1. Rock and age relationships within the Talkeetna forearc accretionary complex in the Nelchina area, southern Alaska
- Author
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Barefoot, John, Nadin, Elisabeth S., Newberry, Rainer J., and Camacho, Alfredo
- Subjects
United States. Geological Survey -- Analysis ,Glaciers -- Analysis ,Mid-ocean ridges -- Analysis ,Tectonics (Geology) -- Analysis ,Zirconium -- Analysis ,Basalt -- Analysis ,Sediments (Geology) -- Analysis ,Earth sciences - Abstract
Subduction zone processes are challenging to study because of the rarity of good exposures and the complexity of rock relationships within accretionary prisms. We report the results of field mapping and petrographic, geochemical, and geochronological analyses of the McHugh Complex accretionary prism melange in south-central Alaska that was recently exposed due to retreat of the Nelchina Glacier. Our new mapping and analyses of the melange, as well as adjacent Talkeetna arc intrusives, suggests that the previously mapped trace of the Border Ranges fault should shift northward in this location. Detailed petrographic analysis places this melange exposure with the Potter Creek assemblage of the McHugh Complex. Blocks of pillow lavas within the melange have both mid-ocean ridge basalt and intra-plate geochemical affinities, attesting to the complex relations of subduction-zone inputs in an alternating erosive--accretionary margin. A new zircon U-Pb age and geochemical analyses of a set of felsic dikes that cross-cut the accretionary sequence provide constraints on the regional tectonic evolution, including near-trench plutonism associated with the migration of a subducting spreading ridge along the southern Alaska margin during the Paleocene--Eocene. The McHugh section and cross-cutting dikes in this location are pervasively hydrothermally altered, which we attribute to elevated temperatures related to ridge subduction. Late-stage motion along the Border Ranges fault system, which is also recorded in the area, may also have contributed to the widespread alteration. Our data indicate that the Talkeetna volcanic arc and associated accretionary prism sediments were in their current configuration by 55 Ma. Key words: McHugh Complex, melange, Chugach terrane, southern Alaska, Talkeetna arc. Les processus associes aux zones de subduction sont difficiles a etudier en raison de la rarete de bons affleurements et de la complexite des relations entre les roches au sein de prismes d'accretion. Nous faisons etat des resultats de travaux de cartographie de terrain et d'analyses petrographiques, geochimiques et geochronologiques du melange de prisme d'accretion du complexe de McHugh, dans le centre-sud de l'Alaska, recemment expose par le retrait du glacier Nelchina. La nouvelle cartographie et les nouvelles analyses du melange, ainsi que des roches intrusives attenantes de l'arc de Talkeetna, donnent a penser que la trace cartographiee anterieurement de la faille des Border Ranges devrait se deplacer vers le nord en cet endroit. L'analyse petrographique detaillee place cette exposition du melange avec l'assemblage de Potter Creek du complexe de McHugh. Des blocs de lave a coussins dans le melange ont des affinites geochimiques avec du basalte de dorsale medio-oceanique et intraplaque, ce qui temoigne des relations complexes des apports de zone de subduction dans une marge alternant entre l'erosion et l'accretion. Un nouvel age U-Pb sur zircons et des analyses geochimiques d'un ensemble de dykes felsiques recoupant la sequence d'accretion permettent de preciser l'evolution tectonique regionale, dont du plutonisme proximal a une fosse associe a la migration d'une dorsale d'expansion en subduction le long de la marge sud de l'Alaska au Paleocene--Eocene. La coupe de McHugh et les dykes discordants dans cette localite temoignent d'une alteration hydrothermale penetrante que nous attribuons a des temperatures elevees associees a la subduction d'une dorsale. Un deplacement tardif le long du systeme de la faille des Border Ranges, qui est egalement preserve dans la region, pourrait aussi avoir contribue a l'alteration repandue. Nos donnees indiquent que la configuration actuelle de l'arc volcanique de Talkeetna et des sediments de prisme d'accretion associes etait deja etablie a 55 Ma. [Traduit par la Redaction] Mots-cles: complexe de McHugh, melange, terrane de Chugach, sud de l'Alaska, arc de Talkeetna., Introduction Arc--forearc relationships are difficult to study but are key to understanding processes that contribute to continental growth. Active margins are offshore and underwater, potentially buried by sediment. Ancient accretionary [...]
- Published
- 2020
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