What Does Subduction Continually Renew the Ocean Floor

Sea Floor Spreading

Sea water near sea floor spreading zones where there is active interchange between hot, forming ocean crust and sea water can be locally altered (van Dover, 2000).

From: Dynamic Aquaria (Third Edition) , 2007

Foreword

In Investigating Seafloors and Oceans, 2017

Seafloor spreading centers proved to be particularly important in nourishing the seafloor with a wealth of minerals directly; and several interesting life forms indirectly. More than 220 active vent sites have been identified along the ~ 58,000 km of global mid-ocean ridge crests, over half of them along spreading ridges in the eastern Pacific Ocean. Hydrothermal venting along the global mid-ocean ridge system plays a major role in cycling elements and energy between the interior and surface of the Earth. Vents are found along zones of tectonic or volcanic activity such as mid-ocean ridges, formed by the Earth's major tectonic plates, where hot magma is near the seafloor.

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HISTORY OF GEOLOGY SINCE 1962

U.B. Marvin , in Encyclopedia of Geology, 2005

The Eltanin Profile: 1966

Sea-floor spreading got off to a slow start partly because neither the Vine–Mathews nor the Mason–Raff papers showed obvious symmetry of linear magnetic anomalies across oceanic ridges. Early in 1966, James Heirtzler, Xavier Le Pichon, and J G Baron at Lamont published the results of an aeromagnetic survey over the Reykjanes Ridge, south of Iceland, which yielded a beautifully symmetrical pattern of magnetic stripes across a spreading ridge. They computed the spreading rate at slightly less than 2 cm per year of separation.

In December 1966, Walter Pitman III, and Heirtzler, at Lamont published the magnetic profiles recorded by the research vessel, Eltanin, during four passes over the Pacific–Antarctic Ridge, south of Easter Island. All four profiles gave similar results but the most southerly one, Eltanin-19, was so spectacularly symmetrical, both in the topography of the ridge and its record of magnetic reversals that it seems to have prompted mass conversions to sea-floor spreading, first at Lamont and then elsewhere. The profile showed each of the dated magnetic epochs of the past 3.4 my, for which it yielded a computed spreading rate of 9 cm per year of separation. Assuming a constant spreading rate within 500 km on either side of the ridge, the profile made it possible to extend the series of geomagnetic reversals back to 10 my. Pitman and Heirtzler documented a good match between the anomalies in the South Pacific and those on the Reykjanes Ridge, adjusted for the slower spreading rate in the Atlantic. Today the Eltanin-19 profile is ranked as one of the most important pieces of evidence in the history of geophysics. And further confirmation was at hand. Approaching the problem by an independent method, Neil Opdyke and his colleagues at Lamont, plotted the magnetic polarities of fossiliferous strata from deepsea cores of the South Pacific floor. Early in 1966, they found a definitive match with the dated record in the basaltic oceanic bedrock.

In February 1966, Vine visited Lamont where Heirtzler gave him a copy of the Eltanin-19 profile. Vine incorporated it into his paper 'Spreading of the Ocean Floor: New Evidence', which appeared in Science the following December, shortly after one by Pitmann and Heirtzler. In it, Vine presented six symmetrical profiles of magnetic anomalies across ridges in the Atlantic, Indian, and Pacific Oceans, and showed that the linear anomalies on the East Pacific Rise match those on the Juan de Fuca Ridge offshore from British Columbia, even though they lie 11 000 km apart. His computed spreading rates for all the ridges ranged from about 2.0 to 3.0 cm of separation per year in the Atlantic and Indian Oceans to 8.8 cm per year across the East Pacific Rise. Vine speculated that the whole history of the ocean basins in terms of ocean-floor spreading must be 'frozen-in' as paired magnetic anomalies in the oceanic crust. Meanwhile, Vine had given a summary of his results in November at the annual meeting of the Geological Society of America, where it startled many geologists with their first serious introduction to sea-floor spreading.

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The Oceans and Marine Geochemistry

C.R. German , W.E. SeyfriedJr., in Treatise on Geochemistry (Second Edition), 2014

8.7.2.4.3 Back-arc systems

Seafloor spreading in back-arc basins contrasts significantly with that along mid-ocean ridges, characterized by very distinct manifestations of hydrothermal activity imposed by a unique combination of mantle dynamics and crustal structure and composition (Baker et al., 2006; Ferrini et al., 2008; Martinez et al., 2006). Thus, it is not surprising that the composition of hydrothermal fluids in the vicinity of the well-studied Lau Basin offers an amazing degree of diversity of trace and major elements from one location to another. Mottl et al. (2011) reported on the composition of vent fluids from six hydrothermal fields along the Eastern Lau Spreading Center (ELSC). The chemistry of the waters reveals enormous variability both within and between fields in response to the wide range of chemical and physical factors that affect the origin and evolution of the fluids, not the least of which is the substrate composition that varies along axis as a function of magmatic processes and input of subducted sediment. Hot spring temperatures range from 229 to 363 °C and show temporal variability where comparisons can be made. Chloride variability, which is likely caused by phase separation effects, has a significant influence on vent chemistry as it does in virtually all other submarine hydrothermal systems due to the effect of chloride complexing on mass-transfer reactions. Although pH for most ELSC hot springs falls in the range observed for all other high-temperature systems, suggesting pH buffering by silicate assemblages at depth, magmatic degassing in back-arc systems is evidenced by unusually low pH for the Mariner and Vai Lili fields when sampled in 2004 and 1989, respectively, much lower than the value of 5.2 at Vai Lili when sampled in 2005 ( Figure 13 ). The combination of low pH and briny fluids at Mariner gives rise to some of the highest heavy metal concentrations ( Figure 13 ). Dissolved H₂S concentrations tend to correlate with dissolved Fe, suggesting phase equilibria control by pyrite in the subsurface, although this would need to be confirmed by a study of redox effects, particularly the determination of dissolved H₂ concentrations.

A striking feature of the chemical composition of ELSC vent fluids is the strong correlation of mobile trace elements with substrate composition. The dissolved concentrations of K, Rb, Cs, and B in vent fluids increase systematically from north to south, consistent with the higher abundance of these species in more slab-influenced felsic rocks as the ridge-arc separation diminishes. Although complex variations in substrate composition and magmatic degassing that are intrinsic to back-arc spreading systems contribute to the systematic variations observed in vent-fluid chemistry, pH buffering by silicate mineral hydrolysis reactions and phase separation are common to mid-ocean ridge hydrothermal systems in general, underscoring some degree of commonality in process in spite of the well-recognized compositional variability.

The chemical and isotopic composition of hydrothermal fluids sampled from vents along the Manus Spreading Center (MSC) and Pual Ridge (PR) in the Manus back-arc basin also reveal unusual diversity owing to the complex interplay of substrate variability, geographical location relative to the New Britain subduction zone, phase separation, conductive cooling, subsurface entrainment of seawater along the fluid flow path, and inputs of acidic magmatic vapor (Craddock and Bach, 2010; Craddock et al., 2010; Reeves et al., 2011). Substrate variability (mafic at the Vienna Woods site on the MSC and felsic for vents at PR) is clearly reflected by the absolute abundances and differing ratios of soluble alkali elements and boron (Reeves et al., 2011), similar to the vent-fluid chemistry–substrate linkage recognized for the ELSC discussed above. Moreover, the Manus Basin study unambiguously demonstrated the profound effect of magmatic acid volatiles on hydrothermal alteration for most or all of the vent fluids in this back-arc system. Evidence of this effect can be found in the relatively high transition metal concentrations and unusually low pH_(25°C) values, as well as the ubiquitous negative δD_H₂O, especially for vent fluids at PR. Although magmatic SO₂ likely plays a key role in lowering pH, addition of other acid volatiles such as HF contributes as well. Craddock et al. (2010) showed that REE can be used as an indicator of the type of magmatic acid volatile (HCl, HF, and SO₂) degassing into the Manus Basin hydrothermal systems, providing a new tool to unravel the sources and sinks of components involved in the temporal and spatial evolution of magmatically and tectonically complex back-arc hydrothermal systems. Vent fluids from magmatic–hydrothermal systems hosted in felsic crust have historically been underrepresented, accentuating the importance of the chemical and isotopic data recently obtained for hydrothermal systems in the western Pacific.

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Paleomagnetism

In International Geophysics, 2000

5.1.1 Sea-Floor Spreading and Plate Tectonics

The sea-floor spreading hypothesis was first formulated by Hess (1960, 1962). The mid-ocean ridges, which are characterized by unusually high heat flow along their crests, are the largest topographic features on the surface of the Earth. On Hess's model the mid-ocean ridges are interpreted as representing the rising limbs of mantle convection where hot magma comes right through to the surface and new oceanic crust is formed as the magma cools. It was originally thought that the intrusion of new material forced the cooling crust to move away from the ridge symmetrically on either side. It is now generally believed that the crust and part of the upper mantle are under tension at a spreading center. Thus, the oceanic crust is pulled apart, allowing magma to rise to the surface and the whole oceanic crust is part of a conveyor belt system, rising up at the mid-ocean ridges and eventually sinking down at the oceanic trenches. The spreading rate across a mid-ocean ridge is defined as the relative rate of separation of the plates on either side of the ridge, sometimes referred to as the "full rate". Consequently, the spreading rate on one side of a ridge is often referred to as the "half rate". Values are usually quoted in km Myr⁻¹ (mm yr⁻¹).

The theory of plate tectonics was formulated by McKenzie and Parker (1967), Morgan (1968) and Le Pichon (1968). The plate tectonics model is now accepted as the first-order explanation of global tectonics. To a first approximation it is possible to divide the Earth's surface into several essentially aseismic plates or blocks bounded by seismicity associated with active ridge crests, faults, trenches, and mountain systems. The plates can be composed entirely of continental crust, or of oceanic crust, or of a combination of both. Figure 5.1 shows the surface of the Earth divided into 17 plates (see DeMets et al., 1990, 1994) together with two microplates. There does not appear to be any formal definition of a microplate. Originally, Le Pichon (1968) proposed there were six major aseismic plates, but since that time it has been found that relative motion exists between North and South America and between India and Australia (Wiens et al., 1985) and that there are several smaller plates. Other minor plates not shown in Fig. 5.1 are Nubia (west Africa) and Somalia (east Africa).

The sea-floor spreading hypothesis maintains that these plates are in constant motion and that seismic boundaries between them delineate zones where oceanic crust is created or destroyed, continental crust extended or compressed, and crustal plates are translated laterally along faults without change in their surface area. Some of the principal points of the plate tectonics model are illustrated in Fig. 5.2. Three flat-lying layers are distinguished. The lithosphere, which generally includes the crust and upper mantle, has significant strength and is of the order of 100 km in thickness. The asthenosphere, which is a layer of effectively no strength on the appropriate time scale, extends from the base of the lithosphere to a depth of several hundred kilometres. The mesosphere, which may have strength, makes up the lower remaining portion of the mantle and is relatively passive, perhaps inert, in tectonic processes.

The lithosphere as defined above corresponds with the seismic lithosphere, a region of high seismic velocity at the top of the mantle overlying a low-velocity zone. The bottom of the seismic lithosphere is characterized by an abrupt decrease in shear-wave velocities at depths of 150–200 km under the continents and 10–50 km under the ocean floor depending on age (Regan and Anderson, 1984). This corresponds roughly to the 600°C isotherm below the ocean floor, which approximates the effective elastic plate thickness (i.e., the thickness that reacts to loads and deformation as an elastic sheet). The asthenosphere is often equated with the seismic low-velocity zone that arises from the increase of temperature with depth. It should be noted that the thermal lithosphere has been defined as having its lower boundary as the depth to a constant isotherm, usually modeled to be in the range 1250–1350°C (McKenzie and Bickle, 1988). Depending on its age, this corresponds to a depth of 10–125 km below ocean floor or, in the case of continental lithosphere, to a thickness of 100–200 km. Old cratonic lithosphere may be up to 400 km in thickness (Jordan, 1975). The boundary between the lithosphere and the asthenosphere is a transition zone referred to as the lower thermal boundary layer. However, the lowermost part of the thermal lithosphere may deform in a ductile fashion over time and thus is not really part of the tectonic plate that moves as a mechanical unit in plate tectonics.

The movement of these plates on the surface of a sphere is best understood in terms of rotations by applying Euler's theorem. If one of two plates is taken to be fixed, the movement of the other plate corresponds to a rotation about some pole (Fig. 5.3) with angular velocity ω. Mathematically, ω is a vector pointing outwards along the rotation axis and is reckoned as positive when the rotation is clockwise looking outwards from the center of the sphere. However, it is commonly found more convenient to visualize these rotations as viewed from the surface of the Earth looking toward the center. The convention has therefore developed to regard rotations as being "clockwise" or "counterclockwise" when viewed from the surface of the Earth, corresponding mathematically to negative and positive angular rotations, respectively. If a is the radius of the Earth (Fig. 5.4) and the angular distance from a point S on one plate to the pole is φ, then the magnitude v _φ of the relative velocity at that point is given by

(5.1.1) $v_{φ} = a ω sin φ .$

The relative velocity thus has a maximum at the "equator" and vanishes at the pole of rotation. The relative velocity vectors must lie along small circles or "latitudes" with respect to the pole, which has no significance other than being a construction point. When several plates are in relative motion, as at the present time (Fig. 5.1), then it is possible to use the property of angular velocities that they behave like vectors. Around any closed circuit crossing several plates, A, B, C, D, etc., the sum of the angular velocities must be zero, that is,

(5.1.2) $_{A} ω_{B} +_{B} ω_{C} +_{C} ω_{D} +_{D} ω_{A} = 0,$

where _A ^ω _B is the angular velocity of the rotation that describes the magnitude and direction of the relative motion between plates A and B. The sense of the rotation is found by moving from plate A to plate B and so on. There are many points on the Earth's surface where three plates meet, called triple junctions. Triple junctions can arise from any combination of the three types of plate boundaries (ridges, trenches, transform faults) meeting at a point. Their stability conditions and evolution have been discussed by McKenzie and Morgan (1969).

The Euler poles of rotation and angular velocity vectors describing the average motion between the plates over the past few million years can be determined by combining ridge spreading rates, transform fault azimuths, and earthquake slip vectors determined from fault plane solutions. Since the original global calculations of Le Pichon (1968) and Morgan (1968), successive new global plate motion models have been constructed by Chase (1978), Minster and Jordan (1978), and DeMets et al. (1990, 1994) as new high-quality data have become available. In addition, current plate motions can be measured at the millimetre per year level using various geodetic techniques such as very long baseline interferometry (VLBI), satellite laser ranging (SLR), and global positioning system (GPS) interferometry. Global analyses using VLBI and SLR at 20 sites on five plates (Australia, Eurasia, Nazca, North America, and Pacific) show excellent agreement between the geodetically determined plate motions over the past two decades and those measured geophysically on the 0–2 Myr time scale (DeMets, 1995). The current instantaneous poles of rotation describing the motion between those pairs of plates that are mainly separated by mid-ocean ridges (Fig. 5.1) are summarized in Table 5.1 from the NUVEL-1A model of DeMets et al. (1994). The "equatorial spreading rate" is that which would occur when φ = 90°. For most ridges (e.g., the Cocos–Pacific ridge) φ never approaches 90° and the fastest actual spreading rate today is about 150 km Myr⁻¹ along the East Pacific Rise separating the Nazca and Pacific plates.

Table 5.1. Instantaneous Poles of Rotation and Magnitudes (ω) of Angular Velocities Describing the Present Motion Between Pairs of Plates (Fig. 5.1) ^a

Plate Pairs	Pole Lat. (°N)	Pole Long. (°E)	ω (°Myr⁻¹)	Equatorial spreading rate (km Myr⁻¹)
North America – Pacific	48.7	−78.2	−0.75	−83.5
Cocos – Pacific	36.8	−108.6	−2.00	−222.6
Nazca – Pacific	55.6	−90.1	−1.36	−151.4
Antarctica – Pacific	64.3	−84.0	−0.87	−96.8
North America – Eurasia	62.4	135.8	0.21	23.4
North America – Africa	78.8	38.3	0.24	26.7
South America – Africa	62.5	−39.4	0.31	34.5
Antarctica – Australia	13.2	38.2	0.65	72.3
Antarctica – Africa	5.6	−39.2	0.13	14.5
Australia – India	−5.6	77.1	−0.30	−33.4
Africa – India	23.6	28.5	0.41	45.6
Africa – Arabia	24.1	24.0	0.40	44.5

Note. The second plate moves clockwise relative to the first plate.

^a: From DeMets et al. (1994).

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Volume 3

Richard Hey , in Encyclopedia of Geology (Second Edition), 2021

Subaerial Examples: Iceland and Afar

The global seafloor spreading system is exposed subaerially in only two places, Iceland and Afar. In both of these areas structural geologists have identified evidence for rift propagation and microplate tectonics, although these areas are considerably more complicated than the oceanic examples.

Iceland is a major melting anomaly and geoid high along the Mid-Atlantic Ridge, often interpreted as a hotspot/mantle plume. The plate boundary zone on Iceland is a complex array of structures bounding areas similar to oceanic microplates. Major rift zones are propagating north and south from the hotspot centered in southeast Iceland, resulting in the progressive abandonment of the Western Rift Zone (Fig. 8). The southern propagator has extended past the South Iceland Seismic Zone, a non-transform boundary characterized by bookshelf faulting, to Surtsey. The northern propagator is linked to the southern end of the receding Kolbeinsey Ridge by the Tjörnes transform zone. This propagator appears to be extending north in incremental steps, leaving a wake of crustal deformation and abandoned Tjörnes fracture zone strands. The microplate-like rotations west of these propagators are clockwise north of the hotspot and anti-clockwise to the south, with the resulting north-south divergence providing a plausible explanation for the Central Rift Zone.

There is also evidence for oceanic rift propagation near Iceland on both the Reykjanes and Kolbeinsey Ridges. Although propagators explain most V-shaped patterns flanking mid-ocean ridges, it was long thought that the most famous such pattern, the V-shaped ridges, troughs and scarps (VSRs) flanking the Reykjanes Ridge south of Iceland, could not be propagator wakes. This was because the VSRs had been thought to be symmetric about the ridge axis, whereas propagators must produce asymmetric wakes because of the lithospheric transfer inherent in rift propagation. However, recent work has shown the VSRs actually have the characteristic asymmetric geometry produced by lithospheric transfer, and detailed magnetic anomaly modeling suggests that there has been extremely rapid small-offset rift propagation within the plate boundary zone both toward and away from Iceland. The dominant propagation both offshore and onshore is away from the Iceland hotspot, consistent with the pattern near other hotspots.

Complex continental rift propagation is exposed in the Afar depression, where the propagator forming the Gulf of Aden is presently breaking west into the African continent in Djibouti, while a Red Sea propagator is simultaneously breaking south into the continent in Eritrea and Ethiopia (Fig. 9). The zone between these dueling propagators is rotating clockwise as a microplate and deforming by pervasive bookshelf faulting similar to that observed in areas of oceanic rift propagation and the cores of oceanic microplates. The southward propagation of the Red Sea propagator and the westward propagation of the Gulf of Aden propagator are toward the presumed Afar hotspot location, an observation for which there is still no compelling explanation.

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Seismology and the Structure of the Earth

W.D. Mooney , in Treatise on Geophysics, 2007

1.11.5.2 Mid-Ocean Ridges

The concept of seafloor spreading from mid-ocean ridges ( Figure 2 ) was first proposed in the early 1960s by several workers, including most prominently the American geologist Harry H. Hess (Hess, 1965). Its major tenets gave great support to the theory of continental drift and provided a conceptual base for the development of plate tectonics.

Mid-Ocean ridges can be separated into three categories: fast spreading, intermediate spreading, and slow spreading. Fast-spreading ridges have a spreading rate of 8–16 cm yr⁻¹; intermediate- spreading ridges have a spreading rate of 4–8 cm yr⁻¹; and slow-spreading ridges have a spreading rate of 1–4 cm yr⁻¹ (Perfit and Chadwick, 1998).

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Deep Earth Seismology

W.D. Mooney , in Treatise on Geophysics (Second Edition), 2015

1.11.5.2 Mid-Ocean Ridges

Mid-ocean ridges can be separated into three categories: fast spreading, intermediate spreading, and slow spreading. Fast-spreading ridges have a spreading rate of 8–16 cm year^{− 1}, with eruptions of 1–5 million m³ of magma occurring every 5 years. Meanwhile, intermediate-spreading ridges have a spreading rate of 4–8 cm year^{− 1}, with 5–50 million m³ of magma erupting every 500 years. Slow-spreading ridges erupt 50–1000 m³ of magma every 5000 years, yielding a spreading rate of 1–4 cm year^{− 1} (Perfit and Chadwick, 1998).

The seismic structure of a fast-spreading ridge shows that the intrusive zone is only 2–3 km wide, and normal oceanic crust is found 5–6 km away from the ridge axis ( Figure 13 ). Directly beneath the ridge axis, an upper crustal low-velocity zone exists that corresponds to a zone of partial melting. This seismic structure is in contrast to the earlier hypothesis that anomalous oceanic crust extends for tens of kilometers away from the axis of a mid-ocean ridge. The seismic structure of the fast-spreading East Pacific Rise has also been examined using seismic P-wave tomography (Dunn et al., 2000). The spreading ridge is characterized by a seismic low-velocity volume that is 5–7 km wide within the crust and three times broader (c. 18 km) in the mantle. Magma accumulates at two levels: at a shallow crustal depth (the top of the magmatic system) and at the Moho transition zone (Dunn et al., 2000). Measurements of seismic anisotropy within the shallow mantle beneath the ridge indicate a mismatch between the locus of mantle melt delivery and the morphological ridge axis. The geometry of this mismatch governs the segmentation of the ridge and controls the intensity of ridge crest processes (Toomey et al., 2007).

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Labrador Sea, Davis Strait, and Baffin Bay

James A. Chalmers , in Regional Geology and Tectonics: Phanerozoic Passive Margins, Cratonic Basins and Global Tectonic Maps, 2012

Early to middle Eocene

The commencement of seafloor spreading in the northern North Atlantic at the beginning of the Eocene changed spreading from a two-plate system (North America and Greenland + Europe) to a three plate system in which Greenland moved away from both North America to the west and Europe to its southeast. This could be achieved only by Greenland moving northward and that meant a change in spreading direction in the Labrador Sea from SW–NE to N–S. Strike-slip motion along the Ungava system also changed and one of the obvious consequences was the change in the Ikermiut Ridge area from extension during the Paleocene to compression during the Eocene (Dalhoff et al., 2003). Chalmers and Pulvertaft (2001) interpreted this compression as resulting from a left-lateral step-over on a sinistral strike-slip fault system. Folding in this compressional system lasted until the middle Eocene and ceased at the same time as spreading in the Labrador Sea slowed after Chron 21 (ca. 49 Ma).

This change in spreading direction changed movement between Greenland and Ellesmere Island from being predominantly strike-slip along northeastern Nares Strait during the Paleocene to entirely compressional during the Eocene, resulting in the Eurekan orogeny. Total shortening appears to be of the order of 100–150 km (De Paor et al., 1989). Compressional movements affected the basins around northern Baffin Bay, probably at the same time, and there may have been an episode of northward-directed strike-slip movement along the western margin of the Cary Basin.

Tectonism along the Ikermiut Fault Zone, the western margin of the Sisimiut Basin, changed from extensional to transpressional and a flower structure developed while it otherwise continued in a similar pattern to that during the late Paleocene. The Sisimiut delta continued to prograde from the north, and smaller deltas built out over the Nukik Platform from the east while deposition in the basin center was fine-grained (Dalhoff et al., 2003). Renewed volcanism affected western Nuussuaq (Storey et al., 1998), and the Palaeogene basalts west of Disko and Nuussuaq appear to have remained sub-aerial during the early part of the Eocene and were finally transgressed shortly before the formation of the mid-Eocene Unconformity. Analysis of apatite fission-track and vitrinite reflectance data suggest that this transgression covered western Nuussuaq, but did not reach as far as inner Disko Bay (Japsen et al., 2005).

In the Labrador Basins, the Paleocene Cartwright Formation was succeeded and overstepped by the Eocene Kanamu Formation, but sedimentation continued in a similar pattern of fine-grained clastic sediments in the basin centers and coarser-grained sedimentation around the margins.

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Extensional tectonics and stratigraphy of the Mesozoic Jeanne d'Arc basin, Grand Banks of Newfoundland

Herman Welsink , Anthony Tankard , in Regional Geology and Tectonics: Phanerozoic Rift Systems and Sedimentary Basins, 2012

NE-oriented Orphan basin extension

By Aptian time, sea-floor spreading had jumped from the southern Newfoundland basin to the edge of the Flemish Cap, thus initiating the final phase of extension between the Orphan basin and Goban Spur (Figs. 14.1 and 14.2). Orphan basin is a 450-km-wide tract of rift basins, typically with only 3–4 km (2 s) of structural relief. Lithoprobe deep seismic line 84–3 images the base of the crust at 11.5 s. Basin-forming normal faults form a crustal-scale listric fan which merges at 15–17 km (9 s) depth with an intracrustal décollement (Keen et al., 1987a; Tankard and Welsink, 1989). Seismic refraction data indicate that this is not a velocity boundary or basement strength anisotropy. We measure about 50% extension by cross-section balancing. The age of this rifting episode is mid-Aptian to late Cenomanian. The Jeanne d'Arc basin also participated in this phase of NE-directed extension, expressed in the asymmetric synrift geometries between the Aptian unconformity and the end-Cenomanian Petrel limestone (Figs. 14.1 and 14.3; Sinclair, 1995; Tankard and Welsink, 1987). Before the Aptian, the Dominion transfer had partitioned strain between the Grand Banks and Orphan provinces, but in this new extensional phase both regions were jointly affected.

After the mid-Aptian, extensional stresses were aligned approximately with the axis of the Jeanne d'Arc basin, thereby imposing a dip-slip sense of displacement on the cross-basin transfer faults, including the Dominion structure (Tankard et al., 1989). NE-directed extension resulted in domino-style rotation of these basement blocks and northward deepening of the basin floor across a series of fault-block steps (Fig. 14.7). The depth to the top of these tilted fault blocks in the Jeanne d'Arc basin coincides with the level of Orphan basin detachment, 18 km and 15–17 km, respectively. We suggest that this array of domino-tilted blocks continued beneath the Orphan intracrustal detachment; line 85–3 is of variable quality and cannot resolve the deep structure. These various seismic structural interpretations together have intriguing consequences. First, Early Cretaceous SE-directed extension was accommodated by an intracrustal detachment at ~26 km beneath the Jeanne d'Arc basin (Fig. 14.5). Second, NE-directed extension of the Orphan basin in the mid-Cretaceous used a detachment at 15–17-km depth, and likely involved tilt-block rotation of the sub-detachment lithosphere as well. This arrangement suggests that the upper plate of the Grand Banks–Iberia extension became the lower plate, possibly an extensional wedge, of the Orphan basin extension.

Substantial basin deepening is attributed to lower crust and mantle flow. Modeled subsidence curves (Keen et al., 1987a) predict that subcrustal extension (δ) is everywhere greater than extension of the brittle crust (β). This would explain the northward plunge of the Jeanne d'Arc basin. The small amount of rotation of the basement blocks (<5% extension) is not enough to explain the observed over-deepening. A large positive gravity anomaly that forms an arcuate rim along the inboard edge of the Orphan basin is not associated with topography in the upper crust, and is attributed to flexure at the crust-mantle level (along latitude 48° in Fig. 14.4A; Welsink et al., 1989a). Regional subsidence and basin over-deepening continued until separation from the European continental plate in the Santonian (84 Ma chron 34; Srivastava et al., 1988) and opening of the Bay of Biscay.

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International Handbook of Earthquake and Engineering Seismology, Part A

Seiya Uyeda , in International Geophysics, 2002

3.2 Supporting Evidence

With the appearance of the sea-floor spreading hypothesis, solid earth science of the late 1960s was abuzz with discussion of it. Findings from marine geology and geophysics were generally consistent with the hypothesis, but some reported that their findings were not.

Oceanic ridges and trenches are in contrast with regard to many features. The former sit in the middle of the ocean, while the latter are found on the margins. The seismic activity (see Chapter 7 by Stein and Klosko) directly below oceanic ridge axis is limited to small magnitude (M < 6) shallow (d < 10 km) earthquakes, apparently caused by the tensile stress associated with sea-floor spreading. In contrast, earthquakes at trench areas include shallow low-angle thrust types (including the M > 8 class great earthquakes) and deep-focus earthquakes, associated with the subduction of the sea floor. The contrast in heat flow (see Chapter 81.4 by Cermak), which is anomalously high over the mid-oceanic ridges, and low in the trench areas, is also supportive of the hypothesis.

By far the most decisive support of sea-floor spreading was provided by the geomagnetic anomalies of the sea floor. The development of electronic devices during the 1950s made it remarkably easy to make continuous and accurate measurements of the total geomagnetic force at sea. Armed with new tools, scientists, mainly from the Scripps Institution of Oceanography of the University of California, led by V. Vacquier, began extensive surface ship geomagnetic surveys over the eastern Pacific, and made a startling discovery. As demonstrated by Raff and Mason (1961), for instance, there are striking zebrastriped patterns of geomagnetic anomalies, with widths of 20–30 km and lengths of several hundred kilometers in the northeastern Pacific off the coast of North America. And that was not all. Here and there across topographic discontinuities called fracture zones, the stripes showed displacements as great as many tens of kilometers. Such strange geomagnetic anomalies have never been seen on land. Vine and Matthews (1963) provided an intriguing answer to the question of the origin of these striped patterns. If the sea floor spreads, the newly produced sea floor would be magnetized parallel to the geomagnetic field of the time. If at the same time the geomagnetic field keeps reversing its polarity, the sea floor would inevitably be magnetized with positive and negative stripes. This model, which can be likened to a magnetic tape recorder, has proven to provide the most powerful time frame for plate tectonics and today leaves little room for doubt. But it was regarded as extreme and too speculative when it was first proposed. To be exact, it would better be called the Vine–Matthews–Morley Hypothesis, including L.W. Morley from Canada, who submitted a paper with almost identical content to scientific journals independently, but was unable to get it published. As this episode tells, the tape recorder model was initially not taken seriously by the general geophysical community.

However, there was a real breakthrough at about 1965 (Vine, 1968). Assuming that the crust is magnetized alternately in stripes, the magnetic field on the ocean surface can be calculated. In early 1965, Pitman obtained a high-quality magnetic profile, the famous Eltanin-19 profile, over the East Pacific Rise south of Easter Island. It was then demonstrated that the pattern of the geomagnetic intensity anomalies on either side of the Rise agreed perfectly with the model based on the geomagnetic reversal history that had been obtained from the paleomagnetic studies of rocks on land (Pitman and Heirtzler, 1966). Vine and Wilson (1965) demonstrated the same for the Juan de Fuca Ridge off Vancouver Island. For instance, Figure 4 shows the remarkable agreement between calculated and observed geomagnetic field anomaly profiles of the Pacific–Antarctic Ridge. In the meantime, the geomagnetic reversal history was pursued in a different way (e.g., Opdyke, 1972). The history of the geomagnetic field for several million years could be continuously analyzed by looking at the magnetization of some 10 m of core samples from sea floor—since the sedimentation rate on the deep sea floor is extremely slow. Their results also quantitatively agreed with the reversal history based on volcanic rocks on land. Thus, identical results were obtained from three independent sources—volcanic rocks on land, deep-sea sediments, and the marine magnetic anomalies. This "trinity" was irrefutable evidence not only of geomagnetic reversals but also of sea-floor spreading.

By the time the Vine–Matthews–Morley hypothesis was established for the past four or so million years, attempts to extend this hypothesis to older sea floors were already underway by Heirtzler and others of the Lamont-Doherty Geological Observatory of Columbia University. Such research was possible only at that institution, thanks to the enormous amount of surface ship geomagnetic data that had been amassed under the leadership of Maurice Ewing. They analyzed the long and numerous geomagnetic profiles for each of the world oceans and, by 1968, had demonstrated that magnetic stripe patterns in different oceans are correlated as far back to some 80 My old sea floor (Heirtzler et al., 1968).

As each stripe represents sea floor produced at the same time, it is called an isochron or chron. Some prominent isochrons were given numbers. The problem they had was that the ages of isochrons were not known for the period older than 4.5 My. Heirtzler and his colleagues, however, extrapolated the age by assuming that the sea-floor spreading rate of the South Atlantic was constant. Their assumption was proved to be correct by the DSDP (Deep Sea Drilling Project) in 1968 (Maxwell and von Herzen, 1969). In this way, a magnetic lineation map consisting of isochrons was translated into a map showing the age of the ocean floor. Thus, by about 1974, the age of almost the entire world's ocean floor was determined (Fig. 5; Pitman et al., 1974).

There are several features to be noted in relation to Figure 5. (1) The oldest sea floor of Jurassic age lies in an area east of the Mariana Arc in the western Pacific. The present Pacific Plate was born there about 190 Ma (Hilde et al., 1977). Of course, this does not mean that the Pacific Ocean at that time was zero in area. On the contrary, the Ocean was much larger but was occupied by other, now subducted, plates. As the present Pacific Plate grew, the spreading ridge (East Pacific Ridge, EPR) migrated eastward and the sea floor produced in the eastern side of the ridge was continuously subducted under North America. Finally, the ridge itself collided with North America and was subducted. The collision of ridges with trenches, however, was rather puzzling from the view that regards oceanic ridges as upwelling zones and oceanic trenches as downwelling zones of mantle convection. How can an upwelling flow go down? Beloussov (1979) used this apparent contradiction to attack the "new view of the Earth." But the observation does require the subduction of spreading ridges, with some important implications. (2) Although not shown in Figure 5, the vast area shown as Cretaceous age is characterized by absence of striped magnetic anomaly lineations. It is therefore called the Magnetic Quiet Zone (MQZ). This vast sea floor is interpreted to have been produced during an abnormally long absence of geomagnetic reversals. Taking into consideration the width of the MQZ and its age span, the rate of sea-floor spreading seems to have been 50–75% faster during the Cretaceous Period (124–83 Ma) than during other periods. Larson and Pitman (1972) called this phenomenon a pulse. When spreading is faster, the area with young age increases. Given the fact that the sea depth increases with age, they proposed that the sea floor must have risen during the pulse period, causing the Cretaceous "transgression of the sea," a long-known fact in geology. This pulse of spreading, with a long period of no geomagnetic reversal, was speculated as related to mantle superplume activity (Larson, 1991). This was the dawn of plume tectonics.

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What Does Subduction Continually Renew the Ocean Floor

Sea Floor Spreading

Foreword

HISTORY OF GEOLOGY SINCE 1962

The Eltanin Profile: 1966

The Oceans and Marine Geochemistry

8.7.2.4.3 Back-arc systems

Paleomagnetism

5.1.1 Sea-Floor Spreading and Plate Tectonics

Volume 3

Subaerial Examples: Iceland and Afar

Seismology and the Structure of the Earth

1.11.5.2 Mid-Ocean Ridges

Deep Earth Seismology

1.11.5.2 Mid-Ocean Ridges

Labrador Sea, Davis Strait, and Baffin Bay

Early to middle Eocene

Extensional tectonics and stratigraphy of the Mesozoic Jeanne d'Arc basin, Grand Banks of Newfoundland

NE-oriented Orphan basin extension

International Handbook of Earthquake and Engineering Seismology, Part A

3.2 Supporting Evidence

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