Juan Pablo Canales

Kinematics and geometry of active detachment faulting beneath the Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge

Newly acquired seismic refraction and microearthquake data from the Trans-Atlantic Geotraverse (TAG) segment of the Mid-Atlantic Ridge at 26°N reveal for the first time the geometry and seismic character of an active oceanic detachment fault. Hypocenters from 19,232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an ∼15-km-long, dome-shaped fault surface that penetrates to depths >7 km below the seafloor on a steeply dipping (∼70°) interface. A tomographic model of compressional-wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall, which appears to roll over to a shallow dip of 20° ± 5° and become aseismic at a depth of ∼3 km. Outboard of the detachment the exhumed lithosphere is deformed by ridge-parallel, antithetical normal faulting. Our results suggest that hydrothermal fluids at the TAG field exploit the detachment fault to extract heat from a region near the crust-mantle interface over long periods of time.

Discovery of a magma chamber and faults beneath a Mid-Atlantic Ridge hydrothermal field

Crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes, with magmatic accretion possibly involving short-lived crustal magma chambers. The reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast-spreading centres, but it has been difficult to image such magma chambers beneath slow-spreading centres, owing to rough seafloor topography and associated seafloor scattering. In the absence of any images of magma chambers or of subsurface near-axis faults, it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow-spreading ridges. Here we report the presence of a crustal magma chamber beneath the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge. The reflection from the top of the magma chamber, centred beneath the Lucky Strike volcano and hydrothermal field, is approximately 3 km beneath the sea floor, 3–4 km wide and extends up to 7 km along-axis. We suggest that this magma chamber provides the heat for the active hydrothermal vent field above it. We also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward-dipping faults cutting through the volcanic edifice, suggesting continuous interactions between tectonic and magmatic processes.

Rift topography linked to magmatism at the intermediate spreading Juan de Fuca Ridge

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 ∼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 rheology of the crust above a magma sill and dike intrusion, rather than episodic magma delivery from the mantle.

Constructing the crust along the Galapagos Spreading Center 91.3°–95.5°W: Correlation of seismic layer 2A with axial magma lens and topographic characteristics

the seafloor) and narrow (� 0.5–1.5 km, cross-axis width). We also image a thin seismic layer 2A (0.24–0.42 km) that thickens away from the ridge axis by as much as 150%. West of 92.7� W, the magma lens is deeper (2.5–4.5 km) and wider (0.7–2.4 km), and layer 2A is thicker (0.36–0.66 km) and thickens off axis by <40%. The positive correlation between layer 2A thickness and magma lens depth supports the interpretation of layer 2A as the extrusive volcanic layer with thickness controlled by the pressure on the magma lens and its ability to push magma to the surface. Our findings also suggest that narrower magma lenses focus diking close the ridge axis such that lava flowing away from the ridge axis will blanket older flows and thicken the extrusive crust off axis. Flow of lava away from the ridge axis is probably promoted by the slope of the axial bathymetric high, which is largest east of 92.5� W. West of � 94� Wt he ‘‘transitional’’ axial morphology lacks a prominent bathymetric high and layer 2A no longer thickens off axis. We detect no magma lens west of 94.7� W where a small axial valley appears. The above changes can be linked to the westward decrease in the magma and heat flux associated with the fading influence of the Galapagos hot spot on the Galapagos Spreading Center. INDEX TERMS: 3035 Marine Geology and Geophysics: Midocean ridge processes; 7220 Seismology: Oceanic crust; 0935 Exploration Geophysics: Seismic methods (3025);

Variable crustal structure along the Juan de Fuca Ridge: Influence of on-axis hot spots and absolute plate motions

Multichannel seismic and bathymetric data from the Juan de Fuca Ridge (JDFR) provide constraints on axial and ridge flank structure for the past 4–8 Ma within three spreading corridors crossing Cleft, Northern Symmetric, and Endeavour segments. Along-axis data reveal south-to-north gradients in seafloor relief and presence and depth of the crustal magma lens, which indicate a warmer axial regime to the south, both on a regional scale and within individual segments. For young crust, cross-axis lines reveal differences between segments in Moho two-way traveltimes of 200–300 ms which indicate 0.5–1 km thicker crust at Endeavour and Cleft compared to Northern Symmetric. Moho traveltime anomalies extend beyond the 5–15 km wide axial high and coincide with distinct plateaus, 32 and 40 km wide and 200–400 m high, found at both segments. On older crust, Moho traveltimes are similar for all three segments (∼2100 ± 100 ms), indicating little difference in average crustal production prior to ∼0.6 and 0.7 Ma. The presence of broad axis-centered bathymetric plateau with thickened crust at Cleft and Endeavour segments is attributed to recent initiation of ridge axis-centered melt anomalies associated with the Cobb hot spot and the Heckle melt anomaly. Increased melt supply at Cleft segment upon initiation of Axial Volcano and southward propagation of Endeavour segment during the Brunhes point to rapid southward directed along-axis channeling of melt anomalies linked to these hot spots. Preferential southward flow of the Cobb and Heckle melt anomalies and the regional-scale south-to-north gradients in ridge structure along the JDFR may reflect influence of the northwesterly absolute motion of the ridge axis on subaxial melt distribution.

A crustal transect through the northern and northeastern part of the volcanic edifice of Gran Canaria, Canary Islands

Abstract Wide-angle reflection and refraction seismic data were obtained during METEOR cruise 24 in the N and NE of Gran Canaria (GC), Canary Islands. Seismic energy was generated with two 32 L air guns fired at 1 or 2 min intervals. Seven ocean bottom hydrophones (OBH) and 8 mobile land stations recorded seismic arrivals over a network of profiles covering the northern and northeastern sector up to 60 km away from coastline of GC. The detailed structure of the volcanic edifice and the adjacent ocean basin is revealed in the data set along three radial profiles. A 4 km thick sediment sequence overlies the 7 km thick igneous oceanic crust. The basement is characterized by a first order discontinuity with a velocity jump from 3.4 km/s in the sediment to 4.5 km/s. A pronounced lateral velocity variation was found beneath the island. A 5–6 km thick low velocity zone within the central volcanic edifice, at roughly 4–12 km depth south of the island center is interpreted as the Miocene syenitic feldspar-rich core with lower velocity than the recent volcanic core of more mafic composition beneath northern GC. The massive volcanic island flank thins rapidly away from the island with velocities decreasing gradually from 5.0 near the coast to 3.5 km/s in the outermost part 50–60 km away from the coastline. The clear doming of the lower crust (>6.6 km/s) to 8–10 km depth beneath the northern part of GC is attributed to relatively young mafic plutonic rocks. The Moho north and northeast of GC lies almost horizontal; its depth increases slightly from 14 km along the N–S oriented Profile 1 in the west to 16 km along the N–E oriented Profile 3 in the east. A minor flexure of 1–2 km is indicated by a very gentle dip of the Moho beginning around 10–15 km offshore. A zone of magmatic underplating at depths as low as 26 km is found beneath GC. The anomalous velocity–depth function of the igneous oceanic crust north and northeast of GC and the crustal structure beneath the island are clear evidence for fundamental modification and disruption of the original crustal structure by the Canarian magmatic and volcanic activity.

Anatomy of an active submarine volcano

Most of the magma erupted at mid-ocean ridges is stored in a mid-crustal melt lens that lies at the boundary between sheeted dikes and gabbros. Nevertheless, images of the magma pathways linking this melt lens to the overlying eruption site have remained elusive. Here, we have used seismic methods to image the thickest magma reservoir observed beneath any spreading center to date, which is principally attributed to the juxtaposition of the Juan de Fuca Ridge with the Cobb hotspot (northwestern USA). Our results reveal a complex melt body, which is ∼14 km long, 3 km wide, and up to 1 km thick, beneath the summit caldera. The estimated volume of the reservoir is 18–30 km3, more than two orders of magnitude greater than the erupted magma volumes of either the A.D. 1998 or 2011 eruption. Our images show a network of sub-horizontal to shallow-dipping (<30°) features that we interpret as pathways facilitating melt transport from the magma reservoir to the eruption sites.

An estimation of the elastic thickness of the lithosphere in the Canary Archipelago using admittance function

A recent compilation of shipborne gravity and bathymetry data has been used to determine the Effective Elastic Thickness (EET) of the lithosphere beneath the Canary Islands volcanic complex by means of the admittance technique. As a first approach, we have computed the admittance, which is interpreted in terms of isotropic elastic plate thickness, with an EET of 23 km. Nevertheless, the proximity of the West African Continental Margin (WACM), together with the lateral variability in lithosphere structure in part caused by the large volcanic history of the Canaries, require a slightly different approach. To minimize the influence of the margin and to check whether or not any significant variation in EET occurs related to the known Canary hotspot, we have computed the admittance following the isochron lines. The results show a higher EET (35 km) than that obtained by the first approach but slightly smaller than that predicted by the cooling plate model.

Seismic reflection imaging of the Juan de Fuca plate from ridge to trench: New constraints on the distribution of faulting and evolution of the crust prior to subduction

We present prestack time-migrated multichannel seismic images along two cross-plate transects from the Juan de Fuca (JdF) Ridge to the Cascadia deformation front (DF) offshore Oregon and Washington from which we characterize crustal structure, distribution and extent of faults across the plate interior as the crust ages and near the DF in response to subduction bending. Within the plate interior, we observe numerous small offset faults in the sediment section beginning 50–70 km from the ridge axis with sparse fault plane reflections confined to the upper crust. Plate bending due to sediment loading and subduction initiates at ~120–150 km and ~65–80 km seaward of the DF, respectively, and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity. Most bend faulting deformation occurs within 40 km from the DF; on the Oregon transect, bright fault plane reflections that extend through the crust and 6–7 km into the mantle are observed. If attributed to serpentinization, ~0.12–0.92 wt % water within the uppermost 6 km of the mantle is estimated. On the Washington transect, bending faults are confined to the sediment section and upper-middle crust. The regional difference in subduction bend-faulting and potential hydration of the JdF plate is inconsistent with the spatial distribution of intermediate-depth intraslab seismicity at Cascadia. A series of distinctive, ridgeward dipping (20°–40°) lower crustal reflections are imaged in ~6–8 Ma crust along both transects and are interpreted as ductile shear zones formed within the ridges accretionary zone in response to temporal variations in mantle upwelling, possibly associated with previously recognized plate reorganizations at 8.5 Ma and 5.9 Ma.

Seismic Imaging in Three Dimensions on the East Pacific Rise

The U.S. R/V Marcus G. Langseth (operated by the Lamont-Doherty Earth Observatory of Columbia University) sailed in late June 2008 from Manzanillo, Mexico, to the 9°50′N area of the East Pacific Rise (EPR), a site of vigorous hydrothermal venting (Figure 1). The cruise, MGL0812, the first research deployment of the Langseths advanced three-dimensional (3-D) seismic imaging capability, had as its objective obtaining high-resolution images of crustal structure beneath the ridge crest and adjacent regions. The benefits of 3-D seismic imaging had been outlined in a U.S. National Science Foundation (NSF)—sponsored workshop in 2005 [Mutter and Moore, 2005]. Short courses on techniques of 3-D survey planning were given at AGU Fall Meetings in 2007 and 2008. This brief report describes experiences during the cruise, with the objective of aiding future researchers in planning cruises using Langseths unique imaging capability for 3-D.