Abstract
K/Ar dating on llite in Upper Cretaceous low-grade metamorphic pelites in the Torres del Paine area was used to set new time constraints on the development of the Patagonian retroarc fold-and-thrust belt (FTB) caused by the subduction of the Antarctic Plate beneath the South American Plate. The combined use of illite crystallinity (Kubler Index), polytype quantification and K/Ar dating of illite fractions (<0.2, <2 and 2-6 µm) allowed to distinguish four distinct periods of illite growth based on their K/Ar ages and degree of regional metamorphism: (1) early Cenomanian (98 Ma) illite crystallization, (2) widespread early Campanian (ca. 80 Ma) diagenetic illite growth under anchizonal metamorphic conditions, (3) a significant period of illite formation in the early Paleocene (ca. 60 Ma), and (4) a late stage of illite growth in the early Eocene (55-46 Ma) under epizonal conditions. The earliest indication for the emergent FTB formation in the hinterland is documented in a metapelitic clast (14-9, <2 µm) within the Upper Cretaceous Cerro Toro conglomerate which yields a K/Ar cooling age of 98.3±1.2 Ma and an epizonal KI value of 0.24 ∆°2Θ. After a certain period of geological quietness an interval of major thrusting and uplift occurred between ca. 60 and 46 Ma. The east dipping Rio Nutria and Rio Rincon thrusts record the onset of thrust and fold activity which can be placed close to 60 Ma. They also mark the frontal thrust towards the less deformed Magallanes foreland basin. In the western part of the internal domain, widespread fault and thrust activity of the frontal wedge and associated thermal overprint continued and is recorded until 46 Ma by K/Ar illite cooling ages. The flexural subsidence that is driven by the thrust sheet loading in the internal domain was responsible for the eastward migration of the foreland depocenter and the rapid increase of sedimentation rate along the monoclinal belt. No Miocene thrusting nor uplift event has been recorded by K/Ar illite dating in the study area.
Highlights
The Patagonian fold-and-thrust belt is located at the southern tip of South America
Based on K-Ar fine fraction ages and Kübler index values, we propose to subdivide the internal domain along the Río Nutria and Río Rincon thrust into a thrust zone with maximum rock uplift in the west and a foreland-vergent imbricate thrust zone in the east
Age constraints allow us to divide the internal domain of the fold-and-thrust belt into a thrust zone with maximum rock uplift and a foreland-vergent imbricate thrust zone
Summary
The Patagonian fold-and-thrust belt is located at the southern tip of South America. Its large curved structure shapes the western Patagonian margin, starting from a N-S orientation at around 51°S to a nearly E-W orientation in Tierra del Fuego (Fig. 1). Thrust loading of heavy oceanic crust due to incipient fold-and-thrust belt formation triggered flexural subsidence, resulting in the development of a foredeep and the birth of the Magallanes foreland basin stage (Natland et al, 1974; Winn and Dott, 1979; Dalziel, 1981; Biddle et al, 1986; Wilson, 1991; Fildani and Hessler, 2005) This dramatic conversion in depositional environment is reflected by a sudden change in sediment supply observed between the Zapata and Punta Barrosa Formations. The foredeep of the Magallanes foreland basin was filled with deep-marine sediments, recorded as the Punta Barrosa and Cerro Toro Formations (Natland et al, 1974; Winn and Dott, 1979; Dalziel, 1981; Wilson, 1991; Fildani and Hessler, 2005; Romans et al, 2009) Due to their syntectonic deposition, both units pinch out toward the east.
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