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

Uncertainties in the relative positions of the Australia, Antarctica, Lord Howe, and Pacific Plates since the Late Cretaceous

We determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the Pacific and Antarctica plates, between Australia and Antarctica, and between the Lord Howe Rise and Australia. We combined these to yield a range of possible finite rotations describing the relative positions of the Pacific, Australia, Antarctica, and Lord Howe plates since the Late Cretaceous. If the Pacific-Australia plate boundary has had its present trend since anomaly 18 time, reconstructions show 330±110 km of motion of the Pacific plate relative to the Lord Howe Rise since anomaly 5 time (9.8 m.y.), 420±110 km since anomaly 6 time (19.5 m.y.), 770±330 km since anomaly 13 time (35.6 m.y.), and 820±260 km since anomaly 18 time (43.0 m.y.). We examined two cases for times prior to anomaly 18, assuming a Late Cretaceous age of Australia-Antarctica separation. If a plate boundary existed between the Lord Howe Rise and Pacific plates since the Late Cretaceous, with no plate boundary in Antarctica, reconstructions with the Lord Howe Rise fixed predict 610 ± 200 km of westward motion of the Pacific plate between the times of anomalies 31 and 22, followed by 260±100 km of northward motion between the times of anomalies 22 and 18. If the Lord Howe Rise was fixed to the Pacific plate until the Eocene, but a plate boundary existed between East and West Antarctica, reconstructions show very little motion across this boundary between the times of anomalies 31 and 22, followed by convergence between the times of anomalies 22 and 18. This second case also brings 70–80 m.y. paleomagnetic poles from the Pacific and East Antarctica plates into better agreement than the first case, but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated

Uncertainties and implications of the Late Cretaceous and Tertiary position of North America relative to the Farallon, Kula, and Pacific Plates

We present updated global plate reconstructions and calculated uncertainties of the Pacific, Kula, and Farallon/Vancouver plates relative to North America for selected times since 68 Ma. Improved magnetic data from the Indian Ocean decrease the uncertainties in. the global plate circuit approach; these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots. Major differences between these results and those of others are due to our use of more detailed Africa-North America reconstructions, separate Vancouver and Farallon plate reconstructions, and the assumption of a rigid Antarctica plate during Cenozoic time. The uncertainties in the relative positions of the Pacific and North America plates since the time of anomaly 7 (26 Ma) range up to ±100 km in position, or from 1 to 3 m.y. in time. If the Mendocino triple junction initiated at about 28.5 Ma, its position would have been at 31.3°N ± 130 km relative to fixed North America. Unacceptable overlap of oceanic crust of the Pacific plate with continental crust of western North America in the anomaly 10 (30 Ma) reconstruction is a minimum of 340±200 km along an azimuth of N60°E and may be accounted for by Basin and Range extension. Pacific-North America displacement in the past 20 Ma is found to be considerably less than that calculated by fixed hotspot reconstructions. Farallon (Vancouver)-North America convergence velocity decreased greatly between the times of anomalies 24 and 21 (56 to 50 Ma), prior to the 43 Ma age of the Hawaiian-Emperor bend and the often quoted 40 Ma “end” of the Laramide orogeny. A change in direction of Farallon-North America convergence occurred sometime between 50 and 42 Ma and also may not correlate with the time of the Hawaiian-Emperor bend. The lack of data from subducted parts of the Farallon and Kula plates permits many possibilities regarding the position of the Kula-Farallon ridge, the age of subducted crust, or the position of oceanic plateaus during the Laramide orogeny, leaving open the question of the relationship between plate tectonic scenarios and tectonic style during Laramide time. Displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed North America during the interval between anomalies 30/31 and 18 (68 to 42 Ma) are found to be: Pacific plate, 1700±200 km northward; Farallon plate, 3200±400 km northeastward; Vancouver plate, 3000±400 km northeastward; Kula plate, if attached to the Pacific plate after A24 time, 2500±400 km northward

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

The Ayyubid Orogen: An Ophiolite Obduction-Driven Orogen in the Late Cretaceous of the Neo-Tethyan South Margin

A minimum 5000-km long obduction-driven orogeny of medial to late Cretaceous age is located between Cyrenaica in eastern Libya and Oman. It is herein called the Ayyubid Orogen after the Ayyubid Empire that covered much of its territory. The Ayyubid orogen is distinct from other Alpide orogens and has two main parts: a western, mainly germanotype belt and an eastern mainly alpinotype belt. The germanotype belt formed largely as a result of an aborted obduction, whereas the alpinotype part formed as a result of successful and large-scale obduction events that choked a nascent subduction zone. The mainly germanotype part coincides with Erich Krenkel's Syrian Arc (Syrischer Bogen) and the alpinotype part with Ricou's  Peri-Arabian Ophiolitic Crescent (Croissant Ophiolitique péri-Arabe). These belts formed as a consequence of the interaction of one of the now-vanished Tethyan plates and Afro-Arabia. The Africa-Eurasia relative motion has influenced the orogen's evolution, but was not the main causative agent. Similar large and complex obduction-driven orogens similar to the Ayyubids may exist along the Ordovician Newfoundland/Scotland margin of the Caledonides and along the Ordovician European margin of the Uralides.SOMMAIREEntre la Cyrénaïque dans l'est de la Libye et Oman, se trouve un ceinture orogénique d’au moins 5 000 km de longueur créé par obduction au Crétacé moyen et tardif.  Nous le nommons ici l’orogène ayyoubide d’après l'empire ayyoubide qui couvrait une grande partie de son territoire.  L'orogène ayyoubide qui est distincte des autres orogènes alpides, comporte deux parties principales : une bande occidentale, principalement germanotype, et une bande orientale principalement alpinotype.  La bande germanotype s’est formée en grande partie à la suite d'une obduction avortée, tandis que la partie alpinotype s’est formée par des épisodes d’obduction à grande échelle qui ont étranglé une zone de subduction naissante.  La partie principalement germanotype coïncide avec l’arc syrien d’Erich Krenkel (Syrischer Bogen), alors que la partie alpinotype correspond au croissant ophiolitique péri-Arabe de Ricou (Croissant ophiolitique péri-Arabe).  Ces bandes se sont formées par l'interaction de l'une des plaques de la Téthys, maintenant disparues, avec l’Afro-Arabie.  Le mouvement relatif Afrique-Eurasie a influencé l'évolution de l'orogène, mais ça n’a pas été le principal facteur.  Des orogènes grandes et complexes résultant de mécanismes d’obduction similaires à l’orogène Ayyoubide peuvent exister le long de la marge des Calédonides de l'Ordovicien de Terre-Neuve/Écosse et le long de la marge européenne des Uralides de l'Ordovicien

The Ayyubid Orogen: An Ophiolite Obduction-Driven Orogen in the Late Cretaceous of the Neo-Tethyan South Margin

A minimum 5000-km long obduction-driven orogeny of medial to late Cretaceous age is located between Cyrenaica in eastern Libya and Oman. It is herein called the Ayyubid Orogen after the Ayyubid Empire that covered much of its territory. The Ayyubid orogen is distinct from other Alpide orogens and has two main parts: a western, mainly germanotype belt and an eastern mainly alpinotype belt. The germanotype belt formed largely as a result of an aborted obduction, whereas the alpinotype part formed as a result of successful and large-scale obduction events that choked a nascent subduction zone. The mainly germanotype part coincides with Erich Krenkel's Syrian Arc (Syrischer Bogen) and the alpinotype part with Ricou's  Peri-Arabian Ophiolitic Crescent (Croissant Ophiolitique péri-Arabe). These belts formed as a consequence of the interaction of one of the now-vanished Tethyan plates and Afro-Arabia. The Africa-Eurasia relative motion has influenced the orogen's evolution, but was not the main causative agent. Similar large and complex obduction-driven orogens similar to the Ayyubids may exist along the Ordovician Newfoundland/Scotland margin of the Caledonides and along the Ordovician European margin of the Uralides.SOMMAIREEntre la Cyrénaïque dans l'est de la Libye et Oman, se trouve un ceinture orogénique d’au moins 5 000 km de longueur créé par obduction au Crétacé moyen et tardif.  Nous le nommons ici l’orogène ayyoubide d’après l'empire ayyoubide qui couvrait une grande partie de son territoire.  L'orogène ayyoubide qui est distincte des autres orogènes alpides, comporte deux parties principales : une bande occidentale, principalement germanotype, et une bande orientale principalement alpinotype.  La bande germanotype s’est formée en grande partie à la suite d'une obduction avortée, tandis que la partie alpinotype s’est formée par des épisodes d’obduction à grande échelle qui ont étranglé une zone de subduction naissante.  La partie principalement germanotype coïncide avec l’arc syrien d’Erich Krenkel (Syrischer Bogen), alors que la partie alpinotype correspond au croissant ophiolitique péri-Arabe de Ricou (Croissant ophiolitique péri-Arabe).  Ces bandes se sont formées par l'interaction de l'une des plaques de la Téthys, maintenant disparues, avec l’Afro-Arabie.  Le mouvement relatif Afrique-Eurasie a influencé l'évolution de l'orogène, mais ça n’a pas été le principal facteur.  Des orogènes grandes et complexes résultant de mécanismes d’obduction similaires à l’orogène Ayyoubide peuvent exister le long de la marge des Calédonides de l'Ordovicien de Terre-Neuve/Écosse et le long de la marge européenne des Uralides de l'Ordovicien

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