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Wilson, DS, Hallenborg E, Harding AJ, Kent GM, Ocean Drilling Program, Leg 206 SSP.  2003.  Site Survey Results from Cruise EW9903. Proceedings of the Ocean Drilling Program, Initial Reports.   10.2973/odp.proc.ir.206.104.2003   AbstractWebsite

Cruise EW9903 of the Maurice Ewing sailed from Panama to San Diego in March and April 1999, with goals of determining the structure of oceanic crust formed at superfast spreading rates and conducting site survey work in support of Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES) proposal 522. Three survey grids were collected in the Guatemala Basin at sites formed at spreading rates >200 mm/yr (Wilson, 1996), and one grid was collected in the Alijos Rocks area west of Baja California (Figs. F1, F2). This report presents basic data bathymetry, magnetics, and gravity data for all of the grids and migrated multichannel seismic reflection (MCS) sections for the Guatemala Basin sites. Seismic refraction results will be reported separately (Harding et al., unpubl. data).

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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.

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.

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 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 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.

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Umhoefer, PJ, Maloney SJ, Buchanan B, Arrowsmith JR, Martinez-Gutierrez G, Kent G, Driscoll N, Harding A, Kaufman D, Rittenour T.  2014.  Late Quaternary faulting history of the Carrizal and related faults, La Paz region, Baja California Sur, Mexico. Geosphere. 10:476-504.   10.1130/ges00924.1   AbstractWebsite

The southwest margin of the Gulf of California has an array of active normal faults despite this being an oblique-divergent plate boundary with spreading centers that localized deformation along the plate boundary 2-3 million years ago. The Carrizal and Centenario faults form the western border fault of the Gulf of California marginal fault system within and south of La Paz Bay, and similar to 20-30 km west of the capital city of La Paz, Baja California Sur, Mexico. Geologic and geomorphic mapping, optically stimulated luminescence (OSL) geochronology, and paleoseismic investigations onshore, compressed high-intensity radar pulse (CHIRP) profiling offshore, and analysis of uplifted marine terraces in the footwall of the offshore Carrizal fault provide some of the first numerical and geometrical constraints on late Pleistocene-Holocene faulting along the Carrizal fault. The onshore Carrizal fault has ruptured with up to similar to 1-2 m of vertical displacement per event, likely producing similar to M 6.3-6.9 earthquakes, and at least two to three surface rupturing earthquakes have occurred since 22 ka. Onshore paleoseismic excavations and uplifted marine terraces on the western side of La Paz Bay both suggest offset rates of 0.1-0.2 mm/yr, with a footwall uplift rate of 0.13 mm/yr since 128 ka, and an approximately constant rate since marine oxygen-isotope stage (MIS) 11 terraces (420 ka). A CHIRP survey identified underwater fault scarps with heights ranging from 21 to 86 m on the Carrizal fault in La Paz Bay and from 3 to 5 m along the Centenario fault. The offshore Carrizal fault lies 8-10 km east of the western edge of La Paz Bay, forming a right step from the onshore Carrizal fault. The offshore Carrizal fault is the oldest fault of the fault system, and the fault likely grew in the latest Miocene to Pliocene in a complex way to the south toward the onshore Centenario and Carrizal faults. When the Alarcon spreading center started its modern rates at 2.4 Ma, the Carrizal fault likely slowed to the 0.1-0.2 mm/yr rates of the late Quaternary determined in this study.

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Tong, CH, White RS, Warner MR, Barton PJ, Bazin S, Harding AJ, Hobbs RW, Kent GM, Orcutt JA, Pye JW, Singh SC, Sinha MC.  2004.  Effects of tectonism and magmatism on crack structure in oceanic crust; a seismic anisotropy study. Geology. 32:25-28.   10.1130/g19962.1   AbstractWebsite

We analyzed 25,675 traveltime residuals from a three-dimensional seismic tomographic inversion to investigate crack-induced seismic anisotropy in the upper oceanic crust. The study covered two regions with contrasting levels of magmatic activity on the western limb of the 9 degrees N overlapping spreading center on the East Pacific Rise. The level of anisotropy gradually decreases with depth in the magmatically and hydrothermally active ridge region. In contrast, we observed a highly variable anisotropic structure in the magmatically and hydrothermally less active tip region at the end of the dying ridge segment: a weakly anisotropic layer beneath strongly anisotropic extrusive volcanic rocks is likely to be the result of relatively shallow cracks closed by hydrothermal precipitation. Strongly anisotropic dikes with inferred narrow and water-saturated cracks provide important along-axis pathways for the circulation of hydrothermal fluids beneath the shallow cracks in the less magmatically active regions. Furthermore, a significant clockwise rotation (20 degrees -30 degrees ) of fast directions occurs in both regions with increasing depth. Such a rotation provides evidence that the geometry of the underlying crack structure of the western limb is significantly different from that defined by the bathymetric ridge crest.

Tong, CH, Pye JW, Barton PJ, White RS, Sinha MC, Singh SC, Hobbs RW, Bazin S, Harding AJ, Kent GM, Orcutt JA.  2002.  Asymmetric melt sills and upper crustal construction beneath overlapping ridge segments: Implications for the development of melt sills and ridge crests. Geology. 30:83-86.   10.1130/0091-7613(2002)030<0083:amsauc>2.0.co;2   AbstractWebsite

A new three-dimensional tomographic velocity model and depth-converted reflection images of the melt sills beneath the 9degrees03'N overlapping spreading center on the East Pacific Rise show that the upper crustal construction at this ridge discontinuity is highly asymmetric with reference to the bathymetric ridge crests of the overlapping limbs. Despite the similarly curved ridge crests, the asymmetries are markedly different under the two limbs and appear to be related to the contrasting evolutionary history of the limbs. The overlap basin is closely related to the propagating eastern limb in terms of its seismic structure. By contrast, the western limb forms a distinct morphologic region that displays little structural relationship to the adjacent overlap basin and other relict basins. As the overlapping spreading center is migrating southward, the differential development of melt sills and ridge crests may be inferred from the results of this study. Ridge propagation appears to involve two major processes: the advancement of the melt sill at the ridge tip and the development of ridge-crest morphology and the neovolcanic axis to the north of the overlap basin region near the existing propagating limb. The latter process may result in the abandonment of the current neovolcanic axis, leading to the self-decapitation of the propagating limb. By contrast, the self-decapitation of the western limb is related to the receding melt sill, which lags behind the anticlockwise rotational motion of the ridge crest.

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.

Tong, CH, Barton PJ, White RS, Sinha MC, Singh SC, Pye JW, Hobbs RW, Bazin S, Harding AJ, Kent GM, Orcutt JA.  2003.  Influence of enhanced melt supply on upper crustal structure at a mid-ocean ridge discontinuity: A three-dimensional seismic tomographic study of 9 degrees N East Pacific Rise. Journal of Geophysical Research-Solid Earth. 108   10.1029/2002jb002163   AbstractWebsite

[1] We present a three-dimensional upper crustal model of the 9degrees03'N overlapping spreading center (OSC) on the East Pacific Rise that assists in understanding the relationship between melt sills and upper crustal structure at a ridge discontinuity with enhanced melt supply at crustal levels. Our P wave velocity model obtained from tomographic inversion of similar to 70,000 crustal first arrival travel times suggests that the geometry of extrusive emplacement are significantly different beneath the overlapping spreading limbs. Extrusive volcanic rocks above the western melt sill are inferred to be thin ( similar to 250 m). More extensive accumulation of extrusives is inferred to the west than to the east of the western melt sill. The extrusive layer inferred above the eastern melt sill thickens from similar to 350 ( at the neovolcanic axis) to 550 m ( to the west of the melt sill). Volcanic construction is likely to be significant in the formation of ridge crest morphology at the OSC, particularly at the tip of the eastern limb. On the basis of our interpretation of the velocity model, we propose that enhanced magma supply at crustal levels at the OSC may provide an effective mechanism for the migration of ridge discontinuities. This "dynamic magma supply model'' may explain the commonly observed nonsteady migration pattern of ridge discontinuities by attributing this to the temporal fluctuations in melt availability to the overlapping spreading limbs.

Tolstoy, M, Harding AJ, Orcutt JA.  1993.  Crustal Thickness on the Mid-Atlantic Ridge - Bulls-Eye Gravity-Anomalies and Focused Accretion. Science. 262:726-729.   10.1126/science.262.5134.726   AbstractWebsite

Spreading segments of the Mid-Atlantic Ridge show negative bull's-eye anomalies in the mantle Bouguer gravity field. Seismic refraction results from 33-degrees-S indicate that these anomalies can be accounted for by variations in crustal thickness along a segment. The crust is thicker in the center and thinner at the end of the spreading segment, and these changes are attributable to variations in the thickness of layer 3. The results show that accretion is focused at a slow-spreading ridge, that axial valley depth reflects the thickness of the underlying crust, and that along-axis density variations should be considered in the interpretation of gravity data.

Tolstoy, M, Harding AJ, Orcutt JA, Detrick RS, Kent GM, Mutter JC, Buhl P.  1997.  Deepening of the axial magma chamber on the southern East Pacific Rise toward the Garrett Fracture Zone. Journal of Geophysical Research-Solid Earth. 102:3097-3108.   10.1029/96jb03226   AbstractWebsite

A wide-aperture profile along the ridge axis from 14 degrees 29'S to 13 degrees 39'S, 120 km to 30 km south of the Garrett Fracture Zone, is analyzed to constrain the thickness of layer 2a and the depth to the axial magma chamber reflector. Five areas along the 90 km line are examined in detail, with several consecutive gathers being analyzed for each area to establish the degree of consistency within each area. A genetic algorithm code is used to find a best fit model from a comparison of the data and WKBJ synthetic seismograms. One hundred starting models are generated using a predefined set of velocity nodes, with a fixed window of allowable depth variations between nodes. An evolutionary process favors the better fitting models in each generation, and a satisfactory misfit is usually obtained within 40 generations. Within individual areas the models were in good agreement with the depth of a given velocity node, generally varying by not more than 20 m, the depth discretization interval for the models. A consistent deepening trend of the axial magma chamber (AMC) is observed across the five areas as the Garrett Fracture Zone is approached. The depth varies from 0.99 km at area 1, which is approximately 100 km south of the Garrett, to 1.23 km at area 5, which is approximately 40 km south of the Garrett. The depth to the axial magma chamber is highly sensitive to any ship wander off axis since layer 2a thickens rapidly off axis with age. For the areas examined here, layer 2a is observed to be relatively constant in thickness along the axis, although it is about 40 m thicker over area 5, where the axial magma chamber is deepest. This variation is within the scatter of previously detailed layer 2a measurements at 13 degrees N on the East Pacific Rise, where an effectively constant thickness is observed. This implies that layer 2a thickening is not a significant factor along this profile and that the AMC deepening is rear rather than apparent. Theoretical modeling suggests that the depth to the lid of the axial magma chamber is related to the rate of heat supply at a given location. Thus the gradual consistent deepening of the axial magma chamber can be taken as an indication of a slightly reduced magma supply toward the Garrett Fracture Zone, which marks a major interruption of hundreds of kilometers of continuous ridge axis. The deepening may also be interpreted as: a downward limb from a central injection point; however, there is no indication of a similar downward trend in the other (southern) direction. Furthermore, there is no accompanying systematic variation in axial depth or axial volume, both of which are proposed to be indicators of central injection and along-axis flow.

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Stephen, RA, Harding AJ.  1983.  Travel Time Analysis of Borehole Seismic Data. Journal of Geophysical Research. 88:8289-8298.   10.1029/JB088iB10p08289   AbstractWebsite

A method is presented for reducing travel time data from multiple offset borehole seismic experiments to velocity-depth structure. The technique, which treats simultaneously data from any number of depths in the borehole in addition to surface data, is based on the linear inversion scheme of Dorman and Jacobson (1981). Given the parameters ζ(p) = T(p) + pX(p) and τ(p) = T(p) − pX(p), the method solves for depth to specified slownesses, assuming linear gradients (in velocity) between slowness values. A practical limitation on the use of surface travel time measurements to resolve velocity-depth structure is the necessity of assuming a surface velocity. This is not necessary for the case of borehole data. For a borehole receiver the velocity at the depth of the receiver can be obtained from the slope of the inflection point of the travel time curve. Thus a direct measure of the uppermost velocity of a section can be obtained from the travel time data of a shallow borehole receiver. Estimation of velocities from the inflection points of deeper receivers improves the resolution of the velocity-depth function, which would be obtained from surface data alone. The technique is applied to data from three borehole seismic experiments in oceanic crust. The experiments were carried out in the western Atlantic (crustal age ∼110 m.y.), the Gulf of California (crustal age ∼1 m.y.) and the Costa Rica Rift Area (crustal age ∼6 m.y.). All three experiments show relatively high upper crustal velocities ( >4.0 km/s), suggesting that layer 2A is not present even in the very young crust. All sites had over 100 m of sediment thickness, and it is postulated that sediment thickness and sediment permeability, not merely age, govern the velocity of the upper crust by accelerating the cementation of fractures and cracks.

Singh, SC, Sinha MC, Harding AJ, Kent GM, Barton PJ, Orcutt JA, White RS, Hobbs RW.  1999.  Preliminary results are in from mid-ocean ridge three-dimensional seismic reflection survey. Eos, Transactions, American Geophysical Union. 80   10.1029/99eo00129   Abstract

The first three-dimensional (3-D) seismic reflection survey of a mid-ocean ridge was shot in 1997 and, while it is still too early for firm interpretations of the data, it can be confirmed that significant crustal melt bodies have been located and one widely accepted model does not seem to apply to the presence of a robust magma supply there. The survey the Anatomy of a Ridge-Axis Discontinuity (ARAD) experiment, was centered over an overlapping spreading center (OSC) system that offsets the ridge axis at 9 degrees 03' north latitude on the East Pacific Rise (EPR) (Figure 1). It was conducted aboard the R/V Maurice Ewing during September and October and included a coincident 3-D crustal seismic tomography experiment.

Singh, SC, Harding AJ, Kent GM, Sinha MC, Combier V, Bazin S, Tong CH, Pye JW, Barton PJ, Hobbs RW, White RS, Orcutt JA.  2006.  Seismic reflection images of the Moho underlying melt sills at the East Pacific Rise. Nature. 442:287-290.   10.1038/nature04939   AbstractWebsite

The determination of melt distribution in the crust and the nature of the crust - mantle boundary ( the 'Moho') is fundamental to the understanding of crustal accretion processes at oceanic spreading centres. Upper-crustal magma chambers have been imaged beneath fast- and intermediate-spreading centres(1-4) but it has been difficult to image structures beneath these magma sills. Using three-dimensional seismic reflection images, here we report the presence of Moho reflections beneath a crustal magma chamber at the 9 degrees 03' N overlapping spreading centre, East Pacific Rise. Our observations highlight the formation of the Moho at zero-aged crust. Over a distance of less than 7 km along the ridge crest, a rapid increase in two-way travel time of seismic waves between the magma chamber and Moho reflections is observed, which we suggest is due to a melt anomaly in the lower crust. The amplitude versus offset variation of reflections from the magma chamber shows a coincident region of higher melt fraction overlying this anomalous region, supporting the conclusion of additional melt at depth.

Singh, SC, Collier JS, Harding AJ, Kent GM, Orcutt JA.  1999.  Seismic evidence for a hydrothermal layer above the solid roof of the axial magma chamber at the southern East Pacific Rise. Geology. 27:219-222.   10.1130/0091-7613(1999)027<0219:sefahl>2.3.co;2   AbstractWebsite

A full-waveform inversion of two-ship, wide-aperture, seismic reflection data from a ridge-crest seismic line at the southern East Pacific Rise indicates that the axial magma chamber here is about 50 m thick, is embedded within a solid roof, and has a solid floor. The 50-60-m-thick roof is overlain by a 150-200-m-thick low-velocity zone that may correspond to a fracture zone that hosts the hydrothermal circulation, and the roof itself may be the transition zone separating the magma chamber from circulating fluids. Furthermore, enhanced hydrothermal activity at the sea floor seems to be associated with a fresh supply of magma in the crust from the mantle. The presence of the solid floor indicates that at least the upper gabbros of the oceanic lower crust are formed by cooling and crystallization of melt in magma chambers.

Singh, SC, Kent GM, Collier JS, Harding AJ, Orcutt JA.  1998.  Melt to mush variations in crustal magma properties along the ridge crest at the southern East Pacific Rise. Nature. 394:874-878.   10.1038/29740   AbstractWebsite

The determination of along-axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle, eruption history along the ridge crest, and the process of crustal accretion. Seismic reflection images(1-4) have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths (250-4,500 m), but variations in its seismic properties and thickness have remained elusive, despite several attempts to constrain these properties(5-7). Here we report that the melt sill along the southern East Pacific Rise, which is about 50 m thick, undergoes abrupt changes in its internal properties, ranging from pure melt to mush. The 60-km-long ridge-crest segment near 14 degrees 00' S is characterized by three 2-4-km sections containing pure melt embedded within a magma chamber rich in mush. These small pure melt pockets may represent a fresh supply of magma from the mantle, capable of erupting and forming the upper crust. Conversely, the 80-90% of the magma chamber which is mushy is unlikely to erupt and may influence the lower-crustal accretion.

Sahakian, V, Kell A, Harding A, Driscoll N, Kent G.  2016.  Geophysical evidence for a San Andreas subparallel transtensional fault along the northeastern shore of the Salton Sea. Bulletin of the Seismological Society of America. 106:1963-1978.   10.1785/0120150350   AbstractWebsite

The southern San Andreas fault (SSAF) accommodates a significant amount of strain between the Pacific and North American plates; thus, the fault represents a major geohazard to the populated areas of southern California, in particular the larger Los Angeles metropolitan area. Paleoseismic chronology of ruptures along the SSAF segment suggests this fault is near the end of its interseismic period (similar to 180 years), because it has not ruptured in historic times (similar to 320 years). A recent active-source seismic experiment performed in the Salton Sea west of the SSAF provides evidence for extensional deformation along the northeastern shore of the Salton Sea. This study posits that the extensional deformation is due to a previously unmapped fault, here named the Salton trough fault (STF). The seismic reflection data image a divergent sediment package that dips toward the northeast with thicknesses up to at least 2 km. Refraction inversion produces a southwestward-dipping velocity discontinuity that crops out east of the SSAF surface trace, consistent with the existence of a southwest to northeast gradient in lithology. If present, the existence of the STF has scientific and societal relevance. First, the STF appears to control the recent Salton trough architecture north of Bombay Beach. Second, from a seismological hazards perspective, the presence of this structure could alter the current understanding of stress transfer and rupture dynamics in the region, as well as community fault models and ground-motion simulations on the SSAF.

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.

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Phipps Morgan, J, Harding AJ, Kent GM, Orcutt JA, Chen YJ.  1994.  An observational and theoretical synthesis of magma chamber geometry and crustal genesis along a mid-ocean ridge spreading center. Magmatic Systems. ( Ryan M, Holton J, Dmowska R, Eds.).:139-178., Burlington: Elsevier Abstract
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Paramo, P, Holbrook WS, Brown HE, Lizarralde D, Fletcher J, Umhoefer P, Kent G, Harding A, Gonzalez A, Axen G.  2008.  Seismic structure of the southern Gulf of California from Los Cabos block to the East Pacific Rise. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005113   AbstractWebsite

Multichannel reflection and coincident wide-angle seismic data collected during the 2002 Premier Experiment, Sea of Cortez, Addressing the Development of Oblique Rifting (PESCADOR) experiment provide the most detailed seismic structure to date of the southern Gulf of California. Multichannel seismic (MCS) data were recorded with a 6-km-long streamer, 480-channel, aboard the R/V Maurice Ewing, and wide-angle data was recorded by 19 instruments spaced every similar to 12 km along the transect. The MCS and wide-angle data reveal the seismic structure across the continent-ocean transition of the rifted margin. Typical continental and oceanic crust are separated by a similar to 75-km-wide zone of extended continental crust dominated by block-faulted basement. Little lateral variation in crustal thicknesses and seismic velocities is observed in the oceanic crust, suggesting a constant rate of magmatic productivity since seafloor spreading began. Oceanic crustal thickness and mean crustal velocities suggest normal mantle temperature (1300 degrees C) and passive mantle upwelling at the early stages of seafloor spreading. The crustal thickness, width of extended continental crust, and predicted temperature conditions all indicate a narrow rift mode of extension. On the basis of upper and lower crust stretching factors, an excess of lower crust was found in the extended continental crust. Total extension along transect 5W is estimated to be similar to 35 km. Following crustal extension, new oceanic crust similar to 6.4-km-thick was formed at a rate of similar to 48 mm a(-1) to accommodate plate separation.

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Nedimovic, MR, Carbotte SM, Harding AJ, Detrick RS, Canales JP, Diebold JB, Kent GM, Tischer M, Babcock JM.  2005.  Frozen magma lenses below the oceanic crust. Nature. 436:1149-1152.   10.1038/nature03944   AbstractWebsite

The Earth's oceanic crust crystallizes from magmatic systems generated at mid-ocean ridges. Whereas a single magma body residing within the mid-crust is thought to be responsible for the generation of the upper oceanic crust, it remains unclear if the lower crust is formed from the same magma body, or if it mainly crystallizes from magma lenses located at the base of the crust(1-3). Thermal modelling(4-6), tomography(7), compliance(8) and wide-angle seismic studies(9), supported by geological evidence(3,10-18), suggest the presence of gabbroic-melt accumulations within the Moho transition zone in the vicinity of fast- to intermediate-spreading centres. Until now, however, no reflection images have been obtained of such a structure within the Moho transition zone. Here we show images of groups of Moho transition zone reflection events that resulted from the analysis of similar to 1,500 km of multichannel seismic data collected across the intermediate-spreading-rate(19) Juan de Fuca ridge. From our observations we suggest that gabbro lenses and melt accumulations embedded within dunite or residual mantle peridotite are the most probable cause for the observed reflectivity, thus providing support for the hypothesis that the crust is generated from multiple magma bodies.

Nedimovic, MR, Carbotte SM, Diebold JB, Harding AJ, Canales JP, Kent GM.  2008.  Upper crustal evolution across the Juan de Fuca ridge flanks. Geochemistry Geophysics Geosystems. 9   10.1029/2008gc002085   AbstractWebsite

Recent P wave velocity compilations of the oceanic crust indicate that the velocity of the uppermost layer 2A doubles or reaches similar to 4.3 km/s found in mature crust in < 10 Ma after crustal formation. This velocity change is commonly attributed to precipitation of low-temperature alteration minerals within the extrusive rocks associated with ridge-flank hydrothermal circulation. Sediment blanketing, acting as a thermal insulator, has been proposed to further accelerate layer 2A evolution by enhancing mineral precipitation. We carried out 1-D traveltime modeling on common midpoint supergathers from our 2002 Juan de Fuca ridge multichannel seismic data to determine upper crustal structure at similar to 3 km intervals along 300 km long transects crossing the Endeavor, Northern Symmetric, and Cleft ridge segments. Our results show a regional correlation between upper crustal velocity and crustal age. The measured velocity increase with crustal age is not uniform across the investigated ridge flanks. For the ridge flanks blanketed with a sealing sedimentary cover, the velocity increase is double that observed on the sparsely and discontinuously sedimented flanks (similar to 60% increase versus similar to 28%) over the same crustal age range of 5-9 Ma. Extrapolation of velocity-age gradients indicates that layer 2A velocity reaches 4.3 km/s by similar to 8 Ma on the sediment blanketed flanks compared to similar to 16 Ma on the flanks with thin and discontinuous sediment cover. The computed thickness gradients show that layer 2A does not thin and disappear in the Juan de Fuca region with increasing crustal age or sediment blanketing but persists as a relatively low seismic velocity layer capping the deeper oceanic crust. However, layer 2A on the fully sedimented ridge-flank sections is on average thinner than on the sparsely and discontinuously sedimented flanks (330 +/- 80 versus 430 +/- 80 m). The change in thickness occurs over a 10-20 km distance coincident with the onset of sediment burial. Our results also suggest that propagator wakes can have atypical layer 2A thickness and velocity. Impact of propagator wakes is evident in the chemical signature of the fluids sampled by ODP drill holes along the east Endeavor transect, providing further indication that these crustal discontinuities may be sites of localized fluid flow and alteration.

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Mutter, JC, Barth GA, Buhl P, Detrick RS, Orcutt J, Harding A.  1988.  Magma Distribution Across Ridge-Axis Discontinuities on the East Pacific Rise from Multichannel Seismic Images. Nature. 336:156-158.   10.1038/336156a0   AbstractWebsite

Detailed studies of the morphology of the East Pacific Rise axis have shown that its linearity is disrupted by many small but distinct non-transform offsets, including overlapping spreading centres (OSCs) and deviations from axial linearity (devals), which display variable geochemical signals1–9. Using multichannel seismic reflection profiling, we have mapped the distribution of a bright, shallow reflector that Detrick et al. 10 have associated with an axial magma chamber. We have found that it is neither continuous across the 9°03' OSC6, nor separated into two parallel bodies2, and that its lateral offset does not conform to that of the topographic offset associated with the 9° 17' deval. These observations provide important insight into a causative relationship between morphological and petrological segmentation in this region of the East Pacific Rise, and we speculate that the discontinuities may be the morphological response to fluctuations in the spatial pattern of magma delivery.