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Abstract: Laramide Inversion of the Tethyan Continental Margin of Southwestern North America

T. F. Lawton

Late Cretaceous-Paleogene deformation in southeastern Arizona and southwestern New Mexico reactivated extensional faults formed during Late Jurassic-Early Cretaceous rifting. Jurassic and Cretaceous marine strata deposited in sedimentary basins related to the mid-Mesozoic extension contain ammonites found in Tethyan strata that encircle the globe. Laramide reactivation of Tethyan normal faults Is therefore analogous to, albeit not synchronous with, Alpine deformation of the Tethyan passive margin of Europe and Africa.

Although exceptions to the general structural grain exist, Laramide faults in southeastern Arizona and southwestern New Mexico dominantly trend northwest fault planes range from flat to nearly vertical. Fault vergence varies systematically: faults that cut thick sections of the mid-Mesozoic Bisbee Group in the southwestern part of the region mostly verge southwest; faults to the northeast that cut thin mid-Mesozoic cratonal strata verge northeast Stratigraphic and structural relations of the Paleozoic-Mesozoic unconformity surface most clearly illustrate tectonic inversion. The most diagnostic indicators of reversal of earlier fault offset include the following: 1) abrupt changes in amount of post-Paleozoic erosional truncation across reverse faults, such that Lower Cretaceous strata est on lower Paleozoic rocks in the Laramide footwall but on Permian strata in the handing wall; 2) thick Bisbee Group sections on structurally high, Laramide-age blocks; 3) changes in stratigraphic separation on a single fault across the Paleozoic-Mesozoic unconformity. In the latter case, Paleozoic strata below the unconformity show normal separation, whereas above the unconformity Paleozoic and Mesozoic strata show reverse separation, indicating that normal offset was not fully recovered by reverse offset.

Basement was widely involved in laramide structures of the southwestern Cordillera, evidenced both by exposed tracts of Precambrian granitic and metamorphic rocks in hanging-wall blocks and by basement detritus in Laramide basins for which uplifts have not yet been clearly delineated. in some cases, basement involvement may be inferred by the presence of basement and lower Paleozoic rocks as structural horses along steep fault zones whose hanging-wall and footwall blocks both consist of younger strata. These structural horses were probably plucked from basement of the extensional footwall block when it was inverted to become the hanging-wall block upon shortening.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada