Geophysical Prospecting, 2005, 53, 755–765 A constrained 2D gravity model of the Sebasti ´ an Vizca´ ıno Basin, Baja California Sur, Mexico J. Garc´ ıa-Abdeslem, 1∗ J.M. Romo, 1 E. G ´ omez-Trevi ˜ no, 1 J. Ram´ ırez-Hern ´ andez, 2 F.J. Esparza-Hern´ andez 1 and C.F. Flores-Luna 1 1 CICESE, Divisi ´ on de Ciencias de la Tierra, km 107 Carretera Tijuana–Ensenada, Ensenada, Baja California, 22860, and 2 Instituto de Ingenier´ ıa, UABC, Av. de la Normal s/n col. Insurgentes Este, Mexicali, Baja California, 21280 M´ exico Received May 2003, revision accepted July 2005 ABSTRACT The subsurface geometry of the Sebasti ´ an Vizca´ ıno Basin is obtained from the 2D inversion of gravity data, constrained by a density-versus-depth relationship derived from an oil exploration deep hole. The basin accumulated a thick pile of marine sed- iments that evolved in the fore-arc region of the compressive margin prevalent along western North America during Mesozoic and Tertiary times. Our interpretation indi- cates that the sedimentary infill in the Sebasti ´ an Vizca´ ıno Basin reaches a maximum thickness of about 4 km at the centre of a relatively symmetric basin. At the location of the Suaro-1 hole, the depth to the basement derived from this work agrees with the drilled interface between calcareous and volcaniclastic members of the Alisitos Formation. A sensitivity analysis strongly suggests that the assumed density function leads to a nearly unique solution of the inverse problem. INTRODUCTION Western North America was an active convergent margin from late Triassic (225 Ma) to middle Miocene (12 Ma) times. In the western margin of the Baja California Peninsula, the Cedros deep, which marks the location of the former subduction zone, and a parallel alignment of outcrops of volcano-plutonic rocks are a relic of this convergence. The geological evidence at hand suggests that the Sebasti ´ an Vizca´ ıno Basin was developed in the fore-arc region of this convergent margin (Baldwin 1996; Sedlock 1996). The geological evolution of this region, as well as the sig- nificant thickness of marine sedimentary rocks that filled the basin during Mesozoic and Tertiary times, aroused the in- terest of oil explorationists (Beal 1948; Mina 1957). Later, Pemex (the Mexican oil company) conducted an assessment of the economic potential of the Sebasti ´ an Vizca´ ıno Basin as an oil and/or natural-gas reservoir (Lozano 1975). As part of an exploration programme, Pemex drilled several exploration ∗ E-mail: jgarcia@cicese.mx holes, finding some gas manifestations in a couple of them, but not enough to pursue their commercial exploitation. Since that time the prospect has been held in reserve for future develop- ment. Current energy needs have led to a renewal of interest in ex- ploration for natural gas in several Tertiary basins in the coun- try. Hence, the reassessment of the Sebasti´ an Vizca´ ıno Basin using up-to-date geological knowledge and modern geophys- ical interpretation tools is a desirable project. The aim of this work is to investigate the geometry and depth of the basin by modelling an 82-km-long gravity profile across the basin. Our interpretation of the gravity data was constrained by in- dependent information obtained from the Suaro-1 hole, which reached a maximum depth of 2636 m below sea-level. Suaro-1 was drilled by Pemex during their exploration programme in the 1970s (Garc´ ıa-Dom´ ınguez 1976), and it passed through a sedimentary section spanning the late Cretaceous to Holocene age. At the bottom of the section, Suaro-1 cut through 355 m of volcanic rocks assigned to the Alisitos Formation. The interpretation of the gravity profile was carried out by simulating the basin with a 2D single-source body, with C  2005 European Association of Geoscientists & Engineers 755