Pressure–temperature–time evolution of the Central East Greenland Caledonides: quantitative constraints on crustal thickening and synorogenic extension

• Published in: Journal of Metamorphic Geology
• Main Authors: White, A. P., Hodges, K. V.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2003
• Subjects: East Greenland; Greenland
• Online Access: http://dx.doi.org/10.1046/j.1525-1314.2003.00489.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1046%2Fj.1525-1314.2003.00489.x; https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1525-1314.2003.00489.x

Abstract Whereas geologists have known for three‐quarters of a century that there was significant crustal thickening in the central East Greenland Caledonides, the crucial role of extensional faulting during Caledonian orogenesis has only been recognized during the past decade. In this paper, new petrographic and thermobarometric observations are presented from migmatitic metasedimentary gneisses of the Forsblad Fjord region ( c. 72.5°N). Samples of the Krummedal Sequence, collected from the footwall of the upper of two significant splays of the main extensional fault system in the region—the Fjord Region Detachment (FRD)—enable us to establish a relative sequence of metamorphism. Our pressure ( P )–temperature ( T ) results imply a clockwise loop in P–T space. As recorded by mineral assemblages in the Krummedal gneisses, prograde metamorphism involved a net increase of c . 4 kbar and 250 °C, with peak conditions of c. 10.5 kbar at 785 °C. Early burial and heating was followed by near‐isothermal decompression of 4.5 kbar, a process which is attributed to roughly 18 km of tectonostratigraphic throw on the upper splay of the FRD. Combining data reported here with the published data, it is estimated that the approximate tectonostratigraphic throw along the lower splay of the FRD was c . 16 km. In situ U–Th–Pb‐monazite electron microprobe dating suggests that the earliest phase of metamorphism recorded in the Krummedal Sequence gneisses of Forsblad Fjord occurred during the Caledonian orogeny. Furthermore, the combination of our new data with existing conventional TIMS U‐Pb and 40 Ar/ 39 Ar data imply that: (1) movement along the uppermost splay of the FRD ( c. 425–423 Ma) occurred at maximum time‐averaged slip‐rates equivalent to c. 9 mm of vertical displacement per year; and (2) that the final stages of metamorphism occurred prior to c. 411 Ma, although part of this denudation was likely accommodated on overlying extensional structures that may have been active more recently. There is close agreement between our ...