Showing papers by "Paul W. Layer published in 2009"

Geochronological constraints on the evolution of the southern Dom Feliciano Belt (Uruguay)

Abstract: New U–Pb sensitive high-resolution ion microprobe, K–Ar and Ar–Ar data from the southernmost Dom Feliciano Belt allow the identification of four major events. Orthogneisses from the Punta del Este Terrane indicate a magmatic episode at c . 770 Ma and high-grade metamorphism at 641 ± 17 Ma. Granitoid emplacement at 627 ± 23 Ma was roughly coeval with peak metamorphism. Volcaniclastic rocks of the Las Ventanas Formation dated at 573 ± 11 Ma can be correlated with the peripheral foreland basin (571 ± 8 Ma). Transpression and coeval high-K calc-alkaline magmatism is recorded in the Maldonado granite dated at 564 ± 7 Ma. The following events are postulated: (1) magmatism at 850–750 Ma related to rifting; (2) metamorphism and granite emplacement at 650–600 Ma; (3) molasse sequences and foreland basins at c . 573 Ma; (4) late magmatism at 580–560 Ma associated with transpression. The data indicate that (1) the Punta del Este Terrane could be a portion of the Coastal Terrane of the Kaoko Belt, (2) granitoid emplacement at 650–600 Ma in the Punta del Este and Nico Perez terranes favours westward subduction, and (3) widespread post-collisional synkinematic magmatism occurred in the Dom Feliciano and Kaoko belts between 580 and 550 Ma. Supplementary material: Analytical methods and data are available at http://www.geolsoc.org.uk/SUP18369.

El Chichón volcanic complex, Chiapas, México: Stages of evolution based on field mapping and 40Ar/39Ar geochronology

Abstract: A new interpretation of the evolution of El Chichon volcano is presented in this paper based on photogeology, fieldwork, 40 Ar/ 39 Ar dating and chemistry of juvenile products. El Chichon volcano belongs to a volcanic complex formed by craters and peripheral domes with a total estimated volume of ~26 km 3 . Our data suggest that inception of magmatism began around 370 ka with the emission of lava domes buried by younger products. The activity continued with the formation of a large andesitic dome complex between 209 and 276 and associated block and ash flows and lahars. The dome complex was subsequently destroyed by a major eruption that left a 1.5-km wide Somma-type crater. The activity continued with the extrusion of the SW dome at 217 ka, that partially disrupted the southwestern Somma crater wall. Later on a series of dome extrusions occurred beyond the northwestern sector of the Somma crater at about 95 ka. Juvenile blocks found in 1982 products yielded ages of 44 and 29 ka attesting to heretofore unidentified late Pleistocene activity. The onset of Holocene activity occurred both outside the Somma crater with explosive eruptions that formed the Guayabal Tuff Cone and inside the Somma crater with the formation of a tuff cone that has been repeatedly reactivated during the Holocene, lastly during the 1982 eruption. All magmas produced during the past 370,000 years are K-alkaline andesites that exhibit minor variation in their chemical composition. An average discharge rate of 0.5 km 3 /ka is calculated during the past 8,000 years (~4 km 3 ).

South Anyui suture: tectono-stratigraphy, deformations, and principal tectonic events

Abstract: . Geochronologic and structural data from the terranes of the South Anyui suture zone record a protracted deformational history before, during and after an Early Cretaceous collision of the passive margin of the Chukotka-Arctic Alaska continental block with the active continental margin of the North Asian continent. Preceding this collision, the island arc complexes of the Yarakvaam terrane on the northern margin of the North Asian craton record Early Carboniferous to Neocomian ages in ophiolite, sedimentary, and volcanic rocks. Triassic to Jurassic amphibolites constrain the timing of subduction and intraoceanic deformation along this margin. The protracted (Neocomian to Aptian) collision of the Chukotka passive margin with the North Asian continent is preserved in a range of structural styles including first north verging folding, then south verging folding, and finally late collisional dextral strike slip motions which likely record a change from orthogonal collision to oblique collision. Due to this collision, the southern passive margin of Chukotka was overthrust by tectonic nappes composed of tectono-stratigraphic complexes of the South Anyui terrane. Greenschists with ages of 115–119 Ma are related to the last stages of this collision. The postcollisional orogenic stage (Albian to Cenomanian) is characterized by sinistral strike slip faults and an extensional environment.

Uplift deduced from remanent magnetization of a proterozoic basic dyke and the baked country rock in the Hoting area, Central Sweden: a palaeomagnetic and 40Ar/39Ar study

Abstract: Remanent magnetization in dyke contact zones in the Hoting area of the Fennoscandian Shield in the central part of Sweden has been studied in order to establish the ambient temperature of the host ...

Age and paleomagnetism of the Okhotsk-Chukotka Volcanic Belt (OCVB) near Lake El'gygytgyn, Chukotka, Russia

Abstract: . Paleomagnetic results from the upper two thirds of the whole section of the Okhotsk-Chukotka Volcanic Belt (OCVB) volcanics exposed in the area around Lake El'gygytgyn, Chukotka yield stable, consistent magnetic vectors and well-preserved reversed directions. The magnetostratigraphy and 40Ar/39Ar geochronologic data reported here indicate that the sampled OCVB volcanics were erupted between about 90 and 67 Ma, and show no significant change in the apparent pole position over that time. The OCVB extends from northeast China, across Northeast Russia to the Bering Straight. This belt is made up of both extrusive and intrusive rocks, with the extrusive rocks and their associated sediments being dominant. The whole belt important in interpreting the paleogeography of the region because it overlies many of the accreted terranes of Northeast Russia. Most importantly, it overlies parts of the Chukotka-Alaska block which is thought to have moved out of the Arctic Ocean region, as well as terranes accreted from the south. These latter terranes have been rafted northwards on the paleo-plates of the Pacific, implying that the present relative paleogeography of all of the terranes overlain by the OCVB were essentially in place by 67 Ma, and possibly as early as 90 Ma. However, comparing our paleomagnetic pole position for the OCVB with those for North America and Eurasia (a proxy for Siberia) shows a statistically significant displacement of the OCVB pole to the south west. This implies that not only the OCVB, but the underlying terranes of northeast Russia, experienced southerly displacement with respect to the Siberian and North American platforms since the Late Cretaceous.

Tectonic reconstruction of Uda-Murgal arc and the Late Jurassic and Early Cretaceous convergent margin of Northeast Asia–Northwest Pacific

Abstract: . A long tectonic zone composed of Upper Jurassic to Lower Cretaceous volcanic and sedimentary rocks is recognized along the Asian continent margin from the Mongol-Okhotsk fold and thrust belt on the south to the Chukotka Peninsula on the north. This belt represents the Uda-Murgal arc, which was developed along the convergent margin between Northeast Asia and Northwest Meso-Pacific. Several segments are identified in this arc based upon the volcanic and sedimentary rock assemblages, their respective compositions and basement structures. The southern and central parts of the Uda-Murgal arc were a continental margin belt with heterogeneous basement represented by metamorphic rocks of the Siberian craton, the Verkhoyansk terrigenous complex of Siberian passive margin and the Koni-Taigonos Late Paleozoic to Early Mesozoic island arc with accreted oceanic terranes. At the present day latitude of the Pekulney and Chukotka segments there was an ensimatic island arc with relicts of the South Anyui oceanic basin in a backarc basin. Accretionary prisms of the Uda-Murgal arc and accreted terranes contain fragments of Permian, Triassic to Jurassic and Jurassic to Cretaceous (Tithonian–Valanginian) oceanic crust and Jurassic ensimatic island arcs. Paleomagnetic and faunal data show significant displacement of these oceanic complexes and the terranes of the Taigonos Peninsula were originally parts of the Izanagi oceanic plate.

The Tommot pluton: a Middle Paleozoic rift-related alkaline gabbro and syenite complex, Yakutia, northeast Russia

Abstract: The Tommot pluton is located within the continen- tal Omulevka terrane of the inner zone of the Verkhoyansk- Kolyma Mesozoic orogen. It is a small complex ( 12 km 2 ) composed of alkaline-ultramafic rocks, alkaline and subalka- line gabbroids, and alkaline and quartz syenites. The plu- ton is unique both in the composition and age of its con- stituent rocks. Mineralogical-petrographical and geochem- ical studies of the rocks indicate that the alkaline rocks re- sulted from the melting of depleted mantle horizons. K-Ar, Rb-Sr, and 40 Ar/ 39 Ar age determinations confirm a Paleozoic age of the rocks. Formation of the alkaline rocks is related to Middle Paleozoic rifting which occurred as two discrete events: a Late Devonian event, which affected the marginal part of the Siberian continent, and a Late Carboniferous event that reflects internal deformation of the Omulevka terrane or late-stage extension. A spatially associated alkali granite, the Somnitel'nyy pluton, is Late Jurassic-Early Cretaceous in age and is synchronous with accretion of the Kolyma- Omolon Superterrane to Siberia in the Mesozoic.