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Sahakian, V, Bormann J, Driscoll N, Harding A, Kent G, Wesnousky S.  2017.  Seismic constraints on the architecture of the Newport-Inglewood/Rose Canyon fault: Implications for the length and magnitude of future earthquake ruptures. Journal of Geophysical Research-Solid Earth. 122:2085-2105.   10.1002/2016jb013467   AbstractWebsite

The Newport-Inglewood/Rose Canyon (NIRC) fault zone is an active strike-slip fault system within the Pacific-North American plate boundary in Southern California, located in close proximity to populated regions of San Diego, Orange, and Los Angeles counties. Prior to this study, the NIRC fault zone's continuity and geometry were not well constrained. Nested marine seismic reflection data with different vertical resolutions are employed to characterize the offshore fault architecture. Four main fault strands are identified offshore, separated by three main stepovers along strike, all of which are 2km or less in width. Empirical studies of historical ruptures worldwide show that earthquakes have ruptured through stepovers with this offset. Models of Coulomb stress change along the fault zone are presented to examine the potential extent of future earthquake ruptures on the fault zone, which appear to be dependent on the location of rupture initiation and fault geometry at the stepovers. These modeling results show that the southernmost stepover between the La Jolla and Torrey Pines fault strands may act as an inhibitor to throughgoing rupture due to the stepover width and change in fault geometry across the stepover; however, these results still suggest that rupture along the entire fault zone is possible.

Tong, CH, Lana C, White RS, Warner MR, Barton PJ, Bazin S, Harding AJ, Hobbs RW, Kent GM, Orcutt JA, Pye JW, Singh SC, Sinha MC.  2005.  Subsurface tectonic structure between overlapping mid-ocean ridge segments. Geology. 33:409-412.   10.1130/g21245.1   AbstractWebsite

Our results from seismic anisotropy analyses reveal for the first time the complex spatial variability of the characteristics of subsurface tectonic structures associated with ridge propagation. The significance lies in the fact that these variations are found at a locality with few lineaments or fissures at seafloor level. The overlap region between mid-ocean ridge segments at 9 degrees N on the East Pacific Rise is characterized by aligned cracks that are structurally more closely related to the propagating-ridge segment. These aligned cracks, which are approximately parallel to the ridge segments, provide conclusive observational evidence for establishing the nontransform nature of overlapping spreading centers, especially those with overlap basins covered by volcanic edifices. The aligned cracks of the 9 degrees 03'N overlapping spreading center are more similar to the ridge-parallel lineaments observed between overlapping axial-summit collapse troughs than those found at larger overlapping spreading centers. Our results therefore suggest that the lithospheric deformation between overlapping ridge segments depends on ridge offset and that this dependency may be thermally related.

Bazin, S, Harding AJ, Kent GM, Orcutt JA, Tong CH, Pye JW, Singh SC, Barton PJ, Sinha MC, White RS, Hobbs RW, Van Avendonk HJA.  2001.  Three-dimensional shallow crustal emplacement at the 9 degrees 03 ' N overlapping spreading center on the East Pacific Rise: Correlations between magnetization and tomographic images. Journal of Geophysical Research-Solid Earth. 106:16101-16117.   10.1029/2001jb000371   AbstractWebsite

We report a three-dimensional (3-D) seismic reflection and tomographic survey conducted at the 9 degrees 03'N overlapping spreading center (OSC) on the East Pacific Rise to understand crustal accretion at this feature. Inversions of travel time data from 19 ocean bottom hydrophones provide a 3-D image of the shallow velocity structure beneath the nontransform offset and associated discordance zone. Seismic analysis indicates that layer 2A thickness varies between 100 and 900 in and averages 430 in throughout the study area. The heterogeneous upper crustal structure at the OSC region contrasts with the simpler symmetric structure flanking the midsegments of the East Pacific Rise. The crust affected by the OSC migration carries evidence for the complex accretion at the axial discontinuity where the overlap basin may act as a lava pond. An area of thick layer 2A covers the southern half of the overlap basin and the propagating ridge tip and shows good correlation with a high magnetization region. Comparison of the magnetic field anomaly derived from the seismic structure model with the observed sea surface magnetic anomaly suggests that a significant portion of the high magnetization can be related to magnetic source thickness variation rather than solely to the geochemistry of the volcanic rocks.

Kent, GM, Singh SC, Harding AJ, Sinha MC, Orcutt JA, Barton PJ, White RS, Bazin S, Hobbs RW, Tong CH, Pye JW.  2000.  Evidence from three-dimensional seismic reflectivity images for enhanced melt supply beneath mid-ocean-ridge discontinuities. Nature. 406:614-618.   10.1038/35020543   AbstractWebsite

Quantifying the melt distribution and crustal structure across ridge-axis discontinuities is essential for understanding the relationship between magmatic, tectonic and petrologic segmentation of mid-ocean-ridge spreading centres. The geometry and continuity of magma bodies beneath features such as overlapping spreading centres can strongly influence the composition of erupted lavas(1) and may give insight into the underlying pattern of mantle flow. Here we present three-dimensional images of seismic reflectivity beneath a mid-ocean ridge to investigate the nature of melt distribution across a ridge-axis discontinuity. Reflectivity slices through the 9 degrees 03' N overlapping spreading centre on East Pacific Rise suggest that it has a robust magma supply, with melt bodies underlying both limbs and ponding of melt beneath large areas of the overlap basin. The geometry of melt distribution beneath this offset is inconsistent with large-scale, crustal redistribution of melt away from centres of upwelling(2,3). The complex distribution of melt seems instead to be caused by a combination of vertical melt transport from the underlying mantle and subsequent focusing of melt beneath a magma freezing boundary in the mid-crust.

Babcock, JM, Harding AJ, Kent GM, Orcutt JA.  1998.  An examination of along-axis variation of magma chamber width and crustal structure on the East Pacific Rise between 13 degrees 30 ' N and 12 degrees 20 ' N. Journal of Geophysical Research-Solid Earth. 103:30451-30467.   10.1029/98jb01979   AbstractWebsite

We investigate the along-axis variations of magma chamber width and crustal structure along the East Pacific Rise (EPR) from 13 degrees 30'N to 12 degrees 20'N through reprocessed common depth point (CDP) reflection profiles. The magma lens is, predominantly, a continuous feature in the study area with an average width of similar to 500 m as determined from migrated cross-axis CDP profiles. This value is similar to widths estimated elsewhere along the EPR, suggesting that the axial magma chamber (AMC) width is not spreading rate dependent once the threshold for a steady state magma chamber is reached. The axial morphology of the 13 degrees N area is generally not a good predictor of magma lens width or continuity. A fairly continuous melt lens is imaged where the triangular axial topography might suggest waning magma supply. In fact, between 13 degrees 05'N and 13 degrees 01'N a shallow melt lens has been imaged which may be indicative of recent or impending eruptive activity. This shoaling is similar to that observed near the 17 degrees 26'S region of the EPR where the rise axis summit is domed and highly inflated. Generally, the thickness of seismic layer 2A beneath the ridge crest is uniform and comparable to that estimated for 9 degrees N, 14 degrees S, and 17 degrees S on the EPR, suggesting that the axial extrusive layer is invariant along fast spreading ridges. Uniformity of layer 2A thickness along-axis implies that variations in magma chamber depth are directly attributed to changes in thickness of the sheeted dike complex (seismic layer 2B). Contrary to expectations of decreasing melt sill depth with increasing spreading rate, the average thickness of seismic layer 2B is slightly less (similar to 165 m) at 13 degrees N than at the faster spreading, more robust 9 degrees N area. Finally, geochemical/petrologic boundaries, which may delineate separate melt supply regions, occurring at the 13 degrees 20'N and 12 degrees 46'N devals (deviation in axial linearity) are observed to coincide with subtle changes in AMC and layer 2A reflection characteristics.

Begnaud, ML, McClain JS, Barth GA, Orcutt JA, Harding AJ.  1997.  Velocity structure from forward modeling of the eastern ridge-transform intersection area of the Clipperton Fracture Zone, East Pacific Rise. Journal of Geophysical Research-Solid Earth. 102:7803-7820.   10.1029/96jb03393   AbstractWebsite

In the spring of 1994, we undertook an extensive geophysical study of the Clipperton Fracture Zone (FZ) on the fast spreading East Pacific Rise. The Clipperton Area Seismic Study to Investigate Compensation experiment (CLASSIC) included surveys to examine the deep structures associated with the fracture zone and adjacent northern ridge segment. In this paper, we report the results from five seismic profiles acquired over the eastern ridge-transform intersection (RTI), including profiles over the RTI high, the northern ridge segment, and the eastern transform region. The travel time data for crustal phases, Moho reflections, and mantle phases were modeled using two-dimensional ray tracing. Seismic profiles reveal that the crust is similar in thickness north and south of the Clipperton FZ, despite differences in axial topography that have previously been interpreted in terms of differences in magma supply. When compared to older crust, the northern ridge axis is characterized by lower seismic velocities and higher attenuation. In our model, a low-velocity zone exists beneath the ridge axis, probably associated with a zone of partial melt and/or very high temperatures. Within the transform zone, we find that the southeastern trough is underlain by nearly normal crustal structure. The crust is slightly thinner than the adjacent aseismic extension but not enough to compensate for the depths of the trough. Toward the RTI, the trough is replaced by an intersection high which appears underlain by a thickened crust, and a thicker upper crustal section. Both characteristics indicate that the intersection high is a volcanic feature produced by excess volcanism at the intersection. The volcanism acts to ''fill in'' the transform trough, creating the thicker crust that extends under the eastern aseismic extension of the transform. Our results show that the northern ridge segment, often identified as magma-starved, displays the crustal thickness and apparent signal-attenuation characteristic of a plentiful, but perhaps episodic, magma supply.

Magde, LS, Detrick RS, Kent GM, Harding AJ, Orcutt JA, Mutter JC, Buhl P.  1995.  Crustal and Upper-Mantle Contribution to the Axial Gravity-Anomaly at the Southern East Pacific Rise. Journal of Geophysical Research-Solid Earth. 100:3747-3766.   10.1029/94jb02869   AbstractWebsite

This paper reassesses the crustal and upper mantle contribution to the axial gravity anomaly and isostatic topography observed at two segments (14 degrees S and 17 degrees S) of the southern East Pacific Rise (SEPR) in order to determine what constraints these data place on the amount of melt present in the underlying mantle. Gravity effects due to seafloor topography and relief on the Moho (assuming a constant crustal thickness and density) overpredict the amplitude of the gravity high at the EPR by 8-10 mGal. About 70% of this mantle Bouguer anomaly (MBA) low (6-7 mGal) can be explained by a region of partial melt and elevated temperatures in the mid-to-lower crust beneath the rise axis. Compositional density reductions in the mantle due to melt extraction are shown to make a negligible contribution to the amplitude of the observed MBA. Temperature-related mantle density variations predicted by a simple, plate-driven, passive flow model with no melt retention can adequately account for the mantle contribution to the observed MRA within the experimental uncertainty (+/- 1 mGal). However, the retention of a small amount of melt (less than or equal to 1-2% at 14 degrees S;less than or equal to 4% at 17 degrees S) in a broad region (tens of kilometers wide) of upwelling mantle is also consistent with the observed gravity data given the uncertainty in crustal thermal models. The anomalous height of the narrow, topographic high at the EPR provides the strongest evidence for the existence of significant melt fractions in the underlying mantle. It is consistent with the presence of a narrow (similar to 10 km wide) partial melt conduit that extends to depths of 50-70 km with melt concentrations up to 2% higher than the surrounding mantle. Along-axis variations in mantle melt fraction that might potentially indicate focused upwelling are only marginally resolvable in the gravity data due to uncertainties,in crustal thermal models. The good correlation between along-axis variations in depth, and changes in axial volume and gravity, argue against the mantle melt conduit as being the major source of this along-axis variation. Instead, this variability can be adequately explained by a combination of along-axis changes in crustal thermal structure and/or alone-axis crustal thickness changes of a few hundred meters.

Mutter, JC, Carbotte SM, Su WS, Xu LQ, Buhl P, Detrick RS, Kent GM, Orcutt JA, Harding AJ.  1995.  Seismic Images of Active Magma Systems Beneath the East Pacific Rise Between 17-Degrees-05' and 17-Degrees-35'S. Science. 268:391-395.   10.1126/science.268.5209.391   AbstractWebsite

Seismic reflection data from the East Pacific Rise between 17 degrees 05' and 17 degrees 35'5 image a magma lens that varies regularly in depth and width as ridge morphology changes, confirming the notion that axial morphology can be used to infer ridge magmatic state. However, at 17 degrees 26'S, where the ridge is locally shallow and broad, the magma lens is markedly shallower and wider than predicted from regional trends. In this area, submersible dives reveal recent volcanic eruptions. These observations indicate that it is where the width and depth of the magma chamber differ from regional trends, indicating an enhanced magmatic budget, that is diagnostic of current magmatism.