Abstract

Paleomagnetic and rock magnetic measurements for 27 sites in the tonalitic ca. 108 Ma Alpine and ca. 100 Ma Ramona complexes in the suspect Yuma terrane or western zone of the Peninsular Ranges Batholith near San Diego show that single- and pseudosingle-domain magnetite carries their characteristic remanence. The Alpine and Ramona paleopoles agree with six published paleopoles from Cretaceous igneous rocks in the Yuma terrane, but all eight paleopoles are discordant for North America. Al-in-hornblende geothermobarometric depth determinations at 16 sites, plus published mapping and depth determinations, suggest that the batholith is divided into blocks by orogen-parallel faults or by orthogonal fault sets with large vertical displacements. The paleomagnetic data show that the fault blocks in the terrane underwent, on average, ENE-side-up tilting of ~10° ± 3° about an orogen-parallel axis between ca. 125 Ma and ca. 89 ± 7 Ma, but they have not been translated northward as a suspect terrane by ≥500 km, excluding Neogene opening of the Gulf of California. The tilting is attributed to torque from ENE shallow underthrusting of the Farallon plate beneath the WSW overthrusting North American plate, where subsequent extension has caused differential uplift of the fault blocks. Nonsignificant dextral displacement of ~4° ± 1° (440 ± 110 km) between the eastern and western zones of the batholith suggests that this boundary represents an intraarc fault. The large paleopole discordance found in younger sedimentary strata of Baja California is attributed mainly to compaction shallowing of the remanence and/or postdepositional remagnetization.

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