Artículos

 

Structural pattern of the Todos Santos Coastal Plain, based on geophysical data

 

Patrón estructural de la Planicie Costera de Todos Santos, con base en datos geofísicos

 

Marco A. Pérez-Flores, Francisco Suárez-Vidal*, Luis A. Gallardo-Delgado, Antonio González-Fernández and Rogelio Vázquez

 

División de Ciencias de la Tierra, CICESE Km 107 Carretera Tijuana-Ensenada Ensenada, Baja California, México. *E-mail: fsuarez@cicese.mx

 

Recibido en marzo de 2003;
aceptado en marzo de 2004.

 

Abstract

A set of gravity and magnetic data taken within the Todos Santos Coastal Plain between 1962 and 2000 from land, sea and air is used to integrate the structural framework and tectonic model of this area. The present work collected all the available data, homogenized them, and maintained them at their original altitude. A new technique of joint inversion for gravity and magnetic data from three-dimensional structures was applied. As a result, a well-defined topographic map of the granitic-andesitic basement was obtained and previously unknown geological structures were depicted. Several cross-sections were made from the three-dimensional model obtained, establishing the Todos Santos Coastal Plain structural pattern as a typical pull-apart basin associated with a tectonic fore-arc during the subduction of the Farallón Plate in Late Mesozoic and Early Cenozoic time.

Key words: structure, coastal plain, geophysical data, gravity, magnetics.

 

Resumen

Se integró el marco estructural y el modelo tectónico de la Planicie Costera de Todos Santos, utilizando un conjunto de datos gravimétricos y magnéticos tomados entre 1962 y 2000 en tierra, aire y mar. En el presente trabajo se integró toda la información disponible, se homogeneizó y se mantuvo a su altitud original. Se empleó una nueva técnica de inversión conjunta de datos gravimétricos y magnéticos para estructuras tridimensionales. Como resultado se obtuvo un mapa topográfico del basamento granítico-andesítico, donde se visualizan rasgos geológicos desconocidos hasta este trabajo. Se hicieron algunas secciones a partir del modelo tridimensional, en los que se establece el patrón estructural de la Planicie Costera de Todos Santos, el cual es típico de una cuenca extensional (pull-apart basin) asociada con un frente de arco en un ambiente tectónico durante la subducción de la placa Farallón a final del Mesozoico y principios del Cenozoico.

Palabras clave: estructura, planicie costera, datos geofísicos, gravimetría, magnetometría.

 

Introduction

The Todos Santos Coastal Plain was defined by Gastil et al. (1975) and includes the city and bay of Ensenada de Todos los Santos and the Ejido Sánchez Taboada (Maneadero) basin, being different from the Ensenada Block defined by Gastil et al. (1975) (fig. 1). The Todos Santos Coastal Plain is an area of about 34,000 ha (Aranda, 1983), forming a half-graben bound by several active faults that generate a high seismic risk and create a geologic hazard to the region (Wong, 1980; González and Suárez, 1984; Rockwell et al., 1987; Ortega, 1988; Suárez et al., 1991). This geomorphic province is a half-graben filled with 2000 m of sediments of unknown age. There are few geological studies and some geophysical surveys on the Todos Santos Coastal Plain that describe the general geology, structural geology and related tectonic process which acted in this region through geological time, among them the aero-magnetic study made by Consejo de Recursos Naturales No Renovables (1962), the geological reconnaissance in Valle Dorado by Schroeder (1967), and the gravity survey made in Maneadero Valley by Dowdy (1977). Vázquez (1980) carried out a geophysical survey, applying electrical resistivity, self-potential, tellurics and potential field. Based on the gravity data, maximum depth of the basement was estimated along two profiles oriented perpendicular to the Agua Blanca fault (fig. 2). One was located along the Estero Beach sand bar that separates the Punta Banda coastal lagoon from the open ocean and in this profile the basement depth was estimated to be 1700 m, while in the second profile, located 5 km to the SE from the previous, the basement was estimated to be 900 m deep. These calculations were made with a density contrast of 0.35 g cm^-3. Fabriol et al. (1982) worked with telluric and gravity measurements from Vázquez (1980) in Maneadero, and reported a density contrast between sediments and andesites of 0.7 g cm^-3 and a fault in Maneadero Valley parallel to Agua Blanca (fig. 2). They interpreted two bidimensional profiles and determined the depth of the Maneadero basin in several places. Cruz (1986) interpreted data from a gravity survey along Arroyo San Carlos and Maneadero and determined the depth to the basement at 650 m, being shallower to the north. Aguero (1986) integrated most of the available potential field data from the Maneadero area. Recently, González-Fernández et al. (2000) conducted a bathymetric, magnetic and gravity survey along the Punta Banda peninsula, and found a set of normal faults that are cross-cutting the peninsula in a N-S direction. González-Serrano (1977) interpreted several gravity and magnetic profiles and described some of the active faults within the bay. Wong (1980), using reflection seismology, made a configuration of Todos Santos Bay and part of the inner continental borderland. González and Suárez (1984) closely analyzed a seismic swarm located offshore and established that such activity was produced by a new branch of the Agua Blanca fault within Ensenada Bay. Pou (1982), Quintanilla-Montoya (1984) and Quintanilla-Montoya and Suárez-Vidal (1992) described the structural geology of the southern Todos Santos island and determined the relationship between the faulting at the island and the major faults located in the continental borderland. Although these studies describe most of the relevant structural features found within the Todos Santos Coastal Plain, there is no tectonic model that describes how, when and under what tectonic regime the coastal plain was developed. Further, if the area is still active, the geologic hazard and seismic risk need to be established.

 

We present the results of processing the magnetic and gravity data from the existent literature (bachelor and master theses, professional papers, etc.), applying the Gallardo et al. (2003) new inversion technique of joint gravity and magnetic data from 3-D geologic structures, although the main objective here is not to describe the method. We demonstrate that this technique, when applied to an area such as the Todos Santos Coastal Plain, results in a map that describes the structural pattern and creates a tectonic model for the region. By comparing our 3-D model with the previous 2-D model, new structural features were obtained.

 

Regional geology

The present topography of the Baja California peninsula closely reflects its geologic history and structure. The area can be subdivided into four regions of distinct topographic character. These, in turn, can be subdivided into geomorphic provinces.

On the west coast, a line approximating the Santillán and Barrera line separates the coastal terraces and continental borderland areas from the plateaus of the central peninsula (Gastil et al., 1975). To the north, between the Santillán and Barrera line and the peninsular plateaus, is a region of rugged topography developed primarily on prebatholithic volcanic rocks. This topographic region continues into California (USA).

In Baja California, six distinct geomorphologic provinces are well defined: Pacific Coastal, Coastal Mountain, Northern Highland Plateau, Highland Valley, Southern Highland Plateau and Gulf of California. The Ensenada region is part of the Pacific Coastal Province and is included within the Santo Tomás Block/Todos Santos Coastal Plain, but it is also considered part of the Coastal Mountain Province, known as the Ensenada Block (Gastil et al., 1975).

The Ensenada Block is characterized by a rough topography and it extends from the Guadalupe Valley to the Agua Blanca fault in the south. To the east, a series of high peaks mark the eastern edge of this block. The area is dissected by the west-flowing Guadalupe River, Santa Clara River, Cañón de la Chispa and Arroyo Santo Tomás, which cut gorges as much as 1000 m deep (Gastil et al., 1975). The Ensenada Block stands 500 m above sea level (on average), giving little evidence of a pre-existing Early Tertiary surface. A lag deposit, the remnant of a fluvial conglomerate, is scattered over a 400 m high plateau in the northwestern corner of the block.

The Todos Santos Coastal Plain covers an area of 150 km² occupied by the city of Ensenada, Maneadero Valley (Ejido Sánchez Taboada) and Todos Santos Bay. The area is a half-graben controlled by the Agua Blanca fault as the southern limit. This half-graben is filled with more than 2000 m of sediments of unknown age. The mesas to the north do not pass beneath these deposits but stand as high bluffs that were once wave-battered headlands (Gastil et al., 1975).

The Todos Santos Coastal Plain is surrounded by the Early Cretaceous (Aptian-Albian) Alisitos Formation, composed of andesites and pyroclastic material. Allison (1955) described the Alisitos Formation as a sequence of 1790 m of thinly bedded (including diorite sills) tuffs. On top of this, there are 1500 m of mudstones and, in lesser proportion, some sandstones. Resting over the mudstone there are more than 2000 m of pyroclastic and epiclastic intermediate volcanic rocks and porphyritic andesite. South of the Agua Blanca fault, the Alisitos Formation terminates with a sequence of biohermal limestones and intercalated beds of pyroclastic material. North of the Agua Blanca fault, the calcareous sedimentary sequence is absent, hence there is no fauna-fossil that can be used for accurate age control of the Early Cretaceous rocks. The ENE part of the Todos Santos Coastal Plain is bound by a pluton of tonalite-granodiorite 120 to 110 Ma from the peninsular batholith (Ortega et al., 1997). To the west-northwest the intrusive rocks are in contact with the Alisitos Andesite that outcrops in the Chapultepec Hills, as well as to the north of them. North of the city of Ensenada, and in the El Sauzal area, the Alisitos Formation is in contact with the Late Cretaceous Rosario Formation. This formation is well represented along the southwestern part of California and northwesterly Baja California, and it is characterized by a thick sequence of clastic material wedged against the Alisitos Formation.

The Rosario Formation represents a continental to deep water marine facies that was deposited along the margin of a Great Valley type fore-arc basin. Sediments were derived from rugged eastern highlands formed by uplifted peninsular ranges, granitic-metamorphic terrain (arc system). In the Ensenada region, the general Rosario stratigraphy reflects an initial phase of non-marine conditions, followed by the widespread marine incursion of the last major eustatic sea-level rise of the Cretaceous.

The Punta Banda peninsula closes the south side of Ensenada Bay. This peninsula rises several hundred meters above sea level and lithologically is formed by the Early Cretaceous Alisitos Formation andesites. In the westerly two-thirds of the peninsula, the andesites are in contact with the Late Cretaceous Rosario Formation. One of the fewest outcrops of the rudist exists in this area, known as Coralliochama orcutti, which forms a bank developed within the lowest sandstone unit, and represents a rudist reef in a wave-washed cliff and rocky headland (Ross, 1981). The city of Ensenada and the rural area to the south are located over a plain filled with sediments eroded from the peninsular ranges and on top of this there is the Quaternary alluvial material. All the above litho-logical units are distributed around Ensenada Bay (mainland and in the continental borderland) and play an important role that has to be considered in a structural and tectonics model.

 

Main structural element

The major structural element within the Todos Santos Coastal Plain is the Agua Blanca fault, which is a dextral strike-slip fault oriented N68°-70°W, oblique, almost perpendicular to the general strike of the San Andreas-Gulf of California fault system. This fault extends for more than 140 km from the San Matías pass, in the east, to Ensenada Bay, which continues offshore. The Agua Blanca fault is one of the active faults in the northern Baja California region. It can be traced as a single structure from the San Matías pass to the Santo Tomás valley (Allen et al., 1960; Rockwell et al., 1987; Ortega, 1988; Suárez et al., 1991). From this valley to the west, the fault branches in two segments that bound the Punta Banda peninsula along the south and northern flanks (fig. 2). In the northern sector along Ejido Sánchez Taboada (Maneadero), the fault has a transition from its dextral strike-slip movement to a normal fault behavior, with significant vertical displacement, and it forms the southwest limit of the half-graben that characterizes the Todos Santos Plain (Gastil et al., 1975). The Agua Blanca North fault passes offshore along the north side of Punta Banda Ridge, where it marks the steeply dipping contact between basement rocks of the peninsula, including the Todos Santos islands, and the thick fill sediments in Todos Santos Bay. The fault makes a sharp turn from nearly N70°W to a more northerly N25°-30°W strike east of the Todos Santos islands (Legg et al., 1991). Along the north branch of the Agua Blanca fault there are several hot springs originated by deep circulation of metheoric water. The NW hot spring is located on the beach at the shore line where the fault intercepts the sand barrier at Estero Beach (Hummel, 1972).

The other major structural element in the Ensenada area is a left lateral strike-slip fault, oriented almost N-S and located just north (600 m) of the entrance to the harbor; González-Serrano (1977) has hypothesized that it extends through the sea to connect with the Agua Blanca fault (fig. 2). The Agua Blanca fault has also been traced with reflection seismology (Wong, 1980; Wong et al., 1987). In the area known as Valle Dorado (fig. 2), the andesites from the Alisitos Formation are deformed developing a syncline oriented in E-W direction. To the south, the Ejido Sánchez Taboada (Maneadero Valley, fig. 1) is located in a half-graben whose southern limit is the Agua Blanca fault and its northern limit is the Estero Beach fault (González and Suárez, 1984) (fig. 2). It extends along the Cañón de San Carlos in a westerly direction and continues offshore just at the mouth of the Estero Beach coastal lagoon (fig. 1).

Within Todos Santos Bay there are two main morphological elements that are well expressed in the bathymetry. The Cañón de Punta Banda (fig. 2) is a typical structure with a bowl-shaped head, located within a large, coastal embayment (Ensenada Bay). This shape may be the result of headward erosion by large scale slumping and sliding of unconsolidated sediments. The Cañón de Punta Banda necks downstream, passing through a narrow gorge cut into older uplifted bedrock that is sub-aerially exposed at Punta Banda and Todos Santos islands (Gastil et al., 1975; Legg, 1985). The expression of sinuousity varies from sharp, almost right-angle turns, implying fault or joint control, to smooth meanders, similar to those seen along many rivers (Reineck and Singh, 1980, p. 260). A significant sharp bend in the Cañón de Punta Banda west of the tip of the Punta Banda peninsula can be evidence of 4 km of right lateral offset associated with the branch of the Agua Blanca fault. The other morphological element is the Bajo de San Miguel (San Miguel topographic low). This bathymetric elevation is located between the Todos Santos islands and Punta San Miguel (figs. 1, 2) and may correspond to the northwest limit of the Ensenada half-graben or to a local uplift of the Cretaceous basement.

 

Data description

The geophysical data used in the present work come from different gravity and magnetic surveys made between 1977 and 2000. The land gravity data come from Vázquez (1980), Cruz (1986) and Aguero (1986) (fig. 3). The marine gravity data come from González-Serrano (1977) and the marine magnetic data are from González-Fernández et al. (2000). Aeromagnetic data are from Consejo de Recursos Naturales No Renovable (1962) and land magnetic data from González-Fernández et al. (2000) (fig. 3). All data sets needed different processes to homogenize them. In figure 3a and 3c we present the gravity and magnetic data used in this project. Although each data set was recorded at a different altitude, these were plotted together as they are only a graphical representation and the inversion process takes into account the differences in altitude.

The available gravity data cover the Maneadero Valley and the coastline from Maneadero in the south to San Miguel in the north. The measurements were made using a Worden gravity meter from San Diego State University (Vázquez, 1980), with a precision of 0.1 mgal. The common corrections made to gravity data were the correction by latitude shift, Bouguer slab and topography. The marine gravity survey was performed with a Lacoste Romberg S-42 from Oregon State University, installed on board the Mexican Navy O/V DM-20. The data published by González-Serrano (1977) were corrected by horizontal and vertical acceleration, cross-coupling and latitude shift. Also, a regional was subtracted.

The magnetic data cover the onshore area from Punta Banda to Maneadero, Ensenada city and toward San Miguel (fig. 3c). The marine survey covers the bay very well, even outside the deep channel and the shallow waters of San Miguel in the NW of the bay. The bathymetry and marine magnetic data were taken by González-Fernández et al. (2000) with a dual frequency echosounder and magnetometer installed on the O/V Francisco de Ulloa owned by the Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE). The land equipment consisted of two EG&G magnetometers. The data from González-Fernández et al. (2000) and Consejo de Recursos Naturales No Renovables (1962) were corrected by diurnal variation and IGRF value. Measurements were plotted together regardless of the differences in altitude, because the inversion process takes this into consideration. The gravity anomaly shows a high close to San Miguel and a low close to the Agua Blanca fault, indicating less dense material or deeper basement. The magnetic anomaly shows the lowest values at the middle of the bay, indicating less magnetic material or deeper magnetic basement.

 

Gravity and magnetic methodology

To model the Todos Santos Coastal Plain, several assumptions were applied. The Gallardo et al. (2003) technique, which served as the basis, uses a grid of vertical prisms. The depth to the top of every prism (referred to as top topography) is fixed to the topography-bathymetry of the sedimentary sequence or to the basement that outcrops around the basin. The depth to each bottom prism is free (bottom topography) (fig. 2). The inversion process tries to fit automatically the bottom topography of the prisms with the boundary of the sedimentary basin with the basement using the gravity and magnetic data available as constraints. The resulting model takes into consideration the different altitude of every measurement and computes the model response at the same altitude for their comparison. It is not necessary to reduce the data to a flat level. The model is well constrained by the information from the exposed basement and by fitting the gravity and magnetic anomaly. A regularization is applied, consisting of the minimization of the second spatial derivatives from the bottom topography. This procedure will find the smoothest model that best fits the data, avoiding unnecessary and fictitious roughness in the model due to the random error added when the measurements were taken.

This algorithm also estimates the density variable with depth and the magnetization vector given by the director cosines. From these director cosines it is possible to compute the magnetization magnitude, inclination and declination. One density function and constant magnetization contrast is allowed. We define the contrast as the difference between the sediments and the basement in the density function or the magnetization. In terms of inversion, the problem is non-linear for the determination of prism depths. A rearrangement of the equations was needed in order to allow equality and inequality constraints in every variable by means of quadratic programming. This advantage gave us the opportunity to introduce additional information, such as exploratory wells or exposed basement, or to constrain some parameters into an interval, according to a previous hypothetical model. The program minimizes the error between the data and the response of the model that is obtained iteratively until it arrives at the best fit. It also minimizes the spatial derivatives of bottom depths of the prisms.

The Todos Santos Coastal Plain data require a versatile program such as that developed by Gallardo et al. (2003). The area of study is located between kilometers 516 and 544 UTM in W-E direction and 3501 and 3529 UTM in S-N direction. The prism grid consists of 28 x 28 = 784 prisms, with a plane section of 1 x 1 km, each one with a total area of 784 km² (fig. 2). The depth to the top of every prism was fixed with the corresponding topography or bathymetry. The prisms located in the exposed basement were fixed following the topography and taken out automatically from the inversion, corresponding to the shaded area in figure 2.

The initial model for bottom depths was a constant basement depth of 500 m. The density was supposed 2.6 g cm^-3 for granites and 1.6 g cm^-3 for sediments. We used an initial model with density contrast of Δρ(z) = (-1.0 - 0.05 z -0.05 z²) g cnr³ and an initial magnetization contrast of ΔJ = -580 emu (I = 23°, D = 6°). After eleven iterations the best model was obtained. We assume that the depth estimated for every prism's bottom fits the topography of the granitic-andesitic basement under the bay and Maneadero Valley (fig. 4). The resulting density contrast was Ap(z) = (-0.657 + 0.0 z + 0.0 z²) g cnr³ and a magnetization contrast of ΔJ = -203 emu (I = 25°, D = 10°). The root mean square (RMS) error was 9.5%. The model obtained was the smoothest model resulting from minimizing the second spatial derivatives of the bottom depths.

The algorithm allows variable altitude, so every gravity and magnetic measurement had (x, y, z) coordinates. The sea has a gravity response but not magnetic. Based on what is known about the bathymetry, we extracted the gravity response of the sea to the gravity anomaly in order to manage just the contrast between the sediments and the granitic-andesitic basement. The gravity and magnetic response of the best model resembles the measurements very well (fig. 3b, d).

 

Geological model

The geological model is supported by the gravity and magnetic model as the common source for both methods. Several irregularities appear (fig. 4) when the smoothest model is obtained (i.e., we only obtained the irregularities needed by the data).

The gravity and magnetic responses of the estimated model are shown in figure 3b and 3d, respectively. Comparing the observed and calculated anomalies, a good match is obtained (RMS = 93% for the whole data set).

The geological model shows a very well-developed basin with the deepest part located within the bay at 1650 m depth. Maneadero Valley is part of the same basin, which reaches its deepest part near the coast, 900 m close to the Punta Banda coastal lagoon. The basement is observed in the SE part of the area near El Zorrillo ranch (fig. 2). The same feature is observed under the city of Ensenada and corresponds to a well-developed basin with a maximum depth of 300 m. Another small depression is observed where the Punta Banda coastal lagoon connects with the bay. The shape of the bay basin is not circular; it is almost an ellipse with the main axis in a NE direction. We may assume that this is the major tensional stress. Another relevant structural feature is located between the Punta Banda peninsula and Maneadero Valley where there is a large step that follows the trace of the Agua Blanca fault. The Punta Banda peninsula is flanked by the two well-known faults: Agua Blanca and Maximinos (also known as Agua Blanca South). This appears as a well-defined linear rise over the basin. Between the Punta Banda peninsula and Todos Santos islands there is another big step presumed to be a continuation of the Agua Blanca fault offshore and correlated with Cañón de Punta Banda.

Between Todos Santos islands and Punta San Miguel there is a rise of the basement that almost reaches sea level in the area known as Bajo San Miguel.

For a better understanding of the basin's shape, seven cross-sections in the SW-NE and N-S directions were constructed. Figure 4 is a gravimetric anomaly map and the lines within represent seven cross-sections. Their detail can be seen in figures 5 and 6. All the cross-sections that are oriented in the NE-SW direction are shown in figure 5, while the profiles oriented NW-SE and N-S are shown in figure 6.

 

 

Figure 5a corresponds to the location of cross-section C in figure 4. Close to the left (SW) side of profile C (between 521.5 and 524.5 UTM), a big promontory is seen that may correspond to the extension of the Punta Banda peninsula offshore, flanked by the Maximinos and North Agua Blanca faults, numbered 11 and 12 in figure 7, respectively. The deepest part of the basin is 1500 m, rising gently in the NE direction (right side of the profile) before reaching the shore line. East of the shore line, another small basin is observed where the city of Ensenada is located.

Line D (figs. 4, 5b) extends from Punta Banda to Valle Dorado. In the SW part, the Punta Banda peninsula (left side of the profile) is flanked by the Agua Blanca and Maximinos faults. In this profile, the deepest part of the bottom basin is at 1300 m and becomes shallower in the NE direction (right side of the profile). Despite the model being smoothed in the inversion, the data required two inflections before arriving at the hills around Valle Dorado. These small changes in the dip are interpreted as faults 5 and 4 (figs. 2, 7).

Line E and its associated profile (figs. 4, 5c) go from Punta Banda to Maneadero Valley. In the SW (left side of the profile) there is a persistent signature of the Punta Banda peninsula, as well as the Agua Blanca and Maximinos faults. In the same line E, the basin can be seen to extend inland under Maneadero Valley where it reaches a maximum depth of 1100 m, which is different than the 1700 m reported by Vázquez (1980). To the NE there is a change in the slope that may correspond to fault 5 in figure 7.

Line F (fig. 4) extends from Punta Banda to Arroyo San Carlos, passing through Maneadero Valley where the basin gets shallower: 700 m, in contrast to the 900 m reported by Vázquez (1980). There are two changes in the slopes associated with faults 9 and 8 (figs. 5d, 7).

Line G runs (figs. 4, 6a) in NW-SE direction parallel to the Punta Banda peninsula, starting at Todos Santos islands and ending in El Zorrillo. In the NW part, the basement becomes shallower near the islands, then falls to the deepest part of the basin (1400 m) and turns complex between kilometers 527 and 533 UTM (fig. 7). This feature reflects the 3-D effects of the peninsula because the inversion was in 3-D. Between kilometers 535 and 540 UTM there are changes in the slope that may correspond to faults 10, 9 and 8 (fig. 7). At the end of the profile there is a small pond that corresponds to the El Zorrillo basin, 300 m in depth.

Profile A (fig. 4, 6b) goes N-S. In the south, persistent features of the peninsula and Agua Blanca faults are visible. The deepest part of the basin (1600 m) is located around kilometer 3515. Between kilometers 3520 and 3529 there are several structural features, among them faults 1 and 15. A more detailed morphological analysis could reveal other features; however, the most important are shown here.

Line B (figs. 4, 6c) shows several features, of which fault 5 is the clearest. It is located in the middle of the valley, splitting it structurally in two: Maneadero Valley to the south and Chapultepec Valley to the north. At kilometer 3520 (fig. 6b), the basement rises and the shape of the Valle Dorado syncline is observable (Schroeder, 1967) (figs. 2, 7). The Ensenada city basin is located to the north, with a maximum depth of 300 m.

Through the profiles and the cross-sections of the 3-D model, a number of different structural features can be seen and correlated. In figure 7, all the structural features picked from the cross-section analysis are shown. The numbers are the faults and the lower case letters indicate volumetric features. Fault 5 correlates very well with the Arroyo San Carlos fault (fig. 2) and its path can be traced into the bay. This normal fault is dipping to the SW and divides Chapultepec Valley (b) from Maneadero Valley (c). From a hydrological point of view, we think that this feature does not affect the aquifers because they are shallower; hence, the only direct effect may be a change in the geometry of the water table. Unfortunately, potential methods are not very sensitive to the aquifer geometry. Fault 2 does not project through any of the profiles, but it correlates well with the strike-slip fault located onshore, which is the proposed harbor fault (fig. 7). It is not possible, however, to support González-Serrano's (1977) hypothesis that it extends to the Agua Blanca fault as shown in figure 2.

The Agua Blanca North and South faults (fig. 7) are distinctive features that control most of the tectonics within the Todos Santos Bay basin. The extension of the faults beyond the islands is not interrupted by the submarine Cañón de Punta Banda (fig. 2); from figure 2 it is obvious that the trace of these faults extends offshore, as shown by Line C (fig. 4, 5a). Following their trace near Todos Santos islands, the azimuth changes and becomes more NNW oriented. Another important feature observed in Line A (fig. 6b) is the difference in altitude of 1750 m between the highest elevation in the Punta Banda peninsula and the lowest in the basin.

The Valle Dorado syncline apparently is an isolated, unusual compressive feature, which affects the early Cretaceous Alisitos Formation andesites (Line B; fig. 6c).

Fault 4 was determined by Line D (fig. 5b). It seems to dip to the SW, forming a step (small basin) in the basement morphology much like faults 8, 9 and 10, which are normal faults with the downthrow block in the direction that the steps induce the sinking of the basement under the bay. At the same time these three faults interact with the principal trace of the Agua Blanca fault and control the morphology of the Maneadero basin.

Fault 5 follows the Arroyo San Carlos river bed and extends offshore in Ensenada Bay, where it possibly connects with fault 2. Fault 7 bounds what we believe is a micro-basin, although further evidence is necessary. These features are indicated by the area of four prismatic bodies and there are enough gravity and magnetic data (fig. 3a, c), though there seems to be relevance to the magnetic data but not to gravity (fig. 3 a-d).

The Ensenada basin (a in fig. 7) seems to be a closed basin, reaching a maximum depth of 350 m. El Zorrillo is another closed area, with a maximum depth of 300 m. Faults 1 and 14 are necessary steps for the sinking of the gravity and magnetic basement into the bay. We suggest that features g, f and h (fig. 7) are topographic highs and do not correspond to the structural features of the andesitic rocks exposed by the erosion process. One of these highs corresponds to El Bajo San Miguel.

The main feature of the basin is the minimum of 1600 m (e) close to the Agua Blanca fault (s), which indicates that in this area the main component of the fault is the vertical sense, and it is therefore responsible for the depth of the basin. The strike of the minimum is close to E-W but when we consider the wider semi-axe, this seems more parallel to the Agua Blanca fault strike, following the direction of the tectonic stresses imposed by the fault.

 

Tectonic environment

The north branch of the Agua Blanca fault plays an important role in the development of Todos Santos Bay. Within this area, the fault passes offshore along the north side of the Punta Banda ridge, where it marks the steeply dipping contact between the basement rocks of the peninsula and the thick fill sediments in Todos Santos Bay. Here, the fault makes a sharp turn from almost E-W orientation to N25°-35°W strike. Along the Punta Banda ridge numerous marine terraces (eleven) are elevated (Ortlieb, 1979), which indicates that several processes have occurred producing a great amount of uplift as well as eustatic movements (Orme, 1974). Gastil et al. (1975) and Legg et al. (1991) interpreted Todos Santos Bay as a typical pull-apart basin and the Punta Banda ridge as a horst. However, it is possible that the Todos Santos basin during the Mesozoic time was acting as a fore-arc basin and during that time some extensional deformation may have occurred as fore-arc sedimentation was taking place. Syndepositional folding occurred at the same time that normal faults trending parallel to the tectonic strike were developed. Later this deformation pattern changed when the tectonic regime changed from subduction to a strike-slip (Dickinson, 1995, p. 237). Nevertheless, the Todos Santos Coastal Plain has been affected by different tectonic regimes. The most recent is associated with the tectonism within the continental borderland where the Agua Blanca fault plays an important role. This major structural feature is considered to be unstable because it changes geometry along the length and through time. This means that if the Agua Blanca fault has a stable configuration, then the lateral slip will take place along a single fault within the shear zone and all other faults will be pure dip-slip and oriented parallel to the strike-slip fault (Legg et al., 1991).

In the 3-D gravity and magnetic model obtained using all available data from different authors, the major problem was to homogenize the different data sets because they were corrected with different slabs for the Bouguer anomaly. Thanks to the software used, it was not necessary to move the vertical position of the original measurements. The responses of the 3-D model computed in the place of the observations fit very well with the original observation. We applied a smoother inversion in order to avoid unnecessary 3-D small effects in the gravity and magnetic basement. This gives us more confidence in the 3-D variations obtained, and we assume that the data needed those structural features.

The geological model corroborates the existence of all structural features mentioned by previous authors, such as the Agua Blanca fault, the San Carlos fault that can also be a step form along the development of the Maneadero half-graben, one of the faults in Maneadero Valley, and the fault oriented almost N-S that extends into Ensenada harbor.

The model also adds new knowledge about the Ensenada harbor fault that extends offshore into the bay, deforming the basin. At least three faults in Maneadero Valley were considered responsible for the sinking of the basement. One fault is considered to extend into Chapultepec Valley and apparently ends against the San Carlos fault. The Agua Blanca faults are drawn between Punta Banda and the islands and projected to the north with a change of strike arriving at Todos Santos islands. Between the islands and San Miguel at least two faults are recognized and justify the rise of the basin on the NW side with the presence of some mounts such as the Bajo San Miguel. The geometry of the Ensenada basin and its maximum depth were determined. The city of Ensenada and El Zorrillo ranch basins are considered close, and a maximum depth was estimated for each one. The elongated shape of the basin confirms that this is tectonically controlled by the Agua Blanca faults with some perpendicular faults as deepen steps explaining how this basement historically sank as a half-graben. It is possible that other faults exist but we do not yet have enough resolution in the data to confirm their existence.

 

Acknowledgements

Thanks to the anonymous reviewers for their comments. Thanks to Humberto Benítez-Pérez for his help in the edition of figures; and to Karem Englander for helping us to improve the final presentation.

 

References

Aguero M., G.A. (1986). Características de la bahía de Todos Santos y áreas costeras adyacentes. Tesis de licenciatura, Universidad Autónoma de Baja California, Ensenada, Baja California.         [ Links ]

Allen, C.R., Silver, L.T. and Stehli, F.G. (1960). Agua Blanca fault: A major transverse structure of northen Baja California, Mexico. Bull. Geol. Soc. Am., 71: 457-482.         [ Links ]

Allison, E.C., (1955). Middle Cretaceous Gastropoda from Punta China, Baja California, Mexico. J. Paleontol., 20 : 400-432.         [ Links ]

Aranda, F.J. (1983). Estudio de minerales pesados como trazadores de la corriente litoral en la Bahía de Todos Santos, B.C. Tesis de licenciatura, Universidad Autónoma de Baja California, Ensenada, Baja California 78 pp.         [ Links ]

Consejo de Recursos Naturales No Renovables (1962). Aeromagnetic data base from Continental Mexico. Presently: Consejo de Recursos Naturales de México.         [ Links ]

Cruz F., A. (1986). Gravimetría de la cuenca de San Carlos. Tesis de maestría, Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California, 98 pp.         [ Links ]

Dickinson, R.W. (1995). Forearc Basin. In C.J. Busby and R.V. Ingersoll (eds.), Tectonics of Sedimentary Basins. Chapter 6, pp. 221-261.         [ Links ]

Dowdy, P.R. (1977). A gravity survey of Valle de Mandadero, Baja California. San Diego State Univ., Geological Sciences, Senior Rep., 52 pp.         [ Links ]

Fabriol, H., Martínez, M. y Vázquez, R. (1982). Mediciones gravimétricas y telúricas en el valle de Maneadero, Ensenada, Baja California. Geofís. Int., 21: 41-55.         [ Links ]

Gallardo-D., L.A., Pérez-Flores, M.A. and Gómez-Treviño, E. (2003). A versatile algorithm for joint 3-D inversion of gravity and magnetic data. Geophysics, 68(3): 1-11.         [ Links ]

Gastil, R.G., Phillips, R. and Allison, C.E. (1975). Reconnaissance geology of the state of Baja California. Geol. Soc. Am. Mem., 140, 170 pp.         [ Links ]

González, J.J. and Suárez, F. (1984). Geological and seismic evidence of a new branch of the Agua Blanca fault. Geophys. Res. Lett., 11(1): 42-45.         [ Links ]

González-Fernández, A., Martín, A.B. y Paz, S. (2000). Identificación de fallamientos en la península de Punta Banda, B.A. a partir de datos de topografía, gravimetría y magnetometría. GEOS, 20: 98-106.         [ Links ]

González-Serrano, A. (1977). Anomalías gravimétricas y magnéticas de la bahía de Todos Santos. Tesis de licenciatura, Universidad Autónoma de Baja California, Ensenada, Baja California, 78 pp.         [ Links ]

Hummel, P. (1972). Geothermal investigation along the Agua Blanca fault. San Diego State Univ., Senior Rep., 21, 37 pp.         [ Links ]

Legg, M.R. (1985). Geologic structure and tectonics of the inner continental borderland offshore northern Baja California, Mexico. U.C. Santa Barbara, unpublished Ph.D. thesis, 408 pp.         [ Links ]

Legg, R.M., Wong, V. and Suárez-V., F. (1991). Geologic structure and tectonics of the inner continental borderland of northern Baja California. In: P. Dauphin and B. Simoneit (eds.), The Gulf and Peninsular Province of the Californias. AAPG Mem., 47: 145-177.         [ Links ]

Orme, A.R. (1974). Quaternary deformation of marine terraces between Ensenada and El Rosario, Baja California. In: G. Gastil and J. Lillegraven (eds.), Geology of Peninsular California. AAPG, SEPM and SEG Pacific section, guidebook, pp. 67-79.         [ Links ]

Ortega, M.A. (1988). Neotectónica de un sector de la falla de Agua Blanca, Valle Agua Blanca (Rancho la Cocina-Rancho Agua Blanca), Baja California, Mexico. Tesis de maestría, Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California, México, 146pp.         [ Links ]

Ortega-Rivera, A.E., Ferrar, J.A., Hames, D.A., Archibald, R.G., Gastil, D.L., Kimbrough, M., Zentilli, M., Lopez-Martínez, G., Féraud and Buffet, G. (1997). Chronological constraints on the thermal and tilting history of the San Pedro Mártir pluton, Baja California, Mexico, from U/Pb, 40 Ar/39 Ar, and fission-track geochronology. Geol. Soc. America Bull., 109(6): 728-745.         [ Links ]

Ortlieb, L. (1979). Quaternary shorelines around Baja California Peninsula, Mexico: Neotectonics implications (abs.). Geological Soc. Am., abstracts with programs, 11(7): 490.         [ Links ]

Pou A., S. (1982). Estudio de la tectónica de las islas de Todos Santos, B.C., México. Tesis de licenciatura, Universidad Autónoma de Baja California, México, 50pp.         [ Links ]

Quintanilla-Montoya, A.L. (1984). Origen del depósito sedimentario de la isla norte de Todos Santos, B.C. Tesis de licenciatura, Universidad Autónoma de Baja California, México, 34 pp.         [ Links ]

Quintanilla-Montoya, A.L. y Suárez-Vidal, F. (1992). Origen del depósito sedimentario de la isla norte de Todos Santos, B.C. Cienc. Mar., 18(1): 1-18.         [ Links ]

Reineck, H.E. and Singh, I.B. (1980). Depositional Sedimentary Environments with Reference to Terrigenous Clastic. Springer- Verlag, Berlin, pp. 260-261.         [ Links ]

Rockwell, K.T., Hatch, E.M. and Schug, L.D. (1987). Late Quaternary rates Agua Blanca and Borderland faults. Final Technical Rep. US Geol. Surv., 122 pp.         [ Links ]

Ross, K.Y. (1981). The stratigraphy and sedimentology of upper Cretaceous sediments of southwestern California and Baja California, Mexico. Ph.D. thesis, Rice University, 603 pp.         [ Links ]

Schroeder, J.E. (1967). Geology of a portion of the Ensenada quadrangle, Baja California, Mexico. M.Sc. thesis, San Diego State University, 74 pp.         [ Links ]

Suárez, F., Armijo, R., Morgan, G., Bodin, P. and Gastil, R.G. (1991). Framework of recent and active faulting in northern Baja California. In: P. Dauphin and B. Simoneit (eds.), The Gulf and Peninsular Provinces of the Californias. AAPG Mem., 47: 285-200 pp.         [ Links ]

Vázquez, G.R. (1980). Estudio de métodos potenciales con aplicaciones a geohidrología del valle de Maneadero. Tesis de maestría, Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California, México.         [ Links ]

Wong, V.M. (1980). Implicaciones tectónicas de la falla de Agua Blanca en la Bahía de Todos Santos. Tesis de maestría, Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California, México, 79 pp.         [ Links ]

Wong, O.V., Legg, M. y Suárez, F. (1987). Sismicidad y tectónica de la margen continental del sur de California (USA) y Baja California norte (México). Geofís. Int., 26(3): 459-478 pp.         [ Links ]