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1.
The motion of Greenland relative to Ellesmere Island along Nares Strait is determined from poles of rotation which provide control for the motion independent of local geology and geophysics. The plate kinematics around the North Atlantic Ocean, the Norwegian and Greenland Seas and the Eurasian Basin of the Arctic Ocean constrain motion along Nares Strait. These motions are checked by examining the stability characteristics of the triple junctions. These junctions are found to be stable. The motion along Nares Strait between anomalies 34 and 13 is a combination of strike-slip and compression. The regional geology is found to support the plate reconstructions. The local geology of the Nares Strait area is reviewed and found not to refute the predicted motions. The geophysical and geological data are interpreted in terms of the Wilson cycle, the opening and closing of an ocean. The Nares Strait area has the characteristics of a cryptic suture, a join between regions of collided continental crust.  相似文献   

2.
A combined analysis of the recently collected aeromagnetic data from the Eurasian Basin with the magnetic data from the Labrador Sea, the Norwegian-Greenland Sea and the North Atlantic yields a plate kinematic solution for the Eurasian Basin which is consistent with the solution for the North Atlantic as a whole. It shows that the Eurasian Basin and Norwegian-Greenland Sea started to evolve at about anomaly 25 time, though active seafloor spreading did not start in either of these regions until anomaly 24 time. It further shows that the spreading in the Eurasian Basin has been a result of motion only between the North American and Eurasian plates since the beginning, with the Lomonosov Ridge remaining attached to the North American plate. The relative motion among the North American, Greenland and Eurasian plates as obtained from the plate kinematics of the North Atlantic shows that from Late Cretaceous to Late Paleocene (anomaly 34 to 25) Greenland moved obliquely to Ellesmere Island. It is suggested that most of this motion was taken up within the Canadian Arctic Islands resulting in little or no motion along Nares Strait between Greenland and Ellesmere Island. From Late Paleocene to mid-Eocene (anomaly 25-21) Greenland continued to move obliquely, resulting in a displacement of 125 km along and of 90 km normal to the Nares Strait. From mid-Eocene to early Oligocene another 100 km of motion took place normal to the Strait, which correlates well with the Eurekan Orogeny in the Canadian Arctic Island. During these times the relative motion between Greenland and Svalbard (Eurasian plate) was mainly strike-slip with a small component of compression. The implication of the resulting motion between the North American and the Eurasian plates onto the Siberian platform are discussed.  相似文献   

3.
The extent of glacier ice in the Canadian High Arctic during the Last Glacial Maximum (LGM) has been debated for decades. One school proposed a regional Innuitian Ice Sheet whereas another proposed a smaller, non-contiguous Franklin Ice Complex. Research throughout western Nares Strait supports coalescent Innuitian and Greenland ice during the LGM, based on widespread glacial and marine deposits dated by 14C and amino acid analyses. This coalescence likely promoted a vigorous regional ice flow westward across Ellesmere Island to Eureka Sound. Post-glacial emergence in Eureka Sound suggests a former ice thickness at least as great as that in Nares Strait (≥ 1 km). Recently, independent field studies elsewhere in the High Arctic also support an Innuitian Ice Sheet during the LGM. Collectively, these studies resolve a long-standing debate, and initiate new opportunities concerning the reconstruction of high-latitude palaeoenvironmental and palaeoclimatic change. © 1998 John Wiley & Sons, Ltd.  相似文献   

4.
The “Nares Strait problem” represents a debate about the existence and magnitude of left-lateral movements along the proposed Wegener Fault within this seaway. Study of Palaeogene Eurekan tectonics at its shorelines could shed light on the kinematics of this fault. Palaeogene (Late Paleocene to Early Eocene) sediments are exposed at the northeastern coast of Ellesmere Island in the Judge Daly Promontory. They are preserved as elongate SW–NE striking fault-bounded basins cutting folded Early Paleozoic strata. The structures of the Palaeogene exposures are characterized by broad open synclines cut and displaced by steeply dipping strike-slip faults. Their fold axes strike NE–SW at an acute angle to the border faults indicating left-lateral transpression. Weak deformation in the interior of the outliers contrasts with intense shearing and fracturing adjacent to border faults. The degree of deformation of the Palaeogene strata varies markedly between the northwestern and southeastern border faults with the first being more intense. Structural geometry, orientation of subordinate folds and faults, the kinematics of faults, and fault-slip data suggest a multiple stage structural evolution during the Palaeogene Eurekan deformation: (1) The fault pattern on Judge Daly Promontory is result of left-lateral strike-slip faulting starting in Mid to Late Paleocene times. The Palaeogene Judge Daly basin formed in transtensional segments by pull-apart mechanism. Transpression during progressive strike-slip shearing gave rise to open folding of the Palaeogene deposits. (2) The faults were reactivated during SE-directed thrust tectonics in Mid Eocene times (chron 21). A strike-slip component during thrusting on the reactivated faults depends on the steepness of the fault segments and on their obliquity to the regional stress axes.Strike-slip displacement was partitioned to a number of sub-parallel faults on-shore and off-shore. Hence, large-scale lateral movements in the sum of 80–100 km or more could have been accommodated by a set of faults, each with displacements in the order of 10–30 km. The Wegener Fault as discrete plate boundary in Nares Strait is replaced by a bundle of faults located mainly onshore on the Judge Daly Promontory.  相似文献   

5.
Approximately 400,000 line kilometers of high quality, low level Arctic aeromagnetic data collected by the Naval Research Laboratory, the Naval Oceanographic Office and the Naval Ocean Reseach and Development Activity from 1972 through 1978 have been analyzed for depth to magnetic source. This data set covers much of the Canada Basin, the Alpha Ridge, the central part of the Makarov Basin, the Lincoln Sea, the Eurasia Basin west and south of the 55°E meridian and the Norwegian-Greenland Sea north of the Jan Mayen Fracture Zone. The analysis uses the autocorrelation algorithm developed by Phillips (1975, 1978) and based on the maximum entropy method of Burg (1967, 1968, 1975). The method is outlined, examples of various error analysis techniques shown and final results presented. Where possible, magnetic source depth estimates are compared with basement depths derived from seismic and bathymetric data.All major known bathymetric features, including Vesteris Bank and the Greenland, Molloy and Spitsbergen fracture zones, as well as the Mohns, Knipovich and Nansen spreading ridges and the Alpha Cordillera appear as regional highs in the calculated magnetic basement topography. Shallow basement was also found under the northeastern Yermak Plateau, the Morris Jesup Rise and under the southern (Greenland-Ellesmere Island) end of the Lomonsosov Ridge. Regional magnetic source deeps are associated with such bathymetric depressions as the Canada, Makarov, Amundsen, Nansen, Greenland and Lofoten basins; more localized magnetic basement deeps are found over the Molloy F.Z. deep and over the Mohns, Knipovich and Nansen rift valleys. A linear magnetic basement deep follows the extension of Nares Strait through the Lincoln Sea toward the Morris Jesup Rise, suggesting the continuation of the Nares Strait or Wegener F.Z. into the Lincoln Sea. A sharp drop in the regional magnetic source depths to the southeast of the Alpha Ridge suggests the Alpha Ridge is not connected to structures in northwest Ellesmere Island as previously postulated from high altitude aeromagnetic collected by Canadian workers. A regional deep under the east Greenland shelf west of the Greenland Escarpment suggests the presence of 5–10 km of post-Paleozoic sediments.  相似文献   

6.
During the last glacial stage, Washington Land in western North Greenland was probably completely inundated by the Greenland Ice Sheet. The oldest shell dates from raised marine deposits that provide minimum ages for the last deglaciation are 9300 cal. yr BP (northern Washington Land) and 7600 cal. yr BP (SW Washington Land). These dates indicate that Washington Land, which borders the central part of Nares Strait separating Greenland from Ellesmere Island in Canada, did not become free of glacier ice until well into the Holocene. The elevation of the marine limit falls from 110 m a.s.l. in the north to 60 m a.s.l. in the southwest. The recession was followed by readvance of glaciers in the late Holocene, and the youngest shell date from Neoglacial lateral moraines north of Humboldt Gletscher is 600 cal. yr BP. Since the Neoglacial maximum, probably around 100 years ago, glaciers have receded. The Holocene marine assemblages comprise a few southern extralimital records, notably of Chlamys islandica dated to 7300 cal. yr BP. Musk ox and reindeer disappeared from Washington Land recently, perhaps in connection with the cold period that culminated about 100 years ago.  相似文献   

7.
Paleogene sediments in fault-bounded basins on Judge Daly Promontory, northeast Ellesmere Island, Canadian High Arctic, are rich in volcanogenic material. Volcanic pebbles within the Cape Back basin near Nares Strait were studied for their petrography, geochemistry, Sr and Nd isotopes, and geochronology to identify and characterize their parent rock. The pebbles are derived from lava flows and ignimbrites of a continental rift-related, strongly differentiated, highly incompatible element enriched, alkaline volcanic suite, the proposed Nares Strait volcanic suite, which is distinct from other alkaline volcanic suites on the northern coasts of Ellesmere Island and Greenland. 40Ar/39Ar amphibole and alkali feldspar ages indicate that volcanism was active around 61–58 Ma and was probably contemporaneous with sedimentation resulting in Middle to Late Paleocene age for deposition within the Cape Back basin and the other Paleogene basins on Judge Daly Promontory.  相似文献   

8.
Nares Strait, a major connection between the Arctic Ocean and Baffin Bay, was blocked by coalescent Innuitian and Greenland ice sheets during the last glaciation. This paper focuses on the events and processes leading to the opening of the strait and the environmental response to establishment of the Arctic‐Atlantic throughflow. The study is based on sedimentological, mineralogical and foraminiferal analyses of radiocarbon‐dated cores 2001LSSL‐0014PC and TC from northern Baffin Bay. Radiocarbon dates on benthic foraminifera were calibrated with ΔR = 220±20 years. Basal compact pebbly mud is interpreted as a subglacial deposit formed by glacial overriding of unconsolidated marine sediments. It is overlain by ice‐proximal (red/grey laminated, ice‐proximal glaciomarine unit barren of foraminifera and containing >2 mm clasts interpreted as ice‐rafted debris) to ice‐distal (calcareous, grey pebbly mud with foraminifera indicative of a stratified water column with chilled Atlantic Water fauna and species associated with perennial and then seasonal sea ice cover) glacial marine sediment units. The age model indicates ice retreat into Smith Sound as early as c. 11.7 and as late as c. 11.2 cal. ka BP followed by progressively more distal glaciomarine conditions as the ice margin retreated toward the Kennedy Channel. We hypothesize that a distinct IRD layer deposited between 9.3 and 9 (9.4–8.9 1σ) cal. ka BP marks the break‐up of ice in Kennedy Channel resulting in the opening of Nares Strait as an Arctic‐Atlantic throughflow. Overlying foraminiferal assemblages indicate enhanced marine productivity consistent with entry of nutrient‐rich Arctic Surface Water. A pronounced rise in agglutinated foraminifers and sand‐sized diatoms, and loss of detrital calcite characterize the uppermost bioturbated mud, which was deposited after 4.8 (3.67–5.55 1σ) cal. ka BP. The timing of the transition is poorly resolved as it coincides with the slow sedimentation rates that ensued after the ice margins retreated onto land.  相似文献   

9.
The formation of the Makassar Strait, situated between southeast (SE) Kalimantan and western Sulawesi, is still subject of much debate. Different authors have proposed several hypotheses to explain its evolution. The only agreement between those several hypotheses is that SE Kalimantan and western Sulawesi once lay close together and that their separation is due to the opening of the Makassar Strait. The age and driving mechanism for this opening are, however, still poorly understood. The strait separates the stable core of the Eurasian Plate to the west from the very active region of the triple junction of three large plates to the east. To the north the strait is bounded by the Sulawesi Sea and to the south by the East Java Sea. The strait is roughly 100–200 km wide and 300 km long and is usually divided into the North and South Makassar basins, separated by the Paternoster Fault. The present study interprets the history of the Makassar Strait using seismic reflection profiles and gravity models, in addition to the compilation of geological information. Implications for the origin of rifting is also discussed. The result of the present study indicates that Makassar Strait was formed by the vertical sinking of a subducting oceanic plate to the east of western Sulawesi, leading to trench roll-back. This vertical sinking was accommodated by extension and rifting of continental crust above the subduction zone at a previous site of collision, causing the opening of Makassar Strait. The time of this trench roll-back marks the cessation of subduction.  相似文献   

10.
Chronology of the last recession of the Greenland Ice Sheet   总被引：1，自引：0，他引：1  
A new deglaciation chronology for the ice‐free parts of Greenland, the continental shelf and eastern Ellesmere Island (Canada) is proposed. The chronology is based on a new compilation of all published radiocarbon dates from Greenland, and includes crucial new material from southern, northeastern and northwestern Greenland. Although each date provides only a minimum age for the local deglaciation, some of the dates come from species that indicate ice‐proximal glaciomarine conditions, and thus may be connected with the actual ice recession. In addition to shell dates, dates from marine algae, lake sediments, peat, terrestrial plants and driftwood also are included. Only offshore and in the far south have secure late‐glacial sediments been found. Other previous reports of late‐glacial sediments (older than 11.5 cal. kyr BP) from onshore parts of Greenland need to be confirmed. Most of the present ice‐free parts of Greenland and Nares Strait between Greenland and Ellesmere Island were not deglaciated until the early Holocene. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

11.
East Greenland forms one of the least understood of the orogenic belts formed during the amalgamation of Rodinia during late Mesoproterozoic times. Recent U–Pb zircon SHRIMP dating on the widespread Krummedal supracrustal succession and associated granites from central East Greenland has shown that metamorphism and intrusion affected the region at around 0.95–0.92 Ga, approximately 150 m.y. later than the main phase of Grenvillian orogenesis (s.s.). These early Neoproterozoic ages may indicate a link with metamorphism and igneous activity in the Sveconorwegian Belt of Scandinavia rather than true ‘Grenvillian’ events on the eastern margin of Laurentia. Previous plate tectonic reconstructions which link Laurentia and Baltica by a collisional margin extending through central East Greenland at 1.1 Ga were based on early conventional U–Pb zircon dating in central East Greenland, and can no longer be considered viable. Instead, new detrital zircon SHRIMP U–Pb dating studies show that the Krummedal supracrustal succession was deposited between ca. 1.0 Ga and no later than 0.95 Ga, during a time of major sediment deposition widely preserved elsewhere in the North Atlantic region. Erosion associated with post-1.1 Ga collapse of the Grenville–Sunsas orogeny is the most likely source for the majority of the detritus, since the corresponding Baltic margin was dominated by A-type magmatism for much of the period 1.4–1.1 Ga material, which is the age of the bulk of detrital zircons in the Krummedal supracrustal succession. We suggest that the Krummedal supracrustal succession was deposited east or south-east of its present location, and was thrust onto Archaean–Palaeoproterozoic orthogneisses, which in turn were displaced across the parautochthonous foreland during the Caledonian orogeny. The early Neoproterozoic orogenic events recorded in central East Greenland therefore involved the metamorphism of a metasedimentary package of Laurentian–Amazonian affinity during the Sveconorwegian orogeny in the final stages of the collision of Baltica and Laurentia.  相似文献   

12.
Investigations indicate that the Iceland Ice Sheet was reduced in size during MIS 3 but readvanced to the shelf break at the LGM. Retreat occurred very rapidly around 15 k–16 k cal. yr BP. By contrast, the margin of the ice sheet on the East Greenland shelf, north of the Denmark Strait, was at or close to the shelf break during MIS 3 and 2 and retreat starting ～17 k cal. yr BP. Quantitative X‐ray diffraction analysis of the <2 mm sediment fraction was undertaken on 161 samples from Iceland and East Greenland diamictons, and from cores on the slopes and margins of the Denmark Strait. Weight% mineralogical data are used in a principal component analysis to differentiate sediments derived from the two margins. The first two PC axes explain 52% of the variance. These associations are used to characterise sediments as being affiliated with (a) Iceland, (b) East Greenland or (c) mixed. The contribution from Iceland becomes prominent during MIS 2. The extensive outcrop of early Tertiary basalts on East Greenland between 68° and 71° N is an alternative source for basaltic clasts and North Atlantic sediments with εNd(0) values close to ±0. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

13.
The precipitation fields of a palaeoatmospheric general circulation model are used to derive estimates of the geographical distribution, and flux, of icebergs from the Laurentide, Fennoscandinavian and eastern Siberian ice‐sheets at the Last Glacial Maximum (LGM). The atmospheric model fields from LGM simulations using CLIMAP or Peltier (ICE‐4G) ice orography were studied, to test the sensitivity of the predicted flux. The estimated Northern Hemispheric LGM iceberg flux is 3500–4000 km3 yr?1, of which about 60% issued directly into the North Atlantic. The iceberg flux from the St Lawrence area is of similar significance to that issuing from Hudson Strait in all estimates. Both the North Pacific and the Arctic received substantial iceberg fluxes (ca. 700 km3 yr?1), with relatively minor differences occurring between the two ice‐sheet reconstructions. Apparent discrepancies between Arctic deep‐sea core samples of ice‐rafted debris and our estimates of mean glacial iceberg flux may be ascribed to coastal trapping of bergs, the existence of floating ice tongues or a rapid exit of icebergs from the Arctic basin into the Greenland Sea through the Fram Strait. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

14.
SIMS U‐Pb zircon dating of metaigneous rocks of the Nordbreen Nappe from the West Ny‐Friesland terrane (Eastern Basement Province of Svalbard) yields crystallization ages of 1,761 ± 4 Ma for a felsic metatuff and 1,373 ± 4 Ma for a metagabbro dyke. The Palaeoproterozoic age of the metatuff is similar to previously obtained ages for various felsic rocks from the study area, whereas the Mesoproterozoic age of the metagabbro has not been thus far documented on Svalbard. However, a similar age pattern has been reported from Northeast Greenland. Therefore, we conclude that the West Ny‐Friesland terrane can be correlated with age equivalent units of Northeast Greenland. We also explore similarities and dissimilarities between the study area and other terranes of Svalbard and speculate on regional‐scale correlations. Together, the presented new ages provide critical piercing points in palaeogeographic reconstructions of the relatively poorly understood circum‐Arctic region.  相似文献   

15.
华北地区奥陶纪岩相古地理   总被引：2，自引：0，他引：2       下载免费PDF全文
通过收集、吸收前人研究以及最新的大地调成果资料,以"构造控盆、盆地控相"为编图思路,在现今纬度下对中国华北地区开展岩相古地理的研究。本文认为,华北板块在早奥陶世和中-晚奥陶世处于不同的构造背景之下,不同的板块边界类型以及原始构造单元的基底形态控制着该时期古地理的展布。在此基础上,采用优势相以及压缩相的方法,总结划分出潮坪、局限台地、开阔台地、蒸发台地、浅(礁)滩、台地边缘斜坡、深水斜坡-盆地等7类沉积环境类型;梳理了华北克拉通沉积盆地在奥陶纪的古地理演化过程,编制了早奥陶世特马豆克期、早奥陶世弗洛期、中奥陶世大坪期-达瑞威尔期、晚奥陶世桑比期-凯迪期早期以及晚奥陶世凯迪期晚期-赫南特期等5个时期的华北地区古地理图。  相似文献   

16.
Holocene climatic and paleoceanographic development of the SE Greenland Shelf is studied from cores MD99-2317 and MD99-2322, at sites north and south of the Denmark Strait, respectively. Lithofacies, IRD counts, calcium carbonate percentages, benthic and planktic foraminiferal assemblages and oxygen isotope analyses, and summer SSTs reveal significant climate variations in the Holocene driven by declining solar insolation and its interaction with waning continental ice sheets, and changing atmospheric pressure patterns. Large changes in the East Greenland and Irminger Currents and the Greenland Ice Sheet are manifested as a 4-part division of the Holocene. An early Holocene cold interval dominated by melting of the Greenland Ice Sheet and Polar Front retreat extends from 11.8 to 9.5 cal kyr BP. A cold interval from 9.5 to 8.1 cal kyr BP involved episodic cooling of the Irminger Current resulting from the last phases of Laurentide Ice Sheet deglaciation and delayed the Holocene optimum off East Greenland by 3 kyr relative to peak summer solar insolation, which likely helped to limit the early Holocene melting of the Greenland Ice Sheet. The period 8.1–3.5 cal kyr BP represents a climatic optimum interval of maximum Greenland Ice Sheet retreat and strong Irminger Current inflow to the Denmark Strait. Between 6.8 and 3.5 cal kyr BP, the Irminger Current penetrated further North into the Nordic Seas than has been observed in recent decades. This signal is consistent with diminished northerly winds, a weaker Greenland High and contracted subpolar gyre. By 5 cal kyr BP, periods of increased Polar Water and decreasing salinity in the Irminger Current suggest a transition toward expansion of the subpolar gyre and increased Polar Water in the EGC. The Neoglacial interval from 3.5 to 0.2 cal kyr BP was cold and variable with increased freshwater forcing from the Arctic Ocean, advance of the Greenland Ice Sheet and southward advance of the Polar Front. Enhanced northerly winds and a strengthened Greenland High are consistent with thicker and more extensive Polar Water and greatly diminished northward advection of Irminger Current in the Denmark Strait.  相似文献   

17.
In contrast to the normal ‘Wilson cycle’ sequence of subduction leading to continental collision and associated mountain building, the evolution of the New Zealand plate boundary in the Neogene reflects the converse—initially a period of continental convergence that is followed by the emplacement of subduction. Plate reconstructions allow us to place limits on the location and timing of the continental convergence and subduction zones and the migration of the transition between the two plate boundary regimes. Relative plate motions and reconstructions since the Early to Mid-Miocene require significant continental convergence in advance of the emplacement of the southward migrating Hikurangi subduction—a sequence of tectonism seen in the present plate boundary geography of Hikurangi subduction beneath North Island and convergence in the Southern Alps along the Alpine Fault. In contrast to a transition from subduction to continental convergence where the leading edge of the upper plate is relatively thin and deformable, the transition from a continental convergent regime, with its associated crustal and lithospheric thickening, to subduction of oceanic lithosphere requires substantial thinning (removal) of upper plate continental lithosphere to make room for the slab. The simple structure of the Wadati–Benioff zone seen in the present-day geometry of the subducting Pacific plate beneath North Island indicates that this lithospheric adjustment occurs quickly. Associated with this rapid lithospheric thinning is the development of a series of ephemeral basins, younging to the south, that straddle the migrating slab edge. Based on this association between localized vertical tectonics and slab emplacement, the tectonic history of these basins records the effects of lithospheric delamination driven by the southward migrating leading edge of the subducting Pacific slab. Although the New Zealand plate boundary is often described as simply two subduction zones linked by the transpressive Alpine Fault, in actuality the present is merely a snapshot view of an ongoing and complex evolution from convergence to subduction.  相似文献   

18.
The West Spitsbergen Fold Belt, together with the Eurekan structures of northern Greenland and Ellesmere Island, are suggested to be the result of Late Cretaceous-Palaeocene intracontinental compressional tectonics. The Late Palaeozoic –Mesozoic rocks of western Spitsbergen are characterized by near-foreland deformation with ramp-flat, top-to-the east thrust trajectories, whereas structurally higher nappes involving Caledonian complexes are typified by more listric thrusts and mylonite zones. A minimum of 40 km of shortening is estimated for the northern part of the West Spitsbergen Fold Belt. The axial trends in the West Spitsbergen and the North Greenland Eurekan fold belts parallel the principal fault zones which accommodated the separation of Greenland and Svalbard after Chron 25/24. In northern Greenland, north directed Eurekan thrusts associated with mylonites and cleavage formation represent at least 10 km of shortening. Between 50 and 100 km of shortening is estimated for the markedly arcuate Eurekan Fold Belt of Ellesmere Island, but the principal tectonic transport is eastwards. Kinematic reconstructions suggest that Svalbard was linked to North America before the opening of the Eurasian Basin and Norwegian — Greenland Sea. In the Late Cretaceous — Palaeocene interval, the relative motion between Greenland and North America was convergent across the Greenland — Svalbard margin, giving rise to the West Spitsbergen Fold Belt and the Eurekan structures of North Greenland.  相似文献   

19.
This paper is the first to summarize research on fluctuations of local glaciers in Greenland (e.g. ice caps and mountain glaciers independent of the Greenland Ice Sheet) during latest Pleistocene and Holocene time. In contrast to the extensive data available for fluctuations of the Greenland Ice Sheet, surprisingly little data exist to constrain local glacier extents. Much of the available research was conducted prior to wide-spread use of AMS radiocarbon dating and the advent of surface-exposure and luminescence dating. Although there is a paucity of data, generally similar patterns of local glacier fluctuations are observed in all regions of Greenland and likely reflect changes in paleoclimate, which must have influenced at least the margins of the Inland Ice. Absolute-age data for late-glacial and early Holocene advances of local glaciers are reported from only two locations: Disko (island) and the Scoresby Sund region. Subsequent to late-glacial or early Holocene time, most local glaciers were smaller than at present or may have disappeared completely during the Holocene Thermal Maximum. In general, local glacier advances that occurred during Historical time (1200–1940 AD) are the most extensive since late-glacial or early Holocene time. Historical documents and more recent aerial photographs provide useful information about local glacier fluctuations during the last 100 yrs. In all but one area (North Greenland), local glaciers are currently receding from Historical extents.  相似文献   

20.
We use updated rotations within the Pacific-Antarctica-Africa-North America plate circuit to calculate Pacific-North America plate reconstructions for times since chron 13 (33 Ma). The direction of motion of the Pacific plate relative to stable North America was fairly steady between chrons 13 and 4, and then changed and moved in a more northerly direction from chron 4 to the present (8 Ma to the present). No Pliocene changes in Pacific-North America plate motion are resolvable in these data, suggesting that Pliocene changes in deformation style along the boundary were not driven by changes in plate motion. However, the chron 4 change in Pacific-North America plate motion appears to correlate very closely to a change in direction of extension documented between the Sierra Nevada and the Colorado Plateau. Our best solution for the displacement with respect to stable North America of a point on the Pacific plate that is now near the Mendocino triple junction is that from 30 to 12 Ma the point was displaced along an azimuth of ～N60°W at rate of ～33 mm/yr; from 12 Ma to about 8 Ma the azimuth of displacement was about the same as previously, but the rate was faster (～52 mm/yr); and since 8 Ma the point was displaced along an azimuth of N37°W at a rate of ～52 mm/yr.

We compare plate-circuit reconstructions of the edge of the Pacific plate to continental deformation reconstructions of North American tectonic elements across the Basin and Range province and elsewhere in order to evaluate the relationship of this deformation to the plate motions. The oceanic displacements correspond remarkably well to the continental reconstructions where deformations of the latter have been quantified along a path across the Colorado Plateau and central California. They also supply strong constraints for the deformation budgets of regions to the north and south, in Cascadia and northern Mexico, respectively.

We examine slab-window formation and evolution in a detailed re-analysis of the spreading geometry of the post-Farallon microplates, from 28 to 19 Ma. Development of the slab window seems linked to early Miocene volcanism and deformation in the Mojave Desert, although detailed correlations await clarification of early Miocene reconstructions of the Tehachapi Mountains. We then trace the post-20 Ma motion of the Mendocino slab window edge beneath the Sierran-Great Valley block and find that it drifted steadily north, then stalled just north of Sutter Buttes at ～4 Ma.  相似文献   

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