{'fraction_non_alphanumeric': 0.02478386167146974, 'fraction_numerical': 0.018443804034582133, 'mean_word_length': 4.618122977346278, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Towards a More Complete Record of Magmatism and Exhumation in Continental Arcs , Using Detrital Fission-Track Thermochrometry

The NE Asian margin was the locus of an Andean-style subduction zone during most of the Mesozoic and Cenozoic. The Ukelayat flysch is a 10 to 15 km thick section of marine turbidites that were deposited in a 500 km long forearc basin along the outboard edge of this convergent margin. It was structurally overridden during the early Tertiary by an obducted island arc, the Olytorsky terrane. We present new fission-track (Ff) grain ages from unreset detrital zircons. These data provide precise information about depositional age of the Ukelayat and the temporal evolution of the Asian margin continental arc, which provided much of the sediment for the Ukalayat. Because all sandstones contain a large fraction of first-cycle volcanic zircons (euhedra\, colorless), we infer that the youngest component of the Ff grain age distribution, designated P l , closely approximates the time of deposition. Twenty-seven samples have yielded P l ages ranging from -44 to 88 Ma, which indicates continuous magmatic activity in the arc from the Late Cretaceous to Middle Eocene. A second component (P2) records progressive exhumation of the basement to the Okhotsk-Chukotka arc. The difference between P l and P2 ages indicates steady exhumation at -200-400 m/Myr from -90 to 44 Ma. This finding implies the removal of 9 to 18 km of rock in the most deeply exhumed parts of the source region. A change in exhumation rates at -70 Ma may coincide with an eastward shift in the locus of volcanism from the Okhotsk-Chukotka arc to the younger Western Kamchatka-Koryak arc. © 2000 Elsevier Science Ltd. All rights reserved.