地球内部的热事件与地壳上地幔结构的相关研究

本文计算并讨论了当地球内部有热脉动事件发生时非稳态的热作用模型及其对地热场和大地热流的影响。理论计算和实际资料表明,构造活动区和稳定地区应有不同的热作用过程,在构造活动区地壳内的低速,低阻层和上地幔的低速,低阻层的埋藏深度之差与该地的热活动史有一定的对应关系。如华北平原地区,观测的二层深度差为40—60公里,推测热脉动持续时间为40百万年,活动史为50—70百万年:而银川盆地二层间隔为36—40公里,活动史为30—40百万年     

Depth distribution of calcareous sediments in the Mesozoic and Cenozoic North Atlantic Ocean

The upper Mesozoic and Cenozoic distribution of calcareous, biogenic particles which are produced by planktonic foraminifers and nannoplankton and which are the most important components of pelagic sediments since mid-Mesozoic times, has been reconstructed using data from North Atlantic deep-sea drill sites. Two phases of sedimentation of carbonate-rich sediments are separated by an interval from 100 to 80 m.y. B.P. when CaCO₃ particles were diluted by chiefly terrigenous material. Prior to 100 m.y. B.P. the highest concentrations of calcareous matter were confined to the deepest part of the then 4–4.5 km deep North Atlantic. After 80 m.y. B.P. sediments with high concentrations of calcareous matter have been deposited above 3 km paleodepth, but during the last 25 m.y. also between 4 and 5.5 km paleo-water depth. The latter occurrence is associated with indications of downslope displacement of calcareous material into the abyssal plains of the deepest parts of the North Atlantic.     

A multi-resolution global land cover dataset through multisource data aggregation

Recent developments of 30 m global land characterization datasets(e.g., land cover, vegetation continues field) represent the finest spatial resolution inputs for global scale studies. Here, we present results from further improvement to land cover mapping and impact analysis of spatial resolution on area estimation for different land cover types. We proposed a set of methods to aggregate two existing 30 m resolution circa 2010 global land cover maps, namely FROM-GLC(Finer Resolution Observation and Monitoring-Global Land Cover) and FROM-GLC-seg(Segmentation), with two coarser resolution global maps on development, i.e., Nighttime Light Impervious Surface Area(NL-ISA) and MODIS urban extent(MODIS-urban), to produce an improved 30 m global land cover map—FROM-GLC-agg(Aggregation). It was post-processed using additional coarse resolution datasets(i.e., MCD12Q1, GlobCover2009, MOD44 W etc.) to reduce land cover type confusion. Around 98.9% pixels remain 30 m resolution after some post-processing to this dataset. Based on this map, majority aggregation and proportion aggregation approaches were employed to create a multi-resolution hierarchy(i.e., 250 m, 500 m, 1 km, 5 km, 10 km, 25 km, 50 km, 100 km) of land cover maps to meet requirements for different resolutions from different applications. Through accuracy assessment, we found that the best overall accuracies for the post-processed base map(at 30 m) and the three maps subsequently aggregated at 250 m, 500 m, 1 km resolutions are 69.50%, 76.65%, 74.65%, and 73.47%, respectively. Our analysis of area-estimation biases for different land cover types at different resolutions suggests that maps at coarser than 5 km resolution contain at least 5% area estimation error for most land cover types. Proportion layers, which contain precise information on land cover percentage, are suggested for use when coarser resolution land cover data are required.     

Case studies of the propagation characteristics of auroral TIDS with EISCAT CP2 data using maximum entropy cross-spectral analysis

In this paper case studies of propagation characteristics of two TIDs are presented which are induced by atmospheric gravity waves in the auroral F-region on a magnetic quiet day. By means of maximum entropy cross-spectral analysis of EISCAT CP2 data, apparent full wave-number vectors of the TIDs are obtained as a function of height. The analysis results show that the two events considered can be classified as moderately large-scale TID and medium-scale TID, respectively. One exhibits a dominant period of about 72 min, a mean horizontal phase speed of about 180 m/s (corresponding to a horizontal wavelength of about 780 km) directed south-eastwards and a vertical phase speed of 55 m/s for a height of about 300 km. The other example shows a dominant period of 44 min, a mean horizontal phase velocity of about 160 m/s (corresponding to a horizontal wavelength of about 420 km) directed southwestwards, and a vertical phase velocity of about 50 m/s at 250 km altitude.Max-Planck-Institut für Aeronomie, Germany. On leave from Department of Space Physics, Wuhan University, China     

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.     

Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania

On September 4, 2007, after 25 years of effusive natrocarbonatite eruptions, the eruptive activity of Oldoinyo Lengai (OL), N Tanzania, changed abruptly to episodic explosive eruptions. This transition was preceded by a voluminous lava eruption in March 2006, a year of quiescence, resumption of natrocarbonatite eruptions in June 2007, and a volcano-tectonic earthquake swarm in July 2007. Despite the lack of ground-based monitoring, the evolution in OL eruption dynamics is documented based on the available field observations, ASTER and MODIS satellite images, and almost-daily photos provided by local pilots. Satellite data enabled identification of a phase of voluminous lava effusion in the 2 weeks prior to the onset of explosive eruptions. After the onset, the activity varied from 100 m high ash jets to 2–15 km high violent, steady or unsteady, eruption columns dispersing ash to 100 km distance. The explosive eruptions built up a ∼400 m wide, ∼75 m high intra-crater pyroclastic cone. Time series data for eruption column height show distinct peaks at the end of September 2007 and February 2008, the latter being associated with the first pyroclastic flows to be documented at OL. Chemical analyses of the erupted products, presented in a companion paper (Keller et al. 2010), show that the 2007–2008 explosive eruptions are associated with an undersaturated carbonated silicate melt. This new phase of explosive eruptions provides constraints on the factors causing the transition from natrocarbonatite effusive eruptions to explosive eruptions of carbonated nephelinite magma, observed repetitively in the last 100 years at OL.     

Global sounding of sporadic E layers by the GPS/MET radio occultation experiment

The GPS radio occultation technique is sensitive for layered structures with horizontal scales of around 100 km and with vertical scales of a few 100 m or more at the Earth's limb. These structures cause strong fluctuations of the GPS L1 and L2 phase paths which have been measured by a GPS receiver onboard of Microlab-1 satellite in 730 km orbit during the GPS/Meteorology experiment (GPS/MET of UCAR, Boulder). By means of GPS/MET radio occultation data, profiles of electron density fluctuations are derived for the mesosphere/lower thermosphere region with a height resolution of around 1 km. Data analysis of 1900 radio occultation events in June/July 1995, 1540 events in October 1995, and 2690 events in February 1997 confirms seasonal dependence of sporadic E layers. The meridian slices of average sporadic E activity show a dominance of plasma irregularities in the summer hemisphere. The irregularities mainly occur at heights 90–110 km. Auroral and equatorial sporadic E, electron density depletions, and multiple ionization layers are also present in the high resolution GPS/MET data. The multiple layers often have a distance of around 5–10 km in height, and appear up to a height of 140 km (upper height limit for 50 Hz sampling rate of GPS receiver). For February and June, the GPS/MET observations are compared to ground-based observations of the Asia/Australia ionosonde chain.     

Love-Rayleigh wave incompatibility and possible deep upper mantle anisotropy in the Iberian peninsula

Love and Rayleigh wave phase velocities are analyzed with the goal of retrieving information about the anisotropic structure of the Iberian lithosphere. The cross-correlation method is used to measure the interstation phase velocities between diverse stations of the ILIHA network at periods between 20 and 120 s. Despite the 2-D structure of the network, the Love wave data are too few to enable an analysis of phase velocity azimuthal variations. Azimuthal averages of Love and Rayleigh wave phase velocities are calculated and inverted both in terms of isotropic and anisotropic structures. Realistic isotropic models explain the Rayleigh wave and short-period Love wave phase velocities. Therefore no significant anisotropy needs to be introduced in the crust and down to 100 km depth in the upper mantle to explain our data. A discrepancy is observed only at long periods, where the data are less reliable. Love wave data at periods between 80 and 120 s remain 0.15 km/s faster than predicted by isotropic models explaining the long-period Rayleigh wave data. Possibilities of biases in the measurements due to interferences with higher modes are examined but seem unlikely. A transversely isotropic model with 8% of S-wave velocity anisotropy in the upper mantle at depths larger than 100 km can explain the whole set of data. In terms of a classical model of mantle anisotropy, this corresponds to 100% of the crystals perfectly oriented in the horizontal plane in a pyrolitic mantle. This is a rather extreme model, which predicts at time delay between 0 and 2 seconds for split SKS.     

Geochronology of Gran Canaria,Canary Islands: Age of shield building volcanism and other magmatic phases

Forty-six new K-Ar age determinations are presented on whole rock samples and mineral separates from volcanic and subvolcanic rocks of Gran Canaria. The main subaerial shield building basaltic volcanism with estimated volume of about 1000 km³ was confined to the interval about 13.7 m.y. to 13.5 m.y. ago in the middle Miocene. Substantial volume (～100 km³) of silicic volcanics (trachyte and peralkaline rhyolite) were erupted with no detectable time break following the basaltic volcanism, essentially contemporaneous with formation of a large collapse caldera at 13.4±0.3 m.y. ago. Trachytic to phonolitic volcanism continued intermittently in the waning states of activity until about 9 m.y. ago. Following a long hiatus there was resurgence of volcanism with eruption of about 100 km³ of basanitic to hauyne phonolitic rocks of the Roque Nublo Group between about 4.4 m.y. and 3.4 m.y. ago in the Pliocene. After a hiatus of less than 1.0 m.y., olivine nephelinite magmas were erupted and this activity continued intermittently until relatively recent times, the younger eruptives being mainly basanitic in composition. The volume of volcanic products in this phase probably does not exceed 10 km³. Thus the volume of all the resurgent volcanism comprises less than 10 percent of the subaerially exposed part of Gran Canaria. The results show that the subaerial main shield building phase of volcanism in Gran Canaria, consisting of mildly alkali to transitional basalts, occurred over a time interval that was less than 0.5 m.y. Magmatic evolution on Gran Canaria appears to be similar to that found on other basaltic volcanoes in oceanic regions. Thus volcanoes in the Hawaiian, Marquesas and Society Islands all were built by basaltic lavas in similar short-lived episodes of volcanism. In some Hawaiian volcanoes, a resurgent phase of volcanism of strongly undersaturated basalts of small volume is recognized following a long hiatus, again similar to that found on Gran Canaria. The relatively large volume of silicic lavas erupted in Gran Canaria immediately following the main basaltic shield building phase is, however, not matched in the Pacific volcanoes mentioned.     

Lunar tidal winds in the upper atmosphere over Collm

The lunar semidiurnal tide in winds measured at around 90 km altitude has been isolated with amplitudes observed up to 4 m s^–1. There is a marked amplitude maximum in October and also a considerable phase variation with season. The average variation of phase with height indicated a vertical wavelength of more than 80 km but this, and other results, needs to be viewed in the light of the considerable averaging required to obtain statistical significance. Large year-to-year variations in both amplitude and phase were also found. Some phase comparisons with the GSWM model gave reasonable agreement but the model amplitudes above a height of 100 km were much larger than those measured. An attempt to make a comparison with the lunar geomagnetic tide did not yield a statistically significant result.     

A three-dimensional crustal geoelectric model of the Ukrainian Shield

A 3-D crustal geoelectric model of the Ukrainian Shield (USh) is constructed from magnetic variation data. High electrical conductivity anomalies with resistivities of 1–100 Ω m are located at depths of up to 30 km from the basement surface. A high conductivity layer with ρ = 25 Ω m and with its upper boundary at a depth of 70 km is supposed to exist in the upper mantle of the southwestern USh.     

Evolution of the Upper Cenozoic Magmatic Arc and plate boundary in northern New Zealand

The Upper Cenozoic Magmatic Arc in northern New Zealand was initiated when the Indian-Pacific plate boundary first spread through the North Island approximately 20 m.y. ago. Six geographically separated magmatic arcs are recognized in succession. The first (20-15 m.y.) was sited over a basement depression; lavas were basic to intermediate and largely submarine; mineralization was minor. Subsequent arcs were sited over basement horst and characterized by sub-aerial intermediate to acid magmas. After prolonged andesitic/dacitic activity (18-6 m.y.) with minor mineralization, prolific rhyolite/ignimbrite eruption began at about 6 m.y., with abundant mineralization. Behind-arc activity produced localized basalt fields in the north, and geographically restricted high-potash andesites in the south.The first four arcs in the series are aligned at about 70° to the active Tonga-Kermadec-Taupo arc. The migration and rotation of the older New Zealand arcs are ascribed to four processes taking place at the plate boundary. These are: (1) anti-clockwise bending of the crust of western North Island, obliquely to the movement of the underlying lithosphere of the Indian plate, beginning at about 3 m.y., accompanying (2) dextral transcurrent displacement of 230 km with respect to eastern North Island; taking place mostly from 3 to 0 m.y.; (3) steepening of the Benioff zone from an initial 18° dip at 20 m.y. to the present 55° to 60°; and (4) fracturing of the west-dipping lithospheric slab to give two parallel, low-potash andesitic arcs between 18 and 15 m.y.Eastern North Island is deduced to have been “floating” while Pacific plate lithosphere passed beneath it throughout the Upper Cenozoic; accordingly it is designated the Hawkes Bay Crustal Microplate.There is good agreement between major tectonic events in the South Pacific deduced by Molnar et al. from magnetic anomaly studies and major tectonic events on land. A tentative history of the Southwest Pacific is proposed for the last 40 m.y.     

Observation of Sea Breeze Front and its Induced Convection over Chennai in Southern Peninsular India Using Doppler Weather Radar

Sea breeze, the onshore wind over a coastal belt during daytime, is a welcoming weather phenomenon as it modulates the weather condition by moderating the scorching temperature and acts as a favourable mechanism to trigger convection and induce precipitation over coastal and interior locations. Sea breeze aids dispersal of pollutants as well. Observational studies about its onset, depth of circulation and induced precipitation have been carried out in this paper for the period April to September, 2004–2005 using a S-band Doppler Weather Radar functioning at Cyclone Detection Radar Station, India Meteorological Department, Chennai, India. The onset of sea breeze has been observed to be between 0900 and 1000 UTC with the earliest onset at 0508 UTC and late onset at 1138 UTC. The frequency is greater during the southwest monsoon season, viz., June – September and the frequency of initial onset is greater in north Chennai. The modal length of sea breeze is between 20 and 50 km with extreme length as high as 100 km also having been observed. Though the inland penetration is on average 10 to 20 km, penetration reaching 100 km was also observed on a number of cases. The induced convection could be seen in the range 50–100 km in more than 53% of the cases. The mean depth of sea breeze circulation is 300–600 m but may go well beyond 1000 m on conducive atmospheric conditions.     

Stratigraphy of the Toba Tuffs and the evolution of the Toba Caldera Complex,Sumatra, Indonesia

During the past 1.2 m.y., a magma chamber of batholithic proportions has developed under the 100 by 30 km Toba Caldera Complex. Four separate eruptions have occurred from vents within the present collapse structure, which formed from eruption of the 2800 km³ Youngest Toba Tuff (YTT) at 74 ka. Eruption of the three older Toba Tuffs alternated from calderas situated in northern and southern portions of the present caldera. The northern caldera apparently developed upon a large andesitic stratovolcano. The calderas associated with the three older tuffs are obscured by caldera collapse and resurgence resulting from eruption of the YTT. Samosir Island and the Uluan Block are two sides of a single resurgent dome that has resurged since eruption of the YTT. Samosir Island is composed of thick YTT caldera fill, whereas the Uluan Block consists mainly of the Oldest Toba Tuff (OTT). In the past 74000 years lava domes have been extruded on Samosir Island and along the caldera's western ring fracture. This part of the ring fracture is the site of the only current activity at Toba: updoming and fumarolic activity. The Toba eruptions document the growth of the laterally continuous magma body which eventually erupted the YTT. Repose periods between the four Toba Tuffs range between 0.34 and 0.43 m.y. and give insights into pluton emplacement and magmatic evolution at Toba.     

Fundamentals of ridge crest topography

A linear relationship between the sea floor depth and the square root of age has been found for ocean lithosphere spreading from mid-ocean ridges. The asymptotic solution of depth as a function of age for the thermally contracting lithosphere predicts a linear dependence of depth ontwith a proportionality involving the initial lithosphere temperature, the thermal diffusivity, and the isostatic expansion coefficient averaged to include any temperature dependent phase changes. Empirical depth observations, when plotted as a function of the square root of age, bear out this prediction well, but there is a variation in the gradient,ht, along the ridge on a fine scale (up to 20% over 200 km). This implies a fundamental variation of the contraction parameter over the same scale, most probably of compositional origin. Details of a more complete cooling model near the ridge crest, including a crust of different thermal parameters than those of the mantle, predict a crestal height about 0.2 km below that of the simplified model. Individual profiles from the southeast Pacific show no such crestal deviation, and it is concluded that by quickly cooling the new crust, hydrothermal circulation may remove any effects of the crust which would be seen in the topography of a lithosphere cooled totally by conduction. The straightness of depth versust for older ocean data (to 80 m.y.) precludes any basal isothermal boundary shallower than 100 km.     

The azimuthal variation of teleseismic P-wave residuals for stations in southern California

A study of teleseismic P-residuals for 13 stations in southern California reveals that these vary systematically with the azimuth of the event; the residuals generally tend to be negative in the NW and SE quadrants. This effect is greatest in the north of the region: a maximum peak-to-peak variation of 1.3 sec with residuals as low as ?0.9 sec was observed at Isabella. The delays become less negative further south in the region, and at the southernmost station, Glamis, delays of up to +0.75 sec were observed to the west. The latter may be associated with velocity decreases in the active geothermal areas of the Imperial Valley. A simple model, consisting of a region some 175 km wide and 100 km thick at a depth of approximately 100 km in which a velocity increase of up to 0.45 km/sec occurs, is proposed to account for the observed variations. This region trends NW-SE approximately parallel to the North America-Pacific plate boundary. The proposed velocity changes are consistent with a pinching out of the low-velocity (partial melting) zone under the area, such as has been proposed to explain the low heat flow in the Sierra Nevada, and may be related to the presence of an ancient (～ 30 m.y.) subduction zone under southern California.     

Magnetotelluric soundings over the northeast Pacific may reveal spatial dependence of depth and conductance of the asthenosphere

A seafloor electrical conductivity profile resulting from a more thorough analysis of magnetotelluric data from station S.F. Revisited than previously presented is compared to an earlier profile at Farewell to Aggy, station III. Both stations are located over the same interfracture zone segment of the Pacific plate, the first roughly 700 km off the coast of California (position 31°18′N, 128°20′W, water depth 4.5 km, plate age 30 m.y. estimated from nearby magnetic reversal number 12), the second approximately 800 km to the NNE of the mainland of Hawaii (position 26°32′N, 151°20′W, depth 5.3 km, age 72 m.y. estimated from adjacent magnetic reversal 30–31).The seafloor impedances at S.F. Revisited are only mildly polarized and their interpretation in terms of an isotropic, horizontally layered structure suggests the occurrence at about 85 km depth of a highly conducting layer with a conductance exceeding by roughly 4 × 10³ S, an otherwise monotonically increasing conductivity trend. The implied dependence of conductivity with depth is therefore similar to that found earlier for station III, however with the following differences: the high-conductivity layer at station III occurs at a greater depth (140 km), it appears to have a slightly reduced excess conductance over the background, 3.5 × 10³ S although this evidence should be used with caution, and the lithospheric conductivity at station III, surprisingly seems to be somewhat higher, an effect possibly related to the proximity of the Hawaiian chain and to its generic processes.     

A post-Transvaal age for the Marydale Formation,Kheis Group,Southern Africa

The metabasic Marydale Formation of the Kheis Group occupies a zone of contact between the Sanama and Kaapvaal structural provinces of South Africa. Stratigraphic relationships between the two provinces are not well understood. Whilst the well-known Kaapvaal basement and supracrustal succession yield radiometric ages older than 1900 m.y., Sanama Province ages reflect a Kibaran(1200 ± 200m.y.) tectogenetic cycle. The age of the Marydale, stratigraphically the oldest Sanama formation, has been variously estimated at2500m.y., about 2000 m.y., or Kibaran, based on controversial field interpretations or on available radiometric data.Rb-Sr data are presented for Marydale samples from a nappe-like body which, having been thrust over the Kaapvaal basement, was shielded from metamorphism. Two types of alteration are described and possible causes of isotopic homogenisation are discussed. It is concluded that an isochron age of 1899± 57m.y. (I = 0.7040 ± 0.0003) represents the age of extrusion of the Marydale volcanics.The stratigraphic controversy is thus resolved, Kheis Group formations being approximately coeval with the Matsap, the youngest formation of the Kaapvaal Precambrian succession. The implications of this and other recent work to theories of crustal evolution are considered. It is suggested that the continental crust of Sanama Province originated partly during the Eburnian(2000 ± 100m.y.) period of African orogeny and partly during the Kibaran tectogenetic cycle during which the province became cratonised and was added to the Southern African cratonic block.     

Geoelectromagnetic measurements across the southern Senegal basin (West Africa)

Magnetotelluric and differential geomagnetic sounding surveys, consisting of nine soundings, were performed in 1984 along a 200-km profile across the southern Senegal basin. They were intended to obtain information concerning the resistivity structure of the crust and upper mantle and the distribution of the induced electric currents. Magnetotelluric data indicate that two-dimensional resistivity models are appropriate for the region. The zone above the basement is highly inhomogeneous in geoelectrical structure. Very conductive sediments (0.6-3 ohm m) appear in the Mesozoic-Cenozoic Senegal basin. These sediments lie at depths of up to 4500 m on the west end of the profile. Below this, a modest resistivity material (10–30 ohm m) extends to a maximum depth of about 3000 m. The material at depth on the cast part of the traverse line is thought to be Palaeozoic sediments of the Bove basin. The depth of the magnetotelluric basement lies between about 250 m (in the east) and 4800 m (in the west). The crust is characterised by a drop in electrical resistivity at a depth of 15 km below the east part of the profile. Considering the total section, we observe a general trend towards lower resistivities at depths in excess of 100 km, the transition from 2000 ohm m to about 2 ohm m occurs in the depth range 100 to 175 km. An analysis of the geomagnetic variation field has identified a concentration of telluric current flow beneath the deep basin. It appears that the additional currents flowing in the striking direction of the Senegal basin are largely controlled by sedimentary rocks of high conductivity lying at depths less than 5 km. Model studies show that the local conductivity distribution is able to explain the currents circulating in the thick well-conducting sediments.     

The evolution of the Parece Vela Basin,eastern Philippine Sea

The Parece Vela Basin is a back-arc basin. It is approximately 5000 m deep and is divided into two topographic provinces by the north-trending Parece Vela Rift. The western province is thinly sedimented and topographically rough. The eastern province is blanketed by a thick apron of volcaniclastic sediments which were derived from the West Mariana Ridge. The Parece Vela Rift is composed of a series of discrete deeps and troughs with depths commonly of 6 km and locally exceeding 7 km.Petrologic and seismic refraction data indicate that the Parece Vela Basin is of oceanic character.Low-amplitude, nort-trending, lineated magnetic anomalies are present in the basin and appear symmetric about a line near the Parece Vela Rift. In the central latitudes of the basin seafloor spreading anomalies 10 (30 m.y. B.P.) to 5E or 5D (18 or 17 m.y. B.P.) can be identified. The uncertainty in identifying the youngest anomaly may be due to ridge jumps near the end of spreading. Spreading may have started slightly later in the northern end of the basin. Anomalies in the eastern province are disrupted and are difficult to correlate. DSDP results indicate post-spreading volcanism on the eastern side of the basin and this may have degraded the anomalies. The age obtained in the western province of the basin at DSDP Site 449 (～25m.y. B.P.) is in close agreement with that obtained from the magnetic data (～26m.y. B.P.).It is hypothesized that subduction was occurring at a west-dipping subduction zone east of the Palau-Kyushu Ridge in the Early Oligocene. This volcanic arc split about 31 or 32 m.y. ago and interarc spreading was initiated between the Palau-Kyushu Ridge (which then became a remnant arc) and the West Mariana Ridge. The Parece Vela Basin formed between the ridges by two-limb seafloor spreading. Spreading stopped about 17 or 18 m.y. ago.Like certain other marginal basins, the Parece Vela Basin is deeper than predicted from depth vs. age curves. The average heat flow for the Parece Vela Basin is in agreement with that predicted from heat flow vs. age curves.The origin of the Parece Vela Rift is unclear. It may represent the extinct spreading center or may be a postspreading feature.