We report structural, paleomagnetic, and magnetic fabric data for mid-Cretaceous plutons of the Peninsular Ranges batholith along a transect at ~30°N latitude. Four plutons in the western sector are characterized by characteristic magnetizations residing in magnetite. In this sector, El Milagro, Aguaje del Burro, La Zarza, and San Telmo plutons yield a combined paleopole at 82.1°N, 169.7°E (K = 137.6, A 95 = 7.9°; n = 4–38 sites), which, rotated for closure of the Gulf of California, falls at 79.3°N, 179.5°E, and it is concordant with the North America reference pole. Plutons in the transition zone, between the eastern and western sectors of the Peninsular Ranges, have magnetizations residing in hematite. El Potrero and San José plutons yield highly discordant paleopoles, indicating apparent clockwise rotation (R) and flattening (F) of 33.0° ± 5.1° and −27.6° ± 6.1°, respectively (San José), and 46.1° ± 5.9° and −31.0° ± 7.0° (El Potrero). The discordance is best explained by west-down tilt of the crustal block between the Main Mártir thrust and the Rosarito fault, which are major compressional structures parallel to the trend of the Peninsular Ranges. The San Pedro Mártir pluton, a large La Posta–type pluton on the eastern sector of the transect, has magnetizations that reside primarily in hematite. The mean paleomagnetic pole (71.3°N, 335.5°E; K = 40.7 and A 95 = 7.2°) is slightly discordant, indicating westward tilt of ~15°. The different paleopoles obtained for individual plutons convincingly show that the Peninsular Ranges batholith has suffered internal deformation, which is more intense along the transition zone. The magnetic fabric for plutons representative of the western, eastern, and transitional sectors of the range show marked contrasts in the deformation recorded by anisotropy of magnetic susceptibility (AMS). Anisotropy is weakly developed in the western sector (El Milagro), very strongly developed in the transition zone (San José), and moderately developed in the eastern sector (Sierra San Pedro Mártir). Within the plutons, El Milagro fabrics record emplacement-related stress. In contrast, San José and San Pedro Mártir appear to record regional stress linked to evolution of the Main Mártir thrust. Overall, our data are consistent with rotation of the crustal block where Potrero and San José plutons are located; rotation was accommodated by major crustal faults in a compressional stress field, as the crustal block moved to occupy the space abandoned by the ascending (and westward expanding) San Pedro Mártir diapir batholith. The rotation could be related to interaction between the large Sierra San Pedro Mártir pluton and the Main Mártir thrust, or to mechanical controls such as wedging against a rigid salient.

Abstract Guanajuato has a long history (450 years) of mineral exploitation and remarkable silver and gold production from a complex system of fault-veins. Despite this, it is only in the past 40 years that the systematic study of its geology has been conducted. Mid-Tertiary epithermal veins occur in all the Mesozoic and Paleogene rock units exposed in the mining district, and mineralization seems to be the result of the combination of several geologic factors, such as the occurrence of greenschists in the basal complex, a thick sequence of Early Paleogene red beds overlain by a thick succession of Oligocene volcanic rocks with the existence of one or more paleolakes when the volcanoes were active. The systematic study of the greenschists and associated plutonic and sedimentary rocks in the basal complex of Sierra de Guanajuato has contributed significant information to the concept of accretion of the Guerrero terrane to the SW end of the North American craton in the Early Cretaceous. Research on the Eocene red bed sequence suggests that early extension occurred creating fault patterns that later were reactivated during Neogene Basin and Range pulses. Immediately east of the city of Guanajuato, a thick volcanic sequence is exposed, with two pyroclastic units formed by felsic ignimbrites that almost certainly are related to a nearby caldera, which was active immediately prior to Ag-Au mineralization. The first activity pulse of the caldera produced the Bufa ignimbrite, a massive unit that displays very large thickness variations (300 to <10 m) in short distances, which we interpret as a signal that it may be an intracaldera deposit. The second explosive pulse originated the Calderones formation, a unit formed by an undetermined but large number of ignimbrites, surge deposits, layers with accretionary lapilli, and epiclastic-volcanic deposits. The Calderones formation is characterized by pervasive chloritization, which points out toward the presence of external water in the system, probably related to one or more shallow lakes within the caldera previously formed by the Bufa eruption. Lithofacies variations and stratigraphic arguments suggest that the Guanajuato caldera was probably located near the Cerro Alto de Villalpando and La Peregrina lava dome complex. Morphological and structural evidence of the caldera are masked by several pulses of younger normal faulting which affected the southern portion of the Mexican Basin and Range Province (i.e., Mesa Central).

The Caltepec shear zone is a dextral transpressional tectonic boundary between the Oaxacan and Acatlán Complexes, which are crystalline basements of the Zapo-teco and Mixteco terranes in southern México, respectively. The terrane boundary (2–6 km wide) reveals protracted and polyphase tectonic activity from at least Early Permian to the present. The major tectonothermal event in the Caltepec fault zone was related to the oblique collision of the metamorphic complexes during the amalgamation of Pangea. An anatectic leucosome and the resulting syntectonic granite (Cozahuico Granite) in the fault zone yielded U-Pb zircon ages of 275.6 ± 1.0 Ma and 270.4 ± 2.6 Ma, respectively. The initial 87 Sr/ 86 Sr ratios (0.70435–0.70686) and Sm-Nd model ages (T DM ) (1.0–1.6 Ga) for the Cozahuico Granite and leucosome indicate a magmatic mixture that originated from melted Proterozoic crust and a component of depleted mantle. The Leonardian age of the cover (Matzitzi Formation) and a 40 Ar/ 39 Ar cooling age (muscovite) of 268.59 ± 1.27 Ma for mylonitic mica schist at the base of the thrust imply high cooling rates (∼180 °C/Ma) and uplift during the Permian. The adjacent sedimentological record indicates intense tectonic reactivation during Early Cretaceous, Paleogene, and Neogene along the long-lived Caltepec fault zone, alternating with periods of relative tectonic quiescence during Triassic, Jurassic, and Mid-Cretaceous times. The trend of the Caltepec fault zone parallel to the Oaxaca fault, 50 km to the east, is interpreted as part of a synchronous and dynamically coupled tectonic system that has been releasing tectonic stresses associated with the rupture of Pangea and the evolution of the Pacific margin of southern México from Jurassic to Holocene times.

U-Pb zircon data from three undeformed to slightly deformed, megacrystic, granitoid plutons in the northern Acatlán Complex of southern México has indicated that all three are part of a larger suite of late Ordovician plutons. 40 Ar/ 39 Ar data from hornblende and biotite show mainly disturbed spectra, but biotite from the Palo Liso and Los Hornos plutons yields plateaus with ages of 305 ± 26 Ma and 157 ± 12 Ma, respectively. These thermal events may be correlated, respectively, with Permo-Triassic and Jurassic tectonothermal events recorded elsewhere in the Acatlán Complex. All three plutons are peraluminous with calc-alkaline affinities, characteristics that are consistent with inherited zircon ages and together suggest a source in Mesoproterozoic calc-alkaline rocks similar to those exposed in the neighboring Oaxaca terrane. We interpret these granites to be related to the early Ordovician separation of peri-Gondwanan terranes from Gondwana during the opening of the Rheic Ocean. Elsewhere in the Acatlán Complex, Ordovician megacrystic granitoids of the Piaxtla Suite were subjected to high-grade metamorphism, which we infer to be related to subduction along the Gondwanan margin during the Devonian–Carboniferous. The three plutons reported here were not affected by Devono-Carboniferous metamorphism and thus are inferred to have remained outside the subduction zone.

The Piaxtla Suite of the Acatlán Complex (southern México) has previously been considered a vestige of the Iapetus Ocean that underwent eclogite-facies metamorphism during Late Ordovician subduction and exhumation. Study of granitoid, mafic, and metasedimentary rocks of the Asis Lithodeme of the Piaxtla Suite reveals a complex tectonothermal history involving: (1) eclogite-facies syntectonic metamorphism preserved as aligned omphacite in mafic lenses dated at 346 ± 3 Ma (concordant U-Pb zircon age), which is inferred to result from subduction; (2) polyphase deformation involving WSW-ENE tectonic transport under amphibolite-facies conditions accompanied by migmatization due to decompression melting dated at ca. 347–330 Ma (SHRIMP [sensitive high-resolution ion microprobe] zircon ages); (3) continued deformation under greenschist facies; and (4) development of several phases of late folds and crenulation cleavage. Pressure, temperature, and time ( P-T-t ) data suggest rapid isothermal decompression from eclogite to upper amphibolite facies during the Visean (Middle Mississippian) followed by cooling. In the absence of age data for the latter stage, nearby unmetamorphosed latest Upper Devonian sedimentary rocks contain metamorphic, Piaxtla Suite clasts suggesting either diachronism in the exhumation process or synchronous exhumation and subsidence in adjacent areas. Such rapid exhumation of eclogites is typical of continent-continent collision zones, and the subhorizontal, WSW-ENE kinematics is compatible with either lateral thrust ramping or extension in the orogen. Devonian–Carboniferous subduction and exhumation are incompatible with an origin within the Iapetus Ocean, because that ocean had closed by Silurian times. However, they are consistent with oblique subduction of the leading edge of Gondwana along the southern flank of the Rheic Ocean during the amalgamation of Pangea.

To better define the cooling history of the northern Oaxacan Complex, titanite and phlogopite from metasedimentary calc-silicate and biotite from a pegmatite were collected. All these rocks were involved in the granulite-facies Zapotecan orogeny between ca. 1004 and 978 ± 3 Ma, inferred to result from underthrusting the Oaxacan Complex beneath an arc or continent. Fragments of 2 × 5 cm 2 titanite crystals yielded a concordant U-Pb age of 968 ± 9 Ma, whereas 40 Ar/ 39 Ar analyses of phlogopite and biotite gave ages of 945 ± 10 Ma and 856 ± 10 Ma, respectively. These ages are inferred to date cooling through 660–700 °C, 450 °C, and 300–350 °C, respectively. When combined with published ages (Sm-Nd garnet, 40 Ar/ 39 Ar hornblende, Rb-Sr biotite and whole rock, and K-Ar biotite and K-feldspar) the data define a two-stage cooling curve: (1) 8 °C/m.y. between 978 and 945 Ma, cooling through 450 °C by which time the rocks had risen through a depth of 15 km; and (2) 2 °C/m.y., which, by extrapolation, brought the rocks to the surface between 710 and 760 Ma. The first stage of exhumation is interpreted in terms of tectonic switching between steep and flat slab subduction, a result of interactions of a ridge, a plume, or an oceanic plateau with the trench. The second stage may be related to thermal relaxation of the lithosphere, ending with the breakup of Rodinia, which brought the rocks to the surface.

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