Pacific-Farallon Ridge Research Articles

Possible partial melting and production of felsic melt in a Jurassic oceanic plateau of the Izanagi Plate: Insights from 159 Ma plagiogranites from northern Japan

ABSTRACT A small tonalite – dacite body has been discovered in the Nakanogawa Group, Hidaka Belt, northern Japan, which is a Palaeogene subduction complex formed in the palaeo-Japan trench along the northeastern margin of Eurasia. The tonalites are characterized by extremely low K2O (0.2–0.3 wt.%) and relatively flat chondrite-normalized rare earth element patterns (La/Yb[N] ~2.3), similar to plagiogranites in ophiolites. The major-and trace-element characteristics are consistent with low-pressure partial melting of oceanic crust outside the garnet stability field. Zircon U – Pb dating of the tonalite yielded a weighted-mean 206Pb/238U age of 159.1 ± 1.6 Ma. Late Jurassic oceanic plateau-type basaltic rocks with a small amount of plagiogranitic rocks are distributed sporadically in subduction complexes along the palaeo-Japan arc – trench system. A similar zircon U – Pb age (151.6 ± 1.8 Ma) has been reported for greenstone in the Cretaceous subduction complex along the palaeo-Kuril arc – trench system. The tonalite – dacite block was probably derived from the subduction complex at the junction of the palaeo-Japan and -Kuril arc – trench systems. Thus, the greenstones and the tonalite body were likely part of a large oceanic plateau, which formed at the Izanagi – Pacific–Farallon ridge triple junction during the Late Jurassic – Early Cretaceous.

Spreading behaviour of the Pacific-Farallon ridge system since 83 Ma

We present improved rotations, complete with uncertainties, for the Pacific-Farallon Ridge (PFR) between geomagnetic chrons 34y (83 Ma) and 10y (28.28 Ma). Despite substantial shortening since ∼55 Ma, this ridge system and its remnants (e.g. the East Pacific Rise) have produced as much as 45per cent of all oceanic lithosphere created since the Late Cretaceous, but reconstructions face the twin challenges of extensive subduction of Farallon crust—which precludes reconstruction by fitting conjugate magnetic anomaly and fracture zone (FZ) traces—and asymmetric spreading behaviour for at least the past 51 Myr. We have calculated best-fit ‘half’-angle stage rotations between nine geomagnetic chron boundaries (34y, 33y, 29o, 24.3o, 20o, 18.2o, 17.1y, 13y and 10y) using combined anomaly and FZ data from both the northern and southern Pacific Plate. For rotations younger than chron 24.3o, estimates for spreading asymmetry, derived using anomaly picks from yet-to-be subducted Farallon/Nazca crust in the south Pacific, allow full stage rotations to be calculated. Between 50 and 83 Ma, where no direct constraints on spreading asymmetry are possible, a ‘best-fit’ full stage rotation was calculated based on the net Nazca:Pacific spreading asymmetry (Pacific spreading fraction f PAC =0.44) over the past 50 Myr, with conservative lower and upper bounds, based on variability in the degree of spreading asymmetry over periods of<15 Myr, assuming f PAC s of 0.5 and 0.36, respectively. Synthetic flowlines generated from our new stage rotation produce a better match to Pacific FZ trends than previously published rotations. With the exception of the chron 18o–20o rotation, the six stage poles for rotations between chrons 33y and 13y (74–33 Ma) all cluster tightly at 60–75°E, 60–68°N, consistent with the relatively constant trend of the major Pacific FZs. This stability spans at least one episode of Farallon Plate fragmentation caused by subduction of PFR segments beneath the Americas, at 55–48 Ma, which appears to have greatly accelerated divergence on the surviving ridge without significantly affecting the location of the instantaneous rotation pole. Together with quasi-periodic 15–20 Myr variations in the degree of spreading asymmetry that also appear to correlate with changes in spreading rate, this indicates that forces other than slab pull may be a factor in determining Pacific-Farallon Plate motions.

Birth of an intraoceanic spreading center

The Cocos-Nazca spreading center is one of the few examples of the formation of a spreading center by splitting of oceanic lithosphere. It was created when the Farallon plate broke up in the early Miocene following the collision of the Pacific-Farallon spreading center with the North American continent. Much of the ancient Farallon plate corresponding to the area of opening is lost to subduction beneath Central America and South America, but new data from the conjugate area on the Pacific plate allow the first detailed reconstruction of the break-up process. The opening began after chron 7 (25 Ma) at a location of focused crustal extension caused by overlapping spreading centers that had evolved in response to a slight reorientation of a Pacific-Farallon ridge segment. Beginning at chron 6B (22.7 Ma), eastward progressing seafloor spreading started along an axis that most likely migrated toward the region of weak lithosphere created by the Galapagos hotspot. By chron 6 (19.5 Ma), plate splitting from the spreading center to the trench was complete, allowing the fully detached Cocos and Nazca plates to move independently. This kinematic change resulted in a significant ridge jump of the newly established Pacific-Nazca spreading center, a change in plate motion direction of the Nazca plate by 20° clockwise, and a large increase in Pacific-Cocos plate velocity in the middle Miocene.

Faster seafloor spreading and lithosphere production during the mid-Cenozoic

Concurrent changes in seawater chemistry, sea level, and climate since the mid-Cretaceous are thought to result from an ongoing decrease in the global rate of lithosphere production at ridges. The present-day area distribution of seafloor ages, however, is most easily explained if lithosphere production rates were nearly constant during the past 180 m.y. We examined spatial gradients of present-day seafloor ages and inferred ages for the subducted Farallon plate to construct a history of spreading rates in each major ocean basin since ca. 140 Ma, revealing dramatic Cenozoic events. Globally, seafloor spreading rates increased by similar to 20% during the early Cenozoic due to an increase in plate speeds in the Pacific basin. Since then, subduction of the fast-spreading Pacific-Farallon ridge system has led to a 12% decrease in average global spreading rate and an 18% or more decrease in the total rate of lithosphere production by the most conservative estimates. These rapid changes during the Cenozoic defy models of steady-state seafloor formation, and demonstrate the time-dependent and evolving nature of plate tectonics on Earth.

Melting of heterogeneous mantle in a slab window environment: examples from the middle Tertiary Masset basalts, Queen Charlotte Islands, British Columbia

Voluminous basalts dominate a middle Tertiary bimodal volcanic assemblage in the Queen Charlotte Islands, British Columbia, Canada. These moderately enriched mid-ocean-ridge basalt (MORB-)like magmas were associated with significant crustal extension related to the subduction of the Pacific–Farallon ridge and the initiation of a slab window environment, which persisted for 35 Ma. These asthenospheric melts were derived from a low degree of melting of a heterogeneous peridotite source mainly in the spinel stability field, as inferred from fractional melting inversion calculations. The basaltic magmas in turn gave rise to both tholeiitic and calc-alkaline volcanic strata up to 3.5 km thick in local grabens. The tholeiitic basalts resemble MORB, with rare-earth element (REE) patterns ranging from slightly depleted to enriched in light REE (LREE). The calc-alkaline basaltic andesites are enriched in LREE, but are not strongly depleted in Nb. The rocks have overlapping Nd, Sr, and Pb isotopic compositions, similar to those of intraplate basalts from the adjacent northeast Pacific seamounts and Cordillera. The difference between the tholeiitic and calc-alkaline rocks is, in part, attributed to polybaric fractional crystallization of different proportions of plagioclase to mafic minerals from a similar tholeiitic magma. This marginal-basin subalkaline volcanic sequence and its genesis describe the slab window environment for the Tertiary tectonics of the western Canadian continental margin and provide a model for older combined tholeiitic and calc-alkaline volcanic successions.

Unusually large Nb [sbnd]Ta depletions in North Chile ridge basalts at 36°50′ to 38°56′S: major element, trace element, and isotopic data

We present new major and trace element data for 28 basalts and 10 basaltic glasses recovered from 16 locations from the North Chile Ridge (NCR) at 36°50′ to 38°56′S. This part of the Chile Ridge consists of three short ridge segments, which are characterized by deep (3200–4100 m) axial valleys. Chemical compositions of the basaltic glasses vary from primitive to moderately fractionated basalts (MgO = 9.54−7.28 wt%). All rocks are incompatible element depleted, mid-ocean ridge basalts (MORB) with average chondrite-normalized (La/Sm) N ratios of 0.46 ± 0.08. The radiogenic isotopic ratios of 10 representative samples display a narrow range in 87Sr 86Sr ratios from 0.70241 to 0.70249. 143Nd 144Nd ratios also vary within a small range from 0.51312 to 0.51318, and the 206Pb 204Pb ratios range from 18.2 to 18.6, with one exception which has a 206Pb 204Pb of 19.1. Overall, isotopic compositions are similar to average depleted MORB from the EPR (East Pacific Rise) and MAR (Mid-Atlantic Ridge), but 207Pb 204Pb ratios are significantly higher. Pb isotopic systematics of East Pacific MORB reflect large-scale heterogeneities, which are probably the result of long-lived differences in Th/U ratios in the mantle. Significant differences exist in the inferred primary melt compositions between the NCR basalts and depleted MORB from the South EPR and Galapagos Spreading Centre (GSC). For a given MgO content, basalt glasses from the NCR have systematically higher Na and Ti and lower Ca concentrations than those from the South EPR and the GSC. This has been interpreted as indicating relatively low average degrees of melting, which is possibly the result of cooling the shallow asthenosphere near transform offsets. NCR basalts are, on average, more primitive than basalts from the EPR and GSC, implying the lack of a robust magmatic system in this part of the Chile Ridge. This, together with the characteristic ridge topography and short average segment length, suggests that the magmatic system is short-lived, similar to slow-spreading ridges. This can be attributed to lower upwelling rates and, consequently, low magma supply rates near the transform offsets. The majority of the basalts from the NCR are unusually depleted in Nb and Ta relative to average depleted MORB. Trace element modelling shows that the distinct trace element characteristics of the NCR lavas could be the result of melting a mantle which has experienced a previous melting episode. This episode was possibly related to upwelling and melting of the mantle beneath the Pacific-Farallon ridge more than a million years ago.

A new Mesozoic isochron chart of the northwestern Pacific Ocean: Paleomagnetic and tectonic implications

A new Mesozoic isochron chart of the northwestern Pacific Ocean has been completed by utilizing all the currently available data from 357 cruises. Our chart provides the first whole view of the Mesozoic magnetic anomaly lineations in the northwestern Pacific Ocean and yields essential information for understanding the problems of the Mesozoic magnetostratigraphic time scale and the evolutional history of the Pacific plate. The Mesozoic magnetostratigraphic time scale must be recalibrated using not only the Hawaiian lineation set but also other Mesozoic lineation sets. The lack of lineation M0 in the lineation sets other than the Hawaiian lineation set indicates that a reorganization of the ridges except the Pacific‐Farallon ridge occurred between chrons M1 and M0 (nearly 120 Ma). The eruption rate of the Shatsky hotspot seems to be comparable to that of the Reunion hotspot at their initial stage. The growth of the Pacific plate began in a small triangular region at about 190 Ma.

Tectonic evolution of the northern Pacific plate and Pacific-Farallon Izanagi triple junction in the Late Jurassic and Early Cretaceous (M21-M10)

Magnetic lineations in the northwestern Pacific, near the Shatsky Plateau, were interpreted to trace the tectonic history of the northern Pacific plate from the time of anomalies M21 to M10. At M22 time the Pacific-Farallon-Izanagi (P-F-I) triple junction was located southwest of the plateau and migrating north-northwest with respect to the Pacific plate. Shortly thereafter the stress field acting on the three plates changed significantly and the Pacific-lzanagi ridge reacted by pivoting 24° clockwise from M21 to M19 time. Several ridge propagation events occurred while the ridge adjusted to the new spreading regime. To the east of the M22 location of the junction it appears that this reorganization triggered a series of at least three ridge jumps along the Pacific-Farallon ridge. The geometry of magnetic isochron M21 suggests that several triple junctions existed simultaneously during the reorganization bordering one or more microplates. Differences in the spreading rate of the Japanese magnetic lineations north of the plateau imply that one microplate, which we call the Shatsky microplate, existed until about M15-M13 time. It was probably captured by either the Farallon or the Izanagi plate and subsequently destroyed by subduction. After M15 the P-F-I triple junction was reformed on seafloor that is now beneath the Shatsky Plateau. The junction drifted generally northeast, either in a ridge-ridge-ridge or in a ridge-fault-fault configuration, perhaps alternating between the two. From M11-M10 time the stress field on the three plates changed again and the Pacific-Farallon ridge southeast of the plateau reoriented 15° clockwise. At this time the P-F-I junction assumed a ridge-ridge-ridge geometry at the northeast end of the plateau.

Late Cretaceous and Paleogene tectonic evolution of the North Pacific Ocean

The Late Cretaceous history of the northern Pacific Ocean has not been adequately deciphered, largely because a major plate reorganization occurred during the Cretaceous magnetic quiet interval. Using primary data to reconstruct plate motions from fracture zone trends and Late Cretaceous seafloor spreading magnetic anomalies allows formulation of a reasonable sequence of events that accounts for all the geologic features of that region, especially the Emperor and Chinook troughs. The primary event in our reconstruction is the subduction of the old northwest Pacific triple junction. New relative plate motions imposed by formation of convergent boundaries along both the northern Pacific and Farallon plates caused the Farallon plate to crack. This subdivision occurred 82 m.y. ago and resulted in the formation of the Kula and Chinook plates. The Chinook plate was bounded on the north by the Chinook-Kula ridge, the western arm of the Great Magnetic Bight, on the west by the southern Emperor trough, a slowly spreading rift valley, on the south by the Mendocino transform, and on the east by the Chinook-Farallon ridge. The northern Emperor trough formed the initial western boundary of the Kula plate. From 82 m.y. ago to 50 m.y. ago this configuration was stable: the Chinook plate expanded, the Chinook-Kula ridge migrated north, and the Kula plate was subducted. With the subduction of this ridge, the Chinook plate adhered to the Pacific plate and the Pacific-Farallon ridge became the north-south-trending feature reflected by anomalies 22 and younger. This reconstruction obviates the awkward Late Cretaceous shifting of triple junctions found in other proposed histories and provides reasonable explanations for the several structural features of the region. To achieve closure of vector triangles representing the relative Late Cretaceous plate motions some extension must occur, presumably across the eastern Mendocino fracture zone, between the northern and southern portions of the Farallon plate. These two plates apparently were independent units between 105 and 50 m.y. ago; the initiation and cessation of rapid North Farallon-North American convergence 82 and 50 m.y. ago may have determined the nature and duration of the Laramide orogeny.

Geologic evolution of Hess Rise, central North Pacific Ocean

Cores from four Deep Sea Drilling Project (DSDP) sites (310, 464, 465, and 466) and seismic-reflection profiles provide data that are used to interpret the geological evolution and paleoenvironments of Hess Rise, a prominent oceanic plateau in the central North Pacific Ocean. Hess Rise apparently formed in the Southern Hemisphere along the western flank of the Pacific-Farallon Ridge 110 to 100 m.y. B.P. Core stratigraphies and lithologies show the response of sedimentation to subsidence and northward movement of Hess Rise on the Pacific plate. Oceanic islands, which crowned Hess Rise during its early evolution, were eroded and subsequently subsided below sea level. Major structural trends include three northwest-trending (∼327°) arms, or ridges, and an east-northeast-trending southern Hess Rise that parallels the Mendocino Fracture Zone (060°). Normal faults offset basement as much as 3,000 m along the southern edge and 1,500 m on the western flank of Hess Rise. Many faults were active during sedimentation. Tholeiitic basalt from the base of Hole 464, trachyte from the base of Hole 465A, and alkalic basalt clasts within sediment of Hole 466 show the diversity of rock types that constitute the igneous basement. A major rock unit is middle Cretaceous limestone, chalk, and minor chert that form the basal sedimentary unit. Some limestone samples, rich in organic carbon, reflect accumulation above the carbonate compensation depth (CCD) within a mid-water oxygen minimum zone. The organic-carbon-rich sediments probably were deposited on the submarine slopes of islands and banks that were at upper bathyal depths as Hess Rise crossed the wide equatorial divergence where increased upwelling and biogenic productivity contributed to high accumulation rates. The source of organic matter was mostly lipid-rich, autochthonous, marine organic matter. Analyses of sediment samples from across the Cretaceous-Tertiary boundary at Site 465 show that there was a significant decrease in surface water temperature and biological productivity. An abrupt increase in transition metals and iridium suggests that an outside source, perhaps extraterrestrial, was the cause for many of the sudden oceanographic, geochemical, and biological changes at the boundary.

Miocene spreading centre south of Isla Guadalupe

The Juan de Fuca, Rivera, Cocos and Nazca plates in the eastern Pacific are small fragments remaining from the fragmentation and partial subduction of the long, narrow Farallon Plate which began as early as 55 Myr ago1. This fragmentation apparently resulted from interaction of the very irregular Pacific–Farallon ridge with the uneven coastlines of the American Plate1–4 and involved the formation of ridge–trench transforms5, creation of new spreading centres6, ridge jumps1,7,8, deactivation of subduction zones2 and other phenomena1. We present here magnetic evidence that a fifth small fragment of the Farallon plate is preserved in the eastern Pacific off the coast of Baja, California. Its preservation is the result of an eastward jump of the Pacific–Farallon ridge ∼12.5 Myr ago which was preceded by asymmetric spreading and perhaps pivotal subduction1. Menard1 first suggested this idea and showed that anomaly 5B is symmetric and parallel to a linear trough trending southwards from Guadalupe Island. He named the small fragment of the Farallon plate preserved between this trough and the margin of Baja California to the east, the Guadalupe plate1. We show that both anomalies 5B and 5C symmetrically flank the Guadalupe trough. Our results and conclusions are not based on new magnetic data, but rather on reinterpretation of aeromagnetic profiles published by Taylor et al.9.

The fixed-hotspot hypothesis and origin of the Easter—Sala y Gomez—Nazca trace

Research Article| July 01, 1981 The fixed-hotspot hypothesis and origin of the Easter—Sala y Gomez—Nazca trace REX H. PILGER, JR.; REX H. PILGER, JR. 1Department of Geology, Louisiana State University, Baton Rouge, Louisiana 70803 Search for other works by this author on: GSW Google Scholar DAVID W. HANDSCHUMACHER DAVID W. HANDSCHUMACHER 2Seafloor Division, Code 360, Naval Ocean Research and Development Activity, NSTL Station, MS 39529 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1981) 92 (7): 437–446. https://doi.org/10.1130/0016-7606(1981)92<437:TFHAOO>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation REX H. PILGER, DAVID W. HANDSCHUMACHER; The fixed-hotspot hypothesis and origin of the Easter—Sala y Gomez—Nazca trace. GSA Bulletin 1981;; 92 (7): 437–446. doi: https://doi.org/10.1130/0016-7606(1981)92<437:TFHAOO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Reconstructions of the Pacific and Nazca plates suggest that both the Nazca and Tuamotu Ridges originated from a melting anomaly which existed beneath the Pacific-Farallon Ridge during the time interval between anomalies 19 and 11, as would be predicted from the fixed-hotspot hypothesis. The hotspot hypothesis, interpreted according to morphology of the Easter-Sala y Gomez-Nazca trace, would further suggest that the hotspot was located entirely beneath the Nazca plate during the time interval between anomalies 11 and 2′. At the end of this interval, the eastern boundary of the Easter plate formed, centered above the hotspot. According to this scheme, the present location of the hotspot is about 200 km west of Easter Island.However, the Easter model is incompatible with the hotspot model for the Hawiian-Emperior chain, if the hotspots are fixed relative to one another, as the age of the point on the Tuamotu Ridge which corresponds with the Hawaiian-Emperor bend (53 to 55 m.y.) is significantly greater than the age of the bend (43 m.y.). The fixed-hostpot hypothesis can be satisfied by the two traces only if one or a combination of the following conditions is met: the kinematic age of the Hawaiian-Emperor bend is substantially greater than the isotopically dated age, the magnetic-anomaly time scale is in significant error, and/or magnetic-anomaly identifications in the vicinity of the Tuamotu Ridge are erroneous.It might appear that other mechanisms for the origin of hotspot traces are preferable to the hotspot hypothesis. The most promising of these involves propagating fractures resulting from intraplate stress. This hypothesis has the advantage of accommodating anomalies in some hotspot traces, such as persistence of igneous activity long after passage of the trace over the hotspot. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Pacific-North American plate interaction and Neogene volcanism in coastal California

Palinspastic and global plate tectonic reconstructions of the southwestern United States, combined with analysis of the distribution of Cenozoic igneous activity suggest that Neogene and younger volcanic rocks in the Continental Borderland and Coast, Transverse and Peninsular Ranges appear to reflect subduction of the Farallon plate south of the Mendocino Fracture Zone and overriding of the Pacific—Farallon ridge. The igneous activity of Neogene age in coastal California appears to be a continuation of fields of magmatism in adjacent areas of southeastern California, Arizona and Baja California. With the close approach of the Pacific—Farallon ridge to the North American continental margin, progressively younger, hotter and thinner Farallon plate underwent subduction beneath California, resulting in more elevated temperatures at shallower depths and closer to the trench. Consequently, the associated volcanic arc apparently shifted to the west. Volcanic activity in coastal California appears to have been most widespread shortly after overriding of the Pacific—Farallon ridge occurred— that is, at that time at which the oceanic lithosphere was thinnest, and hot, partially molten asthenosphere was closest to the subduction zone. The zone of near-coast igneous activity in California was probably limited on the north by the extension of the Mendocino Fracture Zone in the descending Farallon plate. South of the fracture zone the lithosphere was young and hot; to the north it was older and, therefore, colder. Thus, Neogene igneous activity in the Basin-Range Province was associated with more typical subduction of older lithosphere involving a deeper, down-dip locus of melting, resulting in a wider arc-trench gap. The abrupt narrowing of the Neogene arc-trench gap from north to south occurred near the present-day Garlock Fault. The role of oblique and en echelon strike-slip faulting in permitting and localizing middle Cenozoic magmatism in the areas west of the San Andreas Fault is uncertain. Discrepancies in timing of movement on the northern and southern San Andreas Fault seem to require oblique strike-slip faulting, and by association, igneous activity, in the Borderland and Transverse Ranges prior to 10 m.y. B.P. Oblique, divergent strike-slip faulting in the San Francisco Bay area does seem to be responsible for significant igneous activity between 14 m.y. B.P. and present. The Page Mill, Grizzly peak , Sonoma and Clear Lake volcanic fields occur in zones of divergence between faults of the San Andreas, Calaveras and Hayward systems. Further, the age of these volcanic rocks corresponds with the documented displacement history of the San Andreas Fault.