Baja California: The Geology of Rifting

Those of us who live in the Los Angeles region know that this is an area of active tectonics. We have earthquakes; we have many large mountains nearby that are testimony to the great power of the forces that are moving and deforming the surface of the earth here; and we have the San Andreas fault as our local tourist attraction. But this great fault is not just local. Besides extending northward it also continues south toward the Gulf of California, where a series of structures represents its continuation under water. All of these structures are part of the major boundary between the Pacific plate and the North America plate. So even though we don't think of Los Angeles and the Gulf of California as being similar in many ways, they're tectonically connected because they sit on the same plate boundary and suffer many of the same kinds of deformation due to motions between these two plates

Tectónica de placas y la Evolución del Bloque Jalisco, México

El Bloque Jalisco representa lo que se reconoce como un bloque tectónico, o microplaca, mas o mer os rigido (Fig. 1a). Sabemos que se mueve de manera independiente con respecto a las placas circundantes (Rivera y Norte America) a traves de dos zonas de deformación continental (el rift o graben de Tepic-Zacoalco y el rift o graben de Colima) ya lo largo de una zona de subduccion en su límite costero con la placa oceanica de Rivera. Los rifts de Tepic-Zacoalco y de Colima se unen con el rift de Chapala, en el límite NE del bloque Jalisco, dando lugar a lo que es escencialmente un punto triple continental, cerca de Guadalajara, formado por la unión de: el bloque de Jalisco, el bloque de Michoacán y la placa de Norte America. El desarrollo del bloque Jalisco, como bloque independiente, parece estar relacionado geometricamente con la forma y dinámica de la placa de Rivera, asi como también con la evolución del punto triple continental cerca de Guadalajara ya mencionado. El estudio del bloque Jalisco representa un buen laboratorio para el desarrollode modelos tectonicos que nos permitan estudiar el inicio de movimientos de, microplacas, así como el fenómeno de una posible captura de un bloque continental por otra placa escencialmente oceanica (Luhr et al, 1985; Allan, 1990). Por ello resulta interesante saber con detalle como fue su evolución y como es su movimiento actual, con respecto a las placas circundantes

Geophysical Secrets Beneath Antarctic Waters

Cruising around Antarctica is a perk that a group of us from Caltech have enjoyed over the past few years. You might be curious about how we book one of these cruises. First of all, we write a proposal and send it to the National Science Foundation, which has an Office of Polar Programs and an Office of Marine Geology and Geophysics. If the proposal is approved, we're scheduled for time on board one of the NSF ships. We had proposed several projects to answer some nagging place-tectonic questions about the history and evolution of the Antarctica place, which may hold the key to understanding movement of some of the other plates and ocher global geophysical problems, such as relative motions among the hot Spots

Revised Pacific-Antarctic plate motions and geophysics of the Menard Fracture Zone

A reconnaissance survey of multibeam bathymetry and magnetic anomaly data of the Menard Fracture Zone allows for significant refinement of plate motion history of the South Pacific over the last 44 million years. The right-stepping Menard Fracture Zone developed at the northern end of the Pacific-Antarctic Ridge within a propagating rift system that generated the Hudson microplate and formed the conjugate Henry and Hudson Troughs as a response to a major plate reorganization ∼45 million years ago. Two splays, originally about 30 to 35 km apart, narrowed gradually to a corridor of 5 to 10 km width, while lineation azimuths experienced an 8° counterclockwise reorientation owing to changes in spreading direction between chrons C13o and C6C (33 to 24 million years ago). We use the improved Pacific-Antarctic plate motions to analyze the development of the southwest end of the Pacific-Antarctic Ridge. Owing to a 45° counterclockwise reorientation between chrons C27 and C20 (61 to 44 million years ago) this section of the ridge became a long transform fault connected to the Macquarie Triple Junction. Following a clockwise change starting around chron C13o (33 million years ago), the transform fault opened. A counterclockwise change starting around chron C10y (28 millions years ago) again led to a long transform fault between chrons C6C and C5y (24 to 10 million years ago). A second period of clockwise reorientation starting around chron C5y (10 million years ago) put the transform fault into extension, forming an array of 15 en echelon transform faults and short linking spreading centers

A method for bounding uncertainties in combined plate reconstructions

We present a method for calculating uncertainties in plate reconstructions that does not describe the uncertainty in terms of uncertainties in pole positions and rotation angles. If a fit of magnetic anomalies of the same age and fracture zones that were active as transform faults at that time can be found, such a reconstruction can be perturbed and degraded by small rotations about each of three orthogonal axes (partial uncertainty poles). If the uncertainty in the reconstruction is a consequence of independent, small, but acceptable, rotations about these axes, then the uncertainties in reconstructed points will be elliptical in shape. The dimensions and orientation of such ellipses will depend upon the magnitudes of the perturbing rotations and upon the relative geometry of the partial uncertainty poles and the points in question. In a sequence of rotations, each rotation will contribute an elliptical region of uncertainty for each reconstructed point, and these ellipses can be combined as independent statistical quantities to obtain a confidence ellipse for the sequence of rotations. As a test, we calculated uncertainties for three points on the Pacific plate with respect to North America at the time of anomaly 6 (20 Ma). The computed uncertainties are similar in shape to those that we previously obtained for a sequence of marginally acceptable rotations, but the major axes of the ellipses presented here are about 25% shorter

Variations in ridge morphology and depth-age relationships on the Pacific-Antarctic Ridge

Adjacent segments of the Pacific-Antarctic ridge display significantly different morphologies and depth-age relationships over seafloor younger than 36 Ma. The spreading corridor southwest of Fracture Zone XII is characterized by a rift valley and an usually small subsidence constant of 226±13 m/m.y.^(½), while the two spreading corridors immediately northeast of Fracture Zone XII have an axial high and a subsidence constant consistent with the global average. This abrupt variation in ridge morphology is not usually characteristic of medium-rate spreading centers, nor is such an abrupt variation expected of adjacent ridge segments that are spreading at the same rate. We suggest that a thermal anomaly beneath the ridge may influence the first-order effects of spreading rate and lithospheric cooling enough to produce the observed rift valley and axial high and the different subsidence constants. Although we are not certain what would produce the thermal anomaly here, we speculate that when the spreading rate on the Pacific-Antarctic ridge increased from slow to intermediate rates since 20 Ma, so did the need for materials for accretion, which may be supplied in part by along-axis asthenospheric flow from hotspots or a hot region to the northeast. A sufficient supply of hot asthenosphere may still be lacking in the ridge segment with the axial valley to the southwest, leaving it cooler and starved for accretionary materials

Quantitative determination of uncertainties in seismic refraction prospecting

We present a model of the propagation of refracted seismic waves in planar (horizontal or dipping) layered structures in which we quantify the errors from various sources. The model, called the (mixed) variance component model, separates the errors originating on the surface from those due to inhomogeneities of subsurface layers. The model starts with the assumption of homogeneous (constant-velocity) layers, but by taking the principal errors into account, variations from this model (including degree of velocity inhomogeneity, vertical velocity gradients, and gradational interfaces) can be identified.A complete solution to the variance component model by Bayesian methods relies on the Gibbs sampler, a recently well-developed statistical technique. Using the Gibbs sampler and Monte Carlo methods, we can estimate the posterior distributions of any parameter of interest. Thus, in addition to estimating the various errors, we can obtain the velocity-versus-depth curve with its confidence intervals at any relevant point along the line.We analyze data from a crustal-scale refraction line to illustrate both features of this method. The results indicate that the conventional linear regression model for the first arrivals is inappropriate for this data set. As might be expected, geophone spacing strongly affects our ability to resolve the heterogeneities. Differences in the amount of velocity heterogeneity in different layers can be resolved, and may be useful for lithologic characterization. For this crustal-scale problem, a velocity profile derived from this method is an improvement over simple linear interpretations, but it could be further refined by more comprehensive methods attempting to match later arrivals and wave amplitudes as well as first arrivals. The method could also be applied to smaller-scale refraction problems, such as determination of refraction statics, or constraints on the degree of probable lateral variations in velocity of shallow layers, for improved processing of reflection data

Estratigrafía y Petrología de la Secuencia Volcánica de Puertecitos, Noreste de Baja California. Transición de un Arco Volcánico a Rift

En la Provincia Volcánica de Puertecitos (PVP), en el NE de Baja California, una sucesión de depósitos piroclásticos y lavas riolíticas de la etapa temprana del rift del Golfo de California (Mioceno Tardío-Plioceno) sobreyace en discordancia a rocas andesíticas atribuidas al arco volcanico del Mioceno (Tma). En la franja oriental de la PVP se han documentado dos períodos de actividad volcánica contemporáneos al desarrollo del rift: uno a fines del Mioceno Tardío (ca. 6 Ma) y otro en el Plioceno Temprano (ca. 3 Ma). El primero incluye una secuencia de ignimbritas (Tobas El Canelo, Tmec) de más de 300 m de espesor contenida entre dos períodos efusivos de domos riolíticos. Los cambios de espesor de estas ignimbritas (Tmec), indican que su fuente está localizada hacia el NW de la zona de estudio, mientras que las coladas riolíticas son locales y forman una serie de domos sobrepuestos orientados N-S, en la misma dirección del fallamiento. A fines del Plioceno Temprano un segundo período de actividad explosiva produjo una serie de flujos piroclásticos de composición riolítica y dacítica (Tpr). Este paquete incluye hacia la costa central más de 20 unidades de depósito, con un espesor superior a 200 m que disminuye hacia el W y NNE, sugieriendo que la fuente de Tpr se encontraba al E de la costa actual. Hacia el norte, algunas unidades de Tpr se hallan interestratificadas con depósitos marinas someros del Plioceno Temprano. Este período culminó con la erupción del Volcán Prieto (monogenético) y derrames fisurales de composición andesítica durante el Plioceno Tardío y Pleistoceno. Las andesitas asociadas al rift en los dos períodos de actividad volcánica son comparativamente de escaso volumen, y se caracterizan por el bajo contenido de K_2O y contenidos variables de TiO_2 y MgO con relación a las andesitas y basaltos asociados al arco volcánico del Mioceno. La característica común de las andesitas y las riolitas es la asociación clinopiroxeno-ortopiroxeno (y olivino en algunos casos), y bajo o nulo contenido de biotita, hornblenda, feldespato alcalino y cuarzo. Lo anterior sugiere una mezcla de magmas, uno máfico a alta temperatura y con posible afinidad toleítica, y otro más diferenciado y frio posiblemente formado por material de la corteza. La generación del magmatismo está asociado a la tectónica transtensional, que en el NE de la PVP se manifiesta con una extensión en dirección ESE a ENE durante el Mioceno Tardío - Plioceno

Cenozoic Reconstructions of the Australia-New Zealand-South Pacific Sector of Antarctica

Reconstructions are presented documenting the relative motion of the Australia. Antarctic and Pacific plates since Chron 27 (61.1 Ma). In addition to the motion of the major plates, the reconstructions show the relative motion between East and West Antarctica and the continental fragments that make up the South Tasman Rise. Recent observations that are used in making these reconstructions include the mapping of seafloor spreading magnetic anomalies in the Adare basin, northeast of Cape Adare, which recorded roughly 150 km of opening between East and West Antarctica between Chrons 20 (43.8 Ma) and 8 (26.6 Ma). In addition, magnetic and bathymetric observations from the lselin Rift, northeast of the Iselin Bank, and from the Emerald Fracture Zone, along the western boundary of Pacific-Antarctic spreading, document the rotation of the Iselin Bank between Chrons 27 and 24 (53.3 Ma). Our reconstructions indicate that there was a total of about 200 km of separation between East and West Antarctica in the northern Ross Sea region in the Cenozoic. These reconstructions document the development of a deep-water passageway between Australia and Antarctica as the South Tasman Rise clears the final piece of the Antarctic continental margin around Chron 13 (33.5 Ma)

Stress Field at Yucca Mountain, Nevada

Hydraulic fracturing stress measurements performed in four holes (USW G-1, USW G-2, USW G-3, and Ue25P1) indicate that at Yucca Mountain, the least horizontal stress S_h is less than the vertical stress S_v. Values of the greatest horizontal stress S_H are intermediate between S_h and S_v, corresponding to a normal faulting regime with values of Φ = (S_H-S_h)/(S_v-S_h) between 0.25 and 0.7. Drilling-induced hydraulic fractures seen on borehole televiewer logs indicate an S_h direction of N. 60° W. to N. 65° W. in USW G-1, USW G-2, and USW G-3. The same S_h direction is inferred from breakout orientations in USW G-2 and Ue25P1. The S_h values in the upper parts of the three USW G holes are less than the pressure of a column of water filling the borehole to the surface. Thus, the long drilling-induced hydraulic fractures in the shallow parts of these holes could have been formed in attempts to maintain circulation during drilling. These low S_h values may be intimately related to the low water table and fracture-dominated hydrology of Yucca Mountain