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101.
BROXTON W. BIRD MATTHEW E. KIRBY IAN M. HOWAT SLAWEK TULACZYK 《Boreas: An International Journal of Quaternary Research》2010,39(1):131-144
Bird, B. W., Kirby, M. E., Howat, I. M. & Tulaczyk, S. 2009: Geophysical evidence for Holocene lake-level change in southern California (Dry Lake). Boreas , 10.1111/j.1502-3885.2009.00114.x. ISSN 0300-9483.
Ground penetrating radar (GPR) data are used in combination with previously published sediment cores to develop a Holocene history of basin sedimentation in a small, alpine lake in southern California (Dry Lake). The GPR data identify three depositional sequences spanning the past 9000 calendar years before present (cal. yr BP). Sequence I represents the first phase of an early Holocene highstand. A regression between <8320 and >8120 cal. yr BP separates Sequence I from Sequence II, perhaps associated with the 8200 cal. yr BP cold event. Sequence II represents the second phase of the early-to-mid Holocene highstand. Sequence IIIa represents a permanent shift to predominantly low lake stands beginning ∼5550 cal. yr BP. This mid-Holocene shift was accompanied by a dramatic decrease in sedimentation rate as well as a contraction of the basin's area of sedimentation. By ∼1860 cal. yr BP (Sequence IIIb), the lake was restricted to the modern, central basin. Taken together, the GPR and core data indicate a wet early Holocene followed by a long-term Holocene drying trend. The similarity in ages of the early Holocene highstand across the greater southern California region suggests a common external forcing – perhaps modulation of early Holocene storm activity by insolation. However, regional lake level records are less congruous following the initial early Holocene highstand, which may indicate a change in the spatial domain of climate forcing(s) throughout the Holocene in western North America. 相似文献
Ground penetrating radar (GPR) data are used in combination with previously published sediment cores to develop a Holocene history of basin sedimentation in a small, alpine lake in southern California (Dry Lake). The GPR data identify three depositional sequences spanning the past 9000 calendar years before present (cal. yr BP). Sequence I represents the first phase of an early Holocene highstand. A regression between <8320 and >8120 cal. yr BP separates Sequence I from Sequence II, perhaps associated with the 8200 cal. yr BP cold event. Sequence II represents the second phase of the early-to-mid Holocene highstand. Sequence IIIa represents a permanent shift to predominantly low lake stands beginning ∼5550 cal. yr BP. This mid-Holocene shift was accompanied by a dramatic decrease in sedimentation rate as well as a contraction of the basin's area of sedimentation. By ∼1860 cal. yr BP (Sequence IIIb), the lake was restricted to the modern, central basin. Taken together, the GPR and core data indicate a wet early Holocene followed by a long-term Holocene drying trend. The similarity in ages of the early Holocene highstand across the greater southern California region suggests a common external forcing – perhaps modulation of early Holocene storm activity by insolation. However, regional lake level records are less congruous following the initial early Holocene highstand, which may indicate a change in the spatial domain of climate forcing(s) throughout the Holocene in western North America. 相似文献
102.
103.
Silicic Magma Formation in Overthickened Crust: Melting of Charnockite and Leucogranite at 15, 20 and 25 kbar 总被引:23,自引:0,他引:23
Two models of silicic magma formation have been experimentallytested: (1) generation of A-type granite magma by partial meltingof crustal source rocks at depths >50 km; (2) productionof syenite magma by partial melting of quartzofeldspathic rocksat pressures >15 kbar. Melting experiments at 15, 20 and25 kbar were performed on Archaean biotite-bearing charnockiteof opx-bearing granodiorite composition, and on leucogranite.Most experiments were conducted with oxalic acid (OA) to provide2 wt % H2O and 相似文献
104.
The layered syenite series in the Klokken stock formed by continuousin situ fractionation of a trachytic magma in a chamber linedby gabbro with 3000 m of cover rocks. The following mineralsand reactions are assessed as geothermometers and barometers:two feldspars; hypersolvus ternary feldspars; ferrohedenbergite-ß-wollastonite;clinopyroxene-olivine Fe-Mg exchange; Fe-Ti oxides; sanidine-magnetite-annite;ferroedenite stability. Estimates of silica activity are obtainedfrom the silica-magnetite-fayalite assemblage. The gabbros ended magmatic crystallization at > 10001050°C.The less fractionated members of the syenite range probablycrystallized with PH2O < Ptotal, at T > 870°C and,PH2O 800 bars. In the more fractionated syenites PH2O = Ptotalduring intercumulus feldspar growth, and all magmatic phasescrystallized within the interval 940830°C and PH2O< 1100 bars. Magmatic fO2 (bars) was 1 log unit below theQFM buffer. aSIO2 in gabbros was slightly above the albite-nephelinebuffer, but rose suddenly to just <1 in the syenites, a jumpmirrored by minor elements in pyroxenes and opaque oxides. Biotitegrew subsolidus in most rocks, at fO2 < QFM, except in intermediaterocks when fO2 > QFM and was defined by the sanidine-magnetite-biotiteassemblage. In these rocks PH2O of 450 bars at 760°C isobtained using existing experimental data, but application ofthis data to Fe-rich biotites in the layered series (where biotiteis an intercumulus phase) requires P > 10 kb at magmatictemperatures. High TiO2 or F: OH probably accounts for increasedT stability of natural annites at low P. The syenitic liquid fractionated down a low temperature zonein a multicomponent system precipitating alk fsp + ol + cpx+ mt and the more fractionated members of the layered serieshad a negligible crystallization interval, a prerequisite forthe development of the unique Klokken-type inversely gradedmineral layering. 相似文献
105.
NIELSEN FRANK M.; CAMPBELL IAN H.; MCCULLOCH MALCOLM; WILSON J. RICHARD 《Journal of Petrology》1996,37(1):171-193
The 230 km2 Proterozoic BjerkreimSokndal layered instrusionhas a monzonoritic bulk composition and comprises a 6 km thick,broadly leuconoritic Layered Series (LS) overlain by unlayeredmangerite and quartz mangerite. In the Bjerkreim lobe the LScomprises six megacyclic units (MCU 0, IA, IB, II, 11 and IV)in a syncline. This lobe is surrounded by migmatitic gneisses;the roof to the instrusion is missing. The mg-number opx, An% and whole-rock initial 87Sr/86Sr isotope ratios (Sr0) displaybroadly parallel trends through MCUs II and III, and into MCUIV, with decreasing mg-number (7558) and An % (5040)accompanying increasing Sr0 (0.70500.7085). This correlationbreaks down in the upper part of MCU IV and mangerite; the quartzmangerite has Sr0 values of 0.7085. Abrupt reversals occur acrossthe MCU boundaries. The LS crystallized on the floor of a periodically replenishedmagma chamber that was continually assimilating country rockgneisses. Strong compositional zoning of the magma developedas a result of repeated replenishments with relatively dense,primitive ferrobasaltic magma (Sr0 = 0.7049) along the floorand the development of extensive buoyant roof melts. Assimilationtook place on a massive scale (up to 50%) in the upper partsof the chamber. KEY WORDS: assimilation; fractional crystallization; hybrid magma; layered intrusion; Sr-isolopes
*Corresponding author. 相似文献
106.
IAN HUTCHINSON THOMAS S. JAMES JOHN J. CLAGUE J. VAUGHN BARRIE KIM W. CONWAY 《Boreas: An International Journal of Quaternary Research》2004,33(3):183-194
Bracketing ages on marine—freshwater transitions in isolation basins extending from sea level to 100 m elevation on Lasqueti Island, and data from shallow marine cores and outcrops on eastern Vancouver Island, constrain late Pleistocene and Holocene sea-level change in the central Strait of Georgia. Relative sea level fell from 150 m elevation to about —15 m from 14000 cal. yr BP to 11 500 cal. yr BP. Basins at higher elevations exhibit abrupt changes in diatom assemblages at the marine-freshwater transition. At lower elevations an intervening brackish phase suggests slower rates of uplift. Relative sea level rose to about +1 m about 9000 cal. yr BP to 8500 cal. yr BP, and then slowly fell to the modern datum. The mean rate of glacio-isostatic rebound in the first millennium after deglaciation was about 0.11 in a -1 , similar to the peak rate at the centres of the former Laurentide and Fennoscandian ice complexes. The latter feature smooth, exponential-style declines in sea level up to the present day, whereas in the study area the uplift rate dropped to less than one-tenth of its initial value in only about 2500 years. Slower, more deeply seated isostatic recovery generated residual uplift rates of <0.01 m a-1 in the early Holocene after the late-Pleistocene wasting of the Cordilleran ice sheet. 相似文献
107.
108.
109.
PT conditions and prograde PT paths have beencalculated for amphibolite-grade pelites and amphibolites fromCordillera Darwin, Tierra del Fuego, Chile. Peak PT conditionsare nearly all within the kyanite stability field; temperaturesgenerally show an increase with increasing grade, but pressureshave a less consistent trend, possibly increasing slightly fromgarnet to kyanite grade. PT paths from pelites show heatingby 80100C during loading of 023 kbar. Texturalanalysis and previous structural work indicate that this segmentof the path correlates with back-folding deformation. PTpaths from two Mg-rich garnet amphibolites suggest a decreasein pressure of as much as 3 kbar with 2550C of heatingfrom the kyanite stability field to the sillimanite, and areconsistent with pervasive, minor development of fibrolitic sillimanitealong plagioclase grain boundaries. Together, the PTpath segments from pelites and amphibolites constitute a clockwisePT trajectory. The proposed clockwise PT paths are consistent with theinterpretation that Cordillera Darwin represents an extensionallyexhumed metamorphic core complex, in which loading during garnetgrowth in the pelitic rocks was succeeded by differential upliftduring garnet growth in magnesian amphibolites.
* Present address: Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 相似文献
110.
The number of paired horizontal laminae in an ephemeral channel-fill increases systematically down-catchment from stream headwaters, and is shown to be functionally related to tributary confluence. Under moving storm conditions, each tributary sub-catchment delivers a sediment-laden discharge pulse that finds sedimentary expression in paired laminae of light and heavy minerals. Lamina differentiation is attributed to a congregational sorting mechanism. The direct relationship (r, = 0.903) between paired laminae and confluencing tributaries permits palaeoenvironmental inference of drainage-net character, and palaeoclimatic variability. 相似文献