Download New geologic mapping and SHRIMP U

New geologic mapping and SHRIMP U-Pb zircon data in the Peninsular
Ranges batholith, Baja California, Mexico: Evidence for a suture?
Scott E. Johnson*
Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales 2109, Australia
Departamento de Geología, CICESE, Km 107 Carratera Ensenada-Tijuana, Baja California, México
Marcus C. Tate†
Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales 2109, Australia
C. Mark Fanning
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
ABSTRACT
Geologic and SHRIMP U-Pb zircon studies in the Sierra San Pedro Mártir area of northern Baja California, Mexico, suggest that the western and eastern parts of the Peninsular
Ranges batholith originated as separate arcs. They are now juxtaposed along a well-exposed
ductile thrust (Main Mártir thrust) marked by an age discontinuity of at least 10 to 15 m.y., and
sharp changes in rock type, metamorphic grade, and deformation history. The east-dipping
thrust, and adjacent highly deformed rocks, were stitched by 108–97 Ma plutons that were generated in a region of crustal thickening formed during juxtaposition of the two arcs. Lack of
chemical, isotopic, and geologic evidence for continentally derived rocks in the western arc may
preclude an origin by rifting of the continental margin, or development of the arc on a prism of
continentally derived sediments adjacent to the continental margin. Instead, the western arc
may have originated as an island arc above a subduction zone outboard of North America. If so,
a second, concurrent subduction zone along the North American margin must have driven convergence and suturing of the two arcs, and the Main Mártir thrust may mark a nonterminal suture within a wider convergence-related deformation zone that formed ca. 115–108 Ma.
INTRODUCTION AND GEOLOGIC SETTING
The Jurassic to Cretaceous Peninsular Ranges batholith (Fig. 1) contains one of the world’s best-preserved, most completely exposed, and
readily accessed transitions between adjacent island (western) and continental-margin (eastern) arcs. A boundary of variable width between western and eastern parts of the batholith is known from geologic, petrological,
geochemical, and isotopic gradients or discontinuities along its northern
900 km (e.g., Gastil et al., 1975; Todd et al., 1988; Silver and Chappell,
1988; Gastil, 1993). Although interpretations vary, most workers previously
considered the boundary to be a suture between North America and a fringing arc, the two originally separated by a backarc extensional basin (Gastil
et al., 1978, 1981; Rangin, 1978; Phillips, 1993; Busby et al., 1998) or a
basin of unspecified origin (Todd et al., 1988; Griffith and Hoobs, 1993).
Alternatively, Walawender et al. (1991) and Thompson and Girty (1994)
suggested that the batholith may have formed in situ across a poorly understood, pre-Triassic boundary between oceanic and continental crust. The
boundary between the eastern and western parts of the batholith is well
exposed northwest of the Sierra San Pedro Mártir pluton (Fig. 1), where it
is expressed as an east-dipping ductile thrust (Main Mártir thrust) that juxtaposes rocks of different age, type, metamorphic grade, and deformation history. In this paper we present geologic mapping and SHRIMP (sensitive
high-resolution ion microprobe) U-Pb zircon ages1 that provide new con*E-mail: [email protected].
†Present address: Department of Geology, University of Newcastle, Callaghan,
NSW 2308, Australia.
1GSA Data Repository item 9962, SHRIMP U-Pb zircon data, is available
on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301,
[email protected], or www.geosociety.org/pubs/drpint.htm.
straints on the age and origin of this structure (Fig. 1). On the basis of work
in this area, we suggest that the thrust may mark a nonterminal suture
(Dewey, 1977) within a broad deformation zone that formed during collision of the western island arc with North America ca. 115–108 Ma.
NEW MAPPING AND SHRIMP U-Pb ZIRCON AGES
Figure 1 shows the current results of an ongoing mapping, geochemical, and SHRIMP geochronological program in the Sierra San Pedro Mártir
region of Baja California. On the basis of this work we outline the following key observations and relationships.
1. Prebatholithic rocks in the southwest half of the study area (Fig. 1)
form part of the Early Cretaceous Alisitos Formation (Allison, 1974; Gastil
et al., 1975). Volcanogenic rocks of unknown age in the southwest (yellow
in Fig. 1) are dominated by pyroclastic deposits, volcanic flows and
breccias, and volcaniclastic rocks, all of which underwent lower greenschist facies regional metamorphism. Marine volcano-sedimentary basinal
assemblages to the east (pale blue in Fig. 1) contain similar rocks, in addition to reef limestones and calc-silicate and argillitic rocks. Metamorphic
conditions vary from lower greenschist facies in the west to upper greenschist facies or lower amphibolite facies in the east, adjacent to the Main
Mártir thrust. Silver et al. (1963) found Albian fossils in the study area
(Fig. 1), and demonstrated that these volcano-sedimentary rocks are
directly along strike of the late Aptian to early Albian type section at Punta
China, 100 km to the northwest.
2. Prebatholithic rocks in the northeast third of the study area (green in
Fig. 1) are separated from the Alisitos Formation by the Main Mártir thrust,
and include volcanic, volcaniclastic, tuffaceous, calc-silicate, quartzo-feldspathic, and sillimanite-bearing pelitic rocks, all of which have been metamorphosed to at least upper amphibolite facies with locally abundant
migmatite. On the basis of fossils in the Alisitos Formation, and SHRIMP
Data Repository item 9962 contains additional material related to this article.
Geology; August 1999; v. 27; no. 8; p. 743–746; 2 figures.
743
U-Pb zircon ages (Fig. 1), these rocks predate the Alisitos Formation by at
least 10–15 m.y. These older rocks have been intruded by variably deformed
sheets and plutons of Early Cretaceous age (ca. 127–135 Ma), and we suggest that they all formed part of a continental arc on the margin of North
America that was active from at least 140 Ma.
3. A strong eastward-increasing deformation gradient occurs across
the study area. Westernmost volcanogenic rocks of the Alisitos Formation
are gently folded, and regional tectonic foliations are rare. Deformation
intensity increases markedly within 200 m of the contact between the two
major packages of the Alisitos Formation, which is marked as an east-dipping contractional fault in Figure 1. Volcanic rocks on the west side of the
contact show abundant brittle shears with associated folds, whereas marbles
locally present on the east side show annealed mylonitic microstructures.
This faulted contact continues south of the Sierra San Pedro Mártir pluton,
and is apparently continuous with the Rosarito fault described by Schmidt
(1996). From west to east in the volcano-sedimentary rocks (pale blue in
Fig. 1), fold axes rotate progressively from gently plunging to downdip.
Deformation within and east of the Main Mártir thrust was intense and
resulted in widespread mylonites showing consistent east over west shear
sense. Only one phase of regional ductile deformation and folding has been
recognized in the Alisitos Formation, whereas the older rocks east of the
thrust contain folded foliations and lineations, indicating that they had a
more complex deformation history.
4. The two major subdivisions of the Alisitos Formation host different
types of intrusive rocks. In the southwest, recently recognized magmatic
ring complexes contain gabbro, mafic cone sheets, and tonalite that weathers
pink (Johnson et al., 1999; Tate et al., 1999). The tonalite probably formed
in situ by mixing of low-pressure (<2.3 kbar) andesite fractionates and
trondhjemitic partial melts derived from the contact aureole (Tate et al.,
1999). In the volcano-sedimentary rocks (pale blue in Fig. 1), homogeneous
intrusions of hornblende-biotite tonalite that weathers white are associated
with bodies containing variable amounts of gabbro, diorite, and tonalite.
The white-weathered tonalites also intrude the Main Mártir thrust (e.g., the
Cerro de Costilla complex in Fig. 1), as well as metavolcanic, metasedimentary, and metaplutonic rocks east of it. These tonalites are younger than
those associated with the ring complexes to the west, and our preliminary
modeling suggests that they may have formed at source-region pressures of
~10 kbar by mixing of andesite fractionation products with batch melts of
garnet-bearing amphibolites. Although they occurred at deeper crustal
levels, these fractionation and mixing processes closely resembled those
involved with the ring-complex tonalites.
In summary, the relationships described here and shown in Figure 1
indicate that age, lithological, deformational, and metamorphic discontinuities occur across the Main Mártir thrust. This structure corresponds to the
boundary between the western and eastern prebatholithic rocks in this area,
as originally proposed by Gastil et al. (1975). The thrust and adjacent highly
deformed rocks were stitched by a distinctive phase of tonalite plutons and
gabbro-bearing intrusive complexes that have chemical and isotopic affinities with the western part of the batholith.
TECTONIC EVOLUTION
We propose the following history (Fig. 2) for development and stitching of the boundary between the western and eastern parts of the batholith,
as exposed in the study area (Fig. 1).
1. Voluminous Early Cretaceous orthogneisses, sheeted intrusions, and
plutons that intrude volcanogenic rocks east of the Main Mártir thrust suggest that an east-facing subduction zone and related magmatic arc were
active along the continental margin. Our SHRIMP U-Pb zircon data in these
Early Cretaceous intrusive rocks indicate possible inheritance that may be
as old as 180 Ma (see footnote 1), and when combined with ages reported
by Thompson and Girty (1994) they suggest that magmatism in the eastern
arc may date to Late Triassic or Early Jurassic. Our SHRIMP data indicate
that the eastern arc was active until ca. 127 Ma. Although they must be taken
744
as a preliminary data set, these various age determinations may indicate that
the continental-margin arc was active for at least ~60–80 m.y. prior to being
intruded by the mid-Cretaceous stitching plutons.
2. The Alisitos island arc was probably established ca. 140 Ma (Silver
and Chappell, 1988) above an east- or west-facing subduction zone
(Fig. 2A). The volcano-sedimentary part of the Alisitos Formation (pale
blue in Fig. 1) accumulated in a deep- to shallow-marine basin on the eastern flanks of the subaerial Alisitos arc. In the area of Figure 1, no quartz,
granitoid, or metamorphic clasts have been found in these rocks (Silver
et al., 1963). Thus, although it seems probable that the basin received some
arc-derived detritus from the continental margin, there is no published evidence in the study area for continentally derived material.
3. Between ca. 115 and 108 Ma, the basin between the island arc and
continental margin closed (Fig. 2B), and the two arcs were juxtaposed along
the Main Mártir thrust, which shows characteristics of a nonterminal suture
zone (Dewey, 1977). Suturing and the main phase of related deformation
were largely completed by ca. 108 Ma in the study area, as indicated by the
age of the San José pluton, which postdates much or most of the regional
tight folding of the Alisitos Formation. Intrusion of the Main Mártir thrust
by the Cerro de Costilla complex (Fig. 1) and lack of evidence for postintrusion deformation within and around the complex indicate that the thrust
was inactive by ca. 103 Ma.
4. We speculate that crustal thickening during collision placed lower
crustal garnet-bearing amphibolites at appropriate depths for partial melting,
and that these partial melts mixed at depth with andesite fractionates to form
the stitching tonalites, which then ascended and were emplaced at middle to
upper crustal levels. Late in this phase of plutonism, oceanic and continental
sources were both available for magma generation along a linear belt parallel
to, and just east of, the Main Mártir thrust. The combination of these two
sources resulted in the voluminous Sierra San Pedro Mártir pluton and other
La Posta–type plutons that thoroughly stitched the suture over a distance of
~900 km (Walawender et al., 1990; Kimbrough and Gastil, 1997).
5. Considerable displacement along the Main Mártir thrust, combined
with substantial ductile shortening over a wide area, was followed by rapid
denudation east of the thrust. This denudation exposed metamorphic rocks
that equilibrated at pressures of as much as 5.5 kbar (Rothstein, 1997), and
led to abundant plutonic and metamorphic clasts in Cenomanian to middle
Turronian conglomerates on the western margin of the peninsula (e.g.,
Busby et al., 1998; Kimbrough et al., 1998).
DISCUSSION
The tectonic model outlined herein requires the Alisitos arc to initiate
outboard of the North American margin, which is consistent with published chemical and isotopic data from the batholith that show no compelling evidence for a continental component in western arc intrusions
(e.g., Silver and Chappell, 1988; Tate et al., 1999). Our determinations
show that western belt plutons (e.g., San José and Potrero) and ring complexes (e.g., Burro and Zarza) always have 87Sr/ 86Sr I ≤ 0.7040, and ε Nd ≥
+3.7, and no inherited zircon ages have been found in these bodies. The
lack of evidence for a continental component in western arc intrusions may
conflict with previous models that generate the western arc by backarc rifting of the continental margin (e.g., Gastil et al., 1978, 1981; Busby et al.,
1998), because they require that basement of the western arc be partly or
entirely composed of continentally derived rocks.
Although a suture zone was proposed in the southern California part
of the batholith by Todd et al. (1988), there is apparently no equivalent structure to the Main Mártir thrust that separates island-arc volcanic rocks in the
west from continentally derived rocks in the east. In alternative models,
Walawender et al. (1991) and Thompson and Girty (1994) proposed that the
Cretaceous batholith developed in situ over a poorly understood, pre-Triassic boundary between oceanic and continental crust. Walawender et al.
(1991) suggested that a single subduction zone outboard of the continental
margin caused the western arc to develop on oceanic crust, and shallowing
GEOLOGY, August 1999
115o50'W
115 o W
San Diego
Ensenada
?
Main Mártir Thrust
?
?
?
29 o N
ul
fo
rn
Ca
ifo
ia
lifo
rn
ia
Concepción
pluton, 133.6
± 1.9 Ma
?
al
ja
Pacific
Ocean
San José pluton
fC
Ba
Orthogneiss,
133.9 ± 1.5 Ma
108.5 ± 1.2 Ma
?
?
G
Sill or flow,
127.9 ± 1.2 Ma
?
?
Map
area
31o05'N
Orthogneiss,
127.8 ± 1.6 Ma
Albian fossil
localities (Silver
et al., 1963)
33 o N
México
?
Cerro de Costilla
complex, 103.0
± 1.0 Ma
110 o W
USA
?
?
?
?
A'
115o40'W
?
La Paz
?
N
Gabbro
?
?
5 km
0
Mafic cone-sheet
assemblages
?
107.5 ±
1.8 Ma
ex
pl
m
co a
a 6M
os 1.
cin ±
En 10.4
1
Burro complex,
113.6 ± 1.0 Ma
Sierra San Pedro
Mártir pluton, 97
+4/-1 Ma (TIMS)
Zarza complex
Potrero
pluton,
102.5 ±
1.6 Ma
Hornblendebiotite tonalite
Variable gabbro,
diorite and tonalite;
some hypabyssal
30o55'N
Probable magmatic
ring complexes
,
116.2 ± 0.9 Ma
Alisitos Fm.; reef limestones,
volcaniclastic, volcanic, calcsilicate, and argillitic rocks
114.5 ± 0.9 Ma
?
A
?
Alisitos Fm.; volcanic and
volcaniclastic rocks
?
?
Calc-silicate, volcaniclastic
and sedimentary rocks, tuffs,
migmatites,orthogneisses,
sheeted intrusions, mylonites
?
?
?
0
10
K-feldspar-bearing
granitoids
A'
A
?
?
?
?
20
In-situ, low-P
tonalites
(<2.3 kbar)
30
km
Tonalites
sourced at
Higher P
Moho
?
?
Main
Mártir Thrust
Oceanic crust
40
??????
Crestal traces of
selected folds
?
?
Trend lines of bedding in
Alisitos Fm., and bedding or foliation east of
Alisitos Fm.
North American
continental or
transitional crust
Contractional fault
Main Mártir Thrust separating western and eastern
prebatholithic rocks
??????
Figure 1. Preliminary geologic map and accompanying cross section of 1200 km2 in Sierra San Pedro Mártir area, northern Baja California, Mexico. U-Pb zircon ages were determined by using SHRIMP II machine at Research School of Earth Sciences, Australian National
University, Australia. Regional map shows location of
study area and extent of exposed Peninsular Ranges
batholith (colored).Thermal ionization mass spectromE
W
etry (TIMS) age for Sierra San Pedro Mártir pluton is
Gabbros and metaluminous
Type Aptian-Albian Alisitos
from Ortega-Rivera et al. (1997). Moho in cross section
Continental-margin arc
granitoids, some of which fed Fm. deposited here
initiated at least 140 Ma.
projected south from Ichinose et al. (1996).
ring complexes and calderas
North America
~140-110 Ma
A
Figure 2. Possible convergence history between western
island arc and North American continental margin to form
geologic relationships in Figure 1. A: Development of Alisitos
arc, possibly on older arc rocks; final phase of Alisitos
Formation entered marine basin between converging island
and continental arcs. B: Arc collision, development of
suture, waning of deformation, and final emplacement of
plutons that stitch the suture.
Some typical western-belt tonalites occur east
of suture, indicating an eastward dip
?
~115-97 Ma
B
GEOLOGY, August 1999
North America
Sierra San Pedro Mártir
pluton is derived from both
western oceanic and
eastern continental sources
745
of the subduction angle ca. 105 Ma caused an eastward step in the locus of
magmatism resulting in the La Posta–type plutons.
Models of in situ batholith development require that the western arc was
initiated, and remained, adjacent to the continental margin. Although a model
of this type could potentially be applied to the study area (Fig. 1), it would
need to incorporate the following observations. (1) The contact between the
Alisitos Formation and continentally derived rocks is a well-exposed ductile
thrust, and rocks on either side differ in age, origin, metamorphic grade, and
deformation history. (2) Existing SHRIMP U-Pb zircon studies on western arc
plutons show no evidence for inherited ages (this study; Tate et al., 1998;
Premo et al., 1998), and therefore do not support the arc being built on a
wedge of continentally derived sediments. (3) U-Pb zircon ages from various
parts of the batholith suggest that the western and eastern arcs operated separately over specific time intervals, and simultaneously over others, indicating
major shifts in the position, width, and type of active magmatism.
CONCLUSIONS
In the Sierra San Pedro Mártir region of Baja California, the Main
Mártir thrust juxtaposes rocks of different age, type, metamorphic grade,
and deformation history. We suggest that this east-dipping ductile thrust is
a major structure within a broad zone of intense deformation that formed
by collision of the Cretaceous Alisitos island arc with the North American
continental margin ca. 115–108 Ma. The thrust and broad deformation
zone were stitched by mid-Cretaceous plutons that formed in response to
collision-relate crustal thickening. Geological relationships and U-Pb
zircon geochronology suggest that a continental-margin arc was active
prior to, and concurrent with, development of the Alisitos arc. This is consistent with a two-subduction-zone model, in which the Alisitos arc originates above a subduction zone in the Farallon plate, outboard of North
America, and a second, concurrent subduction zone under the continental
margin drives convergence.
ACKNOWLEDGMENTS
This work was supported by a Large Grant (A39700451) and Queen Elizabeth
II Research Fellowship from the Australian Research Council, and grant 4311PT
from the Consejo Nacional de Ciencia y Tecnologia of México (to Johnson). We
thank P. Morgan, S. R. Paterson, K. L. Schmidt, S. E. Shaw, R. H. Vernon, and M. L.
Williams for helpful reviews of the manuscript.
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Manuscript received November 17, 1998
Revised manuscript received April 26, 1999
Manuscript accepted May 5, 1999
Printed in U.S.A.
GEOLOGY, August 1999