Douglas S. Wilson

Confidence intervals for motion and deformation of the Juan de Fuca Plate

Finite-rotation parameters for 17 isochrons from anomalies 1-4A for the Juan de Fuca-Pacific plate pair are reported. Comparison of confidence intervals for plate rotations derived by different techniques indicates that a single-plate misfit criterion significantly underestimates the confidence region. Defining the confidence interval by a threshold level of misfit determined by an F statistic gives similar results to the technique of bootstrap resampling for internally consistent data. Bootstrap resampling offers far greater robustness in the case of inconsistent data. Precision of about 1 km in the direction normal to isochrons is available for the younger anomalies. This precision allows confident recognition of 2–5 km of eastward deformation of the Pacific plate, 50–100 km south of the active Sovanco transform. Slight deformation of the northern Juan de Fuca plate adjacent to the Nootka fault zone is also apparent East of the northern Gorda ridge, younger (< 3 Ma) anomalies do not show measurable deformation near the Blanco fracture zone, but older anomalies are up to 30 km east of their position predicted by rigid behavior of the Juan de Fuca plate. Reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km, even with offset of 75 km in one case and 140 km in another. New techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99% confidence: a clockwise change in motion direction at about 5 Ma and an increase in the gradient of spreading rate since 2 Ma. Two other changes in gradient since 5 Ma are required at 95% confidence.

History of rift propagation and magnetization intensity for the Cocos‐Nazca sspreading Center

Analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the Cocos-Nazca spreading center. Forward models of the magnetic anomalies accounting for spreading rate variations, ridge jumps, asymmetric spreading, magnetization intensity variations, and bathymetry show excellent agreement with observed anomalies. Spreading rates can be constrained to a common finite rotation history through anomaly 4A with three changes in rates. Rate changes at about 1.5 Ma and 4.1 Ma correspond to changes in rate gradients and occur during the well-calibrated part of the reversal timescale, so they can unquestionably be identified as true changes in plate motion. A ∼15% rate decrease at about 5.2 Ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale. The change at 4.1 Ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0–6 Ma. Rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis. Propagation has been predominantly away from the hotspot, with jumps predominantly south-ward. Propagation rates have ranged from 30 to 120 mm/yr, commonly near 70 mm/yr. Origin of most propagation sequences is difficult to interpret, but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot. Magnetic anomaly amplitude appears to be a reliable tracer of Fe content of lavas. Several generalizations can be drawn about along-axis variations in magnetization intensities since 8 Ma: high magnetizations are only observed at the far ends (relative to the Galapagos hotspot) of segments at least 150 km long; offset at the end of a high-magnetization segment is at least 15 km; and there are no offsets larger than 30–45 km between high-magnetization segments and the reconstructed position of the hotspot. We interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along-axis flow of evolved magma. The gradients in supply result from subaxial flow of hotspot-derived asthenosphere in a narrow conduit. This flow is only partly obstructed by an offset of 20–30 km but entirely blocked by an offset of 50 km.

Relative motions of the Pacific, Rivera, North American, and Cocos plates since 0.78 Ma

We use magnetic anomaly and fracture zone crossings from 3°N–24°N along the East Pacific Rise to define the boundaries and relative velocities of the rigid Pacific, Rivera, North American, and Cocos plates since 0.78 Ma and to test several hypotheses regarding the nature of deformation between these plates. Crossings of the 0.78 Ma isochron from the Pacific-Rivera rise north of 22.0°N are significantly better fit by the 0–0.78 Ma Pacific-North America rotation than by a Pacific-Rivera rotation, thereby supporting a recently proposed model for accretion of the northern part of the Rivera plate to North America. A significant misfit of the Pacific-Cocos rotation to crossings of anomaly ln from the East Pacific Rise north of 16.4°N suggests that seafloor north of ∼16°N moves relative to the rigid Pacific or Cocos plates. We also find that nine crossings of the eastern Rivera fracture zone and eight crossings of anomaly ln flanking the Manzanillo spreading segment are fit well by a Pacific-Rivera rotation. The Rivera-Cocos angular velocity derived from the best fitting Pacific-Cocos and Pacific-Rivera angular velocities predicts 19±3 mm yr−1 (95% confidence limit) of ∼N-S convergence near the center of the Rivera-Cocos boundary. Nearby left-lateral, strike-slip earthquakes with north trending fault planes suggest that convergence is accommodated via sinistral shear along a north trending zone. The unambiguous kinematic and seismologic evidence for significant Rivera-Cocos motion indicates that the absence of a well-defined Rivera-Cocos plate boundary cannot be interpreted as evidence that Rivera-Cocos motion is slow or zero, as is postulated in several previous studies. The Rivera-Cocos angular velocity predicts oblique Rivera-Cocos convergence at the El Gordo graben and beneath the Manzanillo trough and southern Colima graben. Any extension across these features since 0.78 Ma thus cannot have resulted from divergence between the subducting Rivera and Cococs plates.

Morphology of the Blanco Transform Fault Zone-NE Pacific: Implications for its tectonic evolution

The right-lateral Blanco Transform Fault Zone (BTFZ) offsets the Gorda and the Juan de Fuca Ridges along a 350 km long complex zone of ridges and right-stepping depressions. The overall geometry of the BTFZ is similar to several other oceanic transform fault zones located along the East Pacific Rise (e.g., Siquieros) and to divergent wrench faults on continents; i.e., long strike-slip master faults offset by extensional basins. These depressions have formed over the past 5 Ma as the result of continual reorientation of the BTFZ in response to changes in plate motion. The central depression (Cascadia Depression) is flanked by symmetrically distributed, inward-facing back-tilted fault blocks. It is probably a short seafloor spreading center that has been operating since about 5 Ma, when a southward propagating rift failed to ‘kill’ the last remnant of a ridge segment. The Gorda Depression on the eastern end of the BTFZ may have initially formed as the result of a similar occurrence involving a northward propagating rift on the Gorda ridge system. Several of the smaller basins (East Blanco, Surveyor and Gorda) morphologically appear to be oceanic analogues of continental pull-apart basins. This would imply diffuse extension rather than the discrete neovolcanic zone associated with a typical seafloor spreading center. The basins along the western half of the BTFZ have probably formed within the last few hundred thousands years, possibly as the result of a minor change in the Juan de Fuca/Pacific relative motion.

Focused mantle upwelling beneath mid-ocean ridges: evidence from seamount formation and isostatic compensation of topography

Abstract Observational evidence for the geometry of mantle flow beneath mid-ocean ridges is often considered limited. I offer two constraints supporting interpretations of strong focusing of upwelling through the depths where MORB magmas are generated. The first is based on a modification of the model of Davis and Karsten [ 1 ] for the formation of near-axis seamounts by the melting of fertile mantle heterogeneities. Several types of observations indicate that most seamounts in the eastern Pacific form in a remarkably narrow range of distance from the axis, about 6–15 km. The outer limit of this range is interpreted to represent the width of the upwelling zone in the upper part of the garnet stability field (depth ∼ 80 km) where the most fertile mantle melts, while the inner limit indicates the width of most magma generation (perhaps depths of 30–50 km). The diverse and depleted chemistry of young seamount lavas is inferred to reflect originally enriched magmas generated at depth that bypass the principal zone of melting and interact with strongly depleted material at intermediate depths. A comparable and entirely independent constraint on the width of most magma generation is available from the assumption that anomalously shallow topography at ridge axes is isostatically compensated by melt at depth. The width of the topographic anomaly limits the melt zone to within 10 km from the axis, while the amplitude of the gravity anomaly, at least in some places, requires most of the melt to be deeper than 15 km. If the fraction of melt is assumed to be small ( ∼ 2%), the partial melt must extend to depths exceeding 40 km.

An astronomical polarity timescale for the late middle Miocene based on cyclic continental sequences

[1] We present an astronomically tuned polarity timescale for the late middle Miocene based on a cyclic shallow lacustrine/mudflat succession exposed in the Orera Composite Section (OCS; Calatayud basin, NE Spain). Spectral analysis and band-pass filtering of high-resolution carbonate and color reflectance records in the depth domain reveal cyclic changes with different cycle lengths, which correspond to lithological alternations observed in the field. An initial age model was constructed by calibrating the OCS magnetostratigraphy to the geomagnetic polarity timescale of Cande and Kent [1995]. Subsequent spectral analysis of the proxy records in the time domain reveals periodicities close to 23, 41, and 400 kyr and, to a lesser extent, 100 kyr, supporting an astronomical origin for the sedimentary cyclicity in the OCS. We established a new age model based on the astronomical calibration of the OCS to the Laskar et al. [1993] (La93) solution by tuning the cycles to the astronomical target curves. Cross-spectral analysis results of the tuned time series followed by band-pass filtering reveal a remarkably good and in-phase relation with precession and with obliquity despite a presumed uncertainty of 20–40 kyr in the tuning in some short intervals. Our tuning provides astronomical ages for sedimentary cycles and subsequently for polarity reversals in the interval between 12.9 and 10.6 Ma. Comparison with Cande and Kent shows that the OCS polarity reversal ages are older by 80 kyr. This age discrepancy decreases with increasing age to 50–60 kyr. INDEX TERMS: 1035 Geochemistry: Geochronology; 1520 Geomagnetism and Paleomagnetism: Magnetostratigraphy; 8105 Tectonophysics: Continental margins and sedimentary basins; 9335 Information Related to Geographic Region: Europe; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: geochronology, magnetostratigraphy, cyclostratigraphy, astronomical timescale, orbital forcing, Miocene

Structural and tectonic evolution of the Ross Sea rift in the Cape Colbeck region, Eastern Ross Sea, Antarctica

The far eastern continental shelf of the Ross Sea, Antarctica, has been relatively unexplored up to now. This region and western Marie Byrd Land are at the eastern limit of the Ross Sea rift, part of the West Antarctic rift system, one of the larger regions of extended crust in the world. The Ross Sea continental shelf west of Cape Colbeck and the Edward VII Peninsula in western Marie Byrd Land was investigated using marine geophysics during cruise 9601 of the research vessel ice breaker Nathaniel B. Palmer. The purpose was to determine the structural framework and tectonic history of the eastern border of the Ross Sea rift and to integrate this with what is known about western Marie Byrd Land. The region mapped is characterized by a passive margin with a flat overdeepened shelf cut by the north trending Colbeck Trough, an erosional feature formed in Miocene and later time by glacial downcutting that followed the locations of existing basement structures. Seismic sequences and unconformities identified in the Ross Sea were correlated into the Colbeck shelf area. The section comprises mostly undeformed glacial marine sequences of late Oligocene and younger age that are unconformably overlying late Early to Late Cretaceous and minor early Tertiary (?) faulted sequences. This unconformity is identified as RSU6, mapped elsewhere in the eastern Ross Sea. Two units are found below RSU6, each separated by an unconformity that is here named RSU7. These sequences fill north trending half grabens in the faulted basement and are interpreted as syn rift units. Unconformity RSU7 is correlated to the West Antarctic Erosion Surface mapped onshore in western Marie Byrd Land. The lack of thick early Tertiary sediments on the shelf suggests significant vertical tectonics. This onshore and offshore region was widely faulted in late Early and Late Cretaceous time, was high above sea level and was beveled by prolonged erosion, while subsiding steadily in Late Cretaceous and Cenozoic time. Subsidence was largely due to lithosphere cooling amplified later by glacial and sediment loading in Cenozoic time. Mylonites that have late Early Cretaceous cooling ages were dredged from the southeast wall of the Colbeck Trough. This finding and normal faults that we mapped in the eastern Ross Sea we attribute to detachment-style extension in late Early Cretaceous time. This extension was directed subparallel to the trend of the present margin edge and occurred prior to the rifting of Campbell Plateau from Marie Byrd Land at ∼79 Ma. Cooling events onshore western Marie Byrd Land suggest the main extension began at ∼105 Ma. This is also the time of transition from subduction to extension elsewhere along the ancient Gondwana margin. Minor west tilting of the shelf during the late Cenozoic was the result of continued subsidence of the continental shelf along with possible uplift of western Marie Byrd Land associated with the Marie Byrd Land dome to the east. Early Tertiary extension in the western Ross Sea rift is not strongly reflected in the east side of the rift. A more robust correlation of the events here with the better known tectonic history on the west side of the Ross Sea rift awaits sampling and dating of the units we mapped on the Colbeck shelf.

Kinematics of overlapping rift propagation with cyclic rift failure

Existing kinematic models for propagation of oceanic spreading ridges that incorporate overlap between ridge segments fail to describe detailed observations of the failed segments. I present a new model which discards the assumption of steady state behavior of the failing rift, permitting inward curvature of both rift tips in the overlap region. The shape of an inward-curving failing rift must continuously change, but is assumed to cyclically return to its original shape by discrete inward ridge jumps. Other assumptions of symmetric spreading and uniform simple shear deformation between the overlapping rift tips are retained from previous models. Inward curvature of failed rift structures provides much better agreement with observations, and is consistent with tensile fracture theory. If the offset between ridge segments is small enough, the inward jumps of the failing rift will cut across deformed structures originally formed at the propagating tip, possibly generating seafloor fabrics that crosscut each other at nearly right angles. Observations of such structures near the Gorda Ridge can be explained by a model incorporating variable cyclieity of the failing rift. chrons curve inward toward the ridge, faithfully recording the shape of the propagating tip in an undeformed part of the plate. Isochrons also curve toward the ridge along the inner pseudofault, but In the original formulation of the rules of plate tectonics, transform faults were assumed to be fixed relative to each other and the ridge segments they offset. The propagating rift model [1], which discards this assumption, has been very successful in describing oblique offsets in magnetic anomalies as a result of the lengthening of a ridge segment through time at the expense of the adjacent segment. Detailed observations of active propagating rifts suggest that the assumption that ridges are necessarily offset by discrete transform faults should also be discarded, with the segments instead offset by zones of distributed shear with widths of 10 or more km [2,3].

A multibeam-sonar, magnetic and geochemical flowline survey at 14°14′S on the southern East Pacific Rise: insights into the fourth dimension of ridge crest segmentation

A detailed bathymetric and magnetic survey of the eastern flank of the East Pacific Rise at 14°14′S covering seafloor ages of 0–10 Ma has been carried out and used, along with a flowline profile on the conjugate western ridge flank, to reveal the spreading history and the temporal ridge crest segmentation. Additional information from basaltic lavas is included to study the relationship between physical and magmatic segment boundaries. The sequence of magnetic reversals indicates a total spreading rate of 150 mm/yr since 10 Ma. Symmetric spreading, however, occurred only since 2.8 Ma. Between 7 and 2.8 Ma spreading was asymmetric, with a higher spreading rate toward the east. Migration events of at least five overlapping spreading centres (OSC) left discordant zones on the Nazca plate consisting of hummocky basins and motley texture of curved lineations striking a few degrees oblique to the strike of the ridge crest. Four of the OSCs were right-stepping and migrated northward and one was left-stepping and migrated southward. By transferring Pacific lithosphere to the Nazca plate, these migration events may account for most of the asymmetric accretion observed. The basaltic samples from the eastern flank have been analysed and back tracked to the position of eruption on the ridge crest. In terms of their geochemical signature (Mg# 0.41–0.68) the samples reveal that the magmatic segment boundary between the Garrett transform and 14°30′S has remained stationary over the last 10 Myr and therefore provide no evidence for a link between magmatic and physical segmentation. We therefore propose that migrating non-transform ridge axis discontinuities are governed by propagating giant cracks; as a crack front advances a melt reservoir is tapped and magma rises passively into the crack and erupts subsequently on the seafloor. Some of the OSCs seem to have originated close to transform faults and therefore argue that far-field stresses, perhaps caused by the evolution of the Bauer microplate, rather than mantle upwelling create non-transform ridge axis discontinuities.

Oligocene development of the West Antarctic Ice Sheet recorded in eastern Ross Sea strata

Seismic-reflection data from the easternmost Ross Sea image buried scour-and-fill troughs and flat-topped ridges interpreted as having formed by glacial erosion and deposition during the Oligo-cene. The NNW-SSE orientation of the troughs and lack of similar Oligocene glacial features within the central Ross Sea suggests that the ice issued from the highlands of Marie Byrd Land located 100 km away and that portions of the West Antarctic Ice Sheet formed earlier than previously accepted. Existing global climate models (GCMs) do not produce West Antarctic ice caps for the Oligocene, in part due to low elevations modeled for that time. Evidence for Oligocene ice beyond the paleocoast suggests a higher elevation for the early Cenozoic Marie Byrd Land and Ross Embayment than at present.