Tanya Atwater

University of California, Santa Barbara | UCSB · Department of Earth Science

Starting with the Laramide orogeny and continuing through the Cenozoic, the U.S. Cordilleran orogen is unusual for its width, nature of uplift, and style of tectonic and magmatic activity. We present teleseismic tomography evidence for a thickness of modified North America lithosphere <200 km beneath Colorado and >100 km beneath New Mexico. Existin...

We use updated rotations within the Pacific-Antarctica-Africa-North America plate circuit to calculate Pacific-North America plate reconstructions for times since chron 13 (33 Ma). The direction of motion of the Pacific plate relative to stable North America was fairly steady between chrons 13 and 4, and then changed and moved in a more northerly d...

Professional geoscientists tend to have specific, elaborate "mental cartoons" that encapsulate their conceptual understandings of earth processes. The computer revolution has made it possible for this mental imagery to be illustrated and animated in ways not previously possible, while the development of the Internet is making it easily shared. The...

The San Andreas system plate boundary developed first in southern California, about 25 Ma. Between 20 and 12 Ma the southern triple junction of this system remained offshore of northernmost Baja California. The Pacific-North American relative plate motion vector during this period was significantly more oblique to the coast than at present, so that...

The Channel Islands occupy the southern edge of the Transverse Ranges block and thus the islands share and illuminate the history of this block. In the Mesozoic and early Cenozoic, the Transverse Ranges block was oriented north-south, so that the Channel Islands probably lay near San Diego. The block occupied the forearc region of a subduction zone...

Tectonic rotation of the western Transverse Ranges block is explained by capture of the partially subducted Monterey microplate by the Pacific plate at about anomaly 6 time (ca. 20 Ma). As Pacific-Monterey spreading slowed and eventually ceased, the slip vector along the gently northeast dipping subduction interface beneath the California margin ch...

Preliminary results are presented from an expedition to the ultra-fast spreading segment on the East Pacific Rise. The combined multibeam and sidescan sonar worked extremely well, and provided the authors with surprising discoveries of abundant off-axis volcanism. Individuals from several institutions have contributed to a collection of public doma...

We present a new, more complete mapping of the fracture zones as they cross the Cretaceous Quiet Zone in the central north Pacific. We compile and combine observations of lineations from three distinct data bases: deflection-of-the-vertical profiles from GEOSAT altimetry measurements, magnetic and topographic profiles collected on closely spaced no...

ALVIN investigations have defined the fine-scale structural and volcanic patterns produced by active rift and spreading center propagation and failure near 95.5 W on the Galapagos spreading center. Behind the initial lithospheric rifting, which is propagating nearly due west at about 50 km m.y.–1, a triangular block of preexisting lithosphere is be...

We have reconstructed the isochron pattern of the Faralion and Vancouver plates in order to predict the thermal state and geometry of subducting slabs beneath western North America during the Cenozoic. Slabs do not last indefinitely; they warm up by conduction when bathed in the asthenosphere. As they warm up, they lose the ability to have earthqua...

Rotation and propagation have both been proposed as mechanisms by which spreading centers can reorient following changes in direction of seafloor spreading. The rotation hypothesis predicts a smooth asymmetric fanning of magnetic anomaly and abyssal hill orientations, whereas the propagation hypothesis predicts that the orientation changes will occ...

We have tested and corroborated the propagating rift hypothesis with high-resolution Sea Beam and Deep-Tow data collected over the Galapagos 95.5°W propagating rift system. The propagating rift is continuously breaking through the Cocos plate at a velocity of about 50 km/m.y. with an azimuth of about 273°, away from the Galapagos hotspot. This proc...

Hydrothermal vents jetting out water at 380° ± 30°C have been discovered on the axis of the East Pacific Rise. The hottest waters issue from mineralized chimneys and are blackened by sulfide precipitates. These hydrothermal springs are the sites of actively forming massive sulfide mineral deposits. Cooler springs are clear to milky and support exot...

Old, cold oceanic lithosphere is denser and therefore gravitationally more unstable than younger, hotter oceanic lithosphere. Hence, whereas old lithosphere will sink under its own weight, subduction of young lithosphere may require an additional force. Interarc spreading occurs or occurred recently in the western Pacific, in the southern Atlantic,...

A major question in seafloor tectonics has been, how does the 2-km-deep rift valley characteristic of slow-spreading ridges evolve into the relatively horizontal undulating relief of the rift mountains? Deep-tow studies of the Mid-Atlantic Ridge suggest that the primary mechanism for transformation of the rift valley topography is normal faulting a...

We report here the results of a near-bottom geophysical survey of the Reykjanes Ridge, a mid-ocean ridge that is oriented obliquely to the perpendicular spreading direction. From a combination of the bathymetric profiles, side-scan sonar data, and regional bathymetric maps we infer that the present center of spreading is made up of a number of N15°...

A COMMON assumption in seafloor spreading is that mid-ocean ridge crests are aligned perpendicular to their transform faults and, hence, to their spreading directions. There are some well known exceptions to this rule, for example, the Reykjanes Ridge. Vogt et al. 1 suggested that spreading systems may take one of two configurations: either a trans...

An intensive paleomagnetic and rock magnetic study has been carried out on the basalt samples from the FAMOUS area of the Mid-Atlantic Ridge at lat 37 °N. In addition to the samples obtained by the submersible Alvin from the floor and walls of the central rift valley, dredge and rock-drill samples were taken on a line perpendicular to the axis of t...

Bathymetric and magnetic anomaly data indicate that the South Pacific Ocean floor between New Zealand and Antarctica formed since late Cretaceous time by sea-floor spreading at the Pacific-Antarctic Rise and the southern portion of the East Pacific Rise. Following the initial breaking apart of the Campbell plateau from Antarctica, the Pacific plate...

A deeply towed instrument package was used to survey the fine details of topography, sediment distribution, and magnetization of the Gorda rise, an active spreading center off northern California. The gross form of the central rift valley is the result of large-scale normal faulting. The surface is broken into long, narrow, tilted steps parallel to...

Reconstruction of the major lithospheric plates 21 and 38 m.y. ago implies that linear island chains and aseismic ridges were not generated by localised sources of volcanism (hot spots) that have been fixed with respect to one another. If the linear volcanic chains are formed over mantle hot spots, the spots move at rates of 0.8 to 2 cm yr-1 with r...

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The present lack of volcanic activity in the Western Aleutian Arc correlates with the lack of underthrusting at the western Aleutian trench predictable by plate tectonic theory. A simple plate model allows one to predict that another plate (Kula plate) and a spreading ridge (Kula ridge) lay south of the arc in early Tertiary time and that the Aleut...

Magnetic data for the northeast Pacific are summarized and interpreted in terms of sea floor spreading. Unclear areas exist in the “disturbed zone” and west of the Juan de Fuca Ridge. The direction of spreading changed about 55 my ago. An intermediate plate was gradually destroyed as segments of the ridge neared the continent and the timing of this...

Fracture zone topography can be accounted for by seafloor spreading and plate tectonics.

A study made of Gorda Rise near 41°15'N with a novel instrument shows that the rift-valley walls have a tilted steplike profile, often with perched, planar sediments. Topography indicates that the steps were formed by block faulting. Distribution of the steps and the character of their tops suggest that they originated in the central 2 or 3 kilomet...