IMPORTANCE OF EXTENSIONAL ARCS IN THE GEOLOGIC RECORD (Invited Presentation)

The preservation potential of upper- to mid- crustal rocks in both continental and oceanic arcs is greatly enhanced by syn-magmatic extension. Yet only decades ago, all continental arcs were assumed to be high-standing “Andean” arcs, and extension was assumed to be restricted to backarc settings, not intra-arc settings. However, in 1988, I proposed that the early Mesozoic continental arc of the southwest US formed within a deep, normal fault-bounded depression. Since that time, dozens of publications from many sites along the length of the arc have confirmed the validity of this model. Distinctive features of extensional continental arcs include: high tectonic subsidence rates, in places dropping the arc below sea level; syndepositional normal faults and associated thick fault talus breccias; potential for trapping craton-derived sediment; and abundant silicic calderas. Silicic calderas in this setting may be very large rectilinear features, up to 50 X 25 km in size.

Silicic calderas are also an important feature of extensional oceanic arcs, a fact that was underappreciated only twenty years ago. Our study of the Ordovician Bald Mountain massive sulfide deposit in Maine (USA) shows that it formed in a primitive oceanic arc-rift basin, with two nested rectilinear silicic calderas bounded by normal faults. This section contains a well-preserved ignimbrite vent that formed at a water depth of 1.45 km, where black smoker chimneys grew. Our work in the Rosario segment of the Cretaceous Alisitos extensional oceanic arc of Baja California (Mexico) also shows that it contains silicic calderas, including a large one with a cross section of 15 km and resurgent magmatism. The Rosario segment exposes the upper- to middle-crustal transition, from volcanic through hypabyssal to plutonic rocks, and is the subject of an entire forthcoming GSA Special Paper¹, as well as a publicly accessible and interactive 3D arc crustal model². We compare the geology of the Alisitos oceanic arc with that of modern oceanic arcs with silicic calderas, which are numerous and largely occur in extensional settings.

¹Busby, C., et al., Geology of a large intact oceanic arc crustal section with superior exposures: Cretaceous Alisitos arc, Baja California (Mexico): GSA Books

²R. Morris et al., 2022; https://cedar.wwu.edu/geology_facpubs/103/