table 1). 

This mean age must be considered a maximum for the 

time of lava extrusion because of the evidence of excess 

40Ar. Nevertheless, the close agreement of the minimum 

ages defined by the age spectra suggests that their mean may 

closely represent the eruption age. An early Miocene age 

approximately equivalent to the latest Saucesian benthic 

foraminiferal stage (Barron and Isaacs, 2001) is thus indicated, an age about 1.5 m.y. older than the oldest Conejo 

Volcanics of the central Santa Monica Mountains (Turner and 

Campbell, 1979).

Fossil Carbonate Strontium Isotopic Ages

Littoral to shallow shelf marine fossils are widespread, 

fairly abundant, and generally well preserved within the 

Topanga Formation (Hoots, 1931) sequence that contains 

basalt flows in the eastern Santa Monica Mountains. Analyses of the strontium isotopes in these fossils can be used to 

assign numeric ages. The Oligocene and earlier Miocene are 

particularly well suited for stratigraphic applications of the 

technique because the 87Sr/86Sr ratio in seawater increased 

rapidly and monotonically during those time intervals. The 

87Sr/86Sr ratio of a fossil can be used to assign an age and 

probable error by comparison with an established curve of 

time versus the seawater 87Sr/86Sr value (for example, McArthur and others, 2001). 

Fossil shells from nine sites along nearly 9 km of outcrop strike in the eastern Santa Monica Mountains were 

selected and prepared for analysis. Fossil type and appearance were the basis for field selection. Laboratory selection, preparation, and analytical methods are outlined in the 

appendix. The results of the isotopic and chemical analyses, 

and assigned ages, are shown in table 2. Results of analyses 

of three fossils from the type Topanga Canyon Formation are 

given for comparison. A nearshore but open marine environment is suggested by the rare but widespread occurrences of 

pectenids (Neuerburg, 1953).

Eight of the samples from fossils associated directly with 

the basalts yielded very similar isotope ratios; their mean 

87Sr/86Sr is 0.708654±16, equivalent to an age of 17.3±0.3 Ma 

(McArthur and others, 2001). 

Two sparry carbonate samples have ratios that are above 

and below the mean of consistent ratios, giving important 

guides to the interpretation of the results. Trace elements 

from both samples show that they recrystallized in the presence of imported components. The sparry gastropod(?) from 

strata interbedded with basalt at locality d (fig. 2) yielded an 

87Sr/86Sr value that is lower than the mean of the best shell 

material, suggesting that basalt-derived solutes contributed 

low-ratio strontium during replacement. The spar-replaced 

oyster fraction from locality i (fig. 2) yielded a ratio higher 

than the mean, showing that the origin of the pore fluids and 

their migration in this sequence was complicated. Nonetheless, these two samples, despite their complete recrystallization and elevated Mn and Fe contents, yielded 87Sr/86Sr ratios 

that are not profoundly different from the mean of samples 

judged to be minimally altered.

The very close agreement between the age of 17.38±0.08 

Ma determined by Ar-Ar analyses of basalts of the eastern 

Santa Monica Mountains and the age of 17.3±0.3 Ma determined by strontium isotopic dating of directly associated 

fossils is shown in figure 3. The importance of the age is discussed in the concluding section.

Age of the Oldest Glendora Volcanics

The type area of the Glendora Volcanics (Shelton, 1946; 

1955; Nourse and others, 1998) is at the northeastern edge of 

the Los Angeles Basin, about 70 km directly east of the eastern end of the Santa Monica Mountains. The overall thickness of volcanic units is variable, partly because of erosion, 

Age of the oldest Glendora Volcanics 
6 Age and Tectonic Significance of Volcanic Rocks in the Northern Los Angeles Basin, California 

and is as much as 914 m in drill holes west of the principal 

outcrops (fig. 1). Nourse and others (1998) reported 40Ar/39Ar 

ages of 16.3±1 and 15.9±0.3 Ma for these volcanic rocks, 

roughly consistent with paleontologic ages of overlying 

Relizian marine strata (West and Redin, 1991; McCulloh and 

others, 2001, p. 12-13, appendix 2).

Four samples of the Glendora Volcanics were collected 

within an area of <0.6 km2 in the easternmost part of the San 

Jose Hills (Shelton, 1955, plate 1). On the basis of Shelton’s 

(1955) geologic mapping and our own reconnaissance field 

studies, all four samples appear to represent the lower part of 

the folded and faulted volcanic sequence, where the flows rest 

either on granitoid rocks of the basement complex or on 27.6-

Ma intrusive dacite (Nourse and others, 1998; McCulloh and 

others, 2001). Three of the analyzed samples are of andesitic 

composition. A fourth sample of basaltic composition, GV17, was collected from a unit of interstratified basalt, sandstone, and conglomerate exposed in roadcuts that postdate 

the mapping of Shelton (1955). The basaltic and andesitic 

units are separated by a fault of undetermined displacement, 

leaving their relative stratigraphic positions uncertain. The 

40Ar/39Ar results and interpretations, summarized in table 1, 

are presented in the appendix (table 3, figs. 6, 7). 

Andesites within the lower part of the Glendora Volcanics (samples GV-9, GV-12, and GV-18) yielded ages 

from 15.4 to 15.1 Ma with uncertainties of 0.05 to 0.16 Ma 

(1σ). Although exact stratigraphic positions are difficult to 

ascertain, GV-12 and GV-18 appear to be near the base of 

the unit as mapped by Shelton (1955), whereas GV-9 was 

collected somewhat higher in the andesite. Although none of 

the age spectra is entirely undisturbed, the agreement of all 

andesite plateau ages within analytical error indicates a mean 

age of about 15.25±0.08 Ma. This compares to an age of 

15.9±0.8 Ma for the base of the Conejo Volcanics (Turner and 

Campbell, 1979, with recalculation using current decay constants). The age of the Glendora Volcanic andesite samples 

is close to the 15.7-Ma age of the Luisian-Relizian provincial 

foraminiferal stage boundary (Barron and Isaacs, 2001) and 

distinctly younger than the submarine basalts of the eastern 

Santa Monica Mountains (table 1).

The olivine-bearing basalt, sample GV-17 from a roadcut 

exposure 560 m northeast of GV-18, introduces uncertainty 

Map 

Symbol 

(figs. 1 

and 6)

Latitude (N) 

Longitude (W) 

(degrees)

Total 

Gas 

Age 

(Ma)

Best 

Estimate 

Age 

(Ma) *

Comments

A 34.114 

118.3294

20.7 ± 0.6 17.38 ± 0.12 Olivine basalt; middle part of multiple flows.

B 34.1161 

118.3414

18.7 ± 0.6 17.38 ± 0.11 Olivine basalt; middle part of multiple flows.

C 34.1283 

118.3708

19.8 ± 0.6 17.27 ± 0.45 Olivine basalt; middle part of multiple flows.

GV-9 34.0796 

117.7889

15.4 ± 0.5 15.08 ± 0.11 Andesite; plagioclase separate; within 

middle part of "hypersthene andesite" unit 

of Shelton (1955).

GV-12 34.0813 

117.7831

15.4 ± 0.5 15.32 ± 0.16 Andesite; plagioclase separate; at base of 

"hypersthene andesite" unit of Shelton 

(1955).

GV-17 34.0759 

117.7837

17.2 ± 0.5 17.2 ± 0.5 Basalt, iddingsite-bearing.

GV-18 34.0746 

117.7857

15.2 ± 0.4 15.28 ± 0.05 Andesite; plagioclase separate; near base 

of "hypersthene andesite" unit of Shelton 

(1955).

Table 1.—Locations and 40Ar/ 39Ar ages of volcanic rock samples from the Topanga Formation (Hoots, 1931) in the eastern 

Santa Monica Mountains and from the Glendora Volcanics in the San Jose Hills, California.

* See appendix for analytical details, including interpretations of complex Ar release patterns. 
7

Map

Symbol 

(fig. 3)

Latitude (N) 

Longitude (W) 

(degrees)

87 Sr/ 86 Sr ∆sw Age (Ma) Mn (ppm) Sr (ppm) Fe (ppm) Comments

a 34.1256 

118.3020

0.708636±16 -53.7 17.5±0.3 253 454 136 Oyster, pinkish foliated fragments from 

grainstone at base of volcanic sequence.

b 34.1358 

118.3092

0.708801±18 -37.2 13.8±1.4 158 635 160 Oyster, gray foliated fragments, minor patina. 

Equal to UCLA locality 2303 (above volcanic sequence).

c 34.1207 

118.3145

0.708673±10 -50.0 17.1±0.2 380 297 60 Oyster, white translucent foliated fragments. At 

top of volcanic sequence.

d 34.1160 

118.3370

0.708503±13 -67.0 19.2±0.3 1364 562 1284 Gastropod(?), white sparry calcite. Within 

upper part of volcanic sequence.

e 34.1165 

118.3418

0.708672±16 -50.1 17.1±0.3 189 239 96 Oyster, pale, fibrous fragments. Within lower 

half of volcanics sequence.

f 34.1177 

118.3501

0.708659±16 -51.4 17.3±0.3 nd nd nd Oyster, pinkish foliated, some chalky. Beneath 

lowest volcanic flow.

g 34.1230 

118.3585

��������±�� ����� ����±��� ��� ��� ��� �������������������������������������������������

��������������������������

� ������������

��������

��������±�� ����� ����±��� ��� ��� ��� �������������������������������������������

�������������������������������������������

� ������������

��������

��������±��������

��������±�������0.708741±19

�������������������������������������������-43.2

����±������������������±�����������������

16.0±0.5

�����������������������������

������������

3480

����������

����������

414

����������

�����������

3899

�������������������������������������������������������������������������������������������������������������������������������Oyster, dark brown spar within thin foliated layers. Close to volcanic base. 

0.708671±19 0.708667±17 0.708678±19

-50.2 -50.6 -49.5 

17.1±0.3 

17.2±0.3 17.1±0.3 

nd nd nd

574 592 

1227

nd nd nd

Shells

Pelecypod 

Gastropod 

Topanga Formation type area (Sec. 35, T. 1 N, R. 17 W., San Bernardino Baseline and Meridian)

������ �������������� ����������� ��������� ���� �������� ���� ����� ����� ��� ������� ���������� �������� ����� ���� �������� ���������� �������� ������ ��� ���� �������� ������ ������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������� ������������������������� ∆������������������������ ���������������������������������� ���������������������

���������������������������������������������������������������������������������������������������������������������������

Age of the oldest Glendora Volcanics 
8 Age and Tectonic Significance of Volcanic Rocks in the Northern Los Angeles Basin, California 

into the timing of initiation of volcanism near the northeastern edge of the Los Angeles Basin. This sample is from a 

sequence of basalt flows with intercalated lenses of unfossiliferous sandstone and conglomerate that crops out adjacent 

to an area mapped by Shelton (1955) as “basement complex”. 

Incremental heating of this sparsely porphyritic basalt with 

its very fine grained groundmass yields a complex 40Ar/39Ar 

age spectrum suggestive of 39Ar recoil. We consider the totalgas age of 17.2±0.5 Ma from this whole-rock sample to be 

the best estimate, but it may be erroneously old if some 39Ar 

has been lost (appendix). Because the stratigraphic relationship between the sampled andesitic and basaltic strata is not 

conclusively known, the 17.2-Ma age of GV-17 cannot be 

discounted and may represent an older, and possibly separate, 

volcanic unit. On the basis of the age of 15.25±0.08 Ma for 

the more abundant and widespread andesites, we consider the 

lower part of the Glendora Volcanics to be the same age as the 

middle part of the Conejo Volcanics and significantly younger 

than the volcanic rocks of the Topanga Formation (Hoots, 

1931) in the easternmost Santa Monica Mountains. Further 

efforts are needed to evaluate the ages of other occurrences 

of basalt mapped as Glendora Volcanics, including those near 

Puddingstone Dam described as “pillow lava” by Shelton 

(1955, plate 1).

Other Los Angeles Basin 

Volcanic Rocks

No direct physical connection has been established, and 

none is believed to exist, between the Glendora Volcanics 

and the volcanics of the Topanga Formation (Hoots, 1931) of 

the eastern Santa Monica Mountains or the Conejo Volcanics 

farther west. Data from deep drill holes allow a western edge 

of the Glendora Volcanics to be mapped roughly coincident 

with longitude 118° W. in the subsurface beneath the San 

Gabriel Valley and western Whittier Hills (McCulloh and 

others, 2001, fig. 6). Volcanic rocks are absent north and 

south of the Whittier Fault southeast of the Hollywood-Raymond Fault and west of approximate longitude 118° W. The 

easterly pinchout of all units of the volcanics in the Santa 

Monica Mountains suggests that no direct connection ever 

existed. However, the great depth to the volcanic-equivalent 

horizon between the Hollywood and Las Cienegas Faults, 

and the dearth of drill holes deep enough to provide control, 

limits mapping except at the Las Cienegas oil field. There 

the volcanic-equivalent horizon is absent at the unconformity 

separating basement from uppermost Topanga Formation 

(Hoots, 1931) or Modelo equivalent strata (Mefford, 1970, 

plate III; Schneider and others, 1996, fig. 9). “Topanga volcanic rocks...entrained in the Las Cienegas fault” along the 

south flank of the anticlinal high (Tsutsumi and others, 2001, 

p. 464) are a questionable exception because they are merely 

tuffaceous sedimentary beds. Thus, for lack of control or 

because of either original absence or posteruption erosional 

removal, a large area of the northwestern flank of the basin is 

shown on figure 1 as lacking volcanics.

Volcanic rocks are present in the basin south of the Whittier Fault and east of about longitude 118° W. Some of these 

clearly represent a continuation of the Glendora Volcanics, 

connecting through the Whittier Hills (with about 9 km of 

right strike slip across the Whittier Fault). Others may represent continuations of another possibly partly younger suite 

erupted from a separate center, the El Modeno volcanic center 

(Yerkes, 1957; Eaton, 1958; Turner, 1970; Luyendyk and 

others, 1998). Topanga Group strata are much too deep and 

(or) economically unpromising to have been drilled beneath 

the central synclinal trough of the basin, except at the northwestern and southeastern (up-plunge) ends. Therefore, neither the presence nor absence of volcanic rocks in this area 

can be demonstrated (fig. 1). Our map reflects the presence of 

volcanics at both ends and both flanks of the syncline and our 

judgement that these suggest their presence throughout the 

undrilled axial part.

Figure 3.—Strontium isotope results from table 2 plotted on the curve 

of 87Sr/ 86Sr variation in seawater during part of the Miocene. The 

curve, from McArthur and others (2001), is based largely on results 

from deep-sea drilling samples. The cluster of eight samples from the 

eastern Santa Monica Mountains (fig. 2) yields a mean age of 17.3±0.3 

Ma. The 17.38±0.08 Ma weighted mean age from the Ar-Ar analyses 

is shown for comparison. Samples d and i have elevated Mn and Fe 

content (table 2) indicative of diagenetic alteration and isotopic exchange. Sample b is from a locality stratigraphically above the volcanic sequence. Δsw = (87Sr/ 86Srunknown - 87Sr/ 86Srmodern seawater) x 105

 .

�

���������

��

��

��

��

���

����������� �� ���

��������

�����

�������������

��������

�� ��

������ ������

��� ��� ���

��

������

�

��������������

������
9

Volcanic rocks have been encountered in some deep drill 

holes along the Newport-Inglewood Fault Zone (White, 1952; 

Graves, 1954; Eaton, 1958; Harris, 1958; Wright, 1991, figs. 

22, 25). Those at Inglewood oil field have been paleontologically dated as about 13.6 Ma to possibly 16.5 Ma (Castle and 

Yerkes, 1976, plate 1). The youngest of those at Dominguez 

oil field might be as young as 13 Ma on the basis of published paleontological data (Graves, 1954). Basaltic lavas 

interbedded with unfossiliferous fine-grained sedimentary 

strata beneath the Long Beach Airport oil field are older than 

13.5-Ma foraminiferal beds but are otherwise undated (Harris, 

1958). Basalts and andesites in multiple wells at and around 

Huntington Beach oil field have been paleontologically zoned 

in the range from about 15 Ma to questionably 16.5 Ma 

(Eaton, 1958). Importantly, volcanic rocks drilled along the 

Newport-Inglewood Fault Zone are largely restricted to local 

fault blocks along the northeast side of the zone; on fault 

blocks southwest of the fault they occur only in limited areas 

at four restricted sites.

The subsurface occurrences at the southeast end of the 

volcanic field along the Newport-Inglewood Fault Zone probably merge with volcanics of the Topanga Formation studied 

in San Joaquin Hills outcrops (Vedder and others, 1957). 

Andesitic flow breccias there were dated radiometrically as 

16.7 to 14.1 Ma (Turner, 1970).

Altogether separate physically from all of the volcanics 

mentioned above is a field of more or less altered olivine 

basalts restricted to the Palos Verdes Hills and largely to 

their seaward flank. Those submarine basalts underlie or 

intertongue with basal Luisian and (or) Relizian strata of the 

Monterey Formation (Woodring and others, 1946; Eaton, 

1958; Conrad and Ehlig, 1983). Although dated paleontologically at about 15.5 Ma, they have not been radiometrically 

dated. They thin and wedge out on the northeast flank of the 

Palos Verdes Hills between the Catalina Schist basement and 

the Luisian base of the Monterey Formation.

Figure 1 shows the locations of the various Los Angeles 

Basin area volcanics described above. The chronostratigraphic relations among them are summarized in figure 4. 

The northwestern and northern limits to the distribution of 

volcanics in the northwestern part of the basin are especially 

important for regional understanding. The thick and nearly 

continuous volcanic sequences present north of the Santa 

Monica-Hollywood Fault Zone, and the absence of such 

sequences to the south, requires explanation that has not been 

previously attempted.

Discussion, Conclusions, and 

Recommendations

The new age determinations reported here show that the 

oldest known Miocene volcanic rocks of the Los Angeles 

Basin crop out in the easternmost Santa Monica Mountains 

north of the Santa Monica-Hollywood Fault Zone. These 

rocks are olivine-bearing basalts that were erupted onto a 

shallow sea floor where subsidence equalled or outpaced a 

very rapid overall accumulation rate of more than 3.3 mm/yr. 

Both the whole rock 40Ar/39Ar ages and the fossil carbonate 87Sr/86Sr ages agree in assigning a 17.4-Ma age to these 

near-tholeiitic basalts. This age is 1.5 m.y. older than the 

older dated part of the Conejo Volcanics in the western Santa 

Monica Mountains and among the oldest ages for Miocene 

volcanics of coastal southern California (fig. 4).

The new age determinations also indicate that andesitic 

rocks in the lower part of the Glendora Volcanics erupted at 

about 15.4 to 15.1 Ma and are therefore about the same age or 

slightly younger than the lowest volcanic strata of the Conejo 

Volcanics dated at 15.9±0.8 Ma (Turner and Campbell, 1979, 

p. E21, with recalculation using current decay constants). 

Furthermore, the andesites in the Glendora Volcanics area are 

clearly younger than the 17.4-Ma basalts in the eastern Santa 

Monica Mountains.

The significance of the age determination of 17.2±0.5 

Ma for basalt in the lower part of the Glendora Volcanics is 

uncertain. No fossil carbonate 87Sr/86Sr ages are obtainable 

for comparison with this age. As discussed in the appendix, it 

is unclear whether the 17.2-Ma age represents an older, previously unrecognized basaltic interval, or instead is erroneously 

too old owing to 39Ar loss. 

Tectonic Implications

Nearshore marine volcanics of the eastern Santa Monica 

Mountains are either the same age as the inferred time of 

onset of western Transverse Ranges clockwise transrotation 

(Luyendyk, 1990, fig. 3; Nicholson and others, 1994, fig. 3) 

or just slightly older (Hornafius and others, 1986, fig. 2; Dickinson, 1996, p. 14 and table 2). Their age therefore places a 

maximum limit on the time of initial opening of Los Angeles 

Basin (Bohannon and Geist, 1998, p. 785). At the time of 

eruption, the present roughly east-west strike of the volcanics 

must have been oriented in a different, pre-transrotational 

position, possibly roughly northeast-southwest. 

The location of these oldest volcanics suggests a way to 

localize the initial extensional failure zone related to early 

stages of the clockwise tectonic rotation of the western Transverse Ranges. The starting position of this failure zone is 

imperfectly defined (Kamerling and Luyendyk, 1979; Hornafius and others, 1986; Crouch and Suppe, 1993; Nicholson 

and others, 1994). If the amount of clockwise rotation of 

the volcanics were firmly established, much of the present 

uncertainty about the locus of the northernmost end of the 

transrotational breakaway would vanish. Clarification of 

the most likely position and orientation of the zone of rifting responsible for the Miocene and later opening of the Los 

Angeles Basin would result. Paleomagnetic declination data 

are needed to firmly fix the pre-transrotational orientation of 

the eastern Santa Monica Mountains volcanic sequence. Such 

data should preferably differentiate between the limited area 

of volcanics northeast of the Cahuenga Pass syncline and the 

lava outcrops to the west.

Discussion, conclusions, and recommendations
10 Age and Tectonic Significance of Volcanic Rocks in the Northern Los Angeles Basin, California 

��

��

��

��

��

��

��

��

��

��

��

��

��

MIOCENE

EARLY MIDDLE LATE

C5

C5A

C5A

A

C5AB

C5AC

C5B

C5C

C5D

C5E

C6

C5AD

CN7

CN6

CN5

CN4

CN3CN2CN1

a

b ����������

Saucesian Relizian

Luisian Mohnian

middle late late early early

and

middle

"Vaqueros" "Temblor" "Margaritan"

Clarendonian

Hemingfordian Barstovian

EPOCH

AGE (Ma)

CHRON

MAGNETIC POLARITY

(black is normal)

CALCAREOUS

NANNOFOSSIL

ZONES

AGE (Ma)

BENTHIC

FORAMINIFERAL

STAGES

MOLLUSCA (PROVINCIAL)

STAGES (Bartow, 1992)

TERRESTRIAL VERTEBRATE

STAGES (Tedford and others, 1987)

GLENDORA VOLCANICS

(Nourse and others, 1998)

EL MODENO VOLCANICS

(Luyendyk and others, 1998)

NE LOS ANGELES BASIN

(Eaton, 1958)

SW LOS ANGELES BASIN

(Eaton, 1958)

SAN JOAQUIN HILLS

(Turner, 1970)

PALOS VERDES HILLS

(Woodring and others, 1946)

SANTA CATALINA ISLAND

(Forman, 1970)

E. SANTA MONICA MTNS. Fossil carbonate ages

Vedder and others, 1979 Luyendyk and others, 1998

��

��

��

��

��

��

��

��

?

?

??

?

?

W. SANTA MONICA MTNS.

(Turner and Campbell, 1979)

��

GLENDORA VOLCANICS

�� ����

�

��

�

�

Weigand, 1982 (corrected

from Turner, 1970)

E. SANTA MONICA MTNS. Volcanic rock ages

Berry and others, 1976

? ?

�

�

�

�

�

�

�

�

�

�

�

�

�

�

�

�

����

�

�

�

�

�

�

�

�

�

�����

���

�

�

�

��

Mean of 8 sample values

Figure 4.—Chronological chart of Miocene volcanic rocks in and around the Los Angeles Basin (including the western Santa Monica Mountains and Santa Catalina Island). Solid lines represent volcanic ages with uncertainties, while dashed lines represent less 

certain age ranges. Chronostratigraphic and biostratigraphic data are adapted from compilation of Barron and Isaacs (2001).
11

Although we believe this study is a major step forward 

in understanding, much remains to be done. We hypothesize 

that the 17.4-Ma volcanic rocks of the Topanga Formation 

(Hoots, 1931) in the eastern Santa Monica Mountains are an 

early expression of deep crustal magmatism accompanying 

the earliest extensional tectonism associated with rifting. If 

further work confirms the questionable 17.2-Ma age of basalt 

at the base of the Glendora Volcanics, the evidence of early 

volcanism would be extended to the northeasternmost edge 

of the Los Angeles Basin. The absence in the eastern Santa 

Monica Mountains of volcanic rocks that are age equivalents 

of the extremely thick younger volcanic pile in the western 

and central parts of the range suggests that the early igneous 

activity in the eastern area was premonitory. Early cessation of that oldest volcanism may indicate a false start in that 

eastern area to the earliest phase of transrotational tectonism. 

Knowledge of the paleomagnetic declination in the 17.4-Ma 

volcanics is therefore a prerequisite to secure knowledge of 

the prerotation orientation and position of the eastern Santa 

Monica-Hollywood Fault Zone and of the adjacent field of 

submarine lava flows.

Intriguing possibilities exist that the source of extrusives 

in the eastern Santa Monica Mountains is related in some 

way to the “dioritic plutons” hypothesized as deep sources 

(Bjorklund and others, 2002) for the sill-like diabase intrusions of the Whittier Fault hanging wall (Yerkes, 1972; 

McCulloh and others, 2000, p. 1167, fig. 11) and for the El 

Modeno Volcanics (Yerkes, 1957). Differences in ages and 

compositions, as well as disparate locations, pose substantial 

questions and complications to exploration of such possible 

connections. We therefore wait with interest for explication 

and full documentation of the bases for these hypothesized 

plutons.

Stratigraphic and Age Relationships

The strata containing littoral molluscan fossils that 

underlie the 17.4-Ma volcanics of the Topanga Formation 

(Hoots, 1931) have been regarded as part of the Topanga Formation by previous workers. Our data show that these strata 

are uppermost lower Miocene, supporting their assignment 

as “Topanga.” We agree with Fritsche (1993, fig. 2), who 

regards the various environmentally diverse lithologic (and 

cartographic) units previously erected and used to describe 

the enormous volume of Oligocene-Miocene strata of the 

western Santa Monica Mountains as complex lithofacies 

equivalents of one another. Thus, the 17.4-Ma volcanics 

of the Topanga Formation (Hoots, 1931) might be viewed 

simply as an older (probably the oldest) unit of the Conejo 

Volcanics. The ages and stratal relationships, rather than the 

name, are important.

 Ar and Sr Isotopic Dating

The two geochronological approaches used for dating the 

volcanic sequence in the eastern Santa Monica Mountains are 

mutually supportive. Because the 40Ar/39Ar and Sr isotopic 

dating approaches are independent, relying on different materials and systematics, the close agreement of the two data sets 

greatly strengthens the view that both sets are correct. Our 

experience with the 87Sr/86Sr method for such precise dating 

of young fossils underscores the need to use multiple sample 

sites, redundant analyses of different sample fractions, care 

in selection of material for analysis, and quantitative analyses 

of Mn, Fe, and Sr to detect diagenetic alteration. Similarly, 

age determinations of such fine-grained, submarine basalts by 

whole-rock incremental 40Ar/39Ar heating may yield useful 

age information, provided attention is paid to identification of effects of excess Ar and 39Ar recoil. The results of 

this study emphasize the need for careful sample selection, 

multiple sample sites, and the use of complementary dating 

techniques. 

Extent of 17.4-Ma Volcanics of Topanga 

Formation (Hoots, 1931)

 Surface geologic mapping (for example, Dibblee, 1991) 

of the volcanics of the easternmost Santa Monica Mountains 

shows that both individual volcanic units and the entire volcanic sequence thin and pinch out to the northeast (fig. 2). Drill 

holes south of the Santa Monica-Hollywood-Raymond Fault 

Zone encountered no volcanic rocks in the basinal region 

north of the Las Cienegas Fault and west of about longitude 

118° W., with one exception (Yeats, 1973, p.135; Wright, 

1991, fig. 18). Palinspastic restoration of 13-14 km of left 

strike slip on the Hollywood-Raymond Fault Zone (McCulloh 

and others, 2001) places the outcrops shown on figure 2 adjacent to a region south of the fault zone where strata younger 

than 16 Ma rest depositionally on a varied basement terrane 

without intervening older strata (or volcanic rocks)(Yerkes 

and others, 1965, fig. 5; Lamar, 1970; Davis and others, 1989, 

fig. 9 and plate 1; Wright, 1991, figs. 4,8; McCulloh and 

others, 2001, fig. 10). 

Whether the Topanga-age volcanics ever extended 

southward across the Hollywood Fault Zone is not known. 

Post-volcanic uplift and erosion, followed by subsidence and 

renewed deposition, is a possibility. Accumulation of the volcanic sequence only on the fault block north (originally northwest) of an ancestral Santa Monica-Hollywood-Raymond 

Fault Zone is conjectural but also possible (Davis and others, 

1989). Possibilities for gaining a conclusive answer are limited by complex structure and structural relief, substantial 

depths in some places to the critical horizon, a paucity of drill 

holes deep enough stratigraphically to provide definite control, and limited means to obtain indirect evidence. The presence of subrounded clasts of Santa Monica Slate basement 

rock types in strata dated faunally as about 15.6 Ma and older 

in deep cored wells on both sides of the Newport-Inglewood 

Fault Zone at Inglewood oil field indicates that such basement 

rocks were exposed to erosion not long after volcanism of 

earliest Topanga age. The lack of associated volcanic detritus 

implies erosion from a region devoid of Topanga age volcanic 

Discussion, conclusions, and recommendations
12 Age and Tectonic Significance of Volcanic Rocks in the Northern Los Angeles Basin, California 

rocks at that time. This suggests to us that the region of Santa 

Monica Slate basement directly southeast of the HollywoodRaymond Fault Zone may never have been covered by volcanic rocks, a view evidently held also by Davis and others 

(1989), although they did not discuss it or cite evidence.

Age Trends of Volcanism in Southern California

The new age determinations from the eastern Santa 

Monica Mountains reported here can be used to critically 

examine two extant hypotheses that propose systematic trends 

in the ages of Miocene volcanic rocks in coastal southern 

California. (1) The first hypothesis (Luyendyk and others, 

1998, fig. 3) noted, on the basis of about 20 age determinations, an apparent eastward decrease in the age of volcanism 

in the western Transverse Ranges and Channel Islands from 

about 18-17 Ma in the west (originally south, prior to transrotation) to about 16 Ma or even younger in the east (originally 

north). This age progression, according to Luyendyk and 

others (1998), may have resulted from the development of a 

northward-propagating rift between the Channel Islands and 

the rest of onshore coastal southern California. That hypothesis appears to predict that volcanic rocks in the eastern Santa 

Monica Mountains should be younger than 16 Ma (Luyendyk 

and others, 1998, fig. 3), considerably younger than our determination of 17.4 Ma for the age of the Topanga Formation 

(Hoots, 1931) volcanics in this area. (2) The second hypothesis (Wilson and others, in press, fig. 7) suggests, on the basis 

of a compilation of more than 30 age determinations, that 

volcanism in coastal southern California migrated from west 

to east (in early Miocene, pre-transrotation coordinates) in 

response to rapid eastward propagation of a slab window that, 

in turn, resulted from breakup and partial subduction of the 

Monterey plate. This hypothesis leads to the prediction that 

volcanism in the eastern Santa Monica Mountains should be 

between 17.5 and 16.0 Ma but closer to the latter age (Wilson 

and others, in press, fig. 7). 

 The apparent discrepancies between the predictions of 

these two hypotheses and the newly determined 17.4-Ma 

age of the Topanga Formation (Hoots, 1931) volcanics in the 

eastern Santa Monica Mountains show that our understanding 

of the tectonic controls on the ages and locations of volcanic 

centers in coastal southern California remains imperfect. This 

lack of agreement underscores the need for further work. 

Acknowledgments

The work reported here was made possible through the cooperation of the University of Texas at Dallas. R. J. Enrico 

provided important paleontological assistance. Early inspiration came during 1999 fieldwork with I. P. Colburn. J. W. 

Hillhouse and D. M. Morton encouraged the work in several 

ways. We are grateful to Linda Barth, Rob Hancock, Steve 

Lipshie, Mark Oborne, Cliff Plumb, Jim Shuttleworth, and 

Mark Stipanovich of the City of Los Angeles Departments of 

Recreation and Parks, Public Works, and Water and Power for 

permissions to collect samples along roadways and on city 

lands. N. R. Miller provided the ICP trace-element analysis 

of fossil carbonates. We are grateful to C. L. Powell and 

Zenon Valin for figure preparation. Critical reviews by R. H. 

Campbell and R. W. Kistler are appreciated.

 References

Arnold, Ralph, 1907, New and characteristic species of fossil 

mollusks from the oil-bearing Tertiary formations of 

southern California: U. S. National Museum Proceedings, v. 32, p. 525-546.

Atwater, Tanya, 1989, Plate tectonic history of the northeast 

Pacific and western North America, in Winterer, E.L., Hussong, D.M., and Decker, R.W., eds., The eastern Pacific 

Ocean and Hawaii: Geological Society of America, The 

Geology of North America, v. N , p. 21-72.

Atwater, Tanya, and Stock, Joann, 1998, Pacific-North America 

plate tectonics of the Neogene southwestern United States; 

an update: International Geology Review, v. 40, p. 375-

402.

Barron, J.A., and Isaacs, C.M., 2001, Updated chronostratigraphic framework for the California Miocene, in Isaacs, 

C.M., and Rullkötter, Jürgen, eds., The Monterey Formation from Rocks to Molecules: New York, Columbia 

University Press, p. 393-395.

Bartow, J.A., 1992, Neogene timescale for southern California: U. S. Geological Survey Open-File Report 92-212, 

2 sheets.

Berry, A.L., Dalrymple, G.B., Lanphere, M.A., and Von Essen, 

J.C., compilers and eds., 1976, Summary of miscellaneous 

potassium-argon age measurements, U. S. Geological 

Survey, Menlo Park, California, for the years 1972-74: 

U. S. Geological Survey Circular 727, 13 p.

Bjorklund, Tom, Burke, Kevin, Zhou, Hua-Wei, and Yeats, R. S., 

2002, Miocene rifting in the Los Angeles basin; evidence 

from the Puente Hills half-graben, volcanic rocks, and Pwave tomography: Geology, v. 30, p. 451-454.

Blake, G. H., Jr., 1991, Review of the Neogene biostratigraphy 

of the Los Angeles basin and implications for basin 

evolution, in Biddle, K. T., ed., Active basin margins: 

American Association of Petroleum Geologists Memoir 

52, p. 135-184.

Bohannon, R. G., and Geist, Eric, 1998, Upper crustal structure and Neogene tectonic development of the California 

continental borderland: Geological Society of America 

Bulletin, v. 110, p. 779-800.

Brand, Uwe, and Veizer, Jan, 1980, Chemical diagenesis of a 

multicomponent carbonate system—1. Trace elements: 

Journal of Sedimentary Petrology, v. 50, p. 1219-1236.

Burgess, R., Kelley, S.P., Parsons, I., Walker, F.D.L., and Worden, 

R.H., 1992, 40Ar/39Ar analysis of perthite microstructures 

and fluid inclusions in alkali feldspars from the Klokken 

syenite, South Greenland: Earth and Planetary Science 

Letters, v. 109, p. 147-167.
13

Burke, W. H., and Hetherington, E. A., 1984, Normalizing 87Sr/ 86Sr by multiple collection and comparison to a standard: 

Chemical Geology, v. 12, p. 265-268.

Castle, R. O., and Yerkes, R. F., 1976, Recent surface movements 

in the Baldwin Hills, Los Angeles County, California: U. 

S. Geological Survey Professional Paper 882, 125 p.

Claesson, S., and Roddick, J. C., 1983, 40Ar/39Ar data on the age 

and metamorphism of the Ottfjallet dolerites, Sarv Nappe, 

Swedish Caledonides: Lithos, v. 16, p. 61-73.

Colburn, I. P., and Novak, G. A., 1989, Paleocene conglomerates 

of the Santa Monica Mountains, California; petrology, 

stratigraphy, and environments of deposition, in Colburn, 

I. P., Abbott, P. L., and Minch, J., eds., Conglomerates in 

basin analysis; a symposium dedicated to A. O. Woodford: 

Society of Economic Paleontologists and Mineralogists, 

Pacific Section, Book 62, p. 227-253.

Conrad, C. L., and Ehlig, P. L., 1983, The Monterey Formation 

of the Palos Verdes Peninsula, California—an example 

of sedimentation in a tectonically active basin within 

the California Continental Borderland, in Larue, D. K., 

and Steel, R. J., eds., Cenozoic marine sedimentation, 

Pacific Margin, U. S. A.: Los Angeles, Society of Economic Paleontologists and Mineralogists, Pacific Section, 

p. 103-116.

Crouch, J. K., 1981, Northwest margin of California Continental Borderland; marine geology and tectonic evolution: 

American Association of Petroleum Geologists Bulletin, 

v. 65, p. 191-218.

Crouch, J. K., and Suppe, John, 1993, Late Cenozoic tectonic 

evolution of the Los Angeles basin and inner California 

borderland; a model for core-complex-like crustal extension: Geological Society of America Bulletin, v. 105, p. 

1415-1434.

Crowe, B. M., McLean, Hugh, Howell, D. G., and Higgins, R. 

E., 1976, Petrography and major-element chemistry of 

the Santa Cruz Island Volcanics, in Howell, D. G., ed., 

Aspects of the geologic history of the California Continental Borderland: American Association of Petroleum 

Geologists, Pacific Section, Miscellaneous Publication 

24, p. 196-215.

Dalrymple, G.B., 1989, The GLM continuous laser system for 40Ar/39Ar dating; description and performance characteristics: U. S. Geological Survey Bulletin 1890, p.89-96.

Dalrymple, G. B., and Duffield, W. A., 1988, High precision 40Ar/39Ar dating of Oligocene rhyolites from the Mogollon-Datil volcanic field using a continuous laser system: 

Geophysical Research Letters, v. 15, p. 463-466.

Davis, T. L., Namson, Jay, and Yerkes, R. F., 1989, A cross section of the Los Angeles area; seismically active fold and 

thrust belt, the 1987 Whittier Narrows earthquake, and 

earthquake hazard: Journal of Geophysical Research, v. 

94, n. B7, p. 9644-9664.

Denison, R. E., Koepnick, R. B., Fletcher, A., Howell, M. W., 

and Calloway, W. S., 1994, Criteria for the retention of 

original seawater 87Sr/86Sr in ancient shelf limestones: 

Chemical Geology, v. 112, p. 131-143.

Dibblee, T. W., Jr., 1982, Geology of the Santa Monica Mountains and Simi Hills, southern California, in Fife, D. L., 

and Minch, J. A., eds., Geology and mineral wealth of 

the California Transverse Ranges: Santa Ana, California, 

South Coast Geological Society, Inc., p. 94-130.

Dibblee, T. W., Jr., 1989, Mid-Tertiary conglomerates and 

sandstones on the margin of the Ventura and Los Angeles basins and their tectonic significance, in Colburn, I. 

P., Abbott, P. L., and Minch, J., eds., Conglomerates in 

basin analysis; a symposium dedicated to A. O. Woodford: 

Society of Economic Paleontologists and Mineralogists, 

Pacific Section, Book 62, p. 207-226.

Dibblee, T. W., Jr., 1991, Geologic map of the Hollywood and 

Burbank (south 1/2) Quadrangles, Los Angeles County, 

California: Dibblee Geological Foundation Map DF-30, 

1:24,000.

Dickinson, W. R., 1996, Kinematics of transrotational tectonism 

in the California Transverse Ranges and its contribution 

to cumulative slip along the San Andreas transform fault 

system: Geological Society of America Special Paper 

305, 46 p.

Dickinson, W. R., 1997, Tectonic implications of Cenozoic 

volcanism in coastal California: Geological Society of 

America Bulletin, v. 109, p. 936-954.

Duffield, W. A., and Dalyrmple, G. B., 1990, The Taylor Creek 

Rhyolite of New Mexico; a rapidly emplaced field of 

lava domes and flows: Bulletin of Volcanology, v. 52, 

p. 475-487.

Durrell, Cordell, 1954, Geology of the Santa Monica Mountains, 

Los Angeles and Ventura Counties [California], in Jahns, 

R. H., ed., Geology of Southern California: California 

Division of Mines Bulletin 170, map sheet 8, scale 1:

126,720.

Eaton, G. P., 1958, Miocene volcanic activity in the Los Angeles Basin, in Higgins, J. W., ed., A guide to the geology 

and oil fields of the Los Angeles and Ventura regions: 

American Association of Petroleum Geologists, Pacific 

Section, p. 55-61.

Fleck, R.J., Sutter, J.F., and Elliot, D.H., 1977, Interpretation 

of discordant 40Ar/39Ar age spectra of Mesozoic tholeiites from Antarctica: Geochimica et Cosmochimica Acta, 

v.41, p.15-32.

Forman, J. A., 1970, Age of the Catalina Island pluton, California: Geological Society of America Special Paper 124, 

p. 37-45.

Fritsche, A. E., 1993, Middle Tertiary stratigraphic terminology for the Santa Monica Mountains, southern California, in Weigand, P. W., Fritsche, A. E., and Davis, G. E., 

eds., Depositional and volcanic environments of middle 

Tertiary rocks in the Santa Monica Mountains, southern 

California: Society for Sedimentary Geology, Pacific 

Section, Book 72, p. 1-12.

Graves, D. T., 1954, Geology of the Dominguez oil field, in 

Jahns, R. H., ed., Geology of southern California: California Division of Mines Bulletin 170, map sheet 32, 

scale 1:12,000.

Harris, P. B., 1958, Long Beach Airport pool, in Higgins, J. 

W., ed., A guide to the geology and oil fields of the Los 

References
14 Age and Tectonic Significance of Volcanic Rocks in the Northern Los Angeles Basin, California 

Angeles and Ventura regions: American Association of 

Petroleum Geologists, Pacific Section, p. 75-77.

Harrison, T. M., and McDougall, Ian, 1981, Excess 40Ar in 

metamorphic rocks from Broken Hill, New South Wales; 

implications for 40Ar/39Ar age spectra and the thermal history of the region: Earth and Planetary Science Letters, 

v. 55, p. 123-149.

Harrison, T. M., Heizler, M. T., and Lovera, O. M., 1993, In 

vacuo crushing experiments and K-feldspar thermochronometry: Earth and Planetary Sciences Letters, v. 117, 

p. 169-180.

Harrison, T. M., Heizler, M. T., Lovera, O. M., Chen, Wenji, 

and Grove, Marty, 1994, A chlorine disinfectant for excess 

argon released from K-feldspar during step heating: Earth 

and Planetary Science Letters, v. 123, p. 95-104.

Hawkins, J. W., Jr., Allison, E. C., and MacDougall, Doug, 

1971, Volcanic petrology and geologic history of Northeast Bank, southern California borderland: Geological 

Society of America Bulletin, v. 82, p. 219-228.

Higgins, R. E., 1976, Major-element chemistry of the Cenozoic 

volcanic rocks in the Los Angeles Basin and vicinity, in 

Howell, D. G., ed., Aspects of the geologic history of the 

California Continental Borderland: American Association 

of Petroleum Geologists, Pacific Section, Miscellaneous 

Publication 24, p. 216-227.

Hoots, H. W., 1931, Geology of the eastern part of the Santa 

Monica Mountains, Los Angeles County, California: U. S. 

Geological Survey Professional Paper 165-C, p. 83- 134, 

map scale 1:24,000.

Hornafius, J. S., Luyendyk, B. P., Terres, R. R., and Kamerling, 

M J., 1986, Timing and extent of Neogene tectonic rotation 

in the western Transverse Ranges, California: Geological 

Society of America Bulletin, v. 97, p. 1476-1487.

Huneke, J. C., and Smith, S. P., 1976, The realities of recoil: 39Ar recoil out of small grains and anomalous age patterns 

in 39Ar/40Ar dating: Geochimica et Cosmochimica Acta 

Supplement 7 (Proceedings of the Seventh Lunar Science 

Conference), p. 1987-2008.

Johnson, C. M., and O’Neil, J. R., 1984, Triple junction magmatism; a geochemical study of Neogene volcanic rocks in 

western California: Earth and Planetary Science Letters, 

v. 71, p. 241-262.

Jones, C. E., Jenkyns, H. C., and Hesselbo, S. P., 1994, Strontium isotopes in Early Jurassic seawater: Geochimica et 

Cosmochimica Acta, v. 58, p. 1285-1301.

Kamerling, M. J., and Luyendyk, B.P., 1979, Tectonic rotations 

of the Santa Monica Mountains region, western Transverse Ranges, California, suggested by paleomagnetic 

vectors: Geological Society of America Bulletin, v. 90, 

p. 331-337.

Kew, W. S. W., 1923, Geologic formations of a part of southern 

California and their correlation: American Association of 

Petroleum Geologists Bulletin, v. 7, p. 411-420.

Lamar, D. L., 1970, Geology of the Elysian Park-Repetto Hills 

area, Los Angeles County, California: California Division 

of Mines and Geology Special Report 101, 45 p., map 

scale 1:24,000.

Lanphere, M.A., and Dalrymple, G.B., 1976, Identification of 

excess 40Ar by the 40Ar/39Ar age spectrum technique: Earth 

and Planetary Science Letters, v. 32, p. 141-148.

Legg, M. R., 1991, Developments in understanding the tectonic evolution of the California Continental Borderland: 

Society of Exploration Paleontologists and Mineralogists 

Special Publication No. 46, p. 291-312.

Ludwig, K.R., 2001, Users Manual for Isoplot/Ex (rev. 2.49); 

a geological toolkit for

Microsoft Excel: Berkeley Geochronology Center Special 

Publication 1a, 56 p.

Luyendyk, B. P., 1990, Neogene-age fault slip in the continental 

transform zone in Southern California: Annales Tectonicae, Special issue, v. IV, no. 2, p. 24-34.

Luyendyk, B. P., Gans, P. B., and Kamerling, M. J., 1998, 

Geochronology of southern California Neogene volcanism, in Weigand, P. W., ed., Contributions to the geology of the Northern Channel Islands, southern California: 

American Association of Petroleum Geologists, Pacific 

Section, Miscellaneous Publication 45, p. 9-35.

McArthur, J. M., Crame, J. A., and Thirlwall, M. F., 2000, Definition of Late Cretaceous stage boundaries in Antarctica 

using strontium isotope stratigraphy: Journal of Geology, 

v. 108, p. 623-640.

McArthur, J. M., Howarth, R. J., and Bailey, T. R., 2001, 

Strontium isotope stratigraphy; LOWESS Version 3; 

best fit to the marine Sr-isotope curve for 0-509 Ma and 

accompanying look-up table for deriving numerical age: 

Journal of Geology, v. 109, p. 155-170.

McCulloh, T. H., Beyer, L. A., and Enrico, R. J., 2000, Paleocene strata of the eastern Los Angeles basin, California; 

paleogeography and constraints on Neogene structural 

evolution: Geological Society of America Bulletin, v. 

112, p. 1155-1178.

McCulloh, T. H., Beyer, L. A., and Morin, R. W., 2001, Mountain 

Meadows Dacite; Oligocene intrusive complex that welds 

together the Los Angeles Basin, northwestern Peninsular 

Ranges, and central Transverse Ranges, California: U. S. 

Geological Survey Professional Paper 1649, 34 p.

McIntyre, G. A., Brooks, C., Compston, W., and Turek, A., 1966, 

The statistical assessment of Rb-Sr isochrons: Journal of 

Geophysical Research, v. 71, p. 5459-5468.

Mefford, M. G., 1970, Murphy area of Las Cienegas oil field: 

California Division of Oil and Gas, California Oil Fields, 

v. 56, n. 1, p. 5-13.

Merrihue, Craig, and Turner, Grenville, 1966, Potassium-argon 

dating by activation with fast neutrons: Journal of Geophysical Research, v. 71, p. 2852-2857.

Miller, F. K., and Morton, D. M., 1980, Potassium-argon 

geochronology of the eastern Transverse Ranges and 

southern Mojave Desert, southern California: U. S. Geological Survey Professional Paper 1152, 30 p.

Neuerburg, G. J., 1953, Geology of the Griffith Park area, Los 

Angeles County, California: California Division of Mines 

Special Report 33, 29 p.

Nicholson, Craig, Sorlien, C. C., Atwater, Tanya, Crowell, J. C., 

and Luyendyk, B. P., 1994, Microplate capture, rotation of 
15

the western Transverse Ranges, and initiation of the San 

Andreas transform as a low-angle fault system: Geology, 

v. 22, p. 491-495.

Nourse, J. A., Weigand, P. W., and Hazelton, G. B., 1998, 

Igneous and tectonic response of the eastern San Gabriel 

Mountains to Neogene extension and rotation of the Transverse Ranges block, in Behl, R. J., ed., Guidebook of 

field trip #10, 94th Annual Meeting, Cordilleran Section: 

Long Beach, California, Geological Society of America, 

p. 10-1 to 10-15.

Redin, T. W., 1991, Oil and gas production from submarine fans 

of the Los Angeles basin, in Biddle, K. T., ed., Active basin 

margins: American Association of Petroleum Geologists 

Memoir 52, p. 239-259.

Samson, S. D., and Alexander, E. C., 1987, Calibration of 

the interlaboratory 40Ar/39Ar dating standard, MMhb-1: 

Chemical Geology (Isotope Geoscience Section), v. 66, 

p. 27-34.

Schneider, C. L., Hummon, C., Yeats, R. S., and Huftile, G. L., 

1996, Structural evolution of the northern Los Angeles 

basin, California, based on growth strata: Tectonics, v. 

15,p. 341-355.

Severinghaus, Jeff, and Atwater, Tanya, 1990, Cenozoic geometry and thermal state of the subducting slabs beneath 

western North America, in Wernicke, B. P., ed., Basin 

and Range extensional tectonics near the latitude of Las 

Vegas, Nevada: Geological Society of America Memoir 

176, p. 1-22.

Shelton, J.S., 1946, Geology of northeast margin of San Gabriel 

Basin, Los Angeles County, California: U. S. Geological 

Survey Oil and Gas Investigation Preliminary Map 63, 

scale 1:24,000.

Shelton, J. S., 1955, Glendora Volcanic rocks, Los Angeles 

basin, California: Geological Society of America Bulletin, v. 66, p. 45-90.

Steiger, R. H., and Jager, E., 1977, Subcommission on 

geochronology; convention on the use of decay constants 

in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, p. 359-362.

Sullwold, H. H., Jr., 1960, Tarzana fan, deep submarine fan 

of late Miocene age, Los Angeles County, California: 

American Association of Petroleum Geologists Bulletin, 

v. 44, p. 433-457.

Tedford, R. H., Skinner, M. F., Fields, R. W., Remsberger, J. M., 

Whistler, D. P., Galusha, Theodore, Taylor, B. E., Macdonald, J. R., and Webb, S. D., 1987, Faunal succession 

and biochronology of the Arikareean through Hemphillian 

interval (late Oligocene through earliest Pliocene epochs) 

in North America, in Woodburne, M. O., ed., Cenozoic 

Mammals of North American: Berkeley, University of 

California Press, p. 153-210.

Tsutsumi, Hiroyuki, Yeats, R. S., and Huftile, G. J., 2001, Late 

Cenozoic tectonics of the northern Los Angeles fault 

system, California: Geological Society of America Bulletin, v. 113, p. 454-468.

Turner, D. L., 1970, Potassium-argon dating of Pacific Coast 

foraminiferal stages, in Bandy, O. L., ed., Radiometric 

dating and paleontologic zonation: Geological Society of 

America Special Paper 124, p. 91-129.

Turner, D. L., and Campbell, R. H., 1979, Age of the Conejo 

Volcanics, in Yerkes, R. F., and Campbell, R. H., eds., 

Stratigraphic nomenclature of the Central Santa Monica 

Mountains, Los Angeles County, California: U. S. Geological Survey Bulletin 1457-E, p. E18-E22.

Turner, G., and Bannon, M.P., 1992, Argon isotope geochemistry of inclusion fluids from granite-associated mineral 

veins in southwest and northeast England: Geochimica 

et Cosmochimica Acta, v. 56, p. 227-243.

Turner, G., and Cadogan, P.H., 1974, Possible effects of 39Ar 

recoil in 40Ar/39Ar dating: Geochimica et Cosmochimica 

Acta Supplement 5 (Proceedings of the Fifth Lunar Science Conference), p.1601-1615.

Turner, G., and Wang, S., 1992, Excess argon, crustal fluids, and 

apparent isochrons from crushing K-feldspar: Earth and 

Planetary Science Letters, v. 110, p. 193-211.

Vedder, J. G., 1973, Geologic framework and correlation of 

Miocene rocks in the Caliente Range: American Association of Petroleum Geologists-Society of Economic 

Paleontologists and Mineralogists-Society of Exploration 

Geophysicists, Pacific Section, Annual Meeting Field Trip 

2, p. 42-60.

Vedder, J. G., 1987, Regional geology and petroleum potential 

of the Southern California Borderland, in Scholl, D. W., 

Grantz, Arthur, and Vedder, J. G., eds., Geology and 

resource potential of the continental margin of western 

North America and adjacent ocean basins—Beaufort 

Sea to Baja California: Houston, Texas, Circum-Pacific 

Council for Energy and Mineral Resources, Earth Science 

Series, v. 6, p. 403-447.

Vedder, J. G., Yerkes, R. F., and Schoellhamer, J. E., 1957, 

Geologic map of the San Joaquin Hills-San Juan Capistrano area, Orange County, California: U. S. Geological 

Survey Oil and Gas Investigations Map OM-193, scale 

1:24,000.

Vedder, J. G., Howell, D. G., and Forman, J. A., 1979, Miocene 

strata and their relation to other rocks, Santa Catalina 

Island, California, in Armentrout, J. M., Cole, M. R., 

and TerBest, H., Jr., eds., Cenozoic paleogeography of 

the western United States: Los Angeles, Society of Economic Paleontologists and Mineralogists, Pacific Section, 

p. 239-256.

Veizer, Ján, Buhl, Dieter, Diener, Andreas, Ebneth, Stefan, Podlaha, O. G., Bruckschen, Peter, Jasper, Torsten, Korte, 

Christoph, Schaaf, Michael, Ala, Davin, and Azmy, 

Karem, 1997, Strontium isotope stratigraphy; potential 

resolution and event correlation: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 132, p. 65-77.

Weigand, P. W., 1982, Middle Cenozoic volcanism of the 

western Transverse Ranges, in Fife, D. L., and Minch, 

J. A., eds., Geology and mineral wealth of the California 

Transverse Ranges: Santa Ana, California, South Coast 

Geological Society, Inc., p. 170-188.

Weigand, P. W., and Savage, K. L., 1993, Review of the petrology and geochemistry of the Miocene Conejo Volcanics 

References
16 Age and Tectonic Significance of Volcanic Rocks in the Northern Los Angeles Basin, California 

of the Santa Monica Mountains, California, in Weigand, 

P. W., Fritsche, A. E., and Davis, G. E., eds., Depositional 

and volcanic environments of middle Tertiary rocks in the 

Santa Monica Mountains, southern California: Society 

for Sedimentary Geology, Pacific Section, Book 72, p. 

93-112.

Weigand, P. W., Savage, K. L., Reid, T., and Chinn, B. D., 1998, 

Composition of volcanic rocks on Santa Rosa, San Miguel, 

and Santa Barbara Islands, California, in Weigand, P. W., 

ed., Contributions to the geology of the Northern Channel Islands, Southern California: Bakersfield, California, 

American Association of Petroleum Geologists, Pacific 

Section, Miscellaneous Publication 45, p. 37-47.

West, J. C., and Redin, T. W., 1990, Correlation section across 

northern Los Angeles basin from Santa Monica Bay to 

Prado flood control basin: Los Angeles, American Association of Petroleum Geologists, Pacific Section, Correlation Section Series, CS 28, scale 1:48,000 (horizontal), 

1:12,000 (vertical).

West, J. C., and Redin, T. W., 1991, Correlation section across 

eastern Los Angeles basin from San Pedro Bay to San 

Gabriel Mts., Correlation Section Series: Los Angeles, 

American Association of Petroleum Geologists, Pacific 

Section, Correlation Section Series, CS 29, scale 1:48,000 

(horizontal), 1:12,000 (vertical).

White, R. T., 1952, Cenozoic correlation section across Los 

Angeles Basin from Palos Verdes Hills to San Gabriel 

Mts., California: American Association of Petroleum 

Geologists, Pacific Section, scale 1:24,000.

Wilson, D.S., McCrory, P.A., and Stanley, R.G., in press, Implications of volcanism in coastal California for the deformation history of western North America: Tectonics.

Woodring, W. P., Bramlette, M. N., and Kew, W. S., 1946, Geology and paleontology of Palos Verdes Hills, California: U. 

S. Geological Survey Professional Paper 207, 145 p.

Wright, T. L., 1991, Structural geology and tectonic evolution 

of the Los Angeles basin, California, in Biddle, K. T., ed., 

Active Basin Margins: American Association of Petroleum 

Geologists Memoir 52, p. 35-134.

Yeats, R. S., 1973, Newport-Inglewood fault zone, Los Angeles basin, California: American Association of Petroleum 

Geologists Bulletin, v. 57, p. 117-135.

Yerkes, R. F., 1957, Volcanic rocks of the El Modeno area, 

Orange County, California: U. S. Geological Survey 

Professional Paper 274-L, p. 313-334.

Yerkes, R. F., 1972, Geology and oil resources of the western 

Puente Hills area, Southern California: U. S. Geological 

Survey Professional Paper 420-C, 63 p.

Yerkes, R. F., and Campbell, R. H., 1979, Stratigraphic nomenclature of the central Santa Monica Mountains, Los Angeles County, California: U. S. Geological Survey Bulletin 

1457-E, p. E1-E31.

Yerkes, R. F., McCulloh, T. H., Schoellhamer, J. E., and Vedder, 

J. G., 1965, Geology of the Los Angeles basin, California—an introduction: U. S. Geological Survey Professional Paper 420-A, 57 p.

York, Derek, Hall, C. M., Yanase, Yotaro, Hanes, J. A., and 

Kenyon, M. J., 1981, 40Ar/39Ar dating of terrestrial minerals with a continuous laser: Geophysical Research Letters, 

v. 8, p. 1136-1138.

Zeitler, P. K., and FitzGerald, J. D., 1986, Saddle-shaped 40Ar/ 39Ar age spectra from young, microstructurally complex 

potassium feldspars: Geochimica et Cosmochimica Acta, 

v. 50, p. 1185-1199.

Appendix

Techniques and Interpretations of 

40Ar/ 39Ar and 87Sr/ 86Sr Dating

40Ar/ 39Ar Analytical Techniques

In the 40Ar/39Ar or Ar-Ar dating technique (Merrihue 

and Turner, 1966), samples are irradiated with fast neutrons, 

converting 39K to 39Ar in potassium-bearing materials. The 39Ar, representing the radioactive parent, and 40Ar, the decay 

product, are measured simultaneously by mass spectrometry, increasing the precision and utility of conventional 

K-Ar dating. In the present study, Ar was released from 

volcanic samples of the Topanga Formation (Hoots, 1931) 

and Glendora Volcanics by the incremental-heating (or “age 

spectrum”) method. In this method, step-wise heating of the 

material evolves the neutron-produced 39Ar together with the 

radiogenic 40Ar, atmospheric 40Ar, and any extraneous 40Ar 

in sequential steps or increments. The 40Ar/39Ar age spectrum 

is a graphical display of apparent age versus cumulative 

percent 39Ar released. The 40Ar/39Ar laser-fusion technique, 

fusing extremely small amounts of material with a continuous laser (York and others, 1981), was used in this study to 

analyze standard minerals (Taylor Creek Rhyolite sanidine; 

see Dalrymple and Duffield, 1988) that determine the neutron-flux.

Ar analyses were performed on the same mass spectrometer using the same argon-extraction system described 

by Dalrymple (1989). Incremental-heating analyses used 

a low-blank, tantalum and molybdenum, resistance-heated 

furnace, commonly releasing all of the Ar in 8 to 15 heating 

increments. Samples used in this study were irradiated for 

16-20 hours in the U.S. Geological Survey TRIGA Reactor Facility in Denver, Colorado. The neutron flux monitor 

used in all irradiations was Taylor Creek Rhyolite sanidine, 

85G003, with an age of 27.92 Ma, as reported by Duffield 

and Dalrymple (1990). This age is standardized to an average age of 513.9 Ma for interlaboratory standard hornblende, 

MMhb1 (Samson and Alexander, 1987) and the Menlo Park 

intralaboratory standard biotite, SB-3. Decay and abundance 

constants for all ages reported are those recommended by 

Steiger and Jager (1977). Analytical errors in 40Ar/39Ar ages 

are reported at the 1σ level. Plateau ages of 40Ar/39Ar age 

spectra are defined as the weighted mean ages of contiguous 

gas fractions representing more than 50 percent of the 39Ar 

released for which no difference can be detected between the 
17

ages of any two fractions at the 95-percent level of confidence 

(Fleck and others, 1977). Analytical results are summarized 

in table 3 and illustrated graphically in figures 5-7.

Interpretation of 40Ar/ 39Ar Incremental-Heating 

Experiments

40Ar/39Ar results from the eastern Santa Monica 

Mountains

The 40Ar/ 39Ar age spectra of the basalts from the eastern Santa Monica Mountains (fig. 5) define “U-shaped” or 

“saddle-shaped” patterns typical of excess radiogenic 40Ar 

(Lanphere and Dalrymple, 1976; Harrison and McDougall, 

1981). In U-shaped spectra, apparent ages decrease from 

values greater than the true age of the sample to values that 

approach that age and then increase again to values often 

far above this minimum. Harrison and McDougall (1981) 

attributed U-shaped spectra to incorporation of argon into 

anion vacancies in mineral phases at temperatures at or below 

about 350°C. Claesson and Roddick (1983) documented 

a strong correlation between the release of excess 40Ar and 

release of Ar isotopes derived from irradiating chlorine. They 

argued that excess 40Ar is related solely to anion lattice sites. 

Studies in which chlorine and excess 40Ar were released by 

crushing in vacuum support the interpretation that fluid-inclusion and grain-boundary sites are sources for these species 

released at low temperatures, whereas excesses released at 

high temperatures are consistent with the higher activation 

energies required to liberate chlorine and other anions from 

mineral lattice sites (Turner and Wang, 1992; Burgess and 

others, 1992; Harrison and others, 1993). At intermediate 

temperatures, an absence or reduced release of excess 40Ar 

coincides with the primary release of radiogenic 40Ar, producing the minima in these U-shaped age spectra. Although this 

central portion of the U-shaped spectrum may give reliable 

estimates of the age of the material, this minimum may only 

approach the true age of the sample, especially where the total 

percentage of excess 40Ar is large (Lanphere and Dalrymple, 

1976; Harrison and McDougall, 1981; Zeitler and FitzGerald, 1986). A valid estimate of the true age of these samples 

depends on obtaining increments with a minimum of excess 

40Ar, which depends in turn on both the total amount of 

excess 40Ar and the resolution of the analysis (the percentage 

of gas released in each increment).

In some cases the correlation of chlorine-derived 38Ar 

with excess 40Ar has been used to identify the excess component and correct 40Ar/ 39Ar ages (Turner and Bannon, 1992; 

Harrison and others, 1993, 1994). The three basalt samples 

(A through C) from the Topanga Formation (Hoots, 1931) all 

yield U-shaped age spectra that decrease moderately to an age 

between 17.2 and 17.4 Ma before increasing irregularly to 

values above 30 Ma (fig. 5, table 3). As shown in figure 5 by 

the variation of Cl/K, this U-shaped pattern is also reflected in 

the release of chlorine-derived 38Ar. Spectra from sample C 

reveal an especially large high-temperature release of excess 

40Ar that is strongly correlated with an increase in Cl/K. If 

the true age of this sample is defined by the intermediatetemperature minimum of 17.27±0.45 Ma in the 875°C step, 

excess 40Ar released in low-temperature increments (625°C 

to 800°C steps) for this sample ranges from about 4 percent to about 8 percent in each step. Although 17.27±0.45 

Ma represents a maximum age for this sample, we accept 

it as the best estimate of the true age. The pattern of Cl/K 

released mimics that for age, with the substantial increase in 

both parameters occurring after the 875°C step.

Like the pattern of Ar release for sample C, the age 

minimum of the 40Ar/ 39Ar spectrum for sample B occurs in 

a single step, as the ages decrease from more than 20 Ma to 

17.38±0.11 Ma before increasing at higher temperatures (fig. 

5, table 3). Cl/K follows a similar, U-shaped pattern with 

substantially higher Cl/K at high temperature. Ages derived 

from increments above 925°C in this sample have larger 

uncertainties and show less systematic variation than in 

sample C. Comparing the total-gas ages of the two samples, 

however, the total amount of excess 40Ar in sample B is substantially less than in sample C. The best estimate age for 

sample B is 17.38±0.11 Ma, but this may represent only a 

maximum age, because ages of adjacent increments are substantially higher.

Incremental heating of sample A provides the most 

interpretable age spectrum of the three samples from the 

eastern Santa Monica Mountains. As with the others, 40Ar/ 39Ar results define a U-shaped pattern, with ages decreasing 

from 19 Ma to a two-step minimum of 17.38±0.12 Ma in the 

725°C and 800°C steps. In this case the age derived from 

the 875°C step, the next higher temperature, is only slightly 

higher, suggesting little or no excess 40Ar and confirming the 

close approach to the true age. The ages derived from subsequent temperature steps, however, increase to values well 

above 30 Ma (fig. 3, table 3). Cl/K values show a similar 

pattern, with an abrupt increase coinciding with a correlative increase in apparent age at 1,100°C. We consider the 

17.38±0.12 Ma average age of the 725°C and 800°C steps to 

represent the true age of the basalt. The agreement of these 

two lowest-age increments that represent more than 30 percent of the 39Ar released, the limited excess 40Ar apparent in 

adjacent steps, and the near coincidence of the age and Cl/K 

minima strongly support this interpretation.

Within analytical uncertainties, the ages derived from 

the four steps representing the minima of the three age spectra are the same, yielding a weighted mean age of 17.38±0.08 

Ma and a mean square of weighted deviates or MSWD (see 

McIntyre and others, 1966) of 0.04. Because of the clear 

evidence of excess 40Ar in these basalts, this age must be 

considered a maximum for the time of lava extrusion. The 

close agreement of the minimum ages of the three samples, 

however, suggests that this age may represent that event 

quite closely. The age is in the early Miocene and approximately equivalent to the latest Saucesian provincial benthic 

foraminiferal stage, about 1.5 m.y. older than the lowest part 

of the Conejo Volcanics (Turner and Campbell, 1979; Barron 

and Isaacs, 2001).

Appendix
18 Age and Tectonic Significance of Volcanic Rocks in the Northern Los Angeles Basin, California 

���������� ������� ��������� �������� ��������� ��������� ����� ���� ���� ����

��� ���� ����� ���� ������ ������ ������� ����� �������

��� ����� ����� ���� ������ ������ ������� ����� �������

��� ����� ����� ����� ������ ������ ������� ����� �������

��� ����� ����� ����� ������ ������ ������� ����� �������

��� ����� ����� ����� ������ ������ ������� ����� �������

��� ����� ����� ����� ������ ������ ������� ����� �������

��� ���� ����� ����� ������ ������ ������� ����� �������

���� ���� ����� ����� ������ ������ ������� ����� �������

���� ���� ����� ����� ������ ������ ������� ����� �������

���� ���� ����� ����� ������ ������� ������� ����� �������

���� ���� ����� ����� ������� ������� ������� ����� �������

��� ���� ����� ���� ������� ������ ������� ����� �������

��� ���� ����� ����� ������ ������ ������� ����� �������

��� ����� ����� ����� ������ ������ ������� ����� �������

��� ����� ����� ����� ������ ������ ������� ����� �������

��� ����� ����� ����� ������ ������ ������� ����� �������

��� ����� ����� ����� ������ ������ ������� ����� �������

���� ���� ����� ����� ������ ������ ������� ����� �������

���� ���� ����� ����� ������ ������� ������� ����� �������

���� ���� ����� ����� ������ ������� ������� ����� �������

���� ���� ����� ����� ������ ������� ������� ����� �������

���� ���� ����� ����� ������� ������� ������� ����� �������

���� ���� ����� ����� ������� ������� ������� ����� �������

��� ���� ���� ���� ������� ������ ������� ����� �������

��� ����� ����� ���� ������� ������ ������� ����� �������

��� ����� ����� ���� ������� ������ ������� ����� �������

��� ����� ����� ���� ������� ������ ������� ����� �������

��� ����� ����� ����� ������ ������� ������� ����� �������

��� ����� ����� ����� ������ ������� ������� ����� �������

��� ���� ����� ����� ������ ������� ������� ����� �������

���� ���� ����� ����� ������ ������� ������� ����� �������

���� ���� ����� ����� ������ ������� ������� ����� �������

���� ���� ����� ����� ������ �������� ������� ����� �������

���� ���� ����� ����� ������ ������� ������� ����� �������

��� ���� ����� ���� ������� ������� ������� ����� �������

��� ���� ����� ����� ������� ������� ������� ����� �������

��� ���� ����� ����� ������ ������� ������� ����� �������

��� ���� ����� ����� ������ ������� ������� ����� �������

��� ����� ����� ����� ������ ������� ������� ����� �������

��� ����� ����� ����� ������ ������� ������� ����� �������

��� ����� ����� ����� ������ ������� ������� ����� �������

���� ����� ����� ����� ������ ������� ������� ����� �������

���� ����� ����� ����� ������ ������� ������� ����� �������

���� ���� ����� ����� ������ ������� ������� ����� �������

���� ���� ����� ����� ������

���� ���� ����� ����� ������ ������� ������� ����� �������

Sample GV-9 PLAGIOCLASE J=0.00286171

��������������

����������

������� ������� ����� �������

��������������

������������������

����������������

�����������������

��������

Sample B BASALT J=0.0034999

��������������

��������������

��������������

Sample A BASALT J=0.00347452

��������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

����������

������������

��������������

��������������

��������������

��������������

��������������

����������

���������������

��������������

��������������

Sample C BASALT J=0.00428292

���������������������� ������������

��������������

��������������

������������������

��������������

��������������

��������������

��������������

��������������

��������������

������������������ ����������

���������������������� ������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

����������������������

������������������ ����������

������������

Table 3.—Incremental heating ��Ar/ ��Ar ages of volcanic rocks of the Topanga Formation (Hoots, 1931) from the eastern 

Santa Monica Mountains and Glendora Volcanics, California. 

[See figures 1 and 2 for sample localities. Constants used for Ar computations are λβ = 4.962x10-10 yr-1

, λε = 0.581x10-10 yr-1, and 40K/KT = 1.167x10-4. Weighting 

factors used in plateau ages are inverse variances. Estimated uncertainties for ages are quoted at one standard deviation.] 
19

40Ar/ 39Ar results from the Glendora Volcanics

The four samples from the Glendora Volcanics in the 

eastern San Jose Hills are all from the lower part of the 

deformed lavas of that area. Samples GV-9, GV-12, and 

GV-18 (table 3; figs. 6, 7) are porphyritic pyroxene-bearing 

andesites from which plagioclase was separated for 40Ar/ 39Ar 

incremental heating. The age spectrum of sample GV-9 is 

irregular, with low- and high-temperature increments yielding ages differing significantly from those of intermediate 

temperature steps at confidence levels greater than 95 percent 

(fig. 6). The 910°C to 1,200°C steps satisfy plateau criteria, 

yielding an age of 15.08±0.11 Ma for 64.4 percent of the 39Ar 

released. Cl/K values reveal a slightly U-shaped pattern, but 

ages derived from the steps with higher Cl/K are only slightly 

elevated above the plateau. The plagioclase may contain 

a small amount of excess 40Ar, but the majority of the gas 

released is consistent with the plateau. The total-gas age of 

Appendix

��� ���� ���� ���� ������� ������� ������ ����� �������

��� ���� ����� ����� ������� ������� ������ ����� �������

��� ���� ����� ����� ������ ������� ������ ����� �������

��� ���� ����� ����� ������ ������� ������ ����� �������

��� ����� ����� ����� ������ ������� ������ ����� �������

��� ����� ����� ����� ������ ������� ������ ����� �������

���� ����� ����� ����� ������ ������� ������ ����� �������

���� ����� ����� ����� ������ ������� ������ ����� �������

���� ����� ����� ����� ������ ������� ������ ����� �������

���� ���� ����� ����� ������ ������� ������ ����� �������

���� ���� ����� ����� ������ ������� ������ ����� �������

������������������

����������������

�����������������

��� ���� ����� ����� ������ ������ ������� ����� ��������

��� ���� ����� ����� ������ ������ ������� ����� ��������

��� ����� ����� ����� ������ ������ ������� ����� ��������

��� ����� ����� ����� ������ ������ ������� ����� ��������

��� ����� ����� ����� ������ ������ ������� ����� ��������

��� ����� ����� ����� ������ ������ ������� ����� ��������

��� ����� ����� ����� ������ ������ ������� ����� ��������

��� ���� ����� ����� ������ ������ ������� ����� ��������

��� ���� ����� ����� ������ ������ ������� ����� ��������

���� ���� ����� ����� ������ ������ ������� ����� ��������

���� ���� ����� ����� ������ ������ ������� ����� ��������

���� ���� ����� ����� ������ ������� ������� ����� ��������

���� ���� ����� ����� ������ ������ ������� ����� ��������

��� ���� ������ ���� ������� ������� ������� ����� �������

��� ���� ����� ����� ������ ������� ������� ����� �������

��� ���� ����� ����� ������ ������� ������� ����� �������

��� ����� ����� ����� ������ ������� ������� ����� �������

��� ����� ����� ����� ������ ������� ������� ����� �������

��� ����� ����� ����� ������ ������� ������� ����� �������

��� ���� ����� ����� ������ ������� ������� ����� �������

���� ����� ����� ����� ������ ������� ������� ����� �������

���� ����� ����� ����� ������ ������� ������� ����� �������

���� ���� ����� ����� ������ ������� ������� ����� �������

���� ���� ����� ����� ������ ������� ������� ����� �������

���� ���� ����� ����� ������ ������� ������� ����� �������

������������

��������������

��������������

��������������

��������������

��������������

Sample GV-12 PLAGIOCLASE J=0.00286171

��������������

��������������

Sample GV-17 BASALT J=0.00283602

��������������

��������������

��������������

��������������

��������������

��������������

��������������

����������

������������

��������������

��������������

��������������

��������������

Sample GV-18 PLAGIOCLASE J=0.00281499

��������������

������������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

��������������

����������

��������������

��������������

��������������

��������������

��������������

������������������ ����������

��������������

��������������

��������������

��������������

���������������� ������������

����������������� ������������

Table 3.—Incremental heating ��Ar/ ��Ar ages of volcanic rocks of the Topanga Formation (Hoots, 1931) from the eastern 

Santa Monica Mountains and Glendora Volcanics, California—Continued. 

���������� ������� ��������� �������� ��������� ��������� ����� ���� ���� ���� ��������
20 Age and Tectonic Significance of Volcanic Rocks in the Northern Los Angeles Basin, California 

Figure 5.—Diagrams of the 40Ar/ 39Ar age spectrum and Cl/K ratio from stepwise heating of basalt samples A, B, and C from the Topanga Formation (Hoots, 1931) in the eastern Santa Monica Mountains (see fig. 2 for sample localities). Where shown, inset drawings represent enlargements of low-amplitude portions of the containing diagrams for the temperature steps labeled. Comparison of U-shaped 40Ar/ 39Ar age spectra and corresponding Cl/K patterns reveals excellent correlation of age with high Cl/K, consistent with excess 40Ar residing in anion sites.

�

����

���������

��� ��� ��� ��� ��� ���

���������������������

������������

�������

�

������������������������

��� ��� ��� ��� ��� ���

����������� �������������

���������������������

�����������������������������������

��������������������

��� ��� ��� ��� ��� ���

�

��������

��� ��� ��� ��� ��� ���

����������������������������������������

��� ��� ��� ��� ��� ���

�

��������

��� ��� ��� ��� ��� ���

����������� ������������� ����������� ������������� ����������� �������������

����������� ������������� ����������� �������������

�������

�

�������

�

�������

�

�������

�

�������

�

����

����

����

����

��������

������������������������ ��

��������������� �

��������������� �

��������������� �

���� ����

������������������������ ��

���� ����
21

����

����

����

����

����

����

����

����

��� ��� ��� ��� ��� ���

����������� �������������

�������������������

������

������

������

������

������

������

������

������

��

��

��

��

��

��

��� ��� ��� ��� ��� ���

����������

��������������������������

�����������������

����

����

���� ����

����

���

���

���

���

���

������

������

������

������

������

������

������

������

���� ���� ���� ���� ���� ���� ����

����� ����

�������� �����

��������������

����������

�����������������

������

������

������

������

������

������

������

������

��� ��� ��� ��� ��� ���

���������������������

������

������

������

������

������

�

��

��

��

��

��

��� ��� ��� ��� ��� ���

����������

�������� ������������������

����������������

����������������

�����������������

���������������

���������������

��������

����������� �������������

����������� ����������� �� ����������� ����������� �� ������� �� �� ���� ����

���������������

����������������

���

��� ���

���

���

������

������

������

������

������

������

���� ���� ���� ���� ����

���������

����� ����

����� ����

�������������

����������� �������������

��������������� ���������������� �

���� ���

����������� �������������

���� ����

Figure 6.—Diagrams of the 40Ar/ 39Ar age spectrum, Cl/K ratio, and inverse isochron from stepwise heating of Glendora Volcanics samples GV-9 

and GV-12 (see fig. 1 for sample localities). Where shown, inset drawings represent enlargements of low-amplitude portions of the containing 

diagrams for the temperature steps labeled. Ages are listed in millions of years with ± of 1 . MSWD, the mean square of weighted deviates (McIntyre and others, 1966), is a measure of the goodness of fit of the data. Samples are from andesites in the lower part of the volcanic sequence. 

Appendix
22 Age and Tectonic Significance of Volcanic Rocks in the Northern Los Angeles Basin, California 

����� ������

�����

�����

�����

�����

�����

�����

��� ��� ��� ��� ��� ���

���������� ��

�� ��������

��

��

��

��

��

��

��

��

��

��� ��� ��� ��� ��� ���

��������� ��� � ���� � ��� �� ����

����� �����������

1400

1275

1175 1100

1025

950

900 850

775

700

������

������

������

������

������

������

������

���� ���� ���� ���� ���� ���� ����

��

���

��

��

��� � ����� � ���� �� ����

�������

��

���

��

�� � ��� � � ��

����� ���

����� �����������

����

����

����

����

����

����

����

��� ��� ��� ��� ��� ���

����� �����������

��

��

��

��

��

��

��� ��� ��� ��� ��� ���

���� � ���

�������� ����� �� ��� ����

����� ������

����� � ��� ��

����� � ���� ��

��������

��

���

��

��

�

�

� ����

���� �

�

�

� ����

���� �

���������� ��

�� ��������

���������� �� ���������� �� �������� ��

�� ��������

Figure 7.—Diagrams of the 40Ar/ 39Ar age spectrum, Cl/K ratio, and inverse isochron from stepwise heating of Glendora Volcanics samples GV-17 

and GV-18 (see fig. 1 for sample localities). Sample GV-18 is from a stratigraphic position near the base of the volcanic sequence. Sample GV17 is from basalt in fault contact with the andesitic sequence. Parameters are as shown in figure 6. 
23

15.4±0.5 Ma, slightly older than the plateau age, is consistent 

with either a minor component of excess 40Ar or with minor 

redistribution of 40Ar without loss.

Plagioclase from sample GV-12 (fig. 6) yields a plateau-type age spectrum with an age of 15.32±0.16 Ma for the 

1,000°C to 1,225°C steps representing 51.8 percent of the 

39Ar released. Only the age difference between the 925°C and 

1,075°C steps prevents recognition of a much broader plateau. 

Underestimation of the uncertainties in either or both of these 

steps could be responsible for limiting this plateau. Ages from 

earlier and later increments differ only slightly above the 95-

percent level of confidence, consistent with only minor disturbance of the age spectrum, as indicated by the total-gas age of 

15.4±0.5 Ma. Cl/K ratios for this sample reveal a U-shaped 

release, but the values are extremely low, consistent with insignificant amounts of excess 40Ar.

Plagioclase from sample GV-18 yields a rather imprecise 

plateau-type age spectrum (fig. 7), with minor complications 

introduced by scatter greater than accommodated by the estimated analytical errors. Using criteria discussed earlier (Fleck 

and others, 1977), a plateau age of 15.13±0.11 Ma is defined 

by the 900°C to 1,175°C steps, representing 50.4 percent of the 

39Ar released. The remaining steps are excluded because of 

age differences greater than 2σ in the 850°C and 1,275°C steps. 

Criteria of Ludwig (2001), which do not require concordance 

between all gas fractions of a plateau, would define a plateau 

age of 15.28±0.05 Ma using all but the 550°C and 625°C steps. 

Those two steps indicate minor loss of 40Ar under either set 

of criteria. An MSWD (McIntyre and others, 1966) of 0.35 

for the 900°C to 1,175°C plateau age compares to an MSWD 

of 1.6 using all steps except 550°C and 625°C, indicative of 

the increased scatter related to the additional points. The ages 

are not different statistically at levels of confidence above 80 

percent, however, nor are they different from the total-gas age 

of 15.2±0.4 Ma. Considering this total-gas age and clear evidence for minor Ar loss in the first two steps, the probability 

of minor internal redistribution of 40Ar prompts us to use the 

larger plateau designation with an age of 15.28±0.05 Ma.

Sample GV-17, a finely crystalline, iddingsite-bearing 

basalt, was collected from a sequence of basalt, sandstone, and 

conglomerate separated from adjacent andesites by faulting. 

Results of incremental heating of a 45-to-60-mesh wholerock aliquant of this sample reveal a continuously decreasing 

age spectrum (fig. 7). The total-gas age from this experiment 

is 17.2±0.5 Ma, but the ages range from an initial step of 

24.7 Ma to about 15.1 Ma at 1,200°C (table 1). Turner and 

Cadogan (1974) provided an understanding of this type of 

age spectrum from their study of fine-grained lunar materials. 

The spectrum is produced by loss of potassium-derived 39Ar 

from near-surface lattice sites of potassium-bearing phases. 

Their results, confirmed by experiments of Huneke and Smith 

(1976), showed that 39Ar would be depleted in a surface layer 

0.08 μm thick on neutron-irradiated grains as a result of recoil 

of the 39Ar nucleus following the 39K(n,p)39Ar reaction. Surface-to-volume ratios show that recoil loss of 39Ar from grains 

larger than about 100 μm is generally unimportant. Basalts 

with fine-grained groundmass grains considerably less than 

100 μm in diameter, however, may exhibit substantial effects 

from recoil (Turner and Cadogan, 1974; Huneke and Smith, 

1976; Fleck and others, 1977). The more or less monotonically declining ages observed in the age spectrum of sample 

GV-17 are a result typical of 39Ar recoil. Interpretation of this 

age spectrum, however, depends critically on whether 39Ar 

was lost completely from the basalt grains or if it recoiled 

from a less retentive phase, such as groundmass plagioclase, 

into a more retentive phase like groundmass pyroxene. In the 

first case, involving bulk loss of 39Ar from the rock, the ages 

of the highest temperature grain-core-dominated increments 

may approach the true age of the rock, but lower-temperature, grain-surface-dominated fractions are too old because 

of greater loss of 39Ar. Alternatively, no net loss of 39Ar may 

have occurred, but the redistribution of 39Ar from less retentive to more retentive phases would result in low-temperature 

increments yielding ages older than the true age and high-temperature increments yielding ages that are too young. In the 

first case the total-gas age would be erroneously old, whereas 

in the latter case it would approximate the true age. Because 

ages of the high-temperature fractions in the basalt sample GV17 approximate the age of the andesite samples, total loss of 

recoil-liberated 39Ar would explain the difference in apparent 

age. 

87Sr/ 86Sr Dating Method and Techniques

The variation of 87Sr/ 86Sr in seawater can be used to 

determine the age of deposition of a variety of marine fossils, 

rocks, and minerals (Veizer and others, 1997; McArthur and 

others, 2001). The precision of an age assignment is dependent on the isotope measurement error, the curve definition, 

and the slope of the 87Sr/ 86Sr variation curve with time. Age 

assignments are more precise for periods of rapid change in 

the seawater 87Sr/ 86Sr (higher curve slopes). Larger errors are 

inherent during times of slow change (lower curve slopes). 

The definition of seawater variation is best in the Cenozoic 

because of numerous measurements on high quality samples 

with precise paleontologic age assignments. This, coupled 

with a steep curve slope between 35 Ma and 15 Ma, make 

the Oligocene and older Miocene ideal for the application of 

strontium isotope stratigraphy.

In order to be useful in stratigraphic studies, marine fossil 

carbonate samples must carry their original seawater 87Sr/ 86Sr 

ratios. Fossil carbonates yield spurious Sr isotope ratios only 

if they grew in waters poorly mixed with open marine water, 

or if their ratios were altered during post-burial diagenesis. 

Shell material that was originally low magnesium calcite is 

more resistant to diagenesis than shell material of aragonite. 

Shell appearance is crucial in the field selection process. Specifically, among Miocene fossils, foliated translucent layers of 

oyster shell and foliated pecten shell are most likely to have 

retained their original 87Sr/ 86Sr ratios.

 During diagenesis of carbonates, manganese and iron 

increase and strontium decreases (see, for example, Brand 

and Veizer, 1980). Strontium imported in pore fluids comAppendix
24 Age and Tectonic Significance of Volcanic Rocks in the Northern Los Angeles Basin, California 

monly evicts and replaces original strontium. Limestones 

with the lowest Fe and Mn and the highest Sr/Mn ratios 

have been shown to be the most likely to retain the original 

strontium isotope ratio of the seawater (Denison and others, 

1994). Trace element analysis has therefore proven useful to 

evaluate Sr retention in shell material (for example, Jones and 

others, 1994; McArthur and others, 2000). Imported strontium may have a ratio either higher or lower than the original 

strontium, depending on the origin of the imported strontium. 

A sample may have undergone profound diagenesis and yet 

appear to retain the original seawater ratio if the strontium 

imported during alteration has an isotope ratio indistinguishable from the original. Consistency and agreement of results 

from stratigraphically related samples are marks of a retained 

marine ratio.

Nine shell samples selected for analysis (see table 2) 

were crushed, washed and sieved. The cleanest and freshest 

fragments were picked for analysis. These were taken into 

solution with 1N acetic acid. A portion of the soluble carbonate was prepared for ICP analysis and the remainder used for 

separation with a Sr-specific resin. The contents of the trace 

elements Mn, Fe, and Sr were determined using a Perkin 

Elmer Optima 3300DV plasma emission spectrophotometer. 

The isotope ratios were measured on a second order, double 

focusing mass spectrometer having a magnetic sector of 60°

with a 13-inch (33.0 cm) radius of curvature and an electric 

sector of 91° with a 15.8-inch (40.1 cm) radius of curvature. 

Isotopes of mass 85, 86, 87, and 88 were measured simultaneously in four separate faraday cups. The 87Sr/86Sr values were 

normalized to 86Sr/88Sr = 0.1194. The isotope ratios of the 

unknown samples were measured by comparison to a standard (Burke and Hetherington, 1984). The standard used in 

most of the measurements was NBS/987, for which a value of 

0.710240 has been assumed. 

More than 100 measurements of modern seawater yield a 

weighted mean of 0.709173±3. To assign ages from the lookup table, we normalized our results to the modern seawater 

value of 0.709175 (McArthur and others, 2001). Results of 

strontium isotopic analyses of fossils from nine localities of 

the study area are summarized in table 2.