Plate Boundary Configuration Research Articles

What constitutes ‘emplacement’ of an ophiolite?: Mechanisms and relationship to subduction initiation and formation of metamorphic soles

Abstract Ophiolites have long been recognized as on-land fragments of fossil oceanic lithosphere, which becomes an ophiolite when incorporated into continental margins through a complex process known as ‘emplacement’. A fundamental problem of ophiolite emplacement is how dense oceanic crust becomes emplaced over less dense material(s) of continental margins or subduction-accretion systems. Subduction of less dense material beneath a future ophiolite is necessary to overcome the adverse density contrast. The relationship of subduction to ophiolite emplacement is a critical link between ophiolites and their role in the development of orogenic belts. Although ophiolite emplacement mechanisms are clearly varied, most existing models and definitions of emplacement concern a specific type of ophiolite (i.e. Oman or Troodos) and do not apply to many of the world’s ophiolites. We have defined four prototype ophiolites based on different emplacement mechanisms: (1) ‘Tethyan’ ophiolites, emplaced over passive continental margins or microcontinents as a result of collisional events; (2) ‘Cordilleran’ ophiolites progressively emplaced over subduction complexes through accretionary processes; (3) ‘ridge-trench intersection’ (RTI) ophiolites emplaced through complex processes resulting from the interaction between a spreading ridge and a subduction zone; (4) the unique Macquarie Island ophiolite, which has been subaerially exposed as a result of a change in plate boundary configuration along a mid-ocean ridge system. Protracted evolutionary history of some ocean basins, and variation along the strike of subduction zones may result in more complicated scenarios in ophiolite emplacement mechanisms. No single definition of emplacement is free of drawbacks; however, we can consider the inception of subduction, thrusting over a continental margin or subduction complex, and subaerial exposure as critical individual stages in ophiolite emplacement.

Observations of ridge‐hotspot interactions in the Southern Ocean

The evolution of ridge‐hotspot systems is not well understood. In this investigation, satellite‐derived marine gravity data are used in conjunction with underway bathymetric and magnetic anomaly profiles to investigate the nature of ridge‐hotspot interaction at four sparsely explored systems in the Southern Ocean. These systems illustrate three different stages of ridge‐hotspot interaction in which a migrating spreading center approaches a hotspot (Pacific‐Antarctic/Louisville), passes over or is captured by the hotspot (Mid‐Atlantic/Shona‐Discovery), and ultimately migrates away from the hotspot (Southeast Indian/Kerguelen). All of these systems show some evidence of discrete ridge jumps in the direction of the hotspot as the spreading center attempts to relocate toward the hotspot by asymmetric spreading. Interestingly, these ridge jumps show no evidence of propagating offsets as have been seen on many other ridge‐hotspot systems. A simple model predicts that typical plume excess temperatures can weaken the lithosphere sufficiently to promote asymmetric spreading and possibly allow a discrete ridge jump. The presence of previously uncharted, obliquely oriented aseismic ridges and gravity lineations between the ridge and the hotspot supports the notion of asthenospheric flux from the plume to the spreading center both before and after the time when the hotspot is ridge centered. The azimuths of the aseismic ridges cannot be explained by plate kinematics alone; they consistently extend from the ends toward the centers of the adjacent spreading segments suggesting some interaction between plume derived asthenospheric flux and local lithospheric structure. The features discussed here also indicate that the transfer of asthenospheric material from the plume to the spreading center is influenced by the local plate boundary configuration and interaction with transform offsets.

Southeastern boundary of the Juan Fernandez microplate: Braking microplate rotation and deforming the Antarctic plate

Data from a 1991 multibeam, sidescan sonar, and geophysical survey of the Juan Fernandez microplate at the Pacific‐Nazca‐Antarctic triple junction strongly support a model of edge‐driven microplate rotation. They reveal the nature of the microplate's southeastern boundary with the Antarctic plate and show that plate boundary interactions are capable of significantly altering plate motions. Microplate rotation slowed progressively from >30°/m.y. to <10°/m.y. between Anomaly 2 time (∼1.9 Ma) and the early Brunhes (∼0.6 Ma). Magnetics, bathymetry, and morphology suggest that this deceleration resulted from coupling across a rapidly evolving Juan Fernandez‐Antarctic (JF‐A) plate boundary. Over the past ∼2 m.y., the JF‐A plate boundary has lengthened, changed orientation, and migrated to the southeast, into the Antarctic plate. This process deformed and transferred to the Juan Fernandez plate about 3000 km2 of the Antarctic plate. Kinematic and morphologic observations support interpreting curved lineaments within this deformed zone as originally N‐S abyssal hills generated at the Pacific‐Antarctic Ridge that were later sheared by dextral slip bookshelf faulting within the migrating sinistral shear zone of the JF‐A boundary. Deformation within this system documents that shear zone migration, not rift propagation, is the key process associated with development of curved lineaments and transferred lithosphere. The JF‐A plate‐boundary shear‐zone may have been quite narrow (∼10–20 km) despite its migration through the Antarctic plate. Narrow migrating plate boundaries are also observed at other systems, such as the Galapagos 95.5°W migrating offset which also has a width of ∼20 km. Such narrow deformation zones may be common along migrating plate boundaries in various tectonic environments. Triple junction systems in this area appear to represent a suite of plates and triple junctions crossing a range of scales. The observations here support models for triple junctions having complex evolutionary histories involving rapid changes in plate boundary configuration and stress field.

The origin of bathymetric highs at ridge-transform intersections: A multi-disciplinary case study at the Clipperton Fracture Zone

Bathmetric highs on the old crust proximal to ridge-transform intersections (RTIs), termed “intersection highs”, are common but poorly understood features at offsets of fast to intermediate rate spreading centers. We have combined new reflection seismic, photographic, and geochemical data with previously published Seabeam, SeaMARC I, and SeaMARC II data to address the nature of the intersection highs at the Clipperton Fracture Zone. The Clipperton Intersection Highs are both topped by a carapace of young lavas at least 100 m thick. These lavas, which were erupted on the intersection highs, are chemically similar to their adjacent ridge segments and different from the surrounding older crust. At least some of the erupted magma traveled directly from the adjacent ridge at a shallow crustal level. Ridge-related magma covers and intrudes at least the upper 500 m of the transform tectonized crust at the RTI. We suspect that additional magma enters the intersection highs from directly below, without passing through the ridge. The young oceanic crust near the western Clipperton RTI is not thin by regional comparison. The 1.4 m.y. old crust near the eastern Clipperton RTI thickens approaching the transform offset. If the thermal effects of the proximal ridge were negligible, the eastern intersection high crust would appear to be in isostatic equilibrium. We believe that thermal effects are significant, and that the intersection high region stands anomalously shallow for its crustal thickness. This is attributable to increased temperature in the mantle below the ridge-proximal crust. Although ridge magma is injected into the proximal old crust, plate boundary reorganization is not taking place. Intersection high formation has been an ongoing process at both of the Clipperton RTIs for at least the past 1 m.y., during which time the plate boundary configuration has not changed appreciably. We envision a constant interplay between the intruding ridge magma and the disrupting transform fault motion. In addition, we envision a nearly constant input of magma from below the high, as an extension of the magma supply to the ridge from the mantle. Because the proximal ridge profoundly affects the juxtaposed crust at the RTI, sea floor fabric along the aseismic extensions of this fast-slipping transform fault is primarily a record of processes at work at the RTI rather than a record of transform tectonism.

A test of present‐day plate geometries for northeast Asia and Japan

Alternative geometries for the present‐day configuration of plate boundaries in northeast Asia and Japan are tested using NUVEL‐1 and 256 horizontal earthquake slip vectors from the Japan and northern Kuril trenches. Statistical analysis of the slip vectors is used to determine whether the North American, Eurasian, or Okhotsk plate overlies the trench. Along the northern Kuril trench, sup vectors are well‐fit by the NUVEL‐1 Pacific‐North America Euler pole, but are poorly fit by the Pacific‐Eurasia Euler pole. Results for the Japan trench are less conclusive, but suggest that much of Honshu and Hokkaido are also part of the North American plate. The simplest geometry consistent with the trench slip vectors is a geometry in which the North American plate extends south to 41°N, and possibly includes northern Honshu and southern Hokkaido. Although these results imply that the diffuse seismicity that connects the Lena River delta to Sakhalin Island and the eastern Sea of Japan records motion between Eurasia and North America, onshore geologic and seismic data define an additional belt of seismicity in Siberia that cannot be explained with this geometry. Assuming that these two seismic belts constitute evidence for an Okhotsk block, two published kinematic models for motion of the Okhotsk block are tested. The first model, which predicts motion of up to 15 mm yr−1 relative to North America, is rejected because Kuril and Japan trench slip vectors are fit more poorly than for the simpler geometry described above. The second model gives a good fit to the trench slip vectors, but only if Okhotsk‐North America motion is slower than 5 mm yr−1.

Terrane amalgamation in the Philippine Sea margin

The Philippine Sea plate includes plateaus of thickened crust interpreted as imbricated ophiolite and arc-related terranes of late Mesozoic-early Tertiary age separated by thinner oceanic crust. The arrival of plateaus at the subducting southwest margin of the Philippine Sea plate has caused the Philippine Trench to propagate southward in increments and caused transfer of terranes to the Philippine margin. New data from the Halmahera region indicate that the position, nature and evolution of plate boundaries have been strongly influenced by the heterogeneous character of the Philippine Sea plate. At present the Philippine Trench terminates at an oceanic plateau which is structurally continuous with an old forearc and ophiolite terrane on Halmahera. The position of this terrane has caused Philippine Sea plate-Eurasia convergence to be transferred from subduction at the Philippine Trench to the Molucca Sea Collision Zone through a broad NE-SW dextral transpressional zone across Halmahera. This plate boundary configuration is unstable and requires the future development of a new subduction zone to the east of Halmahera which will result in amalgamation of the Halmahera ophiolite terrane to the Philippine margin. In the Halmahera region amalgamation of terranes to the evolving Philippine microcontinent is currently in process.

Triple junction reorganization

For at least the past 20 m.y. the Bouvet triple junction appears to have existed predominantly in a ridge‐fault‐fault rather than a ridge‐ridge‐ridge configuration when it has an isosceles velocity vector triangle that allows either configuration to be stable. In this paper two conceptual models originally developed to explain the general orthogonality of ridges and transform faults in spreading center systems are examined to see if they can explain the observed predominance of the ridge‐fault‐fault geometry for this triple junction. The first hypothesis is that a ridge axis will form and evolve to remain oriented perpendicular to the direction of the maximum local tensile stress at the ridge axis; at a triple junction this can be strongly modified by the local plate boundary geometry. The second hypothesis is that plate boundary geometries adjust to minimize the power dissipation associated with plate boundary motions. Both hypotheses successfully predict the observed predominance of the ridge‐fault‐fault geometry. We suspect that the minimum dissipation hypothesis is successful for spreading center problems, despite its questionable theoretical underpinning, because for these problems it is equivalent to plate geometries assuming a minimum resistive force configuration. Since a minimum resistive force configuration is a partial corollary of a local stress‐controlled ridge axis geometry, the resulting minimum dissipation hypothesis plate geometry predictions will be similar to stress‐controlled plate boundary geometries. Because of its simplicity, the minimum dissipation hypothesis may remain a useful tool for predicting plate boundary configurations provided it is realized that this is more an approximate rule of thumb than a hard physical theory. Observations of the occurrence and orientation of episodic overshooting of ridge segments past the stable triple junction location should provide a better test that both fine‐ and large‐scale triple junction tectonics are stress controlled.

Lithospheric stress, deformation, and tectonic cycles: the disruption of Pangaea and the closure of Tethys

Abstract Intra-plate stress is governed on a global scale by plate boundary configuration and the disposition of continents. When continents are assembled into Pangaea-type masses, they are in deviatoric tension and predisposed to rifting and splitting along weak zones and where tension is locally enhanced by compensated uplifts. When continents are dispersed, they are, mainly, in deviatoric compression when local and minor thrusting and strike-slip faulting occur. Cyclic assembly, disruption, separation and reassembly of continental fragments causes a globally episodic, perhaps even periodic, repetition and synchroneity of tectonic events, alkaline and mafic magmatism and sea-level changes.

Neotectonics

The study of neotectonics is carried out by workers from a wide variety of disciplines, including structural geology seismology, regional geophysics, geomorphology and engineering geology. Landsat MSS imagery is a particularly effective tool for the investigation of neotectonic structures. The aim of the meeting was to bring together workers from the various contributory disciplines in order to discuss the development of modern techniques in each subject area. These modern approaches were exemplified by descriptions of neotectonically active regions ranging from NW Europe to China. In order to stimulate a debate on the temporal scope of neotectonics, the meeting commenced with a short discussion paper by Hancock. He argued that the selection of an arbitrary Neogene or Quaternary date to be applied globally for the beginning of the period during which neotectonic structures form is unhelpful. He proposed as an alternative the notion that for a given region the neotectonic phase might be regarded as having begun when the present-day configuration of relevant plate boundaries and motions was established. Blenkinsop suggested modifying Hancock’s proposal by defining the neotectonic phase as having commenced when the contemporary stress field of a region was established. Blenkinsop’s proposal possesses the great practical merit of permitting the geologist to compare regionally significant directions of horizontal compression and extension inferred from structures, known from field evidence to have been active during an appropriate time frame, with those determined from in situ measurements or fault plane solutions. Recent earth movements give rise to geomorphological features indicative of the

Oceanic plateaus, the fragmentation of continents, and mountain building

Many anomalous rises in today's oceans may be submerged continental fragments detached from previous continents, ancient island arcs, or basaltic piles formed by hot spots and spreading centers. These rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins. Where such rises are being consumed at present, e.g., the Nazca Ridge, they cause cessation of volcanism, disruption of the downgoing slab, and possible shifts in plate boundary configuration. Many past rises, including numerous continental fragments, have been recognized within mountain belts as allochthonous terranes. They constitute a large portion of the orogenic belts in the North Pacific from Mexico through western North America, Alaska, east Siberia, Japan and in New Zealand. The orogenic deformation in these belts is possibly the result of the accretion of the allochthonous terranes. Many terranes have been accreted with substantial deformation also in the Alpine chain, well before major continent‐continent collisions. It is suggested, therefore, that the accretion of fragments may be the common process of the deformation phase of mountain building. Subduction of normal oceanic crust may be insufficient for deformation, whereas full continent‐continent collision may not be necessary. The general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the Andes or not. Most of the accreted fragments with continental affinities in the Mesozoic‐Cenozoic orogenic belts of the world can be traced back to the breakup of Gondwana, beginning with a Pacifica domain in the Permian through a larger India domain in the early Mesozoic and continuing through the separation of the Somalia plate in the near future. The reasons for this 250 million year breakup process are not known, but some kind of thermal process, possible of mantle‐wide scale, is implied.

The Tamayo transform fault in the mouth of the Gulf of California

The Tamayo transform fault is located at the north end of the East Pacific Rise where it enters the Gulf of California. This paper presents bathymetric, seismic reflection, magnetic, and gravity data from a detailed survey of the transform fault. The dominant feature of the offset region is a bathymetric ridge trending 120°, parallel to the predicted transform plate boundary. This transform ridge is associated with a large (600 γ) positive magnetic anomaly, and a very small positive free-air gravity anomaly. Magnetic and gravity models indicate either a basalt or serpentinite composition for the ridge, but cannot distinguish between these possibilities. At its eastern end, the modern zone of strike-slip motion is in a narrow valley south of the transform ridge. The transform plate margin appears to pass through a saddle in the transform ridge and meet the western spreading center segment in the trough north of the transform ridge. On the basis of this survey and previous work, the history of the Tamayo from continental breakup to the present has been reconstructed. Initial rifting occurred along a trend of 130° at approximately 3.5 m.y.b.p. Once the transform fault was free of the constraints imposed by continent-continent and continent-oceanic lithospheric interaction, the trend of the transform fault rotated counter-clockwise. This rotation resulted in a ‘leaky’ transform fault and intrusion of a large continuous transform ridge. Further adjustments in the spreading center/transform fault plate boundary configuration have given rise to an incipient zone of rifting cutting across the transform ridge and emplacement of diapiric structures.

Tectonic evolution of the Cocos-Nazca spreading center

Magnetic and bathymetric data from the eastern Pacific have been analyzed and a model for the evolution of the Galapagos region developed. The Farallon plate appears to have broken apart along a pre-existing Pacific-Farallon fracture zone, possibly the Marquesas fracture zone, at about 25 m.y. B.P. to form the Cocos and Nazca plates. This break is marked on the Nazca plate topographically by the Grijalva scarp and magnetically by a rough-smooth boundary coincident with the scarp. The oldest Cocos-Nazca magnetic anomalies parallel this boundary, implying that the early Cocos-Nazca spreading center trended east-northeast. This system soon reorganized into an approximately east-west rise–north-south transform configuration, which has persisted until the present, and the Pacific-Cocos-Nazca triple junction has since migrated north from its original location near lat 5°S. If correct, the combination of these simple geometric constraints produced the “enigmatic” east-trending anomalies south of the Carnegie Ridge. The axes of the Cocos-Nazca spreading center and the Carnegie Ridge are essentially parallel; this can lead to paradoxical conclusions about interpretation of the Cocos and Carnegie Ridges as hotspot tracks. Hey and others (1977) have shown that recent accretion on the Cocos-Nazca spreading center has been asymmetric, resulting at least in part from small discrete jumps of the rise axis. I show here that the geometric objections to both the “hot-spot” and “ancestral-ridge” hypotheses on the origin of the Cocos and Carnegie Ridges can be resolved with an asymmetric-accretion model. However, all forms of the ancestral-ridge hypothesis encounter more severe geometric difficulties, and these results support the hotspot hypothesis. After further elaboration of the hotspot hypothesis by Johnson and Lowrie (1972) and Hey and others (1973), Sclater and Klitgord (1973) examined both the hotspot and ancestral-ridge hypotheses and decided that both should be rejected, concluding that the Cocos and Carnegie Ridges “are not tectonically related” (p. 6973). The difficulty in the Galapagos area arises primarily because the older magnetic anomalies have proven extremely difficult to correlate, surprisingly so considering the high data density and the ease with which the very young anomalies are correlated. An important problem is the reason for this difficulty in correlating older anomalies — either clearly recognizable anomalies were never formed here, or some mechanism has acted in this area to destroy them after they were formed. On the basis of all available evidence, including new data presented here, I conclude that a model based on the hotspot hypothesis, with the modification of asymmetric accretion resulting at least in part from discrete jumps of the rise axis as discussed by Hey and others (1973) and demonstrated by Hey and others (1977), successfully meets the objection of Sclater and Klitgord (1973) and allows us to outline the history of the area from the break-up of the Farallon plate and birth of the Cocos-Nazca spreading center to the present. The “instantaneous” (a term Hey and others, 1977, have examined) configuration of plate boundaries and motions (Fig. 1) has generated a wedge of crust spread from the Cocos-Nazca spreading center, which is characterized by a slow spreading rate, rough topography, and strong magnetic anomalies. This wedge, termed the Galapagos gore by Holden and Dietz (1972) and discussed in detail by Hey and others (1977), is surrounded by crust spread from the Pacific-Cocos and Pacific-Nazca spreading centers which has the smooth morphology common to fast-spreading rises and low-amplitude magnetic anomalies, as both these segments of the East Pacific Rise are oriented nearly parallel to the Earth's magnetic field vector. My model explains the location and orientation of the magnetic and bathymetric rough-smooth boundaries that thus bound the gore and implies that there are two genetically different magnetic and bathymetric boundaries in the area.