标题：Structural and 40Ar/39Ar age constraints on the Kulidjik nappe: A record of an early Alpine thrust tectonics in the northeastern Rhodope Massif, Bulgaria
期刊名称：IOP Conference Series: Earth and Environmental Science
印刷版ISSN：1755-1307
电子版ISSN：1755-1315
出版年度：2008
卷号：2
出版社：IOP Publishing
摘要：In the northeastern Rhodope Massif, a unique juxtaposition of assumed high-grade
basement allochthon onto a low-grade Mesozoic unit is exposed in the Kulidjik
nappe. The thrusting is interpreted as post-Jurassic to pre-Late Eocene. The
tectonostratigraphy comprises the following units: (i) an upper unit of
high-grade basement of the eastern Rhodope consisting of various amphibolite
facies rocks with metaophiolite lenses; (ii) an overlaying Mesozoic (including
Jurassic radiolarians) low-grade unit; (iii) a structurally overlaying
allochthon of the Kulidjik nappe; and (iv) unconformable Late Eocene sedimentary
and Oligocene volcanic cover rocks. The basement unit contains a NNE-SSW
oriented lineation rarely associated with top-to-the NNE ductile shearing.
Greenschist facies retrogression of this unit is diagnostic of local thrusting
of the overlying low-grade unit. The greenschists of the low-grade unit
represent arc metavolcanic rocks overlain by phyllites. The structural elements
in this unit have similar attitudes as the basement unit. Kinematic indicators
in the low-grade unit demonstrate internal top-to-the NNE ductile shearing
during greenschist facies conditions. The allochthon consists of klippen
composed of muscovite-albite gneisses. The flat-lying thrust contact with
ductile-brittle deformation contains mylonites and cataclasites. Previously
reported local top-to-the SSW ductile shearing in the allochthon is probably a
consequence of shear instabilities. Deformation-crystallization relationships
revealed fabrics in the allochthon compatible with greenschist up to lower
amphibolite facies conditions without retrogression and similar ENE-verging fold
patterns in the low-grade unit and the allochthon at the thrust contact. Two
muscovite samples from distinct klippen yielded inverse isochron
40Ar/39Ar ages of 156.07±1.30 Ma and 162.62±2.69 Ma, which
reflect cooling below 350°C of the allochthon associated with its exhumation
during emplacement after greenschist-lower amphibolite facies metamorphism.
Structural data indicate NNE-directed tectonic transport of the low-grade
footwall unit, which is consistent with emplacement in comparable metamorphic
conditions, and the thrust direction of the allochthon in the hanging wall. The
subduction-accretionary origin of the low-grade unit and the new age results
allow us to link this tectono-metamorphic event to Late Jurassic thrust
imbrication of arc units on the Rhodope basement, which is the first time that
early Alpine thrust tectonics in the northeastern Rhodope Massif have been
recognized.
