BURIAL AND EXHUMATION HISTORY OF THE EASTERN LOS ANGELES BASIN FROM APATITE FISSION-TRACK THERMOCHRONOLOGY

The Puente-San Jose Hills form a triangular-shaped block in the eastern Los Angeles basin bounded on the east by the Chino Hills Fault and on the southwest by the Whittier Fault. The Puente-San Jose Hills consist of anticlinal structures that developed in the late Miocene during transpression associated with development of the San Andreas Fault system. These positive inversion structures formed in predominately middle to late Miocene sedimentary strata, which are underlain by middle Miocene volcanics and Late Cretaceous Peninsular Range granitic rocks. The middle Miocene strata were deposited in extensional basins during a middle Miocene transtension phase of Los Angeles basin development. Apatite fission-track dating was used to reconstruct the thermochronologic history of the eastern Los Angeles basin and to assess magnitudes and timing of burial and exhumation of these inversion structures. Late Cretaceous intrusive rocks that underlie the middle Miocene volcanic and sedimentary rocks have AFT ages of 24 (±2) Ma in the San Jose Hills and 62 (±5) to 69 (±3) Ma in the Puente Hills. The 16 Ma Glendora Volcanics have AFT ages of 20 (±3) to 21 (±2) Ma, indicating no resetting after deposition. South of the Chino Hills and San Jose faults in the Puente Hills, AFT ages of granitic rocks indicate cooling and possible exhumation of the intrusions in the Late Cretaceous prior to initial basin development, but with little resetting since. The 24 (±2) Ma AFT age in the San Jose Hills suggests the more recent (late Oligocene) exhumation possibly associated with transrotation of the Los Angeles basin prior to extensional basin development. AFT ages of mid-late Miocene sedimentary rocks are 56 (±4) Ma to 37 (±4) Ma, indicating that burial during the transtensional phase of basin development and prior to basin inversion was insufficient to reset the AFT ages. These new ages will place important constraints on the vertical motion of rocks before and during the development of the eastern Los Angeles basin.