INSIGHT INTO THE ARC TECTONICS OF THE SOUTHWESTERN CORDILLERA VIA THE MESOZOIC DEFORMATION HISTORY OF THE PANAMINT RANGE, CALIFORNIA

New tectonic data from the Panamint Range, California elucidates the strain history of the southwestern Cordillera and is used to examine the relationship between plate motions and the resulting strain in arc-related orogenic belts. The Panamint Range is ideal for study because it preserves a relatively complete record of the deformation due to Mesozoic tectonism of the southwestern Cordillera: D₁ is east-vergent contraction between 175 and 148 Ma; D₂ is an episode of emplacement of numerous dikes and intrusions at 148 Ma with a minor component of sinistral arc-parallel shear; D₃ is intrusion-related folding of the margin of the Manly Peak batholith at 146 Ma; D₄ is top-eastward contraction at 75 Ma, with possibly earlier deformation as old as 107 Ma; D₅ is west-vergent extension at ~72 Ma; and D₆ is dextral arc-parallel at ~60 Ma. Miocene extensional fabrics (D₇) overprint all of these earlier deformations. This tectonic history fits with the tectonic history compiled for the southwestern Cordillera, except the ~60 Ma dextral shear of the Panamint Range, which occurs at 80-90 Ma in the other localities of the southwestern Cordillera.

The Mesozoic strain history recorded in the southwestern Cordillera shows a first order relationship between the direction and magnitude of relative plate convergence vectors for the North American-Pacific Ocean plates. Times of major contractile deformations match times of moderate to high rates of arc-perpendicular convergence. Periods of arc-parallel strike-slip shear occur during times of strong tangential convergence components. Regional ~72 Ma extension does not fit this simple relationship; it occurs during a moderate rate of convergence, immediately after a period of very rapid convergence. The extension therefore fits with critical taper models of orogenic belts, where high topography is built during convergence and must be held up by plate boundary forces.