Dr. Harm Van Avendonk
Publications
Worthington, L. L., H. J. A. Van Avendonk, S. P. S. Gulick, G. L. Christeson, and T. L. Pavlis, Crustal structure of the Yakutat terrane and the evolution of subduction and collision in southern Alaska, J. Geophys. Res., 117, B01102, 2012, doi:10.1029/2011JB008493, #2430 
We present a two-dimensional velocity model to constrain crustal thickness and composition of the Yakutat terrane in the northern Gulf of Alaska. The model was constructed using seismic reflection and refraction data along a ~455 km onshore-offshore profile. Our model shows that the crystalline crust composing the Yakutat terrane is wedge-shaped, with crustal thickness increasing west to east from ~15 km to ~30 km. Crustal velocity and structure are continuous across the terrane, with lower crustal velocities > 7 km/s, suggesting that the Yakutat terrane is an oceanic plateau across its entire offshore extent rather than a composite oceanic-continental terrane as previously proposed. The thickest Yakutat crust is entering the adjacent St. Elias orogen where elevated exhumation rates and concentrated seismicity in this vicinity are likely influenced by incipient Yakutat-North America collision. Our model includes a ~8 km thick low-velocity crustal cap extending across the eastern portion of the profile where shallow basement is imaged on marine seismic reflection data. We interpret this cap as a lithified, metamorphosed remnant accretionary prism, providing evidence of a previous attempt at Yakutat subduction along its eastern margin prior to current emplacement at the southern Alaska margin.
Van Avendonk, H. J. A., W. S. Holbrook, D. Lizarralde, and P. Denyer, Structure and serpentinization of the subducting Cocos plate offshore Nicaragua and Costa Rica, Geochem., Geophys., Geosyst., 12, Q06009, 2011, doi:10.1029/2011GC003592, #2376
The Cocos plate experiences extensional faulting as it bends into the Middle American Trench (MAT) west of Nicaragua, which may lead to hydration of the subducting mantle. To estimate the along strike variations of volatile input from the Cocos plate into the subduction zone, we gathered marine seismic refraction data with the R/V Marcus Langseth along a 396 km long trench parallel transect offshore of Nicaragua and Costa Rica. Our inversion of crustal and mantle seismic phases shows two notable features in the deep structure of the Cocos plate: (1) Normal oceanic crust of 6 km thickness from the East Pacific Rise (EPR) lies offshore Nicaragua, but offshore central Costa Rica we find oceanic crust from the northern flank of the Cocos Nazca (CN) spreading center with more complex seismic velocity structure and a thickness of 10 km. We attribute the unusual seismic structure offshore Costa Rica to the midplate volcanism in the vicinity of the Galápagos hot spot. (2) A decrease in Cocos plate mantle seismic velocities from â˼7.9 km/s offshore Nicoya Peninsula to â˼6.9 km/s offshore central Nicaragua correlates well with the northward increase in the degree of crustal faulting outboard of the MAT. The negative seismic velocity anomaly reaches a depth of â˼12 km beneath the Moho offshore Nicaragua, which suggests that larger amounts of water are stored deep in the subducting mantle lithosphere than previously thought. If most of the mantle low velocity zone can be interpreted as serpentinization, the amount of water stored in the Cocos plate offshore central Nicaragua may be about 2.5 times larger than offshore Nicoya Peninsula. Hydration of oceanic lithosphere at deep sea trenches may be the most important mechanism for the transfer of aqueous fluids to volcanic arcs and the deeper mantle.
Christeson, G. L., S. P. S. Gulick, H. J. A. Van Avendonk, L. L. Worthington, R. S. Reece, and T. L. Pavlis, The Yakutat terrane: Dramatic change in crustal thickness across the transition fault, Alaska, Geology, 38, 895-898, 2010, 3 citations, doi:10.1130/G31170.1, #2267
We present new constraints on the crustal structure of the Yakutat terrane and evidence of the role of the Transition fault in southern Alaska. The Yakutat terrane south of Yakutat Bay includes crystalline crust that is 24ââ¬â27 km thick overlain by sedimentary units that are 4.5ââ¬â7.5 km thick. The Yakutat terrane crustal thickness and velocity structure are consistent with an oceanic plateau origin. The southern edge of the Yakutat terrane is bounded by the Transition fault, which is imaged as a near-vertical fault zone ~1 km wide. The Transition fault is coincident with a dramatic change in Moho depth from 32 km for Yakutat oceanic plateau crust to 11.5 km for Pacific Ocean crust occurring over a horizontal distance of 0ââ¬â5 km. There is no evidence for underthrusting of the Pacific Ocean crust beneath the Yakutat terrane at the Transition fault. We argue that the Yakutat terrane formed on the Kula or Farallon plate and was later juxtaposed next to younger Pacific Ocean crust by the Transition fault.
Van Avendonk, H. J. A., W. S. Holbrook, D. Lizarralde, M. M. Mora, S. Harder, A. D. Bullock, G. E. Alvarado, and C. J. Ramirez, Seismic evidence for fluids in fault zones on top of the subducting Cocos Plate beneath Costa Rica, Geophys. J. Int., 181, 997-1016, 2010, 2 citations, doi:10.1111/j.1365-246X.2010.04552.x, #2269
In the 2005 TICOCAVA explosion seismology study in Costa Rica, we observed crustal turning waves with a dominant frequency of âÃâ ü10 Hz on a linear array of short-period seismometers from the Pacific Ocean to the Caribbean Sea. On one of the shot records, from Shot 21 in the backarc of the Cordillera Central, we also observed two seismic phases with an unusually high dominant frequency (âÃâ ü20 Hz). These two phases were recorded in the forearc region of central Costa Rica and arrived âÃâ ü7 s apart and 30âââ‰â¬Å40 s after the detonation of Shot 21. We considered the possibility that these secondary arrivals were produced by a local earthquake that may have happened during the active-source seismic experiment. Such high-frequency phases following Shot 21 were not recorded after Shots 22, 23 and 24, all in the backarc of Costa Rica, which might suggest that they were produced by some other source. However, earthquake dislocation models cannot produce seismic waves of such high frequency with significant amplitude. In addition, we would have expected to see more arrivals from such an earthquake on other seismic stations in central Costa Rica. We therefore investigate whether the high-frequency arrivals may be the result of a deep seismic reflection from the subducting Cocos Plate. The timing of these phases is consistent with a shear wave from Shot 21 that was reflected as a compressional (SÃÆââ¬âP) and a shear (SÃÆââ¬âS) wave at the top of the subducting Cocos slab between 35 and 55 km depth. The shift in dominant frequency from âÃâ ü10 Hz in the downgoing seismic wave to âÃâ ü20 Hz in the reflected waves requires a particular seismic structure at the interface between the subducting slab and the forearc mantle to produce a substantial increase in reflection coefficients with frequency. The spectral amplitude characteristics of the SÃÆââ¬âP and SÃÆââ¬âS phases from Shot 21 are consistent with a very high Vp/Vs ratio of 6 in âÃâ ü5 m thick, slab-parallel layers. This result suggests that a system of thin shear zones near the plate interface beneath the forearc is occupied by hydrous fluids under near-lithostatic conditions. The overpressured shear zone probably takes up fluids from the downgoing slab, and it may control the lower limit of the seismogenic zone.
Van Avendonk, H. J. A., L. L. Lavier, D. J. Shillington, and G. Manatschal, Extension of continental crust at the margin of the eastern Grand Banks, Newfoundland, Tectonophysics, 468, 131-148, 2009, 16 citations, doi:10.1016/j.tecto.2008.05.030, #1968
Seismic and gravity observations from the rifted margin of the eastern Grand Banks, Newfoundland, support a new model for extension of the continental crust from the shelf edge to ODP Site 1277, where mantle rocks are exhumed. We find that the largest decrease in crustal thickness, from about 28 km to 6 km, occurs beneath the continental slope of the Grand Banks over a distance of just 20 km. This rapid decrease in crustal thickness coincides with anomalously high seismic velocities (7.0â7.2 km·s− 1) in the lower crust of the shelf edge. The thin crust of the continentâocean transition (COT) in this area has a smooth basement surface, void of upper crustal blocks and prerift sediments. We compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere. Both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust. The geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins: Beneath the continental shelf of the Grand Banks, the Moho and the strong lower crust rotated upwards toward to a 50° dip without visible internal deformation. The presence of these strong lower crustal rocks at shallow depth in the rift flank subsequently helped to localize the extension farther seaward. With ongoing extension, some high-angle normal faults may have rotated to a sub-horizontal orientation, which would explain the lack of brittle deformation visible in the seismic reflection data. The two geodynamic models produce different amounts of extension of continental crust in the distal margins. The hot rifting model localizes strain much more rapidly, leaving narrow zones of extended continental crust, and it produces a relatively large amount of melt (> 30%) in the final stages of rifting. Continental breakup may occur rapidly in hot lithosphere (< 5 Myr). On the other hand, a cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin, similar to what we inferred at the eastern Grand Banks. The cold lithospheric model requires about 23 Myr of extension before continental breakup, and it predicts much less melting in the mantle (13%). The long rift duration, wide zones of thinned continental crust, and small amount of magmatism make the cold rifting model the most applicable to NewfoundlandâIberia rift.
Hornbach, M. J., D. M. Saffer, W. S. Holbrook, H. J. A. Van Avendonk, and A. R. Gorman, Three-dimensional seismic imaging of the Blake Ridge methane hydrate province: Evidence for large, concentrated zones of gas hydrate and morphologically driven advection, J. Geophys. Res., 113, B07101, 2008, 9 citations, doi:10.1029/2007JB005392, #1994
Current estimates for the amount of methane trapped below gas hydrate provinces remain highly speculative, and explanations for how this methane is injected into the atmosphere are wide-ranging and unverified. Blake Ridge, one of the largest passive margin gas hydrate provinces on Earth, is traditionally characterized as an expansive yet dilute reservoir of methane hydrate with no significant fluid advection. Previous 2-D seismic analysis and Ocean Drilling Program Leg 164 drilling results show evidence for both concentrated zones of hydrate and possible fluid flow; however, the extent of these phenomena remains ambiguous. Here we analyze high-resolution 3-D seismic data collected at Blake Ridge in 2000 and map seismic indicators of concentrated hydrate and fluid flow. We also use the seismic data to map the base of the gas hydrate stability in 3-D. Our analysis demonstrates that the gas hydrate phase boundary varies significantly in areas of high sedimentation and erosion, suggesting a dynamic hydrate system. Furthermore, evidence of localized bottom-simulating reflector shoaling, particularly at a sediment wave bounding surface, indicates ongoing advection. The analysis reveals that the Blake Ridge gas hydrate system is significantly more dynamic than previous studies suggest, and we hypothesize that fluctuating sedimentation and erosion patterns cause hydrate phase-boundary instability that triggers fluid flow.
Davey, F. J., D. Eberhart-Phillips, M. D. Kohler, S. Bannister, G. Caldwell, S. Henrys, M. Scherwath, T. Stern, and H. J. A. Van Avendonk, 3-D structure of the Southern Alps orogen, South Island, New Zealand, in A Continental Plate Boundary: Tectonics at South Island, New Zealand, edited by D. Okaya, T. Stern, and F. Davey, Washington, D. C., AGU Geophysical Monograph, 175309-330, 2007, #1880
Fuis, G. S., M. D. Kohler, M. Scherwath, U. Ten Brink, H. J. A. Van Avendonk, and J. M. Murphy, A comparison between the transpressional plate boundaries of the South Island, New Zealand and Southern California, USA, in A Continental Plate Boundary: Tectonics at South Island, New Zealand, edited by D. Okaya, T. Stern, and F. Davey, Washington, D. C., AGU Geophysical Monograph, 175, 47-74, 2007, #1881
Hu, C. S., K. D. McIntosh, H. J. A. Van Avendonk, and P. L. Stoffa, Hybrid ray tracer and amplitude calculation with finite difference, graph theory and ray bending , SEG Ann. Meeting, New Orleans, LA, 3408-3412, 2006, #1957
Shillington, D. J., W. S. Holbrook, H. J. A. Van Avendonk, B. E. Tucholke, J. R. Hopper, K. E. Louden, H. C. Larsen, and G. T. Nunes, Evidence for asymmetric nonvolcanic rifting and slow incipient oceanic accretion from seismic reflection data on the Newfoundland Margin, J. Geophys. Res., 111, B09402, 2006, 28 citations, doi:10.1029/2005JB003981, #1834
Prestack depth migrations of seismic reflection data collected around the Ocean Drilling Program (ODP) Leg 210 transect on the Newfoundland nonvolcanic margin delineate three domains: (1) extended continental crust, (2) transitional basement, and (3) apparent slow spreading oceanic basement beyond anomaly M3 and indicate first-order differences between this margin and its well-studied conjugate, the Iberia margin. Extended continental crust thins abruptly with few observed faults, in stark contrast with the system of seaward dipping normal faults and detachments imaged within continental crust off Iberia. Transition zone basement typically appears featureless in seismic reflection profiles, but where its character can be discerned, it does not resemble most images of exhumed peridotite off Iberia. Seismic observations allow three explanations for transitional basement: (1) slow spreading oceanic basement produced by unstable early seafloor spreading, (2) exhumed, serpentinized mantle with different properties from that off Iberia, and (3) thinned continental crust, likely emplaced by one or more detachment or rolling-hinge faults. Although we cannot definitively discriminate between these possibilities, seismic reflection profiles together with coincident wide-angle seismic refraction data tentatively suggest that the majority of transitional basement is thinned continental crust emplaced during the late stages of rifting. Finally, seismic profiles image abundant faults and significant basement topography in apparent oceanic basement. These observations, together with magnetic anomaly interpretations and the recovery of mantle peridotites at ODP Site 1277, appear to be best explained by the interplay of extension and magmatism during the transition from nonvolcanic rifting to a slow spreading oceanic accretion system.
Van Avendonk, H. J. A., W. S. Holbrook, G. T. Nunes, D. J. Shillington, B. E. Tucholke, K. E. Louden, H. C. Larsen, and J. R. Hopper, Seismic velocity structure of the rifted margin of the eastern Grand Banks of Newfoundland, Canada, J. Geophys. Res., 111, B11404, 2006, 26 citations, doi:10.1029/2005JB004156, #1878
We present a compressional seismic velocity profile of the crust of the eastern margin of the Grand Banks of Newfoundland, Canada. This velocity model was obtained by a tomographic inversion of wide-angle data recorded on a linear array of 24 ocean bottom seismometers (OBSs). At the landward side, we imaged a crustal thickness of 27 km in Flemish Pass and beneath Beothuk Knoll, which is thinner than the 35-km-thick crust of the central Grand Banks. We therefore assume that the eastern rim of the Grand Banks stretched uniformly by 25%. Farther seaward, the continental crust tapers rapidly beneath the continental slope to ∼6 km thickness. In the distal margin we find a 60-km-wide zone with seismic velocities between 5.0 and 6.5 km s−1 that thins to the southeast from 6 to 2 km, which we interpret as highly extended continental crust. Contrary to other seismic studies of the margins of the Grand Banks, we find seismic velocities of 8 km s−1 and higher beneath this thin crustal layer in the continent-ocean transition. We conclude that mantle was locally emplaced at shallow levels without significant hydration from seawater or serpentinized mantle was removed along a décollement in the final stages of continental rifting. The outer edge of highly extended continental crust borders a 25-km-wide zone where seismic velocities increase gradually from 6.3 km s−1 just below the top of acoustic basement to 7.7 km s−1 at 5 km below basement. We interpret this area as a relatively narrow zone of exhumed and serpentinized continental mantle. Seaward, we imaged a thin and laterally heterogeneous layer with a seismic velocity that increases sharply from 5.0 km s−1 in basement ridges to 7.0 km s−1 at its base, overlying mantle velocities between 7.8 and 8.2 km s−1. We interpret this area as unroofed mantle and very thin oceanic crust that formed at an incipient, magma-starved, ultraslow spreading ridge. A comparison of the conjugate rifted margins of the eastern Grand Banks and the Iberia Abyssal Plain show that they exhibit a similar seaward progression from continental crust to mantle to oceanic crust. This indicates that before continental breakup, rifting exhumed progressively deeper sections of the continental lithosphere on both conjugate margins. A comparison between the continent-ocean transition of the Grand Banks and Flemish Cap shows that the final phase of continental rifting and the formation of the first oceanic crust required more time at the Grand Banks margin than at the southeastern margin of Flemish Cap.
Shillington, D. J., H. J. A. Van Avendonk, W. S. Holbrook, P. B. Kelemen, and M. J. Hornbach, Composition and structure of the central Aleutian island arc from arc-parallel wide-angle seismic data, Geochem., Geophys., Geosyst., 5, Q10006, 2004, 30 citations, doi:10.1029/2004GC000715, #2054
New results from wide-angle seismic data collected parallel to the central Aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry. The 3-D ray tracing/2.5-D inversion of this sparse wide-angle data set, which incorporates independent phase interpretations and new constraints on shallow velocity structure, produces a faster and smoother result than a previously published velocity model. Middle-crustal velocities of 6.5-7.3 km/s over depths of 20 km indicate an andesitic to basaltic composition. High lower-crustal velocities of 7.3-7.7 km/s over depths of 35 km are interpreted as ultramafic-mafic cumulates and/or garnet granulites. The total crustal thickness is 35-37 km. This result indicates that the Aleutian island arc has higher velocities, and thus more mafic compositions, than average continental crust, implying that significant modifications would be required for this arc to be a suitable building block for continental crust. Lateral variations in average crustal velocity (below 10 km) roughly correspond to trends in major element geochemistry of primitive (Mg > 0.6) lavas. The highest lower-crustal velocities (and presumably most mafic material) are detected in the center of an arc segment, between Unmak and Unalaska Islands, implying that arc segmentation exerts control over crustal composition.
Van Avendonk, H. J. A., Slowness-weighted diffraction stack for migrating wide-angle seismic data in laterally varying media, Geophysics, 69, 1046-1052, 2004, doi:10.1190/1.1778247, #1696
Wide-angle prestack depth migration is an important tool for studying the nature of reflecting boundaries in the earth's crust. The slowness-weighted diffraction stack (SWDS) method has been used to incorporate both two-way traveltime constraints and slowness information in the migration. For this purpose, traveltimes and apparent slownesses of reflected arrivals must be calculated in the image space. Earlier applications of SWDS required a 1D or gently varying seismic velocity structure to obtain these quantities by ray tracing in the image space. I show that the apparent slownesses can also be derived directly from one-way traveltime maps using Fermat's principle. The SDWS is applied to an existing onshoreâoffshore wide-angle data set, and the example shows that the method can be used to image detailed reflectivity structure at great depths
Van Avendonk, H. J. A., W. S. Holbrook, D. Okaya, J. K. Austin, F. J. Davey, and T. Stern, Continental crust under compression: A seismic refraction study of South Island geophysical transect I, South Island, New Zealand, J. Geophys. Res., 109, B06302, 2004, 37 citations, doi:10.1029/2003JB002790, #1697
The 1996 South Island Geophysical Transect (SIGHT) active source seismic survey was designed to show the style of lithospheric thickening due to late Cenozoic oblique convergence across the Australian-Pacific plate boundary in New Zealand. As part of this study, two seismic refraction lines were shot across central South Island and offshore extensions of the continental crust in the Tasman Sea and Pacific Ocean. We present the data and a 603 km long seismic velocity profile of the crust and uppermost mantle along one of these seismic transects. A tomographic inversion of 62,563 travel times from crustal and upper mantle refractions and wide-angle reflections resulted in a model with a two-layer crust. Upper crustal velocities were between 5.9 and 6.3 km/s, and lower crustal velocities were between 6.5 and 7.0 km/s. Continental compression has locally reduced the seismic velocities in the Pacific plate crust by 0.2â0.3 km/s, a possible effect of high strain and fluids in the crust. The thickening of the crust from 28 km at the east coast of South Island to 37 km beneath the Southern Alps can account for about 25% of the 80â110 km shortening of Pacific plate crust, while the rest must be accounted for by rapid erosion of Mesozoic sedimentary rocks on the west side of the orogen. In our model the lower crust forms a continuous 2â6 km thick layer beneath central South Island. The asymmetric topography of the Southern Alps is reflected in the crustal root which has a steeper flank at the west coast. This observation is consistent with westward underthrusting of Pacific lower lithosphere beneath South Island that has been suggested in earlier studies.
Van Avendonk, H. J. A., D. J. Shillington, W. S. Holbrook, and M. J. Hornbach, Inferring crustal structure in the Aleutian island arc from a sparse wide-angle seismic data set, Geochem., Geophys., Geosyst., 5, Q08008, 2004, 15 citations, doi:10.1029/2003GC000664, #1698
Compressional seismic travel times from a relatively sparse wide-angle data set hold key information on the structure of a 800 km long section of the central Aleutian arc. Since the source and receiver locations form a swath along the arc crest that is ∼50 km wide, we trace rays in 3-D for a collection of 8336 seismic refraction and reflection arrivals. We investigate variations in seismic velocity structure parallel to the Aleutian arc, assuming that our result represents average crustal structure across the arc. We explore seismic velocity models that consist of three crustal layers that exhibit smooth variations in structure in the 2-D vertical plane. We consider the influence of additional constraints and model parameterization in our search for a plausible model for Aleutian arc crust. A tomographic inversion with static corrections for island stations reduces the data variance of a 1-D starting model by 91%. Our best model has seismic velocities of 6.0-6.5 km/s in the upper crust, 6.5-7.3 km/s in the middle crust, and 7.3-7.7 km/s in the lower crust and a total crustal thickness of 35-37 ± 1 km. A resolution analysis shows that features having a horizontal scale less than 20 km cannot be imaged, but at horizontal length scales of ∼50 km most model features are well resolved. The study indicates that the Aleutian island arc crust is thick compared to other island arcs and strongly stratified and that only the upper 60% of the arc crust has seismic velocities that are comparable to average seismic velocities in continental crust.
Bazin, S., A. J. Harding, G. M. Kent, J. A. Orcutt, S. C. Singh, C. H. Tong, J. W. Pye, P. J. Barton, M. C. Sinha, R. S. White, R. W. Hobbs, and H. J. A. Van Avendonk, A three-dimensional study of a crustal low velocity region beneath the 9°03N overlapping spreading center, Geophys. Res. Lett., 30, 1039, 2003, 4 citations, doi:10.1029/2002GL015137, #2253
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 9°03′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.