Onshore and offshore apatite fission-track dating from the southern Gulf of California: insights into the time-space evolution of the rifting

We present the results of a apatite fission-track (AFT) study on intrusive rocks in the southern Gulf of California, sampled along the eastern margin of Baja California Sur (western rift margin), as well as from islands and submerged rifted blocks within the Gulf of California, and from the conjugate Mexican margin (Nayarit state). For most of the samples U-Pb zircon and 40Ar-39Ar mineral ages were already available (Duque-Trujillo et al., 2015). Coupledwith the newAFT data these ages provide amore complete information on cooling after emplacement.Our samples span a wide range of ages between 5.5 ± 1.1 and 73.7 ± 5.8 Ma, and show a general spatialdistribution,with lateMiocene AFT ages (about 6 Ma) aligned roughly NW-SE along a narrow offshore belt, parallelto Baja California Peninsula, separating older ages on both sides. This pattern suggests that in LateMiocene,deformation due to plate transtension focused at the eastern rheological boundary of the Baja California block.Some Early Miocene AFT ages onshore Baja California could be related to plutons emplaced at shallow depthsand thermal resetting associated with the onset of volcanism at ~19 Ma in this part of the Peninsula. On theother hand, an early extensional event similar to that documented in the eastern Gulf cannot be ruled out inthe westernmost Baja California.