AN: T11B-07TI: Formation of Continental Crust by Magmatic Accretion During Late    Orogenic ExtensionAU: Andronicos, C.L.AF: Dept. of Geosciences, Princeton Universtiy, Princeton, New Jersey    08544EM: cla@princeton.eduAU: Chardon, D.AF: Dept. of Geosciences, Princeton Universtiy, Princeton, New Jersey    08544EM: AU: Hollister, L.S.AF: Dept. of Geosciences, Princeton Universtiy, Princeton, New Jersey    08544EM: AU: Gehrels, G.AF: Dept. of Geology, University of Arizona, Tucson, Arizona    85721EM: AU: Isachsen, C.AF: Dept. of Geology, University of Arizona, Tucson, Arizona    85721EM: AB: Evidence of emplacement of mafic to intermediate composition sills    synchronously with late orogenic extension in the Coast Plutonic    Complex (CPC) of British Columbia suggests magmatic accretion during    extension may be a fundamental process for the formation of    continental crust. The Eocene Kasiks sill complex, consisting of a    minimum 7 km thickness of shallow dipping sills, produced ~1/5 the    total thickness of the present continental crust in the CPC. Detailed    structural data show the sills intruded during subvertical shortening    partitioned into two directions of extensional shearing. Two kinematic    domains record this partitioning. The first, along the eastern edge of    the sills, records top to the northeast normal shearing. A second    domain dominates the central and western parts of the complex and    records top to the north-northwest normal shearing. Greenschist facies    shear zones overprint magmatic structures suggesting northeastdirected    shearing outlasted north-northwest directed shearing. Preliminary U/Pb    zircon age dates and titanite cooling ages document rapid exhumation    of the sill complex following emplacement. Two U/Pb zircon dates of    the sill complex were obtained, one yielding an age of 53.4 Ma, and a    second yielding an age of 52.5 Ma. A cross cutting pegmatite dike has    yielded a U/Pb zircon date of 52.4 Ma, suggesting sill emplacement and    northwest directed extensional deformation were ending by this time.    Titanite cooling dates from the two samples of the sill complex    yielded cooling ages of 51 Ma. Regionally distributed hornblende and    biotite cooling ages confirm rapid cooling of the sill complex from    ~550 C to <300 C between 50 and 48 Ma. These geochronologic data    combined with the localization of extensional strain during decreasing    temperatures suggests intrusion of the sills was synchronous with and    may have triggered exhumation of the high-grade root of the    CPC.SC: TDE: 8109DE: 8035DE: 8102MN: 1997 Fall Meeting