Tectonometamorphic evolution of the Maclaren Glacier metamorphic belt, south-central Alaska

Tectonometamorphic evolution of the Maclaren Glacier metamorphic belt, south-central Alaska
Davidson, C. M.
Princeton University, NJ
210 p., Illus., Maps
Alaska Resources Library & Information Services: QE84 C64 D38 1991; Rasmuson Library: ALASKA QE84 C68 D38 1991a (mircofiche); Geophysical Institute Library: THESIS NOT UAF
The Maclaren Glacier metamorphic belt is an exhumed portion of a deep-crustal thrust shear zone (the Valdez Creek shear zone) where hot, upper amphibolite facies rocks were emplaced over cooler, lower grade rocks. The shear zone formed during the accretion of Wrangellia, an allochthonous terrane which now occurs in south-central Alaska. $\sp{40}$Ar/$\sp{39}$Ar dating of igneous and metamorphic amphibole and biotite from within the belt indicate that the shear zone was active from at least 77.5 Ma to about 68.1 Ma; and that the entire terrane cooled through the biotite closure temperature ($\sim$300$\sp\circ$C) at about 62 Ma. Metamorphic reactions identified in the hanging wall rocks indicate that the hanging wall experienced nearly isothermal decompression from about 700$\sp\circ$C and 7.5 kbar (minimum) to less than 3.7 kbar before cooling. A wealth of kinematic indicators which occur within the shear zone show that thrusting within the shear zone was top to the south (present-day coordinates). The deformed foreland is characterized by a pronounced inverted metamorphic field gradient which formed in response to the simultaneous emplacement of the hot hanging wall and the intrusion of a 1 km thick tonalite sill. Field data indicate that the tonalite sill was emplaced into the shear zone while the shear zone was still active; these data include the presence of a well-developed foliation within the sill which is concordant to the fabric of the shear zone, the alignment and tiling of plagioclase laths, and the presence of highly strained mafic enclaves within a matrix which shows no evidence of any sub-solidus deformation. Results from thermal modeling show that melt which is intruded into the hot, deep crust can remain partially molten for long periods of time ($>$1 Ma). In the Maclaren Glacier metamorphic belt, the amount of time for the tonalite sill to fully crystallize is calculated to be about 90,000 years. If the bulk of convergence between North America and Wrangellia were concentrated within the tonalite sill, a displacement of at least 10 km could have been accommodated across the sill before it crystallized.
Ph.D. dissertation
Minerals Data and Information Rescue in Alaska (MDIRA)