Crustal thickness variations in the margins of the Gulf of California from receiver functions

Abstract

S U M M A R YReceiver functions (RFs) from teleseismic events recorded by the NARS-Baja array were used to map crustal thickness in the continental margins of the Gulf of California, a newly forming ocean basin. Although the upper crust is known to have split apart simultaneously along the entire length of the Gulf, little is known about the behaviour of the lower crust in this region. The RFs show clear P-to-S wave conversions from the Moho beneath the stations. The delay times between the direct P and P-to-S waves indicate thinner crust closer to the Gulf along the entire Baja California peninsula. The thinner crust is associated with the eastern Peninsular Ranges batholith (PRB). Crustal thickness is uncorrelated with topography in the PRB and the Moho is not flat, suggesting mantle compensation by a weaker than normal mantle based on seismological evidence. The approximately W-E shallowing in Moho depths is significant with extremes in crustal thickness of ∼21 and 37 km. Similar results have been obtained at the northern end of the Gulf by Lewis et al., who proposed a mechanism of lower crustal flow associated with rifting in the Gulf Extensional Province for thinning of the crust. Based on the amount of pre-Pliocene extension possible in the continental margins, if the lower crust did thin in concert with the upper crust, it is possible that the crust was thinned during the early stages of rifting before the opening of the ocean basin. In this case, we suggest that when breakup occurred, the lower crust in the margins of the Gulf was still behaving ductilely. Alternatively, the lower crust may have thinned after the Gulf opened. The implications of these mechanisms are discussed.

Seismic structure beneath the Gulf of California: a contribution from group velocity measurements

“…At depths of 20-40 km, we also interpret the southernmost velocity high in Sonora as associated with the Seri Terrane or southern segment of the Caborca Block. It should, however, be noted that station NE82 located at the southern end of this anomaly has a Moho depth of 25.9 km (Persaud et al 2007). In this case, the deeper shear wave high would instead be associated with the upper mantle.…”

“…A magnetotelluric survey at a latitude of 28 o N across the peninsula imaged the slab at depths of 30-40 km (Romo-Jones 2002). Receiver function analyses also suggested a possible slab top close to the Moho beneath station NE75, with an anisotropic layer at around ∼100 km (Persaud et al 2007). In addition, numerical models show that shearing in a mantle wedge above the slab beneath the peninsula could produce the unique compositional variation in post-subduction volcanism found in Baja (Negrete-Aranda et al 2013).…”

“…In addition to previous surface waves studies, other related investigations that have used NARS-Baja as well as other data sets to determine the large-scale tectonic features in this region include receiver function studies of Moho depth (Persaud et al 2007), as well as SKS (Long 2010) and SKS-receiver function studies of anisotropy van Benthem et al 2008). We analyse our detailed results in light of these studies.…”

“…Station NE80 (Fig. 1 for location), which has a Moho depth of 32.1 km (Persaud et al 2007) is located at the northern end of the easternmost velocity high which extends up to shallower crustal levels of ∼20 km near latitude 30 o , suggesting that the velocity anomaly may indeed be a part of the lower crust rather than the mantle. At depths of 20-40 km, we also interpret the southernmost velocity high in Sonora as associated with the Seri Terrane or southern segment of the Caborca Block.…”

“…Variations in the crustal structure and composition along the Gulf and beneath Baja are found in several studies. Based on receiver function analysis, Persaud et al (2007) found a thinning of the crust from western Baja towards the Gulf, and for the southernmost stations (NE76 down to NE79 in Fig. 1) relative to the northern stations (north of NE75 in Fig.…”