| Popis: |
The Denali fault system, one of the major tectonic elements of southern Alaska, forms an arc 2,100 km long across southern Alaska. In the central Alaska Range, the system consists of a northern Hines Creek strand and a southern McKinley strand, 30 km apart, which divide the area into northern, central, and southern terranes. There is evidence for at least two episodes of deformation in the northern terrane, four in the central, and two in the southern during Paleozoic and Mesozoic time. During each, the inferred axis of maximum compressive strain was subhorizontal and about north-south, but the direction shifted to north-northwest–south-southeast during a late Paleocene–Eocene folding episode. Tectonic stability during Oligocene–middle Miocene time was followed by differential uplift of crustal blocks during late Miocene–Pliocene time. The Hines Creek fault may preserve a record of the early history of the fault system. Strong contrasts between lower and middle Paleozoic rocks juxtaposed along the fault suggest large dextral strike-slip displacement, but major convergent movement cannot be ruled out. Movement throughout the Hines Creek fault ceased by middle Cretaceous time, but local dip-slip movements continued into the Cenozoic era. The McKinley fault is an active dextral strike-slip fault, with Cenozoic offset of probably at least 30 km and possibly much greater. Mean Holocene displacement rates are 1 to 2 cm/yr. These rates would produce a 30-km offset in 1.5 to 3.0 m.y., or a 400-km offset in 20 to 40 m.y. The Denali fault may be part of a transform fault system connecting the Juan de Fuca Ridge and the landward extension of the Aleutian subduction zone. However, it is more likely that the fault forms the northern boundary of a small lithospheric plate caught between the Pacific and American plates. |
