Structural and Paleomagnetic Evidence for Tertiary Bending of the Eastern Brooks Range Flexure, Alaska

  1. John W. Hillhouse
  1. Peter W. Plumley,
  2. Margaret S. Vance and
  3. Giulio Milazzo

Published Online: 18 MAR 2013

DOI: 10.1029/GM050p0127

Deep Structure and Past Kinematics of Accreted Terranes

Deep Structure and Past Kinematics of Accreted Terranes

How to Cite

Plumley, P. W., Vance, M. S. and Milazzo, G. (1989) Structural and Paleomagnetic Evidence for Tertiary Bending of the Eastern Brooks Range Flexure, Alaska, in Deep Structure and Past Kinematics of Accreted Terranes (ed J. W. Hillhouse), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM050p0127

Author Information

  1. Department of Geology, Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070

Publication History

  1. Published Online: 18 MAR 2013
  2. Published Print: 1 JAN 1989

ISBN Information

Print ISBN: 9780875904542

Online ISBN: 9781118666609



  • Geology, Structural—Congresses;
  • Geodynamics—Congresses;
  • Earth—Crust—Congresses


Paleozoic carbonate sedimentary rocks from northeastern Alaska have been sampled for a structural and paleomagnetic study of the relationship of the Porcupine Terrane to both cratonic North America and adjacent Brooks Range terranes. Major structural features investigated includes the Yukon Porcupine megashear and the Northeast Alaskan oroclinal flexure.

Structural analysis of two well-exposed Paleozoic sections along the upper Porcupine River in northeastern Alaska yields estimates of shortening directions for two distinct phases of deformation. In the Upper Ramparts region of the Porcupine Terrane, north-south shortening (D1) occurs in Silurian-Devonian units. In the equivalent units of the Lower Ramparts northwest-southeast shortening occurs.

A much younger deformational event (D2), presumably associated with the Late Cretaceous Brookian Orogeny, overprints these earlier pre-Permian (D1) structural elements. In the Upper Ramparts the orientation of this event is somewhat equivocal but appears to be oriented northwest-southeast with vergence to the southeast. The Lower Rampart Permian section is clearly deformed by the D2 event in an east-west sense with vergence to the west. The general trend between the D1 and D2 deformational events differ by 45 degrees in the Upper Rampart Region. A similar variance between D1 and D2 is observed in the Lower Ramparts, but the structural orientations differ by 45 degrees between the Upper and Lower Ramparts, suggesting large scale block rotations occurred after the D2 shortening event.

The characteristic remanent magnetization postdates the latest phase of deformation (D2) and therefore represents an overprint. This overprint is presumed to be early to middle Tertiary, although the mean paleomagnetic pole lies off the expected apparent polar wander path (APWP) for North America. This paleomagnetic discordance is in declination only, and the sense and magnitude of rotation is clockwise approximately 45 to 90 degrees. It is proposed that the structural-grain of the Brookian Orogeny in northeastern Alaska was aligned with the northwest Canadian Cordilleran. Subsequent bending in the Tertiary produced the eastern Brooks Range orocline.