Ring complexes in the Peninsular Ranges Batholith, Mexico and the

Lithos 61 (2002) 187 – 208
www.elsevier.com/locate/lithos
Ring complexes in the Peninsular Ranges Batholith,
Mexico and the USA: magma plumbing systems
in the middle and upper crust
S.E. Johnson a,*, K.L. Schmidt b, M.C. Tate c
b
a
Department of Geological Sciences, University of Maine, Orono, ME 04469-5790, USA
Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA
c
Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
Received 15 November 2000; accepted 18 December 2001
Abstract
Subvolcanic ring complexes are unusual in that they preserve a rapidly frozen record of intrusive events. This sequential
history is generally lost or complicated in plutons owing to mixing and mingling in a dynamic state. Thus, subvolcanic ring
complexes are more like erupted rocks in their preservation of instantaneous events, but the self-contained nature of the
complexes allows detailed structural and chemical work to be conducted in environments where the relative timing between
individual magmatic events is commonly well preserved. We suggest that development of subvolcanic ring complexes in the
western Peninsular Ranges Batholith (PRB) involved the following three-stage generalized sequence: (1) fracturing of the roof
above a buoyant or overpressured magma chamber, which resulted in moderately inward-dipping conical fractures that locally
hosted cone sheets; (2) subsequent loss of magma from the chamber, combined with degassing of the melt, which facilitated
collapse of the roof along near-vertical ring faults that locally hosted ring dikes; and (3) resurgence of the chamber, and/or
intrusion of a broadly cogenetic nested pluton, which locally destroyed evidence for the earlier history of the system. This
sequence has been repeated twice in one of the ring complexes that we have identified, which resulted in nested intrusive
centers. Calderas, subvolcanic ring complexes and plutons may represent progressively deeper sections through linked magma
plumbing systems, and the systematic occurrences of these features in the western PRB are consistent with progressively deeper
along-strike exposures of the batholith from south to north over a distance greater than 250 km. In addition to subvolcanic
complexes in the western PRB, deeper crustal levels exposed in the transition zone between eastern and western parts of the
batholith preserve ring complexes emplaced at depths of up to 18 km. Occurrence of these deeper-level complexes suggests
either that caldera subsidence can extend to mid-crustal levels or that other processes can produce ring complexes. D 2002
Elsevier Science B.V. All rights reserved.
Keywords: Cone sheet; Magmatism; Peninsular Ranges; Pluton emplacement; Ring complex
1. Introduction
*
Corresponding author.
E-mail address: [email protected] (S.E. Johnson).
The temporal and spatial evolution of magma
plumbing systems remains one of the outstanding
problems in our search for a better understanding of
0024-4937/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 4 - 4 9 3 7 ( 0 2 ) 0 0 0 7 9 - 8
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S.E. Johnson et al. / Lithos 61 (2002) 187–208
how continents grow and evolve. However, magma
chambers are extremely dynamic physical and chemical systems, and multiple processes such as mixing,
mingling, convection and recharge can lead to considerable complexity in the resulting pluton. In contrast,
and much like eruptive sequences, subvolcanic ring
complexes preserve instantaneous magmatic events.
They contain a wide variety of intrusive phases including cone sheets, ring dikes and massive central intrusions, and are potentially a rich source of information
about the evolution of caldera/volcano root zones and
the tops of upper-crustal magma chambers. The
detailed intrusive relationships in these complexes,
and the intimate timing relationships between the
various intrusions and deformational structures, are
preserved in part due to rapid quenching of some units,
and in part due to the sequential series of intrusions
that produce a magmatic stratigraphy. Thus, these
complexes provide an unusual opportunity to evaluate
the evolution of subvolcanic magmatic systems and
upper-crustal magma-transfer zones in general.
In this paper, we describe some of the ring complexes that occur in the Peninsular Ranges Batholith
(PRB) of Baja California Norte, México, and southern
California, USA (Fig. 1), and discuss their roles in
middle- and upper-crustal magma plumbing systems.
Although numerous ring-dike complexes occur in
North America (e.g., Billings, 1943, 1945; Lipman,
1984; Clemens-Knott and Saleeby, 1999), the Baja
California ring complexes are relatively unusual in
that some are multiple-center, subvolcanic, conesheet-bearing complexes. In addition to these subvolcanic complexes, there are also younger ring complexes within 20 km to the east that formed f 10– 15
Ma later, f 10 km deeper in the crust, and during a
different stage in the tectonic history of the batholith.
Thus, both subvolcanic and mid-crustal ring complexes are preserved at the current exposure level.
Our work to date has shown that some of the Baja
California ring complexes are surrounded by ductile
deformation aureoles that record substantial emplacement-related bulk shortening (Johnson et al., 1999a),
Fig. 1. Simplified geological map of the Sierra San Pedro Martir block of the Peninsular Ranges Batholith (PRB). Intrusive bodies discussed in
this paper are, from north to south: PVRC = Paloma Valley Ring Complex; RRD = Ramona ‘‘ring-dike’’; EP = El Pinal tonalite; CIC = Cerro de
Costilla Intrusive Complex; BIC = Burro Intrusive Complex; ZIC = Zarza Intrusive Complex; RIC = Rinconada Intrusive Complex. Map and
data assembled from Gastil et al. (1975, 1990, 1991), Gromet and Silver (1987), Johnson et al. (1999b), Silver et al. (1979), and Taylor and
Silver (1978).
S.E. Johnson et al. / Lithos 61 (2002) 187–208
which makes them unusual in comparison to many
previously described ring complexes (e.g., those in the
British Tertiary Intrusive Province and New England
Appalachians). The subvolcanic complexes provide
insights into what are currently poorly understood
mechanisms by which magma is transported from
upper-crustal magma reservoirs to the surface, and
the deformation processes that help to facilitate this
magma transfer. In addition, the presence of ring
complexes that formed at mid-crustal depths in the
PRB underscore the possibility that: (a) volcanoplutonic subsidence systems can extend deep into
the crust; or (b) some ring complexes may not form
in association with volcanism, which leaves open the
question of what other mechanisms may be responsible for their formation, and what role they play in
magma plumbing systems.
2. Definitions and background
Subvolcanic ring complexes have historically been
viewed as integral parts of ‘‘volcano-plutonic subsidence systems’’, which is a general term encompassing
the complete set of geological features found between,
and including, a high-level caldera and its underlying
magma chamber (definition after Shannon, 1988).
More specifically, subvolcanic ring complexes have
been interpreted as frozen magma pathways between
upper-crustal magma chambers and surface calderas
(e.g., Williams, 1941; Richey, 1948; Turner, 1963;
Smith and Bailey, 1968; Oftedahl, 1978; Dodge,
1979; Lipman, 1984). The best-known ring complexes
occur in the Tertiary plutonic districts of the British
Isles, which include spectacular examples of multiplyintrusive, nested complexes (e.g., Richey, 1932, 1948;
Walker, 1975). Other examples include those in the
Younger Granite complexes of northern Nigeria (e.g.,
Jacobson et al., 1958; Turner, 1963; Bowden and
Turner, 1985), the Peruvian Coastal Batholith (e.g.,
Cobbing and Spencer, 1972; Myers, 1975; Bussell et
al., 1976), the Oslo region of Norway (e.g., Oftedahl,
1953, 1978), the White Mountains and Winnipesaukee
batholiths in New Hampshire, USA (Billings, 1943,
1945), the Mediterranean island of Corsica (Bonin,
1986), the Georgetown Inlier of Queensland, Australia
(Branch, 1966), and the western PRB in Baja California, México (Gastil et al., 1975; Gromet and Silver,
189
1977; Gastil, 1990; Delgado-Argote et al., 1995; Johnson et al., 1999a,b; Tate et al., 1999; this paper).
Since the first documentation of a ring dike in
relation to a collapsed cauldron at Glen Coe (Clough
et al., 1909; the term ‘‘ring-dyke’’ was apparently first
used by Bailey, 1914), nomenclature used to describe
magmatic ring structures has become complex, with
several terms being used loosely or interchangeably.
Examples of these terms include ‘‘caldera’’, ‘‘cauldron’’, ‘‘ring structure’’, ‘‘ring complex’’, ‘‘ring zone’’,
‘‘ring dike’’, ‘‘ring-dike complex’’ and ‘‘cone sheet’’.
Here we define some of the terms used in this paper,
retaining as much consistency with historical usages as
possible. A ‘‘caldera’’ is a large volcanic depression
that is more or less circular, has a diameter many times
that of included vents, and forms by roof collapse into
an underlying magma chamber (Lipman, 2000). ‘‘Ring
complex’’ is a general term used to describe an intrusive complex that contains cone sheets and/or ring
dikes, although the term can be used loosely to describe any intrusive complex with circular, oval, polygonal or arcuate intrusions in plan view. A ‘‘cone sheet’’
is a discordant intrusion that can be circular, oval,
polygonal or arcuate in plan and has gently to steeply
inward-dipping contacts (Harker, 1904 was apparently
the first to document cone sheets, but the term was
apparently first used by Bailey et al., 1924). Sheet
thicknesses are variable, but seldom reach more than a
few tens of meters. Rock-types can be mafic, intermediate or felsic (less common), and commonly show
quench textures. A ‘‘ring dike’’ is a discordant intrusion that can be circular, oval, polygonal or arcuate in
plan and has near-vertical to steeply outward-dipping
contacts. Dike thicknesses are variable, but can reach
up to several kilometers. Rock-types are generally
felsic and do not typically show quench textures.
Implicit in the published use of some of the above
terms is the notion that they describe different structural levels in volcano-plutonic subsidence systems;
plutons lie below ring complexes, which lie below
calderas (e.g., Richey, 1932, 1948; Williams, 1941;
Billings, 1945; Reynolds, 1956; Jacobson et al., 1958;
Smith and Bailey, 1968; Cobbing and Spencer, 1972;
Bussell et al., 1976; Oftedahl, 1978; Bonin, 1986,
1996). Thus, the term ‘‘ring complex’’ is commonly
reserved for relatively deeply eroded subsidence systems, even though cone sheets can be found relatively
high in volcanic edifaces (e.g., Schirnick et al., 1999).
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S.E. Johnson et al. / Lithos 61 (2002) 187–208
S.E. Johnson et al. / Lithos 61 (2002) 187–208
Such an inferred relationship between map pattern and
position in the subsidence system may not be universally applicable. Overlaps or transitions between the
different levels may occur (Lipman, 1984), and some
workers have inferred that ‘‘underground cauldron
subsidence’’ and associated ring-dike formation can
take place without surface volcanism (e.g., Richey,
1932; Billings, 1943; Myers, 1975). Nevertheless,
many studies are consistent with the general relationships between map pattern and position in the subsidence system that are illustrated in Fig. 2, which
shows a small number of possible intrusive histories
and geometries. Fig. 2 illustrates that there need be no
strict correspondence between map pattern and absolute crustal depth. For example, maps 4, 5, 6 and 9 all
come from approximately the same depth, but show
very different map patterns. Thus, the different features
of volcano-plutonic subsidence systems may lie above
or below one another in a predictable sequence in
individual systems, but the absolute crustal depth at
which they occur is dependent on the specific intrusive
history of each system.
3. Geologic setting
The Jura-Cretaceous PRB is one of the great batholiths of western North America. It extends some 1600
km from southern California, USA, to the tip of Baja
California, México (Fig. 1), and is recognized as the
plutonic roots of what was once a vast Andean-type arc
system (Lipman, 1992). In northern Baja California
alone, the batholith contains more than 400 discrete
plutons of mainly tonalitic and granodioritic composition (Gastil et al., 1975), although gabbro comprises
f 20% of the western PRB. Unlike the Sierra Nevada
Batholith to the north, much of the metasedimentary
and metavolcanogenic host rocks to the PRB are
191
preserved between plutons, similar to isolated plutons
of the White –Inyo Ranges of eastern California (e.g.,
Bateman, 1992).
The batholith is divisible into distinct western and
eastern belts on the basis of geological, petrological,
geophysical, geochemical and isotopic characteristics
(Fig. 1, Todd and Shaw, 1985; Silver and Chappell,
1988; Todd et al., 1988; Gastil et al., 1990; Walawender et al., 1991; Magistrale and Sanders, 1995;
Johnson et al., 1999b; Lewis et al., 2000, 2001). A
transition zone occurs between the two belts and is
defined by gradients or sharp breaks in these parameters; the width of this zone depends on the parameter
being considered, and varies from sharp ( f 100 m) to
perhaps 20 km. In northern Baja California, in the area
shown in Fig. 1, the western PRB intruded Mesozoic
volcanic-arc and basinal volcano-sedimentary assemblages that, combined, comprise the Alisitos arc. In
contrast, the eastern part intruded Triassic to Cretaceous flysch and Proterozoic to Paleozoic passivemargin sedimentary rocks. The transition zone between these belts comprises flysch assemblage rocks
and is a zone into which contractional deformation was
focused during the development of much of the batholith. The origin of these west-to-east contrasts in the
PRB has been attributed to a number of tectonic
histories. Most previous workers considered the boundary between western and eastern belts in the PRB to
be a suture between the North American craton and a
fringing arc, which were previously separated by a
back-arc 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 Thomson and Girty (1994) suggested that the
batholith may have formed in situ across a pre-Triassic
boundary between oceanic and continental crust. More
recently, Johnson et al. (1999b) described a major
Fig. 2. Set of diagrams illustrating a small number of possible intrusive histories in a volcano-plutonic subsidence system, and various map
patterns that might be observed. The intrusive history starts with the diagram at the top center (A), which shows a set of conical fractures above a
rising magma chamber. These fractures are stylized, and if present may be occupied by cone sheets. Diagrams B – D show different subsurface
geometries and features that may develop during collapse along ring-faults to form a caldera, which is filled with syn-collapse volcanics. The
ring-faults may host ring-dikes, although this is not necessary. Diagram D evolves to either E, F or G, which show resurgence, new cone-sheet
development and a nested intrusion, respectively. Diagram F evolves to H, which shows a multiple-center ring complex with a central intrusion.
Map patterns 1 – 9 were chosen as representative of what has been described in the literature and in this paper: 1 = caldera, 2 = cauldron, 3 = ring
complex, 4 = ring complex with central intrusion, 5 = multiple-center ring complex with central intrusion, 6 = roots of a ring dike in a pluton,
7 = ring complex intruded by cogenetic intrusion, 8 = nested plutons, 9 = pluton cored by collapsed roof rocks, which may include earlier gabbro.
The arcuate lines in maps 2 – 5 and 7 represent conical fractures that, if present, may host cone sheets.
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S.E. Johnson et al. / Lithos 61 (2002) 187–208
ductile thrust (Main Mártir thrust) that they interpreted
as part of a broader suture zone that formed at f 115 –
108 Ma between the eastern and western PRB. They
suggested that the Alisitos arc originated as an island
arc that collided with North America. The occurrence
of ring complexes in different parts of the PRB
provides an opportunity to compare and contrast
complexes that formed at different times, different
crustal levels and in different tectonic settings.
4. Ring complexes in the PRB
4.1. Introduction
Below, we describe four ring complexes that we
have mapped, namely the Zarza, Burro, Cerro de
Costilla and Rinconada intrusive complexes. We also
discuss three bodies described by Gastil (1990); the El
Pinal tonalite, San Telmo pluton and Ramona ring
dike. Finally, we discuss the Paloma Valley ring
complex described by Morton and Baird (1976).
The four ring complexes we have mapped can be
divided into two distinct types formed at different times
and crustal levels. One type (e.g., Zarza and Burro
complexes) occurs in the Alisitos (western) arc, and
formed f 113 to 117 Ma, prior to, and possibly during,
juxtaposition of the Alisitos arc with the North American continental margin. These complexes are relatively small ( < 10 km2), and formed at depths of < 9
km (Al-in-hornblende barometry, see below; Johnson
et al., 1999a; Tate et al., 1999; Tate and Johnson, 2000).
The other type (Rinconada and Cerro de Costilla
complexes) occurs in the strongly deformed transition
zone between the western and eastern parts of the
batholith, and formed in thickened crust at f 100 to
103 Ma, following arc collision. These complexes are
relatively large ( >40 km2). The Rinconada complex
formed at f 18 km depth (Al-in-hornblende barometry, see below), and although the Cerro de Costilla
complex occurs in a similar structural position in the
batholith, we currently have no reliable constraints on
its emplacement depth. Thus, the PRB provides an
opportunity to examine ring complexes formed at
different crustal levels; upper-crustal complexes are
preserved in the western zone of the PRB, whereas
younger, mid-crustal complexes occur farther east in
the PRB transition zone.
4.2. Western complexes
4.2.1. Zarza intrusive complex
The f 115 Ma, 7 km2 Zarza complex (Johnson et
al., 1999a; Tate et al., 1999) contains three nested
intrusive centers, was emplaced at f 5- to 9-km
depth (2.6 F 0.6 kbar, Al-in-hornblende barometry,
Johnson et al., 1999a), and is elongate approximately
parallel to the northwest–southeast regional structural
trends in the batholith (Fig. 3a). The northern and
central intrusive centers contain inward-dipping cone
sheets overprinted by subvertical shear zones that we
interpret as ductile equivalents of ring faults. Approximately 90% of the intrusive rocks in the complex are
mafic to intermediate, including the basaltic andesite
to andesite cone sheets. The remaining 10% is tonalite
and trondhjemite that occurs in the southern center.
The three centers intruded sequentially beginning with
the northern center (map units G1, G2 and northern
cone sheets, Fig. 3a). The central center (map units G3
and central cone sheets, Fig. 3a) intruded the solidified northern one, as illustrated by the sharp truncations of units G1, G2 and the northern cone sheets.
Finally, the southern center (map units G4 and tonalite/trondhjemite, Fig. 3a) intruded the central one
after it was solid, as shown by the sharp truncation
of unit G3 and the central cone sheets.
The northern and central centers had similar intrusive histories beginning with (1) concentric cone
sheets followed by (2) central gabbros and ending
with (3) the development of ductile shear zones near
the outer margins of the centers. Magmatic foliations
in the cone sheets and gabbros are concentric and dip
moderately to steeply inward (Fig. 3b). In contrast, the
ductile shear zones are approximately vertical, and
shear-sense indicators consistently show that the centers moved down relative to the surrounding country
rocks, which suggests that they are ductile equivalents
of ring faults that accommodated collapse of the
centers (Johnson et al., 1999a). In many documented
ring-dike complexes, ring dikes are preceded by the
development of ring faults and fractures, and we
suggest that the ductile shear zones in the Zarza
complex represent the early stages of ring-dike formation at intermediate to deep levels in a volcanoplutonic subsidence system. The southern center had a
simpler history, with intrusion of unit G4 and the
tonalite/trondhjemite.
S.E. Johnson et al. / Lithos 61 (2002) 187–208
The Zarza complex is surrounded by a f 300-mwide ductile strain aureole with bulk shortening of 38%
perpendicular to its margins (Johnson et al., 1999a).
193
Shortening perpendicular to the margin of the complex
ranges from 71% near the contact to 18% at the outer
margin of the aureole. Bedding is inward-dipping and
Fig. 3. (a) Geological map of the Zarza Intrusive Complex. Units labeled northern and central cone sheets are composed of cone sheets with
intervening, hornfelsic, metavolcanogenic wall-rock screens. (b) Structural map of the complex, showing trend lines for structural fabrics and
outlines of geological units. (c) Block-diagram section through the complex along the line shown in (a).
194
S.E. Johnson et al. / Lithos 61 (2002) 187–208
Fig. 3 (continued).
S.E. Johnson et al. / Lithos 61 (2002) 187–208
195
Fig. 3 (continued).
inward-younging (relatively rare cross-bedded volcanogenic sandstones) around the entire complex, which
is bounded on its western, northern and eastern sides by
anticlines (Fig. 3b). Excellent preservation of the
intrusive history allowed Johnson et al. (1999a) to
evaluate the origin of the puzzling deformation aureole,
which they concluded could not have formed solely by
diapirism or lateral expansion during emplacement of
the intrusive rocks, or by regional deformation. Instead,
they favored models that require downward displacement of the complex, possibly at various stages in its
history in association with collapse along the ductile
shear zones. Thus, vertical material transfer processes
dominated the intrusive and emplacement history.
4.2.2. Burro intrusive complex
The f 114 Ma, 4.5 km2 Burro complex lies 5 km
west –northwest of the Zarza complex, and is elongate
approximately parallel to the northwest– southeast regional structural trends in the PRB (Fig. 4). Unlike the
more mafic Zarza complex, approximately 54% of the
Burro complex is composed of tonalite, of which there
are two texturally distinct types. Geological and structural evidence suggest only one intrusive center, but
relatively late intrusion of gabbro and tonalite has apparently destroyed much of the original complex, lea-
ving only a small remnant of an outer zone (cf. Fig. 2,
map 7) that bears a close resemblance to the Zarza cone
sheets and intervening volcanogenic screens. Alternatively, the complex may have developed asymmetrically. This remnant outer zone includes hornfelsed
country rock that was intruded by a network of narrow,
centimeter-scale, locally quenched, cross-cutting sheets
that attain widths of 50 cm or more. Where they can be
measured, the country rocks and individual sheets all
dip moderately to steeply inward, supporting a conesheet interpretation. Narrow, steeply dipping ductile
shear zones like those found in the Zarza complex
occur at several localities in this outer zone, and shearsense indicators always show that the complex moved
down relative to the surrounding country rocks. On
the basis of our work in the Zarza, we suggest that the
Burro complex had a similar history that involved the
emplacement of cone sheets, followed by intrusion of
the gabbro and tonalite units with associated collapse
of the complex along ductile shear zones.
4.3. Transition-zone complexes
4.3.1. Rinconada intrusive complex
The f 101 Ma, 40 km2 Rinconada complex occurs
approximately 50 km southeast of the Zarza complex,
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S.E. Johnson et al. / Lithos 61 (2002) 187–208
Fig. 4. Simplified geological map and cross section of the Burro Intrusive Complex. Structural symbols are defined in Fig. 3b.
immediately east of the Main Martir thrust that juxtaposes western and transitional belts in the PRB (Fig. 5).
The complex is lithologically heterogeneous at the
meter-scale and largely consists of hornblende gabbro
and anorthositic gabbro with minor mafic tonalite. The
northwest and southeast sides of the complex are
partially ringed by arcuate, elongate hornblende tonalite bodies, and smaller ones occur on the southwestern
side (Fig. 5). All these tonalite bodies dip inward,
towards the center of the complex. Tonalite at the ends
of these partial rings commonly shows intrusive and
mingling relations with gabbro and anorthositic gabbro. A particularly intriguing feature of the Rinconada
complex is the occurrence of multiple wall-rock
screens within several km of the southern margins of
the complex (Fig. 5). These hornfelsic metavolcanic
and metasedimentary screens rarely exceed a meter in
width and extend up to 100 m in length. The screens
also dip moderately to steeply inward, and occur in
both gabbro and tonalite within the complex. The
relatively fine scale (tens of meters) at which these
screens occur suggests that much of the outer complex
was constructed by emplacement of multiple, inwarddipping, arcuate magmatic sheets. However, contacts
between individual sheets are cryptic at this scale.
The inward-dipping tonalite sheets could be called
cone sheets on the basis of their geometry. However,
the term ‘‘cone sheet’’ has never been used to describe
sheets occurring at such deep crustal levels, and all
explanations for their formation have been related to
upper-crustal processes such as upward extension of
rocks above a subvolcanic magma chamber (e.g.,
Anderson, 1936; Phillips, 1974; Walker, 1975; Schirnick et al., 1999). Given the depth at which they were
emplaced, we hesitate to refer to these sheets as ‘‘cone
sheets’’. This hesitation reflects our uncertainty about
how they formed, and what significance they may have
for mid-crustal magma plumbing systems. Neverthe-
S.E. Johnson et al. / Lithos 61 (2002) 187–208
Fig. 5. Geological map of the Rinconada Complex. Structural symbols are defined in Fig. 3b.
197
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S.E. Johnson et al. / Lithos 61 (2002) 187–208
less, we still consider the Rinconada complex to be a
ring complex in the loose sense of the term (as defined
above).
The original geometry of the Rinconada complex
has been modified by syn- to post-intrusive deformation. The complex intruded a f 6-km-thick mylonitic
thrust sheet that forms the western flank of a midcrustal, doubly vergent fan structure preserved across
the Sierra San Pedro Martir. The northwest and southeast tonalite shells in the complex yielded Al-inhornblende pressures of 5.4 and 5.8 kbar, respectively
(C. Kopf, unpublished data, summarized in Schmidt,
2000), with approximate errors of F 1 kbar. These
pressure estimates are consistent with mineral equilibrium studies in metamorphic wall rocks along the
southeast and southern parts of the complex (Kopf
and Whitney, 1999; Schmidt, 2000). Thrusting continued in wall rocks around the Rinconada complex
following its emplacement. The eastern margin of the
complex has been overthrust by its wall rocks on
steeply- to gently-dipping shear zones, and more than
10 km of east-side-up throw is apparent across a
narrow reverse shear zone along the western side of
the complex (Fig. 5; Schmidt, 2000). The degree to
which deformation developed within the complex is
strongly dependent on rock type. Tonalite and much
of the hornblende gabbro near its margins show
penetrative solid-state fabrics that closely parallel
mylonitic fabrics in the wall rocks. Anorthositic gabbro within the complex largely preserves magmatic
foliations that show a weak tendency to dip inward.
Near the sides of the pluton this gabbro is deformed
by mylonitic reverse faults.
4.3.2. Cerro de Costilla intrusive complex
We have mapped the southern third of the Cerro de
Costilla complex (Fig. 6), but the northern two-thirds
are known only in reconnaissance. Below, we briefly
Fig. 6. Reconnaissance geological map of the Cerro de Costilla Intrusive Complex. Structural symbols are defined in Fig. 3b.
S.E. Johnson et al. / Lithos 61 (2002) 187–208
summarize our current knowledge. The f 103 Ma,
80 km2 complex lies approximately 20 km northeast
of the Zarza complex, in a very similar structural
setting to the Rinconada complex, just east of the
Main Martir thrust. The complex contains a concentrically foliated, apparently complete, outer ring of tonalite. This outer ring surrounds a core composed
largely of coarse-grained gabbro with a tonalite shell
that may (from air photo examination) form a second,
inner ring. Between the outer ring and inner core is a
complex zone composed of strongly hornfelsed,
deformed and partially melted metasedimentary and
metaigneous rocks. In addition, there is a zone of
variable width along the southern margin of the
complex that is composed of mixed and mingled
gabbro, diorite and tonalite.
Unlike the Rinconada complex 60 km to the south,
the original geometry of the Cerro de Costilla complex has not obviously been modified by syn- to postintrusive deformation. The emplacement depth of the
complex is presently unknown; metamorphic wallrock assemblages between the outer ring and inner
core record f 5-kbar pressures (C. Kopf, unpublished data), but we are uncertain as to whether these
conditions reflect those at the time of emplacement. If
the complex was emplaced at pressures of f 5 kbar,
then the complex and host rocks must have been
uplifted on a shear zone that would have to occur
structurally below the Main Martir thrust because this
thrust is cut by the outer tonalite of the complex (Fig.
6). Given the similarity of its age and structural
position to the Rinconada complex, it is tempting to
suggest that the Cerro de Costilla may have crystallized at pressures of 5 – 5.5 kbar, but further work is
needed before we can reach a conclusion.
The steeply dipping outer tonalite ring could be
called a ring dike on the basis of its geometry.
However, owing to the possibility of a mid-crustal
emplacement depth, we are reticent to use the term for
the same reasons that we preferred not to use the term
‘‘cone sheet’’ to describe the inward-dipping sheets in
the Rinconada complex. We still consider the Cerro de
Costilla complex to be a ring complex, but as with the
Rinconada complex, we are uncertain about what
controlled the geometry of the intrusions, and what
significance they may have for mid- or upper-crustal
magma plumbing systems. These intrusions may be
part of a volcano-plutonic subsidence system, but
199
more work is required before we can reach such a
conclusion. In particular, if the outer tonalite is a ring
dike associated with caldera collapse, kinematic indicators on both sides of the tonalite should show that
the inner part of the complex dropped down, as with
the Zarza complex. The Cerro de Costilla complex has
a ductile strain aureole around the outer tonalite ring;
various kinematic indicators have been observed, but
detailed mapping is required to determine their distribution and sense of displacement.
4.4. Other ring-like intrusions in the PRB
4.4.1. El Pinal tonalite
The 190 km2 El Pinal tonalite (Fig. 7) is located 35
km south of the international border, and contains three
plutonic units that can be distinguished on the basis of
texture and structure (Duffield, 1968). The core zone
lacks a magmatic foliation, but elongate enclaves
consistently define a concentric fabric that shallows
towards the center of the body and defines a basinal
shape (Fig. 7). In contrast, the vertical zone composed
of granodiorite, and the contact zones within the
pluton’s outer margins, contain a well-developed magmatic foliation defined by alignment of plagioclase,
biotite and hornblende crystals. The vertical zone also
contains a well-developed, down-dip lineation, which
is only locally present in the contact zone (Duffield,
1968). Modal mineralogy indicates that the contactzone tonalite is slightly more quartzo-feldspathic than
the core-zone tonalite. Duffield (1968) suggested that
the granodiorite ring represents the roots of a ring dike
(Figs. 2 (map 6) and 7). Although this remains a
possibility, other interpretations are possible.
4.4.2. San Telmo pluton
The 100 km2 San Telmo pluton (Fig. 1) appears as
a reversely zoned pluton on the reconnaissance geological maps of Gastil et al. (1975), and Gastil (1990)
referred to it as a zoned pluton with a plagiogranite
rim and gabbro core. Gromet and Silver (1977)
instead referred to this body as a ring complex and
stated that the core contains prebatholithic volcanic
rocks, and gabbro that is older than, and intruded by,
more-felsic ring-shaped units. Similarly, on the basis
of field relations and 40Ar/39Ar ages, Delgado-Argote
et al. (1995) concluded that the core contains volcanic
rocks, gabbro, and diorite that predate surrounding
200
S.E. Johnson et al. / Lithos 61 (2002) 187–208
Fig. 7. Geological map and cross section of the El Pinal tonalite, modified after Duffield (1968). Structural symbols are defined in Fig. 3b.
monzodiorite, granodiorite and granite. They reached
a similar conclusion to Gromet and Silver (1977),
interpreting the pluton as a subcaldera magma chamber. On the basis of the above studies, it appears that
the San Telmo pluton may have formed as part of a
volcano-plutonic subsidence system.
4.4.3. Ramona ring dike
The 50 km2 Ramona ‘‘ring dike’’ (Mirriam, 1941),
located in San Diego County, USA, is a mediumgrained tonalite of uniform texture and composition,
and is part of an intrusive complex that contains three
contemporaneous tonalite units and a gabbro (Fig. 8a).
Mirriam (1941) suggested that the Lakeview tonalite
intruded as a ring dike around a subsiding boss of
Green Valley tonalite (Fig. 8b). However, the geologic
relationships also support an intrusive history in which
the Green Valley tonalite intruded the Lakeview tonalite as a nested diapir (Figs. 2 (map 8) and 8c). Mirriam
(1941) largely discounted this possibility on the basis
that it would require large horizontal displacements,
which should have concentrically foliated the Bonsall
tonalite at its contact. However, if the Lakeview
tonalite contained a moderate percentage of melt when
the Green Valley tonalite was emplaced, material transfer could have been mainly vertical, as shown in Fig. 8c
S.E. Johnson et al. / Lithos 61 (2002) 187–208
201
Fig. 8. (a) Simplified geological map of the Ramona ‘‘ring-dike’’ and surrounding geology, modified after Mirriam (1941). Although no
structural data are presented, Mirriam’s (1941) block diagram (b) shows the three-dimensional geometry. (b) Development of the Ramona ‘‘ringdike’’, as proposed by Mirriam (1941). The central plug of Green Valley tonalite subsides, allowing intrusion of the ring-shaped Lakeview
tonalite around it. (c) Alternative intrusive history in which the Lakeview tonalite is emplaced first, and while still a crystal mush it is intruded
by the Green Valley tonalite, which is accommodated by downward flow of the Lakeview tonalite.
202
S.E. Johnson et al. / Lithos 61 (2002) 187–208
(e.g., Paterson and Vernon, 1995). This intrusive history is consistent with the strong magmatic foliation in
the Lakeview tonalite, and concentric foliation in the
older basement rocks. The relative emplacement timing of the different tonalites is uncertain, and so the
lack of margin-parallel foliation in the Bonsall tonalite
may indicate that it postdated the Green Valley and
Lakeview tonalites. More detailed work may be required before this body can be viewed as a ring dike.
4.4.4. Paloma Valley ring complex
The f 75 km2 Paloma Valley ring complex,
located in Riverside County, USA, was interpreted
by Morton and Baird (1976) as containing an older,
single ring dike locally greater than 3 km wide, and a
younger set of arcuate ring dikes up to several meters
wide that occur mainly inside the area bounded by the
older ring dike (Fig. 9). The older dike is composed
largely of quartz monzonite and granodiorite, has
near-vertical contacts and intruded gabbro and minor
metasedimentary rocks. The younger ring dikes, of
which there are more than 200, are granitic pegmatites with dips varying progressively from flat-lying
in the center of the complex to 50– 70j outward near
the inner margin of the older ring dike (Fig. 9).
Morton and Baird (1976) suggested that the older
ring dike intruded the gabbroic and metasedimentary
wall rocks along ring fractures. After emplacement of
the older dike, the younger, inner dikes were
emplaced in domal fracture sets formed by a rapidly
Fig. 9. Geological map and cross-section of the Paloma Valley ring complex, modified after Morton and Baird (1976).
S.E. Johnson et al. / Lithos 61 (2002) 187–208
changing stress field that they related to cauldron
subsidence.
5. Discussion
We have described four ring complexes that occur
at two distinct crustal levels across the PRB, and have
reviewed several other plutons or intrusive complexes
that have been related to volcano-plutonic subsidence
systems by other workers. The four complexes we
have mapped formed during two distinct time periods
(113 – 117 and 100– 104 Ma), and in locations within
the batholith that experienced very different tectonic
and deformation histories. The two older ring complexes formed in the western zone of the PRB, as part
of an island arc (the Alisitos arc) that collided with the
North American margin f 110 Ma. In contrast, the
two younger ring complexes we have described occur
in the highly deformed transition zone between the
eastern and western parts of the batholith. At least one
of these two complexes was emplaced at relatively
deep crustal levels following the collision between the
Alisitos arc and North America, and was exhumed
during continued thrusting in the transition zone. The
formation of these four complexes at different depths
and at different stages in the tectonic evolution of the
composite arc allows us to speculate about magma
plumbing at various crustal levels.
5.1. Subvolcanic levels in the Alisitos arc
The intrusion/collapse cycles we have documented
in the Zarza complex provide compelling evidence for
how pulses of magma escape from some high-level
chambers, and how pathways towards the surface are
created. On the basis of our work, we propose a threestage generalized model for the evolution of volcanoplutonic subsidence systems that can be repeated in an
individual complex.
(1) An initial magma chamber is formed, which
was mafic to intermediate in the Zarza example (Tate
et al., 1999), but may vary in composition. The magma chamber becomes overpressured, and at some
stage exerts enough upward force to fracture and
disrupt the overlying crust. These fractures fill with
magma that rapidly freezes to form cone sheets. We
do not know how vertically extensive these sheets
203
were in the Zarza example, or whether they provided
pathways for magma transport to the surface. Processes that may cause fracturing and cone-sheet
emplacement could include one or all of the following: (a) positive magma buoyancy relative to surrounding country rocks; (b) volatile overpressure in
the magma chamber; or (c) large magmastatic stress in
the magma chamber owing to connectivity (e.g.,
through dikes) with a positively buoyant magma
source at depth.
(2) Disruption of the overlying crust during conesheet emplacement relaxes an important energy barrier to voluminous magma transport from the chamber. The resulting network of fractures and sheet
contacts provides a vast array of potential magma
pathways (e.g., Walker, 1986) through which the
massive central intrusions later stope to form central
conduits that may supply volcanic eruptions at the
surface. Subsequent loss of magma from the chamber,
combined with degassing of the melt, reduces magma
pressure and facilitates collapse of the roof, commonly along near-vertical ring faults. These faults
evolve with increasing depth into brittle/ductile kinematic zones that locally vary in width and amount of
displacement, and provide conduits for the ascent of
relatively felsic melts that form ring dikes. The depth
to which these kinematic zones can transfer subsidence is unknown.
(3) Resurgence of the chamber, and/or intrusion of
a broadly cogenetic nested pluton, can partially or
completely destroy evidence for the earlier history of
the system through a range of exposure levels (e.g.,
Smith and Bailey, 1968; Lipman, 1984), which provides one possible explanation for why ring complexes are only rarely preserved in the geological
record. This point was also made by Jacobson et al.
(1958, p. 7) who said the following with regards to the
Younger Granite complexes in northern Nigeria: ‘‘In
several of the complexes, where the structural relationships of the component phases are clearly revealed, it is apparent that a difference of as little as
1000 ft in erosion level would profoundly modify the
surface plan of the complex.’’ Thus, exposure of wellpreserved ring complexes like the Zarza appears to be
somewhat fortuitous. They may have been more
common in the PRB than preserved examples would
suggest, but massive intrusions may have followed
magma-transfer pathways between calderas and their
204
S.E. Johnson et al. / Lithos 61 (2002) 187–208
linked chambers, possibly destroying the distinguishing features of other ring complexes that may have
occurred in this area.
5.2. Deeper levels in the batholith transition zone
Based on their map patterns, it is possible that the
deeper-level complexes in the transition zone were
linked to volcano-plutonic subsidence systems more
than 10 km above them. The Cerro de Costilla
complex appears to be a well-preserved ring-dike
complex, but there is no clear evidence that it was
tied to caldera processes at higher crustal levels. At
present we can only loosely constrain the emplacement depth of this complex to V 18 km. However,
the f 18-km-deep Rinconada complex also has
strong geometrical affinities with shallower-level ring
complexes. Only partial tonalite rings are preserved in
the complex, but the intricate network of host-rock
screens that occurs in its southern portion suggests
that the complex formed, at least in part, by intrusion
of arcuate-shaped pulses of mafic and felsic magma.
The occurrence of such a deep ring complex raises
the possibility that volcano-plutonic subsidence systems near the surface may in some instances link to
magmatic systems at much deeper crustal levels. The
classical view of caldera formation suggests that caldera floors subside into magma chambers near the
surface, accommodated by the expulsion of magma. If
subsidence continued to relatively deep levels in some
instances, then vertically extensive ring faults or ductile shear zones would be required to transfer material
downward, and thus provide a kinematic link between
upper- and mid-crustal levels.
Alternatively, ring complexes at mid-crustal levels
may have simply provided arcuate, dike-like magma
pathways that fed intrusions, be they plutons or ring
complexes, at higher crustal levels. Connectivity to a
deeper magma plumbing system through these
‘‘rings’’ could provide the high magmastatic head
required by magma at shallower levels to intrude
upward through country rock of equal or lesser density.
As another alternative, deep ring complexes may
indicate that mid-crustal plutons can be partly constructed by steeply dipping arcuate sheets. Concentration of these sheets and the wall-rock screens in the
northwest and southeast parts of the Rinconada complex may indicate that they were dilational sites
during east –west crustal contraction. In this interpretation, such complexes may represent magmatic systems that never reached upper-crustal levels, rather
than systems that were linked to upper-crustal plutons,
complexes or calderas. The various possibilities
remain to be properly tested through detailed structural and petrological analyses.
5.3. Exposure levels along strike in the western PRB
As mentioned earlier, particular map patterns in
volcano-plutonic subsidence systems are indicators
only of their relative position within the system, and
not of their absolute crustal exposure depth. However,
the distribution of map patterns throughout a large area
might provide a qualitative means of evaluating exposure depth. Crustal levels exposed in the PRB vary
from < 5-km depth in the west to >20 km in the east
across a sharp crustal boundary in the transition zone;
however, exposure levels in the western PRB may also
vary systematically from shallow levels in the south to
deeper levels in the north, parallel with the trend of the
batholith. In the Turquesa area, approximately 60 km
south of San Quintin (Fig. 1), calderas were described
by Fackler-Adams (1997) and Fackler-Adams and Busby (1998). The Burra Caldera, which developed at the
summit of a large stratovolcano, is partially bounded by
regional normal faults, and includes an intra-caldera
silicic ignimbrite sequence and resurgent intrusions.
In contrast, approximately 100 km to the north, the
Zarza and Burro complexes may represent the roots of
calderas. The emplacement depths of some of these
complexes is uncertain, but Al-in-hornblende barometry in the Zarza complex (Johnson et al., 1999a)
indicates emplacement at 2.3 F 0.6 kbar ( f 5 – 9 km)
and is consistent with greenschist – facies or lower
regional metamorphism of surrounding country rocks.
Farther north, no unequivocal ring complexes have
been identified, but the El Pinal tonalite has been
interpreted to contain the roots of a granodiorite ring
dike within an otherwise relatively homogeneous tonalite pluton (Duffield, 1968; Fig. 7). Still farther north,
in San Diego and Riverside Counties, much more
extensive geological mapping has defined no unequivocal volcano-plutonic subsidence systems, although
the Paloma Valley ring complex is a possible candidate.
Thus, the following picture is beginning to emerge.
From the Turquesa area north to San Diego County,
S.E. Johnson et al. / Lithos 61 (2002) 187–208
205
over a distance of approximately 250 km, calderas in
the south give way to massive intrusions in the north,
with intermediate-level ring complexes and contemporaneous plutons in between. Calderas like those in
the Turquesa area are commonly exposed at depths of
2 km or less (Fig. 3c in Lipman, 1984), whereas
various studies in San Diego County indicate depths
of between 7 and 14 km just west of the transition
zone between western and eastern belts in the PRB
(Todd et al., 1994, and references therein). Between
these is the Zarza complex, which formed at 5- to 9km depth. Thus, it appears that a difference in
exposure level of f 5 – 12 km may occur over a
north – south strike-length of f 250 km. However,
discontinuities such as major transpeninsular faults
(e.g., the Agua Blanca fault) may disrupt an otherwise
continuous variation in crustal level, or perhaps even
separate markedly different arc components that
formed at different time, and are exposed at different
crustal levels (cf. Gastil et al., 1975, 1981; Schmidt et
al., in press).
PRB, but that their distinguishing features were typically destroyed by upward advance of the magma
chambers that fed them.
(4) Particular map patterns in volcano-plutonic
subsidence systems are indicators only of their relative
position within an individual system, and not of their
absolute exposure depth. However, calderas, ring
complexes and plutons may generally represent progressively deeper sections through volcano-plutonic
subsidence systems, and the systematic occurrences of
these features along strike in the western PRB are
consistent with progressively deeper exposures of the
batholith from south to north over a distance greater
than 250 km. This section exposes calderas at the
shallowest levels, plutons that fed them at deeper
levels, and the frozen remains of magma pathways
(ring complexes) between the two at intermediate
levels. Preservation of these three magmatic elements
in a single region is unusual, and provides an opportunity to determine if and how they linked together in
three dimensions to ‘‘plumb’’ the upper crust.
6. Summary and conclusions
Acknowledgements
(1) Subvolcanic ring complexes that we have mapped in the western PRB apparently developed in a
general, repeatable three-stage sequence of: (a) fracturing of the roof above a buoyant or overpressured
magma chamber, which resulted in conical fractures
that commonly hosted cone sheets; (b) subsequent loss
of magma from the chamber, combined with degassing
of the melt, which facilitated collapse of the roof along
near-vertical ring faults locally hosting ring dikes; and
(c) resurgence of the chamber, and/or intrusion of a
broadly cogenetic nested pluton, which locally destroyed evidence for the earlier history of the system.
(2) Ring complexes in the transition zone of the
PRB represent deeper plutonic levels (as deep as 18
km). These deeper complexes raise the question of
whether caldera/volcano-related processes can, in
some instances, extend to mid-crustal levels. Alternatively, deeper-level complexes may be unrelated to
volcano-plutonic subsidence systems; their mechanisms of formation are poorly understood.
(3) Ring complexes are relatively rarely preserved
at Earth’s surface. We suggest that they may have
been more common at shallower levels in the western
Our work in the PRB has been supported by: (1)
the U.S. National Science Foundation, which provided grants EAR 0087661 (to S.E.J.) and EAR
9614682 (to S.R. Paterson); (2) the Australian
Research Council, which provided Large Grant No.
A39700451 (to S.E.J. and R.H. Vernon) and a Queen
Elizabeth II Research Fellowship (to S.E.J.); and (3)
Grant 4311PT from the Consejo Nacional de Ciencia
y Tecnologia (CONACyT) of México (to S.E.J.). We
thank R.H. Vernon for comments on an early version
of the manuscript, and C.G. Barnes, R.V. Metcalf and
S.R. Paterson for thorough and constructive reviews.
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