Orthogonal flow directions in extending continental crust: An example from the Kigluaik gneiss dome, Seward Peninsula, Alaska

Abstract

A 15-km-thick crustal section is found within the Kigluaik gneiss dome on Seward Peninsula, Alaska. Metamorphism at 91 Ma overprints a pre-120 Ma blueschist-facies event. This overprint increases from greenschist facies in the Nome Group to granulite facies in the structurally lower Kigluaik Group. Core gneisses ceased deforming before cooling from high temperatures. Although the rocks are L-S tectonites, shear-sense indicators are rarely preserved. Rocks higher in the section cooled during deformation and preserve evidence for upward motion of core rocks relative to surrounding rocks. The rocks of the Nome Group contain two foliations. The older fabric formed during the blueschist-facies event. The younger fabric is undated but is parallel to the fabrics in the amphibolite-facies rocks of the dome and thus may be related to the same event. Shear-sense indicators adjacent to the dome show motion down and away from the core; elsewhere, top-to-the-north shear sense indicators prevail.Lineations developed at ~90 Ma record a rotation of stretching directions with depth. Lineations in the core are east-west. An ~4-km-thick transition zone contains lineations which progressively rotate upward in the section to nearly north-south within the Nome Group. The continuous transition of lineation azimuth and the lack of overprinting relationships indicate orthogonal stretching directions at different structural levels. Mid-crustal east-west flow beneath the Seward Peninsula may reflect compensation at depth for differential crustal thinning at higher crustal levels. Thinning may have occurred during regional north-south extension associated with the vertical rise of the gneiss dome.

Geologic map and summary of the evolution of the Kigluaik Mountains gneiss dome, Seward Peninsula, Alaska

“…In the lower parts of the section (Zones I and lower Zone II), peak metamorphic temperatures outlasted deformation, producing coarse-grained equigranular textures and fabrics at microscopic scale, while macroscopic high strain fabrics are preserved. At the higher structural levels, within the isograd region (upper Zone II) and within the Nome Group (Zone III), deformation occurred at lower metamorphic grade and outlasted the temperature peak (Amato et al, 2002). The preferred interpretation for the observed lineations in the Kigluaik Mountains involves the concept of divergent directions of fl ow or stretching within the crust during high-temperature metamorphism.…”

“…A gradual transition is present from east-west-stretching lineations in the core of the dome to nearly north-south (~160°) lineations in the Nome Group. The observed continuous variation in lineation orientations appears to have developed entirely during the second metamorphic event because they are developed in rocks with no apparent structural breaks that exhibit a continuous metamorphic gradient (Amato et al, 2002). These fi eld relationships preclude the hypothesis that the two crustal levels characterized by orthogonal mineral lineations or stretching directions were juxtaposed by subsequent deformation.…”

“…is white mica, "bt" is biotite, "hbl" is hornblende, "mnz" is monazite, "zrn" is zircon. From Amato et al (2002). those in the isograd zone.…”

“…The exposed crustal section in the gneiss dome can be divided into three zones based on structural style of deformation, the textural relationship of structures with respect to peak heating and metamorphism, and on the orientation of mineral elongation lineations, which vary in orientation depending on structural depth (Amato et al, 2002). These zones are described from structurally deepest to shallowest.…”

“…The geologic mapping used for this compilation was carried out over fi ve fi eld seasons between 1989 and 1994 by the authors and those listed as making a contribution to this publication (see page 1 footnote). The summary of geologic relations in the Kigluaik gneiss dome is based on Miller et al (1992), Amato et al (1994), , Hannula and McWilliams (1995), Dumitru et al (1995), , Calvert et al (1999), Amato et al (2002), and Amato et al (2003). The Kigluaik Mountains were originally mapped in reconnaissance by Brooks et al (1901), with more detailed maps of selected areas produced by Moffi t (1913), Hummel (1962), Sainsbury (1972Sainsbury ( , 1974, Sainsbury et al (1972), Till (1980), Till et al (1986), Bundtzen et al (1994), and others listed in the mapping credits (Plate 1).…”