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EOCENE TECTONIC EVOLUTION OF NW MEXICO
(SONORA and BAJA CALIFORNIA)
Luigi RADELLI(1) and Ariel G. NAVARRO-HERRERA(2)
Departamento de Geología, Universidad de Sonora
E-mail:
(1) [email protected].
(2) [email protected]
RESUMEN
Se toman en cuenta la evolución del Arco Volcánico Continental (Arco Huepac) del Eoceno de
Sonora por un lado; y los datos paleomagnéticos disponibles sobre el Conglomerado Poway del Eoceno
de San Diego, California. Se presenta una interpretación paleogeográfica y paleotectónica del NW de
México que permite conciliar los datos geológicos y geofísicos.
ABSTRACT
The evolution of the Eocene Volcanic Arc (Huepac Arc) of Sonora and the available paleomagnetic
data concerning the Eocene Poway Conglomerate of San Diego are considered. A paleogeographic and
paleotectonic interpretation of NW Mexico (Sonora and Baja California) during the Eocene is worked
out that is in agreement with both the geological and the paleomagnetic data.
PROLOGUE
During the Eocene the global system of
plates underwent a general reorganisation. This
is attested, for instance, by the following events
of that time: the main phase of the Alpine
Orogeny, one of the main phases of the Andean
Orogeny, the opening of several marginal seas
in Eastern Asia, the change of direction of the
movement of the Pacific and North America
Plates above the hot spots of Hawaii and Yellowstone respectively.
In connection with, and as part of, this global geodynamic revolution, the events discussed herebelow occurred in north-western Mexico.
* During the Early and Middle Eocene (from
±50 to ±40 Ma), a huge Continental Volcanic
Arc was built in Sonora, which has been called
Huepac Arc. It is a NNW-SSE-trending belt of
the axial part of the State, that contains the socalled Metamorphic Core Complexes (MCC) of
Magdalena and Mazatan–Huepac (fig. 1),
* As shown by paleomagnetic data, during
the Late Eocene (from ±40 to ±35 Ma) the western part of north-western continental México
together with adjacent California and Baja California – the Baja-Borderland terrane of Abbott
and Smith (1989) – was displaced 14º ± 5º (say,
some 1500 km, of which 300 km accounted for
by the San Andreas Fault) northward rotated
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L. RADELLI and A. G. NAVARRO-HERRERA
30º clock-wise (Abbott and Smith 1989: Gastil,
1991) at San Diego.
* During the same time interval (± 40 to ±35
Ma) the Huepac Arc was deformed and uplifted.
In particular, the shear zone in question has
been observed: (a) in the vicinities of Hermosillo, (b) along the Ures-Mazocahui road on the
THE HUEPAC ARC
(1) During the Early and Middle Eocene (±
50 to ±40 Ma), a huge Continental Volcanic
Arc was built in Sonora, where it was called
Huepac Arc. It occur as a NNW-SSE-trending
belt of the axial part of the State of Sonora, that
contains the so-called Metamorphic Core
Complexes (MCC) of Magdalena and MazatanHuepac (Fig. 1A and 1B) but is wider than the
belt of the MCC itself.
The evolution of the Huepac Arc has been
dealt with previously (Radelli et al., 1992; Lucero and Radelli, 1993; Radelli et al., 1995;
Radelli and Lucero, 1996a; Radelli and Lucero,
1996b; Radelli, 1998a; Radelli, 1998b; Radelli,
1999). Here, such discussion is not repeated
only its results are summarised.
The Huepac Arc was characterised by the
emplacement of both intrusive and effusive
rocks. These include deep-seated two mica ±
garnet-bearing granites and, at higher crustal
levels, cupolas of granitoids, and rhyolites, all
of them genetically related with a magma
underplating of the North American continental
crust.
Fig. 1 A – The Huepac Arc: the Cordilleran Inner Belt of two mica ± garnetbearing granites (of Eocene age in
Sonora) and of Metamorphic Core
Complexes; belt of Eocene cupolas.
(2) During the Late Eocene, at about 40Ma,
the Huepac Arc was affected by a low angle
zone of ductile shear that became a mylonitic
zone (Bronner and Radelli, 1996). In several
segments this zone has been subsequently folded to give way to the so-called metamorphic
core complexes, according to the model
proposed by Reynolds and Lister (1990); then
the Huepac Arc was uplifted.
Fig. 1B – The Cordilleran Inner Belt of
two mica ± garnet-bearing granites (of
Eocene age in Sonora) and of Metamorphic Core Complexes.
left side of the Valley of Rio Sonora and (c) in
the lowland at the base of the Mazatan Massif
(Anderson, Silver and Salas, 1980); (d) along
the Hermosillo-Tecoripa-Yecora road upon its
L. RADELLI and A. G. NAVARRO-HERRERA
143
Fig. 2 – Structure of a zone of ductile shear. Characters of a zone of simple
shear, as projected on the plane parallel to X and Z of the ellipsoid of the finite
deformation. The schistosity/foliation is defined by the X (main) and Y
(intermediate) axes of said ellipsoid. The direction of rotation is defined by the
pitch of the stretching X in relation to the surfaces limiting the deformation of
the shear zone. [After Steck, 1990]
crossing of the Yaqui River; (e) in the area of
Onavas-Realito near the Yaqui River, and (f) in
the sector of Santa Rosa northwards.
In all of these instances the shear zone cuts
through the Laramide (Late Cretaceous – Paleocene, 67 ±10 Ma) granitoids, and at (a), (b), (c)
and (d), it is easily recognized thank to the
deformation of the andesitic (s.l.) dykes that cut
through those granitoids and corresponding to a
late pulse of the Laramide magmatic phase, that
cut through those granitoids.
This point has to be stressed: the shear zone
is not developed between a basement and a
cover, but within two portions of the Laramide
batholite. The situation is similar of that of
ductile shear zones of the Central Alps (Western
Swiss Alps), that have been carefully studied by
Steck (1990).
A zone of ductile shear corresponds to a slab
of rocks affected by a unique schistosity/foliation. A zone of ductile shear (mylonitic zone)
is usually divided from the undeformed domains
by two roughly parallel sufaces (Fig.2).
At the sections of Ures-Mazocahui and Mazatan the zone of ductile shearing corresponds
to the tectonite gneiss level that, in the so-called
metamorphic core complexes divides the nontectonite basement (below) from, where it
exists, the décollement zone beginning with a
microbrecciated level, and from a slab of undeformed units (above) that includes the two mica
± garnet-bearing Eocene granite.
A basic character of this shear zone is that it
contains muscovite which, unlike the muscovite
of the two micas ± garnet-bearing granites
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L. RADELLI and A. G. NAVARRO-HERRERA
which is magmatic, is metamorphic in origin
and related to the deformation itself.
de granitoides are cut, at a higher level, by a
near horizontal zone of fault.
Also, along such fault the upper slab, containing Tarahumara volcanics (Upper Cretaceous
– Paleocene) and Eocene granitoid cupolas,
appears to have moved north-eastwards.
Thus, along the hereabove indicated shear
zone(s) the overriding un-deformed slab(s)
became allochthonous and was (were) translated north-eastwards after the Early and Middle
Eocene magmatism and prior to the Oligocene
(±35 Ma) deposition of the andesite and rhyolite of the Sierra Madre Occidental (see Radelli,
this volume), that is during the Late Eocene.
The time interval of such motion is the same
as that of the northward translation of the
western part of California, Sonora and of Baja
California, the Baja Borderland of Abbott and
Smith (1989), discussed by Gastil (1991).
It is worth noting that the north-eastern motion along the above discussed shear zone is
similar to that of about 150 km observed in the
western USA across the western Idaho suture
zone and northern Rocky Mountains (Leeman
et al., 1992).
Fig. 3 – Distribution of the MCC in Sonora and
Arizona. 1: Mazatan; 2: Madera; 3: Magdalena;
4: Pozo Verde; 5: Comobabi-Coyote; 6: Catalina-Rincon; 7: Tortolita; 8: Picacho; 9: Santa
Teresa-Pinaleno; 10: South Moutain-White
Thank; 11: Harquahalla; 12: Harcuvar; 13:
Whipple.
EAST OF THE HUEPAC ARC:
THE SIERRA MADRE OCCIDENTAL
Along the shear zone in question the undeformed upper slab, which contains the two
mica ± garnet-bearing granite and pegmatites,
has been moved north-eastwards.
The section of the Tecoripa-Yecora road
near the Yaqui River exposes the upper part of
the shear zone.
In the sector of Onavas-Realito, as well as at
that of Santa Rosa north-eastwards, the Larami-
East of the Huepac Arc there is the Sierra
Madre Occidental (SMOc). There,upon the
uplift of the Huepac Arc a basin was opened. It
will be filled up first by a mollasse, the
conglomerate, the Novosaigame Conglomerate (Bockhoven, 1980; Cochemé, 1985; Cochemé and Demant, 1991), and then (from ±35 to
±24 Ma), following a short lived andesitic
pulse, by the mostly Oligocene rhyolites and
ignimbrites that constitute the bulk of the
SMOc (Cochemé, 1985; Cochemé and
Demant, 1991).
The basement of this stratigraphic pile, and
of said basin as a whole, can be observed on
L. RADELLI and A. G. NAVARRO-HERRERA
145
morphism, and penetrative deformation (schistosity/foliation). This block corresponds to
the paleomagnetically defined “Baja – Borderland terrane” of Abbott and Smith (1989)
discussed here-below.
Fig. 4 - Simplified structural map showing the sub-horizontal attitude of the
Yaqui River Fault in the Yaqui Valley in
the vicinities of Onavas. 3: Post-Yaqui
River Fault volcanic and sedimentary
deposits. 2: Upper Triassic strata, Upper
Cretaceous to Paleocene volcanic and
sedimentary deposits, Sierra Madre
Occidental Volcanics, Baucarit Formation. 1 : Laramide granitoids.
scanty outcrops and as xenoliths in the
Oligocene rhyolites. It is characterized by
Rodinian metasedimentary rocks (Stewart al.,
2002), Palaeozoic and Triassic sedimentary
rocks, and Laramide (Late Cretaceous –
Paleocene: 67 ±10 Ma [Damon et al., 1983])
both intrusive (granitoids) and effusive (andesite and rhyolite of Tarahumara Formation)
igneous rocks. These igneous rocks shows
neither a metamorphic facies nor a penetrative
deformation, such as schistosity/foliation, that
could be assigned a Eocene geological age.
WEST OF THE HUEPAC ARC:
THE BAJA-BORDERLAND BLOCK
West of the Huepac Arc there is a block,
comprised of western Sonora and Baja, which
is entirely free from Eocene magmatism, meta-
Fig. 5 – Some localities of Sonora cited
in the text.
In Sonora, said block includes, in its northern part, the famous dolomitic sequences of
Caborca and San Luis, and their respective crystalline basements, Bamori and San Luis (Radelli et al., 1997; De la Cruz and Dorame, 2000);
the shallow water Cambrian deposits which do
not occur East of it; the Lower Jurassic volcaniclastic deposits of the Santa Rosa Formation,
that do not occur east of it, where only Upper
Jurassic volcaniclastic deposits are found. Furthermore and above all, the autochthonous Bisbee and Cerro de Oro Lower Cretaceous se-quences (sandstones and limestone), widespread east of it, are lacking in the BajaBorderland block. Within the Baja-Borderland
Block the Lower Cretaceous is represented in
Baja California by the Alisitos Arc and the
Olvidada Nappe (Radelli, 1988a, 1988b, 1990),
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L. RADELLI and A. G. NAVARRO-HERRERA
and in Sonora by several allochthonous bodies
(nappes) such as those of El Chanate and Cerro
Chino (Dorame, 2003).
Finally, between the latitudes of Hermosillo
and Guaymas the block in question consists of a
basement made up by un-deformed Upper Cretaceous granitoids, with minor bodies of possibly Palaeozoic and/or Mesozoic sedimentary
rocks, and a cover of Miocene volcanics (MoraAlvarez and McDowell, 2000).
On its western side the Baja-Borderland
Block is bordered by Eocene sedimentary
deposits: the already mentioned Poway Formation in the southern California (San Diego), the
Tepetate Formation in the Baja California.
THE EOCENE MOUNTAIN
CHAIN OF SONORA
In the area of San Diego (California) there
is the Eocene Poway Conglomerate, which had
its source in the area of Tajitos in northern
Sonora (Abbott and Smith, 1989).
Dealing with the above exposed paleomagnetic data concerning the Poway Conglomerate, Abbott and Smith (1989) stated that “any
boundary separating a paleomagnetically defined Baja-Borderland terrane” (or rather BajaBorderland block) “from the craton since Eocene time was at least 100 km east of the Gulf
of California in northernmost Sonora”.
The Baja-Borderland terrane of Abbott and
Smith (1989) appears to be an equivalent of the
here-above indicated block bounding to the
west the Huepac Arc. Thus, concerning such
block, the conclusion reached by said authors is
in agreement with that exposed here-above.
Gastil (1991) addressed directly the question
of the clock-wise rotation of 30º and the 14º±5º
northward translation of the Baja-Borderland
terrane of Abbott and Smith (1989) - a minimum of 1500 km. as far as the Californias are
concerned, [The subsequent displacement of
about 300 km along the San Andreas Fault,
from 4.5 Ma onward, occurred between Baja
and Sonora, that is, actually, between two
parts of the Baja-Borderland terrane, or block.]
Gastil (1991) assumed that the terrane (block)
in question could have travelled north along a
California-Oaxaca megashear.with a convexity
towards the West (implying, possibly, an accordance with the subduction zone supposed at
work along the pacific margin). It would seem
that he conceived this megashear as a simple
“linear” structure similar, for instance, to that of
the Insubric Line of the Alps or, say, that of the
Northern Fault of the Pyrenees, or even that of
the San Andreas Fault. Then, he devised three
optional positions of said supposed California –
Oaxaca megashear and looked for it in the
field – but he found none. Because of this
he concluded that “a variety of geological data
across the borderland-batholith boundary,
within the peninsula, across the Gulf of California, and within the State of Sonora, Mexico
appear to preclude the existence of a megashear along which peninsular California could
have travelled northward. […] the data of
paleomagnetism and other elements of geology
are in direct conflict. The resolution of this
enigma will require an important revision of
current assumption”.
At this point the question of the tectonic
significance of the belt of the Huepac Arc in
relation with the adjacent areas of NW Mexico
(Sonora and Baja) ought to be addressed directly. The main points to be considered are as
follows:
(a) The axial part of Sonora corresponds to
a belt that contains Early and Middle Eocene
magmatic products genetically related to a
magmatic underplating of the continental crust
of North America, which are lacking both eastand westwards of it.
(b) During the Late Eocene such belt has
been deformed into a general east-vergent
L. RADELLI and A. G. NAVARRO-HERRERA
structure to become a real arc, the Huepac Continental Volcanic Arc properly. The deformation is the result of a gently dipping, and subsequently folded, zone of ductile shear – i.e., the
mylonitic, and metamorphic, zone of its so-called metamorphic core complexes, the southernmost of which is that of Mazatan, Sonora.
(c) After the Eocene and prior to the opening
of the Gulf of California at ±4.5 Ma, the State
of Sonora was affected by at least three main
extensional phases (at ± 24 Ma, ± 20 Ma, and ±
10 Ma), which are discussed in a companion
paper (Radelli, this volume; see also Valenzuela et al., this volume).
Taken together, the points (a) and (b) establish the Huepac Arc as a generally east vergent belt of crustal deformation and thickening,
i.e. as an Eocene Mountain Chain, of which
the region of the present-day SMOc is the
foreland.
What stated at point (b) suggests that Gastil’s (1991) California-Oaxaca megashear(s)
was (were) could not show a convexity towards
the West; and that its convexity, if any, should
have been towards the East instead.
On the other hand, what indicated at point
(c) clearly suggests that an Eocene “ linear”
megashear, as that considered, and looked for,
by Gastil (1991), cannot have preserved such a
continuous simple character, for it has been
necessarily disrupted by the already mentioned
post-Eocene extensional deformations (see Valenzuela et al., this volume; Radelli a, this volume).
Thus, the lacking in north-western Mexico
of a self-evident, well preserved, Eocene “megashear” is not indication that “the data of paleomagnetism and other elements of geology
are in direct conflict”. Under the circumstances
147
the original megashear should be recognized
taking into account the contrasting stratigraphic
and deformational histories of the blocks that
by such megashear were divided: the Baja –
Borderland Block and the Huepac Arc.
PALEOGEOGRAPHY AND
TECTONIC EVOLUTION
To restore the Baja-Borderland Block to its
Eocene position taking into account the paleomagnetic data obtained on the Eocene Poway
Formation of San Diego, the Baja-Borderland
Block should be (a) moved some 1200 km
south-west-wards along the eastern limit of the
Baja-Borderland Block, bringing San Diego at
the latitude of Mazatlan (Fig. 6a); and, (b)
concomitantly rotated about 30º counter-clockwise at the latitude of San Diego.
The question is to establish around which
pivotal point such rotation took place.
It seems reasonable to associate the genesis
of the low angle deformation we have dealt
with in discussing the MCC of the Huepac Arc
of Sonora with the coeval clockwise rotation of
the Baja-Borderland Block.
As already mentioned, such deformation has
been recognized as far south as Onavas/Rio
Chico and Realito (Cuatro Hermanos mining
prospect), that is some 50 km north of the latitude of Guaymas. Still southwards, the Eocene
Huepac Volcanic Arc continues [it is well
known, for instance, at the Tayoltita Mine, Durango (Horner,1998)] but the Eocene deformation does not occur, at least on surface.
Thus, although the overall structural geology
of the area is incompletely explored as yet, it
seems permissible to think that the rotation in
question occurred around a pivot located not far
from the original location of the point of where
is Guaymas, at present (Fig. 6b), and where the
148
L. RADELLI and A. G. NAVARRO-HERRERA
Fig. 6 – Eocene motion of the Baja-Borderland Block according to paleomagnetic data on the
Poway Formation of San Diego, southern California.
On fig. 6a the Baja-Borderland Block is moved 1500 km southwestwards. On fig. 6b the BajaBorderland Block is then rotated 30º counter-clockwise. Thus, fig. 6b shows the original Eocene
location of the Baja-Borderland Block according to the paleomagnetic data.
The curve lines indicate the clockwise rotation of the Baja-Borderland Block during the Late
Eocene. (See text for discussion)
edge of the Proterozoic crust of North America
it is worth noting that the distribution and
would be (Valencia–Moreno et al., 2001). And
extent of the MCC of Sonora and Arizona ap-
L. RADELLI and A. G. NAVARRO-HERRERA
149
pears to be well in accordance with this scheme (Fig. 3)
A large space is comprised between the position of the eastern limit of the Baja-Borderland
Block prior to its clockwise rotation and that
consequent to said rotation (Fig. 6b).
There is no trace in Sonora of any oceanic
Eocene formation. As a matter of consequence,
it is not permitted to suppose that the rotation of
the Baja-Borderland Block occurred thanks to
the consumption of an oceanic crust. It has to
be thought instead that its rotation was made
possible by the crustal shortening shown by the
Huepac Arc, in fact by the genesis of the Eocene mountain chain of Sonora, the Huepac
Mountain Chain.
How did such shortening occur ?
To answer this question it is very much
worth remembering Ampferer’s (1906) with his
Verchluckung
(French:
engloutissement),
Hobbs’ (1914), Argand’s (1916), and Amstutz’s (1955) ideas on the genesis of the
arcuate geological structures as that of the
Huepac Mountain Chain. Their ideas (Radelli
and Desmons, 1997), that later on were
expressed as “A-subduction, underthrust flexures, or “intracontinental subduction” were
clearly formulated by Hobbs (1914) as follows:
Arcuate structure representing a reduplcation of strata in recumbent and ruptured
folds requires that the duplicated material shall
have migrated centripetally from outside the
arc […] The active force (thrust) which produced rock folds, instead of operating from
behind and above the anticline, is applied
below and in front. Continuation of the process
yields therefore not “overturned” and “over-
Fig. 7 – The Baja-Borderland Block
after the Eocene motion and prior to the
Pliocene (±4.5 Ma) opening of the Gulf
of California. Black:: Eocene Mountain
Chain of Sonora. Compare with Fig. 6
and Fig. 3.
150
L. RADELLI and A. G. NAVARRO-HERRERA
thrust” but underturned and underthrust
flexures […] Applied to the Alps, this requires
that the main active force concerned in their
folding came from the northwest, instead of the
southeast, as generally assumed.
Fig. 8 – Generalized structure of the Canadian Rocky Mountains. (After Debelmas and
Mascle, 1991)
According to this principle and to the fact
that the Baja-Borderland Block does not record
an Eocene deformation (as the case would be
had this unit “pushed” the adjacent one) the
origin of the east-vergent (s.l.) Huepac Mountain Chain should be related not to a eastvergent “pushing” of the Baja-Borderland, but
to a west-oriented (s.l.) underthrusting of the
block on the east of that chain itself, to an
intracontinental A-subduction, similar, for instance, to that of the Rocky Mountains of
Canada (Fig. 8). At least as an educated guess
said underthrusting may in turn be related to
the Eocene opening of the Rio Grande Rift.
North and east of Hermosillo several
allochthonous
Lower Cretaceous bodies
(nappes) have been put in evidence and
assigned a Oregonian (“Mid-Cretaceous”)
age of emplacement (e.g., Dorame, 2003).
The rationale for this was that the Oregonian
Orogeny (and Mountain Chain) was the
youngest such structure known in Sonora.
However, this cannot be maintained any
longer. In fact, on the one hand, it has been
shown in the foregoing that a huge, deepseated crustal shortening has taken place
there during the Eocene; and, on the other
hand, it has recently been proven that at
Sierra de los Chinos (Sahuaripa) an
allochthonous body of Lower Cretaceous
rocks (nappe) was also emplaced during the
Eocene at a higher structural level (see
Radelli et al., this volume). Thus, the
chronology of the emplacements of certain
nappes of Sonora shall have to be revised.
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