Bol. Depto. Geol. UniSon, 2005, Vols. 18 y 19, p. 153 - 174

OLIGO-MIOCENE EVOLUTION OF SONORA-BAJA CALIFORNIA, MEXICO - A SEGMENT OF THE NORTH AMERICAN MARGIN

Luigi RADELLI Departamento de Geología, Universidad de Sonora, Hermosillo, Sonora, México. email: lradelli@guaymas.uson.mx

ABSTRACT

The data obtained during a revision of the Cainozoic stratigraphic sequence of the eastern part of Southern Baja California (SBC) were integrated with previuos knowledge on the Cainozoic stratigraphy of Sonora. In this way it has been possible to recognize the main events that determined the Cainozoic evolution of this segment of the North America margin, prior to the Pliocene (±4.5 Ma) inception of the San Andreas Transform System.

During the Eocene a clastic sedimentation took place in southern California (Poway Con-glomerate) and in Southern Baja California (Tepetate Formation of conglomerate and sandstone). Toward the uppermost Eocene/lowermost Oligocene, upon the uplift of the Huepac Eocene Volcanic Arc, and of the Eocene Mountain Chain of Sonora (see Radelli and Navarro, this volume), the basin of SMO took origin eastwards.

In Sonora, during the Oligocene (±35 to ±25 Ma) the Novosaigame Conglomerate was covered by the famous rhyolitic-ignimbritic volcanic pile of Sierra Madre Occidental (SMOc). At about 25 Ma an extensional phase intervened: within the western part of SMO the volcanism became bi-modal, whereas still new basins, the so-called "Belleza" and Magdalena" basins, were opened westwards. These latter were filled up by lacustrine, detrital, sometimes boratiferous, deposits and interlayered flows of basalts, that in places show a trachyandesitic character. At ±20 Ma a short-lived pulse of fracturing allowed the empla-cement of a number of huge rhyolitic dykes. It was followed by one of the main extensional phases, the Baucarit extensional phase. Then, a number of intramontane, fluviatile basins were opened in western Sonora. There, unconformably upon all of the pre-existing rocks, were accumulated, prior to 12.5 Ma, conglomerates, sandstones and flows of basaltic andesites, collectively termed Baucarit Formation.

In Western Sonora new deposits began to be accumulated upon a Mesozoic basement only during the early Miocene : Gipo Baleado Tuff (22.5 Ma), unconformably followed by the LaEsperanza Andesite (22.7 Ma). Also in this block the ±20 Ma old rhyolitic dykes occur (Rancho El Gavilan).From 18.8 to 15.3 Ma, whereas the Baucarit Formation is depsited eastwards, in this western sonoran block the El Mezquite Dacite is accumulated, which is intruded by a granophyre. And between 15 and 12.5 Ma both the El Mezquite Dacite and the granophyre intrusive into it were unconformably covered by La Pasion Andesite.

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In the eastern part of Southern Baja California during the Oligocene-middle Miocene time span (until ±12. 5 Ma) a thick (> 1000 m) detrital continental sequence (sandstone and conglomerate) with interlayered andesitic and basaltic flows, aglomerates, and tuff, was deposited on its eastern part. This sequence is comprised of El Salto and Pelones Formations in the area of Concepción Bay, and of Las Parras Formation in the area of Loreto and Sierra de la Giganta. It appears to grade westwards into the marine formations of San Gregorio and San Isidro of the Southern Baja's Pacific coast. The unconformities of this time-span noted in Sonora do not occur in Southern Baja. A link between these formations and the coeval ones of Sonora is established, however, by interlayered or cross-cutting magmatic products (for instance: 28 Ma tuff of El Salto Formation, 17 to 15 Ma andesitic dykes cutting the lower part of the Las Parras Formation, 13.3 Ma basaltic flows interbedded in the upper part of this same formation). Also, to the ±20 Ma rhyolitic dykes of Sonora correspond in SBC coeval tonalitic intrusions into the Pelones Fomation and dacitic intrusions into the Las Parras Formation.

At 12.5 Ma the subduction zone, active until then at the western limit of Baja, ceased to exist and its place was taken by the Tosco-Abreojos Transform Fault. This lithospheric reorganization brought about a new extensional phase all over the area. In Sonora, the Baucarit deposits were tilted and rotated by listric faults. In western Sonora, they were subsequently unconformably covered by the ±12-10 Ma acidic volcanics, the Lista Blanca Volcanics. In SBC the Las Parras Formation was affected by a regional extensional cleavage, and, at least locally, gently tilted. Then, between ±12 and ±10 Ma, the Las Parras Formation of Sierra de la Giganta and, in the southern part of the Concepción Peninsula, the Pelones Formation were unconformably, or at least disconformably, covered by the clastic deposits with a tuffaceous matrix of the Comondú and Hornillas Formations, respectively. Further eastwards, beginning with the Minitas Formation of the northern part of the Concepción Peninsula, a conspicuous gypsiferous belt appears at El Carmen Island and extends itself northwards to Mulegé, Santa Rosalia and as far as Laguna del Gato (a total distance of some 250 Km). This gypsiferous structure, the main result of the aforementioned extension, can be interpreted as an aborted rift, or at least as a Protogulf of California.

In western Sonora the La Ventana Formation was unconformably covered by the tuff of the Sanat Ursula Formation (12.3 to 11.4 Ma). In eastern Southern Baja California a short-lived acidic volcanic activity occurred in the area of Santa Rosalia (copper bearing Boleo Formation) and, westwards, in that of San Ignacio - Sierra San Francisco.

A new extensional phase intervened at ±10 Ma, which both in geological age and tectonic style corresponds to the classic Basin and Range extension of the western USA. In western Sonora, the ±12-10 Ma Lista Blanca Volcanics, together with the upper part of their basement, were rotated and attenuated by a set of ±SW-dipping listric faults. Such crustal attenuation was strong enough to give way to an elongated marine embayment, which is known as the Paleogulf of California. In western Sonora, the afore mentioned tilted rocks were unconformably covered by still flat-lying ±8-6 Ma old basalts. The ±10 Ma old listric faults of Sonora do not occur at surface in SBC, meaning that before reaching it they had already entered a ductile zone. As a consequence the Paleogulf of California was an asymmetrical structure, which is still revealed by the morphology and the bathymetry of the continental crust on either side of the Gulf of California. Nevertheless, that deep-seated extensional conditions existed then in Southern Baja is clearly suggested by the huge blanket

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of almost flat-lying basalts and trachyandesitic basalts (Comondú Trachyandesitic Basalt, Ricason Formation, Pilares Formation, Esperanxa basalt) erupted there between ±10 and ±6 Ma. As a whole, the structural pat-tern under discussion suggests the prelude of a rift. It was superseded anyway by the Pliocene (±4.5 Ma) intervention of the San Andreas Transform System, that opened the Gulf of California, and caused Sonora and Baja to belong in two different plates.

RESUMEN

Los datos obtenidos en una revisión de la secuencia estratigráfica cainozoica de la parte oriental de Baja California Sur han sido integrados con conocimientos previos de la estratigrafía cainozoica de Sonora. De esta manera ha sido posible reconocer los principales eventos que determinaron la evolución de este segmento de la margen de Norte América durante el Oligoceno y el Mioceno, es decir antes de la intervención del Sistema Transformante de San Andrés a los 4.5 Ma (Plioceno).

Durante el Eoceno una sedimentación clástica tuvo lugar en la parte occidental del Sur de Cali-fornia (Conglomerado Poway) y de Baja California Sur (conglomerados y areniscas de la Formación Tepetate). Hacia el final del Eoceno/comienzo del Oligoceno, en relación con el levantamiento del Arco Volcánico Huepac del Eoceno, y de la Cadena de Montañas del Eoceno de Sonora (veáse Radelli and Navarro, este volumen), tomó origen al Este la cuenca de la Sierra Madre occidental (SMOc), donde se depositó en Conglomerado Novosaigame.

Durante el Oligoceno (±35 a ±25 Ma), en Sonora oriental el Conglomerado Novosaigame fue cubierto por la famosa secuencia riolítico-ignimbrítica de la SMOc. Alrededor de los 25 Ma intervino una fase distensiva : en la parte occidental de la SMOc el volcanismo se volvió bimodal, mientras que nuevas cuencas – las así llamadas Cuencas « Tubutama », « Magdalena » y « Belleza » se abrieron más al Oeste. Estas cuencas se rellenaron de depósitos lacustres, detríticos a veces boratíferos con intercalaciones de flujos basálticos, que a veces presentan un caracter traquiandesítico. A ± 20 Ma una corta fase de fracturación permitió el emplazamiento de numerosos grandes diques riolíticos. Dicha fase fue seguida por una de las mayores fase distensivas, la fase Báucarit. Varias cuencas fluviales intramontanas se abrieron entonces en Sonora occidental. Entre ±20 y 12.5 Ma se acumularon en esas cuencas, y discordantes sobre todas las rocas pre-existentes, conglomerados, areniscas con flujos intercalados de andesitas basálticas, depósitos conocidos bajo el nombre de Formación Báucarit.

En Sonora Occidental los depósitos empezaron solamente en el Miocene inferior: Tobas Gipo Baleado (22,7 Ma), seguida en discordancia por la Andesita La Esperanza (22.7 Ma). En este bloque occidental de Sonora también se encuentran los diques riolíticos a los cuales se asigna una edad radiométrica de ±20 Ma. Entre los 18.8 y los 15.3 Ma, cuando al Este se acumula la Formación Báucarit, en el bloque occidental ocurre la Dacita El Mezquite, misma que es afectada por la intrusión de un granofiro. Entre 15 y 12.5 Ma tanto la Dacita El Mezquite como la intrusión granofírica estám cubiertas en discordancia por la Andesita La Pasión.

En la parte oriental de Baja California Sur durante el Oligoceno y Mioceno inferior y medio (hasta ±12.5 Ma) se depositó una espesa (> 1000 m) secuencia continental de areniscas y conglo-merados con intercalaciones de flujos andesíticos y basálticos, aglomerados y tobas. Esta secuencia

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incluye las Formaciones El Salto y Pelones del área de Bahía Concepción, y la Formación Las Parras en el área de Loreto y de Sierra de la Giganta. Hacia el Oeste esta secuencia pasa progresivamente a las Formaciones marinas de San Gregorio y San Isidro de la costa pacífica de la Baja California Sur. Las discordancias que en el periodo considerado ocurrieron en Sonora no existen en Baja Califoria Sur. Un enlace entre las formaciones citadas de Baja California Sur y las de Sonora se establece sin embargo gracias a las rocas ígneas que se intercalan en ellas o las cortan (por ejemplo : toba fechada 28 Ma de la Formación El Salto; intrusiones de ±20 Ma, tonalíticas en la Formación Pelones y dacíticas en la Formación Las Parras, que corresponden a los diques riolíticos de la misma edad radiométrica de Sonora ; diques andesíticos de 17 y 15 Ma que cortan la parte inferior de la Forma-ción Las Parras, y flujos basálticos de 13.3 Ma intercalados en la parte superior de esa misma formación.

A los 12.5 Ma la zona de subducción activa hasta entonces al límite occidental de Baja dejó de existir, su lugar siendo tomado por la Transformante Tosco-Abreojos. A esta reorganizción litosférica siguió en toda la región considerada una nueva fase distensiva. En Sonora los depósitos Báucarit fueron basculados y rotados por fallas lístricas. Tanto los depósitos Báucarit como la Andesita La Pasión fueron entonces cubiertos discordantemente, entre ±12 y ±10 Ma, por las rocas volcánicas ácidas de la Formación Lista Blanca y de la Formación La Ventana (12.3–11.4 Ma), respectivamente. Contem-poráneamente, en Baja California Sur la Formación Las Parras fue afectada en su conjunto por un clivaje distensivo y localmente ligeramente inclinada. Y entre ±12 y ±10 Ma la Formación Las Parras de la Sierra de la Giganta y la Formación Pelones de la parte sur de la Peninsula Concepción fueron cubiertas en discordancia angular, o por lo menos en disconformidad, por los depósitos clásticos con una matriz tobácea de la Formaciones Comondú y Hornillas respectivamente. Más al Este, empezando con la Formación Minitas de la parte norte de la Peninsula Concepción, aparece un conspicuo cinturón yesífero, mismo que se desarrolla desde la Isla El Carmen hasta la Laguna del Gato, pasando por Mulegé, la Isla San Marcos y Santa Rosalía, es decir sobre una distancia de unos 250 km. Esta estructura yesífera es el resultado principal de la ya mencionada distensión y puede ser interpretada como un rift abortado, un Protogolfo de California.

En Sonora occidental la Formación La Ventana fue entonces cubierta discordantemente por las tobas de la Formación Santa Ursula (12.3 a 11.4 Ma). Mientras, en la parte oriental de la Baja California Sur una corta fase de actividad volcánica ácida tuvo lugar en las áreas de Santa Rosalía (Formación cuprífera El Boleo) y de San Ignacio-Sierra de San Francisco más al Oeste.

Una nueva fase distensiva intervino a los ±10 Ma, la cual tanto por su estilo como por su edad corresponde a la clásica distensión Basin and Range del Oeste de los Estados Unidos de América. En Sonora occidental las rocas volcánica de la Formación Lista Blanca (±12 a 10 Ma), junto con la parte superior de su basamento estratigráfico. Fueron rotadas y atenuadas por un conjunto de fallas lístricas con echado al SW. Esta atenuación cortical fue lo bastante importante por dar lugar a una cuenca marina alargada, conocida como Paleogolfo de California. Las formaciones de Sonora occidental mencionadas arriba fueron entonces cubiertas discordantemente por basaltos todavía horizontales de ±8 a 6 Ma. Las fallas lístricas de ±10 Ma que afloran en Sonora no ocurren en la superficie en Baja California Sur, lo que sugiere que antes de alcanzarla habían llegado a la zona dúctil. Como consecuencia de este hecho, la de Paleogolfo de California era una estructura

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asimétrica, todavía mostrada por la morfología y la batimetría de los dos lados del Golfo de California. Sin embargo, la existencia en ese tiempo de condiciones distensivas profundas en Baja California Sur está claramente indicada por una espesa cubieta de basaltos y traquiandesitas basálticas (basaltos traquiandesíticos Comondú, Formaión Ricasón, Formacón Pilares. basalto Esperanza) emplazados entre ±10 y ±6 Ma. El modelo estructural resultante sugiere el preludio de un rift. Este sin embargo ha sido suplantado por la intervención del Sistema Transformante de San Andrés que abrió el Golfo de California e hizo que Sonora y Baja pertenezcan a dos placas diferentes.

INTRODUCTION

Prior to the Pliocene (± 4.5 Ma) inception of the San Andreas Transform System, and the consequent opening of the Gulf of California (fig. 1), Sonora and Baja California were two adjacent strips of a single structure, the Cainozoic North American Margin, bounded, until 12.5 Ma (Klitgord and Mammerickx, 1982 ; Mammerickx, J. and Klitgord, K.D., 1982) by a subduction zone.

However, these two strips have seldom been considered together in order to clarify the Cainozoic evolution of said margin as a whole. And when this has been done, it has been done

taking into account, essentially, its volcanic products (e.g., Gastil et al., 1979; Radelli et al., 1993), thus mostly disregarding its sedimen-tary formations.

There are two reasons for this. The first one is that most of the geologists who worked in Baja California did not know Sonora, and vice versa. The second one is an unruly employment in Baja California, and particularly along its western side, of the litho-stratigraphic terms (formations), which precluded all stratigraphic and tectonic

reasoning and interpretation. For instance, the term of Co-mondú Formation was introduced by Heim (1922) for a Upper Miocene sequence beginning

Fig. 1 – General outline of Sonora, Gulf of California and Baja California.

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with clastic rocks and containing at its top ±10-6 Ma old volcanics of the Comondú area. But, later on , this same term has been applied to a number of clastic and volcanic occurrences of western Baja, irrespectfully of their stratigraphic position and geological age (Radelli and Ortiz, 1995). As a consequence, whereas the Comondú Formation of Heim (1922) is capping the Sierra de la Giganta, according to the post-Heim (1922) geological literature, a supposed “Comondú Formation” would constitute the base of the Cainozoic stratigraphic pile of Western Southern Baja California, in particular at Loreto, Bahia Con-cepción and Santa Rosalia, where Oligocene-Lower Miocene both sedimentary and igneous rocks actually occur.

It is the purpose of this paper to try and fill

up this gap. What follows is based on previous knowledge of the Cainozoic stratigraphy and tectonics of Sonora, acquired by the author during some 20 years of work there; on a field revision of the Cainozoic stratigraphy of Southern Baja California carried out, from 1995 to 1998, by the author and one of his students (Ortiz, 1998); and, finally, on a chro-nostratigraphic correlation between the Caino-zoic formations of both areas.

STRATIGRAPHY AND TECTONICS

EOCENE

The stratigraphy and tectonics of the Eocene has been dealt with in a companion paper (Radelli and Navarro-H., this volume), so that they will be only summarized here. During the Early Eocene (±50 to ±40 Ma), western Sonora and Baja California – the

Baja–Bor-derland terrane of Abott and Smith (1989) – were located some 1500 km southwestwards of their present position, so that San Diego was oughly at the latitude of Mazatlan.

During that time span (±50 to ±40 Ma) due to a magma underplating, a huge Conti-nental Volcanic Arc, the Huepac Arc, which included both intrusive and effusive products, was built up in what is now axial Sonora (Radelli et al., 1992; Lucero and Radelli, 1993; Radelli et al., 1995; Radelli and Lucero, 1996a; Radelli and Lucero, 1996b; Radelli, 1999). During the late Eocene (±40 to ±35 Ma) the Baja-Borderland terrane of Abbott and Smith (or Baja-Borderland Block) was moved north-eastwards and rotated some 30º clockwise (Fig. 2). Then the Huepac Arc was affected by a strong compressional phase (Bronner and Radelli, 1996), and uplifted.

Upon its uplift such arc became the struc-tural backbone of Northwest Mexico, boun-ding the basin of the Sierra Madre Occidental

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Fig. 2 – A outline of the Baja-Borderland Block and of the Eocene Huepac Arc. Baja California is shown in its position prior to the opening of the Gulf of California. 1 : Baja-Borderland Block and 2 : its Eocene deposits ; 3 : Early Eocene

Huepac Arc ; 4 : Oligocene basin of the SMOc

(SMOc) on the east [It is worth noting that, in order to explain the clastic sediments of Baja (see in particular their fig. 2 A), Karig and Jensky (1972) had already postulated such an arc. However, for wanting of valid stratigraphic and chronological data they postulated its existence at a wrong date (25 Ma).] During latemost Eocene, the erosion of the Eocene Arc furnished detritus to the Novosaigame Conglo-merate with which began the Cainozoic history of the basin of the SMOc (Bockoven, 1980); Cochemé, 1985; Cochemé and Demant, 1991). During the Eocene, the western side of Southern Baja California was characterised by the deposit of the chiefly conglomeratic Tepe-tate Formation.

OLIGOCENE to EARLY MIOCENE (± 20 Ma) OF SONORA

In eastern Sonora, during the Oligocene-early Miocene (±35 to ±20 Ma) the Novo-saigame Conglomerate was covered by the volcanic pile of the Sierra Madre Occidental. This pile consists, upwards, and above some andesiditic flows, of the huge rhyolitic-ignim-britic sequence that actually characterizes that Sierra, and, beginning at about 25 Ma, of a bi-modal volcanic sequence of alternating rhyolite and andesitic basalt (Cochemé, 1985; Cochemé and Demant, 1991).

It should be noted that the rhyolitic-ignimbritic activity is divided from the bi-modal volcanism by an intervening defor-mational phase. Such phase is revealed by a number of syncline-like structures that affect the rhyolites-ignimbrites exclusively (Radelli,

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1986). The bimodal sequence is accompanied in place by sedimentary clastic deposits, often misinter-preted in the past as bilonging to the Baucarit Formation (see below).

In the axial part of Sonora (fig. 3), to the

west of the Sierra Madre Occidental proper (Baviacora, La Estrella, Mazatan, Magdalena-Tubutama), a concomitant extensional phase occurred as well at about 25 Ma. It gave way to the exhumation of the Sonora Metamorphic Core Complexes of Magdalena and Mazatan and to the opening of several lacustrine basins, generally called « Magdalena », « Tubutama », and « Belleza » basins.

Fig. 3 – Map showing the localities of Sonora mentioned in the text.

Concomitantly with the bimodal volcanism

of the SMOc, these basins were filled up by clastic and sometimes boratiferous deposits, and by interlayered sub-alkaline basaltic (s.l.)

flows (Miranda et al, 1998; Calles, 1999). The above mentioned syncline-like structure of the SMOc that con-tain the bimodal volcanic rocks and the clastic deposits that accompany them appear as correlative with the « Mgdalena », « Tubutama » and « Belleza » basins of axial Sonora.

It is possible that other basins of this type existed also in western Sonora, as for instance in the Sierra of Santa Ursula, near Guaymas, where 23.5 Ma old volcanic rocks of inter-mediate composition (Grijalva and Roldan, 1998) occur below the 12-10 Ma Lista Blanca Volcanics (see below).

In western Sonora (Mora-Alvarez and

McDowell, 2000) the first Cainozoic deposit resting upon the Paleo-( ?)-Mesozoic base- ment is that of the ignimbritic Gipo Baleado Formation (23.5 Ma). This formation is un-conformably followed by the La Esperanza Andesite Formation (22.7 Ma) , which consists of andesite and interlayered sandstone hori-zons.

Thus, prior to ±20 Ma (see below) western Sonora still preserves its individuality from the rest of Sonora as it had done during the Eocene, when it was divided from it by a transcurrent fault (Radelli and Navarro-H., this volume)

OLIGOCENE to MIDDLE MIOCENE (±35 to ±20 Ma) OF SOUTHERN BAJA

During the Oligocene-middle Miocene (until 12.5 Ma) interval, a detrital sedimentation occurred in eastern Southern Baja (Figs. 2 and 4).

In the area of Concepción Bay (McFall,

1968) this sedimentary pile includes two

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formations: the El Salto Formation, and the Pelones Formation (Fig. 5).

The El Salto Formation unconformably co-vers a pre-Cainozoic basement, including some Cretaceous (?) intrusive body. It consists of mostly quartzitic, fine to middle grained, poorly sorted and poorly rounded, cross bedded red-dish sandstones and conglomerates, with inter-bedded tuff radiometrically dated (K/Ar) 28 Ma (late Oligocene).

The Pelones Formation rests conformably upon the El Salto Formation. It consists of a huge pile (> 1000 m) of volcaniclastic deposits (mostly andesitic agglomerates and conglome- rates), with interbededd basaltic flows and tuffaceous sandstone, intruded by ± 20 Ma old tonalitic bodies (see below).

Fig. 4 – Map showing the localities and the geographic units of Southern Baja California mentioned in the text.

In the area of Loreto and Sierra de la Giganta, the morphologic backbone of Sou-thern Baja, the equivalent of the El Salto and the Pelones Formations is the also detrital, about 1000 m thick, sub-horizontal to gently tilted, Las Parras Formation (Ortiz, 1998).

The lowermost part of Las Parras Formation, possibly in contact with the pre-Cainozoic basement, does not crop out in the here considered area. Its exposed section begins with cross-bedded, fine grained depo-sits (shales) interfingering with conglomerate, sandstones containing lenticular bodies of microconglomerate with volcanic fragments, and some interbedded basaltic flows. Up-wards, the Las Parras Formation becomes chiefly conglomeratic, and includes inter-bedded andesitic and basaltic flows as young as 13.3 Ma (Gastil et al., 1979). The matrix of the conglomerates is generally sandy, some-times tuffaceous. Its colour varies from brown to orange, and to red. The clasts of the conglomerates are of volcanic rocks, including andesite, dacite, basalts, and tuff. In the eastern part of Sierra de la Giganta they are poorly sorted and gently rounded, and vary in size from some 5 cm to about 70 cm; but the deposits of this formation become much less coarse towards the west.

The Las Parras Formation has been

intruded first by ±20 Ma old dacitic bodies (see below), and then, in its lower part, by from 5 to 20 m thick andesitic dykes, radiometrically (K/Ar) dated 16±2.0 and 15.4±0.8 Ma (Gastil et al. 1979).

The Las Parras Formation is affected by a strong, regional, extensional vertical cleavage related to a normal faulting , a character that is not shown by the overlying, younger forma-tions (e.g., the Comondú Formation - see below).

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The age of the upper limit of the Las Parras and its equivalent formations is not perfectly constrained.

However, taking into account (i) the 13.3 Ma radiometric age of some of its basalt on the one hand, and (ii) the uncon-formity that divides it from the overlying Comondú Formation, it is permissible to relate such limit, as well as the regional clevage of the Las Parras Formation with the 12.5 Ma old ceasing of the subduction at the western limit of Norhtwest Mexico (Klitgord and Mamme-rickx, 1982; Mammerickx and Klitgord, 1982), it can be assigned to the Oligocene-middle Miocene (<12.5 Ma) interval.

In western Southern Baja the equivalent of

the Las Parras, El Salto and Pelones Forma-tions are the Oligocene San Gregorio Forma-tion of the area of La Purisima (Applegate, 1986), and the early Miocene San Isidro Formation, into which they appear to grade, the transition occurring below the cover of the overlying Comondú Formation.

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Fig. 5 – Geological Map of Concepcion Bay area (after McFall, 1968, simplified). 1 : Quaternary ; 2 : Infierno Formation (Pliocene) ; 3 : Pilares and Ricason Formations (uppermost Miocene, ±10 to ± 6 Ma basalts). 4 : Hornillas and Minitas Formations (upper Miocene, ±12 to ±10 Ma, detrital-tuffaceous and partly gypsiferous deposits) : 5 : ±20 Ma Tonalites ; 6 : El Salto and Pelones Formations (pre-12.5 Ma, Oligocene to Miocene, clastic, volcaniclastic and volcanic deposits) ; 7 : Pre-Cainozoic basement (mainly intrusive rocks)

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Fig. 6 – Geological Map of the area of San javier (Sierra de la Giganta, west of Loreto).

1 : Undiffeterentiated Quaternary ; 2 : Young, probablu Quaternary basalt ; 3 : Comondú Trachyandesitic Basalt (uppermost Miocene, ±10 to ±6 Ma) ; 4 : Comondú Formation (upper Miocene, ±12 to ±10 Ma) ; 5 : ±20 Ma Intrusive dacitic bodies ; 6 : Las Parras

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Formation (pre-12.5 Ma, Oligocene to Miocene, clastis, volcaniclastic and volcanic deposits).

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The San Gregorio Formation, which uncon-formably covers the Tepetate Formation, con-sists of phosphatic shales, siliceous shales, diatomites, phosphatic sandstones, with inter-bedded volcanic rocks dated 25 and 23.4 Ma (the time of the bi-modal volcanism of Sierra Madre Occidental and of the Belleza-type basins of Sonora).

The San Isidro Formation of the western slope of Sierra de la Giganta consists of conglomerate with well rounded clasts, sandstone, tuffaceous sandstone and shales. These are near-shore, shallow water, neritic deposits, probably accumulated under lagoonal conditions (Hausback, 1984).

THE ± 20 Ma EPISODE

At about 20 Ma (Miranda et al., 1998), a number of tens of meters wide and several kilometers long, rhyolitic dykes, which are either ±E-W (Puerta del Sol, north of Her-mosillo, Rancho Gavilan) or ±NW-SE (Mag-dalena) oriented where emplaced in Sonora west of the Sierra Madre Occidental, including into the western Sonora Block, formerly part of the Baja-Borderland Block (Abbott and Smith, 1989).

The Pelones Formation is crossed by tonalitic bodies dated (McFall, 1968; Gastil et al., 1979) 20±2 Ma (fig. 5).

A number of dacitic bodies appear in the lower and middle part of the Las Parras For-mation (fig. 6). They are porphyritic, with well developed (1 to 2 cm long) phenocrists of hornblende, and less developed plagioclase (< 1 cm). They occur as sills and dykes, these lat-ter blowing up, locally, to form stocks and/or plugs. Their radiometric (K/Ar) age is 20.6±1.4 and 22.9±2.7 Ma (Gastil et al., 1979) as. Thus, and this is an important point, they appear as fairly coeval with the rhyolitic

dykes of Sonora and the tonalites of Con-cepción Bay.

As we shall see herebelow, the empla-cement of the ±20 Ma rhyolitic dykes was followed in Sonora by the Baucarit extension and related sedimentation. Under these circum-stances, it seems safe enough to conclude that the emplacement of the ±20 Ma rhyolitic dykes of Sonora, of the tonalites of Concepción Bay, and of the dacites of Sierra de la Giganta is related to a huge fracturing of the whole area (see Valenzuela et al., this volume). And it is worth noting that said fracturig and the emplacement of the ±20 Ma old intrusive bodies appear as the first events affecting both axial Sonora and the former Baja-Borderland Block. MIOCENE, FROM ±20 TO 12.5 Ma, OF SONORA

Following the emplacement of the already mentioned ±20 Ma rhyolitic dykes, a major extensional phase, that I call the Baucarit extensional phase, developed itself in Sonora (fig. 7), affecting the western part of the Sierra Madre Occidental and the eastern part of central Sonora (Cochemé, 1985; Radelli, 1989; Bartolini et al., 1991, 1994). There, this extension gave way to several elongated, parallel basins, which at

L. RADELLI 164

Fig. 7 – Sketch-map showing the area where the Baucarit Extension occurred.

(See text for discussion) present correspond mostly to valleys. These basins were filled up by clastic deposits, produced by the erosion of the sequences of the ranges delimiting them, and by volcanics. The clastic deposits in question constitute the Baucarit Formation, which con-sists of well consolidated fluvial and fluvio-lacustrine conglomerate and sandstone.

The volcanics occur as flows of basaltic andesite interlayered within the clastics, and also overlying them, as in the case, for instance, of the basaltic andesites, that began at 17 Ma, of the Tepoca’s (Cochemé, 1985, Cochemé and Demant, 1991).

The Baucarit Formation unconformably co-vers the bimodal volcanics of Sierra Madre Occidental, the “Belleza” basins, as well as (Puerta del Sol) the ± 20 Ma rhyolitic dykes.

It has been argued, including by this author and one of his students (Sosa, 2000), that the Baucarit basins were opened by listric faults. However, such an interpretation and therefore it has to be rejected for it cannot give a reason

for the volcanics of those same basins. Possibly, they opened as a series of rifts, or pull-apart basins.

The Baucarit Formation does not occur in western Sonora, that is west of the La Colorada - El Gavilan Line (fig. 7), and thi is a point worth discussing. We have seen that the intrusions of the ±20 Ma old rhyolitic dykes affected both axial Sonora and the Baja-Borderland of western Sonora. However, the western limit of the Baucarit extension, the Gavilan-La Colorada Line, suggests that during Baucarit time (±20 to 12.5 Ma) the Baja-Borderland Block mantained a sort of indivi-duality.

In Southern Baja the Baucarit tectonic and sedimentary phase is not recognizable (or could not have been recognized yet) as a sepa-rated entity. Nevertheless, its influence is clear-ly suggested by the 16 ± 2.0, 15.4 ± 0.8 Ma, and 13.3 Ma (Gastil et al., 1979) igneous pulses that occurred there during the Las Parras interval. There, the ±20 to 12.5 Ma time-span (the Baucarit time in Sonora) is represented by the the volcaniclastic and beds covering the Oligocene-middle Moi-cene San Isidro Formation and the Las Parras Formation of Sierra de la Giganta, constituting the Comondú Formation as redefined in the next chapter..

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Fig. 8 - ±20 to 12.5 Ma Baucarit sedimentation in the eastern part of Sonora and Comondú sedimentation in the western part of the Baja-Borderland Block.

LATE MIOCENE, FROM 12.5 TO ±10 Ma

A new tectonic setting took place in North-west Mexico at 12.5 Ma, when subduction ceased, and the place of the former subduction zone was taken by the Tosco-Abreojos Transform Fault (Klitgord and Mammerickx, 1982; Mammerickx and Klitgord, 1982), which is shown on fig.1.

Consequent to this lithospheric reorga-nization, new tectonic, sedimentary, and igneous conditions were put in force at crustal level both in Sonora and Southern Baja.

(a) Sonora

In Sonora the Baucarit deposits were tilted, probably by west-dipping normal listric faults, and then, roughly from the Yaqui River westwards, unconformably covered by a huge pile of rhyolites, ignimbrites and dacites (fig. 9).

Fig. 9 – Map showing the areal distribution of the ±12-10 Ma acidic Lista Blanca Volcanics of Sonora.

This volcanic pile, sometimes termed

the Lista Blanca Volcanics, has been radiome-trically dated 12 to 10 Ma (Bartolini et al., 1991, 1994; Radelli et al., 1993).

The thickness of this volcanic unit increases westwards, reaching its maximum in the Sie-rras of El Bacatete, La Ventana and Santa Ursula, and near the coast of Sonora in the vicinities of Guaymas-San Carlos (Sheridan et al., 1979; Cochemé, 1985; Radelli et al., 1993).

In the Sierra Santa Ursula the position of the Lista Blanca is taken by the La Ventana and the Santa Ursula Formations

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(Mora and McDowell, 2000). The La Ventana Formation, which unconformably rests on La Pasion An-desite, consists of lava flows, domes, and minor ignimbrites radiometricaly dated 12.3 to 11.4 Ma. Thus, beginning at 12.5 Ma the Baja-Bor-derland Block and axial Sonora underwent a common geologic evolution. The Santa Ursula Formation, which rests unconformably on that of La Ventana, consists of ignimbrites, youn-ger than 11.4 and older than 8.5 Ma. In fact. 10.3 to 8.5 Ma old tholeiitic lava flows, Las Trincheras Basalt, are interlayered in the upper part of the Santa Ursula Formation (Mora and McDowell, 2000). (b) Southern Baja

For reasons of clarity, before dealing with the post-12.5 Ma and pre-4.5 Ma stratigraphy of Southern California, the term Comondú Formation should be discussed, and actually re-defined.

Originally, the term Comondú Formation was introduced by Heim (1921, 1922) to indicate (i) the volcaniclastic and (ii) the overlying volcanic beds covering the Oligocene-middle Miocene San Isidro Formation in the vicinities of the town of Comondú, in the Sierra de la Giganta. However, later on this term was assigned different meanings (Radelli and Ortiz, 1995). Beal (1948) extended the term Comondú Formation to include all the magmatic rocks (porphyritic and dense intrusive, rhyolite, andesite, basaltic lavas and tuffs) and the fragmental rocks derived from them (agglomerates, mud flows, conglomerates, sandstones, and clays) resting upon the San Isidro Formation, and covered by the Pliocene Salada Formation. Following this new fashion, McFall (1968) spoke of a Comondú

Group, that included the ±20 Ma old igneous rocks of Concepción Bay. In the same way, Wilson (1955) applied this term of Comondú to all the igneous rocks underlying the copper bearing El Boleo Formation of Santa Rosalia. In contrast, Gastil et al. (1979) used such term, and then with much caution, only for the ±10-6 Ma old trachyandesitic basalts capping the reliefs of Comondú.

This led to such a confusing situation that several authors (e.g., Sawlan and Smith, 1984; Radelli et al., 1993) questioned the usefulness of this term, and suggested that it should be either altogether disregarded or re-defined.

Having revisited the area of Comondú stu-died by Heim; having recognized (see here-below) the relationships between the rocks of Heim’s Comondú with the other country rocks (Radelli and Ortiz, 1995; Ortiz, 1998); and having given new thought to this question, my present standpoint is as follows: (iii) the term Comondú Formation should be restricted to indicate the clastics rocks of Heim’s Comondú only; (iv) the upper volcanic member of He-im’s Comondú Formation should be disso-ciated from it, and termed Comondú Trachyan-desitic Basalt instead.

In the Sierra de la Giganta the Las Parras Formation is covered by the Comondú Forma-tion (as re-defined hereabove). This formation consists, upwards, of sub-horizontal to gently tilted, thick beds of partly interfingering volca-niclastic conglomerate and breccia, sandstone and conglomerate, and sandstone, all of these facies with a chiefly tuffaceous matrix. Its clasts, generally poorly rounded, include ande-site, dacite,

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basalt, and sometimes tonalite. Its thickness does not exceed some 150 m.

According to Heim (1922), on the western side of Southern Baja this formation unconfor-mably overlies the Oligocene to middle Mioce-ne marine San Isidro Formation.

In the Sierra de la Giganta this Comondú Formation overlies the Las Parras Formation. In the vicinities of Rancho Palo Chino (rou-ghly midway between the villages of San Javier and Comondú) the contact appears as a gentle angular unconformity. But elsewhere this rela-tionship between the Comondú and Las Parras Formations has not been observed so far.

However, three facts always clearly indicate that a tectonic phase divided from each other the sedimentation of the Comondú Formation from that of the Las Parras Formation. These facts are : (i) the Comondú Formation does not show the widespread, conspicuous cleavage affecting the underlying Las Parras Formation; (ii) the dacitic intrusions which occur within the Las Parras Formation do not within the Comondú Formation; (iii) the clasts of the tonalitic bodies crossing the Pelones Forma-tion are founf within the Comondú Formation, and this implies a previous exposure of such bodies,.

Northeast of Sierra de la Giganta, the

Oligocene - early Miocene Pelones Formation (older than 12.5 Ma) is covered: in the southern part of the Concepción Peninsula by the Hornillas Formation; and in the northern part of said Peninsula as well as in the area of Mulegé by the Minitas Formation.

The Hornillas Formation begins with tuffaceous sandstone but, essentially, it con-sists of tuffaceous volcanogenic conglome-

rate and sandstone, for a total thickness of 150-200 m.

In the Concepción Peninsula, the Minitas Formation comprises itself of tuffaceous sand-stone and conglomerate cut by manganesi-ferous veinlets, with a ± 50 m thick basal mem-ber of gypsiferous tuffs.

Then, in the area of Mulegé the Minitas Formation becomes gypsiferous tuff and gyp-sum, with much less conglomerate, with both volcanic and tonalitic clasts. As in the case of the Comondú Formation, the occurrence of these latter clasts clearly imply the erosion either of the pre-Cainozoic basement (McFall, 1968) or of the igneous bodies intrusive into the Pelones Formation, and therefore at least a disconformity of the Minitas Formation with the underlying Pelones Formation.

Thus, the Hornillas Formation appears as the stratigraphic link between the Comondú Formation and the Minitas Formation.

On the other hand, and this is an important point, the gypsum content of the Minitas For-mation clearly speaks by itself for an equiva-lence of this formation with the gypsiferous belt of Santa Rosalia, cropping out widely at Island El Carmen, Island San Marcos, Santa Rosalia, and Laguna del Gato (fig. 9), presen-tly being mined at Island San Marcos and at Santa Rosalia.

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Fig. 10 – Map showing the aproximate western limit of the gypsiferous belt of Southern California. 2 : ±10 to ±6 Ma basaltic cover ; 1 : Gypsiferous outcrops.

Although it crops out discontinuosly, this more than 250 km long evaporitic belt appears as a major structure of Southern Baja. Much work is still needed on this point, but it seems permissible, at least as a working hypothesis, to interpret such structure as corresponding to a rift (aborted later on), and more generally to a Protogulf of California.

In the area of San Ignacio-Sierra San Francisco, some 50 km westwards of Santa Rosalia, an acidic (rhyolitic?) sequence

occurs, which disappear southwards. Its base is still unknown, but it is certainly directly covered, same as the Boleo Formation, by a ± 10 Ma old basalt (Gastil et al., 1979).

The famous copper-bearing Boleo

Formation of Santa Rosalia consists of several levels of strongly altered acidic tuffs (which contain the copper orebody, and numerous mangane-sifeous veinlets as well) divided from each other by interlayered conglomerates. It occurs below the ±10 Ma old Esperanza basalt (Sawlan and Smith, 1984) that will be discussed in the following chapter. It has been generally admitted that the Boleo Formation overlies the gypsiferous body of Santa Rosalia. However, recent drilling results have shown that several thin beds of clay, that in places contain some copper minerals, exist within said gypsiferous body as well.

Thus, at present level of knowledge, it

seems permissible to conclude, at least as an educated guess, that also this volcanic episode of South-ern Baja was related to the rifting that gave way to the Protogulf of California.

LATEMOST MIOCENE, FROM ±10 TO ±6 Ma

A nex extensional phase occurred in North-west Mexico at ±10 Ma, which corresponds, both in geological age and tectonic style, to the classic Basin and Range extension of the western USA (Zoback et al., 1981).

(a) Sonora

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This tectonic phase affected the western

part of Sonora, where it gave way to the ope- ning of new basins (Radelli et al., 1993). The mode of this deformation can be parti-

cularly well studied and defined in western-most Sonora, from the latitude of Hermosillo to that of Guaymas. There, indeed, near and up to the shore of the Gulf of California, it can be observed that a number of ±SW-dipping, lis-tric faults strongly rotated and progressively attenuated the ±12 to ±10 Ma Lista Blanca Volcanics, as well as their basement, including in particular some Laramide granitoids. The deformation in question occurred, therefore, according to a simple shear mode.

Fig. 11 – The upper Miocene (±10 Ma) Paleogulf of California and its marine trans-gression in coastal Sonora. Note that such transgression was limited to the area where the ±12–10 Ma Lista Blanca Volcanics and equi-valent formations are rotated and atte-

nuated. (After Radelli et al. 1990, modified).

3 : Eastern limit of upper Miocene marine transgression ;

2 : Rotated and attenuated blocks of Lista Blanca and equivalent Volcanics :

1 : Unrotated blocks of ±12-10 Ma Lista Blanca and coeval rocks.

As discussed previously (Radelli et al.,

1990; Radelli et al., 1991), such faulting did not affect the bajacalifornian side of the pre-sent-day Gulf of California - a situation still reflected by the morphology, and in particular the bathymetry, of its two opposite sides. Thus, the result of said faulting has been an asymmetric rifting, with the Baja side con-stituting the western limit of the Sonoran Basin and Range Province, same as the Sierra Nevada constitute the western limit of the Basin and Range Province of western USA.

With one exception, that will be dis-cussed herebelow, all of these newly created basins were intramontane basins, that were filled up by continental clastic deposits (ge-nerally, poorly consolidated conglomerate and sandstone).

The exception mentioned hereabove is

that of westernmost Sonora and of the pre-Pliocene crust of the Gulf of California. There, this Miocene rifting allowed the opening of the marine embayment known as the Paleogulf of California, that reached, northwards, the area of Lake Mead, Nevada (Noble, 1931; Anderson, 1969; Mandra and Mandra, 1972; Gastil et al, 1979; Radelli, 1986).

As noted previously (Radelli et al., 1990; Radelli et al., 1991), definite proof

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that this marine transgression was indeed allowed by such rifting is provided by the strict coin-cidence between the limit of the rotated and unrotated blocks of the Lista Blanca Volca-nics and coeval formations on the one hand, and, on the other hand, that of said trans-gression itself in westernmost Sonora (fig. 11).

In western Sonora the deformation dis-

cussed hereabove has been followed by a new igneous phase, during which still flat-lying basalts unconformably covered the pre-existing rocks, including the ±12 to ± 10 Ma old, tilted Lista Blanca Volcanics (Gastil and Krummenacher., 1977; Sheridan et al., 1979; Gastil et al., 1979; Cochemé, 1985; Radelli, 1986; Radelli et al., 1993). Their radiometric (K/Ar) age falls in the 8 to 6 Ma bracket (Gastil et al., 1979).

(b) Southern Baja

As pointed out above, Southern Baja has not been affected by the ±10 Ma Basin and Range faulting. Consequent to this, during the whole of the ±10 to ±6 Ma interval Southern Baja underwent an evolution different from, and less differentiated than, that of Sonora. In fact, Southern California was extensively covered by a blanket of basaltic rocks, radiometrically (K/Ar) dated ±10 to ±6 Ma (Gastil et al., 1979; Sawlan and Smith, 1984).

Possibly because of local differences of facies these basalts have received different na-mes in different places. Thus, they are known as: Ricason Formation in the southern part of the Concepción Peninsula and in the area of Mulege (McFall, 1968), Pilares Formation in the northern part of the Concepción Peninsula (McFall, 1968), Esperanza Basalt in the area of Santa Rosalia (Sawlan and Smith, 1984),

“basalt capping the Sierra” (Gastil et al., 1979) or Comondú Trachyandesitic Basalt (Ortiz, 1998; and this paper) in the area of Comondú. However, for their general facies and radiometric age, it is undoubtful that they correspond to a single, huge volcanic event. This is confirmed by their stratigraphic posi-tion. In fact, to consider only the formations already discussed in this paper, they directly overly : the clastic deposits of the Comondú and Hornillas Formations, the gypsiferous deposits of the Minitas Formation, and, as already noted, the Boleo Formation of Santa Rosalia, as well as the acidic volcanics and/or volcaniclastics of San Ignacio-Sierra San Fran-cisco.

Unfortunately, geochemical data on these basalts are rather scanty. According to Sawlan and Smith (1984), the Esperanza Basalt would be tholeiitic; and, according to Gastil et al. (1979), the basalts capping the Sierra in the general area of Comondú are, at least in part, trachyandesitic, that is of an alkaline affinity. Nevertheless, it can be postulated safely enough that they are related to extensional conditions. If this assumption is correct, then it would appear once again that the simple shear extensional conditions of Sonora became pure shear deep-seated extensional condi-tions in Baja.

CONCLUSIONS

The present survey ends here, at ±6 Ma, because the following event of that occured area is that of the Pliocene (4.5 Ma) ope-ning of the Gulf of California by the inter-vening San Andreas System. Then, Baja became a part of the Pacific Plate, and only

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Sonora remained a part of the Ameri-can Plate, both of them acquiring the character of a passive margin in the Gulf of Cali-fornia.

During the Eocene (±50 to ±35 Ma) a huge Volcanic Arc, the Huepac Arc, was built up in western Sonora east of the Baja-Borderland Block, that furnished detritus to the basins at its western margin. The uplift of the Huepac Arc individualized the basin of Sierra Madre Occidental in the east. But after the uplift of said Arc the history of the region was one dictated by successive pha-ses of crustal extension, in spite of the fact that until 12.5 Ma the region was in a Su-pra Subduction Zone setting.

During the late Eocene the erosion of this Arc furnished clastic sediments both eastwards : Novosaigame conglomerate in Sonora and adjacent Chihuahua.

During the Oligocene (±35 to ±25 Ma interval) in Sonora, east of the Eocene Arc, was accumulated the classic rhyolitic-ignim-britic sequence of Sierra Madre Occidental.

An extensional phase intervened there at ± 25 Ma. Then, from about 25 to 20 Ma (latemost Oligocene - early Miocene), in the Sierra Madre Occidental the volcanism be-came first bimodal and successively basal-tic-andesitic, whereas in central Sonora (and probably in western Sonora as well) new, lacustrine and sometimes boratiferous ba-sins (Tubutana-Magdalena-Belleza type ba-sins) were opened. Possibly, the origin of the San Isidro Basin of the western side of Baja, that contains 25 and 23 Ma volcanics, is also related to this extensional phase.

This stratigraphic subdivision cannot be recognized in western Southern Baja. There, a monotonous clastic and largely conglomeratic sequence (El Salto, Las Parras and Pelones Formations) derived from the continuing ero-

sion of the Baja-Borderland Block corre-sponds to the whole of the interval Oligocene - middle Miocene (prior to12.5 Ma).

A short lived phase of fracturing followed at ± 20 Ma., recognizable both in Sonora and in Southern Baja. It allowed the emplacement of a number of rhyolitic dykes in Sonora, and that of dacitic and tonalitic bodies in Southern Baja, cutting across its Las Parras and Pelones Formations.

This was followed by one of the main

extensional phases, the Baucarit extensional phase.. In the eastern part of Sonora, and particularly in the Sierra Madre Occidental, faulting, gave way to a number of intramontane basins - the Baucarit Basins. During the inter-val ±20 to 12.5 Ma these were filled up by continental, mostly fluviatile, clastic deposits and interlayered basaltic andesites, which rest unconformably over all of the older formations, including those of the Tubutama-Magdalena-Belleza Basins. In Central Sonora the western limit of the Baucarit extension is on the Gavilan-La Colorada Line. From that line west-wards no Baucarit basin occurs, but the influ-ence of the Baucarit extension is recognizable as far as Southern Baja, however. There, in fact, the chiefly clastic Las Parras Formation con-tains interlayered 13.3 Ma old basalt in its upper part, and it is crossed by 16±2 and 15.4±0.8 Ma andesitic dykes in its lower part.

A new lithospheric setting was put in

force at 12.5 Ma with the ceasing of the

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subduction, its place being taken up by the Tosco-Abreojos Transform Fault. In the study area a new crustal extensional setting corres-ponds to such new lithospheric setting.

In Sonora, the Baucarit Formation is til-ted, and then unconformably covered, in western Sonora, by the ±12 to ±10 Ma Lista Blanca acidic volcanics and coeval, equivalent formations.

In Southern Baja the Las Parras Forma-tion is affected by a strong extensional cleavage. This Formation and its coeval ones are then deformed in such a way that either a gentle unconfortmity or a regional disconformity divides them from the over-lying, younger formations. Above this sur-face of unconformity or disconformity, the coeval, clastic and tuffaceous, Comondú Formation (Sierra de la Giganta) and Hor-nillas Formation (southern part of the Con-cepción Peninsula) cover the Las Parras Formation and the Pelones Formation re-spectively.

The main crustal structure produced by this tectonic phase is an elongated, evapo-ritic, gypsiferous belt that, from El Carmen Island in the south to the area of Laguna del Gato in the north, extend itself along the coast, and appears as well in several neigh-bouring islands, of the Gulf of California. This structure, which at any rate is exten-sional in origin, corresponds at least to a Protogulf of California, and probably to a rift.This gypsiferous episode has been followed, at Santa Rossalia and at San Ignacio-Sierra San Francisco, by a short-lived acidic volcanic phase, to which seems to be related the copper mineralization of Santa Rosalia (the famous Boleo).

The last pre-Gulf of California event of the area is also a major extensional phase. It took place at ±10 Ma , and corresponds to the

classic Basin and Range extension of the western USA.

In Sonora, and particularly in western Sonora, the ±12-10 Ma Lista Blanca volca-nics where then affected and rotated, toge-ther with parts of their basement, by a set of ±SW-dipping, normal listric faults. In the process, a crustal attenuation occurred west-wards, which was strong enough to give way to an elongated marine embayment, the late Miocene Paleogulf of California, the eastern shore of which was in coastal Sonora.

The extensional conditions were still in force there during the ±8-6 Ma interval, when still flat-lying islanditic basalts unconformably covered the tilted and attenuated ±12-10 Ma Lista Blanca volcanics and coeval formations.

As still shown by the morphology and the bathymetry of the continental crust of either side of the Gulf of California, the Paleogulf of California was asymmetrical. This is related to the fact that the listric faulting that at ±10 Ma occurred in Sonora did not reach present-day Southern Baja. As a consequence, the ±10 to 6 Ma differentiated stratigraphy of Sonora does not exist in Southern Baja. There, only a huge, and rather monotonous, basaltic sequence, was accumulated during that same ±10-6 Ma inter-val. It can be said that Baja constitutes the western boundary of the Sonoran Basin and Range Province, just as the Sierra Nevada is that of the Basin and Range Province of the western USA. However, its ±10-6 Ma con-spicuous basaltic volcanism, which is trachy-andesitic at least in the area of Comondú, bears witness of extensional conditions in force at that time also in Southern Baja. Under the cir-cumstances, one can suggest

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that, as had been previously the case in western Sonora during the Baucarit extension, in correspondence of Baja the listric faulting responsible for the ope-ning of the Paleogulf of California had reached the ductile zone, causing the deformation to change there from a simple shear mode to a pure shear one.

Thus, it has been recognized that from the

late Eocene to the Miocene, both inclusive, the structure of the Sonora-Baja California seg-ment of the margin of North America was de-termined by five successive phases of crustal extension. The first three of them (Oligocene basins, Belleza basins, Baucarit basins of Sonora and Southern Baja) occurred at ±35 Ma, ±25 Ma, and ±20 Ma respectively, that is when said segment was still bounded by a east-dipping subduction zone. The other two (Protogulf and Paleogulf of California phases) took place, respectively, soon after 12.5 Ma and at ±10 Ma, that is after the

subduction had ceased and the place of the subduction zone had been taken up by the Tosco-Abrejos Transform Fault. As far as it can be judged from the cases of the Baucarit, the Protogulf, and the Paleogulf phases, to the change of the lithospheric setting did not correspond a change in the mode of these successive crustal extensions. However, and this seems worth noting, said extensional phases did not preclude the occurrence of successive pulses of magmatic activity du-ring the life-span of the basins themselves. Thus, one ought to conclude that those ba-sins were not opened by listric faults, but in a pure shear mode. Much remains to be done, however, in order to recognize the actual relationships between these crustal structures and the underlying lithospheric ones.

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FIGURE CAPTION

Fig. 1 - Present setting of Sonora (North America Plate) and Baja (Pacific Plate). It was established by the Pliocene intervention of the San Andreas Transform System and consequent opening of the Gulf of California. The study area is comprised of Sonora and Southern Baja California. Fig. 2 - Diagrammatic sketch of the Eocene Continental Volcanic Arc of Sonora and of the Eocene-Oligocene Basins of Sonora and Southern Baja California. Sonora and Baja are shown in their mutual position (Gastil et al., 1979) prior to the Pliocene opening of the Gulf of California. Fig. 3 - Map showing the localities of Sonora mentioned in the text. Fig. 4 - Map showing the localities and geographic units of Southern Baja mentioned in the text. Fig. 5 - Geological map of Concepción Bay area (after McFall 1968, simplified)

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1 : Quaternary; 2 : Infierno Formation (Pliocene); 3 : Pilares and Ricason Formations (uppermost Miocene, ±10 to ±6 Ma, basalts). 4 : Hornillas and Minitas Formations (upper Miocene, ± 12 to ±10 Ma, detrital-tuffaceous and partly gypsiferous deposits); 5 : ± 20 Ma Tonalites; 6 - El Salto and Pelones Formations (pre-12.5 Ma, Oligocene to Miocene, clastic, volcanoclastic and volcanic deposits); 7 - Pre-Cainozoic basement (mainly intrusive rocks). Fig. 6 - Geological map of the area of San Javier (Sierra de la Giganta, west of Loreto. 1 : Undifferentiated Quaternary; 2 : Young, probably Quaternary basalt; 3 : Comondú Trachyandesitic Basalt (uppermost Miocene, ±10 to ±6 Ma) 4 : Comondú Formation (upper Miocene, ±12 to ±10 Ma, detrital-tuffaceous deposits); 5 : ± 20 Ma intrusive dacitic bodies; 6 : Las Parras Formation (pre-12.5 Ma, Oligocene to Miocene, clastic, volcanoclastic, and volcanic deposits).

Fig. 7 - Sketch-map showing the area where the Baucarit extensional deformation was rotational (simple shear). There, its listric faults, which bottom down in a single master fault, the Yaqui River Fault, could give way to a number of intramontane Baucarit basins, which were filled up by detrital and basaltic andesitic deposits. It is suggested that in correspondence of the Gavilan-La Colorada the Yaqui River Master Fault reached a ductile zone, so that southwestwards, and as far as Southern Baja, the Baucarit extension developed itself in a pure shear (coaxial) mode. - Simplified structural map showing the sub-horizontal attitude of the Yaqui River Fault, the master fault of the Baucart extension, as it appears in the Yaqui Valley in the vicinities of Onavas. 3 : Post-Yaqui River Fault volcanic and sedimentary deposits; 2 : Upper plate (Upper Triassic strata, Upper Cretaceous to Paleocene volcanic and sedimentary deposits, Sierra Madre Occidental Volcanics, Baucarit Formation). 1 : Lower plate (mostly Laramide granitoids).

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Fig. 9 - Map showing the areal distribution of the ±12-10 Ma acidic Lista Blanca Volcanics.

Fig. 10 - Map showing the approximate western limit of the gypsiferous belt of Southern California. 2 : Gypsiferous outcrops: 1 : ±10 to ±6 Ma basaltic cover.

Fig. 11 - The upper Miocene (±10 Ma) Paleogulf of California and its marine transgression in coastal Sonora. Note that such transgression was limited to the area where the ±12-10 Ma Lista Blanca Volcanics are rotated and attenuated (After Radelli et al., 1990, modified). 3 - Eastern limit of the upper Miocene marine transgression; 2 - Rotated and attenuated blocks of Lista Blanca Volcanics; 1 - Unrotated blocks of ±12-10 Ma Lista Blanca Volcanics and coeval rocks.

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