Ángel Francisco Nieto-Samaniego

Variation of Cenozoic extension and volcanism across the southern Sierra Madre Occidental volcanic province, Mexico

The middle to late Cenozoic tectonic-magmatic evolution of the Sierra Madre Occidental volcanic province south of the Tropic of Cancer is summarized and analyzed for the first time, based on new geologic and structural work and published information. In the eastern part of the study region (Mesa central physiographic province) silicic volcanism occurred in a short-lived episode culminating at ca. 30 Ma and was followed by crustal-scale extension between 30 and 27 Ma. In the western part of the study area (Sierra Madre Occidental physiographic province) a voluminous episode of ignimbrite volcanism at 24‐21 Ma was succeeded by east-west extension that produced regularly spaced grabens affecting only the upper crust. In the westernmost part of the study region, an andesitic to rhyolitic arc, formed between 17 and 12 Ma, was affected by crustal-scale, north-northwest‐trending, extensional faulting, leading to the formation of the Gulf of California. In the Mesa central the maximum extension was oriented approximately east-west and amounted to ~20%. In the eastern Sierra Madre Occidental physiographic province extension was only 8% and oriented approximately east-west. We observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late Oligocene, early Miocene, and late Miocene time. Comparison with the offshore tectonics indicates that the first two tectonic-magmatic pulses coincide with periods of fast spreading at the Pacific-Farallon boundary, south of the Shirley fracture zone. We propose that increases in the spreading rate are related to periods of high subduction rate, which in turn correspond to episodes of retreating subduction. A retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge, producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate. Boundary conditions (i.e., plate tectonics) ultimately determined timing, magnitude, and orientation of extension, whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism.

Effect of strain rate in the distribution of monogenetic and polygenetic volcanism in the Transmexican volcanic belt

In the Transmexican volcanic belt, polygenetic and monogenetic volcanism has taken place concurrently with extensional deformation since the late Miocene. At a regional scale, the deformation is manifested by two groups of faults. The dominant group consists of normal faults nearly parallel to the arc. In the other group are north-northwest–trending normal faults that cross the arc and, in places, form the boundaries of crustal blocks. The larger stratovolcanoes of the Transmexican volcanic belt are aligned in north-south volcanic chains along some of these faults, whereas monogenetic volcanoes are usually located along arc-parallel normal fault systems. Because the arc-parallel faults are 15° oblique to the subduction plate boundary, and assuming stretching perpendicular to the trench, the extensional deformation field facilitates activation of both arc-parallel and arc-transverse structures, the former having a higher displacement rate than the latter. We observe that in the Transmexican volcanic belt polygenetic volcanoes develop along faults having small strain rate and monogenetic volcanoes are emplaced along faults having higher strain rate. The agreement with the theoretical model in which monogenetic or polygenetic volcanism depends on the magmatic input rate and the regional stress is true only assuming a linear relation between regional differential stress and local strain rate, as in a continuous and homogeneous medium. We propose that the local strain rate rather than the regional stress field controls the coexistence of both types of volcanism in the Transmexican volcanic belt.

Rhyolitic volcanism in extension zone associated with strike-slip tectonics in the Taxco region, southern Mexico

Abstract The Taxco Volcanic Field (TVF) is part of a broad magmatic province in southern Mexico. It constitutes an isolated zone of deeply dissected volcanic rocks encircled by outcrops of Mesozoic sedimentary and volcano-sedimentary units. A thick unit of rhyolitic lava flows associated with domes and at least two ignimbrite units forms the TVF. This volcanic sequence is distributed within a well defined zone, it overlies and is in part contemporaneous with continental sedimentary beds limited by major faults. Geochronologic data indicate that most rhyolitic volcanism in the area is Oligocene in age and synchronous with episodes of strike-slip faulting. We document two successive phases of strike-slip faulting for the late Eocene–early Oligocene interval, the first with NNW extension and the second with NE extension. In both cases pre-existing structures were reactivated and sedimentary basins were developed in response to displacement along major faults. The stratigraphic sequence gives evidence that the TVF is located in an extensional basin associated to strike-slip faults. The evolution of the basin underwent a change from sedimentary deposition with subsidence to piling up by volcanism. The result of this change was the development of a volcanic pile with elevations higher than the surrounding Mesozoic rocks. According to the fault kinematics, stratigraphy and the volume of volcanic rocks, the rhyolitic volcanism was emplaced in the area of maximum extension, showing that magma flowed into low pressure zones. The small number of faults within the Oligocene volcanic sequence suggests that volcanism inhibited normal faulting and that magma partially filled the space generated in the extended zone produced by the strike-slip faulting.

Origin and tectonic interpretation of multiple fault patterns

Abstract It is shown that in two-dimensional and three-dimensional deformation accommodated by fracture, the symmetry of the fault patterns is an intrinsic attribute because it reflects the symmetry of either stress or strain tensors. The deformation accommodated by sliding along pre-existing planes, when there is kinematic interaction between that planes, forms multiple fault pattern and multiple slickenline sets during a single deformation event. These fault patterns have no restrictions with respect to symmetry, number of fault sets or fault orientation. The kinematic analysis developed here shows that an interacting system is formed by two cross cutting faults and three slickenlines. One slickenline must be parallel to the intersection line between the planes. Also, it is demonstrated that the slickenlines generally do not correspond to the shear stress solution on the planes. Thus, the interaction between planes does not satisfy the assumption of parallelism between shear stress and slip vector. We conclude that the inversion methods to calculate paleostress tensors can lead to erroneous interpretations in structurally complex zones with many pre-existing planes of weakness. We propose four possibilities to form multiple fault patterns: (1) two or more events of faulting obeying Coulombs law with a change of orientation of the principal stresses in each event; (2) reactivation of non-interacting planes according to the Bott (1959) model; (3) one three-dimensional strain event that obeys the “Slip Model”; this mechanism will form an orthorhombic four-fault pattern and two slickenline sets in a single strain event; and (4) one or more events obeying the interacting block model proposed here, with or without rotation of the principal stresses. We propose the last origin as the most common in continental regions.

Origin of rhyolitic lavas in the Mesa central, Mexico, by crustal melting related to extension

The emplacement of a voluminous sequence of rhyolitic lava flows and domes characterizes Oligocene volcanism in the Mesa Central (MC) of Mexico. Its dominant effusive style of emplacement contrasts deeply with the predominantly explosive volcanism of the Sierra Madre Occidental Volcanic Province toward the west. Whole rock geochemical (major- and trace elements) and Sr-Nd isotopic data of the MC Oligocene rhyolitic lavas document a marked change in magma composition at around 30 Ma, allowing us to distinguish between a lower and an upper sequence. Lavas from the lower sequence are geochemically similar to the high-K rhyolitic rocks of the eastern Sierra Madre Occidental. Major- and trace-element variations are characteristic of mantle-derived magmas evolving through fractional crystallization. The initial 87 Sr/ 86 Sr and e Nd values are nearly constant (0.70644-0.70770 and -1.2 to -2.1 respectively) and indicate some contribution from crustal material. Lavas from the upper sequence are high-silica, peraluminous rhyolites, with strong enrichment in fluorine and in some incompatible lithophile elements (Rb, La, Sm, Yb, Y, Th, U, Nb, Ta), and strong depletion in the feldspar-compatible elements Sr, Ba, Eu. Initial 87 Sr/ 86 Sr ratios of the upper sequence lavas are high and variable (0.70812-0.72190), and decrease as silica content increases, whereas the e Nd values are relatively constant (-1.4 to -2.8). The trace element behavior indicates an origin by variable degrees of non-modal partial melting of granulitic low-crustal rocks and chemical disequilibrium during melting processes. The high and variable Sr isotopic ratios could also be related to isotope disequilibrium melting processes if the isotopic heterogeneities between individual mineral phases were preserved during heating of the source rocks. The changes in geochemical compositions are related to the onset of crustal extension at high strain rates documented for the MC. Crustal extension promoted crustal melting at high melting rates, high melt segregation rates, rapid ascent of low-viscosity fluorine-rich magmas, and inhibited melt stagnation in magma chambers. Such conditions favored the effusive volcanic style and support the possibility of melting under disequilibrium conditions.

Stress, strain and fault patterns

If faulting is treated as a stress-controlled phenomenon, the generation of a single fault set, or two sets in conjugate arrangement are inevitably predicted implying plane strain. Alternatively, considering faulting as a strain-controlled process, multiple-set patterns can be predicted. The analysis of multiple-set patterns requires identifying the type of fault pattern from four possibilities: Coulomb, isolated, orthorhombic and complex fault patterns. There are techniques that permit a unique solution of strain tensor for Coulomb and orthorhombic fault patterns. For isolated fault patterns, the principal paleostress directions could be used to approximate the principal strain directions. In this case, we need to assume a homogeneous stress field, independence between faults, and parallelism between shear stress and slip vector on the sliding plane. For complex fault patterns, it is not possible to uniquely determine the total strain tensor without knowledge of all the slip planes. Furthermore, inverting fault-slip data to determine the stress tensor is not correct because the assumptions of the inversion methods are not satisfied. Only a rough approximation is possible assuming that strain produced by major faults represents the total strain tensor. # 1999 Elsevier Science Ltd. All rights reserved.

A graphical technique to predict slip along a pre-existing plane of weakness

Abstract A graphical technique is proposed to determine whether a pre-existing plane of weakness will be reactivated by slip under a stress field. This technique is based on Coulomb-Navier criteria and the method of Yin and Ranalli (Yin, Z., Ranalli, G., 1992. Critical stress difference, fault orientation and slip direction in anisotropic rocks under non-Andersonian stress systems. J. Struct. Geol. 14, 237–244). It consists of calculating which mechanism, rupture or sliding, needs the smaller stress difference to liberate the deformation. Using the results of calculations over a wide range of plane orientations, we plotted, in an equal-area net, the line which separates the orientation fields where rupture needs less stress difference from the fields where slip on pre-existing plane is favored. We named these plots slip-rupture graphs. For the three Andersonian fault regimes, the graphs are presented as dendrograms. These dendrograms show the variation of the range of orientations favorable for reactivation as a function of cohesion and friction of the plane of weakness, depth, pore fluid pressure and the stress ratio. The slip-rupture graphs are compared with the Mohr diagram and slip-tendency graphs (Morris et al., 1996). Relative to Mohr diagrams, our graphs have the advantage that it is possible to work with geographic orientations of planes and principal stresses, and it is not necessary to transform the field data to a stress space. The slip-rupture graphs are similar to slip-tendency graphs; however, the former can lead to estimate physical parameters that make reactivation possible along planes with unfavorable orientations.

Influence of the structural framework on the origin of multiple fault patterns

Abstract We demonstrate that the general equation for three-dimensional strain by slip on orthorhombic faults can be rearranged to take a form that applies to two-dimensional strain due to slip on pre-existing planes of weakness. Therefore, either two-dimensional or three-dimensional strain may result from the same stress state. We deduce that the kinematic interaction between planes of weakness in a body is a fundamental factor to determine the type of strain produced by a stress state. Whether deformation occurs by forming new fractures or by slip on existing planes depends upon which requires a lower stress difference. The stress difference necessary to initiate slip along a plane is highly sensitive to variations in orientation, cohesion and depth. We propose a model for crustal deformation composed of an anisotropic body with existing planes of weakness that interact kinematically. The critical stress difference necessary to initiate sliding is that required by the interacting plane that needs the highest stress difference to slip. Because the stress difference will rise until it reaches a value that can cause slip on all interacting planes, once slip initiates it will occur simultaneously on all planes that require stress differences lower than the critical value. The anisotropic body model proposed here provides a mechanism for forming multiple fault sets and may pertain to the formation of low-angle normal faults.

Assessing Fault Reactivation with the ReActiva Program

In this paper, we offer ReActiva, a shareware program that will aid in understanding the role of reactivation of preexisting planes of weakness in the deformation of a cortical block by faulting. Re- Activa is based on the slip and rupture Coulomb- Navier criterion. It is a program that renders graphically the orientations of planes amenable to reactivation under a given set of physical conditions. These conditions can be modified interactively by the user, and the graphical display is updated continuously. Rather than rendering the orientations of the planes in Mohr space, the program plots the poles of planes amenable to slip on an equal-area projection net. This allows the resulting diagram to be oriented relative to geographic space. ReActiva is available at http://geologia.igeolcu.unam.mx/Tolson/SoftWare/ReActivaEng.htm.

Palynological evidence for Middle Miocene vegetation in the Tehuacán Formation of Puebla, Mexico

Palynological assemblages from the Tehuacán Formation (TF), geochronologically dated as Middle Miocene (15.6 ± 0.4 Ma), provide evidence of a highly diverse flora that, at the generic level, is similar to the extant flora in the Tehuacán Valley. We propose that, during Miocene times, plant communities may have been formed of similar botanical elements to those seen today in the region, with some taxa adapted to semiarid conditions. While major temperate floristic elements of Pinus, Quercus, Juniperus, cloud forest and mexical vegetation can be recognized, components of tropical deciduous forests, such as Burseraceae, Leguminosae and Cactaceae, are also present, indicating semiarid conditions. Semiarid local conditions are also inferred from the geological record, consisting of lacustrine and alluvial fan deposits, which contain abundant evaporites beds. This lithology was formed under high evaporation and moderate precipitation conditions, as usually occurs in small basins fed by a seasonal input of water in semiarid environments. Important differences in the vegetation from the TF palynoflora as compared to older associations from south-central Mexico can be inferred, such as an increase in the diversity of semiarid taxa, belonging to Leguminosae and Burseraceae, and the oldest abundant occurrence of the Cactaceae.