Tectonic sequence diagrams and the structural evolution of schists and gneisses in multiply deformed terranes

• Published in: Journal of the Geological Society Vol. 165; no. 5; pp. 923 - 939
• Main Authors: Forster, Marnie A, Lister, G. S
• Format: Journal Article
• Language: English
• Published: London Geological Society of London 01.09.2008; The Geological Society of London; Geological Society; Geological Society Publishing House
• Subjects: Aegean Sea; Alps; Australasia; Crystalline rocks; deformation; Earth sciences; Earth, ocean, space; East Mediterranean; Europe; Exact sciences and technology; exhumation; faults; Field study; folds; France; Geology; gneisses; Greece; Haute-Savoie France; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Mediterranean Sea; Melanesia; metamorphic rocks; nappes; New Caledonia; New Zealand; Oceania; Otago Schist; overprinting; P-T-t paths; polyphase processes; recumbent folds; Savoy Alps; schists; sequence diagrams; shear zones; Sifnos; South Island; Southern Europe; Structural geology; tectonics; terranes; Vanoise; Western Alps; Western Europe; nappes; extension; outcrops; gneisses; exhumation; schists; focus; recumbent folds; metamorphic rocks; lead; duration; currents; correlation; interpretation; overprinting; Terranes; shear zones; absolute age; deformation; growth; tectonite; tectonics
• ISSN: 0016-7649; 2041-479X
• DOI: 10.1144/0016-76492007-016

The current practice of numbering deformation phases (D1...Dn) in a multiple deformed terrane obscures the link between data and inference, and can lead to erroneous interpretations. Routine production of tectonic sequence diagrams (TSDs) allows a method whereby all deformation and metamorphic mineral growth events, their overprinting and duration (based on relative timing) are systematically recorded. Different hypotheses as to correlation of TSDs across a region can then be assessed objectively, including correlation of characteristic sequences that can often be recognized in TSDs from single outcrops. TSDs ensure that absolute dating is applied to specific events within a sequence, eliminating the practice where a date is imprecisely correlated. TSDs also place a focus on the independent character of events, here confirmed by absolute dating. Here we demonstrate how TSDs show that many fold nappes are recumbent folds transected by later (unrelated) ductile shear zones. TSDs also show that P-T-t paths and exhumation rates must include starting and stopping of events, periods of quiescence and changes in tectonic mode from shortening to extension. TSDs allow objective analysis of the complexity present in many metamorphic tectonites, but which is obscured by conventional methodology.