Pacific Lithospheric Plate Research Articles

Linear volcanic chains — recording plate motions?

Volcanic island chains, showing a progression from high volcanic cone to coral atoll, are especially well developed in the Pacific Ocean, well removed from the active plate margins. These chains include the Hawaiian, Marquesas, Society and Austral Islands, all of which are subparallel and approximately perpendicular to the axis of the East Pacific Rise. Potassium—argon age measurements made a decade ago demonstrated that the locus of volcanism in the Hawaiian chain has migrated toward the southeast in a regular manner, confirming what James Dana suggested more than a century ago. Regression of available K-Ar data on the age of shield building volcanism for sixteen volcanoes of the Hawaiian chain against distance from the site of present activity at Kilauea show a strong linear correlation, yielding an average migration rate for the volcanism of 9.66 (±0.27) cm/yr over the last 27 m.y. This analysis indicates that non-linear models for the migration of volcanism in the Hawaiian chain are unnecessary. A similar monotonic decrease in age of volcanism toward the southeast occurs in the Marquesas, Society and Austral Islands in the South Pacific, with rates of migration in the order of 11 cm/yr. These results together with those from the Hawaiian Islands, the Pitcairn—Gambier Islands and the Pratt-Welker Seamounts are consistent with movement of the Pacific lithospheric plate over magma sources that are fixed in the upper mantle relative to one another. A virtually perfect fit of all the data is obtained for a Pacific plate rotation pole located at 70°N 101°W and a rate of rotation of the Pacific plate about this pole of 1.00 (±0.02)°/m.y. over at least the last 10 m.y. of geological time. These data from the Pacific provide strong support for the Wilson-Morgan hot spot or plume model for the origin of island chains, although the means by which the thermal anomaly underlying each volcanic chain in maintained within the mantle still remains unclear.

Age of shield-building volcanism of Kauai and linear migration of volcanism in the Hawaiian island chain

Tholeiitic basalts of the Napali Formation comprise the bulk of the Kauai shield volcano. Potassium-argon ages measured on 16 samples from three separate areas in this formation lie in the range 5.14 ± 0.20 to 3.81 ± 0.06 m.y. The scatter in the measured ages in each area is greater than that expected from experimental error alone, and variable loss of radiogenic argon is regarded as at least partly responsible. Nevertheless an interval of eruption in the order of 0.8 m.y. is deduced for the Napali Formation. The results from the Napali Formation taken together with K-Ar ages measured earlier on basalts of the Makaweli Formation, the youngest formation of the dome-building phase, yield a mean age of 4.43 ± 0.45 m.y. for the construction of the main subaerial shield volcano of Kauai. When this result from Kauai is combined with estimates of the average age for the shield-building volcanism in 16 other volcanoes in the Hawaiian island chain, extending over a distance of more than 2800 km, the data are found to conform to migration of the centre of volcanism from north-northwest to south-southeast at a uniform rate of 9.4 (±0.3) cm/yr over the last 28 m.y. Non-linear models of propagation of volcanism in the Hawaiian chain are quite unnecessary, especially when uncertainties in the data base are taken into account. These results are consistent with an origin of the Hawaiian volcanic chain by eruption from a magma source situated below the Pacific lithospheric plate, as proposed under hot spot or plume models. Depending upon choice of the pole for the Pacific plate, rates of rotation about the pole of 0.9° to 1.0°/m.y. are derived. By extrapolation of the Hawaiian Island chain data an age estimate of 37.8 m.y. is derived for the Hawaiian-Emperor Seamount intersection.

Implications of north pacific plate tectonics in central Alaska: Focal mechanisms of earthquakes

The focal mechanisms for 86 selected earthquakes ( 3.0⩽ m b⩽ 5.5 ) located in central Alaska have been investigated from P-wave first motions; the data were gathered by local seismic networks. The results show a depth-dependent characteristic to the fault-plane solutions. For earthquakes having focal depths shallower than 60–70 km, the focal mechanisms indicate either strike-slip or normal faults, while for earthquakes with foci at intermediate depths the focal mechanisms correspond to thrust faults. The nature of the seismicity indicates the hinge line of the Pacific lithospheric plate under the study area to be striking N17°E from Cook Inlet towards interior Alaska. The comparison of the focal mechanisms with the seismicity shows that the strike-slip and normal faults are the predominant processes of stress release along the shallow section of the plate. The earthquakes with intermediate foci systematically occur along the inclined section of the plate. If the gently dipping nodal planes for these earthquakes are chosen as the fault planes, the focal mechanisms correspond to underthrust motions at the foci. In these, the slip vectors are oriented either to the west or north with the resultant being in the N30°W direction. The tension axes for the underthrust solutions are also found to be parallel to the local dip of the plate, indicating that the subducted plate in interior Alaska is undergoing gravitational sinking.

Early Tertiary “Incaic” tectonism, uplift, and volcanic activity, Andes of central Peru

Potassium-argon age determinations on volcanic strata that overlie an extensive postorogenic erosion surface in the Western Cordillera show that the “Incaic pulse” of compressive deformation as well as subsequent uplift and erosion of the Andes of central Peru had taken place before 40 to 41 m.y. ago. Intense volcanic activity, which began before the end of uplift and erosion, continued for about 6 m.y. into early Oligocene time. The timing of these events suggests that uplift and volcanic activity, and perhaps also the preceding tectonic pulse, may have been initiated by the same global disturbance that produced the abrupt change in direction of movement of the Pacific lithospheric plate reflected by the bend in the Hawaiian-Emperor volcanic chain.

THE RELATIONSHIP BETWEEN VOLCANIC ISLAND GENESIS AND THE INDO-AUSTRALIAN PACIFIC PLATE MARGINS IN THE EASTERN OUTER ISLANDS, SOLOMON TSLANDS, SOUTH-WEST PACIFIC

The Eastern Outer Islands form a group of small islands situated in the south-west Pacific Ocean, to the south-east of the main Solomon Islands chain, and to the north of the New Hebrides chain.They are founded upon the northern part of the submarine Fiji Plateau and flanked by deep sea trenches on the western, northern and eastern sides, but by an east-west trending fracture zone on the southern side. Two south-easterly trending island chains can be recognized, the western chain includes the islands of Tinakula, Nendo, Vanikolo and Utupua, and the eastern chain includes the islands of the Duff Group, Anuta and Fatutaka. The island of Tikopia is situated midway between both chains. The Torres and Vitiaz Trenches form the western and eastern flanks respectively of the north Fiji Plateau, which is itself bordered to the west and east respectively by the Indo-Australian and Pacific lithospheric plates. A westerly extension of the Vitiaz Trench, known as the Cape Johnson Trough, forms the northern boundary. The south side of the region is delimited by the Hazel Holme Fracture Zone.Volcanic rocks within the group range from picrite basalts, through basalts and andesites, to dacites. Mafic lavas predominate in the older islands of the western chain, two islands of the eastern chain and in samples dredged from the Vitiaz Trench. More sialic lavas are exposed in two islands of the eastern chain, the isolated island of Tikopia and also in the active volcanic island of Tinakula.Petrologic, petrochemical, seismic and heat flow evidence suggests that the Eastern Outer Islands represent two, discrete south-easterly trending island arcs, each associated with the adjacent trench. The sequence of volcanic episodes can be best explained in terms of plate tectonics, in which the islands of the western chain were produced during two episodes of volcanic activity associated with an easterly inclined Torres subduction zone. Late Oligocene to early Miocene subduction produced the island of Nendo, but the islands of Utupua, Vanikolo and Tinakula were produced during the late Pliocene to Recent. The westerly-dipping Vitiaz subduction zone is postulated as a source from which the east facing Duffs-Anuta-Fatutaka island arc was produced during the Middle Miocene to late Pliocene.

Biostratigraphic and tectonic implications of 40Ar-39Ar dates of ash layers from the northeast Gulf of Alaska

Ash layers from Deep Sea Drilling Project site 178 in the northeast Pacific Ocean have been dated by the 40 Ar- 39 Ar stepwise heating technique to resolve published discrepancies concerning the length of time explosive volcanism has affected the eastern Aleutian arc and Alaskan Peninsula. The results of the investigation indicate that the record of ash-fall deposition at site 178 extends back at least 6.5 m.y. Assuming that 6.5 m.y. ago marks the onset of renewed calc-alkalic volcanism of the volcanic arc, proposed models of continuous and discontinuous motion between the Pacific and North American lithospheric plates can be evaluated. If appreciable time elapsed between the onset of subduction and the onset of arc volcanism, the 6.5-m.y. record of ash-fall deposition in the north-east Pacific is most compatible with models of continuous plate motion throughout late Cenozoic time.

Stress fields in central portions of the Pacific Plate: Delineated in time by linear volcanic chains

Many linear island chains in the Pacific appear to consist of individual volcanic shields that lie on relatively short, curved loci sometimes of sigmoidal form. These loci, in turn, lie at an angle to the directions of propagation of the chains. Those chains that now trend just south of east (the Pratt-Welker, Hawaiian, Tuamotu, and Austral chains) consist of en echelon sets of loci with right lateral (clockwise) sense of stepwise overlap, whereas the supposed older extensions of these chains that lie nearly north-south (the Emperor and Ellice-Gilbert-Marshall chains) consist of sets of loci with left lateral (counterclockwise) sense of stepwise overlap. Rift zones of isolated volcanoes in the chains, that is, those volcanoes whose local stress fields were not influenced by the buttressing effects of near-neighbor volcanoes at the time of their formation, show the same orientation as the loci. First-motion studies of earthquakes in the Pacific lithosphere to date are few in number and give a variety of solutions with inferred directions of maximum principal compression (P) nearly horizontal and trending in both northeasterly-southwesterly and northwesterly-southeasterly directions. However, the number of such solutions is small, in some cases related to edifice effects, and interpretations relating compression and dilatation to principal stress directions in magmatic source regions are open to question. We consider it likely that the dominant orientation patterns have been due to dynamic effects related to the overall kinematic patterns of Pacific plate motions. Although a large number of different factors can influence the inherent and transmitted stresses in the lithosphere, and thereby influence the locally dominant stress field, we conclude that the effective stress orientations in the very recent past and for about the last 40–50 m.y. can be considered to have been caused by dynamic forces reflected in a right lateral rotational couple acting within the plane of the Pacific plate. These forces have induced maximum (S1) principal compressional stress directions that have had surface traces trending about northwest-southeast, relating to minimum (S3) principal stress directions that have trended northeast-southwest. Prior to 40–50 m.y. ago the dominant stress orientations in the plate were caused by a tendency for left lateral stress rotations, which induced maximum (S1) principal compressional stress directions with surface traces that trended just west of north and minimum (S3) principal stress directions that trended just north of east. These data and interpretations are independent of whether or not a change in direction of motion of the Pacific lithospheric plate took place roughly at the time represented by the bends in the island chains and are also independent of arguments as to whether melting anomalies of the Pacific are rigidly fixed or whether they are better explained in terms of thermal plumes or gravitational anchors. We conclude that the trends and age correlations of volcanic loci in the Pacific accurately track and identify the evolution of states of stress in the Pacific lithosphere with time. As more age data for linear island edifices become available, it should be possible to construct a Pacific-wide chronology of volcanism independent of but similar to that developed from magnetic reversals. Further, it appears that changes in stress directions in the plate have been episodic and that they may correlate with episodic magma generation not only in the central part of the Pacific plate but also around its margins.

Part 4. Plate boundary evolution in the Solomons region: Ophiolite basement complex in a fractured, island chain, Santa Isabel, British Solomon Islands

As part of a continuing programme involving the structural, petrological, and geochemical evolution of the British Solomon Islands, attention has now centred, among other islands, on Santa Isabel, where a well developed basement ophiolite sequence is exposed. Further work in the area will involve the geochemical relationships between the various members of the ophiolite basement. The Solomons lies within Suess's outer 1st Australian Arc, now regarded as the margin between the India and Pacific lithospheric plates (Le Pichon, 1968) and have been termed a composite, fractured island chain (Coleman and Hackman, 1974). The Group comprises a double en echelon chain of islands, which is believed to reflect a system of basins and anticlinal horsts now progressively offset by sinistral shear (Carey, 1968; Krause, 1967; Hackman, 1973). Five Provinces were originally recognised in the region (Coleman, 1965) and these were subsequently amended to four (Hackman, 1973).

Plate tectonics, volcanism and the lithosphere in British Columbia

THE most important feature of the present tectonic situation off the west coast of Canada is the ridge–trench–fault type triple junction between the Juan de Fuca, Pacific and American lithospheric plates near the continental margin, at approximately 51°N (Fig. 1). The half spreading rate for the Juan de Fuca Ridge, based on the magnetic lineation pattern, is 2.9 cm yr−1, and the calculated rate of motion along the Queen Charlotte transform fault between the American and Pacific Plates is 6 cm yr−1 (ref. 1). The main part of the Juan de Fuca Ridge is probably spreading parallel to the north–east trending Blanco fracture zone which marks the southern edge of the plate at approximately 43.5°N. The vector diagram for the relative motion of the three plates during the late Cainozoic suggests that the Juan de Fuca Plate is being thrust obliquely below the American Plate at 2.5 cm yr−1 in a direction of N 35° E (ref. 1).

Recent studies of Macquarie Island and the Macquarie Ridge Complex

The Macquarie Ridge Complex is an arcuate system of ridges and troughs which forms the boundary between the Pacific and lndian lithospheric plates, from the southern tip of New Zealand to the triple junction between the Indian, Antarctic and Pacific lithospheric plates at approximately 62°S (figure 1 ). Macquarie lsland occurs at 54.5°S, 159°E in the central region of the Macquarie Ridge Complex. In the letter region the Macquarie Trench occurs to the east of the Ridge, in contrast to the Puysegur Trench in the north, and the Hjort Trench in the south, which occur to the west of the Ridge.

Possible permian oceanic crust and plate junction in central Shikoku, Japan

The Osugi greenstones of central Shikoku, Japan, a portion of the so-called Mikabu green rocks, appear to constitute a typical ophiolite complex; original premetamorphic rocks include serpentinized ultramafics, gabbros, diabases, pillow breccias, tuffaceous agglomerates and siliceous sediments, in roughly ascending stratigraphic order. This suite is thought to represent a section of oceanic crust juxtaposed along the Japanese continental margin in latest Paleozoic (mid-Permian?) time during a period of active lithospheric plate convergence. The northern margin of the greenstone complex evidently marks a temporary suture between the Asiatic and Pacific lithospheric plates at this time. Motion apparently continued and, through a later northward stepping of the Benioff zone, is thought to have been responsible for the subsequent regional development of a (Late Jurassic?) relatively highpressure, low-temperature subduction-type metamorphism throughout the entire Sanbagawa-Chichibu terrane.

Implications of Heat Flow and Bottom Water Temperature in the Eastern Equatorial Pacific

Summary The distribution of terrestrial heat flow through the sea floor of the eastern equatorial Pacific Ocean is compared to recent tectonic interpretations of this complex region. Some 500 values between 25° N to 20° S latitude, including 146 newly reported here, show that the theoretical distribution of heat flow for sea-floor spreading models matches well with smoothed data over portions of the Pacific, Cocos and Nasca lithospheric plates created within about the past 18 My from the East Pacific Rise. For the Galapagos Rift Zone, the fit is reasonable for lithosphere formed within the last 5 My only if a significant proportion of low heat-flow values is ignored. The heat flow on the Mathematicians’ Ridge and Galapagos Rise indicates that these were sea-floor spreading axes along which activity terminated about 5 and 10 My ago, respectively; this interpretation and its time scale are generally consistent with tectonic histories deduced from magnetic and topographic evidence. Detailed surveys and repeated measurements at stations suggest that localized heat-flow variability is frequently associated with topographic variability of the sea floor. Bottom water temperature profiles show that the East Pacific Rise is a barrier to eastward flow of waters at depths greater than 3 km, the approximate sill depth over the Nasca plate.

Geology of Eastern Papua

Eastern Papua is a southeasterly trending mountainous peninsula with islands to the east and southeast, between 7° to 12° S. and 146° to 155° E. The peninsula and islands consist of a linear core of Mesozoic sialic metamorphic rocks flanked by Mesozoic and younger mafic rocks, and partly concealed by still younger sedimentary and volcanic rocks. The Mesozoic sialic rocks are exposed in a belt 900 km long and up to 60 km wide. They consist of Cretaceous (and possibly older) sediments which were metamorphosed during the Paleocene or Eocene. Metamorphic grade is generally greenschist facies, but is higher in the D9Entrecasteaux Islands and on part of Misima Island; lawsonite occurs mainly within a few kilometers of the Owen Stanley fault which bounds the mainland metamorphics on the north and northeast. Mesozoic mafic rocks include peridotite, gabbro and basalt of the Papuan ultramafic belt, metabasalt exposed in the Suckling-Dayman mountain block and on Normanby and the Deboyne Islands, and unmetamorphosed basalt elsewhere on the mainland. These rocks are, at least in part, Cretaceous. Younger mafic rocks include Eocene basalt on the southeastern mainland, and upper Oligocene and lower and middle Miocene tuff and lava at scattered localities. Eocene sediments consist of chert and deep-water limestone along the south coast of the mainland at Port Moresby and Magarida, and Eocene clastic sediments inland from Port Moresby and near Tapini. No lower or middle Oligocene rocks are known. Upper Oligocene, Miocene and Pliocene sediments in the Aure Trough (145° to 146° E.) have a maximum thickness of about 12,000 m; the sediments are mainly alternating mudstone and graywacke, and are probably turbidites. Middle Miocene reef limestone on the eastern hinge line of the trough at 146° E. is 1,000 m thick. Middle Miocene and younger clastic sediments in the Cape Vogel Basin (150° E.) are at least 4,000 m thick. Pliocene and Quaternary volcanic rocks range in composition from basalt to rhyolite and include some potash-rich rocks. Intrusive rocks include Eocene tonalite in the ultramafic belt, Oligocene? gabbro near Port Moresby, middle Miocene granodiorite west of Salamaua, middle Miocene potash-rich intrusives on the southeastern mainland, Pliocene andesitic porphyries near Wau, and Pliocene granodiorite in the D9Entrecasteaux Islands. The Cretaceous (and possibly older) sialic sediments are thought to have been metamorphosed during the early Eocene at the time of emplacement of the Papuan ultramafic belt. The ultramafic belt is thought to be part of a thrust slice of oceanic mantle and crust which rode over or was underridden by the Cretaceous sediments on a low-angle fault (the Owen Stanley fault). The metamorphic rocks were partly exposed during the middle or late Eocene but there was no major land mass in the area until the upper Oligocene. During the upper Oligocene, Miocene and Pliocene the sialic metamorphic rocks emerged, perhaps isostatically. Erosion of the resulting mountain block and contemporary volcanism and limestone development contributed great volumes of sediment to the Aure Trough and lesser volumes to the Cape Vogel basin. The history of the area can be interpreted in terms of interaction between Australian and Pacific lithospheric plates and opening of the Coral Sea by rifting.

The evolution of the Lau Ridge, Fiji Islands

The Lau Ridge lies about 150 miles west of the Tonga Ridge and is sub-parallel to it. It may be an example of a “dead” island arc, activity having transferred to the Tonga-Kermadec region after an Upper Miocene change in the pattern of sea floor spreading in the Pacific. As a consequence of this change the Fiji Plateau has become detached from the main Pacific lithospheric plate. The continental-type Fiji Islands are seen as having evolved inside a sharp bend in the former arc.

Structural Evolution of Bering Continental Margin: Cretaceous to Holocene: ABSTRACT

Spreading in a broad arc from Kamchatka to the tip of the Alaska Peninsula, the Bering continental margin separates the deep Bering Sea from its fringing shelf. Rock dredgings and geophysical data indicate that beneath a veneer of Cenozoic deposits the central sector of the margin (between Alaska and Siberia) is in part underlain by folded sedimentary strata of Late Cretaceous (Campanian) age. These rocks can be traced seaward beneath the deep basin of the Bering Sea, as well as landward to coastal exposures of Mesozoic rocks in Siberia and western Alaska. The deeply submerged Cretaceous rocks appear to be the eroded remnants of a continental margin that was uplifted and deformed in latest Cretaceous or earliest Tertiary time. We speculate that tectonism of the continental margin may be linked with the convergence of a North Pacific lithospheric plate and an American-Eurasian plate in the late Mesozoic; normal or direct convergence off southe n Alaska and the Kamchatka-Koryak region where coastal mountain building was intense, and oblique convergence along the less deformed central sector of the Bering margin. By Oligocene time the seaward fringes of the coastal mountains were beveled to low relief and submerged to form the foundation of the Neogene shelf and continental margin. Submergence was very extensive along its central sector. There narrow grabens parallel with the margin formed in response to extensional rifting and filled with as much as 2,600 m of neritic and upper bathyal deposits. Faults bordering outer-shelf basins and flanking structural highs in the central sector may connect on the west with major fractures offsetting Cretaceous rocks underlying the Koryak Mountains (Cape Navarin area). Eastward from the central sector outer-shelf structural trends appear to turn inside or north of the Alaska Peninsula rather than merge with Pacific marginal structures. During the Neogene, 500 m or more of pelagic and terrigenous sediment accumulated over the collapsing margin. These deposits were deformed by slumping and basement faulting and, in the late Cenozoic, largely stripped from some areas by an episode of intense canyon cutting. End_of_Article - Last_Page 2503------------

Tectonic Overview of Bering Sea-Aleutian Ridge Region: ABSTRACT

Known and inferred surficial structural features of the Aleutian Ridge, Terrace, and Trench in the central and western Aleutian arc, Alaska, appear to reflect predominantly tensional strain of diverse orientations. Features on the ridge are inferred to have formed from igneous activity-related tectonism including distension of the surficial skin over rising and spreading epizonal plutons, volcanotectonic subsidence, and rifting; features on the terrace and landward trench wall may have formed from distension of a decoupled surficial skin above a prolongated zone of imbrication and uplift; and those in the trench and on the seaward wall from dynamic loading of the Pacific lithospheric plate by overthrusting and southward migration of the Aleutian tectogene. The plutonism, mbrication, and dynamic loading are interpreted as resulting from a first-order process of crustal foreshortening normal to the arc. This interpretation is consistent with evidence from natural seismicity which indicates predominantly compressional strain across the central and eastern Aleutian arc. Surficial structural features in the eastern Aleutian arc between the continental margin and the volcanic arc appear to reflect directly the first-order compressional strain, whereas the features in the bordering Aleutian Trench are predominantly of tensional origin. Movement vectors for the Pacific plate based on a pole of rotation at lat. 53°N, long. 47°W., indicate normal convergence against the continental margin near Kodiak Island, movement tangential to the arc trend in the vicinity of the Near Islands, and tension oblique to the arc trend in the vicinity of the Commander Islands. If the Aleutian Trench has resulted from crustal foreshortening its presence along the entire arc suggests movement of the Bering Sea plate independent of the Pacific plate. Structural features such as Bowers Ridge in the western Aleutian arc may reflect foreshortening parallel with the arc. Such features may be related to Pacific plate movement, whereas the arc itself there may have resulted from movement of the Bering Sea plate. The eastern Aleutian arc could have resulted from south-to-southeast drift of the Alaskan continental mass hinged to the Canadian mainland along the axis of the Alaska orocline combined with Pacific plate normal convergence. Body forces such as those that might be derived from perturbations of rotational parameters (angular momentum and moments of inertia) of the core-mantle system seem better suited to explain the suggested plate movements than are forces derived from mantle convection. End_of_Article - Last_Page 2467------------