Geology of a Large Intact Extensional Oceanic Arc Crustal Section with Superior Exposures: Cretaceous Alisitos Arc, Baja California (Mexico)

Abstract

The Rosario segment of the Early Cretaceous Alisitos oceanic arc exposes the transition from upper-crustal volcanic and hypabyssal rocks to middle-crustal plutons, which formed in an extensional environment. The Rosario segment forms a structurally intact, unmetamorphosed, spectacularly well-exposed, gently tilted section that is 50 km long and 7 km deep. The top of the exposed section is unconformably overlain by flat-lying Late Cretaceous sedimentary rocks (Rosario Group, described elsewhere), and the base of the section passes downward into ductilely deformed metamorphic rocks (not mapped herein). We divided the Rosario segment into three subsegments: a central subaerial edifice, underpinned by the La Burra pluton; a southern volcano-bounded basin (dominantly shallow marine), underpinned by the San Fernando pluton; and a northern fault-bounded basin (dominantly deep marine), underpinned by the Los Martires pluton. Using a combination of published and new geochronologic data, we infer that the time span represented by the arc crustal section could be as little as 1.7 m.y., dated at ca. 111–110 Ma. Volcanic and plutonic samples show a continuum from basalt/basaltic andesite to rhyolite, are low to medium K, and are transitional tholeiite to calc-alkaline in character. Hf isotopic data from zircons indicate primitive magma, consistent with previously published whole-rock isotopic data. The volcanic stratigraphy can be correlated across all three subsegments using the tuff of Aguajito (Ki-A), a distinctive rhyolite welded ignimbrite that fills the 15-km-wide, >3.6-km-deep La Burra caldera on the central subaerial edifice. Additionally, a second caldera is preserved below the tuff of Aguajito (Ki-A) in the northern fault-bounded basin, floored by a large rhyolite sill complex, up to 700 m thick with a lateral extent of >7 km. Up section from the tuff of Aguajito (Ki-A), there is an abrupt shift to dominantly mafic volcanism that we correlated across all three subsegments of the Rosario segment, dividing the section into two distinct parts (phase 1 and phase 2). The pluton beneath the central subaerial edifice (La Burra) is associated with the caldera that produced the tuff of Aguajito (Ki-A) during phase 1. Plutons beneath the northern fault-bounded basin (Los Martires) and the southern volcano-bounded basin (San Fernando) were emplaced during phase 2. However, we infer that the La Burra pluton, which is associated with the phase 1 La Burra caldera, continued to grow incrementally during phase 2 because it intruded and tilted both phase 1 and phase 2 strata. The Rosario segment escaped postmagmatic deformation, other than gentle tilting (25°–35°) to the west as a single rigid block. The Rosario segment of the Cretaceous Alisitos arc represents an extensional oceanic arc with abundant silicic pyroclastic rocks, culminating in arc rifting with outpouring of mafic magmas. The excellent exposure and preservation provide us with the opportunity to herein describe the following: (1) caldera collapse features and the products of varying explosive eruptive styles; (2) caldera plumbing systems, including silicic sill complexes; (3) the transition from plutons through hypabyssal intrusions to eruptive products; (4) incremental pluton growth and its effects on the structure of the roof rocks; (5) the products of deep-water mafic to silicic eruptions; and (6) flow transformations that occur when hot pyroclastic flows enter marine basins on gentle slopes versus steep slopes. We also used this data set to address questions highly complementary to the work being done on understanding the growth of continental crust at subduction zones. Finally, this volume serves as a model for detailed geologic study of paleo-arcs.

Lower Cretaceous (Aptian-Albian) Bisbee Group, Arizpe area, northern Sonora, Mexico: Integrated biostratigraphy, age and provenance from U-Pb and Hf isotopes

“…Early Cretaceous plutons from the western zone of the Peninsular Ranges batholith in southern California yielded positive zircon εHf(t) values from 3.1 to 11.4, corresponding to T DM model ages of ca. 0.4 to 0.9 Ma (Shaw et al (2014)), while Jurassic metaigneous rocks and Early Cretaceous plutons and xenoliths from this batholith in the northern Baja California Peninsula have positive zircon εHf(t) values (Contreras-López et al, 2021).Similarly, Hf positive values are reported from volcanic and plutonic rocks of the Alisitos arc in this region (Busby et al, 2023) (Figure 1).…”

Mesozoic drift of the Wrangellia superterrane revisited: the way forward from paleomagnetic data

“…The southernmost slab of the western belt that cannot be explained by Caribbean arcs is the Socorro slab (van der Meer et al, 2010;, at a latitude of ~15°N (Figure 1). It is also not likely that this slab correlates to subduction records of the Guerrero terrane (in Mexico), which was built since Triassic time on accretionary prism rocks formed at the North American margin and was only temporarily separated from North America by a short-lived and likely narrow back-arc basin (Boschman et al, 2018a(Boschman et al, , 2018bBusby et al, 2023;Martini et al, 2014). Because the This manuscript has been submitted to Tectonics 21 Bonanza arc is the southernmost intra-oceanic arc complex in the Cordilleran orogen, we therefore discuss in our scenarios S and N whether this slab may have been linked to the Bonanza arc.…”