D. Lizarralde

Fracture propagation to the base of the Greenland Ice Sheet during supraglacial lake drainage

Surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change. The process whereby such a pathway is created through thick, cold ice has not, however, been previously observed. We describe the rapid (<2 hours) drainage of a large supraglacial lake down 980 meters through to the bed of the Greenland Ice Sheet initiated by water-driven fracture propagation evolving into moulin flow. Drainage coincided with increased seismicity, transient acceleration, ice-sheet uplift, and horizontal displacement. Subsidence and deceleration occurred over the subsequent 24 hours. The short-lived dynamic response suggests that an efficient drainage system dispersed the meltwater subglacially. The integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup.

Structure and composition of the Aleutian island arc and implications for continental crustal growth

We present results of a seismic reflection and refraction investigation of the Aleutian island arc, designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust. The Aleutian arc has the requisite thickness (30 km) to build continental crust, but it differs strongly from continental crust in its composition and reflectivity structure. Seismic velocities and the compositions of erupted lavas suggest that the Aleutian crust has a mafic bulk composition, in contrast to the andesitic bulk composition of continents. The silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the Aleutian intraoceanic arc. Therefore, if island arcs form a significant source of continental crust, the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin. The pervasive deformation, intracrustal melting, and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc-continent collision or through subsequent establishment of a continental arc. The volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two.

Mantle thermal structure and active upwelling during continental breakup in the North Atlantic

Seismic reflection and refraction data acquired on four transects spanning the Southeast Greenland rifted margin and Greenland^Iceland Ridge (GIR) provide new constraints on mantle thermal structure and melting processes during continental breakup in the North Atlantic. Maximum igneous crustal thickness varies along the margin from s 30 km in the near-hotspot zone (6 500 km from the hotspot track) to V18 km in the distal zone (500^1100 km). Magmatic productivity on summed conjugate margins of the North Atlantic decreases through time from 1800 ˛ 300 to 600 ˛ 50 km 3 /km/Ma in the near-hotspot zone and from 700 ˛ 200 to 300 ˛ 50 km 3 /km/Ma in the distal zone. Comparison of our data with the British/Faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist. Joint consideration of crustal thickness and mean crustal seismic velocity suggests that along-margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature. The thermal anomaly (vT) at breakup was modest (V100^125‡C), varied little along the margin, and transient. Data along the GIR indicate that the potential temperature anomaly (125 ˛ 50‡C) and upwelling ratio (V4 times passive) of the Iceland hotspot have remained roughly constant since 56 Ma. Our results are consistent with a plume^impact model, in which (1) a plume of radius V300 km and vT of V125‡C impacted the margin around 61 Ma and delivered warm material to distal portions of the margin; (2) at breakup (56 Ma), the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin; and (3) by 45 Ma, both the remaining plume head and the distal warm layer were exhausted, with excess magmatism thereafter largely confined to a narrow (6 200 km radius) zone immediately above the Iceland plume stem. Alternatively, the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre-existing thermal anomaly unrelated to the plume. fl 2001 Elsevier Science B.V. All rights reserved.

Variation in styles of rifting in the Gulf of California

Constraints on the structure of rifted continental margins and the magmatism resulting from such rifting can help refine our understanding of the strength of the lithosphere, the state of the underlying mantle and the transition from rifting to seafloor spreading. An important structural classification of rifts is by width, with narrow rifts thought to form as necking instabilities (where extension rates outpace thermal diffusion) and wide rifts thought to require a mechanism to inhibit localization, such as lower-crustal flow in high heat-flow settings. Observations of the magmatism that results from rifting range from volcanic margins with two to three times the magmatism predicted from melting models to non-volcanic margins with almost no rift or post-rift magmatism. Such variations in magmatic activity are commonly attributed to variations in mantle temperature. Here we describe results from the PESCADOR seismic experiment in the southern Gulf of California and present crustal-scale images across three rift segments. Over short lateral distances, we observe large differences in rifting style and magmatism—from wide rifting with minor synchronous magmatism to narrow rifting in magmatically robust segments. But many of the factors believed to control structural evolution and magmatism during rifting (extension rate, mantle potential temperature and heat flow) tend to vary over larger length scales. We conclude instead that mantle depletion, rather than low mantle temperature, accounts for the observed wide, magma-poor margins, and that mantle fertility and possibly sedimentary insulation, rather than high mantle temperature, account for the observed robust rift and post-rift magmatism.

Northeastern Pacific mantle conductivity profile from long-period magnetotelluric sounding using Hawaii-to-California submarine cable data

We present results of a long-period magnetotelluric (MT) investigation of the electrical structure beneath the eastern North Pacific. The electric field data consist of ∼2 years of continuously recorded voltages across an unpowered, ∼4000-km-long submarine telephone cable (HAW-1) extending from Point Arena, California, to Oahu, Hawaii. The electric field measurements are coherent to some degree with magnetic field measurements from Honolulu Observatory at periods of 0.1 to 45 days. This coherence is enhanced at long periods over that observed with point electric field sensors due to horizontal averaging of the motional electric fields of spatial scale smaller than the cable length, significantly diminishing their effect. Robust, controlled leverage MT response estimates and their jacknife confidence limits are computed for the HAW-1 to Honolulu data. An equivalent scalar MT response obtained from Honolulu magnetic variations data is used to correct the HAW-1 MT response for static shift and to extend the MT response estimate to periods of 100 days. The composite response function satisfies necessary and sufficient conditions for consistency with a one-dimensional conductivity structure and is most sensitive to structure between 150 and 1000 km. Inversion of the MT response reveals a conductive zone (0.05–0.1 S/m) between 150 and 400 km depth and a positive gradient below 500 km; these observations are consistent with previous MT studies in the North Pacific. This upper mantle conductivity is too high to be explained by solid-state conduction in dry olivine using reasonable mantle geotherms. Calculations based on measurements of hydrogen solubility and diffusivity in olivine indicate that H+ dissolved in olivine, possibly combined with a lattice preferred orientation consistent with measured seismic anisotropy, provide sufficient conductivity enhancement to explain the inversion results. The high conductivity may also be explained by the presence of gravitationally stable partial melt. Comparison of the HAW-1 results with long-period MT studies conducted on land reveals differences in upper mantle conductivity between different tectonic regimes. In particular, the upper mantle beneath the Pacific Ocean is considerably more conductive than that beneath the Canadian shield and similar in conductivity to that beneath the Basin and Range.

Migration of methane gas through the hydrate stability zone in a low-flux hydrate province

New high-resolution seismic data show clear evidence for upward injection of methane gas well into the hydrate stability zone at the stable, low-methane-flux Blake Ridge crest. This movement of gaseous methane, through a thermo-dynamic regime where it should be trapped as hydrate, suggests that dynamic migrations of gas play an important role in the interaction of subseafloor methane with the ocean. In the study area, none of the seismic amplitude anomalies that provide evidence for gas migration reaches the seafloor; instead they terminate at the base of a highly reflective, unfaulted capping layer. Seismic inversions of anomalous regions show (1) increased velocities beneath the hydrate stability zone, suggesting less gas, and (2) increased velocities within the hydrate stability zone associated with observed low-amplitude chimneys and bright spots, indicating increased hydrate concentrations. These observations and analyses indicate that methane migrates upward as free gas hundreds of meters into the hydrate stability zone before forming hydrate. The observations strongly imply that given appropriate permeable pathways, free gas can escape into the ocean. Even in a low-flux environment, the hydrate stability zone is not an impermeable barrier to free-gas migration.

Spreading-rate dependence of melt extraction at mid-ocean ridges from mantle seismic refraction data.

A variety of observations indicate that mid-ocean ridges produce less crust at spreading rates below 20 mm yr-1 (refs 1–3), reflecting changes in fundamental ridge processes with decreasing spreading rate. The nature of these changes, however, remains uncertain, with end-member explanations being decreasing shallow melting or incomplete melt extraction, each due to the influence of a thicker thermal lid. Here we present results of a seismic refraction experiment designed to study mid-ocean ridge processes by imaging residual mantle structure. Our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo-spreading rate. Changes in mantle velocity gradient, basement topography and crustal thickness all correlate with this spreading-rate change. These observations can be explained by variations in melt extraction at the ridge, with a gabbroic phase preferentially retained in the mantle at slower spreading rates. The estimated volume of retained melt balances the ∼1.5-km difference in crustal thickness, suggesting that changes in spreading rate affect melt-extraction processes rather than total melting.

Structure and serpentinization of the subducting Cocos plate offshore Nicaragua and Costa Rica

The Cocos plate experiences extensional faulting as it bends into the Middle American Trench (MAT) west of Nicaragua, which may lead to hydration of the subducting mantle. To estimate the along strike variations of volatile input from the Cocos plate into the subduction zone, we gathered marine seismic refraction data with the R/V Marcus Langseth along a 396 km long trench parallel transect offshore of Nicaragua and Costa Rica. Our inversion of crustal and mantle seismic phases shows two notable features in the deep structure of the Cocos plate: (1) Normal oceanic crust of 6 km thickness from the East Pacific Rise (EPR) lies offshore Nicaragua, but offshore central Costa Rica we find oceanic crust from the northern flank of the Cocos Nazca (CN) spreading center with more complex seismic velocity structure and a thickness of 10 km. We attribute the unusual seismic structure offshore Costa Rica to the midplate volcanism in the vicinity of the Galapagos hot spot. (2) A decrease in Cocos plate mantle seismic velocities from ∼7.9 km/s offshore Nicoya Peninsula to ∼6.9 km/s offshore central Nicaragua correlates well with the northward increase in the degree of crustal faulting outboard of the MAT. The negative seismic velocity anomaly reaches a depth of ∼12 km beneath the Moho offshore Nicaragua, which suggests that larger amounts of water are stored deep in the subducting mantle lithosphere than previously thought. If most of the mantle low velocity zone can be interpreted as serpentinization, the amount of water stored in the Cocos plate offshore central Nicaragua may be about 2.5 times larger than offshore Nicoya Peninsula. Hydration of oceanic lithosphere at deep sea trenches may be the most important mechanism for the transfer of aqueous fluids to volcanic arcs and the deeper mantle.

Direct seismic detection of methane hydrate on the Blake Ridge

Seismic detection of methane hydrate often relies on indirect or equivocal methods. New multichannel seismic reflection data from the Blake Ridge, located approximately 450 km east of Savannah, Georgia, show three direct seismic indicators of methane hydrate: (1) a paleo bottom‐simulating reflector (BSR) formed when methane gas froze into methane hydrate on the eroding eastern flank of the Blake Ridge, (2) a lens of reduced amplitudes and high P‐wave velocities found between the paleo‐BSR and BSR, and (3) bright spots within the hydrate stability zone that represent discrete layers of concentrated hydrate formed by upward migration of gas. Velocities within the lens (∼1910 m/s) are significantly higher than velocities in immediately adjacent strata (1820 and 1849 m/s). Conservative estimates show that the hydrate lens contains at least 13% bulk methane hydrate within a 2‐km3 volume, yielding 3.2 × 1010kg [1.5 TCF (4.2 × 1010 m3] of methane. Low seismic amplitudes coupled with high interval velocities with...

Crustal construction of a volcanic arc, wide‐angle seismic results from the western Alaska Peninsula

[1] Results from the 1994 Aleutian Seismic experiment delineate basic oceanic arc crustal architecture, constrain magmatic flux rates and bulk arc composition, and address questions of continental crustal genesis. Here we present results from a transect across protocontinental crust of the westernmost Alaska Peninsula (line A3) and compare this structure to a purely oceanic arc transect farther west. Arc crustal structure is similar along these two transects. Magmatic accretion occurs at the top and bottom of preexisting oceanic crust as a 5- to 10-km-thick upper crustal carapace of low velocity (2-5.8 km s -1 ) volcaniclastics, flows and small plutons, and a mafic lower crustal underplate (∼7.0 km s -1 ) of variable thickness, for a maximum arc crust thickness of,25-30 km. Lateral lower crustal velocity gradients and high velocities (>7.5 km s -1 ) beneath the forearc suggest dominantly vertical lower crustal accretion above a focused melt source and a forearc underlain by little magmatic crust but rather partially intruded and/or serpentinized mantle. The ratio of upper to lower crustal volume is ∼1, and the total arc crust volume implies a magmatic flux of ∼67 km 3 km -1 m.y. -1 , more than twice previous estimates for this arc and global productivity. The crust is thinner and more mafic than continental crust, and it lacks a massive tonalitic upper crust characteristic of the continents. An interpreted accumulation of upper crustal carapace material at midcrustal depths on line A3 has a velocity of ∼6.4 km s -1 , suggesting an intermediate composition. Accretionary complex terranes consisting of accumulations of this type material would thus have bulk compositions similar to continental crust.