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Kent, GM, Babcock JM, Driscoll NW, Harding AJ, Dingler JA, Seitz GG, Gardner JV, Mayer LA, Goldman CR, Heyvaert AC, Richards RC, Karlin R, Morgan CW, Gayes PT, Owen LA.  2005.  60 k.y. record of extension across the western boundary of the Basin and Range province: Estimate of slip rates from offset shoreline terraces and a catastrophic slide beneath Lake Tahoe. Geology. 33:365-368.   10.1130/g21230.1   AbstractWebsite

Deformation across three major fault strands within the Lake Tahoe basin has been mapped by using a novel combination of high-resolution seismic chirp, airborne laser-and acoustic-multibeam-derived bathymetry, and deep- and shallow-water sediment cores. Submerged erosional terraces of late Pleistocene age (19.2 +/- 1.8 ka) record vertical deformation across fault strands that ranges between 10 and 15 m; offset of 10 m is observed across the southern part of the West Tahoe fault. Avalanche deposits from the catastrophic McKinney Bay slide (ca. 60 ka) are offset across the Stateline fault by at least 21-25 m. The submerged shoreline terraces and debris avalanche provide marker beds with which to constrain the extensional history of the region for the past 60 k.y. This history is then used to assess the future seismic hazard of the region. Data on deformation across these two important marker beds, combined with chronological control from C-14 and optically stimulated luminescence measurements, yield an estimate of extension across the Lake Tahoe basin that is 0.4-0.5 mm/yr. On the basis of these measurements, there exists the potential for a large, seiche wave-generating M7 earthquake every similar to 3 k.y. Late Pleistocene and Holocene vertical deformation rates within the Tahoe basin are characteristic of Basin and Range faulting and place the Tahoe basin within the western limits of the extensional Basin and Range province.

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   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.

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.

Bazin, S, van Avendonk H, Harding AJ, Orcutt JA, Canales JP, Detrick RS, Grp M.  1998.  Crustal structure of the flanks of the East Pacific Rise: Implications for overlapping spreading centers. Geophysical Research Letters. 25:2213-2216.   10.1029/98gl51590   AbstractWebsite

Tomographic inversion of seismic refraction data from the flanks of the East Pacific Rise (EPR), 17 degrees 15'S, shows that the thickness of layer 2 varies by as much as 500 meters off axis. A thick layer 2 is found in crust affected by migration paths of overlapping spreading centers (OSC). However, no significant variation in crustal thickness is detected throughout the study area. The crustal structure differences documented in this paper are primarily related to this paleo-tectonic setting rather than the east-west asymmetries characteristic of this region of the southern EPR.

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.

Henig, AS, Blackman DK, Harding AJ, Canales JP, Kent GM.  2012.  Downward continued multichannel seismic refraction analysis of Atlantis Massif oceanic core complex, 30 degrees N, Mid-Atlantic Ridge. Geochemistry Geophysics Geosystems. 13   10.1029/2012gc004059   AbstractWebsite

Detailed seismic refraction results show striking lateral and vertical variability of velocity structure within the Atlantis Massif oceanic core complex (OCC), contrasting notably with its conjugate ridge flank. Multichannel seismic (MCS) data are downward continued using the Synthetic On Bottom Experiment (SOBE) method, providing unprecedented detail in tomographic models of the P-wave velocity structure to subseafloor depths of up to 1.5 km. Velocities can vary up to 3 km/s over several hundred meters and unusually high velocities (similar to 5 km/s) are found immediately beneath the seafloor in key regions. Correlation with in situ and dredged rock samples, video and records from submersible dives, and a 1.415 km drill core, allow us to infer dominant lithologies. A high velocity body(ies) found to shoal near to the seafloor in multiple locations is interpreted as gabbro and is displaced along isochrons within the OCC, indicating a propagating magmatic source as the origin for this pluton(s). The western two-thirds of the Southern Ridge is capped in serpentinite that may extend nearly to the base of our ray coverage. The distribution of inferred serpentinite indicates that the gabbroic pluton(s) was emplaced into a dominantly peridotitic host rock. Presumably the mantle host rock was later altered via seawater penetration along the detachment zone, which controlled development of the OCC. The asymmetric distribution of seismic velocities and morphology of Atlantis Massif are consistent with a detachment fault with a component of dip to the southeast. The lowest velocities observed atop the eastern Central Dome and conjugate crust are most likely volcanics. Here, an updated model of the magmatic and extensional faulting processes at Atlantis Massif is deduced from the seismic results, contributing more generally to understanding the processes controlling the formation of heterogeneous lithosphere at slow-rate spreading centers.

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.

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.

Brothers, D, Harding A, Gonzalez-Fernandez A, Holbrook WS, Kent G, Driscoll N, Fletcher J, Lizarralde D, Umhoefer P, Axen G.  2012.  Farallon slab detachment and deformation of the Magdalena Shelf, southern Baja California. Geophysical Research Letters. 39   10.1029/2011gl050828   AbstractWebsite

Subduction of the Farallon plate beneath northwestern Mexico stalled by similar to 12 Ma when the Pacific-Farallon spreading-ridge approached the subduction zone. Coupling between remnant slab and the overriding North American plate played an important role in the capture of the Baja California (BC) microplate by the Pacific Plate. Active-source seismic reflection and wide-angle seismic refraction profiles across southwestern BC (similar to 24.5 degrees N) are used to image the extent of remnant slab and study its impact on the overriding plate. We infer that the hot, buoyant slab detached similar to 40 km landward of the fossil trench. Isostatic rebound following slab detachment uplifted the margin and exposed the Magdalena Shelf to wave-base erosion. Subsequent cooling, subsidence and transtensional opening along the shelf (starting similar to 8 Ma) starved the fossil trench of terrigenous sediment input. Slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes. Citation: Brothers, D., A. Harding, A. Gonzalez-Fernandez, W. S. Holbrook, G. Kent, N. Driscoll, J. Fletcher, D. Lizarralde, P. Umhoefer, and G. Axen (2012), Farallon slab detachment and deformation of the Magdalena Shelf, southern Baja California, Geophys. Res. Lett., 39, L09307, doi:10.1029/2011GL050828.

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.

Blackman, DK, Canales JP, Harding A.  2009.  Geophysical signatures of oceanic core complexes. Geophysical Journal International. 178:593-613.   10.1111/j.1365-246X.2009.04184.x   AbstractWebsite

P>Oceanic core complexes (OCCs) provide access to intrusive and ultramafic sections of young lithosphere and their structure and evolution contain clues about how the balance between magmatism and faulting controls the style of rifting that may dominate in a portion of a spreading centre for Myr timescales. Initial models of the development of OCCs depended strongly on insights available from continental core complexes and from seafloor mapping. While these frameworks have been useful in guiding a broader scope of studies and determining the extent of OCC formation along slow spreading ridges, as we summarize herein, results from the past decade highlight the need to reassess the hypothesis that reduced magma supply is a driver of long-lived detachment faulting. The aim of this paper is to review the available geophysical constraints on OCC structure and to look at what aspects of current models are constrained or required by the data. We consider sonar data (morphology and backscatter), gravity, magnetics, borehole geophysics and seismic reflection. Additional emphasis is placed on seismic velocity results (refraction) since this is where deviations from normal crustal accretion should be most readily quantified. However, as with gravity and magnetic studies at OCCs, ambiguities are inherent in seismic interpretation, including within some processing/analysis steps. We briefly discuss some of these issues for each data type. Progress in understanding the shallow structure of OCCs (within similar to 1 km of the seafloor) is considerable. Firm constraints on deeper structure, particularly characterization of the transition from dominantly mafic rock (and/or altered ultramafic rock) to dominantly fresh mantle peridotite, are not currently in hand. There is limited information on the structure and composition of the conjugate lithosphere accreted to the opposite plate while an OCC forms, commonly on the inside corner of a ridge-offset intersection. These gaps preclude full testing of current models. However, with the data in hand there are systematic patterns in OCC structure, such as the 1-2 Myr duration of this rifting style within a given ridge segment, the height of the domal cores with respect to surrounding seafloor, the correspondence of gravity highs with OCCs, and the persistence of corrugations that mark relative (palaeo) slip along the exposed detachment capping the domal cores. This compilation of geophysical results at OCCs should be useful to investigators new to the topic but we also target advanced researchers in our presentation and synthesis of findings to date.

Blackman, DK, Slagle A, Guerin G, Harding A.  2014.  Geophysical signatures of past and present hydration within a young oceanic core complex. Geophysical Research Letters. 41:1179-1186.   10.1002/2013gl058111   AbstractWebsite

Borehole logging at the Atlantis Massif oceanic core complex provides new information on the relationship between the physical properties and the lithospheric hydration of a slow-spread intrusive crustal section. Integrated Ocean Drilling Program Hole U1309D penetrates 1.4km into the footwall to an exposed detachment fault on the 1.2Ma flank of the mid-Atlantic Ridge, 30 degrees N. Downhole variations in seismic velocity and resistivity show a strong correspondence to the degree of alteration, a recorder of past seawater circulation. Average velocity and resistivity are lower, and alteration is more pervasive above a fault around 750m. Deeper, these properties have higher values except in heavily altered ultramafic zones that are several tens of meters thick. Present circulation inferred from temperature mimics this pattern: advective cooling persists above 750m, but below, conductive cooling dominates except for small excursions within the ultramafic zones. These alteration-related physical property signatures are probably a characteristic of gabbroic cores at oceanic core complexes. Key Points Borehole T indicates shallow present circulation, conductive regime > 750 mbsf Narrow fault zones have seismic, T, resistivity signal indicating localized flow Hydration of gabbroic oceanic core complexes is limited below fault damage zone

Dingler, J, Kent G, Driscoll N, Babcock J, Harding A, Seitz G, Karlin B, Goldman C.  2009.  A high-resolution seismic CHIRP investigation of active normal faulting across Lake Tahoe Basin, California-Nevada. Geological Society of America Bulletin. 121:1089-1107.   10.1130/b26244.1   AbstractWebsite

We measured extension rates across Lake Tahoe Basin for the last 60 ka. based on measured displacement of offset marker surfaces across three active faults beneath Lake Tahoe. Seismic chirp imaging with submeter accuracy, together with detailed multibeam and light detection and ranging (LIDAR)-derived bathymetry, was used to measure fault offset, thickness of earthquake-derived colluvial wedges, depth of wave-cut paleoterraces, and other geomorphic features. An analysis of these features provides refined estimates of extension rates and new information on Holocene faulting, and places Tahoe Basin deformation into the larger context of Walker Lane and Basin and Range tectonics. Measured offset marker surfaces include submerged wave-cut paleoterraces of Tioga age (19.2 +/- 1.8 ka), McKinney Bay slide deposits (ca. 60 ka), and a winnowed boulder surface of Tahoe age (ca. 62 ka). Estimated vertical offset rates across submerged geomorphic surfaces are 0.43-0.81 mm/a for the West Tahoe fault, 0.35-0.60 mm/a for the Stateline-North Tahoe fault, and 0.12-0.30 mm/a for the Incline Village fault. These offset rates indicate a combined east-west extension rate across Lake Tahoe Basin, assuming 60 degrees fault dips, of 0.52-0.99 mm/a. This estimate, when combined with the Genoa fault-slip rate, yields an extension rate consistent with the magnitude of the extension deficit across Carson Valley and Lake Tahoe Basin derived from global positioning system (GPS) velocities. The Stateline-North Tahoe, Incline Village, and West Tahoe faults all show evidence for individual Holocene earthquake events as recorded by either colluvial wedge deposits or offset fan-delta stratigraphy.

Forsyth, DW, Scheirer DS, Webb SC, Dorman LM, Orcutt JA, Harding AJ, Blackman DK, Morgan JP, Detrick RS, Shen Y, Wolfe CJ, Canales JP, Toomey DR, Sheehan AF, Solomon SC, Wilcock WSD, Team MS.  1998.  Imaging the deep seismic structure beneath a mid-ocean ridge: The MELT experiment. Science. 280:1215-1218.   10.1126/science.280.5367.1215   AbstractWebsite

The Mantle Electromagnetic and Tomography (MELT) Experiment was designed to distinguish between competing models of magma generation beneath mid-ocean ridges. Seismological observations demonstrate that basaltic melt is present beneath the East Pacific Rise spreading center in a broad region several hundred kilometers across and extending to depths greater than 100 kilometers, not just in a narrow region of high melt concentration beneath the spreading center, as predicted by some models. The structure of the ridge system is strongly asymmetric: mantle densities and seismic velocities are lower and seismic anisotropy is stronger to the west of the rise axis.

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.

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.

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.

Michael, PJ, Forsyth DW, Blackman DK, Fox PJ, Hanan BB, Harding AJ, Macdonald KC, Neumann GA, Orcutt JA, Tolstoy M, Weiland CM.  1994.  Mantle Control of a Dynamically Evolving Spreading Center - Mid-Atlantic Ridge 31-34-Degrees-S. Earth and Planetary Science Letters. 121:451-468.   10.1016/0012-821x(94)90083-3   AbstractWebsite

A segment of the slow-spreading Mid-Atlantic Ridge (MAR) at 33-degrees-S changes dramatically as its center is approached. Towards the center of the segment, the axis shoals from 3900 to 2400 m and a deep median valley nearly disappears. There is a prominent bullseye gravity low centered over the shallow summit, indicating thicker crust or lower density mantle or both. Incompatible element and radiogenic isotope ratios in MORB increase, creating a 'spike high' centered on the summit of the segment. The basalts' enrichment is confined to this robust ridge segment alone and is geochemically unlike the nearby hotspots at Tristan da Cunha, Gough and Discovery Islands. The average extent of mantle melting for the entire segment, as determined from mid-ocean ridge basalt (MORB) major element chemistry, is slightly greater than for adjacent segments. The segment has lengthened to 100 km by ridge propagation at both ends during the past 3.5 m.y., and is presently the longest and shallowest segment in the region. Although the ridge crest anomalies of this ridge segment strongly resemble those caused by the interaction of mid-ocean ridges with mantle hotspots, the geochemical and geophysical evidence suggests that they may instead be related to interaction of the ridge with a passively embedded chemical heterogeneity in the mantle.

Brothers, DS, Kent GM, Driscoll NW, Smith SB, Karlin R, Dingler JA, Harding AJ, Seitz GG, Babcock JM.  2009.  New Constraints on Deformation, Slip Rate, and Timing of the Most Recent Earthquake on the West Tahoe-Dollar Point Fault, Lake Tahoe Basin, California. Bulletin of the Seismological Society of America. 99:499-519.   10.1785/0120080135   AbstractWebsite

High-resolution seismic compressed high intensity Radar pulse (CHIRP) data and piston cores acquired in Fallen Leaf Lake (FLL) and Lake Tahoe provide new paleoseismic constraints on the West Tahoe-Dollar Point fault (WTDPF), the western-most normal fault in the Lake Tahoe Basin, California. Paleoearthquake records along three sections of the WTDPF are investigated to determine the magnitude and recency of coseismic slip. CHIRP profiles image vertically offset and folded strata along the southern and central sections that record deformation associated with the most recent event (MRE) on the WTDPF. Three faults are imaged beneath FLL, and the maximum vertical offset observed across the primary trace of the WTDPF is similar to 3.7 m. Coregistered piston cores in FLL recovered sediment and organic material above and below the MRE horizon. Radiocarbon dating of organic material constrained the age of the MRE to be between 3.6 and 4.9 k.y. B.P., with a preferred age of 4.1-4.5 k.y. B. P. In Lake Tahoe near Rubicon Point, approximately 2.0 m of vertical offset is observed across the WTDPF. Based on nearby core data, the timing of this offset occurred between similar to 3-10 k.y. B.P., which is consistent with the MRE age in FLL. Offset of Tiogaaged glacial deposits provides a long-term record of vertical deformation on the WTDPF since similar to 13-14 k.y. B.P., yielding a slip rate of 0.4-0.8 m/yr. In summary, the slip rate and earthquake potential along the WTDPF is comparable to the nearby Genoa fault, making it the most active and potentially hazardous fault in the Lake Tahoe Basin.

Canales, JP, Detrick RS, Bazin S, Harding AJ, Orcutt JA.  1998.  Off-axis crustal thickness across and along the East Pacific Rise within the MELT area. Science. 280:1218-1221.   10.1126/science.280.5367.1218   AbstractWebsite

Wide-angle seismic data along the Mantle Electromagnetic and Tomography (MELT) arrays show that the thickness of 0.5- to 1.5-million-year-old crust of the Nazca Plate is not resolvably different from that of the Pacific Plate, despite an asymmetry in depth and gravity across this portion of the East Pacific Rise. Crustal thickness on similarly aged crust on the Nazca plate near a magmatically robust part of the East Pacific Rise at 17 degrees 15'S is slightly thinner (5.1 to 5.7 kilometers) than at the 15 degrees 55'S overlapping spreading center (5.8 to 6.3 kilometers). This small north-south off-axis crustal thickness difference may reflect along-axis temporal variations in magma supply, whereas the across-axis asymmetry in depth and gravity must be caused by density variations in the underlying mantle.

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.

Carbotte, SM, Detrick RS, Harding A, Canales JP, Babcock J, Kent G, van Ark E, Nedimovic M, Diebold J.  2006.  Rift topography linked to magmatism at the intermediate spreading Juan de Fuca Ridge. Geology. 34:209-212.   10.1130/g21969.1   AbstractWebsite

New seismic observations of crustal structure along the Juan de Fuca Ridge indicate that the axial rift topography reflects magma-induced deformation rather than alternating phases of magmatism and tectonic extension, as previously proposed. Contrary to predictions of the episodic models, crustal magma bodies are imaged beneath portions of all ridge segments surveyed at average depths of 2.1-2.6 km. The shallow rift valley or axial graben associated with each Juan de Fuca segment is similar to 50-200 m deep and 1-8 km wide and is well correlated with a magma body in the subsurface. Analysis of graben dimensions (height and width) shows that the axial graben narrows and graben height diminishes where the magma body disappears, rather than deepening and broadening, as expected for rift topography due to tectonic extension. We propose an evolutionary model of axial topography that emphasizes the contribution of dike intrusion to subsidence and fault slip at the seafloor. In this model an evolving axial topography results from feedbacks between the rheollogy of the crust above a magma sill and dike intrusion, rather than episodic magma delivery from the mantle.

Hussenoeder, SA, Collins JA, Kent GM, Detrick RS, Harding AJ, Orcutt JA, Mutter JC, Buhl P.  1996.  Seismic analysis of the axial magma chamber reflector along the southern East Pacific Rise from conventional reflection profiling. Journal of Geophysical Research-Solid Earth. 101:22087-22105.   10.1029/96jb01907   AbstractWebsite

The thickness and internal properties of the magma sill located at the top of the axial magma chamber (AMC) along the southern East Pacific Rise (EPR) have been investigated through a combination of waveform modeling the near-vertical incidence reflections from this body and analysis of reflection amplitude variation as a function of source-receiver offset (or slowness). Our results show that the AMC reflector observed along the southern EPR is best modeled by a thin (< 100 m thick) sill of partial melt (V-s not equal 0 km/s) sandwiched between higher-velocity material, and that the thickest sills are generally associated with the lowest P and S wave velocities. The comparatively high P wave velocities and nonzero shear wave velocities inferred for this sill indicate that it is filled with partially molten magma which in some locations has a high crystal content. This may have important implications for eruption mechanisms and along-axis mixing of magma at the EPR. There is no simple relationship between morphologic indicators of magma supply (e.g., axial depth or volume) and sill thickness, depth, or velocity. Magma sill properties may be closely tied to the eruption and replenishment cycle of the AMC and thus may vary on a much shorter spatial and temporal scale than axial morphology, which reflects longer-term variations in magma supply to the ridge.

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.