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41.
Marie-No?lle Guilbaud Claus Siebe Paul Layer Sergio Salinas 《Bulletin of Volcanology》2012,74(5):1187-1211
The 690?km2 Tacámbaro-Puruarán area located at the arc-front part of the Michoácan-Guanajuato volcanic field in the Trans-Mexican Volcanic Belt (TMVB) records a protracted history of volcanism that culminated with intense monogenetic activity in the Holocene. Geologic mapping, 40Ar/39Ar and 14C radiometric dating, and whole-rock chemical analyses of volcanic products provide insights to that history. Eocene volcanics (55–40?Ma) exposed at uplifted blocks are related to a magmatic arc that preceded the TMVB. Early TMVB products are represented by poorly exposed Pliocene silicic domes (5–2?Ma). Quaternary (<2?Ma) volcanoes (114 mapped) are mainly scoria cones with lavas (49 vol.%), viscous lava flows (22 vol.%), and lava shields (22 vol.%). Erupted products are dominantly either basaltic andesites (37 vol. %), or andesites (17 vol.%), or span across both compositions (28 vol.%). Basalts (9 vol.%), dacites (4 vol.%), shoshonites (2 vol.%), and other alkali-rich rocks (<3 vol.%) occur subordinately. Early-Pleistocene volcanism was bimodal (dacites and basalts) and voluminous while since 1?Ma small-volume eruptions of intermediate magmas have dominated. Higher rates of lithospheric extension in the Quaternary may have allowed a larger number of small, poorly evolved dikes to reach the surface during this period. Eruptive centers as old as 1.7?Ma are aligned in a NE direction parallel to both, basement faults and the direction of regional compressive stress, implying structural control on volcanic activity. Data suggest that volcanism was strongly pulsatory and fed by localized low-degree partial melting of mantle sources. In the Holocene, at least 13 eruptions occurred (average recurrence interval of 800?years). These produced ~3.8?km3 of basaltic andesitic to andesitic magma and included four eruptions dated at ~1,000; 4,000; 8,000; and 11,000?years bc (calibrated 14C ages). To date, this is one of the highest monogenetic eruption frequencies detected within such a small area in a subduction-related arc-setting. These anomalous rates of monogenetic activity in an area with thick crust (>30?km) may be related to high rates of magma production at depth and a favorable tectonic setting. 相似文献
42.
Direct 40Ar/39Ar K‐feldspar dating of Late Permian—Early Triassic brittle faulting in northern Norway
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While the offshore post‐Caledonian extensional history of the north Norwegian passive margin is well constrained, the tectonic relationship between onshore and offshore regions is less clear because of limited age constraints on the timing of rifting onshore. 40Ar/39Ar dating of K‐feldspar from hydrothermally altered fault rocks in a Precambrian gneiss complex in northern Norway was used to study the timing of extensional faulting onshore. In addition, 40Ar/39Ar dating of K‐feldspar from the host rock provided insight into the regional rock cooling history prior to brittle deformation. Results indicated a dominant Late Permian–Early Triassic (~265–244 Ma) faulting event and found no evidence for later reactivation, which has been documented offshore. The region cooled to below the closure temperature for 40Ar/39Ar K‐feldspar in the Carboniferous to Early Permian, prior to the main brittle faulting event. 40Ar/39Ar dating of fault zone K‐feldspar products provided a means to date brittle faulting events. 相似文献
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44.
J. L. Macías J. L. Arce A. García-Palomo J. C. Mora P. W. Layer J. M. Espíndola 《Bulletin of Volcanology》2010,72(1):33-53
During late Pleistocene time, the extrusion of an andesitic dome at the summit of Tacaná volcano caused the collapse of its
northwestern flank. The stratocone collapse was nearly parallel to the σ
min stress direction suggesting that failure was controlled by the regional stress field. The event produced a debris avalanche
that was channelized in the San Rafael River and moved 8 km downstream. The deposit covered a minimum area of 4 km2, had a volume of 0.8 ± 0.5 km3, with an H/L (vertical drop to horizontal transport distance ratio) of ~0.35, defining a degree of mobility that is atypical
for volcanic debris avalanches. The flank failure undermined the summit dome leading to its collapse and the generation of
a series of block-and-ash flows that were emplaced in quick succession and covered the avalanche surface. The collapse event
left a 600-m-wide summit amphitheatre with a 30-degree opening to the northwest, and >200 m thick debris that blocked the
San Rafael River. Remobilization of this material produced debris flows that eroded the primary deposits and cascaded into
the Coatán River. After the collapse, the activity of Tacaná continued with the emission of the Agua Zarca lava flow dated
at 10 ± 6 ka (40Ar/39Ar), and pyroclastic surges dated at 10,610 + 330/−315 yr BP (14C), which provide a minimum age for the collapse event. During the Holocene, Tacaná has been very active producing explosive
and effusive eruptions that ended with the extrusion of two summit domes that today occupy the amphitheatre. The 1950 and
1986 phreatic outbursts occurred along the Pleistocene collapse scar. Currently ~300,000 inhabitants live within a 35 km radius
of Tacaná, and could conceivably be impacted by future events of similar magnitude. 相似文献
45.
青藏高原第四纪冰川发育期次及时代确认吸引着众多地质学家的关注,尽管它是一个众所周知且极为重要的地球科学问题,但古冰川发育时代的确定一直悬而未决. 相似文献