The Sierra Mazatán in northwestern Mexico is the southernmost metamorphic core complex in the North American Cordillera. Large‐magnitude Tertiary extension at Sierra Mazatán involved both ductile and brittle slip along a major normal fault that presently dips 10°–15° west. Extension was polyphase and involved an early period of extension from 25 to 23 Ma followed by major slip from 21 to 16 Ma. Total slip was ≤20 km and occurred at rates of 3–4 mm/a. This extension predated the plate boundary change to transtension at ∼12 Ma and was largely decoupled from relative Pacific–North American plate motion. Numerous lines of evidence suggest that the presently low‐angle normal fault initiated at a steep dip (50°–60°) and was rotated to lower angles during slip. When corrected for this tilting, fault corrugations at Sierra Mazatán had a similar geometry to the segmentation of many active normal faults, which is compatible with their origin as primary fault features. Many aspects of the Sierra Mazatán are comparable to large active normal faults, indicating that this core complex formed owing to prolonged extension on an otherwise typical high‐angle normal fault. Therefore, core complexes need not represent a fundamentally unique mode of crustal extension.

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The Sierra Nevada batholith (California, USA) hosts multiple shear zones of different ages and different styles of deformation. In this study we present new data syntheses and maps of U-Pb zircon and hornblende and biotite Ar age distributions through the batholith in order to examine the temporal and thermal settings under which contractional and transpressional shear zones arose. These maps highlight the localization of intrabatholithic shear zones at the boundaries between swaths of some of the oldest and youngest plutons, and help to distinguish deformation styles in the southern and central Sierran arc. We also present new 40 Ar/ 39 Ar ages and crystallization and deformation temperatures from along the Kern Canyon fault system in the southern part of the batholith, and contrast these new constraints with previously published thermochronological conditions for shear zones to the north. The transpressional proto-Kern Canyon fault was continuously active from ca. 95 to 85 Ma. Deformation temperatures along the fault increase by ~100 °C from north to south, following the trend of increasing pluton emplacement pressures. These observations, in conjunction with a steep cooling path for the southeastern section of the batholith east of the proto-Kern Canyon fault, support previous interpretations that rapid exhumation in the southernmost part of the batholith followed the arrival and low-angle subduction of an oceanic plateau (the Shatsky Rise conjugate). We suggest that local forces such as these triggered mid-Cretaceous shear zone development in the southern Sierra Nevada batho lith, while shear zones in the central part of the batholith, which record the transition from 100-90 Ma compression to 90-80 Ma transpression, were triggered by changing kinematic patterns in regional subduction forcing. This idea is supported by our thermochronologic color contour maps, which reveal an east-west-trending older (and colder) swath within the batholith that may have prevented the more northern shear zones from propagating southward. We contrast the cooling paths of regions within the southernmost part of the batholith, and propose a two-part deformation history in which (1) the 95-85 Ma ductile proto-Kern Canyon fault initiated in the southeast as a compressional structure that transitioned to transpressional, and (2) the 85-75(?) Ma ductile to brittle Kern Canyon-White Wolf fault initiated in the southwest, accommodated extrusion of shallowly subducted accretionary material and thinning of the overlying batholith, and then overprinted the proto-Kern Canyon fault along its northern end. Shear zone activity within the central Sierra Nevada batholith can thus be distinguished by location: in the central zone deformation arose from regional forces, and in the southern zone deformation was triggered by local forces.

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