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Structure and emplacement history of a multiple-center, cone-sheet–bearing
ring complex: The Zarza Intrusive Complex, Baja California, Mexico
S. E. Johnson* Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales 2109,
Australia, and Departamento de Geología, CICESE, Km 107 Carratera, Ensenada-Tijuana,
Baja California, México
S. R. Paterson Department of Earth Sciences, University of Southern California, California 90089-0740
M. C. Tate
Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales 2109,
Australia
ABSTRACT
The Cretaceous Zarza Intrusive Complex,
located in the Peninsular Ranges of Baja California Norte, Mexico, is perhaps the bestpreserved multiple-center, cone-sheet–bearing
ring complex documented in North America.
The 7 km2 elliptical complex hosts three
nested, non-concentric intrusive centers that
are successively younger to the south. The
northern and central centers show the same
evolutionary sequence of (1) intrusion of concentric gabbroic cone sheets, (2) intrusion of
massive core gabbros, and (3) development of
subvertical, ductile ring faults. Ring-fault kinematics indicate that both centers moved down
relative to the surrounding country rocks, suggesting collapse into an underlying magma
chamber. The southern center is composed of
approximately equal proportions of gabbro
and tonalite and lacks cone sheets. Aluminumin-hornblende barometry on the tonalite indicates a maximum emplacement depth of 2.3 ±
0.6 kbar. The Zarza Intrusive Complex is surrounded by a ductile deformation aureole, and
bedding is inward dipping and inward younging around the entire complex. Excellent
preservation of the intrusive history allowed us
to evaluate the origin of the aureole, and the
three most applicable models are (1) collapse
of the complex into its underlying magma
chamber, (2) sinking of the complex and its
chamber after solidification, and (3) formation
of the aureole prior to emplacement of the
complex. The preserved structural and intrusive relationships provide information on the
dynamic evolution of subvolcanic magma
*E-mail: [email protected].
chambers and suggest that the complex may
have been overlain by a caldera.
INTRODUCTION
Because much of Earth’s continental crust was
formed and/or influenced by magmatic processes
(e.g., Taylor and McLennan, 1985; Hamilton,
1989; Saleeby, 1990; Lipman, 1992; Yanagi and
Yamashita, 1994; Brown and Rushmer, 1997),
the temporal and spatial evolution of magma
plumbing systems remains one of the outstanding problems in our search for a better understanding of how continents grow and evolve.
Particularly interesting and important parts of
these systems are the pathways taken by magma
travelling from shallow magma chambers to volcanoes. Where the surface expression of magmatism is a caldera, the magma pathways commonly manifest themselves as ring complexes,
which contain a wide variety of intrusive phases
including cone sheets, ring dikes, and massive
central intrusions (e.g., Richey, 1948; Smith and
Bailey, 1968; Lipman, 1984). Because of their
well-defined spatial and geologic context, ring
complexes provide an unparalleled opportunity
to evaluate the evolution of subvolcanic magmatic systems and upper-crustal magma-transfer
zones in general.
In this paper, we evaluate the intrusive history
of the shallow (2.3 ± 0.6 kbar) Zarza Intrusive
Complex, which we suggest may be the solidified remains of a magma-transfer zone between a
caldera and its underlying magma chamber. The
Zarza Intrusive Complex is located in the western
Peninsular Ranges batholith of Baja California
Norte, Mexico (Fig. 1), where it intruded calcalkalic volcanogenic rocks of the Alisitos Formation. The complex is relatively small and com-
GSA Bulletin; April 1999; v. 111; no. 4; p. 607–619; 15 figures; 1 table.
607
pletely accessible, and structural patterns are well
developed within the complex and surrounding
country rocks. Thus, we were able to collect a detailed structural data set, and evaluate its intrusive
history and emplacement mechanisms (a detailed
petrological/geochemical study of the complex
can be found in Tate et al., 1999). The complex
consists of three nested intrusive centers, two of
which contain some of the best-preserved cone
sheets in North America. The Zarza Intrusive
Complex differs significantly from most other
cone-sheet–bearing intrusive complexes in that it
is surrounded by an intense, concentric, ductile
deformation aureole in the adjacent country
rocks. This aureole is intriguing, and we focus
particular attention on evaluating its formation.
BACKGROUND AND DEFINITIONS
In this paper we present evidence that the Zarza
Intrusive Complex is a cone-sheet–bearing ring
complex. Because ring complexes are relatively
rarely described in recent literature, we provide
the following definitions of relevant terms used in
this paper. These definitions partially incorporate
definitions and descriptions provided by Billings
(1943), Jacobson et al. (1958), Walker (1975), and
Bates and Jackson (1980).
Ring Dike. A ring dike consists of a discordant intrusive body that can be circular, elliptical,
polygonal, or arcuate in plan and has steeply dipping to subvertical contacts. Widths are variable,
but can reach up to several thousand meters, and
rock types are generally felsic. The first description of a ring dike in relation to a collapsed cauldron was made at Glen Coe by Clough et al.
(1909), but the term “ring-dyke” was first used by
Bailey (1914).
Cone Sheet. A cone sheet is a discordant intru-
JOHNSON ET AL.
sive body that is arcuate in plan and has variably
inward-dipping contacts. Collectively, a swarm of
cone sheets can be circular or elliptical in plan.
Thicknesses are highly variable; mafic sheets seldom reach more than a few tens of meters,
whereas felsic sheets can reach widths greater
than 60 m. Cone sheets were first described in the
Cuillin district of Skye by Harker (1904), who
called them “inclined sheets.” The term “conesheets” was later introduced by Bailey et al.
(1924), reflecting the fact that they are generally
of conical form surrounding intrusive centers.
Ring Complex. A ring complex is a general
term used to describe an intrusive complex that
contains cone sheets and/or ring dikes. Complexes containing only cone sheets or ring dikes
are occasionally called cone-sheet complexes
and ring-dike complexes, respectively. Ring
complexes have long been thought to represent
transitional links between calderas and their underlying magma chambers (e.g., Williams, 1941;
Richey, 1948; Turner, 1963; Smith and Bailey,
1968; Oftedahl, 1978).
Ring Zone. The part of a ring complex that
contains cone sheets and/or ring dikes is termed
the ring zone.
Notable Occurrences of Ring Complexes
Well over 100 ring complexes have been described around the world, but most of them lack
cone sheets and are defined as ring complexes on
the basis of ring dikes and arcuate intrusions. Notable examples of cone-sheet–bearing ring complexes have been previously described in (1) the
British Tertiary intrusive centers of Mull, Ardnamurchan, Skye, southern Arran, and Carlingford
(e.g., Richey, 1932, 1948; Walker, 1975); (2) the
Georgetown Inlier of Queensland, Australia
(Branch, 1966); (3) the Baie-des-Moutons syenitic
complex of Quebec, Canada (Lalonde and Martin,
1983); (4) the Mediterranean island of Corsica
(Bonin, 1986); (5) the Younger Granite province of
northern Nigeria (Jacobson et al., 1958); and
(6) the Canary Islands (Schmincke, 1967).
GEOLOGIC SETTING
The Zarza Intrusive Complex occurs in the
Jurassic to Cretaceous Peninsular Ranges batholith, which extends ~1600 km from Riverside,
California, USA, to the southern tip of the Baja
California peninsula, Mexico (Fig. 1; Todd
et al., 1994). The batholith is divisible into western and eastern belts on the basis of several criteria summarized in Figure 1. The western belt
is thought to represent a relatively static arc constructed on oceanic crust, whereas the younger
eastern belt apparently developed as a continental-margin arc that migrated east with time
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Figure 1. Reconnaissance geology of the Peninsular Ranges batholith in Baja California Norte,
Mexico, between La Calentura and Bahia Camalu. The black rectangle in the lefthand box (see
arrow) shows the location of the main map. The Zarza Intrusive Complex is located in the southcentral part of the main map, which shows Mesozoic plutons and intrusive complexes in the western and eastern belts. Differences between the belts, summarized above the map, are based on information from Gastil et al. (1975, 1990, 1991), Walawender et al. (1990), Gromet and Silver
(1987), Silver and Chappell (1988), and Rothstein (1997). Geology after Gastil et al. (1975) and
this study.
(Gastil et al., 1981; Silver and Chappell, 1988;
Todd et al., 1988; Walawender et al., 1990). A
semicontinuous, well-exposed oblique section
occurs across the batholith in Baja California
Norte (Fig. 1), with shallow-level rocks exposed
in the west and middle-crustal rocks from
depths as great as ~20 km (Rothstein, 1997) exposed in the east. The Zarza Intrusive Complex
is located in the western belt, ~25 km from the
west coast (Fig. 1).
ZARZA INTRUSIVE COMPLEX
The Zarza Intrusive Complex consists of
three discrete intrusive centers (Fig. 2), which
we describe below in terms of rock types,
crosscutting relationships, chronology, barometry, and structural patterns. Figures 2 through
4 show the geology, bedding and foliation data,
and a trend analysis of the bedding and foliation in the complex and surrounding country
rocks. Figures 5 and 6 show block diagrams
along the cross-section lines in Figure 4.
Geological Society of America Bulletin, April 1999
General Description and Rock Types
The northern intrusive center is composed of
variably distinct, concentric, fine- to mediumgrained cone sheets with anorthositic gabbro
compositions (Streckeisen, 1976). The sheets,
which intruded volcanogenic country rocks, vary
in width from ~0.1–10 m, lack chilled margins,
strike subparallel to the margins of the center, and
have an average inward dip of ~65°. Individual
sheets vary markedly in length, and some of the
wider ones are continuous for at least several hundred meters around the center. Two coarsergrained units of modally similar anorthositic gabbro (G1 and G2, Fig. 2) intruded much of the ring
zone. G1 commonly occurs as lenticular intrusions elongated subparallel to the margins of the
center, whereas G2 forms a relatively large body
in which isolated rafts and blocks of ring-zone
rocks locally form more than 50% of the outcrop.
Thus, the ring zone originally occurred throughout much of the currently exposed center, and
stoping was an important material-transfer
ZARZA INTRUSIVE COMPLEX, BAJA CALIFORNIA, MEXICO
assemblages dominated by plagioclase and either two pyroxenes or hornblende; some of the
gabbros of the northern intrusive center also
contain minor olivine. Overall, they also have
similar major-element oxide, trace element, and
rare-earth element patterns that require a somewhat similar parent with high-alumina basalt
characteristics. Cone sheets and hornblende diabase dikes in the northern and central intrusive
centers approximate near-parental material,
which probably required accumulation of the
gabbros from structurally equivalent cone-sheet
magmas in the underlying magma chamber
(Tate et al., 1999).
SHRIMP Geochronology
Figure 2. Geologic map of the Zarza Intrusive Complex. The northern intrusive center is composed of the northern ring zone and gabbros G1 and G2. The central intrusive center is composed
of the central ring zone and gabbro G3. The southern intrusive center is composed of gabbro G4
and tonalite T1.
process during emplacement of G1 and G2. The
ring zone and gabbros G1 and G2 are cut by volumetrically minor radial and concentric dikes of
hornblende diabase, epidotite, tonalite, and aplite.
The central intrusive center largely mimics the
northern center and is nested in its southern half.
The ring zone is composed almost entirely of
cone sheets with anorthositic gabbro compositions, but the origin of some fine-grained rocks
with feldspar phenocrysts is unclear; they are either rapidly cooled sheets or volcanogenic
screens. The ring zone was intruded by coarsegrained anorthositic gabbro (G3; Fig. 2), which
contains rafts and blocks of ring-zone rocks, the
abundance of which drops off markedly from the
ring zone’s margins to its center. Thus, it is unclear whether the entire intrusive center originally
contained cone sheets. Late radial and concentric
dikes as described in the northern center are rare.
The southern intrusive center instead contains
coarse-grained anorthositic gabbro (G4) cut by a
slightly more abundant hornblende-biotite
tonalite (T1). These intrusive rocks are more
hornblende rich than those in the two preceding
centers, and they cut ring-zone rocks that may
represent the southern extent of those in the
northern and/or central centers. G4 contains
abundant rafts and blocks of ring-zone rocks,
which suggests that they previously occupied
much of the area intruded by G4. The ring-zone
rocks are cut by volumetrically minor radial and
concentric dikes as described above for the northern center, but tonalite and aplite dikes are more
abundant.
All of the mafic intrusive rocks in the Zarza
Intrusive Complex have similar modal mineral
Geological Society of America Bulletin, April 1999
Absolute ages were determined by U-Pb (zircon) geochronology of units G3 and T1 in the
central and southern intrusive centers, respectively, and all data were collected with the Australian National University SHRIMP (sensitive
high-resolution ion microprobe) II instrument.
Only nonmagnetic zircons were analyzed, and all
of them showed ubiquitous oscillatory zonation
consistent with a magmatic origin. Figure 7
shows that most of the analyses plot close to concordia and must be dominated by radiogenic Pb.
A weighted mean of all 206Pb/238U ratios determined for G3 yielded an age of ca. 116.2 Ma; excess scatter reflects analyses 7.1 and 22.1 (indicated in Fig. 7), which came from low-U growth
zones that contain cracks and apparently lost radiogenic Pb. Eliminating these two analyses gave
an age of 116.2 ± 0.9 Ma at the 2σ confidence
level. T1 gave a more reliable age of 114.5 ± 0.9
Ma at the 2σ confidence level. Although both
ages overlap slightly at the limits of analytical uncertainty, the data are consistent with the observed crosscutting relationships, which indicate
that T1 is younger than G3. The oldest intrusive
units exposed in the northern intrusive center
were not dated, and so the above ages provide
minimum emplacement estimates for the complex as a whole.
Al-in-Hornblende Barometry
Biotite and K-feldspar crystals are rare in T1
(<2 vol%), as reflected generally by wholerock compositions that contain extremely low
K2O concentrations (<0.6 wt%). Cobaltinitrite
staining revealed K-feldspar as both a groundmass and phenocryst phase in only the most
potassic tonalites, which also contain titanite
and have the most potential for Al-in-hornblende barometry (e.g., Hammarstrom and Zen,
1986; Johnson and Rutherford, 1989). We conducted single-mineral analyses of these samples by using the Cameca SX-50 scanning elec-
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JOHNSON ET AL.
tron microprobe at Macquarie University,
which was calibrated with geologic standards
and operated in wavelength-dispersive mode
with a 20 nA beam current, a 15 kV accelerating voltage, a 2–5 µm spot diameter, and an integrated counting time of 40 s. Amphibole rims
adjacent to the buffer assemblage are tremolitic
hornblendes (mostly <7.5 Si and >1.6 Ca pfu
[per formula unit], respectively; Mg# = (Mg/Mg
+ Fe), ~53) that provide a temperature of 705 °C
after consideration of the intermediate (An26)
plagioclase compositions (Blundy and Holland,
1990). In conjunction with the general absence
of saussuritization and other manifestations of
deuteric alteration throughout T1, this temperature value suggests that the critical mineral assemblage equilibrated entirely above a wet
solidus (Anderson and Smith, 1995). Depending on the calibration curve employed, pressure
estimates range widely between 1.7 and
2.9 kbar and overlap with the lower boundary
applicable for the technique (Johnson and
Rutherford, 1989). Thus, we estimate a maximum emplacement depth of 2.3 ± 0.6 kbar for
the Zarza Intrusive Complex, which accounts
for the low total K-feldspar content and assumes that the experiments of Schmidt (1992)
most reliably reproduce the intratelluric oxidation state of T1. Although there is current debate regarding temperature controls on element-partitioning behavior in amphiboles,
suitable temperature corrections did not reduce
our pressure estimates beyond the analytical
uncertainties involved (Ague and Brandon,
1996). The shallow emplacement that we infer
is consistent with (1) the rare presence of miarolitic cavities and edenitic biotite compositions in T1 and (2) the ring-complex characteristics of the Zarza Intrusive Complex in
general.
Structures, Microstructures, and Kinematics
Most intrusive rocks in the Zarza Intrusive
Complex contain only one foliation, which is
magmatic (hypersolidus to near-solidus conditions) and characterized by alignment of igneous
plagioclase and/or hornblende crystals. The cone
sheets commonly contain variably developed
magmatic foliations oriented subparallel to sheet
boundaries, and mineral elongation lineations that
plunge approximately downdip. Microstructural
analysis of the sheets showed only rare evidence
for minor solid-state deformation (on the basis of
the criteria of Paterson et al., 1989a), and we have
found no field evidence of boudinaged sheets. G1,
G2, and G3 generally contain moderate to strong
magmatic foliations, but in G3, the foliation progressively weakens toward its central “bull’s-eye,”
where it forms a basinal shape (Figs. 5 and 6). T1
610
Figure 3. Foliation and bedding attitudes in the Zarza Intrusive Complex. See Figure 4 for
composite map of foliation and bedding trends and geology.
and G4 generally contain weak magmatic foliations, and lineations are either absent or very difficult to recognize in all of the larger intrusive
bodies. This observation may be due partly to a
lack of appropriate foliation-parallel exposures,
but in general we infer that these bodies do not
contain a strong linear fabric.
Country-rock screens that lie between sheets
in the northern center commonly contain a welldeveloped solid-state foliation that strikes parallel to, but locally dips less steeply inward than,
the magmatic foliations in adjacent sheets. They
also commonly contain a well-developed mineral-elongation lineation that generally plunges
approximately downdip. Many of these screens
contain microstructures and mineral assemblages
of either the hornblende hornfels or pyroxene
hornfels facies and were locally intruded by narrow felsic and quartzo-feldspathic veins.
Geological Society of America Bulletin, April 1999
Remarkably, foliations (Fig. 4) and lithologic
contacts (Fig. 2) in the northern and central intrusive centers are abruptly truncated by the central
and southern centers, respectively. Neither the
contacts nor the foliations in the older centers are
deflected as younger intrusive contacts are approached, and they both locally strike into these
contacts at high angles (e.g., Figs. 3 and 4). Furthermore, no penetrative solid-state fabrics were
observed near these margins, which indicates that
at least the two younger intrusive centers were
emplaced with little or no lateral expansion or
shearing of preexisting markers. These observations imply that older centers were relatively
solid before intrusion of each younger center.
Spectacular kinematic indicators are present
in the northern and central intrusive centers and
consist primarily of subvertical, discrete, ductile shear zones that are heterogeneously
ZARZA INTRUSIVE COMPLEX, BAJA CALIFORNIA, MEXICO
Structures
Figure 4. Composite map of foliation and bedding trends, folds, and geology in the Zarza Intrusive Complex. Block-diagram cross sections along the lines A–B and C–D are shown in Figures 5 and 6, respectively.
spaced on the centimeter to decimeter scale
(Fig. 8). Locally, in zones of relatively high
strain, asymmetrical indicators are surrounded
by a pervasively developed, subvertical foliation. These subvertical shear zones and locally
pervasive foliations cut across the cone sheets
and country-rock screens and are concentrated
near the outer margins of each center in what
we refer to as kinematic zones (Figs. 2, 5, and
6). The sense of shear in these zones always indicates that the area they enclose moved down
relative to the area outside (Fig. 8), and so we
interpret them as ductile equivalents of ring
faults. Variably abundant felsic and quartzofeldspathic intrusive rocks are localized in
these zones, which suggests that they may be
incipient ring dikes.
ADJACENT COUNTRY ROCKS
Rock Types
The Zarza Intrusive Complex lies entirely
within the Cretaceous Alisitos Formation, which
is thought to represent calc-alkalic volcanogenic
rocks deposited mainly in a shallow-marine environment (Gastil, 1983; Beggs, 1984). Country
rocks surrounding the complex are dominantly
lithic and crystal-lithic tuffs; basalt, andesite, and
dacite flows; and minor siltstones and sandstones. Bedding is generally defined by contacts
between different units and by variably developed alignment of clasts. Locally present sedimentary structures consistently indicated upward
younging.
Geological Society of America Bulletin, April 1999
Regional structures near the Zarza Intrusive
Complex are largely undocumented outside of
the map area shown in Figures 2–4, but the following generalizations can be made on the basis
of our reconnaissance mapping: (1) bedding regionally dips moderately to the southwest or
northeast, (2) upright, macroscale folds have
been observed with axial-plane foliations developed locally in their hinges, and (3) no major
faults have been observed in the vicinity of the
complex.
Approaching the complex, bedding is progressively deflected and folded into steep inward-dipping attitudes around the entire complex (Figs. 3–6). On the west side, bedding is
folded over into an anticline to attain the steep
inward dips (Figs. 4 and 5). This fold is discontinuous and upright to moderately south plunging; it becomes progressively tighter and overturned to the south. It is not clear whether the
fold resulted entirely from emplacement of the
complex or from both emplacement-related and
synemplacement regional deformation. Two
other folds are present to the north and east of the
complex. The northern fold is only locally developed, whereas the eastern fold is regionally
extensive, continuing off the eastern edge of the
map (Fig. 4). Bedding attitudes southeast of the
complex indicate a possible syncline, but alternatively could indicate a vertical fan of bedding
(Fig. 6); the structure is complicated owing to
the unmapped intrusive body with a steeply outward–dipping contact in the southwest corner of
Figures 2–4.
In the country rock, a bedding-parallel foliation
and downdip mineral-elongation lineation become well developed within a few hundred meters of the complex and intensify toward its margins to define a deformation aureole. Although we
recognize three intrusive centers in the complex,
there is only one aureole, which is spatially associated with the first and largest intrusive center.
Kinematic indicators in this aureole were only locally observed and appear to vary from rare, discrete, steeply inward–dipping cleavage seams that
crosscut bedding near the outer edge of the aureole, to asymmetrical clasts and zones of heterogeneously partitioned shear within the beddingparallel foliation of the inner aureole. All such
structures indicated downward movement of the
inner aureole relative to the outer aureole.
Strain Analysis
Ideally, we would like to quantify deformation
in the aureole caused by both rigid rotation and
ductile flow of units, which can be done by determining the shortening of bedding in roofs above
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JOHNSON ET AL.
chambers or the rigid rotation and internal strain at
many individual locations along a single bedding
horizon throughout the aureole (Schwerdtner,
1995). However, neither of these approaches can
be applied to the Zarza Intrusive Complex because
none of the roof is preserved and no appropriate
bedding horizons are traceable throughout the aureole. We have previously noted that all bedding
near the complex was rotated from regional orientations into consistently steep inward dips, which
required that material was transported out of the
exposed map plane (Figs. 5 and 6). For this reason,
an exact value of emplacement-related host-rock
strain can never be determined, but some useful information about strain can be obtained by evaluating (1) the shapes and orientations of ellipsoids
and (2) the amount of bulk shortening along transects perpendicular to the margins of the complex
(e.g., Bateman, 1984; Paterson and Fowler, 1993).
Lithic tuffs around the complex provide good
samples for strain analysis. Fine-grained siltstones and volcanic rocks are locally present between the sampled tuffs and are generally more
highly strained, indicating that deformation was
preferentially partitioned into them. Thus, our
analysis probably underestimates the amount of
shortening. Five suitable samples were collected
along a southern transect, three from a western
transect, and an additional sample from near the
southwest margin of the complex (Fig. 2). The
two transects were chosen because they lie in the
regional “strain shadow” and are nearly perpendicular to the regional strike of folded bedding, respectively (Fig. 4). Three mutually perpendicular
cuts were made through each sample, and the cut
surfaces were sprayed with acrylic and labeled
with a right-handed coordinate system. Axial ratios and orientations of 35 or more lithic clasts
were measured in each face. The magnitude of
cleavage deflection near clasts and the percentage
of clasts versus matrix were used to estimate viscosity contrasts (usually 1, for no viscosity contrast) between clasts and matrix (Paterson et al.,
1989b).
Shimamoto and Ikeda (1976) and Wheeler
(1986) noted that any population of elliptical
markers with different ratios and orientations
can be represented collectively by determining
a single average ellipsoid. If this population is
then deformed, the average ellipsoid defined by
the deformed objects will reflect strain plus any
original fabric (Dunnet and Siddans, 1971;
Seymour and Boulter, 1979). We have used the
techniques of Shimamoto and Ikeda (1976),
Miller and Oertel (1979), and Wheeler (1986)
to calculate final average ellipsoids for the deformed clasts, which were then corrected for
the presence of primary fabrics by using the
procedure outlined in Paterson and Yu (1994).
These corrected ellipsoids (Table 1) approxi-
612
Figure 5. Block-diagram cross section along the line A–B in Figure 4. Displacements along
kinematic zones stylized. See Figure 2 for geology legend. Vertical and horizontal scales are equal.
Figure 6. Block-diagram cross section along the line C–D in Fig. 4. Displacements along kinematic zones stylized. See Figure 2 for geology legend. Vertical and horizontal scales are equal.
mate the tectonic strain at each locality with uncertainties in the range of 10% to 20% strain at
the 95% confidence level. Strain intensities of
samples within the aureole range from moderate values (1.52) near the complex to low “regional” values near the outer margin of the aureole, and Lode’s parameters indicate that all
but one of the samples yield oblate strain ellipsoids (Table 1, Fig. 9).
Two of the samples (BC 220 and BC 221)
came from immediately outside of the deformation aureole and have very low strains with x-y
planes of the strain ellipsoid subparallel to moderately dipping bedding. Thus, we suggest that
they reflect strain caused only by primary compaction plus regional deformation. Another sample (BC 413), which came from the fold hinge to
the west of the Zarza Intrusive Complex, has a
prolate strain ellipsoid with the x-axis of strain at
a low angle to the fold axis. Thus, we suggest that
Geological Society of America Bulletin, April 1999
this sample reflects heterogeneous strain in the
fold hinge. All other samples came from within
the deformation aureole. Figure 10 shows an
equal-area plot of the x-y planes and x-axes of
strain for the oriented samples. The average
Zarza Intrusive Complex margin orientations and
x-y planes of strain correspond well with one another, and so it is particularly useful to examine
shortening along the z-axis (shortening approximately perpendicular to the complex margin).
These values range from 71% near the complex
contact to 18% at the margin of the aureole and
12% outside of the aureole (Table 1 and Fig. 11).
From Figure 11 we calculated bulk shortening
in the aureole perpendicular to the Zarza Intrusive Complex margin by integrating the area under the best-fit curve to all the strain data (the two
transects having essentially identical shortening
gradients). We defined this outer margin approximately at sample BC 220, corresponding to a re-
ZARZA INTRUSIVE COMPLEX, BAJA CALIFORNIA, MEXICO
Figure 7. Tera and Wasserburg (1972) U-Pb (zircon) concordia curves for gabbro G3 and
tonalite T1 analyzed by SHRIMP. MSWD—mean square of weighted deviates.
gional shortening strain of 18%. The bulk shortening obtained depends on how we treat this regional strain, and so two integrations were made:
one in which the regional shortening strain of
18% was included in the integration and one in
which the 18% value was used as the base of the
curve. These integrations resulted in bulk shortening strains of 59% and 38%, respectively, and
we make use of this information in a later section.
PROPOSED INTRUSIVE HISTORY FOR
THE ZARZA INTRUSIVE COMPLEX
This study has revealed a six-stage sequential
intrusive history of the Zarza Intrusive Complex
that involves three distinct, south-migrating episodes (Fig. 12). The cross-sectional view shown
is the same as that in Figure 6, which is reproduced by stage 6 of the sequence. We evaluate the
surrounding deformation aureole in the following
section.
Stage 1. The northern intrusive center (Fig. 2)
was initiated by emplacement of cone sheets derived from an underlying magma chamber. On
the basis of a linear subsurface projection of the
northern-center cone sheets, the underlying
chamber was probably situated no more than
2–3 km below the present level of exposure. At
some point in the development of this chamber, it
presumably became overpressured and began to
dome or expand upward, creating the conditions
required to fracture the overlying country rocks
and intrude the cone sheets (e.g., Anderson,
1936; Roberts, 1970; Phillips, 1974).
Stage 2. After the cone sheets solidified, the
ring zone was intruded and partially destroyed by
gabbros G1 and G2. Rafts and blocks of ring-zone
rocks are common in the gabbros, indicating that
stoping was an important emplacement process.
Formation of the northern-center kinematic zone,
which appears to be a ductile ring fault, resulted
from collapse of the enclosed area into the underlying magma chamber. Although we have no
clear evidence for the relative timing between
displacement along this zone and intrusion of G1
and G2, we suggest that collapse resulted from
deflation of the chamber during intrusion of the
cone sheets and gabbros G1 and G2.
Stage 3. As with the northern center, the central
center was initiated by intrusion of cone sheets,
which formed a second ring zone contained
within the southern half of the northern center.
The diameter of the central center is smaller than
the northern, but the cone sheets have approximately the same range of dips (Fig. 3). Conesheet dips are apparently related to the depth of
the magma chamber below the exposure level, the
shape of the chamber, and the position on the
chamber margin from which the sheets were derived (e.g., Anderson, 1936; Phillips, 1974).
Given the smaller diameter of the central center
and the fact that the dips of the cone sheets are
Geological Society of America Bulletin, April 1999
similar to those of the northern center, we suggest
that the central-center chamber (or some portion
of it) rose closer to the surface to 1–1.5 km below
the present level of exposure (Figs. 5, 6, and 12).
The gabbros and cone sheets of the northern center are cut sharply by the cone sheets of the central
center, which suggests that the entire northern
center was fully crystallized when the central-center sheets were emplaced.
Stage 4. After the cone sheets solidified, they
were intruded by gabbro G3, and the central
center collapsed along a kinematic zone similar
to the one that surrounds the northern center.
Rafts and blocks of ring-zone rocks in G3 indicate that, again, stoping was an important emplacement process. As in the northern center, we
have no clear evidence for the relative timing
between the formation of the kinematic zone
and G3 intrusion, but the basinal form of magmatic foliations in G3 may indicate backflow of
magma, possibly in conjunction with displacement along the zone. In this situation, formation
of the kinematic zone would postdate the initial
intrusion of G3, but predate its complete crystallization, consistent with collapse in response to
deflation of the magma chamber during intrusion of the cone sheets and G3.
Stage 5. At this stage, the sequence of intrusion of cone sheets followed by massive gabbro
appears to have changed, and initiation of the
southern center was marked by intrusion of gabbro G4. Abundant rafts and blocks of ring-zone
rocks in G4 indicate that stoping remained an important emplacement process. On the basis of geologic and structural patterns in Figures 2–4, we
interpret the preserved ring-zone rocks in the
southern center to be remnants of the northern
and/or central centers. As with the central center,
the southern center migrated to the south, but is
contained within both previous centers.
Stage 6. The intrusive history was completed
(except for emplacement of minor radial and concentric dikes) by intrusion of T1, which cuts G4 at
rare field localities. The central-center cone sheets
and G3 are cut sharply by T1, which suggests that
the entire central center was fully crystallized
when T1 was emplaced. This interpretation is consistent with the SHRIMP geochronology discussed previously.
ORIGIN OF THE HOST-ROCK
DEFORMATION AUREOLE
The most puzzling structural feature of the
Zarza Intrusive Complex is the intense ductile deformation aureole in the adjacent country rocks.
In this section, we present and evaluate five models of events that might have contributed to this
aureole in the context of the intrusive history discussed above. Cone sheets of the northern and
613
JOHNSON ET AL.
central centers show only rare evidence for solidstate deformation, and the ring zone around the
outer margin of the complex is remarkably continuous. These observations indicate that even
moderate lateral expansion of the complex during emplacement of internal gabbro units can be
ruled out. Thus, we do not consider the popular
models of chamber expansion (“ballooning”—
Sylvester et al., 1978; Holder, 1981; Bateman,
1985) or diapirism (e.g., Cruden, 1990; Weinberg
and Podladchikov, 1994) as mechanisms for aureole development.
1. Accumulated Shortening during ConeSheet Emplacement. Cone sheets of the northern
and central intrusive centers appear to have originally occupied a large area of the Zarza Intrusive
Complex, prior to being intruded by later gabbros
and tonalite. Collectively, the sheets displaced a
considerable area of preexisting rock, and any lateral component of this displacement could potentially have contributed to the deformation aureole.
However, if progressive cone-sheet emplacement
had caused lateral expansion of the complex, we
would expect to see clear evidence for solid-state
deformation in at least some of the northerncenter sheets, apart from that associated with the
kinematic zone. Microstructural analysis of the
sheets showed only rare evidence for minor solidstate deformation, and we have found no examples of boudinaged sheets. Furthermore, emplacement of the central-center sheets had no
recognizable deformational effect on adjacent
rocks of the northern center. For these reasons, we
suggest that any lateral deformation caused by
cone-sheet emplacement was accommodated
mainly by country-rock screens between the
sheets. These screens were metamorphosed under
conditions of hornblende hornfels to pyroxene
hornfels facies and thus reached temperatures at
which they may have partially melted and could
be relatively easily deformed. The downdip lineations in the screens suggest a large vertical component of material transfer during their deformation, and we also speculate that vertical
displacements may have occurred during opening
of the brittle fractures filled by cone sheets (e.g.,
Le Bas, 1971; Walker, 1975).
2. Synemplacement and Postemplacement
Regional Shortening. Northeast-southwest
shortening in the Peninsular Ranges batholith resulted in a regionally consistent pattern of northwest-trending bedding and folds and locally developed axial-plane foliations. If regional strain
occurred in the aureole, it should have had two
effects: (1) increased shortening gradients along
the western transect compared to the southern
transect, because the latter is in the regional strain
shadow, and (2) prolate strain ellipsoids in the
southern transect because regional and emplacement-related shortening would have occurred at
614
Figure 8. Example of subvertical, discrete, ductile shear zones found in the kinematic zones of
the northern and central intrusive centers. Sense of shear in these zones consistently indicates
downward displacement of the Zarza Intrusive Complex relative to surrounding country rocks.
In this example, the shear zones and sheared rocks have been disrupted and intruded by
quartzo-feldspathic melts.
high angles to one another (e.g., Guglielmo,
1993, 1994). Figure 11 indicates that the two
transects have essentially identical shortening
gradients within the uncertainties of the method,
and the only data yielding a prolate strain ellipsoid were from sample BC 413 from the western
transect (Table 1 and Fig. 9). Thus, we conclude
that the contribution of regional deformation to
the aureole was effectively negligible.
3. Collapse of the Zarza Intrusive Complex
into Its Underlying Chamber. Kinematic zones
within the northern and central intrusive centers
indicate collapse of their contained areas into the
underlying magma chamber. We could not quantify displacements in these zones, but we speculate that they may have accommodated several
hundred meters of collapse. Space for this collapse was likely achieved largely by removal of
magma from the chamber to form the cone sheets
Geological Society of America Bulletin, April 1999
and larger gabbros. During collapse, the entire
complex may possibly have moved downward,
the kinematic zones accommodating only part of
the total displacement. Figure 13 shows collapse
of the complex into its underlying chamber for
two initial configurations: emplacement of the
magmas (1) into host rocks with originally flatlying bedding or (2) into the western limb of the
regional anticline whose axial plane is to the east
(Figs. 4–6). Partitioned strains across the northern ring zone during collapse could also explain
the solid-state deformation and inward dips of the
country-rock screens.
4. Sinking of the Zarza Intrusive Complex
and Its Chamber after Solidification. Glazner
(1994) and Glazner and Miller (1997) noted that
many plutons of intermediate and mafic composition reach a level in the crust that, after they
crystallize, is at or above their level of neutral
ZARZA INTRUSIVE COMPLEX, BAJA CALIFORNIA, MEXICO
TABLE 1. STRAIN DATA FROM THE ZARZA INTRUSIVE COMPLEX (ZIC)
Sample
number
BC 472
BC 209
BC 208
BC 506A
BC 207
BC 220
BC 387
BC 413
BC 221
Axial ratios
Elongations
x
y
z
6.48
7.12
3.56
3.45
2.19
1.46
2.33
1.48
1.27
6.33
3.53
2.96
2.63
1.90
1.26
2.00
1.11
1.17
1
1
1
1
1
1
1
1
1
x
87.9
143.1
62.4
65.4
36.2
19.2
39.5
25.4
11.3
y
83.6
20.5
35.0
26.1
18.1
2.8
19.7
–5.9
2.5
Ellipsoids
z
–71.0
–65.9
–54.4
–52.1
–37.8
–18.4
–40.1
–15.3
–12.4
SI
1.52
1.41
0.97
0.92
0.59
0.27
0.64
0.29
0.17
LP
0.98
0.29
0.71
0.56
0.64
0.22
0.64
–0.47
0.31
Orientations
x-y plane
(strike/dip)
318/66
235/63
235/58
15/69
212/54
————332/54
27/13
————-
Distance
from ZIC
x axis
(trend/plunge)
————336/63
240/8
113/69
321/52
————91/50
136/12
————-
(m)
55
75
190
220
330
495
680
720
785
Note: Samples ordered by distance from the complex (locations in Fig. 2). SI = strain intensity, defined by Hossack (1968) as Es = 1/3 [(e1 – e2)2
+ (e2 – e3)2 + (e3 – e1)2]1/2, where e1, e2, and e3 are the principal natural strains. LP = Lodes parameter.
buoyancy. In the process of crystallization, these
plutons generally increase their density by as
much as 10%, which causes an important density
contrast between the plutons and surrounding
host rocks. These authors have argued that, given
country-rock temperatures sufficient for ductile
deformation, such a density contrast may cause
the solidified plutons to sink some finite distance
through the crust after their initial emplacement
to a higher level. Because the bulk composition
of country rocks near the complex approximates
andesite, substantial sinking of solidified intrusive rocks would effectively require them to be
gabbros. Currently exposed intrusive rocks in the
complex are 90% gabbro, and we also infer a
mafic composition for the underlying chamber
(Tate et al., 1999); thus, this model can be effectively applied to the complex (Fig. 14). Rigorous
Figure 9. Strain intensity and Lode’s
parameter plotted against distance from the
Zarza Intrusive Complex (ZIC) margin.
Strain intensity increases toward the margin,
apparently accompanied by a weak increase
in Lode’s parameter. The one sample (BC 413)
with a negative Lode’s parameter comes from
the hinge of the anticline that bounds the west
side of the complex (Fig. 4).
application of the model is complicated by poor
constraints on several parameters that are important for the required calculations, including (1)
the regional thermal gradient; (2) the lithological
and density stratification of the crust adjacent to
the complex and with increasing depth; and (3)
the size, shape, and composition of the underlying magma chamber. Nevertheless, by using
Figure 10. Equal-area plot of x-y planes and
x-axes of strain for the oriented samples shown
in Table 1 and located in Figure 2 (with BC
prefixes). The x-y planes (solid great circles)
and the average orientations of the Zarza Intrusive Complex margin (large dashes) are
plotted as different line widths corresponding
to each transect (see Fig. 2): BC 207–209 come
from the southern transect, BC 387 and 506A
come from the western transect, and BC 472
comes from the southwestern margin of the
complex. BC 472 gave a Lode’s parameter of
0.98 (very oblate strain ellipsoid). Thus, the xaxis orientation is unconstrained within the
uncertainties of the method and is not plotted.
Sample BC 413 came from the hinge of the anticline that bounds the west side of the complex
(Fig. 4), as reflected by the orientation and
prolate shape (Table 1) of the strain ellipsoid.
Geological Society of America Bulletin, April 1999
ranges of what we considered to be acceptable
possible values for these and other parameters
and the methodology described in Glazner
(1994), we calculated strain rates varying over
four orders of magnitude from 10–11 to 10–14 s–1,
which corresponded to sinking rates of 680
km/m.y. to 680 m/m.y. Strain rates on the order of
10–11 to 10–13 s–1 lead to acceptable time scales
for development of the deformation aureole.
5. Formation of the Aureole prior to Emplacement of the Zarza Intrusive Complex.
Development of the complex involved multiple
stages of collapse along subvertical kinematic
zones. We suggest that the deformation aureole
may have formed by a similar collapse episode
prior to emplacement of what we recognize as
the first intrusive rocks in the complex—the cone
sheets of the northern intrusive center. In Figure
15, A and B, collapse occurs partly along subvertical ring faults and partly by “downsagging”
(e.g., Walker, 1984). The overall collapse process
results in distributed ductile shear zones at depth,
within which the country rocks are deflected
downward into a funnel shape. The required geometrical relationships can be produced if the
complex is then emplaced over this preexisting
structure (Fig. 15C).
Discussion of the Aureole
Although the processes described in all five of
these models may have contributed to the deformation aureole around the Zarza Intrusive Complex, we suggest that models 1 and 2 played minor or negligible roles. Models 3–5 are all similar
in that they require downward sinking or collapse
of material inside the aureole (Figs. 13–15). We
favor this general process for aureole formation,
but we are unable to suggest, on the evidence at
hand, which of these three models is most appropriate. We prefer to consider them as three end
members that are not mutually exclusive; they
could have acted together, or they could have
acted separately at different stages of the intrusive complex’s history. If the complex did actu-
615
JOHNSON ET AL.
ally collapse into, or sink with, its underlying
chamber (models 3 and 4), it is possible that
cone-sheet dips increased during downward displacement, effectively decreasing the dihedral
angle of the cone, which might help explain the
highly strained country-rock screens. However, if
this process occurred, it remains puzzling to us
why the outer cone sheets do not show more evidence for solid-state deformation.
We previously calculated bulk shortening
strains of 59% and 38% in the aureole, depending
on how the background regional strain was
treated. In the situations presented by models 3
and 4, if we assume an approximately symmetrical aureole (supported by field observations and
Figs. 4 and 11) and symmetrical displacement of
the complex (supported by the orientation data in
Fig. 3), this bulk shortening can be used to constrain the amount of relative downward displacement. If we assume that the intrusive complex is
a rigid cone with a specific dihedral angle, we can
calculate how far this cone would have to sink to
shorten the surrounding country rocks by the required amount. Cone sheets in the northern intrusive center have an average dip of ~65°, and so if
a cone with a dihedral angle of 50° is considered,
the country rocks would shorten by 47 m for
every 100 m of sinking. Bulk shortening strains
of 59% and 38% would correspond to lateral
shortening of country rocks in the aureole by 710
and 300 m, respectively. These values could be
entirely accounted for by sinking of the complex,
either into or with its underlying chamber, ~1.5
km and 640 m, respectively.
Figure 11. Shortening strain plotted against distance from the Zarza Intrusive Complex (ZIC)
margin. We consider the strain value for sample BC 387 to be anomalous, occurring in a highstrain zone just off the hinge of the anticline that bounds the west side of the complex (Fig. 4).
Figure 12. Sequential intrusive history for the Zarza Intrusive Complex. See text for discussion.
616
Geological Society of America Bulletin, April 1999
ZARZA INTRUSIVE COMPLEX, BAJA CALIFORNIA, MEXICO
In the situation presented by model 5, if we assume that the aureole is essentially a vertical shear
zone, we can also use the increasing axial ratios,
or the change in foliation and bedding orientations, toward the complex to calculate the shear
strain (γ). Equations summarized by Ramsay and
Huber (1983, 1987) relate the amount of displacement in a simple-shear zone to shear strain and to
the orientations and axial ratios of strain ellipsoids. This approach is more difficult to constrain
than the one applied to models 3 and 4 because
the original orientation of bedding has an important effect on the total required displacement. By
using the initial bedding orientations shown in
Figure 13, A and B, and an average aureole width
of 500 m, we calculated total displacements of
550 m and 2.5 km, respectively.
Both of these approaches overlap the 1.5 km to
650 m displacement range, and our preferred interpretation is that any such displacements probably amounted to no more than 1.5 km.
DISCUSSION
Figure 13. Model that requires collapse of the Zarza Intrusive Complex (ZIC) into its partially evacuated underlying chamber to explain the deformation aureole and inward-dipping
and inward-younging bedding around the complex. There are two possible initial configurations: emplacement of the complex into (A) flat-lying bedding or (B) dipping bedding in the
western limb of the regional anticline to the east of the complex. (C) The required final configuration. Sequence A followed by C would require synemplacement to postemplacement regional
deformation to form the anticlines on the western and eastern sides of the complex, whereas sequence B to C adequately produces the anticlines during collapse.
Multiple Processes for Host-Rock
Material Transfer During Emplacement
of the Zarza Intrusive Complex
The Zarza Intrusive Complex preserves evidence for the following five processes of hostrock material transfer during its emplacement:
(1) downward transport of country rocks in the
deformation aureole relative to those outside
the aureole, (2) stoping during ascent and emplacement of the four massive gabbros (G1 to
G4), (3) collapse along the kinematic zones in
the northern and central intrusive centers,
(4) country-rock partial melting directly adjacent to the intrusive complex and in the country-rock screens, and (5) deformation of
country-rock screens caused by cone-sheet
emplacement. We are impressed by the magnitude of stoping by the Zarza massive gabbros,
and we can imagine that if they had occupied
more area at the present level of exposure, they
could have completely removed evidence for
some or all of the other material-transfer
processes. Even though we recognize multiple
processes, the vast majority of material transfer
during emplacement of the Zarza Intrusive
Complex was vertical; the upward movement
of magma was accompanied by downward
movement of country rocks and parts of the
complex.
Implications for Subvolcanic
Magma-Chamber Dynamics
Figure 14. Model that requires sinking of the Zarza Intrusive Complex (ZIC) and its underlying chamber to explain the deformation aureole and inward-dipping and inward-younging
bedding around the complex. See Figure 13 caption for further explanation.
Geological Society of America Bulletin, April 1999
The intrusion and collapse cycles we have
documented in the Zarza Intrusive Complex pre-
617
JOHNSON ET AL.
serve compelling evidence for how pulses of
magma escape from some high-level chambers
and how pathways toward the surface are created. The magma chamber under the complex
became overpressured and, at some stage, exerted enough upward force to fracture and disrupt the overlying crust. These fractures were
filled with magmas that crystallized to form the
cone sheets; we do not know how vertically extensive these sheets were or whether they provided pathways for magma transport to the surface. The disruption of the overlying crust
relaxed an important energy barrier to voluminous magma transport from the chamber. The resulting network of fractures and sheet contacts
provided a vast array of potential magma pathways (e.g., Walker, 1986), and the massive core
gabbros used them to stope their way upward
through the cores of the ring zones and thus form
central conduits that may have supplied mafic
volcanic eruptions at the surface. Although we
have no conclusive proof that the complex was
overlain by a caldera, its ring-complex characteristics, evidence for calderas in the western
Peninsular Ranges batholith (e.g., Gastil, 1990;
Delgado-Argote et al., 1995; Fackler-Adams,
1997), and the common occurrence of mafic and
bimodal calderas around the world (Walker,
1993), lead us to favor this interpretation.
SUMMARY AND CONCLUSIONS
1. The Zarza Intrusive Complex preserves
structural and intrusive relationships that suggest
it may have been overlain by a caldera.
2. Cone sheets preserve evidence for a dense
population of fractures that disrupted the crustal
integrity above the complex’s magma chamber.
Massive core gabbros stoped their way through
this disrupted zone and formed a pathway for
continued magma ascent.
3. The presence of both sheets and massive intrusive bodies and the evidence for both brittle and
ductile host-rock deformation illustrate that multiple ascent and emplacement processes can act
together to facilitate subvolcanic magma transfer.
4. Nested intrusive centers in the Zarza complex and in other ring complexes indicate that
pathways are commonly reused, even though they
may completely solidify between intrusive pulses.
5. The Zarza Intrusive Complex provides a rare
example in which we can demonstrate that (1) the
intense ductile deformation aureole was not
formed by diapirism or lateral expansion of the
massive intrusive rocks and (2) host-rock displacements were mainly vertical during ascent
and emplacement of these intrusive rocks. Stoping played an important role in both the ascent
and emplacement of the massive core gabbros,
and a moderate increase in their areas at the cur-
618
Figure 15. Model that requires formation of the deformation aureole prior to emplacement of
the intrusive units that define the Zarza Intrusive Complex. See Figure 13 caption for further
explanation.
rent exposure level could have destroyed crucial
evidence for the complex’s ring zones. In this instance, we may have encountered a zoned mafic
pluton (plus or minus tonalite) surrounded by a
ductile deformation aureole, and we may not have
been able to rule out lateral expansion and diapirism as mechanisms for aureole development.
ACKNOWLEDGMENTS
This project was supported by an Australian
Research Council Large Grant A39700451 and
Queen Elizabeth II Research Fellowship (to
Johnson), grant 4311PT from the Consejo Nacional de Ciencia y Tecnologia (CONACyT) of
Mexico (to Johnson), and a Macquarie University Research Fellowship (to Tate). We thank
Mark Brandon (Associate Editor), Allen
Glazner, and Brendon McNulty for constructive
reviews, which led to important improvements
to the manuscript.
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