Margarita López-Martínez

The area of sierras Las Jarillas-El Potrero in northern Sonora is interpreted as a portion of the lower plate of the Magdalena metamorphic core complex. Rock units that characterize this locality range in age from Jurassic to Eocene. Jurassic units comprise a small area of metasedimentary rocks that is probably correlative with the regional Middle Jurassic La Jojoba metasandstone and, is intruded by the La Cebolla granite of 158.1 ± 1 Ma (U-Pb, zircon). A younger succession consists from base upwards of a lower metasedimentary unit, a metalimestone unit, and an upper metasedimentary unit. The lower and upper units have similar lithologies of metapelites, metasandstones, and metavolcanic rocks for which maximum depositional ages (U-Pb, zircon) of ca. 119 to ca. 112 Ma were obtained. The intermediate unit consists of recrystallized limestone with interbedded metapelite and metasandstone. These three units that probably correlate with nearby unmetamorphosed outcrops of the Bisbee Group are intruded by the two-mica and garnet leucocratic mylonitic granites Las Jarillas, dated (U-Pb, zircon) at 60.1 ± 0.9 Ma (MSWD= 7), and La Yegua, dated at 46.9 ± 0.3 Ma (MSWD= 0.8). At the outcrop scale, Las Jarillas granite is fine-grained and saccaroid, whereas the La Yegua granite is medium- to coarse-grained, with common pegmatitic zones and abundant xenoliths of quartzo-feldspathic schists. Mylonitic foliation in the area predominantly strikes NNW-SSE and dips gently NE and SW, with a NE65°SW trending lineation and average plunge of 16° NE and SW. The mylonitic granites record ductile to brittle-ductile deformation with foliation defined by bands of dynamically recrystallized quartz that envelop feldspar porphyroclasts. The metasedimentary and metavolcanic rocks show continuous and/or spaced cleavage defined by alignments of muscovite, biotite and epidote and subordinate tourmaline, titanite, garnet, hornblende, staurolite, actinolite, zoisite, and opaque minerals. These minerals suggest upper greenschist to lower amphibolite facies metamorphism for the rocks in the area. Mica-fish and S-C foliation kinematic indicators consistently indicate a top-to-the-SW tectonic transport direction. A concentrate of muscovite and quartz from a metasandstone sample yielded a cooling age of 24.14 ± 0.57 Ma (40Ar/39Ar), similar to muscovite ages reported by Wong et al. (2010) for the nearby Magdalena-Madera metamorphic core complex. We consider Sierras Las Jarillas-El Potrero to be part of the Magdalena-Madera complex that was exhumed in the footwall of the regional Magdalena-Tubutama detachment fault, which is inferred to pass directly southwest of the study area. A secondary, west-dipping detachment fault in the study area separates the Jurassic and Lower Cretaceous footwall successions near Cerro Picacho. The identified Jurassic through Eocene units, structural overprint, and new geochronologic results are consistent with large-magnitude late Oligocene-early Miocene extension that has affected the region.

Most arcs show systematic temporal and spatial variations in magmatism with clear shifts in igneous rock compositions between those of the magmatic front (MF) and those in the backarc (BA). It is unclear if similar magmatic polarity is seen for extensional continental arcs. Herein, we use geochemical and isotopic characteristics coupled with zircon U‐Pb geochronology to identify the different magmatic style of the Iran convergent margin, an extensional system that evolved over 100 Myr. Our new and compiled U‐Pb ages indicate that major magmatic episodes for the NE Iran BA occurred at 110–80, 75–50, 50–35, 35–20, and 15–10 Ma. In contrast to NE Iran BA magmatic episodes, compiled data from MF display two main magmatic episodes at 95–75 and 55–5 Ma, indicating more continuous magmatism for the MF than for the BA. We show that Paleogene Iran serves as a useful example of a continental arc under extension. Our data also suggest that there is not a clear relationship between the subduction velocity of Neotethyan Ocean beneath Iran and magmatic activity in Iran. Our results imply that the isotopic compositions of Iran BA igneous rocks do not directly correspond to the changes in tectonic processes or geodynamics, but other parameters such as the composition of lithosphere and melt source(s) should be considered. In addition, changes in subduction zone dynamics and contractional versus extensional tectonic regimes influenced the composition of MF and BA magmatic rocks. These controls diminished the geochemical and isotopic variations between the magmatic front and backarc.

This study aims to analyze the modalities of strain accommodation within a highly oblique rift, taking the Gulf of California as a prototype. Rifting in the Gulf of California is accomplished by intra‐Gulf strike‐slip (transform) faults, and mostly dip‐slip displacement on the rift‐margin faults. We have collected fault‐slip data and samples for radiometric dating at selected sites in southeastern Baja California, which is host to the southwestern margin of the rift. We have identified three styles of faulting, particularly (1) WSW‐dipping normal faults, (2) E‐ENE‐dipping normal faults, and (3) steep NNE‐NE‐trending left‐lateral faults. The E‐ENE‐dipping normal faults define the western margin of the Gulf of California rift and are most likely coeval (late Miocene to recent) with both the ~NNE‐NE‐trending left‐lateral faults and some of the WSW‐dipping faults. Fault‐slip data have often been collected on potentially active Gulf of California rift‐margin faults, which invariably display dominant dip‐slip kinematics (generally with minor dextral component). Distribution of extension directions determined from stress inversion of brittle fault kinematic data indicates a peak of 080°–090°, which is strikingly similar to the orientations of T axes from earthquake focal mechanisms of both rift‐margin normal/faults and intra‐Gulf strike‐slip faults. These findings suggest that this stretching may have been occurring throughout the protracted rift history. Furthermore, highly oblique rifts do not show across‐rift variations in the orientation of local extension, which is instead typical of continental rifts with lower obliquity.

We present a geologic map for the Eocene-Oligocene Nanchititla mafic dike swarm consisting of over 140 NNW-SSE trending dike segments; whose emplacement was influenced by the left-lateral transtensional deformation regime prevailing in southern México during the Cenozoic. The hosting rock is a mechanically heterogeneous continental sedimentary sequence interbedded with volcanic rocks. Interaction among magma and host rock promoted the formation of dike-sill systems, segmentation and sinuous intrusions. Dike thickness observed is between 1 and 45 m with a peak in the range from 1 to 5 m. Six giant dikes of more than 30 m thick were documented. The 80% of the dikes have porphyritic textures rich in plagioclase phenocrysts with size ranging from 0.5 to 2.5 cm. New and compiled 40Ar-39Ar ages constrain the swarm emplacement between 30 and 34.5 Ma, whereas plagioclase phenocrysts entrained by the magma have older ages between 35 and 42 Ma.

Volcaniclastic units are exposed at the base of the Puerto Blanco Formation in the Caborca region, northwestern Mexico. The lower unit reveals the presence of Early Cambrian mafic volcanism in this region. It consists of a volcano-sedimentary sequence represented by tuffaceous conglomerates, agglomerates, lapillistones, tuffs, and altered mafic volcanic flows. Petrographic analysis classified the volcanic clasts as albite-sphene-calcite-actinolite granofels, with a moderate to intense hydrothermal alteration, precisely characterized by EPMA analysis. Albite-actinolite geothermometry indicates temperatures from 400 to 500°C, suggesting metamorphic conditions in the upper temperature greenschist facies. Geochemistry analysis shows a high TiO₂ basic–ultrabasic volcanism that originated the volcanic clasts. Rock protoliths were studied using immobile trace elements, which classified them as OIB-type alkaline basalts with the characteristic spider hump-shaped pattern, situated in an anorogenic intracontinental tectonic setting with enriched mantle signatures. ⁴⁰Ar/³⁹Ar geochronology shows metamorphic ages of 52.58 ± 2.0 and 91.67 ± 0.55 Ma, consistent with the emplacement of Laramidic granitoids identified in the region. Possible correlations of this alkaline volcanism include the Southern Oklahoma Aulacogen and the late stages of the rifting of north western Laurentia represented in western United States.