An investigation of the magnetic fabrics and the paleomagnetism of the Ghost Rocks Formation, Kodiak Islands, Alaska

• Main Author: Gallen, Sean F.
• Format: Text
• Language: unknown
• Published: Western Washington University 2008
• Subjects: Kodiak; Alaska
• Online Access: https://dx.doi.org/10.25710/1c48-0457; https://cedar.wwu.edu/wwuet/11

Recent tectonic models based on the hypothesized existence of the Resurrection plate between the Kula and Farallon plates have questioned the location(s) of trench-ridge-trench (TRT) triple junction(s) along the Northern Cordilleran margin during Paleocene to Eocene time. The Paleocene Ghost Rocks Formation, located in the Kodiak islands, Alaska (latitude ~57°N), consists of pillow lavas and hypabyssal sills interbedded with turbidites, and is interpreted to have formed in a trench slope or slope basin during the passage of a TRT triple junction. A previous paleomagnetic study (Plumley et al., 1983) on the volcanic flows of the Ghost Rocks Formation suggests these rocks formed at latitudes significantly south of their present-day locations, at a latitude of ~41ºN during Paleocene time. Tectonic models, based on the assumed existence of the Resurrection plate, reject the conclusions of Plumley et al.'s paleomagnetic study, and instead suggest that these rocks have been remagnetized. Our study revisited the Ghost Rocks Formation in an effort to resolve the disputed location of this TRT triple junction. The focus of this thesis is on magnetic fabrics and paleomagnetism of two localities within the Ghost Rocks Formation: Jap Bay and Alitak Bay. More than 300 oriented core samples were obtained primarily from sedimentary rocks in two coherent sections of Jap Bay, Unit A and Unit B; and over 500 oriented core samples were taken from the turbidites and volcanic flows of Alitak Bay. The anisotropy of magnetic susceptibility was used to study the magnetic fabrics of these rocks. The majority of the sedimentary rocks showed magnetic fabrics typical of weakly deformed sediments with magnetic foliations oriented parallel to bedding, and cryptic magnetic lineations oriented perpendicular to the shortening direction. However, sediments from Unit B of Jap Bay showed a large portion of magnetic lineations oriented approximately parallel to the direction of slip on bedding parallel faults, becoming more pronounced in fold hinges. Magnetic lineations oriented parallel to the slip direction are not typical of weakly deformed sediments. The volcanic samples from Alitak Bay contained magnetic fabrics that can qualitatively be defined as foliated, lineated, and scattered. The paleomagnetism of the majority of the sedimentary rocks were magnetically unstable. Those from Unit A however, exhibited good magnetic behavior but the high unblocking temperature components fail the fold test. The magnetic behavior of the volcanic flows from Alitak Bay was good. Results from a series of fold tests using various structural corrections yield inconclusive results. However, "rotation tests" show positive results. The "rotation corrected" directions from Alitak Bay and in-situ directions of Kiliuda Bay from Plumley et al. (1983) pass a regional fold test yielding a mean paleomagnetic direction for the Ghost Rocks Formation corresponding to a latitude of ~41º. However, the somewhat arbitrary nature of these rotation corrections and failed conglomerate tests suggest that remagnetization of the rocks at Alitak Bay is also a likely possibility.