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1.
The fixity of hotspots and mantle plume locations has long been axiomatic. If the assumption of fixed hotspots is granted, ‘absolute’ plate motions and movements of the spin axis with respect to the hotspot framework, defined by some as True Polar Wander (TPW), can be determined. However, this assumption can be tested by paleomagnetic data, and such tests are gradually raising some doubts about the fixity of hotspots. The result is that discrepancies between Cretaceous and Tertiary hotspot and paleomagnetic reference frames are now beginning to be interpreted as the result of plume drift within a convective mantle. In the Indo-Atlantic, hotspots have remained relatively stationary with respect to the spin axis for the last 95 million yr. However, the Pacific hotspots, notably Hawaii, appear to have undergone large-scale southward drift with respect to the spin axis during the Early Tertiary. Global paleomagnetic data do not indicate that any TPW occurred during the Late Cretaceous or Tertiary. Although the Early Cretaceous paleomagnetic and hotspot frames for the Indo-Atlantic realm can be interpreted as slow TPW, direct estimates of paleolatitude and hotspot motion, in particular the Kerguelen hotspot, challenge TPW as a global phenomenon. At present, we consider that the large Early Cretaceous discrepancy between hotspot and paleomagnetic data is best explained by southward drift of the Atlantic hotspots prior to ∼95 Ma.  相似文献   

2.
Pacific plate equatorial sediment facies provide estimates of the northward motion of the Pacific plate that are independent of paleomagnetic data and hotspot tracks. Analyses of equatorial sediment facies consistently indicate less northward motion than analyses of the dated volcanic edifices of the Hawaiian-Emperor chain. The discrepancy is largest 60–70 Ma B.P.; the 60- to 70-Ma equatorial sediment facies data agree with recent paleomagnetic results from deep-sea drilling on Suiko seamount [1] and from a northern Pacific piston core [2]. Equatorial sediment facies data and paleomagnetic data, combined with K-Ar age dates along the Emperor chain [3], indicate a position of the spin axis at 65 Ma B.P. of 82°N, 205°E in the reference frame in which the Pacific Ocean hotspots are fixed. This pole agrees well with the position of the spin axis in the reference frame in which the Atlantic Ocean hotspots and the Indian Ocean hotspots are fixed [4,5], supporting the joint hypotheses that (1) the Pacific Ocean hotspots are fixed with respect to the hotspots in other oceans, (2) the hotspots have shifted coherently with respect to the spin axis, and (3) the time average of the earth's magnetic field 65 Ma B.P. was an axial geocentric dipole. Global Neogene paleomagnetic data suggest that a shift of the mantle relative to the spin axis has been occurring during the Neogene in the same direction as the shift between 65 Ma B.P. and the present. All data are consistent with a model in which the hotspots (and by inference the mantle) have shifted with respect to the spin axis about a fixed Euler pole at a constant rate of rotation for the last 65 Ma.  相似文献   

3.
The Comores Islands together with the Tertiary volcanic province of northern Madagascar form a sublinear trend of alkali olivine basalt shield volcanoes across the northern entrance of the Mozambique Channel. Potassium-argon dating of shield-building lavas confirms an eastward increase in age of volcanism along the chain, consistent with a hotspot origin for the lineament. The velocity of the Somali plate over the mantle magma source is 50 mm/yr.

We use the distribution of ages along the Comores-Madagascar chain in conjunction with existing age data for the Reunion-Mascarene Plateau hotspot track to model the absolute motion of the Somali plate for the last 10 m.y. We calculate the relative motion across the East African Rift by subtracting the Somali plate absolute motion from African plate absolute motion during this period. The model predicts 320 km of total separation across the East African Rift during the past 10 m.y. which is greater than has been estimated from surface geological evidence

The geometry of older portions of the Comores and Reunion trends indicates that there was no significant relative motion between the African and Somali plates prior to about 10 m.y. ago.  相似文献   


4.
New K/Ar dating and geochemical analyses have been carried out on the WNW–ESE elongated oceanic island of S. Jorge to reconstruct the volcanic evolution of a linear ridge developed close to the Azores triple junction. We show that S. Jorge sub-aerial construction encompasses the last 1.3 Myr, a time interval far much longer than previously reported. The early development of the ridge involved a sub-aerial building phase exposed in the southeast end of the island and now constrained between 1.32 ± 0.02 and 1.21 ± 0.02 Ma. Basic lavas from this older stage are alkaline and enriched in incompatible elements, reflecting partial melting of an enriched mantle source. At least three differentiation cycles from alkaline basalts to mugearites are documented within this stage. The successive episodes of magma rising, storage and evolution suggest an intermittent re-opening of the magma feeding system, possibly due to recurrent tensional or trans-tensional tectonic events. Present data show a gap in sub-aerial volcanism before a second main ongoing building phase starting at about 750 ka. Sub-aerial construction of the S. Jorge ridge migrated progressively towards the west, but involved several overlapping volcanic episodes constrained along the main WNW–ESE structural axis of the island. Mafic magmas erupted during the second phase have been also generated by partial melting of an enriched mantle source. Trace element data suggest, however, variable and lower degrees of partial melting of a shallower mantle domain, which is interpreted as an increasing control of lithospheric deformation on the genesis and extraction of primitive melts during the last 750 kyr. The multi-stage development of the S. Jorge volcanic ridge over the last 1.3 Myr has most likely been greatly influenced by regional tectonics, controlled by deformation along the diffuse boundary between the Nubian and the Eurasian plates, and the increasing effect of sea-floor spreading at the Mid-Atlantic Ridge.  相似文献   

5.
Reconstructions to total closure of the Australia-Antarctic continents causes an unacceptable overlap of Broken Ridge and Kerguelen Plateau. This has been partially resolved in the past by supposing that the northern part of Kerguelen, that is principally involved in the overlap, is younger than the remainder. We have revised the early reconstructions using a newly proposed breakup chronology of Australia and Antarctica which suggests that opening began at least 90 m.y. B.P. at an initially slow rate. This eliminates the overlap problem without invoking major age differences within the Kerguelen Plateau. We also suggest that the northeastern flank of Kerguelen Plateau may be underlain by the “missing” westward continuation of the Diamantina Zone. It may have been isolated on the Antarctic plate by a ridge crest jump at about anomaly 24 time that also formed the Ob Trench.  相似文献   

6.
We report detailed rock-magnetic and paleomagnetic directional data from 35 lava flows (302 standard paleomagnetic cores) sampled in the Central-Northern region of Uruguay in order to contribute to the study of the paleosecular variation of the Earth’s magnetic field during early Cretaceous and to obtain precise Cretaceous paleomagnetic pole positions for stable South America. The average unit direction is rather precisely determined from 29 out of 35 sites. All A95 confidence angles are less than 8°, which points to small within-site dispersion and high directional stability. Normal polarity magnetizations are revealed for 19 sites and 10 are reversely magnetized. Two other sites yield well defined intermediate polarities. The mean direction, supported by a positive reversal test is in reasonably good agreement with the expected paleodirection for Early Cretaceous stable South America and in disagreement with a 10° clockwise rotation found in the previous studies. On the other hand, paleomagnetic poles are significantly different from the pole position suggested by hotspot reconstructions, which may be due to true polar wander or the hotspot motion. Our data suggest a different style of secular variation during (and just before) the Cretaceous Normal Superchron and the last 5 Ma, supporting a link between paleosecular variation and reversal frequency.  相似文献   

7.
The geochemical characteristics of mildly alkalic basalts (24–25 Ma) erupted in the southeastern Kerguelen Archipelago are considered to represent the best estimate for the composition of the enriched Kerguelen plume end-member. A recent study of picrites and high-MgO basalts from this part of the archipelago highlighted the Pb and Hf isotopic variations and suggested the presence of mantle heterogeneities within the Kerguelen plume itself. We present new helium and neon isotopic compositions for olivines from these picrites and high-MgO basalts (6–17 wt.% MgO) both to constrain the enriched composition of the Kerguelen plume and to determine the origin of isotopic heterogeneities involved in the genesis of Kerguelen plume-related basalts. The olivine phenocrysts have extremely variable 4He / 3He compositions between MORB and primitive values observed in OIB (∼90,000 to 40,000; i.e., R / Ra ∼8 to 18) and they show primitive neon isotopic ratios (average 21Ne / 21Neext ∼0.044). The neon isotopic systematics and the 4He / 3He ratios that are lower than MORB values for the Kerguelen basalts clearly suggest that the Kerguelen hotspot belongs to the family of primitive hotspots, such as Iceland and Hawaii. The rare gas signature for the Kerguelen samples, intermediate between MORB and solar, is apparently inconsistent with mixing of a primitive component with a MORB-like source, but may result from sampling a heterogeneous part of the mantle with solar 3He / 22Ne and with a higher (U, Th) / 3He ratio compared to typically high R / Ra hotspot basalts such as those from Iceland and Hawaii.  相似文献   

8.
Geochemical and isotope results are presented from a new study of the most southern basalts in the Deccan Trap, India. Three chemical formations are recognised, two of which can be correlated with the established stratigraphy in Mahabaleshwar and imply a regional southerly dip of 0.06° over a distance of 250 km. In detail Sr-isotope variations within the Ambenali and Mahabaleshwar Formations can be shown to reflect three distinct end-members which provide new constraints for petrogenetic models. Pb-isotope data for selected basalts exhibit a wide range with206Pb/204Pb= 16.87–22.45, and a linear correlation on a Pb—Pb diagram. The least contaminated Ambenali basalts plot within the Pb-array, and interaction with mantle lithosphere involves a shift to less radiogenic Pb whereas contamination with crust is characterised by more radiogenic Pb. Unlike the Karoo and Parana continental flood basalt provinces only four flow units within the southern Deccan appear to contain a significant contribution from mantle lithosphere. The Mahabaleshwar and Ambenali Formation basalts exhibit a striking negative Pb—Sr isotope trend which is presently regarded as one of the features of interaction with shallow level lithospheric mantle. It further suggests that basalts from the Walvis Ridge, Kerguelen and Ninetyeast ridge all remobilised such shallow level material, and that the Deccan basalts which were not affected by crustal contamination reflect interaction between asthenospheric material similar to T-type MORB, but related to the Reunion hotspot, and continental mantle lithosphere of the Indian plate.  相似文献   

9.
Seamount magnetic anomaly inversions as well as DSDP paleomagnetic and equatorial sediment facies data constrain a paleomagnetic pole for the Pacific plate of Late Eocene age. The location of the pole at 77.5°N, 21.2°E implies 12.5 ± 1.6° of apparent polar wander for the Pacific plate during the last 41 ± 5 m.y. The Late Eocene pole is significantly different from the Pacific Maastrichtian pole at the 95% confidence level and indicates 7.2° of apparent polar motion of the Pacific between 69 and 41 m.y. B.P. The data source locations for the Late Eocene pole are scattered over a large area of the North Pacific and thus the consistency of the data supports the hypothesis that the north central Pacific plate has been rigid since the Eocene. The agreement of the Late Eocene pole with the motion predicted for the Pacific from hotspot models suggests that relative motion between the spin axis and hotspots has been small since that time. Additionally, this finding dictates that the significant amounts of hotspot versus spin axis motion inferred by other authors to have occurred since the Cretaceous must have instead occurred at a faster rate and concluded before the Eocene.  相似文献   

10.
Interaction of deformation axes during pure translation of a hanging-wall over a footwall composed by frontal and oblique ramps is carefully evaluated together with the evolution of associated paleomagnetic vectors. Four different cases are distinguished on the basis of the deflection on the paleomagnetic vectors when the bedding correction is applied during the restoration process. Two cases (frontal and oblique ramp without mutual interaction) do not produce any deflection. But two cases in the transition zone between both ramps will undergo non-coaxial axis of tilting during progressive deformation. One of them will produce spurious rotation if the bedding correction is applied. These errors will affect the oroclinal bending diagram as well as the fold test producing an apparent oroclinality and an apparent syn-folding magnetization respectively. A well-known geometry and kinematics of the thrust system is needed to properly restore the beds (and vectors) and to avoid the spurious rotations and its collateral effects in paleomagnetic investigations. A paleomagnetic study in the Pyrenean External Sierras is shown as an example. Primary Eocene vectors underwent a clockwise rotation (40° about) during the emplacement of the South Pyrenean sole thrust, however the Rasal-Gabardiella system of oblique ramps display spurious rotations ranging from –8° up to 13° if the inappropriate bedding correction is performed.  相似文献   

11.
A paleomagnetic study was carried out on the mid-Cretaceous sedimentary strata in west-central Kyushu Island, southwest Japan, to elucidate the origin of sedimentary basins along the Asian continental margin in the Cretaceous. We collected paleomagnetic samples from a total of 34 sites of the mid-Cretaceous Goshonoura Group, shallow-marine clastic deposits in west-central Kyushu, and characteristic remanent magnetizations were recognized from 18 horizons of red beds. Thermal demagnetization has revealed that the red beds contain three magnetization components, with low (<240°C), intermediate (240-480°C), and high (480-680°C) unblocking temperatures. The low unblocking temperature component is present-field viscous magnetization, and the intermediate one is interpreted as chemical remanent magnetization carried by maghemite that was presumably formed by post-folding, partial oxidation of detrital magnetite. Rock magnetic and petrographic studies suggest that the high unblocking temperature component resides largely in hematite (martite and pigmentary hematite) and partly in maghemite. Because of the positive fold test, this high temperature component can be regarded as primary, detrital remanent magnetization. The tilt-corrected mean direction of the high temperature component is Dec=65°, Inc=63° with α95=5°, which yields a paleomagnetic pole at 39°N, 186°E and A95=8°. A combination of this pole with those of the Late Cretaceous rocks in southwest Japan defines an apparent polar wander path (APWP), which is featured by a cusp between the Late Cretaceous and the Paleogene. A comparison of this APWP with the coeval paleomagnetic pole from northeast Asia suggests an approximately 50° post-Cretaceous clockwise rotation and 18±8° southward drift with respect to northeast Asia. The southward transport of the Cretaceous basin suggests that the proto-Japanese arc originated north of its present position. We propose that the coast-parallel translation of this landmass was caused by dextral motion of strike-slip faults, which previous geodynamic models interpreted to be sinistral through the Mesozoic. The change in strike-slip motion may have resulted from Mesozoic collision and penetration of exotic terranes, such as the Okhotsk microcontinent, with the northeastern part of Asia.  相似文献   

12.
Bathymetry, gravity and deep-tow sonar image data are used to define the segmentation of a 400 km long portion of the ultraslow-spreading Knipovich Ridge in the Norwegian-Greenland Sea, Northeast Atlantic Ocean. Discrete volcanic centers marked by large volcanic constructions and accompanying short wavelength mantle Bouguer anomaly (MBA) lows generally resemble those of the Gakkel Ridge and the easternmost Southwest Indian Ridge. These magmatically robust segment centers are regularly spaced about 85-100 km apart along the ridge, and are characterized by accumulated hummocky terrain, high relief, off-axis seamount chains and significant MBA lows. We suggest that these eruptive centers correspond to areas of enhanced magma flux, and that their spacing reflects the geometry of underlying mantle upwelling cells. The large-scale thermal structure of the mantle primarily controls discrete and focused magmatism, and the relatively wide spacing of these segments may reflect cool mantle beneath the ridge. Segment centers along the southern Knipovich Ridge are characterized by lower relief and smaller MBA anomalies than along the northern section of the ridge. This suggests that ridge obliquity is a secondary control on ridge construction on the Knipovich Ridge, as the obliquity changes from 35° to 49° from north to south, respectively, while spreading rate and axial depth remain approximately constant. The increased obliquity may contribute to decreased effective spreading rates, lower upwelling magma velocity and melt formation, and limited horizontal dike propagation near the surface. We also identify small, magmatically weaker segments with low relief, little or no MBA anomaly, and no off-axis expression. We suggest that these segments are either fed by lateral melt migration from adjacent magmatically stronger segments or represent smaller, discrete mantle upwelling centers with short-lived melt supply.  相似文献   

13.
New paleomagnetic investigations on the Ethiopian trap series have been undertaken at the Abbay and Kessem gorges in an attempt to better constrain the 30 Ma paleomagnetic pole of Africa. We sampled six thick massive basaltic lava flows, totaling 230 m, from Abbay Gorge and 10 lava flows, 180 m in thickness, from Kessem Gorge. Detailed paleomagnetic analyses disclosed that the carriers of the characteristic remanent magnetization (ChRM) are different in different lava flows. These are mostly titanomagnetites, titanomaghemites, and magnetite minerals with a broad range of coercive force and blocking temperatures. The heating and cooling susceptibility vs. temperature curves, many of which are irreversible, may indicate chemical remagnetization, notably low temperature maghemitization. Only one flow (KS04) with a clear 580°C Curie temperature was apparently unaffected by chemical remagnetization. The ChRM direction of this flow is identical to that in other flows, which suggests that if and when remagnetization occurred, this was shortly after emplacement of the lava flows. All of the flows sampled have normal polarity. However, a reversed component of low to medium coercive force and low to medium unblocking temperature occurs in flow KS01 at Kessem Gorge. The ChRM directions for the 16 sites are D=3.1°, I=5.8° (α95=12.7°). The paleomagnetic pole obtained from these is at λ=83.0°N, φ=193.3°E (A95=9.0°). Comparison with three previous studies of the traps shows remarkable consistency and a number of means are derived and discussed. Two final preferred poles for the traps are at λ=79.0°N, φ=196.9°E (A95=2.8°) when all 112 published flows are used, and λ=78.7°N, φ=209.4°E (A95=3.4°) when only the 76 flows from the four more recently analyzed sections are included. Both are compatible with the recent reference synthetic pole for Africa of Courtillot and Besse [J. Geophys. Res. (2002) in press]. In that sense, the Ethiopian trap pole is not anomalous and does not require more of a non-dipolar contribution than indicated by analyses of the global paleomagnetic data base covering the last few million years.  相似文献   

14.
The segmentation of the Mid-Atlantic Ridge between 29°N and 31°30′ N during the last 10 Ma was studied. Within our survey area the spreading center is segmented at a scale of 25–100 km by non-transform discontinuities and by the 70 km offset Atlantis Transform. The morphology of the spreading center differs north and south of the Atlantis Transform. The spreading axis between 30°30′N and 31°30′N consists of enéchelon volcanic ridges, located within a rift valley with a regional trend of 040°. South of the transform, the spreading center is associated with a well-defined rift valley trending 015°. Magnetic anomalies and the bathymetric traces left by non-transform discontinuities on the flanks of the Mid-Atlantic Ridge provide a record of the evolution of this slow-spreading center over the last 10 Ma. Migration of non-transform offsets was predominantly to the south, except perhaps in the last 2 Ma. The discontinuity traces and the pattern of crustal thickness variations calculated from gravity data suggest that focused mantle upwelling has been maintained for at least 10 Ma south of 30°30′ N. In contrast, north of 30°30′N, the present segmentation configuration and the mantle upwelling centers inferred from gravity data appear to have been established more recently. The orientation of the bathymetric traces suggests that the migration of non-transform offsets is not controlled by the motion of the ridge axis with respect to the mantle. The evolution of the spreading center and the pattern of segmentation is influenced by relative plate motion changes, and by local processes, perhaps related to the amount of melt delivered to spreading segments. Relative plate motion changes over the last 10 Ma in our survey area have included a decrease in spreading rate from 32 mm a−1 to 24 mm a−1, as well as a clockwise change in spreading direction of 13° between anomalies 5 and 4, followed by a counterclockwise change of 4° between anomaly 4 and the present. Interpretation of magnetic anomalies indicates that there are significant variations in spreading asymmetry and rate within and between segments for a given anomaly time. These differences, as well as variations in crustal thickness inferred from gravity data on the flanks of spreading segments, indicate that magmatic and tectonic activity are, in general, not coordinated between adjacent spreading segments.  相似文献   

15.
The two parallel loci of recent Hawaiian volcanoes, Kea and Loa, have been regarded as the best targets to interpret the chemical structure of an upwelling mantle plume derived from the lower mantle. Here we show that the Sr–Nd–Hf–Pb isotopic data of the shield-building lavas along the Loa locus form a systematic trend from the main shield stage of Koolau (> 2.9 Ma) to the active Loihi volcanoes. During the growth of the Koolau volcano, the dominant material in the melting region successively changed from the proposed KEA, DMK (depleted Makapuu), to EMK (enriched Makapuu) components. The proportion of EMK, dominated by a recycled mafic component, is typified by some Koolau Makapuu-stage and some Lanai lavas. Subsequently, the EMK component decreased and LOIHI component increased toward the Loihi lavas. The temporal coincidence between the episodically elevated magma production rate and the abrupt appearance of the typical Loa-type lavas that is restricted to the last 3 Myr should be linked to magma genesis. We suggest that the abrupt appearance of Loa-type magmatism should be attributed to the transient incorporation of the relatively dense recycled material and surrounding less degassed lower mantle material that accumulated near the core–mantle boundary into the upwelling plume. This episodic involvement could have been trigged by episodic thermal pulses and buoyancy increases in the plume. The continuous appearance of Kea-type lavas during the long history of Hawaiian-chain magmatism and the larger magma volume of Kea-type lavas relative to that of the Loa-type lavas in the last 3 Myr indicate that the Kea locus is closer to the thermal centre of the Hawaiian plume relative to that of the Loa locus.  相似文献   

16.
New paleomagnetic data are reported for the dikes, sills, and lava flows in the Arctic part of the Siberian Platform, which has not been covered by previous systematical paleomagnetic investigations. The analysis of the newly obtained and previously published data provides important time constraints for the duration and character of evolution of the Permian-Triassic magmatic events in the studied regions. Our results once again illustrate the conclusion that, in order to obtain an exact estimate for the location of the paleomagnetic pole in the northern paleolatitudes, at least 30?C40 rapidly cooled magmatic bodies (dikes, flows, or minor sills) should be sampled if secular variation is commensurate with the intensity of the presentday variations.  相似文献   

17.
The Earth's magnetic field changed its polarity from the last reversed into today's normal state approximately 780 000 years ago. While before and after this so called Matuyama/Brunhes reversal, the Earth magnetic field was essentially an axial dipole, the details of its transitional structure are still largely unknown. Here, a Bayesian inversion method is developed to reconstruct the spherical harmonic expansion of this transitional field from paleomagnetic data. This is achieved by minimizing the total variational power at the core–mantle boundary during the transition under paleomagnetic constraints. The validity of the inversion technique is proved in two ways. First by inverting synthetic data sets from a modeled reversal. Here it is possible to reliably reconstruct the Gauss coefficients even from noisy records. Second by iteratively combining four geographically distributed high quality paleomagnetic records of the Matuyama/Brunhes reversal into a single geometric reversal scenario without assuming an a priori common age model. The obtained spatio-temporal reversal scenario successfully predicts most independent Matuyama/Brunhes transitional records. Therefore, the obtained global reconstruction based on paleomagnetic data invites to compare the inferred transitional field structure with results from numerical geodynamo models regarding the morphology of the transitional field. It is found that radial magnetic flux patches form at the equator and move polewards during the transition. Our model indicates an increase of non-dipolar energy prior to the last reversal and a non-dipolar dominance during the transition. Thus, the character and information of surface geomagnetic field records is strongly site dependent. The reconstruction also offers new answers to the question of existence of preferred longitudinal bands during the transition and to the problem of reversal duration. Different types of directional variations of the surface geomagnetic field, continuous or abrupt, are found during the transition. Two preferred longitudinal bands along the Americas and East Asia are not predicted for uniformly distributed sampling locations on the globe. Similar to geodynamo models with CMB heatflux derived from present day lower mantle heterogeneities, a preference of transitional VGPs for the Pacific hemisphere is found. The paleomagnetic duration of reversals shows not only a latitudinal, but also a longitudinal variation. Even the paleomagnetically determined age of the reversal varies significantly between different sites on the globe. The described Bayesian inversion technique can easily be applied to other high quality full vector reversal records. Also its extension to inversion of secular variation and excursion data is straightforward.  相似文献   

18.
Understanding patterns of expansion, contraction, and disconnection of headwater stream length in diverse settings is invaluable for the effective management of water resources as well as for informing research in the hydrology, ecology, and biogeochemistry of temporary streams. More accurate mapping of the stream network and quantitative measures of flow duration in the vast headwater regions facilitate implementation of water quality regulation and other policies to protect waterways. We determined the length and connectivity of the wet stream and geomorphic channel network in 3 forested catchments (<75 ha) in each of 4 physiographic provinces of the Appalachian Highlands: the New England, Appalachian Plateau, Valley and Ridge, and Blue Ridge. We mapped wet stream length 7 times at each catchment to characterize flow conditions between exceedance probabilities of <5% and >90% of the mean daily discharge. Stream network dynamics reflected geologic controls at both regional and local scales. Wet stream length was most variable at two Valley and Ridge catchments on a shale scarp slope and changed the least in the Blue Ridge. The density and source area of flow origins differed between the crystalline and sedimentary physiographic provinces, as the Appalachian Plateau and Valley and Ridge had fewer origins with much larger contributing areas than New England and the Blue Ridge. However, the length and surface connectivity of the wet stream depended on local lithology, geologic structure, and the distribution of surficial deposits such as boulders, glacially derived material, and colluival debris or sediment valley fills. Several proxies indicate the magnitude of stream length dynamics, including bankfull channel width, network connectivity, the base flow index, and the ratio of geomorphic channel to wet stream length. Consideration of geologic characteristics at multiple spatial scales is imperative for future investigations of flow intermittency in headwaters.  相似文献   

19.
再论印度与亚洲大陆何时何地发生初始碰撞   总被引：10，自引：2，他引：8       下载免费PDF全文
印度与亚洲大陆碰撞形成了喜马拉雅造山带.该造山带是当今固体地球科学研究的重点和热点,是建立新的大陆动力学理论的最佳天然实验室.印度与亚洲大陆碰撞时限是正确认识和理解该造山带形成与演化、高原隆升的动力学过程等的起点.近南北向陆陆碰撞的最直接证据是碰撞带两侧块体在古纬度上的相互重叠.本文拟通过对相关古地磁资料的分析,结合近年来在拉萨地块南缘林子宗群火山岩和沉积岩夹层上获得的最新古地磁结果,探索当今古地磁数据所限定的印度和亚洲大陆发生初始碰撞的时间和古地理位置.结果表明,拉萨地块林子宗群形成时期(约64~44 Ma)古亚洲大陆最南缘的古地理位置(~10°N)限定了印度与亚洲大陆的初始碰撞最可能发生在65~50 Ma之间;如果以由印度洋海底地形所限定的东冈瓦纳大陆裂解前的印度板块形状为大印度模型,则印度与亚洲大陆的初始碰撞很可能发生在60~55 Ma之间.  相似文献   

20.
Gabbroic cumulates drilled south of the Kane Transform Fault on the slow-spread Mid-Atlantic Ridge preserve up to three discrete magnetization components. Here we use absolute age constraints derived from the paleomagnetic data to develop a model for the magmatic construction of this section of the lower oceanic crust. By comparing the paleomagnetic data with mineral compositions, and based on thermal models of local reheating, we infer that magmas that began crystallizing in the upper mantle intruded into the lower oceanic crust and formed meter-scale sills. Some of these magmas were crystal-laden and the subsequent expulsion of interstitial liquid from them produced ‘cumulus’ sills. These small-scale magmatic injections took place over at least 210?000 years and at distances of ∼3 km from the ridge axis and may have formed much of the lower crust. This model explains many of the complexities described in this area and can be used to help understand the general formation of oceanic crust at slow-spread ridges.  相似文献   

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