Large-magnitude Oligo-Miocene extension in southern Sonora: Implications for the tectonic evolution of northwest Mexico

The older units are complexly faulted and steeply tilted by several generations of normal faults. Early closely-spaced subhorizontal to moderately-dipping, NW-striking normal faults repeat tilted sections and document large magnitude extension oriented ~N50¡E. North of a NE-trending accommodation zone, tilts are consistently to the northeast and normal faults have mainly top-to-the-southwest displacement. South of this zone, the polarity of faulting and tilting is reversed. Younger, widely spaced NS- to NNW- trending, high-angle normal faults cut the previously faulted and tilted sections and are associated with modest ~E-W extension. Kinematic indicators on some of the steep faults suggest late-stage oblique-slip or strike-slip displacement. Palinspastic reconstruction of a section north of the accommodation zone indicates cumulative extension of ~90%. Angular unconformities and growth fault relations within the Neogene sequence indicate that large-magnitude NE-SW extension occurred between 26 and ~20 Ma, and that extension in the northern tilt domain is slightly older than that in the southern one. Subsequent E-W extension of ²10-15% and associated basin formation is largely bracketed between 20 and 17 Ma.

The magnitude of Neogene extension in southeastern Sonora is substantially greater and the timing is older than previously recognized. Reconnaissance of selected areas between the unextended Sierra Madre Occidental and the coast at this latitude, together with previously published data, suggests that most extension in Sonora occurred between ~27 and 12 Ma, while remnants of the Farallon plate were still being subducted and the magmatic arc shifted westward. The inception of extension was often associated with mafic volcansim, following a period of relative tectonic and magmatic quiescence during the Eocene.

Plate-tectonic models that require substantial extension in Sonora during proto-Gulf (~10 - 5 Ma) transtensional deformation may need to be re-evaluated. An alternative model, presented here, is that Baja California began moving with close to Pacific plate motion coincident with the ~11 Ma termination of subduction at this latitude. In this model, post-11 Ma northwest motion of Baja relative to mainland Mexico would total ~500 km and would be accomplished by both proto-Gulf (10-5 Ma) and modern Gulf (5-0 Ma) distributed right lateral shear and oblique rifting within the previously extended and thermally weakened Mid-Miocene magmatic arc. This speculative reconstruction has the advantage of obviating the need for rapid, large magnitude post-11 Ma NE-SW extension in Sonora and may provide a better match of the pre-Late Miocene geology on either side of the Gulf.

Figures

Figure 1: Map of Sonora and adjacent areas showing location of study area and selected geographic and tectonic features. Boundaries of the Gulf extensional province (hatchured line) and the relatively unextended Sierra Madre Occidental, regional tilt domains, and metamorphic core complexes (dark shading) modified from Stewart and Roldan-Quintana (1994), Stock and Hodges (1989); and Nourse (1994). Abbreviations: SGO = Suaqui Grande - Onabas area. IT = Isla Tiburon, Mz = Mazatan, Mg = Magdalena, SSU = Sierra Santa Ursula.	Figure 2 : Simplified geologic/tectonic map of the Santa Rosa area. Based on 1:25,000 mapping and interpretation of aerial photographs. Only the shaded strips along the major roads were examined in detail. In these areas, units I, III, and IV are further subdivided. Note that the area straddles an "accommodation zone" between southwest directed normal faulting (NE-tilts) in the north and northeast directed faulting (SW-tilts) in the south.
Figure 3: Composite columnar sections for the northern and southern parts of the Santa Rosa area, illustrating typical thicknesses and existing age constraints.	Figure 4 : Composite age spectra and reconstructed cooling history for sample SR-83 from the San Nicolas granodiorite. Estimated cooling curve assumes closure temperatures of 525±40¡C for hornblende and 325±30¡C for biotite during moderately fast cooling. Temperature-time curve for K-feldspar based on diffusion modeling of incremental heating data and assumes multiple diffusion domains following the methods of Lovera (1992). Estimated uncertainites in temperature for this part of the history are estimated to be ±30¡C. See text for discussion.
Figure 5: Schematic cross sections of the Santa Rosa area. The cross section of the northern domain is better constrained and illustrates the general structural style: Multiple generations of SW-directed normal faults repeat NE-tilted sections, with older gently-dipping faults cut by successively steeper faults. "Cut and paste" reconstruction of part of the northern cross section yields 87% extension. Note that the lines of section cross the young high angle faults at an acute angle resulting in a low apparent dip.	Figure 6 : Index map of the Suaqui Grande-Onabas area, showing approximate distribution of Neogene rocks, lines of schematic cross sections (Fig. 8), and selected structural information. Unpatterned area underlain mainly by older rocks (Triassic to early Tertiary) but may include additional exposures of Neogene. Typical attitudes of early Neogene volcanic and sedimentary rocks based on reconnaissance field studies and interpretation of aerial photographs. Only the youngest and most conspicuous high angle faults are shown, some of which have evidence for dextral slip. Many additional high- and low-angle normal faults cut the early Neogene and older units, but are not shown. Some of these older faults are illustrated in the cross sections of Figure 8.
Figure 7: Composite columnar section for the Suaqui Grande-Onabas area. Succession of rock types and typical thicknesses generalized from numerous well exposed, tilted sections throughout the area.	Figure 8: Schematic cross sections from the Suaqui Grande-Onabas area. Sections based on 1:25,000 reconnaissance mapping (Gans, 1996, unpublished data) and illustrate typical structural and stratigraphic relations observed within Neogene rocks. See Figure 6 for locations. Geographic names of section localities: (1) Arroyo El Negro, (2) Arroyo San Javier, (3) Arroyo Cosari, (4) Rancho Sombrerete, (5) Arroyo El Obispo, (6) Arroyo El Capulin
Figure 9: Timing of magmatic and deformational events in the Santa Rosa region and their plate tectonic setting. Local magmatism occurred episodically from the late Cretaceous to middle Miocene, but was was most intense during the Laramide (65-50 Ma) and during the Oligo-Miocene (shaded parts of strip). Vertical bars correspond to dated samples. Extensional deformation was most intense between 25 and 18 Ma, and then waned rapidly. An earlier episode of regional shortening is inferred for the Laramide, but there is no direct evidence for this from the Santa Rosa region. The passage of the Rivera triple junction at this latitude is assumed to have occurred at approximately 13 Ma, following the reconstructions of Stock and Hodges (1989).	Figure 10: Present day configuration and generalized reconstructions of northwest Mexico at ~ 6, 11, 16, and 23 Ma, illustrating the speculative model discussed in text. Maps for the present day and 6 Ma reconstruction are modified from Stock and Hodges (1989). Between 10 and 6 Ma, Baja California is assumed to have moved with close to Pacific plate motion and is restored an additional ~200 km to the southeast. Note that this reconstruction results in some overlap of coastal Sonora and Baja California (shaded area in 11 Ma panel) - an overlap that is attributed to subsequent transtensional deformation. Between 27 and 10 Ma, cumulative extension between the Sierra Madre Occidental and Baja California is estimated to be 50-100% and the locus of extension and magmatism is inferred to have shifted progressively southwestward from the margin of the Sierra Madre Occidental to the area now occupied by the Gulf of California. Abbreviations: SF = San Felipe, H = Hermosillo, G = Guaymas, SR = Santa Rosa, L = Loreto, LP = La Paz, M= Mazatlan, T = Tepic. See text for discussion.

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