Timing of rifting in the southern Gulf of California and its conjugate margins: Insights from the plutonic record

Abstract

The Gulf of California is a young example of crustal stretching and transtensional shearing leading to the birth of a new oceanic basin at a formerly convergent margin. Previous studies focused along the southwestern rifted margin in Baja California indicated rifting was initiated after subduction and related magmatism ceased at ca. 14-12.5 Ma. However, the geologic record on the Mexico mainland (Sinaloa and Nayarit States) indicates crustal stretching in the region began as early as late Oligocene. The timing of cooling and exhumation of pre-and synrift plutonic rocks can provide constraints on the timing and rate of rifting. Here, we present results of a regional study on intrusive rocks in the southern Gulf of California sampled along the conjugate Baja California and Nayarit-Sinaloa rift margins, as well as plutonic rocks now exposed on submerged rifted blocks inside the gulf. Forty-one samples were dated via U/Pb zircon and 40 Ar/ 39 Ar mineral ages, providing emplacement age and thermochronological constraints on timing and rate of cooling. We found an extensive suite of early and middle Miocene plutons emplaced at shallow depths within the basement Cretaceous-Paleocene Peninsular Range and Sinaloa-Jalisco Batholiths. Early Miocene granitoids occur in an elongated WNW-ESE belt crossing the entire southern gulf from southern Baja California to Nayarit and Sinaloa. Most have an intermediate composition (<67 SiO 2 wt%), but a distinctive group of high-silica granites (>75 SiO 2 wt%) was emplaced 20.1-18.3 Ma, near the end of the early Miocene. Age span and chemical composition of the early Miocene silicic plutons essentially overlap ignimbrites and domes exposed in the southern Sierra Madre Occidental and in southern Baja Cali fornia, suggesting that eruptive sources for the early Miocene ignimbrite fl are-up may also have been located within the southern Gulf of California. Early Miocene plutons cooled below the 40 Ar-39 Ar biotite closure temperature (350-400 °C) in less than 2.5 m.y., which we interpret as evidence of a regional extensional event leading to the opening of the Gulf of California. A less widely distributed suite of intermediate-composition, middle Miocene granitoids (15-13 Ma) was sampled from the central-western part of the gulf, west of the Pescadero Basin, and these correspond to an episode of scarce volcanism recorded by the middle and upper members of the onshore Comondú Group in Baja California. Our widely spaced sampling of the generally sediment-covered igneous crust suggests that middle Miocene primary volcanic rocks are much less abundant than implied by previous models in which the gulf was the site of a robust Comondú arc. Thermobarometry data also indicate a very shallow depth (<5 km) of emplacement for the middle Miocene plutonic rocks. Some of these rocks also show a distinctive inequigranular texture indicative of at least two crystallization stages at different pressure. Early and middle Miocene granitoids away from the gulf axis yielded 40 Ar-39 Ar cooling ages very close to...

Rayleigh wave dispersion measurements reveal low‐velocity zones beneath the new crust in the Gulf of California

“…However, Ferrari et al [] describe an earlier period of extension in the margins of the southern Gulf in the Late Oligocene (~29–24 Ma) with extension along the Sinaloa coast ending before ~11–10 Ma, and a considerable amount of crustal thinning (up to 100%) having occurred before that time. Extension in this region was ongoing in the Early Miocene (18–12 Ma) during the age of the supposed Comondú arc [ Duque‐Trujillo et al ., ]. This predates the commonly accepted ~12–6 Ma period of minor extension and contradicts the traditional view of a well‐developed arc existing at ~6 Ma, instead a zone of extended crust occupied the site of the future southern Gulf [ Bryan et al ., ].…”

Oblique rifting: the rule, not the exception

“…The first phase of the Gulf of California rift is closely linked to the Greater Basin and Range extensional zone. Tectono-stratigraphy and dated rift-related magmatic rocks show that the onset of 30 continental extension must have occurred before the mid-Miocene (Ferrari et al, 2013;Duque-Trujillo et al, 2015). This first phase of slow rifting is marked by a wide rift style characteristic of the present-day Basin and Range region.…”

Description of the Geology of the Tecolotlan Graben, Jalisco, Mexico

“…Ferrari et al ( , 2012 suggest that in the western portion of the Trans-Mexican Volcanic Belt, the earliest phase of volcanism was dominated by mafi c, locally alkaline, eruptions between 11 and 7 Ma. This was followed by a lull in volcanic and extensional activity (Ferrari et al, 2012(Ferrari et al, , 2013Duque-Trujillo et al, 2015). Between ca.…”

“…Along the northern boundary of the Jalisco block, andesitic to rhyolitic ashfl ow tuffs and lavas of the Sierra Madre Occidental that erupted between 20 and 25 Ma are cut by normal faults and thus predate the formation of the Tepic-Zacoalco rift (Moore et al, 1994;Ferrari et al, 2002Ferrari et al, , 2007McDowell and McIntosh, 2012;Bryan et al, 2014). These extensional faults and associated Early Miocene intrusive and volcanic rocks are thought to be associated with the opening of the Gulf of California (Ferrari et al, 2013;Duque-Trujillo et al, 2015). Late Oligocene to Early Miocene volcanism, however, has not been identifi ed in the Jalisco block itself .…”

“…Age probability diagram for dated volcanic rocks in the Tepic-Zacoalco and Colima grabens. Data from Allan (1986), Moore et al (1994), Nieto-Samaniego et al (1999), Ferrari et al ( , 2013Ferrari et al ( , 2014, Frey et al (2004Frey et al ( , 2007, Centeno-García et al (2011), Valencia et al (2013, Duque-Trujillo et al (2015).…”