Eocene (51 Ma) end to northward translation of the Coast Plutonic Complex: paleomagnetism and K-Ar dating of the White Pass Dikes

The White Pass mafic dikes are located in the Coast Belt, inboard of the Coast Shear Zone-Denali Fault System, east of Skagway, Alaska, astride the Alaska-British Columbia border. The dikes intrude Eocene felsic plutons in the Coast Plutonic Complex. Three K-Ar analyses date the dikes at 51.9+/-0.8 Ma(biotite), 50.1+/-1.0 Ma and 45.0+/-0.8 Ma (both whole rock), Structural analysis on these vertical dikes indicates that the study area has not been tilted significantly since dike emplacement. Conventional alternating field and thermal step-demagnetization and saturation-remanence paleomagnetic methods were used to analyse 407 specimens from 27 dike and five host rock sites. Three contact tests show that the specimens retain a primary thermoremanent magnetization (TRM) carried in pseudosingle to multidomain magnetite, although some magnetization in a few dikes is carried by pyrrhotite. The dual-polarity TRM is mostly reversed and the 24 accepted dike sites give a pole position of 245.7 degrees E, 76.3 degrees N (d(p)=4.2 degrees, d(m)=4.4 degrees), indicating a paleolatitude discordance of 8+/-4 degrees south and a clockwise rotation of 40+/-9 degrees with respect to the North American craton. These data, along with other paleomagnetic data on Eocene rocks, indicate that the allochthonous eastern Coast and Intermontane Belt terranes had halted their northward translation by about 51 Ma but continued to be rotated clockwise by an average of 24+/-10 degrees thereafter, probably on underlying sole faults. (C) 2000 Elsevier Science B.V. All rights reserved.