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Backscattering and geophysical features of volcanic ridges offshore Santa Rosalia, Baja California Sur, Gulf of California, Mexico
Journal of Volcanology and Geothermal Research 93 Ž1999. 75–92 www.elsevier.comrlocaterjvolgeores
Backscattering and geophysical features of volcanic ridges offshore Santa Rosalia, Baja California Sur, Gulf of California, Mexico b Hubert Fabriol a , Luis A. Delgado-Argote a,) , Juan Jose´ Danobeitia , ˜ c a,c a Diego Cordoba , Antonio Gonzalez , Juan Garcıa-Abdeslem , ´ ´ ´ a,c Rafael Bartolome´ b, Beatriz Martın-Atienza , Vıctor Frias-Camacho a ´ ´ a
b
DiÕision ´ Ciencias de la Tierra, CICESE, Carr. Tijuana-Ensenada km 107, Ensenada, B.C., 22830, Mexico Dipartamento Geofısica, Instituto Ciencias de la Tierra ‘‘ Jaume Almera’’, CSIC, Sole´ i Sabaris s r n, 08028 Barcelona, Spain ´ c Dipartimento Geofısica, Facultad Ciencias Fısicas, UniÕersidad Complutense de Madrid, 28040 Madrid, Spain ´ ´ Received 28 January 1998; accepted 10 September 1998
Abstract Volcanic ridges formed by series of volcanic edifices are identified in the central part of the Gulf of California, between Isla Tortuga and La Reforma Caldera–Santa Rosalıa ´ region. Isla Tortuga is part of the 40-km-long Tortuga Volcanic Ridge ŽTVR. that trends almost perpendicular to the spreading center of the Guaymas Basin. The Rosalıa ´ Volcanic Ridge ŽRVR., older than TVR, is characterized by volcanic structures oriented towards 3108, following a fracture zone extension and the peninsular slope. It is interpreted that most of the aligned submarine volcanic edifices are developed on continental crust while Isla Tortuga lies on oceanic-like crust of the Guaymas Basin. From a complete Bouguer anomaly map, it is observed that the alignments of gravity highs trending 3108 and 2908 support the volcanic and subvolcanic origin of the bathymetric highs. Volcanic curvilinear structures, lava flows and mounds were identified from backscattering images around Isla Tortuga and over a 400-m high ŽVırgenes High., where the TVR and the RVR intersect. A refractionrwide-angle seismic ´ profile crossing perpendicular to the Vırgenes High, together with gravity and magnetic data indicate the presence of shallow ´ intrusive bodies presumably of basaltic or andesitic composition. It is inferred that most volcanic edifices along the ridges have similar internal structures. We suggest that the growth of different segments of the ridges have a volcano-tectonic origin. The older RVR lies along the extension of a fracture zone and it probably is associated with Pliocene NE–SW extension. q 1999 Elsevier Science B.V. All rights reserved. Keywords: volcanic ridge; sidescan images; gravity and magnetic anomaly; seismic refraction
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Corresponding author. Division ´ de Ciencias de la Terra, CICESE, P.O. Box 434843, San Diego, CA 92143-4843, USA. Tel.: q1-5261-744501; fax: q1-5261-744933. E-mail address: [email protected] ŽL.A. Delgado-Argote.
1. Introduction The Gulf of California is the southern extension of the San Andreas transform system. The Baja
0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 8 4 - 0
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California peninsula is moving northwest relative to the continent at a full spreading rate of about 5.6 cmryr since about 6 Ma ago. The origin of this marginal sea is related to the Neogene Basin and
Range disturbance. An oceanic-like crust has been formed in spreading centers during the development of the transform system. Based on the tectonic map of Lonsdale Ž1989., it is observed that the eight
Fig. 1. Tectonic map of the Gulf of California region Žadapted from Lonsdale, 1989.. Approximate backscattering coverage from Hesperides ´ cruise CORTES-P96 is indicated with heavy line.
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largest basins with oceanic crust represent about 20% of the sea floor in the gulf. The remaining 80% is subsided-rifted continental crust of unknown thickness. The crust in the central part of the peninsula is close to 40 km thick ŽNava and Brune, 1982.. The geometry of the structures developed along the gulf margins and associated volcanism are poorly known. The most complete geophysical survey in the gulf was conducted by the CONMAR Group from the Oregon State University and Instituto Oceanografico de Manzanillo in 1981 and 1984. ´ Most of the results of these surveys are included in Dauphin and Simoneit Ž1991. and AtlasrMemoria of the Mexican Secretarıa ´ de Marina Ž1987.. During the CORTES-P96 ŽCrustal Offshore Research Transect by Extensive Seismic Profiling. project, the Spanish RrV Hesperides recorded more ´ than 3000 km of geophysical profiles ŽFig. 1. within the Gulf of California. Data collected along these profiles included multibeam bathymetry and backscatter images, gravimetry, magnetometry, multichannel near-vertical seismic reflection and refractionrwide angle reflection seismics ŽDanobeitia et ˜ al., 1996, 1997.. From these new geophysical data, we selected data lying along three transects located northeast of Santa Rosalıa, ´ offshore of the Pliocene La Reforma Caldera and the Las Tres Vırgenes ´ volcanic field Žbetween 27.408N and 27.708N., where remarkable bathymetric highs are observed ŽFig. 2.. This bathymetric high is the apparent northern continuation of the NNW–SSE Tortuga ridge that ends in the Guaymas Basin ŽLonsdale, 1989., and it is also characterized by a magnetic high ŽSanchez´ Zamora et al., 1991.. These data, combined with pre-existing data, are used to better define the structural, volcanic and bathymetric patterns of ridges located NW of the Guaymas Basin and around Isla Tortuga. This knowledge is important for determining the significance of volcanic ridges and their implications for the geology of the margins of the early Gulf of California.
2. Geologic and structural setting Before the beginning of the San Andreas–Gulf of California transform system, the circum-Gulf region experienced an E–W-oriented extensional deforma-
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tion during the Basin and Range disturbance ŽAngelier et al., 1981; Henry, 1989.. Large-scale normal faulting occurred before and during Miocene time along the Gulf Escarpment. Nearby areas, like Bahıa ´ de los Angeles ŽDelgado-Argote et al., 1999., show extensive explosive volcanism during and after that period of time. It has been documented that many normal faults were reactivated at the beginning of the transcurrent deformation in the western Gulf margin ŽMendoza-Borunda and Axen, 1995; Romero-Espejel, 1996; Delgado-Argote et al., 1999.. Miocene time is also recognized as one episode of intense volcanic activity along the margins of the gulf ŽGastil et al., 1979; Sawlan, 1991; Bigioggero et al., 1995; Delgado-Argote and Garcıa-Abdeslem, ´ 1999; Delgado-Argote et al., 1999. when the Neogene magmatic arc-axis was located near the present eastern margin of the peninsula. The area where simultaneous volcanism and extension took place during the Basin and Range period is primarily the same as the region where the transform system was later developed. Paleozoic metamorphic and Cretaceous granitic rocks ŽGastil et al., 1990, 1991. which are affected and partially covered by mostly Neogene volcanic rocks form the basement on both sides of the gulf. Near the study area, drill holes from the Las Tres Vırgenes volcanic field indicate that the ´ basement is formed by late Cretaceous granitic rocks ŽSanchez-Velasco, 1996.. The granitic and some Pa´ leozoic metasedimentary rocks crop out 50 km west of the study area. These basement rocks are extensively covered by the Oligocene andesitic sequence of the Sierra La Giganta and the Miocene to Pleistocene andesitic and dacitic lavas and pyroclastic rocks from the Las Tres Vırgenes volcanic field ´ ŽCapra et al., 1998. ŽFig. 2.. The oldest spreading centers of the Gulf of California initiated their development about 6 Ma ago. In many areas, less than 5-Ma-old oceanic-type crust is in tectonic contact with continental crust along transform faults. The oldest boundaries of the basins connecting transform faults are interpreted to be constituted by transitional crust consisting of stretched continental crust and igneous rocks of the early magmatic activity in the embryonic basins. Gastil and Fenby Ž1991. and Axen Ž1995. have proposed that regional tilting associated with low-angle detachment faults resulted from extension during
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the process of rifting of the Gulf of California. In areas like Bahıa ´ de los Angeles–Isla Angel de la Guarda, no evidence has been found to interpret important degrees of tilting associated with detachment; instead, discrete blocks have distinct histories of pre-Miocene extension ŽDelgado-Argote and
Garcıa-Abdeslem, 1999; Delgado-Argote et al., ´ 1999.. It is observed in Fig. 2 that the peninsular margin south of Cabo Vırgenes is characterized by normal ´ faulting, probably associated with the fracture zones that limit the southern part of Guaymas Basin. Based
Fig. 2. Tectonic map of the central Gulf of California Žadapted from Lonsdale, 1989. and lithology of the Las Tres Vırgenes volcanic field ´ and La Reforma Caldera region ŽINEGI, 1983, 1984.. Merged bathymetry adapted from INEGI Ž1:1,000,000., Lonsdale Ž1989. ŽGuaymas Basin. and the CORTES-P96 cruise. Heavy dashed lines indicate the track of the RrV Hesperides, dots along the margins indicate ´ transform faults active before and during initial phase of the opening of the Gulf, and solid lines with ticks indicating relative movement along the peninsular margin indicate fracture zone extension. 3V, Tres Vırgenes volcanic field; A, Aguajito caldera; R, Reforma Caldera; ´ CV, Cabo Vırgenes; GB, Guaymas Basin; ISM, Isla San Marcos; IT, Isla Tortuga; SPMB, San Pedro Martir Basin. Lithology: Ž1. ´ ´ Quaternary sediments, Ž2. Quaternary lava flows and tuffs, Ž3. late Tertiary basalt and basaltic breccia, Ž4. late Tertiary tuffs and ignimbrites, Ž5. late Tertiary andesite, Ž6. late Tertiary sedimentary and volcano sedimentary units, Ž7. Miocene volcaniclastic rocks, Ž8. Miocene sandstone, Ž9. undifferentiated volcanic rocks, Ž10. Cretaceous granitoids, Ž11. Paleozoic metamorphic rocks.
H. Fabriol et al.r Journal of Volcanology and Geothermal Research 93 (1999) 75–92 Fig. 3. Bathymetric map showing the side scan images and the rose diagrams of structures interpreted in Figs. 4 Žsegment A., 5 Žsegment B. and 6 Žsegment C.. VH, Vırgenes ´ high; CV, Cabo Vırgenes; IT, Isla Tortuga; TVR and RVR indicate the axis of the Tortuga Volcanic Ridge and the Rosalıa ´ ´ Volcanic Ridge, respectively. 79
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on the spreading rate estimates, Batiza Ž1978. suggested that Isla Tortuga was constructed on 1.7 Ma old oceanic crust and that the island is the result of a northward-migrating volcanic activity. However, the 1000 bathymetric line suggests that the island is part of a WNW-oriented ridge. South of the island, the ridge is oriented NW, and it narrows reaching the Guaymas Basin ŽFig. 2.. It has been reported ŽBatiza, 1978. that the old volcanic series of the island were subaerially extruded and it is inferred that it has been tectonically submerged. The suite of basalts on Isla Tortuga is similar to ocean–ridge tholeiite suites ŽBatiza, 1978; Sawlan, 1991.. In the Guaymas Basin, an oceanic-like crust of about 125 km across has been developed. The basin shows active tholeiitic volcanism and, in its southern spreading center, the lava flows and dikes are interfingered with sediments ŽEinsele et al., 1980; Curray et al., 1982.. Sawlan Ž1991. has classified the basaltic rocks of the Guaymas Basin as transitional rift-tholeiites.
3. Bathymetry and sidescan images of the Tortuga Volcanic Rift In order to identify regional structures from the morphology, we merged the 1:1,000,000 bathymetric map of INEGI for the whole region, the detailed bathymetry of the Guaymas Basin ŽLonsdale, 1989., and the multibeam data collected by the RrV Hesperides of four segments of tracks for the study ´ area ŽFig. 2.. This last data set provides high-resolution bathymetry down to 20 m in a swath of about 3.5 times water depth. This new map shows a number of hills and mounds interpreted to be volcanic in origin, and linear features interpreted to be of tectonic origin. The most conspicuous structure is the Rosalıa ´ Volcanic Ridge ŽRVR in Fig. 3. formed by a series of volcanic edifices trending 3108, following
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interpreted fracture-zone extensions of transform faults offshore the peninsula ŽLonsdale, 1989.. This ridge is about 75 km long, from 27.98N to 27.48N ŽFig. 2., and it is located west of Isla Tortuga. The island is located in another important morphologic feature, named Tortuga Volcanic Ridge, which is observed from the bathymetric map ŽTVR in Fig. 3.. This about 7-km-wide and at least 40-km-long ridge, trending 2858 is well defined by the isobath of 1000 m. For clarity, Fig. 3 shows the backscatter images of Figs. 4–6 overlapped on the detailed bathymetric map. The first section ŽFig. 4, named A in Fig. 3. is located south of the Isla Tortuga in the NW limit of the Guaymas Basin, and east of the RVR. The topographic and textural data ŽFig. 4. allowed the interpretation of structural lineaments and the recognition of the distinctive signature of relatively inelastic sediments from relatively elastic volcanic rocks. It is observed from Figs. 3 and 4 that sediments cover most of the area south of Isla Tortuga. The image shows poorly defined 0308, 2708 and 3108 trending lineaments inferred to be fractures ŽFig. 4.. The topography gently rises towards the coast from 1475 to 1200 m Ž2%-slope; Fig. 4d.. A 2-km wide body of lava flows is interpreted in the southern part of Isla Tortuga that apparently flowed down about 6 km from the island reaching a depth of more than 1400 m. The lavas are intersected by NE and SW dipping lineaments trending towards 3208 in average. The wrinkled lava flows of the eastern side, interpreted to be pillow lavas ŽPL in Fig. 4a., are about 50 m high with respect to the surrounding floor. In contrast, the smooth lavas of the western side are interpreted as sheet-like lava flows ŽSF in Fig. 4a.. The area occupied by the last flows shows abundant mounds interpreted to be vents, while the pillow lavas show concave features indicating a southward flow. Fractures in the pillow lavas are oriented towards 0358 and 0008 suggesting the presence of
Fig. 4. Ža. Sidescan sonar image with bathymetric lines of the southern part of Isla Tortuga. Light areas are low backscatter; the dark areas are interpreted to be lava flows probably extruded from Isla Tortuga. Žb. Structural interpretation of the image indicating the presence of curvilinear lineaments associated with subvolcanic bodies. Žc. Rose diagram of lineaments rotated in the sense of the ship-track; notice the strong tendency of the structures towards NW, parallel to the orientation of the regional fracture zones near Guaymas Basin. Žd. Profile along the track of the ship indicating the presence of interpreted subvolcanic bodies; dashed lines are the lava flows located in the right side of the image.
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fissures. On the western side of this transect, about 25 km offshore Santa Rosalıa, ´ a third series of lava flows is observed with mounds aligned towards 3108 ŽFig. 4a.. A large canyon and other fractures that cut volcanic structures are oriented in the same direction, suggesting that they are part of a fracture-zone extension ŽFig. 3. and that deformation and volcanic activity occurred simultaneously. The second section Žnamed B in Fig. 3. is oriented towards 3208 and lies between the RVR and La Reforma caldera. The presence of a field of lavas in the southern half of the image is the most notable feature ŽFig. 5.. Although it is presumable that the lavas flowed eastwards from vents related to the La Reforma volcanic field, it is also possible that they were extruded from local fissures. The profile along the transect shows a domic shape cut in its southern part by a narrow canyon bounded by normal faults ŽFig. 5b.. The northwestern half of the image is characterized by a relatively inelastic sedimentary cover where nested mounds Žroughly oriented towards N–S and 3208. and curvilinear features are interpreted to have a volcanic association. The whole image shows an orthogonal arrangement of lineaments trending N–S and E–W ŽFig. 5b and c.. Although this structural geometry is distinct from that reported onshore in the Las Tres Vırgenes vol´ canic field, where intense seismicity of tectonic origin has been reported ŽMunguıa ´ and Wong, 1995., it is similar to the regional structures reported by Garduno-Monroy et al. Ž1993. in El Aguajito Caldera. ˜ The section C ŽFigs. 3 and 6. has an almost perpendicular orientation with respect to the RVR, and the most conspicuous volcanic and structural features are observed in this image ŽFig. 6.. The Vırgenes High is clearly defined in this section by a ´ 3.5-km-wide bathymetric high where the western side has a 25% slope, elevated about 200 m, and the abrupt eastern side has an elevation of 400 m with a slope of 50%. The morphology of this portion of the
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ridge suggests the presence of a complex volcanic edifice bounded by normal faults forming a horst-like structure. The high, oriented towards 3008 and slightly tilted eastward, shows a series of curvilinear volcanic and subvolcanic lineaments on the top. A series of feeder dikes are interpreted in the section of Fig. 6d, which could explain the observed curvilinear features. We also interpret the construction of this portion of the ridge to result from the combination of normal faulting and uplifting during the emplacement of magma bodies. A remarkable 6-km diameter curvilinear structure similar to a plateau is located in the eastern side of the ridge. This flat structure has some mounds and its relatively inelastic sediment cover suggests that the plateau is older than the horst-like Vırgenes High. The most abundant and ´ continuous structural lineaments and alignments of volcanic mounds are oriented towards 3008 and 0308. In general, the main structure of the high is almost parallel to the TVR, and the NE-oriented lineaments located in the northeastern portion of the image could be older features belonging to the early Guaymas Basin. The three rose diagrams of structural and volcanic lineaments of the images show important differences with respect each other ŽFigs. 3, 4c, 5c and 6c.. In the southern transect ŽFig. 4., the dominant features are oriented 3208, parallel to the coast and the peninsular slope. They can be correlated with transcurrent faults developed before and concurrently to the initial phases of the opening of the gulf. From the same figure, other not so well defined structures trend 0358 and 0808; the first group is more or less parallel to the Guaymas Basin and the second group has no correlation with other regional features. The section B ŽFig. 5. shows N–S and E–W orthogonal features ŽFig. 5c. that apparently are unrelated to regional structures. In the section C ŽFig. 6., the main tendency of structures towards 3008–3108 clearly correlates with the orientation of the RVR ŽFig. 5c..
Fig. 5. Ža. Sidescan sonar image of the western part of Isla Tortuga. The dark areas are interpreted as fissure related lavas; notice the lack of lavas in the northwestern part of the image. Žb. Structural interpretation of the image where it is remarkable the absence of circular structures in the southeast compared with the northwestern part. Žc. Rose diagram of lineaments rotated in the sense of the ship-track; E–W oriented lineaments are related to fissures of the southeastern part of the image and the NNW lineaments are parallel to the peninsular slope end the submarine canyon located to the west of Isla San Marcos ŽFig. 2.. Žd. Profile indicating the subvolcanic structures below the lava flows and normal faulting in the southeastern end of the image.
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Fig. 6. Ža. Sidescan sonar image of the Vırgenes High zone with bathymetric lines every 25 m. Žb. Structural interpretation of the image ´ where the presence of a large curvilinear structure in the eastern side of the ridge is remarkable. This structure is volcanic in origin and apparently older than the ridge, where nested subvolcanic structures are similar to these located eastward. Žc. Rose diagram indicating two main tendencies of lineaments towards NW and NE. The NW-trending lineaments are parallel to the Tortuga Volcanic Ridge. Žd. Profile indicating important normal faulting developed during the evolution of the Vırgenes High showing shallow intrusive bodies. ´
H. Fabriol et al.r Journal of Volcanology and Geothermal Research 93 (1999) 75–92 Fig. 7. The complete Bouguer anomaly and bathymetry are shown with contours every 10 mgal and 100 m, respectively. Although the data distribution is sparse, as shown by the ship tracks from CONMAR Žcontinuous line. and CORTES-P96 Ždashed line. cruises, the gravity anomaly shows a direct correlation with the expected excess of mass at the Guaymas Basin. Fifty kilometers long gravity highs Ž40–50 mgal. aligned in a 2908 direction ŽVH to B. is interpreted to be produced by magma bodies, which define the Tortuga Volcanic Ridge. Parallel to the peninsular slope, the gentle gravity gradient is interpreted to be associated with a tectonic structure where magma was emplaced describing the 3108-trending Rosalıa ´ Volcanic Ridge. Along the RVR axis, both aligned volcanic edifices and local gravity anomalies are observed ŽVH to C.. 85
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Finally, it is important to mention that temporal seismological networks in the region of the La Reforma caldera and the Las Tres Vırgenes volcanic ´
field do not show any seismicity in the Gulf region. Active faulting and volcanic seismicity ŽMunguıa ´ and Wong, 1995. is concentrated onshore, which
Fig. 8. Geophysical modeling of the transect over the Vırgenes High of Fig. 6, indicating the record sections of the two land seismic ´ stations. Observed data in Ža. and Žb. has been band-pass filtered between 3 and 8 Hz. Times are reduced to 6 kmrs and amplitudes have been normalized. Superposed lines in Ža. and Žb. indicate calculated travel times. Žc. Velocity–depth model along a 60-km-long profile indicating the coast line with CT. For clarity, one of each five rays have been printed in the ray diagram. Žd. Free-air anomaly data profile; crosses represent the observed gravity data and the solid line indicates the calculated gravity values. Že. Residual magnetic anomaly data profile. Crosses represent the observed magnetic data and the solid line indicates the calculated magnetic values. Žf. Adjusted density and susceptibility model used to produce the calculated gravity and magnetic data profiles shown in Žd. and Že.; vertical exaggeration is five times and a 1:1 representation is also shown below.
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Fig. 8 Ž continued ..
suggest that the largest structures in the study area, with the exception of the Guaymas Basin, are mostly inactive. 4. Regional gravimetry The large density contrast between the sea water and the underlying rocks and sediments means that
the contours of the free-air gravity anomalies generally follow bathymetric features. However, where the contours do not follow bathymetric features, we expect lateral changes in the structure of the basement andror the thickness of sediments. With the purpose of suppressing such a well-known effect, we have obtained a complete Bouguer anomaly map for a region that includes the Guaymas Basin and the
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RVR, over the western margin of the Gulf of California ŽFig. 7.. The data used for this analysis include land topography data from the ETOPO-5 digital model ŽNGDC, 1988., the coastline ŽINEGI, 1983, 1984. and free-air gravity anomaly and bathymetric data collected during CONMAR-IOM cruises ŽBAJA-76, MARSUR78, GOLFO-81, GOLFO-84; Dauphin and Ness, 1991., and the CORTES-P96 cruise ŽDanobeitia et ˜ al., 1996, 1997.. Bathymetric and topographic data were merged and gridded by kriging to obtain a regular grid with nodes at every four kilometers. A rectangular prism was then placed at the center of each grid node, and their gravity effect was computed at underway free-air gravity stations position Ž1996.. The comby the method of Garcıa-Abdeslem ´ plete Bouguer correction was carried out up to 167 km. Onshore, the gravity effect is caused by topography with a density of 2670 kgrm3. Offshore, water was replaced by sediments with a density of 2000 kgrm3. By adding this correction to the free-air gravity anomalies, we have obtained the complete Bouguer anomaly map shown in Fig. 7. One remarkable feature in the Bouguer anomaly map ŽFig. 7. is a broad, up to 50 mgal, gravity high located near the center of the Guaymas Basin, roughly following the Guaymas-North spreading center ŽLonsdale, 1989.. The 40-mgal contour approaches the geometry of the Guaymas Basin. Another feature that emerges from this data is an elongated gravity high reaching up to 40–50 mgal ŽVH, A, IT, and B in Fig. 7. that extends along the TVR ŽFig. 3. for about 50 km, following a 2908 direction, perpendicular to the Guaymas-North spreading center. Parallel to the peninsular slope, the gentle gravity gradient with decreasing values towards the SW, observed from Cabo Vırgenes to the Guaymas Basin, is in´ ferred to be associated with the 3108-trending RVR. Along this ridge, aligned volcanic edifices and local gravity anomalies are observed ŽFig. 7.. The ridge is parallel to the fault structures of the initial phase of the opening of the gulf shown in Fig. 2. The Tortuga and Rosalıa ´ ridges are deviated about 308 from each other, suggesting different periods of formation. We suggest that the RVR is older than the TVR, since the apparently most recent structures that follow the orientation of the TVR in the Vırgenes High ŽA in ´ . Fig. 7 cut older volcanic structures, and that re-
cently active volcanic activity has been reported in Isla Tortuga ŽBatiza, 1978.. 5. Geophysical modeling The geophysical data collected along the transect C of Fig. 3 consist of deep near-vertical seismic reflection, refractionrwide-angle seismic reflection, gravity and magnetic data. The energy of the 2850 in.3 Ž47 l. air gun array used to obtain deep nearvertical seismic reflection data Žunder process. was also recorded by two portable instruments on land, providing a densely sampled Ža seismogram every 80 m. refractionrwide-angle reflection data set. Clear wide-angle seismic arrivals up to 100 km distance were recorded. In order to model the uppermost crustal structure in the Vırgenes High, seismograms ´ of the first 60 km were selected. The record sections of Fig. 8a and b, corresponding to the two seismic stations, show a neat first arrival identified as Pg phase. This phase shows an advance of 0.5 s reduced time from 41 to 52 km epicentral Žstation-shot. distance from the seismic station 1, and from 35 to 46 km epicentral distance from the seismic station 2. To obtain a good fit of the calculated travel times with the observed arrival times, a ray-tracing forward modeling of the seismic data has been iteratively applied, adjusting P-wave velocities and depths to the discontinuities ŽFig. 8c.. We have used the method proposed by Zelt Ž1989. and Zelt and Smith Ž1992.. Densely sampled potential field measurements Žgravimetry and magnetometry. were also collected along the transect. The residual magnetic anomaly profile has been obtained by reducing the magnetic data, using the coefficients of the International Geomagnetic Reference Field of 1995. To model the potential field data, a 250-m sample spacing was selected to further constrain the structural interpretation. The Free Air gravity anomaly along the transect ŽFig. 8d. shows a gravity high that reaches up to 25 mgal and is located over the Vırgenes High. The ´ main feature in the residual magnetic anomaly ŽFig. 8e. is a large magnetic high of about 120 nT that is slightly displaced towards NE of the Vırgenes High. ´ Both gravity and magnetic anomalies are located in the same position of the travel time seismic anomaly mentioned previously ŽFig. 8a and b..
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A Talwani et al. Ž1959.-type algorithm ŽCooper, 1991. was used to obtain both the 2D1r2 gravity and magnetic models. The densities used in this model were obtained from the seismic velocities, employing the empirical relationship proposed by Ludwig et al. Ž1971. and Zelt Ž1989.. Induced magnetization was assumed for the magnetic model. The values of magnetic susceptibility were obtained from Carmichael Ž1989.. The results of the refractionrwide-angle reflection seismic ray-tracing modeling profile and the 2D1r2 gravity and magnetic modeling are presented in Fig. 8e and f. The sedimentary cover consists of two layers, where the uppermost sedimentary layer Žsand, marine sandstone and pyroclastic flows. is characterized by a P-wave velocity of 2.0 kmrs, a density of 1900 kgrm3 and a susceptibility of 25 = 10y6 e.m.u. Based on a number of drill-holes in the Las Tres Vırgenes volcanic field, the next layer is ´ formed by about 1-km thick pile of volcanic rocks of the Comondu´ group and Santa Lucıa ´ andesite ŽBallina-Lopez, 1985.. This second sedimentary layer is ´ characterized by a velocity of 3.3 kmrs, a density of 2300 kgrm3 and a susceptibility of 120 = 10y6 e.m.u. These values are confirmed by previous results obtained from the interpretation of seismic refraction, bore-holes and electric soundings carried out in the volcanic area of Reforma–Tres Vırgenes ´ ŽBallina-Lopez, 1985.. Based on our seismic and ´ gravity data, it is observed that the sedimentary cover slightly thins until reaching the Vırgenes High, ´ where the basement crops out. The sedimentary thickness to the northeast of this structure appears to be thinner than to the southwest. The sedimentary layer rests on a ca. 82 Ma granitic basement ŽViggiano-Guerra, 1992., with a P-wave velocity of 6.1 kmrs, a vertical velocity gradient of 0.04 km sy1 kmy1 , a density of 2700 kgrm3 and a susceptibility of 500 = 10y6 e.m.u. The magnetic anomalies can be explained by the presence of three bodies in the Vırgenes High area, between 40 and 50 km from the ´ coast, extending from 0.8 to 1.8 km depth below sea level ŽFig. 8f.. These bodies are characterized by a density of 2900 kgrm3 and a high susceptibility contrast of 10,000 = 10y6 e.m.u. In order to model the influence of these bodies in seismic data, a velocity of 6.6 kmrs has been assigned to this portion of the Vırgenes High. ´
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6. Discussion and conclusions We have defined volcanic ridges based on the integration of magnetic, gravimetric and bathymetric data, as well as structural features and alignments of interpreted volcanic edifices and associated lava flows around the Isla Tortuga. The structural detail based on the textural description of the sea floor shows a complex pattern of curvilinear fractures that can result from the emplacement of subvolcanic intrusive bodies. The largest structures are more rectilinear, show well defined orientations ŽFigs. 3– 6., and their origin can be attributed to the Basin and Range extensional disturbance, and the opening of the Gulf of California ŽHenry, 1989; Lonsdale, 1991; Sawlan, 1991.. In the eastern margin of the Baja California peninsula, along the Gulf Escarpment, the presence of large volcanic rift systems where subcircular concentric faults and joints are associated with NNW– SSE to NW–SE fracture systems has been documented ŽZanchi, 1994.. Uplifting is observed in this area where the fracture systems apparently coexist with dike swarms and complex sets of fractures showing a radial pattern of magmatic origin. A similar association of Miocene extensional deformation and development of basaltic volcanic rifts have been documented in the Bahıa ´ de los Angeles basin, about 150 km north of the Las Tres Vırgenes vol´ canic field ŽDelgado-Argote and Garcıa-Abdeslem, ´ 1999. In this last area, normal faulting bounds elongated hills where lava flows are connected with feeder dike swarms. Both areas where extension and volcanic activity are recognized to coexist indicate that this association is a common phenomenon during Neogene time in several places of the western margin of the Gulf. The extensions associated with the Basin and Range and the San Andreas–Gulf of California regional deformations have not been symmetric or contemporaneous along the circum-gulf region. In a recent synthesis, Lee et al. Ž1996. indicated that extension could be as large as 35%–58% in the Loreto Basin ŽPliocene. or 10% in the northeastern part of the peninsula since late Miocene time. In the Santa Rosalıa ´ area, the same authors mentioned that extension during late Miocene was oriented NE–SW and it changed to WNW–ESE and E–W during
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Pliocene time. Near this region, in the Las Tres Vırgenes volcanic field, Angelier et al. Ž1981. docu´ mented ENE–WSW followed by ESE–WNW extensional deformation, associated to NW–SE to NNW– SSE and N–S to NNE–SSW strike-slip faulting. In addition, Sawlan and Smith Ž1984. and Stock and Hodges Ž1989. mentioned that the Gulf Escarpment, near the volcanic field, is younger than the 10-Ma-old tholeiitic Basalt of Esperanza and proposed that the NNW trending normal faults do not affect 3-Ma-old units. Since the orientation of the RVR is approximately perpendicular to the direction of extension during Pliocene time and lies along a fracture-zone extension, it is assumed here that it is about 5 Ma old, similar to the age of the Guaymas Basin. In that sense, the RVR is constituted by transitional crust consisting of stretched continental crust and igneous rocks of the early magmatic activity in the embryonic Guaymas Basin. The Isla Tortuga is formed by tholeiitic basaltic and andesitic flows younger than 1.7 Ma constructed over a ‘‘leaky-fracture zone’’ ŽBatiza, 1977.. This fracture zone is now interpreted as a volcanic ridge ŽFigs. 3 and 7.. It has been suggested that the oldest volcanic activity on the island initiated in its southeastern portion and later migrated NW to the present position and that normal faulting, perpendicular to the fracture zone is for tectonic submergence ŽBatiza, 1978.. The normal faults oriented towards 0408 are parallel to the trend of the Guaymas spreading center and other volcanic and structural lineaments interpreted from the backscattering images in the southern and northeastern sides of the island ŽFigs. 4 and 6.. The alignment of gravity and bathymetry highs of Figs. 3 and 7 indicates that the island is part of the 2908 trending TVR, which intersects the RVR in the Vırgenes High. In spite of the lack of detailed ´ bathymetry around the island, we assume that its internal structure is similar to that interpreted from geophysical data in the Vırgenes High ŽFigs. 7 and ´ 8.. Because the Vırgenes High is located at the ´ intersection of two regional volcanic ridges, it is characterized by the presence of 2808–3108-oriented conspicuous fractures and normal faults, which bound this structural high. The gravity high in the Vırgenes High ŽFig. 7. ´ results from mass excess related to the accumulation of basaltic magma bodies Ždensity of 2900 kgrm3 .
in shallow intrusives enclosed in granitic country rock. This interpretation is supported by the geophysical models presented in Fig. 8, where it is observed that the tops of a dike and sill-like bodies are located at about 800 m below the surface extending downward to a depth of about 1800 m. Similar structures have been documented from gravity and structural data in the Bahıa ´ de los Angeles area, where Miocene basaltic andesite flows are related to shallow magma bodies enclosed by granitic country rocks ŽDelgadoArgote and Garcıa-Abdeslem, 1999.. We interpret ´ most of the circular lineaments located on the Vırgenes High to be fractures formed during the ´ emplacement of magma bodies as those interpreted in the magnetic model. Subvolcanic magma bodies with characteristic velocities, densities and magnetic susceptibilities of basaltic to andesitic material therefore makeup the Vırgenes High and, in consequence, ´ most prominent highs of the ridges. One to three kilometers deep sill-like reservoirs connected to subvolcanic dikes have been documented in other localities ŽGudmundsson, 1995.. Ryan et al. Ž1983. have remarked that such sill geometry can explain surface deformation, especially normal faulting. The last authors have shown that large-scale deformation patterns require large plan-view intrusives, such as those interpreted from the backscatter images. The Vırgenes High gives us a reference internal ´ structure for the individual volcanic edifices associated with regional structures. The growth of different segments formed by two or more volcanic and subvolcanic structures define a volcanic ridge that can be related to regional tectonic structures, like the fracture zone extensions, which resulted from the transtensive regime of the San Andreas–Gulf of California system. We assume that magma emplacement is a passive phenomena, leading us to suggest that the RVR developed perpendicular to a regional E–W extension to which is also related the deformation and volcanic activity in the volcanic fields of Las Tres Vırgenes, El Aguajito and La Reforma ´ since Pliocene time. Since the early stages of the development of the proto-Gulf of California ŽKarig and Jensky, 1972. the Gulf region has experienced contemporaneous extension and volcanism. One of the main contributions of this study is the correlation of large-scale fracture zones with the emplacement of mafic magma defining volcanic ridges. Under-
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standing the scale and internal geometry of volcanic ridges and their association with tectonic events is an important key to visualizing the evolution of a large part of the circum-Gulf region during Neogene time. Acknowledgements We want to thank the Mexican authorities for all kind of facilities to work within the exclusive economic zone of Mexico. Our gratitude to the captains ´ and crews of the RrV’s Hesperides, Altair and ´ Humboldt for their support during the experiment. Also, we thank the many technicians and students from Spain and Mexico that participated in the dif´ ferent activities inland on the peninsula. This survey was supported by the CICYT projects ANT94-0182C02-01r02 and ANT94-0182-C02-02, the Secretarıa ´ de Marina, CICESE Ž644107. and a cooperative support by CONACyT-CSIC. The Direccion ´ General de Investigacion y Ensenanza Superior ´ Cientıfica ´ ˜ EX96-0002876832, Ministerio de Educacion ´ y Cultura ŽSpain., supported A. Gonzalez in CICESE, ´ Ensenada. We thank Rodey Batiza and an anonymous reviewer for their constructive comments. Finally, we want to thank Lance Forsythe for critic final revision of the manuscript. References Angelier, J., Colletta, B., Chorowicz, J., Ortlieb, L., Rangin, C., 1981. Fault tectonics of the Baja California Peninsula and the opening of the Sea of Cortez, Mexico. J. Struct. Geol. 3, 347–357. Axen, G., 1995. Extensional segmentation of the main gulf escarpment, Mexico and United States. Geology 3, 347–357. Ballina-Lopez, H.R., 1985. Estudio geofısico en la zona geotermica ´ ´ ´ de Tres Vırgenes, B.C.S. Geoterm. Rev. Mex. Geoenerg. 1, ´ 21–43. Batiza, R., 1997. Oceanic crustal evolution: evidence from the petrology and geochemistry of isolated oceanic central volcanoes. PhD Thesis, Univ. California, San Diego, 1977, 295 pp. Žunpublished.. Batiza, R., 1978. Geology, petrology, and geochemistry of Isla Tortuga, a recently formed tholeiitic island in the Gulf of California. Geol. Soc. Am. Bull. 89, 1309–1324. Bigioggero, B., Chiesa, S., Zanchi, A., Montrasio, A., Vezzoli, L., 1995. The Cerro Mencenares volcanic center, Baja California
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Sur: source and tectonic control on postsubduction magmatism within the Gulf Rift. Geol. Soc. Am. Bull. 107, 1108–1122. Capra, L., Macıas, J.M., Siebe, C., 1998. Holocene ´ J.L., Espındola, ´ plinian eruption of La Virgen volcano, Baja California, Mexico. J. Volcanol. Geotherm. Res. 80, 239–266. Carmichael, R.S., 1989. Magnetic properties of minerals and rocks. In: Carmichael, R.S. ŽEd.., Practical Handbook of Physical Properties of Rocks and Minerals. CRC Press, FL, pp. 299–358. Cooper, G.R.J., 1991. Geomodel. 2.5 Dimensional Interactive Magnetic and Gravity Modelling v. 1.3. Dept. Geophysics, Univ. Witwatersrand, Johannesburg, South Africa. Curray, J.R., Moore, D.G., Kelts, K., Einsele, G., 1982. Tectonics and geological history of the passive continental margin at the tip of Baja California. In: Curray, J.R., Moore, D.G. ŽEds.., Init. Rep. D.S.D.P. 64. U.S. Gov. Print. Office, Washington, DC, pp. 1089–1121. Dauphin, J.P., Ness, G.E., 1991. Bathymetry of the Gulf and Peninsular Province of the Californias. In: Dauphin, J.P., Simoneit, B.R. ŽEds.., The Gulf and Peninsular Province of the Californias. Am. Assoc. Pet. Geol. Mem. 47, 21–23. Dauphin, J.P., Simoneit, B.R., 1991. The Gulf and Peninsular Province of the Californias. In: Dauphin, J.P., Simoneit, B.R. ŽEds.., The Gulf and Peninsular Province of the Californias. Am. Assoc. Pet. Geol. Mem. 47, 793 pp. Danobeitia, J.J., Delgado-Argote, L., Michaud, F., Carbonell, R., ˜ Canales, J.P., Ayarza, P., Grupo Marino CORTES-P96, 1996. Informe de la Campana del margen conti˜ evolucion ´ tectonica ´ nental oeste de Mexico: fosa Mesoamericana y Golfo de ´ California, 123 pp. Žunpublished.. Danobeitia, J.J., Cordoba, D., Delgado-Argote, L.A., Michaud, F., ˜ ´ Bartolome, ´ R., Farran, ´ M., Carbonell, R., Nunez-Cornu, ´˜ ´ F., CORTES-P96 Working Group, 1997. Expedition gathers new data on crust beneath Mexican west coast. EOS 78, 565–572. Delgado-Argote, L.A., Garcıa-Abdeslem, J., 1999. Shallow ´ Miocene basaltic magma reservoirs in the Bahia de los Angeles Basin, Baja California, Mexico. J. Volcanol. Geotherm. Res. 88, 29–46. Delgado-Argote, L.A., Lopez-Martınez, M., Perrilliat, M.C., 1999. ´ ´ Geologic reconnaissance and age of volcanism and associated fauna from sediments of Bahia de los Angeles, Central Gulf of California. In: Stock, J., Aguirre, G., Delgado, H. ŽEds.., Cenozoic Tectonics and Volcanism of Mexico. Geol. Soc. Am. Special Paper Žin press.. Einsele, G., Gieskes, J.M., Curray, J., Moore, D.M., Aguayo, E. et al., 1980. Intrusion of basaltic sills into highly porous sediments, and resulting hydrothermal activity. Nature 283, 441–445. Garcıa-Abdeslem, J., 1996. GL2D: a Fortran program to compute ´ the gravity anomaly of a 2-D prism where density varies as a function of depth. Comput. Geosci. 22, 823–826. Garduno-Monroy, V.H., Vargas-Ledezma, H., Campos-Enrıquez, ˜ ´ J.O., 1993. Preliminary geologic studies of Sierra El Aguajito ŽBaja California, Mexico.: a resurgent-type caldera. J. Volcanol. Geotherm. Res. 59, 47–58.
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Gastil, R.G., Fenby, S.S., 1991. Detachment faulting as a mechanism for tectonically filling the Gulf of California during dilatation. In: J.P., Simoneit, B.R. ŽEds.., The Gulf and Peninsular Province of the Californias. Am. Assoc. Pet. Geol. Mem. 47, pp. 371–375. Gastil, R.G., Krummenacher, D., Minch, J., 1979. The record of Cenozoic volcanism around the Gulf of California. Geol. Soc. Am. Bull. 90, 839–857. Gastil, R.G., Diamond, J., Knaack, Ch., Wallawander, M., Marshall, M., Boyles, C., Chadwick, B., Erskine, B., 1990. The problem of the magnetiterilmenite boundary in southern and Baja California. In: Anderson, J.L. ŽEd.., The Nature and Origin of Cordilleran Magmatism. Geol. Soc. Am. Mem. 174, pp. 19–32. Gastil, R.G., Miller, R., Anderson, P., Crocker, J., Campbell, M., Bunch, P., Lothringer, C., Leier-Engelhardt, P., Delattre, M., Hobbs, J., Roldan-Quintana, J., 1991. The relation between the ´ Paleozoic strata on opposite sides of the Gulf of California. In: Perez-Segura, E., Jacques-Ayala, C. ŽEds.., Studies of Sonoran ´ Geology. Geol. Soc. Am. Spec. Pap. 254, pp. 7–17. Gudmundsson, A., 1995. Infrastructure and mechanics of volcanic systems in Iceland. J. Volcanol. Geotherm. Res. 64, 1–22. Henry, C.D., 1989. Late Cenozoic Basin and Range structure in western Mexico adjacent to the Gulf of California. Geol. Soc. Am. Bull. 101, 1147–1156. INEGI, 1983. Carta Geologica 1:250,000, Isla Sn. Esteban H ´ 12-10. Aguascalientes, Mexico. ´ INEGI, 1984. Carta Geologica 1:250,000, Santa Rosalıa ´ ´ G12-1. Aguascalientes, Mexico. ´ Karig, D.E., Jensky, W., 1972. The proto-Gulf of California. Earth Planet. Sci. Lett. 17, 169–174. Lee, J., Miller, M.M., Crippen, R., Hacker, B., Ledesma Vazquez, ´ J., 1996. Middle Miocene extension in the Gulf Extensional Province, Baja California: Evidence from the southern Sierra Juarez. Geol. Soc. Am. Bull. 108, 505–525. Lonsdale, P., 1989. Geology and tectonic history of the Gulf of California. In: Winterer, E.L., Hussong, D.M., Decker, R.W. ŽEds.., The Eastern Pacific Ocean and Hawaii. Geol. Soc. Am., The Geology of North America, N, pp. 499–521. Lonsdale, P., 1991. Structural patterns of the Pacific floor offshore of Peninsular California. In: Dauphin, J.P., Simoneit, B.R. ŽEds.., The Gulf and Peninsular Province of the Californias. Am. Assoc. Pet. Geol. Mem. 47, pp. 87–125. Ludwig, W.J., Nafe, J.E., Drake, C.L., 1971. Seismic refraction. In: Maxwell, A.E. ŽEd.., The Sea. New Concepts of Sea Floor Evolution 4 Wiley-Interscience, New York, pp. 53–84, Part 1. Mendoza-Borunda, R., Axen, G., 1995. Preliminary analysis of the late Cenozoic structural history of the southeastern Sierra Juarez fault zone in the vicinity of the main Gulf Escarpment. III Int. Meet. Geol. Baja California Peninsula, Peninsular Geol. Soc., pp. 126–127. Munguıa, ´ L.y., Wong, V., 1995, Estudio de la sismicidad en la zona geotermica Las Tres Vırgenes, Baja California Sur. In: ´ ´ Medina-Martınez, F., Delgado-Argote, L.A., Suarez-Reynoso, ´ ´
G. ŽEds.., La Sismologıa 10 anos ´ en Mexico: ´ ˜ despues ´ del temblor de Michoacan ´ del 19 de septiembre de 1985 ŽMs8.1., Monografıa Mexicana, pp. 212–228. ´ No. 2, Union ´ Geofısica ´ Nava, A., Brune, J.N., 1982. Source mechanism and surface wave excitation for two earthquakes in northern Baja California. Geophys. J. R. Astron. Soc. 73, 739–763. NGDC ŽNational Geophysical Data Center, NOAA., 1988. ETOPO5 Digital relief of the Surface of the Earth, Boulder, CO. Romero-Espejel, H., 1996. Estructura y petrologıa ´ en el norte de Sierra Juarez, Baja California. MS Thesis, CICESE, 155 pp. ´ Žunpublished.. Ryan, M.P., Blevins, J.Y.K., Okamura, A.T., Koyangi, R.Y., 1983. Magma reservoir subsidence mechanics: theoretical summary and application to Kilauea Volcano, Hawaii. J. Geophys. Res. 88, 4147–4181. Sanchez-Velasco, R., 1996. Aspectos generales del proyecto ´ geotermico de Las Tres Vırgenes, B.C.S., Mexico. Geoterm. ´ ´ ´ Rev. Mex. Geoenerg. 12, 115–124. Sanchez-Zamora, O., Doguin, P., Couch, R.W., Ness, G.E., 1991. ´ Magnetic anomalies of the northern Gulf of California: structural and thermal interpretations. In: Dauphin, J.P., Simoneit, B.R.T. ŽEds.., The Gulf and Peninsular Province of the Californias. Am. Assoc. Pet. Geol. Mem. 47, pp. 377–401. Sawlan, M.G., 1991. Magmatic evolution of the Gulf of California Rift. In: Dauphin, J.P., Simoneit, B.R.T. ŽEds.., The Gulf and Peninsular Province of the Californias. Am. Assoc. Pet. Geol. Mem. 47, pp. 301–369. Sawlan, M.G., Smith, J.G., 1984. Petrologic characteristics, age and tectonic setting of Neogene volcanic rocks in northern Baja California Sur, Mexico. In: Frizzell, V.A. Jr. ŽEd.., Geology of the Baja California Peninsula. Soc. Econ. Paleontol. Mineral. Pac. Sec. 39, pp. 237–251. Secretarıa ´ de Marina, 1987. AtlasrMemoria del levantamiento geofısico de la Zona Economica Exclusiva y margen continen´ ´ tal oeste de Mexico, SMPO8710, Mexico. ´ ´ Stock, J., Hodges, K.V., 1989. Pre-Pliocene extension around the Gulf of California and the transfer of Baja California to the Pacific plate. Tectonics 8, 99–115. Talwani, M., Worzel, J.L., Landesman, M., 1959. Rapid gravity computation for two-dimensional bodies with application to the Mendocino submarine fracture zone. J. Geophys. Res. 64, 49–59. Viggiano-Guerra, J.C., 1992. El pozo desviado LV-2A ŽLas Tres Vırgenes, B.C.S..: petrologıa ´ ´ e interpretacion. ´ Geoterm. Rev. Mex. Geoenerg. 8, 373–394. Zanchi, A., 1994. The opening of the Gulf of California near Loreto, Baja California, Mexico: from basin and range extension to transtensional tectonics. J. Struct. Geol. 16, 1619–1639. Zelt, C.A., 1989. Seismic structure of the crust and upper mantle in the Peace River Arch region. PhD Thesis, Univ. B.C., Vancouver. Zelt, C.A., Smith, R.B., 1992. Seismic traveltime inversion for 2-D crustal velocity structure. Geophys. J. Int. 108, 16–34.
Backscattering and geophysical features of volcanic ridges offshore Santa Rosalia, Baja California Sur, Gulf of California, Mexico b Hubert Fabriol a , Luis A. Delgado-Argote a,) , Juan Jose´ Danobeitia , ˜ c a,c a Diego Cordoba , Antonio Gonzalez , Juan Garcıa-Abdeslem , ´ ´ ´ a,c Rafael Bartolome´ b, Beatriz Martın-Atienza , Vıctor Frias-Camacho a ´ ´ a
b
DiÕision ´ Ciencias de la Tierra, CICESE, Carr. Tijuana-Ensenada km 107, Ensenada, B.C., 22830, Mexico Dipartamento Geofısica, Instituto Ciencias de la Tierra ‘‘ Jaume Almera’’, CSIC, Sole´ i Sabaris s r n, 08028 Barcelona, Spain ´ c Dipartimento Geofısica, Facultad Ciencias Fısicas, UniÕersidad Complutense de Madrid, 28040 Madrid, Spain ´ ´ Received 28 January 1998; accepted 10 September 1998
Abstract Volcanic ridges formed by series of volcanic edifices are identified in the central part of the Gulf of California, between Isla Tortuga and La Reforma Caldera–Santa Rosalıa ´ region. Isla Tortuga is part of the 40-km-long Tortuga Volcanic Ridge ŽTVR. that trends almost perpendicular to the spreading center of the Guaymas Basin. The Rosalıa ´ Volcanic Ridge ŽRVR., older than TVR, is characterized by volcanic structures oriented towards 3108, following a fracture zone extension and the peninsular slope. It is interpreted that most of the aligned submarine volcanic edifices are developed on continental crust while Isla Tortuga lies on oceanic-like crust of the Guaymas Basin. From a complete Bouguer anomaly map, it is observed that the alignments of gravity highs trending 3108 and 2908 support the volcanic and subvolcanic origin of the bathymetric highs. Volcanic curvilinear structures, lava flows and mounds were identified from backscattering images around Isla Tortuga and over a 400-m high ŽVırgenes High., where the TVR and the RVR intersect. A refractionrwide-angle seismic ´ profile crossing perpendicular to the Vırgenes High, together with gravity and magnetic data indicate the presence of shallow ´ intrusive bodies presumably of basaltic or andesitic composition. It is inferred that most volcanic edifices along the ridges have similar internal structures. We suggest that the growth of different segments of the ridges have a volcano-tectonic origin. The older RVR lies along the extension of a fracture zone and it probably is associated with Pliocene NE–SW extension. q 1999 Elsevier Science B.V. All rights reserved. Keywords: volcanic ridge; sidescan images; gravity and magnetic anomaly; seismic refraction
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Corresponding author. Division ´ de Ciencias de la Terra, CICESE, P.O. Box 434843, San Diego, CA 92143-4843, USA. Tel.: q1-5261-744501; fax: q1-5261-744933. E-mail address: [email protected] ŽL.A. Delgado-Argote.
1. Introduction The Gulf of California is the southern extension of the San Andreas transform system. The Baja
0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 8 4 - 0
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H. Fabriol et al.r Journal of Volcanology and Geothermal Research 93 (1999) 75–92
California peninsula is moving northwest relative to the continent at a full spreading rate of about 5.6 cmryr since about 6 Ma ago. The origin of this marginal sea is related to the Neogene Basin and
Range disturbance. An oceanic-like crust has been formed in spreading centers during the development of the transform system. Based on the tectonic map of Lonsdale Ž1989., it is observed that the eight
Fig. 1. Tectonic map of the Gulf of California region Žadapted from Lonsdale, 1989.. Approximate backscattering coverage from Hesperides ´ cruise CORTES-P96 is indicated with heavy line.
H. Fabriol et al.r Journal of Volcanology and Geothermal Research 93 (1999) 75–92
largest basins with oceanic crust represent about 20% of the sea floor in the gulf. The remaining 80% is subsided-rifted continental crust of unknown thickness. The crust in the central part of the peninsula is close to 40 km thick ŽNava and Brune, 1982.. The geometry of the structures developed along the gulf margins and associated volcanism are poorly known. The most complete geophysical survey in the gulf was conducted by the CONMAR Group from the Oregon State University and Instituto Oceanografico de Manzanillo in 1981 and 1984. ´ Most of the results of these surveys are included in Dauphin and Simoneit Ž1991. and AtlasrMemoria of the Mexican Secretarıa ´ de Marina Ž1987.. During the CORTES-P96 ŽCrustal Offshore Research Transect by Extensive Seismic Profiling. project, the Spanish RrV Hesperides recorded more ´ than 3000 km of geophysical profiles ŽFig. 1. within the Gulf of California. Data collected along these profiles included multibeam bathymetry and backscatter images, gravimetry, magnetometry, multichannel near-vertical seismic reflection and refractionrwide angle reflection seismics ŽDanobeitia et ˜ al., 1996, 1997.. From these new geophysical data, we selected data lying along three transects located northeast of Santa Rosalıa, ´ offshore of the Pliocene La Reforma Caldera and the Las Tres Vırgenes ´ volcanic field Žbetween 27.408N and 27.708N., where remarkable bathymetric highs are observed ŽFig. 2.. This bathymetric high is the apparent northern continuation of the NNW–SSE Tortuga ridge that ends in the Guaymas Basin ŽLonsdale, 1989., and it is also characterized by a magnetic high ŽSanchez´ Zamora et al., 1991.. These data, combined with pre-existing data, are used to better define the structural, volcanic and bathymetric patterns of ridges located NW of the Guaymas Basin and around Isla Tortuga. This knowledge is important for determining the significance of volcanic ridges and their implications for the geology of the margins of the early Gulf of California.
2. Geologic and structural setting Before the beginning of the San Andreas–Gulf of California transform system, the circum-Gulf region experienced an E–W-oriented extensional deforma-
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tion during the Basin and Range disturbance ŽAngelier et al., 1981; Henry, 1989.. Large-scale normal faulting occurred before and during Miocene time along the Gulf Escarpment. Nearby areas, like Bahıa ´ de los Angeles ŽDelgado-Argote et al., 1999., show extensive explosive volcanism during and after that period of time. It has been documented that many normal faults were reactivated at the beginning of the transcurrent deformation in the western Gulf margin ŽMendoza-Borunda and Axen, 1995; Romero-Espejel, 1996; Delgado-Argote et al., 1999.. Miocene time is also recognized as one episode of intense volcanic activity along the margins of the gulf ŽGastil et al., 1979; Sawlan, 1991; Bigioggero et al., 1995; Delgado-Argote and Garcıa-Abdeslem, ´ 1999; Delgado-Argote et al., 1999. when the Neogene magmatic arc-axis was located near the present eastern margin of the peninsula. The area where simultaneous volcanism and extension took place during the Basin and Range period is primarily the same as the region where the transform system was later developed. Paleozoic metamorphic and Cretaceous granitic rocks ŽGastil et al., 1990, 1991. which are affected and partially covered by mostly Neogene volcanic rocks form the basement on both sides of the gulf. Near the study area, drill holes from the Las Tres Vırgenes volcanic field indicate that the ´ basement is formed by late Cretaceous granitic rocks ŽSanchez-Velasco, 1996.. The granitic and some Pa´ leozoic metasedimentary rocks crop out 50 km west of the study area. These basement rocks are extensively covered by the Oligocene andesitic sequence of the Sierra La Giganta and the Miocene to Pleistocene andesitic and dacitic lavas and pyroclastic rocks from the Las Tres Vırgenes volcanic field ´ ŽCapra et al., 1998. ŽFig. 2.. The oldest spreading centers of the Gulf of California initiated their development about 6 Ma ago. In many areas, less than 5-Ma-old oceanic-type crust is in tectonic contact with continental crust along transform faults. The oldest boundaries of the basins connecting transform faults are interpreted to be constituted by transitional crust consisting of stretched continental crust and igneous rocks of the early magmatic activity in the embryonic basins. Gastil and Fenby Ž1991. and Axen Ž1995. have proposed that regional tilting associated with low-angle detachment faults resulted from extension during
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the process of rifting of the Gulf of California. In areas like Bahıa ´ de los Angeles–Isla Angel de la Guarda, no evidence has been found to interpret important degrees of tilting associated with detachment; instead, discrete blocks have distinct histories of pre-Miocene extension ŽDelgado-Argote and
Garcıa-Abdeslem, 1999; Delgado-Argote et al., ´ 1999.. It is observed in Fig. 2 that the peninsular margin south of Cabo Vırgenes is characterized by normal ´ faulting, probably associated with the fracture zones that limit the southern part of Guaymas Basin. Based
Fig. 2. Tectonic map of the central Gulf of California Žadapted from Lonsdale, 1989. and lithology of the Las Tres Vırgenes volcanic field ´ and La Reforma Caldera region ŽINEGI, 1983, 1984.. Merged bathymetry adapted from INEGI Ž1:1,000,000., Lonsdale Ž1989. ŽGuaymas Basin. and the CORTES-P96 cruise. Heavy dashed lines indicate the track of the RrV Hesperides, dots along the margins indicate ´ transform faults active before and during initial phase of the opening of the Gulf, and solid lines with ticks indicating relative movement along the peninsular margin indicate fracture zone extension. 3V, Tres Vırgenes volcanic field; A, Aguajito caldera; R, Reforma Caldera; ´ CV, Cabo Vırgenes; GB, Guaymas Basin; ISM, Isla San Marcos; IT, Isla Tortuga; SPMB, San Pedro Martir Basin. Lithology: Ž1. ´ ´ Quaternary sediments, Ž2. Quaternary lava flows and tuffs, Ž3. late Tertiary basalt and basaltic breccia, Ž4. late Tertiary tuffs and ignimbrites, Ž5. late Tertiary andesite, Ž6. late Tertiary sedimentary and volcano sedimentary units, Ž7. Miocene volcaniclastic rocks, Ž8. Miocene sandstone, Ž9. undifferentiated volcanic rocks, Ž10. Cretaceous granitoids, Ž11. Paleozoic metamorphic rocks.
H. Fabriol et al.r Journal of Volcanology and Geothermal Research 93 (1999) 75–92 Fig. 3. Bathymetric map showing the side scan images and the rose diagrams of structures interpreted in Figs. 4 Žsegment A., 5 Žsegment B. and 6 Žsegment C.. VH, Vırgenes ´ high; CV, Cabo Vırgenes; IT, Isla Tortuga; TVR and RVR indicate the axis of the Tortuga Volcanic Ridge and the Rosalıa ´ ´ Volcanic Ridge, respectively. 79
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on the spreading rate estimates, Batiza Ž1978. suggested that Isla Tortuga was constructed on 1.7 Ma old oceanic crust and that the island is the result of a northward-migrating volcanic activity. However, the 1000 bathymetric line suggests that the island is part of a WNW-oriented ridge. South of the island, the ridge is oriented NW, and it narrows reaching the Guaymas Basin ŽFig. 2.. It has been reported ŽBatiza, 1978. that the old volcanic series of the island were subaerially extruded and it is inferred that it has been tectonically submerged. The suite of basalts on Isla Tortuga is similar to ocean–ridge tholeiite suites ŽBatiza, 1978; Sawlan, 1991.. In the Guaymas Basin, an oceanic-like crust of about 125 km across has been developed. The basin shows active tholeiitic volcanism and, in its southern spreading center, the lava flows and dikes are interfingered with sediments ŽEinsele et al., 1980; Curray et al., 1982.. Sawlan Ž1991. has classified the basaltic rocks of the Guaymas Basin as transitional rift-tholeiites.
3. Bathymetry and sidescan images of the Tortuga Volcanic Rift In order to identify regional structures from the morphology, we merged the 1:1,000,000 bathymetric map of INEGI for the whole region, the detailed bathymetry of the Guaymas Basin ŽLonsdale, 1989., and the multibeam data collected by the RrV Hesperides of four segments of tracks for the study ´ area ŽFig. 2.. This last data set provides high-resolution bathymetry down to 20 m in a swath of about 3.5 times water depth. This new map shows a number of hills and mounds interpreted to be volcanic in origin, and linear features interpreted to be of tectonic origin. The most conspicuous structure is the Rosalıa ´ Volcanic Ridge ŽRVR in Fig. 3. formed by a series of volcanic edifices trending 3108, following
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interpreted fracture-zone extensions of transform faults offshore the peninsula ŽLonsdale, 1989.. This ridge is about 75 km long, from 27.98N to 27.48N ŽFig. 2., and it is located west of Isla Tortuga. The island is located in another important morphologic feature, named Tortuga Volcanic Ridge, which is observed from the bathymetric map ŽTVR in Fig. 3.. This about 7-km-wide and at least 40-km-long ridge, trending 2858 is well defined by the isobath of 1000 m. For clarity, Fig. 3 shows the backscatter images of Figs. 4–6 overlapped on the detailed bathymetric map. The first section ŽFig. 4, named A in Fig. 3. is located south of the Isla Tortuga in the NW limit of the Guaymas Basin, and east of the RVR. The topographic and textural data ŽFig. 4. allowed the interpretation of structural lineaments and the recognition of the distinctive signature of relatively inelastic sediments from relatively elastic volcanic rocks. It is observed from Figs. 3 and 4 that sediments cover most of the area south of Isla Tortuga. The image shows poorly defined 0308, 2708 and 3108 trending lineaments inferred to be fractures ŽFig. 4.. The topography gently rises towards the coast from 1475 to 1200 m Ž2%-slope; Fig. 4d.. A 2-km wide body of lava flows is interpreted in the southern part of Isla Tortuga that apparently flowed down about 6 km from the island reaching a depth of more than 1400 m. The lavas are intersected by NE and SW dipping lineaments trending towards 3208 in average. The wrinkled lava flows of the eastern side, interpreted to be pillow lavas ŽPL in Fig. 4a., are about 50 m high with respect to the surrounding floor. In contrast, the smooth lavas of the western side are interpreted as sheet-like lava flows ŽSF in Fig. 4a.. The area occupied by the last flows shows abundant mounds interpreted to be vents, while the pillow lavas show concave features indicating a southward flow. Fractures in the pillow lavas are oriented towards 0358 and 0008 suggesting the presence of
Fig. 4. Ža. Sidescan sonar image with bathymetric lines of the southern part of Isla Tortuga. Light areas are low backscatter; the dark areas are interpreted to be lava flows probably extruded from Isla Tortuga. Žb. Structural interpretation of the image indicating the presence of curvilinear lineaments associated with subvolcanic bodies. Žc. Rose diagram of lineaments rotated in the sense of the ship-track; notice the strong tendency of the structures towards NW, parallel to the orientation of the regional fracture zones near Guaymas Basin. Žd. Profile along the track of the ship indicating the presence of interpreted subvolcanic bodies; dashed lines are the lava flows located in the right side of the image.
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fissures. On the western side of this transect, about 25 km offshore Santa Rosalıa, ´ a third series of lava flows is observed with mounds aligned towards 3108 ŽFig. 4a.. A large canyon and other fractures that cut volcanic structures are oriented in the same direction, suggesting that they are part of a fracture-zone extension ŽFig. 3. and that deformation and volcanic activity occurred simultaneously. The second section Žnamed B in Fig. 3. is oriented towards 3208 and lies between the RVR and La Reforma caldera. The presence of a field of lavas in the southern half of the image is the most notable feature ŽFig. 5.. Although it is presumable that the lavas flowed eastwards from vents related to the La Reforma volcanic field, it is also possible that they were extruded from local fissures. The profile along the transect shows a domic shape cut in its southern part by a narrow canyon bounded by normal faults ŽFig. 5b.. The northwestern half of the image is characterized by a relatively inelastic sedimentary cover where nested mounds Žroughly oriented towards N–S and 3208. and curvilinear features are interpreted to have a volcanic association. The whole image shows an orthogonal arrangement of lineaments trending N–S and E–W ŽFig. 5b and c.. Although this structural geometry is distinct from that reported onshore in the Las Tres Vırgenes vol´ canic field, where intense seismicity of tectonic origin has been reported ŽMunguıa ´ and Wong, 1995., it is similar to the regional structures reported by Garduno-Monroy et al. Ž1993. in El Aguajito Caldera. ˜ The section C ŽFigs. 3 and 6. has an almost perpendicular orientation with respect to the RVR, and the most conspicuous volcanic and structural features are observed in this image ŽFig. 6.. The Vırgenes High is clearly defined in this section by a ´ 3.5-km-wide bathymetric high where the western side has a 25% slope, elevated about 200 m, and the abrupt eastern side has an elevation of 400 m with a slope of 50%. The morphology of this portion of the
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ridge suggests the presence of a complex volcanic edifice bounded by normal faults forming a horst-like structure. The high, oriented towards 3008 and slightly tilted eastward, shows a series of curvilinear volcanic and subvolcanic lineaments on the top. A series of feeder dikes are interpreted in the section of Fig. 6d, which could explain the observed curvilinear features. We also interpret the construction of this portion of the ridge to result from the combination of normal faulting and uplifting during the emplacement of magma bodies. A remarkable 6-km diameter curvilinear structure similar to a plateau is located in the eastern side of the ridge. This flat structure has some mounds and its relatively inelastic sediment cover suggests that the plateau is older than the horst-like Vırgenes High. The most abundant and ´ continuous structural lineaments and alignments of volcanic mounds are oriented towards 3008 and 0308. In general, the main structure of the high is almost parallel to the TVR, and the NE-oriented lineaments located in the northeastern portion of the image could be older features belonging to the early Guaymas Basin. The three rose diagrams of structural and volcanic lineaments of the images show important differences with respect each other ŽFigs. 3, 4c, 5c and 6c.. In the southern transect ŽFig. 4., the dominant features are oriented 3208, parallel to the coast and the peninsular slope. They can be correlated with transcurrent faults developed before and concurrently to the initial phases of the opening of the gulf. From the same figure, other not so well defined structures trend 0358 and 0808; the first group is more or less parallel to the Guaymas Basin and the second group has no correlation with other regional features. The section B ŽFig. 5. shows N–S and E–W orthogonal features ŽFig. 5c. that apparently are unrelated to regional structures. In the section C ŽFig. 6., the main tendency of structures towards 3008–3108 clearly correlates with the orientation of the RVR ŽFig. 5c..
Fig. 5. Ža. Sidescan sonar image of the western part of Isla Tortuga. The dark areas are interpreted as fissure related lavas; notice the lack of lavas in the northwestern part of the image. Žb. Structural interpretation of the image where it is remarkable the absence of circular structures in the southeast compared with the northwestern part. Žc. Rose diagram of lineaments rotated in the sense of the ship-track; E–W oriented lineaments are related to fissures of the southeastern part of the image and the NNW lineaments are parallel to the peninsular slope end the submarine canyon located to the west of Isla San Marcos ŽFig. 2.. Žd. Profile indicating the subvolcanic structures below the lava flows and normal faulting in the southeastern end of the image.
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Fig. 6. Ža. Sidescan sonar image of the Vırgenes High zone with bathymetric lines every 25 m. Žb. Structural interpretation of the image ´ where the presence of a large curvilinear structure in the eastern side of the ridge is remarkable. This structure is volcanic in origin and apparently older than the ridge, where nested subvolcanic structures are similar to these located eastward. Žc. Rose diagram indicating two main tendencies of lineaments towards NW and NE. The NW-trending lineaments are parallel to the Tortuga Volcanic Ridge. Žd. Profile indicating important normal faulting developed during the evolution of the Vırgenes High showing shallow intrusive bodies. ´
H. Fabriol et al.r Journal of Volcanology and Geothermal Research 93 (1999) 75–92 Fig. 7. The complete Bouguer anomaly and bathymetry are shown with contours every 10 mgal and 100 m, respectively. Although the data distribution is sparse, as shown by the ship tracks from CONMAR Žcontinuous line. and CORTES-P96 Ždashed line. cruises, the gravity anomaly shows a direct correlation with the expected excess of mass at the Guaymas Basin. Fifty kilometers long gravity highs Ž40–50 mgal. aligned in a 2908 direction ŽVH to B. is interpreted to be produced by magma bodies, which define the Tortuga Volcanic Ridge. Parallel to the peninsular slope, the gentle gravity gradient is interpreted to be associated with a tectonic structure where magma was emplaced describing the 3108-trending Rosalıa ´ Volcanic Ridge. Along the RVR axis, both aligned volcanic edifices and local gravity anomalies are observed ŽVH to C.. 85
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Finally, it is important to mention that temporal seismological networks in the region of the La Reforma caldera and the Las Tres Vırgenes volcanic ´
field do not show any seismicity in the Gulf region. Active faulting and volcanic seismicity ŽMunguıa ´ and Wong, 1995. is concentrated onshore, which
Fig. 8. Geophysical modeling of the transect over the Vırgenes High of Fig. 6, indicating the record sections of the two land seismic ´ stations. Observed data in Ža. and Žb. has been band-pass filtered between 3 and 8 Hz. Times are reduced to 6 kmrs and amplitudes have been normalized. Superposed lines in Ža. and Žb. indicate calculated travel times. Žc. Velocity–depth model along a 60-km-long profile indicating the coast line with CT. For clarity, one of each five rays have been printed in the ray diagram. Žd. Free-air anomaly data profile; crosses represent the observed gravity data and the solid line indicates the calculated gravity values. Že. Residual magnetic anomaly data profile. Crosses represent the observed magnetic data and the solid line indicates the calculated magnetic values. Žf. Adjusted density and susceptibility model used to produce the calculated gravity and magnetic data profiles shown in Žd. and Že.; vertical exaggeration is five times and a 1:1 representation is also shown below.
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Fig. 8 Ž continued ..
suggest that the largest structures in the study area, with the exception of the Guaymas Basin, are mostly inactive. 4. Regional gravimetry The large density contrast between the sea water and the underlying rocks and sediments means that
the contours of the free-air gravity anomalies generally follow bathymetric features. However, where the contours do not follow bathymetric features, we expect lateral changes in the structure of the basement andror the thickness of sediments. With the purpose of suppressing such a well-known effect, we have obtained a complete Bouguer anomaly map for a region that includes the Guaymas Basin and the
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RVR, over the western margin of the Gulf of California ŽFig. 7.. The data used for this analysis include land topography data from the ETOPO-5 digital model ŽNGDC, 1988., the coastline ŽINEGI, 1983, 1984. and free-air gravity anomaly and bathymetric data collected during CONMAR-IOM cruises ŽBAJA-76, MARSUR78, GOLFO-81, GOLFO-84; Dauphin and Ness, 1991., and the CORTES-P96 cruise ŽDanobeitia et ˜ al., 1996, 1997.. Bathymetric and topographic data were merged and gridded by kriging to obtain a regular grid with nodes at every four kilometers. A rectangular prism was then placed at the center of each grid node, and their gravity effect was computed at underway free-air gravity stations position Ž1996.. The comby the method of Garcıa-Abdeslem ´ plete Bouguer correction was carried out up to 167 km. Onshore, the gravity effect is caused by topography with a density of 2670 kgrm3. Offshore, water was replaced by sediments with a density of 2000 kgrm3. By adding this correction to the free-air gravity anomalies, we have obtained the complete Bouguer anomaly map shown in Fig. 7. One remarkable feature in the Bouguer anomaly map ŽFig. 7. is a broad, up to 50 mgal, gravity high located near the center of the Guaymas Basin, roughly following the Guaymas-North spreading center ŽLonsdale, 1989.. The 40-mgal contour approaches the geometry of the Guaymas Basin. Another feature that emerges from this data is an elongated gravity high reaching up to 40–50 mgal ŽVH, A, IT, and B in Fig. 7. that extends along the TVR ŽFig. 3. for about 50 km, following a 2908 direction, perpendicular to the Guaymas-North spreading center. Parallel to the peninsular slope, the gentle gravity gradient with decreasing values towards the SW, observed from Cabo Vırgenes to the Guaymas Basin, is in´ ferred to be associated with the 3108-trending RVR. Along this ridge, aligned volcanic edifices and local gravity anomalies are observed ŽFig. 7.. The ridge is parallel to the fault structures of the initial phase of the opening of the gulf shown in Fig. 2. The Tortuga and Rosalıa ´ ridges are deviated about 308 from each other, suggesting different periods of formation. We suggest that the RVR is older than the TVR, since the apparently most recent structures that follow the orientation of the TVR in the Vırgenes High ŽA in ´ . Fig. 7 cut older volcanic structures, and that re-
cently active volcanic activity has been reported in Isla Tortuga ŽBatiza, 1978.. 5. Geophysical modeling The geophysical data collected along the transect C of Fig. 3 consist of deep near-vertical seismic reflection, refractionrwide-angle seismic reflection, gravity and magnetic data. The energy of the 2850 in.3 Ž47 l. air gun array used to obtain deep nearvertical seismic reflection data Žunder process. was also recorded by two portable instruments on land, providing a densely sampled Ža seismogram every 80 m. refractionrwide-angle reflection data set. Clear wide-angle seismic arrivals up to 100 km distance were recorded. In order to model the uppermost crustal structure in the Vırgenes High, seismograms ´ of the first 60 km were selected. The record sections of Fig. 8a and b, corresponding to the two seismic stations, show a neat first arrival identified as Pg phase. This phase shows an advance of 0.5 s reduced time from 41 to 52 km epicentral Žstation-shot. distance from the seismic station 1, and from 35 to 46 km epicentral distance from the seismic station 2. To obtain a good fit of the calculated travel times with the observed arrival times, a ray-tracing forward modeling of the seismic data has been iteratively applied, adjusting P-wave velocities and depths to the discontinuities ŽFig. 8c.. We have used the method proposed by Zelt Ž1989. and Zelt and Smith Ž1992.. Densely sampled potential field measurements Žgravimetry and magnetometry. were also collected along the transect. The residual magnetic anomaly profile has been obtained by reducing the magnetic data, using the coefficients of the International Geomagnetic Reference Field of 1995. To model the potential field data, a 250-m sample spacing was selected to further constrain the structural interpretation. The Free Air gravity anomaly along the transect ŽFig. 8d. shows a gravity high that reaches up to 25 mgal and is located over the Vırgenes High. The ´ main feature in the residual magnetic anomaly ŽFig. 8e. is a large magnetic high of about 120 nT that is slightly displaced towards NE of the Vırgenes High. ´ Both gravity and magnetic anomalies are located in the same position of the travel time seismic anomaly mentioned previously ŽFig. 8a and b..
H. Fabriol et al.r Journal of Volcanology and Geothermal Research 93 (1999) 75–92
A Talwani et al. Ž1959.-type algorithm ŽCooper, 1991. was used to obtain both the 2D1r2 gravity and magnetic models. The densities used in this model were obtained from the seismic velocities, employing the empirical relationship proposed by Ludwig et al. Ž1971. and Zelt Ž1989.. Induced magnetization was assumed for the magnetic model. The values of magnetic susceptibility were obtained from Carmichael Ž1989.. The results of the refractionrwide-angle reflection seismic ray-tracing modeling profile and the 2D1r2 gravity and magnetic modeling are presented in Fig. 8e and f. The sedimentary cover consists of two layers, where the uppermost sedimentary layer Žsand, marine sandstone and pyroclastic flows. is characterized by a P-wave velocity of 2.0 kmrs, a density of 1900 kgrm3 and a susceptibility of 25 = 10y6 e.m.u. Based on a number of drill-holes in the Las Tres Vırgenes volcanic field, the next layer is ´ formed by about 1-km thick pile of volcanic rocks of the Comondu´ group and Santa Lucıa ´ andesite ŽBallina-Lopez, 1985.. This second sedimentary layer is ´ characterized by a velocity of 3.3 kmrs, a density of 2300 kgrm3 and a susceptibility of 120 = 10y6 e.m.u. These values are confirmed by previous results obtained from the interpretation of seismic refraction, bore-holes and electric soundings carried out in the volcanic area of Reforma–Tres Vırgenes ´ ŽBallina-Lopez, 1985.. Based on our seismic and ´ gravity data, it is observed that the sedimentary cover slightly thins until reaching the Vırgenes High, ´ where the basement crops out. The sedimentary thickness to the northeast of this structure appears to be thinner than to the southwest. The sedimentary layer rests on a ca. 82 Ma granitic basement ŽViggiano-Guerra, 1992., with a P-wave velocity of 6.1 kmrs, a vertical velocity gradient of 0.04 km sy1 kmy1 , a density of 2700 kgrm3 and a susceptibility of 500 = 10y6 e.m.u. The magnetic anomalies can be explained by the presence of three bodies in the Vırgenes High area, between 40 and 50 km from the ´ coast, extending from 0.8 to 1.8 km depth below sea level ŽFig. 8f.. These bodies are characterized by a density of 2900 kgrm3 and a high susceptibility contrast of 10,000 = 10y6 e.m.u. In order to model the influence of these bodies in seismic data, a velocity of 6.6 kmrs has been assigned to this portion of the Vırgenes High. ´
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6. Discussion and conclusions We have defined volcanic ridges based on the integration of magnetic, gravimetric and bathymetric data, as well as structural features and alignments of interpreted volcanic edifices and associated lava flows around the Isla Tortuga. The structural detail based on the textural description of the sea floor shows a complex pattern of curvilinear fractures that can result from the emplacement of subvolcanic intrusive bodies. The largest structures are more rectilinear, show well defined orientations ŽFigs. 3– 6., and their origin can be attributed to the Basin and Range extensional disturbance, and the opening of the Gulf of California ŽHenry, 1989; Lonsdale, 1991; Sawlan, 1991.. In the eastern margin of the Baja California peninsula, along the Gulf Escarpment, the presence of large volcanic rift systems where subcircular concentric faults and joints are associated with NNW– SSE to NW–SE fracture systems has been documented ŽZanchi, 1994.. Uplifting is observed in this area where the fracture systems apparently coexist with dike swarms and complex sets of fractures showing a radial pattern of magmatic origin. A similar association of Miocene extensional deformation and development of basaltic volcanic rifts have been documented in the Bahıa ´ de los Angeles basin, about 150 km north of the Las Tres Vırgenes vol´ canic field ŽDelgado-Argote and Garcıa-Abdeslem, ´ 1999. In this last area, normal faulting bounds elongated hills where lava flows are connected with feeder dike swarms. Both areas where extension and volcanic activity are recognized to coexist indicate that this association is a common phenomenon during Neogene time in several places of the western margin of the Gulf. The extensions associated with the Basin and Range and the San Andreas–Gulf of California regional deformations have not been symmetric or contemporaneous along the circum-gulf region. In a recent synthesis, Lee et al. Ž1996. indicated that extension could be as large as 35%–58% in the Loreto Basin ŽPliocene. or 10% in the northeastern part of the peninsula since late Miocene time. In the Santa Rosalıa ´ area, the same authors mentioned that extension during late Miocene was oriented NE–SW and it changed to WNW–ESE and E–W during
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Pliocene time. Near this region, in the Las Tres Vırgenes volcanic field, Angelier et al. Ž1981. docu´ mented ENE–WSW followed by ESE–WNW extensional deformation, associated to NW–SE to NNW– SSE and N–S to NNE–SSW strike-slip faulting. In addition, Sawlan and Smith Ž1984. and Stock and Hodges Ž1989. mentioned that the Gulf Escarpment, near the volcanic field, is younger than the 10-Ma-old tholeiitic Basalt of Esperanza and proposed that the NNW trending normal faults do not affect 3-Ma-old units. Since the orientation of the RVR is approximately perpendicular to the direction of extension during Pliocene time and lies along a fracture-zone extension, it is assumed here that it is about 5 Ma old, similar to the age of the Guaymas Basin. In that sense, the RVR is constituted by transitional crust consisting of stretched continental crust and igneous rocks of the early magmatic activity in the embryonic Guaymas Basin. The Isla Tortuga is formed by tholeiitic basaltic and andesitic flows younger than 1.7 Ma constructed over a ‘‘leaky-fracture zone’’ ŽBatiza, 1977.. This fracture zone is now interpreted as a volcanic ridge ŽFigs. 3 and 7.. It has been suggested that the oldest volcanic activity on the island initiated in its southeastern portion and later migrated NW to the present position and that normal faulting, perpendicular to the fracture zone is for tectonic submergence ŽBatiza, 1978.. The normal faults oriented towards 0408 are parallel to the trend of the Guaymas spreading center and other volcanic and structural lineaments interpreted from the backscattering images in the southern and northeastern sides of the island ŽFigs. 4 and 6.. The alignment of gravity and bathymetry highs of Figs. 3 and 7 indicates that the island is part of the 2908 trending TVR, which intersects the RVR in the Vırgenes High. In spite of the lack of detailed ´ bathymetry around the island, we assume that its internal structure is similar to that interpreted from geophysical data in the Vırgenes High ŽFigs. 7 and ´ 8.. Because the Vırgenes High is located at the ´ intersection of two regional volcanic ridges, it is characterized by the presence of 2808–3108-oriented conspicuous fractures and normal faults, which bound this structural high. The gravity high in the Vırgenes High ŽFig. 7. ´ results from mass excess related to the accumulation of basaltic magma bodies Ždensity of 2900 kgrm3 .
in shallow intrusives enclosed in granitic country rock. This interpretation is supported by the geophysical models presented in Fig. 8, where it is observed that the tops of a dike and sill-like bodies are located at about 800 m below the surface extending downward to a depth of about 1800 m. Similar structures have been documented from gravity and structural data in the Bahıa ´ de los Angeles area, where Miocene basaltic andesite flows are related to shallow magma bodies enclosed by granitic country rocks ŽDelgadoArgote and Garcıa-Abdeslem, 1999.. We interpret ´ most of the circular lineaments located on the Vırgenes High to be fractures formed during the ´ emplacement of magma bodies as those interpreted in the magnetic model. Subvolcanic magma bodies with characteristic velocities, densities and magnetic susceptibilities of basaltic to andesitic material therefore makeup the Vırgenes High and, in consequence, ´ most prominent highs of the ridges. One to three kilometers deep sill-like reservoirs connected to subvolcanic dikes have been documented in other localities ŽGudmundsson, 1995.. Ryan et al. Ž1983. have remarked that such sill geometry can explain surface deformation, especially normal faulting. The last authors have shown that large-scale deformation patterns require large plan-view intrusives, such as those interpreted from the backscatter images. The Vırgenes High gives us a reference internal ´ structure for the individual volcanic edifices associated with regional structures. The growth of different segments formed by two or more volcanic and subvolcanic structures define a volcanic ridge that can be related to regional tectonic structures, like the fracture zone extensions, which resulted from the transtensive regime of the San Andreas–Gulf of California system. We assume that magma emplacement is a passive phenomena, leading us to suggest that the RVR developed perpendicular to a regional E–W extension to which is also related the deformation and volcanic activity in the volcanic fields of Las Tres Vırgenes, El Aguajito and La Reforma ´ since Pliocene time. Since the early stages of the development of the proto-Gulf of California ŽKarig and Jensky, 1972. the Gulf region has experienced contemporaneous extension and volcanism. One of the main contributions of this study is the correlation of large-scale fracture zones with the emplacement of mafic magma defining volcanic ridges. Under-
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standing the scale and internal geometry of volcanic ridges and their association with tectonic events is an important key to visualizing the evolution of a large part of the circum-Gulf region during Neogene time. Acknowledgements We want to thank the Mexican authorities for all kind of facilities to work within the exclusive economic zone of Mexico. Our gratitude to the captains ´ and crews of the RrV’s Hesperides, Altair and ´ Humboldt for their support during the experiment. Also, we thank the many technicians and students from Spain and Mexico that participated in the dif´ ferent activities inland on the peninsula. This survey was supported by the CICYT projects ANT94-0182C02-01r02 and ANT94-0182-C02-02, the Secretarıa ´ de Marina, CICESE Ž644107. and a cooperative support by CONACyT-CSIC. The Direccion ´ General de Investigacion y Ensenanza Superior ´ Cientıfica ´ ˜ EX96-0002876832, Ministerio de Educacion ´ y Cultura ŽSpain., supported A. Gonzalez in CICESE, ´ Ensenada. We thank Rodey Batiza and an anonymous reviewer for their constructive comments. Finally, we want to thank Lance Forsythe for critic final revision of the manuscript. References Angelier, J., Colletta, B., Chorowicz, J., Ortlieb, L., Rangin, C., 1981. Fault tectonics of the Baja California Peninsula and the opening of the Sea of Cortez, Mexico. J. Struct. Geol. 3, 347–357. Axen, G., 1995. Extensional segmentation of the main gulf escarpment, Mexico and United States. Geology 3, 347–357. Ballina-Lopez, H.R., 1985. Estudio geofısico en la zona geotermica ´ ´ ´ de Tres Vırgenes, B.C.S. Geoterm. Rev. Mex. Geoenerg. 1, ´ 21–43. Batiza, R., 1997. Oceanic crustal evolution: evidence from the petrology and geochemistry of isolated oceanic central volcanoes. PhD Thesis, Univ. California, San Diego, 1977, 295 pp. Žunpublished.. Batiza, R., 1978. Geology, petrology, and geochemistry of Isla Tortuga, a recently formed tholeiitic island in the Gulf of California. Geol. Soc. Am. Bull. 89, 1309–1324. Bigioggero, B., Chiesa, S., Zanchi, A., Montrasio, A., Vezzoli, L., 1995. The Cerro Mencenares volcanic center, Baja California
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