TECTONICS,VOL. 8, NO. 1, PAGES99-115, FEBRUARY
1989
PRE-PLIOCENE
EXTENSION
AROUND
THE GULF OF
CALIFORNIA
AND
THE
TRANSFER
OF
BAJA
CALIFORNIA
TO THE PACIFIC PLATE
J. M. Stockand K. V. Hodges
Departmentof Earth, Atmospheric,and PlanetarySciences,
Massachusetts
Instituteof Technology,Cambridge
Abstract. Late Miocene (12-5 Ma) extensionaroundthe
edgesof theGulf of Californiahasbeenalternatively
attributed
to "Basin and Range" extension, back arc extension, or
developmentof the Pacific-NorthAmericaplate boundary.
Thisextension
wasENE directedandsimilarin structural
style
to extensionin the Basin and Range province. Timing
constraints permit nearly synchronous onset of this
deformationin a belt extendingSSE from northernmost
Baja
Californiato the mouthof the gulf. Where this extensional
faulting continuedthroughPliocenetime to the present,
synchronouswith motion on the modern transformplate
boundaryin the Gulf of California,no changein directionof
extension can be resolved.
Revised constraints on Pacific-
North Americaplatemotionsupportthe development
of this
late Miocene extensionas a componentof Pacific-North
Americadisplacement
that couldnot be accommodated
by
strike-slipdisplacement
alongtheexistingplateboundarywest
of the Baja Californiapeninsula.This scenarioimpliesthat
transferof BajaCaliforniafromtheNorthAmericaplateto the
Pacific plate was a gradualprocess,beginningabout 12-10
Ma, when motion of the Pacific plate relative to North
Americawaspartitioned
intoseparate
regimesof strike-slip
anddip-slipdisplacement
onopposite
sidesof BajaCalifornia.
INTRODUCTION
Openingof the Gulf of Californiais often attributedto two
sequential extensional events: middle to late Miocene
"protogulf"
extension
[MooreandBuffington,1968;Karigand
Jensky,1972;Moore, 1973]andthePliocenedevelopment
of
thePacific-NorthAmericaplateboundary,fromabout5.5 Ma
to thepresent[Larsonet al., 1968;CurrayandMoore, 1984].
Copyright 1989
by theAmericanGeophysical
Union.
Papernumber88TC03753.
0278-7407/89/88TC-03753510.00
Originally, the protogulfconceptwas usedto explainan area
of anomalouslyold oceanic crust adjacentto the Mexican
margin at the mouth of the Gulf of California [Moore and
Buffington, 1968]. More recently, this concepthas been
expandedto include late Miocene extensionalfaulting and
marine sedimentsfrom areas surroundingthe northern and
centralpartsof the Gulf of California [e.g.,Karig and Jensky,
1972; Moore, 1973; Gastil et al., 1979]. These late Miocene
extensionalstructuresand sedimentsare exposedaroundthe
gulf in the "Gulf ExtensionalProvince"[Gastil et al., 1975]
on the east side of Baja California and the west coast of
mainlandMexico (Figure 1). The amountand directionof
extensionof the pre-5.5 Ma protogulf, and its relation to
Pacific-North America motion, is not well known.
Several causes have been proposed for late Miocene
circumgulfextension.Karig andJensky [1972] suggested
that
it was back arc extension,but more preciseconstraintson the
timing of cessationof subductionwest of Baja California
[Mammerickxand Klitgord, 1982;Lonsdale,1989b] showthat
it was not contemporaneouswith active subduction. The
geographiccontinuity,and similarityin extensiondirection,of
the Gulf
Extensional
Province
and the southern Basin and
Range extensionalprovince (in Arizona and Sonora)invited
suggestionsthat this entire Miocene extensionalbelt resulted
from the sameprocessof "BasinandRangeextension"[Gastil,
1968;Dokka andMerriam, 1982;CurrayandMoore, 1984]. In
both of thesescenarios,late Miocenecircumgulfextensionis
assumedto havebeenobliqueto Plioceneextensionassociated
with the developmentof the Pacific-North America plate
boundary,and it is thoughtto have weakenedthe crustand
hencefacilitatedthepropagationof thePacific-NorthAmerica
plate boundary into the gulf. By contrast, Gastil and
Krummenacher[1977] viewedpost-10Ma extensionin Sonora
as a continuous interval of "rhombochasmic extension",
implicitly related to the developmentof the Pacific-North
America plate boundary. Spencerand Normark [1979] and
Hausback [1984] proposedthat late Miocene circumgulf
extension accommodatedthe componentof Pacific-North
Americadisplacement
perpendicular
to thepreexistingstrikeslip faults west of Baja California, after counterclockwise
100
StockandHodges:Pre-PlioceneExtensionAroundtheGulf of California
/
!
I O0
200
i
0
I00
m,
ß
200
km
RIVERA PLATE
I
PACIFIC PLATE
Fig. 1. Map of the Baja CaliforniapeninsulaandadjacentmainlandNorth America,showingthe coastline,the
southern
BasinandRangeprovince,andtheGulf Extensional
Province.The Tosco-Abreojos
faultzoneis located
according
to Spencer
andNormark[1979];otherborderland
faultsaregeneralized
fromKrause[1965]. Gulf plate
boundary
faultsandlimitsof thecircumgulf
extensional
province(darktickedlines)aregeneralized
fromreferences
in thetext. Bathymetriccontours(500 m, 2000 m) andbasemapare from theNorthAmericanMap Committee
[1965]. ENS is Ensenada.
StockandHedges:Pre-PlioceneExtensionAroundtheGulf of California
101
rotation
ofthedirection
ofPacific-North
America
plate
motion.
Possible
explanations
forthe
widespread
Tertiary
extension
in westernNorth America have been summarizedby Stewart
\$ YUM
'\
[1978] and Christiansenand McKee [1978]. Some of these,as
well as more recentexplanations,invokemechanismsrelated
to the geometryand kinematicsof the Pacific-NorthAmerica
plate boundary:extensionwithin a broadzone of right-lateral
•
ß PHX
•lJ
shear [Atwater, 1970], extension above a "slab window"
[Dickinsonand Snyder,1979], extensiondue to the geometric
instability of triple junctions [Ingersoll, 1982], or extension
"'-.ME
ENS/'•
due to flexure above the Mendocino fracture zone [Glazner and
Bartley, 1984]. Othersinvoke lithosphericor asthenospheric
causes unrelated to the development of the Pacific-North
America plate boundary'extensiondue to relaxationof crust
thickened under compression [Molnar and Chen, 1983;
Wernicke et al., 1987] or extensiondue to steepeningof a
formerly shallowly dipping subductedslab [Davis, 1980].
Probably, Tertiary extensionin westernNorth America had
multiple causes. Observation of extension in areas
surroundingthe Gulf of California, where both "Basinand
Range"extensionandPlioceneextensionrelatedto thePacific-
,
X
ß TUC
'\
ß MD
c
•-.
$
400
k rn----.,,, '" ' '
y
ß
,,
ß
G
.,.•?')'
'•
,,
NorthAmerica
plateboundary
haveoccurred,
mayprovide
Y
f
?
.,.
)
y
y
y
some
constraints
onpossible
causal
links
between
extension
_
and the development
of the Pacific-North
Americaplate
boundaryelsewhere.
Thispaper
examines
(1)thedifferences
instyleanddirection
betweenlate Miocene circumgulfextensionalstructuresand
later structures
of thePacific-NorthAmericaboundaryzonein
regionswhereMioceneto Recentdeformationhasoccurred,(2)
the timing of late Miocenecircumgulfextensionand cessation
of subduction,and (3) the direction and amount of PacificNorth America plate motion in Miocene time. Extensionand
tectonic disruption in this area were much simpler than in
areasto the north,so the earliershapeof the platemargincan
"
-4
= 4
bereconstructed
andincorporated
intheanalysis.
4
We suggest that late Miocene extension in the Gulf
m--•
Extensional
Province
was
similar
instyle
tothat
intheBasin
and
Range
province
but
may
have
been
kinematically
a
component of Pacific-North America plate boundary
displacement.This implies that, in late Miocene time, plate
boundarydisplacementoccurredbothon the borderlandfaults
west of Baja Californiaand on extensionalfaultseastof Baja
California,isolatingBaja Californiaas a rigid block withinthe
Pacific-NorthAmericaplateboundaryzone.
LATE MIOCENE
EXTENSIONAL
EXTENSION
PROVINCE
IN TI-•
4
MAZ
-
•
4
-!
GULF
TEP
I
0
100
200 KM
(• Geologic
tiepoint:
distinctive
conglomerate
{Gastil et al., 1981)
The Gulf Extensional Province [Gastil et al., 1975]
encompasses
the easternedgeof the Baja Californiapeninsula
and the part of mainland Mexico bordering the Gulf of
California, includingparts of the statesof Baja California,
Baja California Sur, Sonora,Sinaloa,Nayarit, Jalisco,and
Durango(Figures 1 and 2). We draw the westernlimit of the
Gulf
Extensional
Province
at the western limit
of known
continuousor pervasiveMiocene or Pliocene normal faults
along the west side of the gulf, inferred from geologic
summariesreferencedin the text. In the centralpart of the
Baja California peninsulathere is no evidence of Miocene
extension; Miocene extensional structureson the west side of
thepeninsula
andin thecontinental
borderland
therefore
belong
to a separategeologicprovince. The westernlimit of the Gulf
ExtensionalProvince(GEP) in the stateof Baja California
coincidesapproximatelywith the Main Gulf Escarpment,
a
topographic
breakof varyingsteepness
thatfollowsthelength
Fig. 2. Reconstructed width of the Gulf Extensional
Province,after closureof the gulf transformfaultsby 300 km
(usingthe 5.5 Ma reconstruction
discussed
in the text). Base
map as in Figure 1. MD, MZ are Sonoran"metamorphiccore
complexes"[Andersonet al., 1980]. Other abbreviations:BC
is Bahia Concepci6n;ENS is Ensenada;LA is Los Angeles;
LOR is Loreto; LP is La Paz; MAZ is Mazatlfin; MEX is
Mexicali; P is Puertecitos;PHX is Phoenix;SB is San Borja;
SF is San Felipe; SJC is San Jos6del Cabo Trough; SR is
Santa Rosalfa; SRB is Santa Rosa Basin; TEP is Tepic, in
Nayarit; TIB is Isla Tibur6n; TIJ is Tijuana; TUC is Tucson;
VCH is Valle Chico; YUM is Yuma; 3V is Tres Vfrgenes;
3M is Islas Tres Marias. Geologictie point acrossthe gulf
(circledx) is a distinctiveconglomerate
[Gastilet al., 1981].
102
StockandHodges:Pre-PlioceneExtensionAroundtheGulf of California
of the Baja California peninsula and is generally faultcontrolled [Gastil et al., 1975; Lindgren, 1888, 1890]. We
draw the eastern limit
of the GEP as the eastern limit
of
knownextensionalstructures
eastof thegulf, as deducedfrom
referencesin the text; there is no pronouncedtopographic
escarpment
alongthiseasternboundary.On mainlandMexico
the northern
and southern
limits
of the Gulf
Extensional
Province are gradational;the GEP mergeswith extensional
structures of the Trans-Mexican
volcanic belt on the south and
with the southernBasin and Rangeprovinceon the north. On
the west side of the gulf the northernlimit of extensional
structures is the San Andreas fault.
In much of the Gulf Extensional Province the structures and
timing of Tertiary extensionhave not been studiedin detail.
Regionalsyntheses
of availabledata[Gastilet al., 1975, 1979]
and more recent mappingand geochronologyprovide the
constraints summarized below.
For most of these areas,
detailedfault slip dataandregionaltransportdirectionsarenot
available; thus, althoughfault orientationsare known, the
exact direction of extension is not.
BajaCalifomia
SierraSanFelipeandSierraSantaRosa. Eastof theSierra
SanFelipe,in theSantaRosaBasin,a sequence
(upto 1 km
thick)of alluvialsediments
interbedded
withmiddleto upper
Miocenevolcanicstratais tilted 30ø to the westabovea planar
low-anglenormalfault dipping20ø to theeastwith 3-5 km of
normal displacement. The extension direction was
approximately
SE [Bryant,1986]. No evidencefor progressive
tilting, growthfaulting,or unconformities
is foundwithin the
section. Pliocenefanglomeratesoverlie the fault trace and
unconformablyoverlie the tilted Miocene strata,suggesting
late Miocene-early Pliocene slip on this fault.
A
monolithologic megabreccia of granitic rocks deposited
betweentuffs datedat 12.3+1.8Ma and 14.9+0.9Ma [Bryant,
1986; Gastil et al., 1979] is the oldest indication of Miocene
tectonism.(Uncertaintiesin agesare givenas + lo throughout
this paper. All ages are conventionalK/Ar unless noted
otherwise.) A changein the provenanceof basin deposits
occurredjust abovea basalthorizondatedat 8.9+1.2 Ma and
wasinterpretedby Gastilet al. [ 1979] asindicativeof normalfault relateduplift of rangesto the west. Minor ENE, NNE,
and NNW strikinghigh-anglestrike-slipfaults disruptall of
the units present and are attributed to accommodationof
extension,or to conjugatestrike-slipfaulting,that postdates
the low-anglenormalfault and is still active. Pliocenemarine
stratawest of the SierraSan Felipe dip up to 45ø and contain
interbedded megabreccias of tonalire, suggesting that
significantdeformationcontinuedlocally into Pliocenetime
[Gastil et al., 1979].
Southern Sierra San Pedro Mfirtir and Valle Chico. Dokka
andMerriam [1982] estimatedthatextensional
faultingbegan
here
sometime
between
17 and 9 Ma
and that it was
approximatelyE-W directedbecausethe dominantstrikeof the
normal faults is north-south. More detailed mappingand
geochronologyconstrainthe initiation of extensionto the
interval between depositionof two rhyolite tuffs dated at
10.85+0.16
Ma (40Ar/39Ar
totalfusion
of anorthoclase)
and
6.10+0.06 Ma (averageof two K/Ar ageson anorthoclase)
[Stock, 1989]. This time of onset of extension is based on the
absenceof regionalunconformities
in a 17-11Ma ignimbrite
sequence,absenceof changesin thicknessof this sequence
acrossthe currentstructures
of the Main Gulf Escarpment,and
absenceof faults predatingthe 10.85 Ma ignimbrite[J. M.
Stock and K. V. Hodges, Miocene to recent structural
development
of an extensional
accommodation
zone,NE Baja
California, Mexico, submitted to Journal of Structural
Geology,1988]. Extensionin thisareaoccurred
by high-angle
normal faulting east of listric faults along the Main Gulf
Escarpment,
whichhadno topographic
relief until after11 Ma.
About 10% extension occurred in this area, with about half
priorto 6 Ma andhalf since,onthesamesetof NNW striking
normalfaults. A changein extensiondirectionwith time was
not recognizedbut cannotbe ruledout. In this area,Holocene
scarps
indicatecontinued
normalfaultingontheNNW striking
SanPedroMfirtir fault (Main Gulf Escarpment
) [Gastilet al.,
1975; Brown, 1978].
Puertecitosregion. Sommerand Garcia [ 1970] andGastilet
al. [1975, 1979] reportedagesof 3.1+0.5 Ma and 5.9+0.2 Ma
for flat-lyingrhyolitesoverlyinga seriesof steeplydipping
acidic volcanic rocks yielding ages from 7.3+1.5 Ma to
8.3+0.8 Ma [Gastilet al., 1975, 1979]. They thusinferred
that the major phaseof tilting in this area is late Miocene
(post-9Ma) in age. About 6 km northof their studyarea,
undated (probably Miocene) volcanicsand unconformably
overlyingPliocenemarinebeds,cappedby water-lainash,are
tilted westup to 30ø on a seriesof eastdippingnormalfaults,
and similarfaults cut Quaternarydeposits(J. M. Stockand J.
T. Smith, unpublished mapping, 1987). This suggests
considerable
variabilityin the amountsof faultingandtilting
affectingPlioceneto Recentstratain this regionandindicates
thatpost-Miocenenormalfaultsmaybe locallysignificantand
of the same orientation as late Miocene normal faults.
Dokka and Merriam [1982] observedthat Miocene stratanear
Puertecitos
dip 25ø to 40ø eastor westbeneathmoreshallowly
dipping Pliocene volcanic strata and inferred that these
Miocene units correlatedwith the 7.3 to 8.3 Ma sequence
describedabove. If all of the faults in the poorly mapped
Puertecitosvolcanicprovinceare inferredto dip 60ø, the area
experienced10-18%extensionsincedepositionof theMiocene
strata(see,e.g., Figure 3 of Wernicke and Burchfiel [1982]).
Lower assumed dip angles would correspondto more
extension. Dokka and Merriam [1982] found that only a
single geometryof normal faults has been active since this
time. Many of theseNNW to northstrikingnormalfaultscut
the post-6 Ma volcanic strata [Dokka and Merriam, 1982;
Gastilet al., 1975],indicatingtheimportanceof post-Miocene
deformation.
Santa Rosalfa. In this region a conformablesequenceof
Miocenevolcanicstratawastiltedup to 45ø alonga few north
to NNW striking, west dipping normal faults [Wilson and
Veytia, 1949] prior to the depositionof overlying lower
Pliocene [Wilson and Rocha, 1955] or uppermostMiocene
[Sawlan and Smith, 1984] Boleo Formation. Here the tilted
volcanic units yielded K/Ar dates from 19.9+0.6 Ma to
10.7+0.3 Ma (andesire of Sierra Santa Lucia; Sawlan and
Smith [1984]). The Main Gulf Escarpment near Tres
Virgenes,immediatelyto the north,postdated10 Ma basalt
[Sawlan and Smith, 1984]. NNW striking normal faults
affected 10 Ma units but not 3 Ma units; however,upper
Plioceneto Holocenealkalic volcanismin NNW trending
grabensappearsto have been coeval with faulting of the
grabenmargins[Sawlanand Smith, 1984], suggesting
more
recentactivity on someNNW strikingnormalfaults. Wilson
[1948] and Wilson and Rocha [1955] reported numerous
normalfaultsstrikingN10øW to N45øW. Thesecut Pliocene
units and tilted them up to 15ø near Santa Rosalia.
Pleistoceneunitsin the sameareawere generallyunaffected;
thus the age of this episode of normal faulting must be
Pliocene. A studyof fault striaein thisarea [Angelieret al.,
1981] showedthat the late MioceneNNW strikingnormal
faults were a responseto ENE oriented least horizontal
StockandHodges:
Pre-Pliocene
Extension
AroundtheGulfof California
principalstress(Oh)andthatPliocenemotionon faultsof the
same orientationwas more oblique, with the Oh direction
oriented WNW-ESE
or E-W.
Bahia (70ncepci6nand Sierra de la Giganta. Near Bahia
Concepci6n,McFall [1968] documented
NE tilting of up to
45ø in upperMiocenevolcanicrocksaffectedby NW andNE
striking normal faults. The uppermoststrata in this
conformablesection(Ricas6nFormationof McFall [1968])
yieldeddatesof 9.8_+1.6Ma and 8.0+3.7 Ma [Gastilet al.,
1979]. Volcanicdepositscorrelatedwith the entiresequence
yielded ages rangingfrom 19.9_+0.6Ma to 10.7_+0.3Ma
(andesiteof SierraSantaLucia; Sawlanand Smith[1984]).
On theConcepci6n
peninsulathelowerpartof thissequence
is
intrudedby diorite dikes strikingN70øW and N10øE and
numerousNE to ENE striking pyroclasticdikes [McFall,
1968]. These dikes are not observedin either the lower or
upperpartof thesequence
whereit is exposedelsewhere,
and
they have not beendated,so their implicationsfor the late
Miocene stress direction cannot be assessed at present.
Pliocene marine strata overlie these units in angular
unconformity,suggestingthat tilting and normal faulting
occurred before the end of Pliocene time. If these Pliocene
strata correlate with 3.3 Ma to 1.9 Ma strata of the Loreto
embayment
to the south[McLean,1988],the normalfaulting
predates3.3 Ma. Freshfault scarpsoccuralongtheN45øW
strikingBahiaConcepci6n
fault zone,whichmayhaveabout
30 km of right-lateralstrike-slipdisplacement
[McFall, 1968].
West of Bahia Concepci6n,in the Sierra Giganta,normal
faultsstrikingN10øW to N35øW cut the Comond6Formation
andoverlyingbasalts[McLeanet al., 1985, 1987]. This setof
faultsdoesnot persistvery far westof thepeninsulardrainage
divide, and hence we include it within the Gulf Extensional
Province. The basaltshave not been datedwhere they are cut
by the faults, but flows in similar stratigraphicpositions
nearbyyield agesfrom 15 to 7 Ma [McLeanet al., 1985]. No
othergenerations
of faultsaffectthearea. As thesefaultshave
only beenmappedin reconnaissance,
mainlyfrom air photo
identification,theirexactdisplacement
directions,dips,andthe
amountof extensiontheyrepresentare unknown.
L0reto. About 50 km southof Bahia Concepci6n,NW to N
strikingnormal faults eastof the peninsulardrainagedivide
displaceNeogenevolcanicsand sedimentsdown to both the
eastand west [McLean, 1988]. One of the major faultswhich
cutsQuaternaryalluviumalongpart of its traceoccursat the
westernedgeof a alepositional
basinof Pliocenemarinestrata
includingtuffs 1.9 and 3.3 Ma in age [McLean, 1988]. Dips
of Pliocenestrataare typically10ø to 30ø. UpperOligoceneto
middle Miocene strata locally dip more steeply, indicating
somepre-Pliocenedeformation.McLean [1988] suggests
that
the major offsetson thesefaults involve pre-Pliocenestrata,
but the exact age of initiation of this faulting is not well
constrained
by theexistingdata.
La Paz.
The Gulf Extensional
Province
is confined to a
narrowstripalongtheeastcoastof BajaCaliforniaSur(Figure
1). Even within this region, Miocene units may be flat-lying
and little deformed. East dipping normal faults striking
N15øW-N35øW in the La Paz area postdatethe Comond6
Formationand may still be active. The youngestdatedunit in
the Comond6 Formation here has yielded a K/Ar age of
12.5_+1.4Ma [Hausback, 1984], thus limiting the earliest
possibleonset of extensionalfaulting. The latest possible
onsetis not well constrained
becauseof theabsenceof younger
units. Hausback[1984] reporteda localangularunconformity
beneath the 18-16 Ma San Juan tuff, but the regional
significanceof the correspondingdeformationalevent is
probablysmall.
Strike-slip faulting has also been important in this area.
Offset facies trendssuggestsome tens of kilometersof left-
lateraldisplacement
on the La Paz fault [Hausback,1984].
Partof this fault is still seismicallyactive[Molnar, 1973], and
geologicrelationsindicatepost-12Ma normaldisplacement
[Hausback,1984]. The timing of initial displacementalong
this fault is not well knownbut may haveprecededdeposition
of Miocene volcanics [Hausback, 1984].
Mainland Mexico
CoastalSonora. Extensionalfaultingandtilting in a variety
of directionsaffectedIsla Tibur6nandadjacentSonorabetween
12 and 9 Ma [Gastil and Krummenacher,1977]. Volcanic and
sedimentary
rocks10 Ma andolderare tiltedup to 60ø to the
eastalonga seriesof NW strikingnormalfaults[Gastilet al.,
1974]. Locally, on Isla Tibur6n, 19-15 Ma units are tilted
moresteeplythan13-11Ma units,suggesting
thatextensional
faultingbeganbetween15 and 13 Ma [Neuhauset al., 1988].
Post-7 Ma units in coastalSonoraare flat-lying, suggesting
thattilting andmajorextensiontherewasmostlypre-Pliocene
in age. NE strikingfaultson Isla Tibur6ncutyoungalluvium
and were related by Gastil and Krummenacher[1977] to
modern dilatation of the Gulf; faults of this strike are not
recognizedfurthereastin mainlandSonora.Somemajorfaults
strikingN30øW to N40øW eastof Isla Tibur6nare likely to
havesignificantstrike-slipdisplacement,
althoughthe amount
has not been established[Gastil and Krummenacher,1977].
Coastal Sina10a. An area extending50 to 100 km inland
from the west coastof southernSinaloa,adjacentto Mazatlfin,
is broken into east tilted blocks boundedby NNW to NW
strikingnormalfaults[HenryandFredrikson,1987]. The tilt
of Oligoceneto middleMiocenestratareaches60ø in places,
and extensionis estimatedto be 20-50% [C. D. Henry, Late
CenozoicBasin and Range structureadjacentto the Gulf of
California, submitted to Geological Society of America
Bulletin, 1988]. This extension postdates16.8_+0.3Ma
becausethis is the youngestdateobtainedfrom a conformable
sequenceof equallytilted volcanicstrata. Thesestrataform
fault-bounded basins filled by unconformably overlying
nonmarinesedimentsthat may be Mioceneor Pliocenein age
[Henry and Fredrikson,1987]. UnfaultedQuaternarybasalts
postdatethesesediments.In theabsence
of furtherinformation
the time of faultingis only bracketedbetween17 and2 Ma.
Nayarit. In southernNayarit, SE of the mouthof the gulf,
pre-Pliocenestrata, including a regional volcanic package
yielding datesfrom 21.3_+0.9Ma to 13.8_+3.0Ma, are tilted
200-30ø to the NE alongNW strikingnormalfaults [Gastilet
al., 1978]. 10-8 Ma basaltlies in angularunconformityabove
the older strata [Gastil et al., 1979]. The normal faults are
overlain by, and rarely cut, Pliocene to Holocene volcanic
rocks. The reconnaissance
natureof the studyprecludedtight
constraintson the timing of this faulting, and it is possible
that it is similarin timing and orientationto the late Miocene
faultingknownfrom elsewherearoundthegulf. Fault-bedding
relationships
shownon interpretivecrosssectionsby Gastilet
al. [1978] limit the extensioncausedby thisfaultingto <10%.
SouthernBasinandRangeProvince
The Gulf Extensional
Province widens northward
into the
southernBasin and Range provinceof Arizona and northern
Sonora (Figure 1). The geology of most of this part of
Mexico is not well studied,andit is prematureto concludethat
the studiedareasadequatelyrepresentthe entireextensional
province. It is also importantto note that if the Baja
0/4
StockandHodges:Pre-PlioceneExtensionAroundtheGulf of California
California peninsulais restoredto its pre-5.5 Ma position
relative to mainland Mexico, by closure of the Gulf of
California by 300 km, the northern extent of the Gulf
Extensional Province in Baja California (Mexicali) lies
adjacentto northernSonora,250 km southof Yuma and280
km west of Tucson(Figure 2). Thus in late Miocene time,
southernArizona was not geographicallycloseto the limits of
theextensionalprovincepresentlyexposedwestof thegulf.
Low-angledetachment
faultingin southernArizonabeganat
about 24-20 Ma [Glazner and Bartley, 1984]; direction of
transporton thesefaultswasgenerallyNE-SW to ENE-WSW
[Wust, 1986]. YoungerMiocene sedimentsfrom SE Arizona
containunconformities
andmegabreccias
indicatingcontinued
extensionalfaulting[Eberlyand Stanley,1978;Zobacket al.,
1981]. A distinct post-13 Ma faulting event, forming NW
strikingstructuraltroughsandassociated
with a majorregional
unconformity,endedsometimebetween10.5and6 Ma [Eberly
and Stanley, 1978]. Although the 24-20 Ma extensionin
southernArizona predatesextensionin the Gulf Extensional
Province,post-13 Ma extensionin southernArizona may be
coevalwith late Miocenecircumgulfextension;the orientation
of structures is similar.
Late Cretaceousand early Tertiary plutonsin NW Sonora
crop out in "metamorphiccore complexes",associatedwith
NE trendinglineations[Andersonet al., 1980] suggesting
SWNE extension[Wust, 1986]. The age of this extensionis not
well constrained. This region is often included within the
southernBasin and Range physiographicprovince;note that
the southernmost
of thesecore complexeslies 200 km eastof
Isla Tibur6n and coastal Sonora where late Miocene extension
beganat about 12 Ma (Figure 1).
ProtogulfMarine Sediments
The record of Miocene marine sediments from around the
gulf shedsadditionallight on the distributionof extendedand
subsidingregionsnear the gulf. The oldestknownMiocene
marine deposits,about 13 Ma in age, are interbeddedwith
12.9+0.4 Ma volcanics and underlie 11.2_+1.3 Ma volcanics on
Isla Tibur6n [Gastil et al., 1979; Smith et al., 1985; Neuhaus
et al., 1988]. Upper Miocene marine rocks on Isla Mafia
Madre in the southernmost
gulf (location 3M, Figure 2)
indicate a pre-8.2 Ma subsidenceevent [McCloy, 1987].
Uppermost Miocene (6.0 to 5.5 Ma) diatomiresoccur in a
Miocene-Pliocenemarinesequencenear SanFelipe [Boehm,
1984], and upper Miocene strata are reportedfrom wells
beneaththe coastalplain of westernSonora[G6mez, 1971].
Pliocenedepositsin SW Arizonaand the SaltonTroughare
underlainby uppermost
Miocenemarinesediments
[Olmstedet
al., 1973; Mattick et al., 1973; Ingle, 1987]. The upper
MioceneHualapailimestonein the GrandWashTroughnear
Lake Mead, Nevada, has also been attributed to the marine
protogulf[e.g., Blair, 1978], althougha lacustrineorigin for
this unit hasnot beenruled out (seediscussion
by Bohannon
[1984]).
The depositionaltroughrepresented
by theseupperMiocene
marinesedimentsmay havebeencloseto the presentaxis of
the gulf, with an outletto the Pacific nearthe presentmouth
of the gulf [e.g., Winker and Kidwell, 1986; Smith, 1987].
Further paleontological studies are needed to detail the
temporaldevelopmentof thismarinebasin.
STYLE AND DIRECTION
OF LATE MIOCENE
EXTENSION
Post-12 Ma extension in the Gulf Extensional Province
producedtopographic
basinsandrangesin NE BajaCalifornia
which are oriented NNW, parallel to those in the southern
Basin and Rangeprovince. Justas in the Basinand Range
province, extension-relatedfaults in the Gulf Extensional
Province include high-angle,listric, and planar low-angle
normal faults, as well as strike-slipfaults accommodating
differential motion of crustal blocks.
The dominantstrikeof the high-anglenormalfaultsin the
Gulf ExtensionalProvince was NNW. Becauseregional
mappinghasnot yet revealedpiercingpointson mostof these
faults,the net extensiondirectionis unknown. The simplest
assumptionis that dip-slipdisplacementdominatedand hence
that extension was ENE directed in late Miocene time.
This
directionof extensional
strain,perpendicular
to thepresentaxis
of the Gulf of California and to the late Miocene continental
margin to the west, is consistentwith observationsof fault
striae[Angelieret al., 1981;J. M. StockandK. V. Hodges,
Miocene to recentstructuraldevelopmentof an extensional
accommodation
zone,NE Baja California,Mexico, submitted
to Journal of Structural Geology, 1988]. Without more
detailedgeologicstudythisdirectionis subjectto considerable
uncertainty,
because
extensional
displacement
maybe oblique
to thedip directionof bothbeddingandfaults[e.g.,Walkeret
al., 1986]. Although dikes have been observedin several
studies[e.g., McLean, 1988; McFall, 1968], dike swarms
suitable for estimating the direction of least horizontal
principalstressCSh
havenot beendated. Again,the simplest
assumption is that CShwas oriented ENE near the Gulf
ExtensionalProvince in late Miocene time, althoughthis
stress direction cannot be confirmed to the level of detail
possibleto the northin the BasinandRangeprovince[e.g.,
Zoback et al., 1981].
The CSh
directionin the southernBasinandRangeprovince
was ENE-WSW
in the interval 20-10 Ma and rotated to the
presentESE-WNW direction at about 10 Ma [Zobacket al.,
1981]. The principalfaultsin thesouthern
BasinandRange
provincestrikeNW to NNW andmostlikely formedundera
pre-10 Ma ENE-WSW extensiondirection,prior to the circa
10 Ma rotationof the stressfield [Zobacket al., 1981]. These
faultsare similarin striketo the late Miocenecircumgulf
extensional
faults. If thecircumgulfextensional
faultsindicate
an ENE-WSW directionfor C•h,it appearsthat the southern
Basin and Rangeprovinceand the circumgulfextensional
province experiencedextensiondirectionsthat may have
differedby as muchas 45ø in late Miocenetime, despitetheir
similar tectonicsettingseast of the strike-slipPacific-North
Americaplateboundary.
AMOUNT
OF EXTENSION
In two areasaroundthe gulf with well-constrained
estimates
for theamountof late Mioceneextensionon high-anglefaults,
local valuesrangefrom 5% [J. M. Stockand K. V. Hodges,
Miocene to recent structuraldevelopmentof an extensional
accommodation
zone,NE Baja California,Mexico, submitted
to Journal of Structural Geology, 1988] to 20-50% [C. D.
Henry, Late CenozoicBasinand Rangestructureadjacentto
the Gulf of California,submittedto GeologicalSocietyof
America Bulletin, 1988]. Dips of faults and beddingon
publishedcrosssectionsof otherareasmay be interpretedto
indicate extension values between these amounts (see
discussionabove). The difficulty in obtaining accurate
estimatesof extensionfrom such techniques,in areas of
detachment faulting, is illustrated by a comparisonof
publishedestimatesfrom the northernBasin and Range
province(10% to 100%, or 75 to 325 km; seesummaries
by
Davis [1980] andWernickeet al. [1982])with a valueobtained
Stock
andHodges:
Pre-Pliocene
Extension
Around
theGulfofCalifornia
fromgeologicpiercingpointsacross
detachment
faultsfurther
south (250-300 km, or 250%, at the latitude of Las Vegas
[Wernickeet al., 1988]). More detailedmappingaroundthe
Gulf of California is needed for an accurate assessmentof the
net extension there.
The presentgeographiclimits of the Gulf Extensional
Province include 300-350 km of post-Miocene transform
displacement
within the gulf (seediscussion
below). This
transformdisplacement
wasaccompanied
by diffuseextension
of continental crust and the formation of "transitional" crust
within the gulf, as well as by the formationof sometrue
oceaniccrustin the southerngulf. The excessareagenerated
by thisdisplacement
mustberemovedto reconstruct
thelatest
Miocene width of the extensional province. This is
straightforward
if the boundaries
of the extensional
province
havenot changedsincethe end of Miocenetime, as indicated
by the availablestructuraldata [e.g.,J. M. Stockand K. V.
Hodges, Miocene to recent structuraldevelopmentof an
extensionalaccommodation
zone,NE Baja California,Mexico,
submittedto Journalof StructuralGeology, 1988; Hausback,
1984]. Then,regardless
of how thePliocenedisplacement
was
partitionedwithin the Gulf ExtensionalProvince,its effecton
the width of the provincecanbe estimatedby closingup the
edgesof the Gulf Extensional
Provinceby the 5.5 Ma (300
km) reconstruction
(Figure2). If this properlyaccountsfor
the Pliocene deformation, it indicates that at the end of
Miocene time the Gulf Extensional Province varied in width
from .-.400 km in the northwest to --250 km at its southern end
(Figure2). Note thattheseboundaries
includea largeareaof
Sonora,includingthe "metamorphic
corecomplexes",
where
the age of extension is not known. Within the Gulf
ExtensionalProvince,200% EN-E extensionwould correspond
to 270 km in the north and 170 km in the south; 20%
extensioncorrespondsto 70 km and 40 km, respectively.
Thesevaluesmerelyillustratethepossiblerangein valuesof
extensionthat might have occurredwithin the late Miocene
Gulf Extensional Province.
There is no reason to believe that
either the total amount of extension,or total percentageof
extension,wasconstantthroughoutthe extensionalprovince.
105
southernfragmentof the Farallon plate [e.g., Dickinsonand
Snyder, 1979]. South of the Rivera triple junction,
subductionof Farallon plate crust (the Guadalupeplate of
Menard [1978] and smaller microplates that broke from it
[Lonsdale,1989b]) continued. The lengthof this subduction
zonedecreasedas the Rivera triplejunctionmovedsouthward
by unevenlytimedridge deathsand microplatecapturealong
theBaja Californiamargin[Lonsdale,1989b].
At 13 Ma, subductionwas still occurringfrom the Vizcafno
Peninsulasouthto the tip of Baja California. This subduction
stopped at about 12 Ma, when this part of the PacificGuadaluperidgestoppedspreading[MammerickxandKlitgord,
1982; Lonsdale,1989a,b]. The marginadjacentto the former
Guadalupe-NorthAmericatrenchdevelopeda strike-slipfault
zone (the Tosco-Abreojosfault) of the Pacific-NorthAmerica
plateboundary[SpencerandNormark,1979]. Many modelsof
gulf evolutionportray this fault zone as the plate boundary
from 12 Ma until the time that the plate boundarybegan to
move into the Gulf of California. This is thought to have
occurred at about 5.5 Ma, becauseextension and subsidenceof
this age occurred at the mouth of the Gulf of California,
possiblylinked to similardiffuseextensionfurthernorthin the
gulf [Currayand Moore, 1984;Lonsdale,1989a]. About300
km of fight-lateraldisplacementon the southernSan Andreas
systemandgulf transformfaults[Larsonet al., 1968;Gastilet
al., 1981] are usuallyattributedto post-5.5Ma motionof the
Baja California peninsula,on the Pacific plate, away from
North America.
The following reconstructions
of the Mioceneplate tectonic
configurationswest of Baja California (Figure 3) address
constraintson threeissues:(1) the positionof theRivera triple
junction with time, relative to Baja California; (2) the
constraintson directionsof relative plate motion; and (3) the
orientation of the plate boundaries. These reconstructions
differ from previousglobalplatecircuitreconstructions
of the
Rivera triple junction [Atwater, 1970; Atwater and Molnar,
1973; Blake et al., 1978; Dickinson and Snyder, 1979;
Klitgord and Mammerickx, 1982; Hausback,1984] because
they are based on updated Pacific-North America
reconstructionsand uncertainties[Stock and Molnar, 1988] for
TERTIARY
PLATE
BAJA CALIFORNIA
TECTONIC
SETTING
OF
The historyof plateinteractions
westof Baja Californiamay
have affectedthe timing and locationof extensionwithin the
Gulf ExtensionalProvince. During most of Tertiary time,
BajaCaliforniawasattachedto westernNorthAmerica,andthe
Farallonplatewasbeingsubducted
eastwardbeneaththeBajaMexico margin. Volcanismrelatedto thissubduction
occurred
in the Sierra Madre Occidental of mainland Mexico from 100-
45 Ma and 34-23 Ma [McDowell and Clabaugh,1979] and in
easternBaja Californiaandwesternmost
Sonorafrom 24 to 10
or 8 Ma [Gastil et al., 1979; Sawlan and Smith, 1984;
Hausback, 1984]. Baja California crust lay west of the
Cenozoic volcanic arc until Miocene time; much of the east
side of the Baja California peninsulacontainsstratafrom the
west side of the Miocene volcanic arc.
The Pacific plate came into direct contact with North
America
at the latitude of California
sometime after 28.5 Ma
[Atwater,1970] andprobablycloserto 25 Ma [Atwater,1989]
at a point northof Baja California[AtwaterandMolnar, 1973;
Stock and Molnar, 1988] if the Gulf of California is closedby
300 km. The new Pacific-North America strike-slip plate
boundaryhad a triple junction at both ends [Atwater, 1970]:
the Mendocino (trench-transform-transform)triple junction,
adjacentto the northernfragmentof the Farallonplate,and the
Rivera (ridge-trench-transform)
triplejunction,adjacentto the
the times of anomalies5 and 6. (Theseanomaliescorrespond
to 10.59 Ma and 19.96 Ma, respectively,on the Decadeof
North American Geology (DNAG) time scaleof Berggrenet
al. [1985]. This magnetic reversal time scale is used
throughoutthe followingdicussion.).The reconstructions
and
uncertaintiespresentedhere are for more closelyspacedand
slightlydifferenttimes,in orderto takeadvantageof the most
informativepartsof the seafloorspreading
recordwestof Baja
California. Implicationsfor the positionof the Rivera triple
junctionare similarto theresultsof Lonsdale[1989b].
Reconstructed
Positionof Baja Californiaat 5..5Ma
In the following discussionand figures, Pacific plate crust
west of the presentSan Andreasfault and west of the gulf
spreadingcenters has been reconstructedto the margin of
mainlandMexico by a rotationof 4.69ø about48.62øN,75.15ø
W (Figures 2 and 3). This 5.5 Ma reconstructionwas
extrapolated
from MinsterandJordan's[1978] RelativeMotion
2 (RM2) angularvelocitiesand uncertaintiesfor the PacificNorth America plate circuit (seeStockand Molnar [1988] for
detailsof this extrapolation.)This reconstruction
removes290
km of displacementalongthe San Andreasfault in the Salton
troughand closesthe mouthof the Gulf of California by 308
km along an azimuth of 127ø. The shelf edge north of the
Marfa Magdalena rise is then aligned with the shelf edge of
Baja CaliforniaSur at 23.7øN, 109.3øW,followingthe detailed
106
StockandHodges:Pre-PlioceneExtensionAroundtheGulf of California
a
55 ø N
30 ø N
25 ø N
CIFIC
GUADALUPE
uncertainty
reconstructed
position
points on
Pac•fm plate
120 ø W
Anomaly 6 (old edge)
55øN
110ow
20.4
Ma
30 ø N
25øN
LOR
PACIFIC
uncertainty
130
W
120øW
Anomaly 5D
110øW
17.7 Ma
Fig. 3. Reconstructions
of thePacificplaterelativeto thefixedNorthAmericaplatein theMioceneandPliocene,
for thetimesof (a) theold edgeof anomaly6, 20.45 Ma on theDNAG timescaleof Berggrenet al. [1985];(b)
centerof anomaly5D, 17.74Ma; (c) centerof anomaly5C, 16.22Ma; (d) centerof anomaly5AA, 12.92Ma; (e)
old edgeof anomaly5, 10.59Ma; (f) old edgeof anomaly3A, 5.89 Ma. Map is an obliqueMercatorprojection
about44.02øN,83.71øW,the stagepolefor Pacific-NorthAmericamotionbetween20 and 10 Ma [Stockand
Molnar, 1988]. The Gulf of Californiais closedas in Figure2, with the 1000fathom(1828 m) contourshown.
Locations,
abbreviated
asin Figure2, aresquares
(onNorthAmerica)andtriangles
(onBajaCalifornia).Heavyline
indicates
positionof Pacific-Rivera
spreading
centerandis dashed
whereinferred.Positionof subruction
zoneand
Pacific-NorthAmericastrike-slipmarginis schematic.Thin linesareisochrons
on thePacificplate[Atwaterand
Severinghaus,
1989]. Rotationdataandshadeduncertainty
ellipses,representing
95% confidence
limitsof the
reconstructed
positionof thePacificplate,arebasedon theglobalplatecircuit(seeStockandMolnar[1988]and
discussion
in text). Seetextfor discussion
of uncertainties.
Theunacceptable
overlapof Pacificplateoceaniccrust
ontoNorthAmericancontinental
crust(Figures3a-3e)resultsfromassuming
a rigidNorthAmericaplateandwould
beremoved
by including
BasinandRangeextension
in thereconstructions
(seediscussion
in text).
StockandHodges:Pre-Pliocene
Extension
AroundtheGulf of Califomia
35 ø N
107
30 ø N
25 ø N
LOR
-:--:..:-
:.-.
uncer
PACIFIC
130øW
120 ø W
Anomaly 5C
d
55 ø N
1 10øW
16.2 Ma
50 ø N
25 ø N
LOR
uncertainty
PACIFIC
13C
W
120øW
Anomaly
5AA
12.9
11oow
Ma
_
Fig. 3 (continued)
bathymetry of Mammerickx [1980] and reconstructionsof
Lonsdale [1989a], and is similar to the reconstructionof
CurrayandMoore [1984].
Thisreconstruction
assumes
complete
rigidityof thepresent
Pacificplate(BajaCalifornia
andsouthern
California)
westof
theSanAndreas
fault. TheBajaCalifornia
peninsula
southof
theAguaBlancafault(nearEnsenada,
Figure1), between
the
partof theplatemarginshouldbe adequate
for timesbetween
20 and5 Ma. (The presence
of a possibletranspeninsular
strike-slipfault in Baja CaliforniaSur,[e.g., Crouch,1979]
has not been conclusivelyestablished.) The continental
marginnorthof Ensenada,
including
theCaliforniacontinental
essentially
rigid in Mioceneandyoungertime. Although
borderland
andthe Transverse
Ranges,
wasmorehighly
deformed
in Miocene
andPliocene
time[DoyleandGorsline,
1977;Crouch,
1981;Hornafius
etal., 1986].Thustheshape
of thispartof the margin,as determined
by rigid plate
significantly
reorient
thewestern
margin
of theBajaCalifornia
reconstructions
(Figure2), is lessvalid for oldertimes.
This reconstructionincludeswithin its uncertaintiesthe
extrapolation of the instantaneousPacific-North America
peninsula. In this case the reconstructedorientationof this
angular
velocity
of Chase[1978].An extrapolation
usinga
Main GulfEscarpment
andthecontinental
borderland,
hasbeen
Miocene extension occurred in the continental borderland
[Normark et al., 1988], we assumethat this extensiondid not
108
Stock
andHodges:
Pre-Pliocene
Extension
Around
theGulfofCalifornia
e
55 ø N
30 ø N
25" N
LOll
.....
:::::.
.........
........
'-'-'-'
uncertainty
PACIFIC
I
1 20" W
13oøw
10.6
Anomaly 5 (old edge)
35øN
110øW
30øN
Ma
25 ø N
.:!.:
ß:..:.. uncertainty
PACIFIC
150
120 ø W
W
Anomaly 3A (old edge)
110øW
5.9 Ma
Fig. 3 (continued)
more recent instantaneous
angularvelocity model known as
NUVEL-1 [DeMets et al., 1987] would closethe mouthof the
California[Lonsdale,1989a]. Geologicconstraints
fromthe
gulfby onlyabout260km butalongthesameazimuthasthis
separation
of a pre-15 Ma conglomerate
channelwith
reconstruction.All of thesereconstructions
result in overlap
of extendedcontinentalcrust within the gulf becausecrust
between the Pliocene transformfaults has extendedby the
formation of "transitional", rather than true oceanic, crust
[Larsonet al., 1972; Moore, 1973].
A value of about300-350 km for strike-slipmotionon the
California(Figure2). Our 5.5 Ma reconstruction
appearsto
overrotate
theoutcrops
of theconglomerate
withrespect
to one
another;however,majorNW strikingstrike-slipfaultsoccur
eastof theSonoran
outcrop[GastilandKrummenacher,
1977]
andmayaccount
for someof thisdiscrepancy.
Threehundred
gulf transformfaults seemsto be constrainedwell by
displacements
onmajorfaultsin southern
California[Crowell,
1981] and is consistentwith the amountof oceaniccrust,and
extendedcontinental
crust,presentat themouthof theGulf of
Gulf ExtensionalProvince [Gastil et al., 1981] includethe
distinctivePermianclasts,foundon both sidesof the Gulf of
kilometers of motion is about 30 km less than the total
displacement
on theSanAndreas,
SanGabriel,SanJacinto,
and Elsinore faults in southernCalifornia [Crowell, 1981;
Hornafiuset al., 1986]. However,pre-5.5Ma displacement
on
StockandHodges:Pre-Pliocene
ExtensionAroundtheGulf of California
35ø N
ß
GLOBAL
ß
HOTSPOT
POINT
PLATE
109
30 ø N
25 øN
CIRCUIT
CIRCUIT
A
POIN'
17
POINT
Ma
C
0 Ma
PACIFIC
POINT
D
130øW
120øW
20.0
Ma
10.6 Ma
1100W
Fig. 4. Uncertainties
in pastpositionsof fourpointson thePacificplaterelativeto fixed North Americasince
Miocenetime. Baja Californiahasbeenmovedbackto NorthAmerica300 km by thereconstruction
discussed
in
the text. Ellipsesare 95% confidence
regionsfor pointpositionsat 5.5 Ma, 10.6 Ma, and20.0 Ma from global
platecircuitreconstructions
[StockandMolnar,1988]. Trianglesarepositions
of samepointsat 5, 9, and17 Ma,
according to the fixed hotspot reconstructionsof Engebretsonet al. [1985]. Note that for both sets of
reconstructions
the averagedirectionof motionin the interval5-10 Ma is obliqueto the trend of the continental
marginwestof BajaCalifornia(theSanBenito-Tosco-Abreojos
faultzone). Map projectionasin Figure3.
the San Gabriel fault should not be included in the 5.5 Ma
of the Miocene-Pliocene
boundary(the old edgeof anomaly
reconstruction.
3A, 5.9 Ma; Figure 3f) was derived,fromMinster and Jordan's
[1978] RM2 model and uncertainties,as describedabove for
the 5.5 Ma fit of Baja Californiato mainlandMexico.
In thesereconstructions
the platesin the globalplate circuit
(Pacific,Antarctica,Africa, andNorth America)are assumedto
have been rigid since20 Ma. For 10.6 Ma and older times
(Figures3a-3e),reconstructed
oceaniccrustof thePacificplate
overlapscontinentalcrustof the Baja Californiaand southern
Californiamargins. The amountof overlapfor sometimes
The exact timing of this 300-350 km of motion is not
constrained
well. In the southerngulf, northof the Tamayo
transformfault, the spacingof anomalies0, 2 (2 Ma), and2A
(2.5 Ma to 3.4 Ma) indicatesthe formationof about 175 km
of oceaniccrustbetweenthePacificandNorthAmericaplates
since3.4 Ma [Larsonet al., 1968;KlitgordandMammerickx,
1982; DeMets et al., 1987; Lonsdale, 1989a]. South of the
Tamayo fracture zone, 300 km of seafloorformed sinceabout
5 Ma [MammerickxandKlitgord, 1982],but thisrepresents
Pacific-Riveraspreadingwhich was fasterthanPacific-North
Americaspreading
dueto convergence
betweentheRiveraand
North America plates [Atwater, 1970; Larson, 1972; Molnar,
1973; Nixon, 1982]. The additional 125-175 km of transform
motion in the Gulf of California is inferred to have occurred in
the interval5.5-3.4 Ma [e.g.,Lonsdale,1989a;Currayand
Moore, 1984], but the time of initiation of this motion is not
knowndirectlyandmaybe olderthan5.5 Ma. The 8 Ma (or
earlier)initiationof motionon the San Gabriel fault [Crowell,
1981]suggests
thatpre-3.4-Mastrike-slipdisplacement
in the
Gulf of Californiamayhavebeendistributed
throughout
the
interval8-3.4 Ma, perhapspartly accommodated
within the
Gulf Extensional Province.
PastPositionof the Rivera Triple Junction
Reconstructions
for timesin the Miocene (Figures3a-3e)
wereinterpolatedfrom thereconstructions
of StockandMolnar
[1988], assumingconstantrelativemotionbetweenthe Pacific
and North America plates throughout this interval.
Uncertainties
in these reconstructions are at least +50 km in
latitudeand+70 km in longitudeandareestimated
to be larger
(+100 km) if the reconstructionwas not closein time to the
end pointsof the interpolation.A reconstruction
for the time
exceedstheuncertainty
in thereconstructions,
indicatingsome
problem with either the reconstructionparametersor the
assumption
of rigid platesin the plate circuit. This overlap
alsooccursin reconstructions
basedon theassumption
of fixed
hotspots (e.g., Engebretson et al. [1985]; Figure 4),
suggesting
thatit is unlikelyto be dueto nonrigiddeformation
in the Africa or Antarcticaplates.It is most likely due to
nonrigiddeformation
of theNorthAmericaplate,considering
the severalhundredkilometersof Neogeneextensionknown
from the Basin and Range province[Wernickeet al., 1988].
Extensionaland strike-slipdeformationhere and alongthe
plate boundary limits the level of precision of the
reconstructions
of Figure3.
Despitetheselimitations,somegeneralinferencescan be
maderegardingthe positionand the lengthof the strike-slip
Pacific-NorthAmericaplateboundary.At 20.4 Ma theRivera
triplejunction(RTJ) was nearthe reconstructed
positionof
Los Angeles, so subductionwas still occurringalong the
entireBajaCaliforniapeninsula.The strike-slip
marginnorth
of this subduction
zonewasonly about400 km long. By
17.7 Ma the RTJ could not have been much further south than
Ensenada;the strike-slipmargin to the northwas about750
km long,andsubduction
wasstill occurringundermostof the
Baja Californiapeninsula.Synchronous
with this subduction,
between22 and 15 Ma, Neogenebasinson the continental
marginwestof BajaCaliforniaexperienced
theirmajorepisode
StockandHodges:
Pre-Pliocene
Extension
Around
theGulfofCalifornia
20 I•
'''"
"•:•••:•••'••ssage
..............
'"•''
:•:"'
' ..........
ofRivera
Triple
Junction
I• Earliest
possible
onset
•:•!:-::!!i%!!!!ii::•::i•1%•
..........
__ofext•alfaulti•9
ß TEP
VCHe.?
ß
Latest
possible
onset
SR•
of extensional
MAZ
faulting
o
o
500
1000
1500
Distance along Gulf of California,
Km
Fig.5. Comparison
of thetimeof onsetof extensional
faultingin theGulfExtensional
Province
withthetimeof
passage
of theRiveratriplejunction
to thewest.Locations
(abbreviated
asin Figure2) areprojected
paralleltothe
westcoastof theBajaCalifornia
Peninsula
(andparallelto thetrendof theSanBenito-Tosco-Abreojos
faultzone)
TIB andSRB are observations
of localdeformation(seediscussion
in text) whoseregionalsignificancehasnotbeen
established.
Ma, subductionceasedalongall of northernBaja California,
and the strike-slipmarginnorthof the Rivera triplejunction
grew to 1400 km in length, extending south through the
southernCaliforniaborderlandto the Vizcainopeninsula.At
12.9 Ma, subductionwas still ocurringwest of the Vizcafno
peninsula.By 10.6 Ma, subductionhad ceasedalongmostof
southern
BajaCalifornia,andtheformercontinental
slopewest
of Baja California Sur (the Tosco-Abreojosfault of Spencer
and Normark [1979]) waspart of the 2100-km-longstrike-slip
margin. Thus the strike-slipmargin lengthenedby about
1700 km between 20 and 10 Ma, even thoughthe relative
displacementbetweenthe Pacific and North Americaplates,
west of Baja California, was only 235+100 km duringthis
(SBTAFZ) after reconstructionof the Baja Peninsula to
mainland Mexico and are consistentwith all of the plate
motion occurring as strike-slip displacement along the
SBTAFZ. However, for the period 5.5-10.6 Ma, the net
azimuth of displacementof the Pacific plate varied from
294ø+11ø to 298ø+12ø, demonstrablyat an angle to the
azimuthof the SBTAFZ (Figure6). Betweenabout 10 and 5
Ma, the net platedisplacement
wasobliqueto the SBTAFZ in
an orientationthatshouldhavecausedobliqueextensionacross
it. However,neitherthe Tosco-Abreojos
fault nor the former
trench extended obliquely [Spencer and Normark, 1979;
Lonsdale,1989b], and subsidenceof the continentalmargin
wasminor [Normarket al., 1988]. This lack of a component
of extensionon the plate boundary,noted by Spencerand
interval.
Normark [ 1979] and Hausback [ 1984], indicatesthat additional
A comparisonbetween the time of onset of extensional
faultingand thetime thattheRiveratriplejunctionlay westof
pointsin the(reconstructed)
Gulf Extensional
Province(Figure
5) shows no clear evidence for late Miocene circumgulf
extensionwhile subductionwas activeto the west. There may
be a temporalcorrelationbetweenthe initial late Miocene
circumgulfextensionand the southwardmotionof theRivera
triplejunctionwestof Baja California,althoughthiscannotbe
confirmed by existing data. It is also possiblethat late
Miocene circumgulf extension started approximately
synchronously
throughoutthe lengthof the gulf and,at least
in someareas,considerably
laterthanthepassage
of theRivera
triplejunctionto the west.
displacement,sufficient to sum to the total relative plate
motion, must have occurred elsewhere within the plate
boundaryzone. The plate reconstructions
suggestthat the
amount of this displacement,measuredin an ENE direction
(perpendicular
to the trendof the Tosco-Abreojos
fault zone),
varied from 160+80 km to 110+80 km along the plate
boundarywest of Baja California (Figure 6). The closest
obviousplace for this displacement
is in the Gulf Extensional
of subsidence[Normark et al., 1988]. Between 17.4 and 12.9
Direction and Amount of PacificrNorthAmerica Motion
The above reconstructions
can be usedto obtainthe average
direction of Pacific-North
America motion, and its
uncertainties,for the time intervals between reconstructions
(Figure4). Sucha computationfor the intervals0-5.5 Ma,
5.5-10.6 Ma, and 10.6 to 20 Ma (Figure 6) indicatesthat
between 20.0 and 10.6 Ma, total Pacific-North America
Province.
Although the dominant extensional faults in the Gulf
Extensional Province strike NNW, the exact direction of
extension is not known.
If we assume that this extension
direction was ENE, then the plate reconstructionsimply
160+80 km of extensionin the northernpart of the Gulf
ExtensionalProvince,decreasingto 110+80 km of extension
in the southernpart of the Gulf ExtensionalProvince. This
corresponds
to 66% extension
in thenorthand78% extension
in the south,with upperandlower limits of 150%-25%in the
north and 300%-13% in the south. A different overall
extension direction in the Gulf Extensional Province would
imply slightlyhigherpercentages
of extension.Thesevalues
are within reasonablelimits of the extensionthat might have
motion was 25+10 mm/yr at an azimuth varying from
3030+20ø to 317ø+20ø alongthewestcoastof Baja California
occurredin the Gulf ExtensionalProvince(20%-200%) andare
Sur.
ExtensionalProvinceaccommodated
the "missing"component
of plate boundarymotion. A northwardincreasein absolute
These azimuths include within their uncertainties the
320ø azimuth of the San Benito-Tosco-Abreojos
fault zone
consistent
with
a kinematic
model
in which
the Gulf
StockandHodges:Pre-PlioceneExtensionAroundtheGulf of California
]]]
50øN
10.6
5.5
25øN
to
M
fault
';60•8
km
-Abreojos
•
rend
o•''' •
'ault•
110ñ80
km
110øW
120øW
Fig. 6. Intervalvelocitiesanduncertainties
for Pacificplatepointswestof Baja California,for the intervals20.0 to
10.6 Ma (large ellipses),10.6 to 5.5 Ma (mediumellipses,shaded)and 5.5 to 0 Ma (smallellipses). Map
projection
asin Figure3. TheSanBenito-Tosco-Abreojos
faultzoneis dotted.Vectordiagrams
in thelowerpartof
theplot showthebestfit anduncertainties
in thedisplacement
of thePacificplaterelativeto North Americafor the
time interval10.6-5.5Ma, thedirectionof the SBTAFZ, andtheamountof platemotionthatcouldnot havebeen
accommodated
by strike-slipmotionalongtheSBTAFZ but whichmighthavebeenaccommodated
withintheGulf
Extensional Province.
extensionin the Gulf ExtensionalProvinceis suggested
by the
relative plate motions (Figure 6) and can be tested with
improvedgeologicconstraintsfrom the Gulf Extensional
Province.
Becausethesereconstructions
correspond
to specifictimesin
thepast,the differencebetweentwo reconstructions
indicates
the net displacement,not the actual displacementpath, for
plate motion betweenthesetwo times. The time interval
duringwhichPacific-NorthAmericaplatemotionwasoblique
to the SBTAFZ is not preciselyconstrainedand could have
been 15 to 7 Ma or 9 to 5 Ma rather than the 10.6 to 5.5 Ma
intervalshownby thereconstructions.
More preciseconstraint
on this time interval awaits both closer time spacingsof
globalplate reconstructions
and moredetailedchronologyof
the Miocene extension within the Gulf Extensional Province.
DISCUSSION
Of all the extended areas in western North America the Gulf
Extensional Province shows the best constrained correlation
betweenthe onsetof extensionoutsideof the existingplate
boundaryzoneanda changein thedirectionof relativemotion
betweenthe Pacific and North Americaplates. The onsetof
extensionheremaybe linkedto thetransitionfrom subduction
to strike-sliptectonics,as inferredfor otherextensionalareas
in western North America [Dickinson and Snyder, 1979;
Glazner and Bartley, 1984].
However, geometric
considerations
suggestthat the extensionreflectsa resolvable
changein thedirectionof relativeplatemotionafterthe strikeslipregimewasalreadyestablished
alongpartof themargin.
The localization of this extensioninland, well removed from
the formerplateboundary,is intriguing.The geometryof late
Mioceneplate motionshouldhavecausedobliqueextension
alongthe SanBenito-Tosco-Abreojos
fault, whichmighthave
become reoriented into en echelon transform faults.
This did
not happen;instead, the plate boundarydisplacementwas
partitionedbetweencontinuedstrike-slipmotion along the
Tosco-Abreojos
fault zoneand extensionfurtherinland. There
are two obviousreasonsfor suchkinematics:eitherthe plate
boundaryat depth was not confinedto the Tosco-Abreojos
fault or else a zone within the continent was closer to failure
by extensionthanwas theplateboundary.The earlierhistory
of the strike-slipboundaryas a subructionzone, and of the
Gulf Extensional
Provinceasan activevolcanicarc,mayhave
contributedto either of thesesituations.The former arc on the
NorthAmericaplatehadbeenrecentlyactive,soit mayhave
been both thermally weak and topographicallyhigh. The
subducted
Guadalupeplate lay somedistancebeneathBaja
California,sotheattachment
of microplates
(brokenfromthe
Guadalupe
plate)to thePacificplateasthePacific-Guadelupe
ridgediedmayhaveprovideda mechanism
for wideningthe
Pacific-North
Americaboundary
at depth.
The strainpartitioningwe infer for thispart of the PacificNorth America plate boundary is similar in scale to the
partitioning of displacementobservedalong obliquely
convergentplate boundaries(Figure 7). Often, in these
regions,subductionis not parallel to the plate motion but
ratherperpendicular
to theplateboundary;the component
of
strike-slipdisplacement
is accommodated
on faultsparallelto
the subductionzone in the arc region [Fitch, 1972]. The
similarityin scaleof thesetwo processes
mayreflecttypical
arc-trench distances.
If this kinematicpictureappliesto extensionin the Gulf
ExtensionalProvince,what are the consequences
for other
extended areas in western North America? The northernmost
limit of extension
attributable
to plateboundary
displacement
is difficult to identify, becauselater deformationnorth of
Ensenadapreventsaccuratereconstructionof the Miocene
shapeof theplateboundaryalongthecontinental
margin. If,
northof Ensenada,
thecontinental
marginaccommodated
all of
the plate boundarydisplacement,then the Gulf Extensional
Provinceshouldhavea well-defined
northern
limitbounded
by
112
StockandHodges:
Pre-Pliocene
Extension
AroundtheGulfof California
OBLIQUE
CONVERGENCE
(F tch,
OBLIQUE
EXTENSION
1972)
Oceanic
plate
Pacific
North
America
plate
Av
S!
B
Continental
plate
o
A
arc
•
0__•C
ean
2oo
A'
B
Province
i ••
ontinental
•ithosl}here
Gulf Extensional
•(D
lithosphere
_ continental
young
'•
lithosphere
former I •'"• --.--•
oceanic
/-•
subduction
former
zone
arc
i
plate
B'
plate
boundary
i
boundary
zone
zone
Fig.7. Comparison
ofpartitioning
ofrelative
platedisplacement
in oblique
strike-slip
regimes
[Fitch,1972]andin
the caseof the Gulf of California. Note similarityof scale. Thicknessof the lithosphere
beneathBaja California
andadjacent
NorthAmerica
isnotwellconstrained,
andthedirection
ofplatemotion
(heavy
arrows)
isschematic.
transfer
structures
(probably
strike-slip
faults)extending
west
across
theBajaCalifornia
Peninsula
tojointheplateboundary
in thecontinental
borderland.
TheAguaBlancafault,northof
Ensenada,
is of a likely orientation
to be sucha transfer
structure,but it does not correspond
to any structural
discontinuity
in the extensional
provinceto the east. The
localextension,
computed
by reconstruction
of displacements
on high-angle
normalfaults,or inferredfromfault-bedding
relationships,
variesfrom 5% to 50%. This extension
may
havebegunsynchronously
all alongthemargins
of thepresent
continuity
between
theGulfExtensional
Province
andthe
gulfor mayhavecloselyfollowed
thetimeof passage
of the
Riveratriplejunctionandthetransition
to a strike-slip
plate
boundary.Noneof thisextensionoccurredduringactive
southern
BasinandRangeprovince
suggests
thatlateMiocene
Pacific-North
Americadisplacement
(manifested
asextension)
episode
of Miocene
extension
recognized
immediately
adjacent
subduction. This 12-6 Ma ENE extension is the earliest
alsocontinuedfurthernorth. The directionof this extension
to the Gulf of California; its relationshipto early to mid-
neednothavebeenperpendicular
to theplateboundary
to the
west,as longthesumof theextension
andthestrike-slip
Miocene"metamorphic
corecomplexes"
knownfromsouthern
Arizona, and similar undatedstructuresin NW Sonora,is
displacement
equaled
thetotaldisplacement
between
thePacific
andNorthAmericaplates.Thus,lateMiocene
WNW directed
extensionfurther north in the Basin and Range province
[Zoback
et al., 1981]mayhavebeenanexpression
of thesame
change
in direction
of platemotionsuperposed
on earlier
extensional structures.
ENE directed
normalfaultingcontinued
intothePliocene
in
manyareas
around
theGulfofCalifornia;
in some
areas
ofNE
BajaCalifornia
it continues
today[Gastiletal., 1975;Dokka
and Merriam, 1982]. Thus this directionand style of
extensional
faultingpersisted
evenafterPacific-North
America
transform motion was well establishedwithin the gulf,
although
it appears
to havecaused
muchlessextension.
Its
occurrence
todaymaybe a smallerscaleexampleof strain
partitioning
in thePacific-North
America
boundary
zone.
SUMMARY
Sinceabout12 Ma, NNW strikingnormalfaultsadjacentto
the presentGulf of Californiahave causedextensionin a
broadlyENE direction,perpendicular
to the Pacific-North
Americaplateboundary
andto thepresentorientation
of the
Gulf of California. In widely separated
areasthe amountof
unknown.
Plate tectonic constraints are consistent with a plate
kinematic cause for this extension, as follows. When
subduction
ceasedwestof BajaCaliforniaat about12 Ma, by
cessation
of spreading
onpartof thePacific-Guadalupe
ridge,
Pacific-NorthAmerica motionwas accommodated
by strike-
slipmotiononfaultzonesparallelto,andcloseto, theformer
trench(theSanBenito-Tosco-Abreojos
faultzone).However,
at or after about 12-10 Ma, the directionof Pacific-America
motionbecameorientedobliqueto thesestrike-slipfaultzones
andshouldhavecaused
obliqueextension
alongthem.Instead,
the plateboundarydisplacement
was partitionedbetween
strike-slipmotion on NNW strikingfaults west of Baja
California, and broadly ENE directedextensionon NNW
strikingfaultseastof BajaCalifornia,alongthetraceof the
formervolcanicarc. This extensioncouldhavedisplacedBaja
California as much as 100-150 km WSW relative to North
America before about 5.5 Ma. If more extension occurred to
the norththanto the south,Baja Californiawould alsohave
beenrotatedvery slightly counterclockwise.During this
extensionalfaulting, Baja Californiamay also havebeen
displaced
NNW relativetoNorthAmerica
by minorstrike-slip
faultingwithintheextensional
belt,linkedto thepost-8Ma
StockandHodges:Pre-Pliocene
Extension
AroundtheGulf of California
strike-slipmotion along the San Gabriel fault in southern
California. Thus for the period betweenabout 12 Ma and
perhaps3.5 Ma, Baja Californiawas a rigid "microplate"
bounded
on bothsidesby displacement
betweenthePacificand
North America plates. The partitioningof displacement
betweenthesetwo parallelzonesof thePacific-NorthAmerica
plateboundarychangedslowlywith time, until by perhaps
about 3.5 Ma most of the plate boundary motion was
occurringeastof Baja California. The transferof the Baja
Californiapeninsulafrom the North Americaplate to the
Pacificplate wasa gradualprocess,whichtookat leastthe
period12-4Ma, andperhaps
is notentirelycomplete
today.
Existinggeologicconstraints
areconsistent
with,butdo not
prove,this scenario.Clearly,muchbetterconstraints
are
neededon the timing and amountof extensionin the Gulf
ExtensionalProvinceand on its relationshipto areasfurther
northin the BasinandRangeprovince. The implicationsof
thiskinematicmodelfor the dynamicsandscaleof strike-slip
platemarginscanbe testedwith futureacquisition
of geologic
andgeophysical
dataandmaybe usefulfor understanding
plate
marginprocesses
elsewhere.
Acknowledgments.
TanyaAtwater,Christopher
Henry,and
PeterLonsdaleprovidedpreprintsin advanceof publication.
Jeff Severinghaus
providedmagneticanomalyidentifications
in
advanceof publication. GordonGastil, Dick Sedlock,and
Hugh McLean reviewed the manuscript. T. Byrne, B.C.
Burchfiel, W. F. Brace, and T. Grove made helpful
suggestions.
J. M. Stockwassupported
by a FannieandJohn
Hertz FoundationFellowship. Acknowledgmentis made to
the Donorsof the PetroleumResearchFund, administered
by
the American Chemical Society,for partial supportof this
research.
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(ReceivedJuly 11, 1988;
revisedSeptember27, 1988;
acceptedSeptember30, 1988.)