Mid‐Cretaceous transtension in the Canadian Cordillera: Evidence from the Rocky Ridge volcanics of the Skeena Group

摘要: The age relations, geochemistry, and sedimentology of the Rocky Ridge Formation Skeena Group are used to test competing tectonic reconstructions for mid-Cretaceous Canadian Cordillera as well timing location accretion Insular Superterrane. Pollen macrofossil assemblages indicate that these intrabasinal basalts were erupted along southern margin Bowser basin in Early Albian Cenomanian. Single-crystal fusion step-heating 40Ar/39Ar dating hornblendes one basalt flow from uppermost part formation yielded Middle Cenomanian ages 94.3 ± 0.4, 95.6 1.6, 95.0 1.6 Ma. Vesicular flows interbedded with crystal-rich tuff breccias contain evidence hot emplacement pyroclastic flows. Individual eruptive centers identified by their proximal facies, paleoflow indicators within lava flows, volcaniclastic fluvial deposits, geochemical differences, geographic isolation volcanic deposits. Major trace-element geochemistry 20 sampled indicates an alkali composition volcanics. northern center show enrichment light rare earth large ion lithophile elements strong negative Nb-Ta anomalies whereas Tahtsa Lake depletion slight elements, minimal anomalies. combined paleogeographic regional reconstruction suggests a continental arc setting intraarc extension. presence deeper marine facies west lack western sediment source technically active Superterrane was not study area during time. Thus we reconsider Omineca Belt main axis arc, placing Intermontane forearc position volcanics side arc. Coeval strike-slip faulting reconstructed oblique plate convergence suggest transtensional An electronic supplement Tables A1–A2 may be obtained on diskette or Anonymous FTP KOSMOS.AGU.ORG (LOGIN AGU's account using ANONYMOUS username GUEST password. Go right directory typing CD APEND. Type LS see what files available. GET name file get it. Finally, type EXIT leave system.) (Paper 95TC03496, Mid-Cretaceous transtension Cordillera: Evidence Group, Kari N. Bassett Karen L. Kleinspehn). Diskette ordered American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order.