CC - CATEGORY CODES: 05A-Igneous-and-metamorphic-petrology
DT - DOCUMENT TYPE: Thesis-or-Dissertation
BL - BIB LEVEL: Monograph
MC - MAP COORDINATES: LAT: N045300; N045300; LONG: W0752200; W0752200.
RF - REFERENCE SOURCE: GeoRef, Copyright 2000, American Geological Institute.
AV - AVAILABILITY: University Microfilms, Ann Arbor, MI, United States
AN - ACCESSION NUMBER: 1996-063648
UD - UPDATE CODE: 199620
Registro 4856 de 5614 - GeoRef Disc 4: 1993-1996
BK - BOOK TITLE: Paleomagnetism and accretion events in the Northern Andes.
BA - BOOK AUTHORS: Estrada-Juan-Jose
CP - COUNTRY OF PUBLICATION: United-States
PY - PUBLICATION YEAR: 1995
DG - DEGREE GRANTED: Doctoral
DI - DEGREE GRANTING INSTITUTION: SUNY at Binghamton. Binghamton, NY, United States. Pages: 172. 1995.
LA - LANGUAGE: English
AB - ABSTRACT: New paleomagnetic results are presented from a complex of allochthonous terranes of oceanic origin, named here the Gorgona, Choco, Western (Occidental) Cordillera and Amaime, that have been accreted to the NW Andes of Colombia. The paleomagnetic data was obtained mainly from pillow basalts, although some massive flows and sedimentary horizons were also investigated. The study was extended to a series of "stitching" plutons of Late Tertiary age (10 Ma) to evaluate post accretionary structural events. These samples were demagnetized using mainly multistep thermal demagnetization procedures and to a minor extent, alternating field procedures. The Gorgona terrane, mainly below sea level, has Gorgona island as the only known exposure. This terrane is made of Mesozoic mafic and ultramafic rocks, including some komatiitic flows, probably accreted before Early Miocene. Strike-slip faulting and a break up of the originally accreted Gorgona terrane possible followed along the Buenaventura fault zone. The Choco terrane includes basic Late Mesozoic-Early Cenozoic volcanic rocks of island arc affinity in the Baudo Range; the Early Cenozoic Mande batholith and a sequence of associated volcanic rocks characterizes the Western Cordillera; and a thick Eocene-Miocene sedimentary sequence lies in the intervening Atrato-San Juan basin. The Choco terrane was accreted along the Uramita suture zone in Middle Miocene. The Western Cordillera terrane, includes most of the Western Cordillera except its NW portion. This terrane contains Cretaceous oceanic volcanic and sedimentary rocks accreted along the Cauca suture zone in Late Cretaceous-Early Tertiary. The Amaime terrane lies on the western flank of the Central Cordillera and contains Late Jurassic-Early Cretaceous oceanic basalts of possible mid-oceanic and oceanic flood affinity accreted along the Romeral suture zone in Early Cretaceous. Each one of these terranes may ultimately prove to be an assemblage of terranes. It is thought that the terranes investigated here were formed in the Pacific basin and driven into place by the motion of the Phoenix, Pacific, Farallon and Caribbean plates. The paleomagnetic results from the Gorgona terrane indicate latitudes of origin of about 30 degrees S and a clockwise tectonic rotation of 130 degrees relative to the mean of Cretaceous platform poles for South America since middle Cretaceous. The paleomagnetic results from the Choco terrane are consistent with up to 15 degrees paleolatitudes of origin. The paleomagnetic data from the Western Cordillera terrane are compatible with equatorial paleolatitudes of origin. It is estimated that this terrane formed at 2 to 4 degrees from the Equator. The paleomagnetic results from the Amaime terrane are yet to be evaluated due to the lack of bedding indicators. In this terrane the in situ orientation of the characteristic direction is I = 40.6 degrees , D = 216.2 degrees , alpha (sub 95) = 8.9 degrees . When using AMS as a proxy for bedding, the paleomagnetic results yield two possibilities: in one case the mean inclination is shallow, 0 degrees , indicating an origin close to the equator. In the other case the mean of the characteristic direction is steep, up to 40 degrees , compatible with latitudes of origin of 20 degrees . Two 40Ar-39Ar age dates from samples of the Anserma basalts have yielded 762 Ma that are younger than the expected Early Cretaceous age. The porphyritic intrusions of andesitic and dacitic compositions, are located at the north end of the Cauca Basin which lies between the Central and Western Cordilleras. They intrude the Amaime terrane near the boundary with the pre-Mesozoic continental margin and also the overlapping sediments. The results from this study indicates an unusual pattern of paleomagnetic vectors being dispersed in a vertical plane with inclinations ranging from horizontal to nearly vertical and declinations being parallel to the trend of the adjacent Romeral suture zone. Two types of rotations can be inferred from the patterns of the paleomagnetic vectors: 1) non-coherent rotations of individual bodies about horizontal axes, and 2) coherent rotation about vertical axes. These rotations come about during transpression and "kinking" of accreted rock panels migrating along the resistant buttress of the paleocontinental margin now marked by the Romeral suture zone.