Louden, Keith

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Open Access
Evidence for asymmetric nonvolcanic rifting and slow incipient oceanic accretion from seismic reflection data on the Newfoundland margin
(2006-09) Shillington, Donna J.; Holbrook, W. Steven; Van Avendonk, Harm J. A.; Tucholke, Brian E.; Hopper, John R.; Louden, Keith E.; Larsen, Hans Christian; Nunes, Gregory T.
[ 1] Prestack depth migrations of seismic reflection data collected around the Ocean Drilling Program (ODP) Leg 210 transect on the Newfoundland nonvolcanic margin delineate three domains: ( 1) extended continental crust, ( 2) transitional basement, and ( 3) apparent slow spreading oceanic basement beyond anomaly M3 and indicate first-order differences between this margin and its well-studied conjugate, the Iberia margin. Extended continental crust thins abruptly with few observed faults, in stark contrast with the system of seaward dipping normal faults and detachments imaged within continental crust off Iberia. Transition zone basement typically appears featureless in seismic reflection profiles, but where its character can be discerned, it does not resemble most images of exhumed peridotite off Iberia. Seismic observations allow three explanations for transitional basement: ( 1) slow spreading oceanic basement produced by unstable early seafloor spreading, ( 2) exhumed, serpentinized mantle with different properties from that off Iberia, and ( 3) thinned continental crust, likely emplaced by one or more detachment or rolling-hinge faults. Although we cannot definitively discriminate between these possibilities, seismic reflection profiles together with coincident wide-angle seismic refraction data tentatively suggest that the majority of transitional basement is thinned continental crust emplaced during the late stages of rifting. Finally, seismic profiles image abundant faults and significant basement topography in apparent oceanic basement. These observations, together with magnetic anomaly interpretations and the recovery of mantle peridotites at ODP Site 1277, appear to be best explained by the interplay of extension and magmatism during the transition from nonvolcanic rifting to a slow spreading oceanic accretion system.
Open Access
Heat-Flow, Depth, and Crustal Thickness of Marginal Basins of South Philippine Sea
(1976) SCLATER, JG; KARIG, D.; LAWVER, LA; LOUDEN, K.
No abstract available.
Open Access
Crustal structure of the Labrador Sea conjugate margin and implications for the formation of nonvolcanic continental margins
(1995-12) Chian, DP; Louden, KE; Reid, I.
Wide-angle seismic studies have determined the detailed velocity structure along a 350-km-long profile across the Labrador margin. Combination of this model with a previously published cross section for the southwestern Greenland margin constitutes the first combined conjugate margin study based on seismic velocity structure. The results indicate three distinct zones across the Labrador margin, similar to the structure of the conjugate Greenland margin. Zone 1 represents 27 to 30-km-thick continental crust thinning gradually seaward over similar to 100 km distance. Farther seaward, zone 2 is 70-80 km wide, characterized by a distinct lower crust, 4-5 km thick, in which velocity increases with depth from 6.4 to 7.7 km/s. Interpretation for this lower crustal block favors an origin by serpentinized peridotite rather than by magmatic underplating. Zone 3 represents two-layered, normal oceanic crust. The cross sections from both margins are reconstructed to an early drift stage at Chron 27. This demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of Chrons 31 and 33 on both margins. Zone 1 shows a marked asymmetry, with a gradual thinning of continental crust off Labrador contrasted with a rapid thinning off Greenland. The abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are both probably related to a very slow rate of continental rifting which produced little if any melt.
Open Access
Extinct spreading center in the Labrador Sea: Crustal structure from a two-dimensional seismic refraction velocity model
(1995-02/10) Osler, J. C.; Louden, K. E.
The Labrador Sea contains a rare example of an abandoned mid-ocean ridge where active accretion of oceanic crust ceased due to a change in the spreading geometry of lithospheric plates. Seismic refraction data were collected along a line which transversely crosses the extinct spreading center. Two-dimensional analyses of the refraction data, using ray-tracing and synthetic seismogram techniques reveal major variations in crustal thickness and velocity in relation to the axis of the extinct spreading center. In the extinct spreading center, a crustal thickness of approximately 4 km is determined, compared with 5.5 km for the flanks. Substantial lateral variations in P wave velocities of the upper and lower crust are observed with a marked decrease within the extinct spreading center. Low velocities are also observed in the uppermost mantle underlying the extinct spreading center and are interpreted as being the result of hydrothermal alteration. The anomalously low crustal velocities and crustal thinning are attributed to a decreasing supply of partial melt and increasing degree of tectonism at the slow spreading rates preceding extinction
Open Access
Paleomagnetism of Dsdp Sediments, Phase-Shifting of Magnetic-Anomalies, and Rotations of West Philippine Basin
(1977) LOUDEN, KE
No abstract available.
Open Access
Deep structure of the ocean-continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: Ocean Drilling Program (Legs 149 and 173) transect
(1999-04) Chian, DP; Louden, KE; Minshull, TA; Whitmarsh, RB
We present a wide-angle seismic refraction study of an 80x40 km region of the southern Iberia Abyssal Plain, south of Galicia Bank. An intersecting grid of two E-W and four NS wide-angle reflection/refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean-continent transition (OCT). These structures can be systematically linked to variations in acoustic basement morphology and to results from Ocean Drilling Program (ODP) boreholes. Lateral changes in the velocity structure of the basement occur abruptly over distances of similar to 20 km where complex variations may be found. Thinned upper continental crust, 2-5 km thick with velocities of 5.0-6.6 km/s, is limited to a series of N-S fault blocks immediately south of Galicia Bank. This crust is underlain by a high-velocity layer (7.3-7.9 km/s) of weakly serpentinized (i.e., 0-25%) peridotite, which exists throughout the eastern part of the survey area. Basement within the OCT appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally. In the southeast sector of our survey where basement topography deepens and becomes subdued, continental fault blocks are absent; instead, basement contains an upper layer of more pervasively serpentinized (i.e., 25-45%) peridotite that is similar to 2 km thick. This layer is characterized by low velocity at the top of basement (4.2 km/s) that increases rapidly with depth, and it probably corresponds to a seismically unreflective layer, previously identified in reflection profiles to the south of our survey. In the western section of our survey, beneath a series of elevated basement ridges, velocities are reduced within both the upper basement layer (3.5-6.0 km/s) and lower layer (6.4-7.5 km/s). These changes suggest that both upper and lower layers have become more highly serpentinized (with values of 60-100% in the upper layer and 25-45% in the lower layer) probably during the last stages of rifting and immediately before formation of oceanic crust, A normal or slow spreading oceanic crustal structure is not found within the survey region. Thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at ODP Site 897 and east of the J magnetic anomaly.
Open Access
Thermal Conduction Across Fracture Zones and Gravitational Edge Effect
(1976) LOUDEN, KE; FORSYTH, DW
No abstract available.
Open Access
Variations in heat flow across the Goban Spur and Galicia Bank continental margins
(1991) Louden, K. E.; Sibuet, J. -C; Foucher, J. -P
Presents the results of 44 new heat flow stations which were taken in 1984 and 1989 in profiles across the Goban Spur and Galicia Bank continental margins (NE Atlantic Ocean). Simple extensional models indicate that the heat flow across both these Early Cretaceous rifted margins should increase from values of 45-50 mW/m2 over oceanic crust to 65-80 mW/m2 on the continents. The rate of this increase should help to constrain the mechanism (simple versus pure shear) by which the upper, more radiogenic continental crust is thinned
Open Access
The continent-ocean crustal transition across the southwest Greenland margin
(1994-05/10) Chian, Deping; Louden, K. E.
The complete crustal transition across the nonvolcanic, southwest Greenland continental margin of the Labrador Sea is examined using wide-angle and coincident vertical-incidence seismic profiles. Six ocean bottom seismometers and a sonobuoy record P and S wave first and multiple arrivals from the crust and upper mantle, which are analyzed by 2D dynamic ray tracing and 1D reflectivity modeling. The resulting seismic velocity model requires that the preexisting 30-km thick continental crust is thinned abruptly to ~3 km across the continental slope, primarily by removal of the lower crust. Farther seaward, the crust thickens to ~6 km primarily through the addition of a high-velocity (7.0-7.6 km/s) layer in the lower crust. This lower crustal layer is 4-5 km thick, extends for a horizontal distance of ~80 km, and is interpreted as partially serpentinized upper mantle. It is overlain by a low-velocity (4.0-5.0 km/s), upper layer which is interpreted as highly fractured upper continental crust. The authors' model suggests that seafloor spreading did not start until chrons 27-28, 13 Ma younger than previously suggested. This interpretation is supported by 2D modeling of gravity and magnetic data along the refraction line. The results are consistent with a simple shear mechanism for the initial rifting, with the SW Greenland margin as the upper plate. However, a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin, east of Labrador
Open Access
Seismic study of the transform-rifted margin in Davis Strait between Baffin Island (Canada) and Greenland: What happens when a plume meets a transform
(2007-04) Funck, Thomas; Jackson, H. Ruth; Louden, Keith E.; Klingelhoefer, Frauke
[ 1] The Davis Strait transform margin was studied using a 630-km-long wide-angle reflection/ refraction seismic transect extending from SE Baffin Island to Greenland. Dense airgun shots were recorded by 28 ocean bottom seismometers deployed along the line. A P wave velocity model was developed from forward and inverse modeling of the wide-angle data and incorporation of coincident deep multichannel reflection seismic data. Off Baffin Island in the Saglek Basin, 7 to 11-km-thick two-layered continental crust (5.8 - 6.6 km/s) is observed. Off Greenland, continental crust is divided into three layers (5.4 - 6.8 km/s) with a maximum thickness of 20 km. Farther offshore Greenland the crust thins to 7 - 12 km and the lower crust disappears. Between the continental blocks a 140-km-wide zone with oceanic crust ( layer 2 is 5.4 - 6.2 km/s and layer 3 is 6.7 - 7.0 km/s) is located. The western half of this zone is interpreted to be part of a volcanic margin with seaward dipping reflectors; the eastern part is associated with the Ungava fault zone (UFZ), the major transform fault in Davis Strait. The UFZ thus acted as leaky transform fault during phases of transtension. Southward flow of material from the Iceland plume created a 4 to 8-km-thick underplated layer (7.4 km/s) beneath the thinned portions of the continental crust and beneath previously emplaced oceanic crust. Plume related Paleogene volcanism is indicated by an up to 4-km thick layer (4.3 - 5.8 km/s) with basalts and interbedded sediments that can be traced from SE Baffin Island 400 km toward the east.
Open Access
Heat flow in the Balearic and Tyrrhenian basins, western Mediterranean
(1985-01/10) Hutchison, I.; Von Herzen, R. P.; Louden, K. E.; Sclater, J. G.; Jemsek, J.
Presents the results of three detailed heat flow surveys which are used to investigate the variations of heat flow and age of the Balearic and Tyrrhenian basins in the western Mediterranean. The heat flow values are within the range predicted by simple plate cooling models for the Late Miocene ages of the deep Tyrrhenian basin. Thus the observations suggest that although the mode of crustal formation of these deep marginal basins is less well defined than that of the major ocean basins, the thermal signature is similar. Also, the trend of increasing heat flow from west to east through the Balearic and Tyrrhenian basins is discussed. In all three areas the measured flux shows significant local variability
Open Access
Crustal structure of the ocean-continent transition at Flemish Cap: Seismic refraction results
(2003-11) Funck, T.; Hopper, JR; Larsen, HC; Louden, KE; Tucholke, BE; Holbrook, WS
[1] We conducted a seismic refraction experiment across Flemish Cap and into the deep basin east of Newfoundland, Canada, and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots. The continental crust at Flemish Cap is 30 km thick and is divided into three layers with P wave velocities of 6.0-6.7 km/s. Across the southeast Flemish Cap margin, the continental crust thins over a 90-km-wide zone to only 1.2 km. The ocean-continent boundary is near the base of Flemish Cap and is marked by a fault between thinned continental crust and 3-km-thick crust with velocities of 4.7 - 7.0 km/s interpreted as crust from magma-starved oceanic accretion. This thin crust continues seaward for 55 km and thins locally to similar to 1.5 km. Below a sediment cover (1.9 - 3.1 km/s), oceanic layer 2 (4.7 - 4.9 km/s) is similar to 1.5 km thick, while layer 3 (6.9 km/s) seems to disappear in the thinnest segment of the oceanic crust. At the seawardmost end of the line the crust thickens to similar to 6 km. Mantle with velocities of 7.6 - 8.0 km/s underlies both the thin continental and thin oceanic crust in an 80-km-wide zone. A gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth. Normal mantle velocities of 8.0 km/s are observed similar to 6 km below basement. There are major differences compared to the conjugate Galicia Bank margin, which has a wide zone of extended continental crust, more faulting, and prominent detachment faults. Crust formed by seafloor spreading appears symmetric, however, with 30-km-wide zones of oceanic crust accreted on both margins beginning about 4.5 m.y. before formation of magnetic anomaly M0 (similar to 118 Ma).
Open Access
Three-dimensional structure of the Torngat Orogen (NE Canada) from active seismic tomography
(2000-10) Funck, T.; Louden, KE; Wardle, RJ; Hall, J.; Hobro, JW; Salisbury, MH; Muzzatti, AM
The crustal velocity structure and the Moho depth of the Proterozoic Torngat Orogen, NE Canada, is determined by active seismic tomography using travel times of crustal turning rays and Moho reflections. The orogen developed during oblique convergence of the Archean Superior and Nain Provinces, which trapped an interior belt of Archean crust (Core Zone) between them, with the Torngat Orogen evolving between the Core Zone and the Nain Province. Beneath the central orogen a crustal root is found with a preserved depth of >52 km and a width of similar to 100 km. To the north, the root shallows to
Open Access
Crustal structure of the northern Nova Scotia rifted continental margin (eastern Canada)
(2004-09) Funck, T.; Jackson, HR; Louden, KE; Dehler, SA; Wu, Y.
[ 1] The Nova Scotia continental margin off eastern Canada marks a transition from a volcanic to a nonvolcanic style of rifting. The northern ( nonvolcanic) segment of the margin was studied by a 490-km-long refraction seismic line with dense air gun shots, coincident with previous deep reflection profiles. A P wave velocity model was developed from forward and inverse modeling of the wide-angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles. The continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5.7 - 6.9 km/s. Crustal thinning down to 3 km occurs in a 180-km-wide zone and the sediment cover in this area is up to 15 km thick. Farther seaward, a 150-km-wide transition zone is observed with a 5-km-thick lower layer (7.2 - 7.6 km/s) interpreted as partially serpentinized mantle. At the landward end, this layer is overlain by highly altered continental crust (5.4 km/s) extending up to the seaward limit of the Jurassic salt province. Farther seaward, the upper layer is interpreted as exhumed and highly serpentinized mantle (5.1 km/s) separated from the lower layer by subhorizontal reflectivity, which probably represents a serpentinization front. Oceanic crustal thickness is 4 km with layer 2 velocities of 4.6 - 5.0 km/s. Layer 3 velocities of 6.4 - 6.55 km/s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile. This reduced magma production might be related to the proximity of the Newfoundland transform margin.
Open Access
Wide-angle seismic imaging of a Mesoproterozoic anorthosite complex: The Nain Plutonic Suite in Labrador, Canada
(2000-11) Funck, T.; Louden, KE; Reid, ID
The Mesoproterozoic Nain Plutonic Suite (NPS) of Labrador (Canada), one of the largest anorogenic plutonic terranes, was studied by a refraction/wide-angle seismic experiment. Four ocean bottom seismometers and 18 land stations were deployed along a 330-km profile and recorded air gun shots from the easternmost 160 km with the NPS located in the center of the line at the suture of the Nain and Churchill Provinces. P and S wave velocity models were developed by forward modeling of travel times and amplitudes. Upper and middle crustal P wave velocities outside and beneath the NPS range from 5.9 to 6.5 km/s, lower crustal P wave velocities range from 6.55 to 7.0 kmls. Within the anorthositic rocks, velocities are as high as 6.8 km/s, and reflections define the base of the NPS to be 8 km deep in the SE Churchill Province and 11 km in the Nain Province, a variation that may be the result of lateral density changes within the country rocks or the anorthosites. The total crustal thickness is 39 km west of the NPS but is only 32-34 km beneath the NPS, some 5 km less than Nain Province crust distal from the NPS. The inferred crustal thinning is possibly related to anatexis of the lowermost crust by a thermal plume that generated the plutonism. The Poisson's ratios are 0.275 within the anorthosite plutons, 0.27 in the upper and middle crust, and 0.285 in the lower crust. These values are some 0.03 higher than in the Archean Nain crust distal to the NPS, indicating a higher plagioclase content at all crustal levels as result of the plutonism. We postulate that a crustal root, similar to the root observed farther north in the Torngat Orogen, was completely removed by anatexis and the silicic and basic magmas probably ascended to midcrustal levels along preexisting zones of weakness at the Nain-Churchill boundary.
Open Access
Wide-angle seismic transect across the Torngat Orogen, northern Labrador: Evidence for a Proterozoic crustal root
(1999-04) Funck, T.; Louden, KE
A refraction/wide-angle reflection seismic transect across the Labrador peninsula covers the Core Zone of the SE Churchill Province, the Paleoproterozoic Torngat Orogen, and the Archean Nain Province including a portion of the Labrador continental margin. An airgun array was used as source, and 11 ocean-bottom seismometers and 16 land stations recorded the shots. Forward modeling of travel times and amplitudes reveals a deep asymmetric crustal root beneath the Torngat Orogen, with a crustal thickness of >49 km and with P-wave velocities of 6.9-7.0 km/s. The geometry of the velocity model and the root can be best explained by either westward dipping subduction or westward underthrusting of the Nain crust. Gravity modeling suggests a correlation of the crustal root with a gravity low that extends similar to 100 km in an E-W direction and similar to 200 km from north to south. The preservation of the crustal root is attributed to the lack of postorogenic heating and ductile reworking consistent with the lack of abundant postcollisional magmatism in the SE Churchill Province. A discontinuity possibly cutting through the entire crust is interpreted as a zone of major transcurrent shearing associated with the main phase of deformation. West of the Torngat Orogen, the crustal thickness in the Core Zone of the Churchill Province varies between 35 and 38 km (P-wave velocities of 5.8-7.0 km/s). East of the orogen, the crystalline crust in the Nain Province is similar to 38 km thick (velocities from 5.8 to 6.9 km/s) but thins to 9 km in the shelf area of the Labrador margin, where it is covered with up to 8 km of sediments. No high-velocity zone was found beneath the thinned continental crust at the margin.
Open Access
Deep structure of the ocean-continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: The IAM-9 transect at 40 degrees 20 ' N
(2000-03) Dean, SM; Minshull, TA; Whitmarsh, RB; Louden, KE
We present a crust and mantle velocity structure for the West Iberia passive continental margin derived from a 320-km-long wide-angle seismic profile acquired in the southern Iberia Abyssal Plain. We observe a 170-km-wide ocean-continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault-bounded blocks of continental crust. The profile lies similar to 40 km south of the transect sampled by Ocean Drilling Program (ODP) Legs 149 and 173. The transition zone structure can be divided into an upper layer, 2-4 km thick with velocities of between 4.5 and 7.0 km s(-1) and generally a high-velocity-gradient (1 s(-1)), and a lower layer up to 4 km thick with a velocity of similar to 7.6 km s(-1) and a low-velocity-gradient. A weak Moho reflection in this zone was seen only on wide-angle profiles at an offset of similar to 30 km. The upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope. Conversely, the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust. On the coincident seismic reflection profile, fault-bounded crustal blocks, identified in unequivocal extended continental crust, are not observed in the transition zone. The upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges, but no oceanic layer 3 velocity structure is present. While magnetic anomalies have been identified within the transition zone, they have not been modeled successfully as seafloor spreading magnetic anomalies, nor do they generally form long linear margin-parallel features. Finally, ODP boreholes, similar to 40 km north of our profile and within the interpreted transition zone, have recovered up to 140-m-thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material. We conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust. Although current melting models predict a considerably thicker crust of decompression melt products, we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite.
Open Access
Seismic velocity structure of the rifted margin of the eastern Grand Banks of Newfoundland, Canada
(2006-11) Van Avendonk, Harm J. A.; Holbrook, W. Steven; Nunes, Gregory T.; Shillington, Donna J.; Tucholke, Brian E.; Louden, Keith E.; Larsen, Hans Christian; Hopper, John R.
[ 1] We present a compressional seismic velocity profile of the crust of the eastern margin of the Grand Banks of Newfoundland, Canada. This velocity model was obtained by a tomographic inversion of wide-angle data recorded on a linear array of 24 ocean bottom seismometers (OBSs). At the landward side, we imaged a crustal thickness of 27 km in Flemish Pass and beneath Beothuk Knoll, which is thinner than the 35-km-thick crust of the central Grand Banks. We therefore assume that the eastern rim of the Grand Banks stretched uniformly by 25%. Farther seaward, the continental crust tapers rapidly beneath the continental slope to similar to 6 km thickness. In the distal margin we find a 60-km-wide zone with seismic velocities between 5.0 and 6.5 km s(-1) that thins to the southeast from 6 to 2 km, which we interpret as highly extended continental crust. Contrary to other seismic studies of the margins of the Grand Banks, we find seismic velocities of 8 km s(-1) and higher beneath this thin crustal layer in the continent-ocean transition. We conclude that mantle was locally emplaced at shallow levels without significant hydration from seawater or serpentinized mantle was removed along a decollement in the final stages of continental rifting. The outer edge of highly extended continental crust borders a 25-km-wide zone where seismic velocities increase gradually from 6.3 km s(-1) just below the top of acoustic basement to 7.7 km s(-1) at 5 km below basement. We interpret this area as a relatively narrow zone of exhumed and serpentinized continental mantle. Seaward, we imaged a thin and laterally heterogeneous layer with a seismic velocity that increases sharply from 5.0 km s(-1) in basement ridges to 7.0 km s(-1) at its base, overlying mantle velocities between 7.8 and 8.2 km s(-1). We interpret this area as unroofed mantle and very thin oceanic crust that formed at an incipient, magma-starved, ultraslow spreading ridge. A comparison of the conjugate rifted margins of the eastern Grand Banks and the Iberia Abyssal Plain show that they exhibit a similar seaward progression from continental crust to mantle to oceanic crust. This indicates that before continental breakup, rifting exhumed progressively deeper sections of the continental lithosphere on both conjugate margins. A comparison between the continent-ocean transition of the Grand Banks and Flemish Cap shows that the final phase of continental rifting and the formation of the first oceanic crust required more time at the Grand Banks margin than at the southeastern margin of Flemish Cap.

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