CENOZOIC INTRAPLATE TECTONICS--LITHOSPHERIC RIGHT-LATERAL BULK SHEAR DEFORMATION IN THE NORTHERN TALKEETNA MOUNTAINS, SOUTH-CENTRAL ALASKA

Contiguous, rhomboidal megascopic blocks that are bounded by lengthy physiographic lineaments are common directly south of the Denali fault in the northern Talkeetna Mountains. The geometric and angular relationships between these lineaments and rhomboidal blocks are strikingly similar to those relationships between shears and polygons developed in experimental models of bulk shear. The shears in these models are both antithetic and synthetic fault zones formed in a passive layer above a penetratively deformed lower layer. Physiographic lineaments in the northern Talkeetna Mountains are associated with complex, wide zones of deformation mapped on the ground; enclosed rhomboidal blocks are manifested as 1) wedge uplifts bounded by high-angle reverse faults, 2) pull-apart grabens, or 3) early Tertiary volcanic fields derived from the partial melting of subjacent lithospheric mantle. The origin of the physiographic lineaments and associated rhomboidal blocks are interpreted to have formed in response to post-accretionary intraplate tectonics related to deep-seated crustal strike-slip faults and shear zones. Supracrustal Mesozoic sedimentary rocks in the northern Talkeetna Mountains are envisioned to have deformed passively above a rigid underlying lithosphere that was deformed by right-lateral bulk shear. The right-lateral Denali fault zone of south-central Alaska that is tangential to this area appears to be closely related to the mechanism driving bulk shear in the subjacent lithosphere.