Export 8 results:
Sort by: Author Title [ Type  (Asc)] Year
Journal Article
Van Avendonk, HJA, Harding AJ, Orcutt JA, McClain JS.  2001.  Contrast in crustal structure across the Clipperton transform fault from travel time tomography. Journal of Geophysical Research-Solid Earth. 106:10961-10981.   10.1029/2000jb900459   AbstractWebsite

A three-dimensional (3-D) seismic refraction study of the Clipperton transform fault, northern East Pacific Rise, reveals anomalously low compressional velocities from the seafloor to the Moho, We attribute this low-velocity anomaly to intensive brittle deformation, caused by transpression across this active strike-slip plate boundary. The seismic velocity structure south of the Clipperton transform appears unaffected by these tectonic forces, but to the north, seismic velocities are reduced over 10 km outside the zone of sheared seafloor. This contrast in seismic velocity structure corresponds well with the differences in mid-ocean ridge morphology across the Clipperton transform. We conclude that the amount of fracturing of the upper crust, which largely controls seismic velocity variations, is strongly dependent on the shallow temperature structure at the ridge axis. Intermittent supply of magma to the shallow crust north of the Clipperton transform allows seawater to penetrate deeper, and the cooler crust is brittle to a greater depth than south of the transform, where a steady state magma lens is known to exist. The crustal thickness averages 5.7 km, only slightly thinner than normal for oceanic crust, and variations in Moho depth in excess of similar to0.3 km are not required by our data. The absence of large crustal thickness variations and the general similarity in seismic structure imply that a steady state magma lens is not required to form normal East Pacific Rise type crust. Perhaps a significant portion of the lower crust is accreted in situ from a patchwork of short-lived gabbro sills or from ductile flow from a basal magma chamber as has been postulated in some recent ophiolite studies.

Van Avendonk, HJA, Harding AJ, Orcuttt JA, Holbrook WS.  2001.  Hybrid shortest path and ray bending method for traveltime and raypath calculations. Geophysics. 66:648-653.   10.1190/1.1444955   AbstractWebsite

The shortest path method (SPM) is a robust ray-tracing technique that is particularly useful in 3-D tomographic studies because the method is well suited for a strongly heterogeneous seismic velocity structure. We test the accuracy of its traveltime calculations with a seismic velocity structure for which the nearly exact solution is easily found by conventional ray shooting. The errors in the 3-D SPM solution are strongly dependent on the choice of search directions in the "forward star," and these errors appear to accumulate with traveled distance. We investigate whether these traveltime errors can be removed most efficiently by an SPM calculation on a finer grid or by additional ray bending. Testing the hybrid scheme on a realistic ray-tracing example, we find that in an efficient mix ray banding and SPM account for roughly equal amounts of computation time. The hybrid method proves to be an order of magnitude more efficient than SPM without ray bending in our example. We advocate the hybrid ray-tracing technique, which offers an efficient approach to find raypaths and traveltimes for large seismic refraction studies with high accuracy.

Van Ark, EM, Detrick RS, Canales JP, Carbotte SM, Harding AJ, Kent GM, Nedimovic MR, Wilcock WSD, Diebold JB, Babcock JM.  2007.  Seismic structure of the Endeavour Segment, Juan de Fuca Ridge: Correlations with seismicity and hydrothermal activity. Journal of Geophysical Research-Solid Earth. 112   10.1029/2005jb004210   AbstractWebsite

[ 1] Multichannel seismic reflection data collected in July 2002 at the Endeavour Segment, Juan de Fuca Ridge, show a midcrustal reflector underlying all of the known high-temperature hydrothermal vent fields in this area. On the basis of the character and geometry of this reflection, its similarity to events at other spreading centers, and its polarity, we identify this as a reflection from one or more crustal magma bodies rather than from a hydrothermal cracking front interface. The Endeavour magma chamber reflector is found under the central, topographically shallow section of the segment at two-way traveltime (TWTT) values of 0.9 - 1.4 s ( similar to 2.1 - 3.3 km) below the seafloor. It extends approximately 24 km along axis and is shallowest beneath the center of the segment and deepens toward the segment ends. On cross-axis lines the axial magma chamber (AMC) reflector is only 0.4 - 1.2 km wide and appears to dip 8 - 36 degrees to the east. While a magma chamber underlies all known Endeavour high-temperature hydrothermal vent fields, AMC depth is not a dominant factor in determining vent fluid properties. The stacked and migrated seismic lines also show a strong layer 2a event at TWTT values of 0.30 +/- 0.09 s ( 380 +/- 120 m) below the seafloor on the along-axis line and 0.38 +/- 0.09 s ( 500 +/- 110 m) on the cross-axis lines. A weak Moho reflection is observed in a few locations at TWTT values of 1.9 - 2.4 s below the seafloor. By projecting hypocenters of well-located microseismicity in this region onto the seismic sections, we find that most axial earthquakes are concentrated just above the magma chamber and distributed diffusely within this zone, indicating thermal-related cracking. The presence of a partially molten crustal magma chamber argues against prior hypotheses that hydrothermal heat extraction at this intermediate spreading ridge is primarily driven by propagation of a cracking front down into a frozen magma chamber and indicates that magmatic heat plays a significant role in the hydrothermal system. Morphological and hydrothermal differences between the intermediate spreading Endeavour and fast spreading ridges are attributable to the greater depth of the Endeavour AMC and the corresponding possibility of axial faulting.

Vera, EE, Mutter JC, Buhl P, Orcutt JA, Harding AJ, Kappus ME, Detrick RS, Brocher TM.  1990.  The Structure of 0.2-MY Old Oceanic Crust at 9-Degrees-N on the East Pacific Rise from Expanded Spread Profiles. Journal of Geophysical Research-Solid Earth and Planets. 95:15529-15556.   10.1029/JB095iB10p15529   AbstractWebsite

We analyze four expanded spread profiles acquired at distances of 0, 2.1, 3.1, and 10 km (0–0.2 m.y.) from the axis of the East Pacific Rise between 9° and 10°N. Velocity-depth models for these profiles have been obtained by travel time inversion in the τ-p domain, and by x-t forward modeling using the WKBJ and the reflectivity methods. We observe refracted arrivals that allow us to determine directly the uppermost crustal velocity structure (layer 2A). At the seafloor we find very low Vp and VS/Vp values around 2.2 km/s and ≤ 0.43. In the topmost 100–200 m of the crust, Vp remains low (≤ 2.5 km/s) then rapidly increases to 5 km/s at ∼500 m below the seafloor. High attenuation values (Qp < 100) are suggested in the topmost ∼500 m of the crust. The layer 2–3 transition probably occurs within the dike unit, a few hundred meters above the dike-gabbro transition. This transition may mark the maximum depth of penetration by a cracking front and associated hydrothermal circulation in the axial region above the axial magma chamber (AMC). The on-axis profile shows arrivals that correspond to the bright AMC event seen in reflection lines within 2 km of the rise axis. The top of the AMC lies 1.6 km below the seafloor and consists of molten material where Vp ≈ 3 km/s and VS = 0. Immediately above the AMC, there is a zone of large negative velocity gradients where, on the average, Vp decreases from ∼6.3 to 3 km/s over a depth of approximately 250 m. Associated with the AMC there is a low velocity zone (LVZ) that extends to a distance no greater than 10 km away from the rise axis. At the top of the LVZ, sharp velocity contrasts are confined to within 2 km of the rise axis and are associated with molten material or material with a high percentage of melt which would be concentrated only in a thin zone at the apex of the LVZ, in the axial region where the AMC event is seen in reflection lines. Away from the axis, the transition to the LVZ is smoother, the top of the LVZ is deeper, and the LVZ is less pronounced. The bottom of the LVZ is probably located near the bottom of the crust and above the Moho. Moho arrivals are observed in the profiles at zero and at 10 km from the rise axis. Rather than a single discontinuity, these arrivals indicate an approximately 1-km-thick Moho transition zone.

Harding, AJ, Orcutt JA, Kappus ME, Vera EE, Mutter JC, Buhl P, Detrick RS, Brocher TM.  1989.  Structure of Young Oceanic-Crust at 13-Degrees-N on the East Pacific Rise from Expanding Spread Profiles. Journal of Geophysical Research-Solid Earth and Planets. 94:12163-12196.   10.1029/JB094iB09p12163   AbstractWebsite

We present the results of the analysis of expanding spread profiles (ESPs) collected on and near the axis of the East Pacific Rise at 13°N. These profiles were collected at 0, 1.1, 2.1, 3.6, and 9.5 km from the rise axis, and all but the most distant profile show a distinct low-velocity zone (LVZ) located within layer 3 of the oceanic crust. At the ridge crest, the top of the magma chamber is at the base of layer 2, while 3.6 km off axis, the roof of the LVZ is 1.1 km below the top of layer 3. The profile farthest from the ridge could possibly have a residual LVZ confined to the lower 1.5 km of the crust. The total width of the LVZ, as determined from the ESP data, is at least 6 km, and possibly much greater. This wide LVZ apparently contradicts multichannel seismic data which show cross-axis reflections from the magma chamber with a width of<5 km. We suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt. The ESP data are sensitive to this larger region, while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast. We identify the presence of a layer at the top of the oceanic crust with initial P wave velocities between 2.35 and 2.6 km/s, while the S wave velocity is estimated as being ≤0.8 km/s. The layer thickness lies between 100 and 200 m. These velocities are consistent with previous estimates for the Pacific and recent results for the Atlantic. The thickness of this layer is consistent with that of layer 2A determined from geophysical measurements at Deep Sea Drilling Project hole 504B.

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.

Bazin, S, Harding AJ, Kent GM, Orcutt JA, Singh SC, Tong CH, Pye JW, Barton PJ, Sinha MC, White RS, Hobbs RW, Van Avendonk HJA.  2003.  A three-dimensional study of a crustal low velocity region beneath the 9 degrees 03 ' N overlapping spreading center. Geophysical Research Letters. 30   10.1029/2002gl015137   AbstractWebsite

[1] Overlapping spreading centers (OSCs) play a key role in models of magma distribution at fast spreading ridges. To investigate the relationship between ridge-axis discontinuities and magma supply, we conducted a three-dimensional seismic reflection and tomography experiment at the 9degrees03'N OSC along the East Pacific Rise. Tomographic analysis imaged a broad mid-crustal low velocity zone (LVZ) beneath parts of the overlapper and the associated overlap basin, demonstrating that it is magmatically robust. The complementary datasets reveal a complex storage and tapping of melt: the LVZ and melt sill at either end of the overlap basin are not simply centered beneath the rise crest but are skewed inwards. The subsequent focussing of the LVZ and sill beneath the axis of the eastern limb appears to be due to melt migration toward the tip. The OSC western limb is less magmatically robust and may be in the process of dying.

Van Avendonk, HJA, Harding AJ, Orcutt JA, McClain JS.  1998.  A two-dimensional tomographic study of the Clipperton transform fault. Journal of Geophysical Research-Solid Earth. 103:17885-17899.   10.1029/98jb00904   AbstractWebsite

From the marine refraction data recorded on five instruments during the Clipperton Area Seismic Survey to investigate Compensation (CLASSIC) experiment in 1994 we construct a compressional velocity model for a 108 km long profile across the Clipperton transform. We apply a new seismic tomography code that alternates between ray tracing and linearized inversions to find a smooth seismic velocity model that fits the observed refraction travel times. The solution to the forward ray-tracing problem is a hybrid of the graph (or shortest path) method and a ray-bending method. The inversion is performed with least squares penalties on the data misfit and first derivatives of the seismic structure. Starting with a one-dimensional compressional velocity model for oceanic crust, the misfit in the normalized travel time residuals is reduced by 96%, decreasing the median travel time residual from 110 to 25 ms. The compressional velocity structure of the Clipperton transform is characterized by anomalously low velocities, about 1.0 km/s lower than average, beneath the median ridge and parallel troughs of the transform domain. The low compressional velocities can be explained by an increased porosity due to fracturing of the oceanic crust. We found crustal thicknesses of 5.6-5.9 km under the transform fault to produce the best fit of the PmP phase arrivals and Pg/Pn crossovers. Since the crust is not thin beneath the transform parallel troughs and the velocity anomaly is not confined to the median ridge, we find uplift by serpentinite diapirs unlikely as an explanation for the relief of the median ridge. A median ridge that is the result of brittle deformation due to compression across the transform domain is, however, compatible with our results. The upper crust is thicker to the north of the transform than to the south, which is likely a consequence of the contrast in temperature structure of these two spreading segments.