There is a new article in press in the journal Icarus titled, "Geologic Mapping of the Zal region of Io," by Melissa Bunte, David Williams, and Ron Greeley. A summary of their results was presented in March at the Lunar and Planetary Sciences Conference and was reported on here. Now the full paper is available on the Icarus website (subscription required to view paper). This paper is based on imagery acquired during the I25 and I27 Galileo encounters with Io.
Like many of the regions mapped by the ASU previously, such as near Camaxtli Patera, Tohil Mons, Amirani, and Thor, the authors mapped 5 basic units in the region: mountains/plateaus, smooth/layered plains, patera floor material, flow material, and diffuse materials. The flow features in this area appear to be generated from a small patera lying near the western margin of South Zal Montes (they propose the name "Rustam Patera" for this volcano) or from a fissure that runs north from "Rustam" along the western margin of South Zal Montes and the eastern margin of North Zal Montes. The flows include bright flows (possibly of sulfurous composition) radiating out from "Rustam" and dark flows which flow east across part of Zal Patera from the northern part of the fissure. Additional flow features are also seen within Zal Patera, but these appear to be older in age based on their brighter appearance.
One interesting hypothesis made in this paper is that the various components of Zal Montes, which surround Zal Patera to the west, east, and south, were originally part of a single structure. This feature then broke-up due to strike-slip then extensional faulting, opening up Zal Patera. Similar plate tectonics-in-miniature is theorized for formation of Hi'iaka Patera. The paper goes on to describe the degradational processes that have occurred at the mountains in the region.
One feature I wished the paper expounded on further is a small volcano west of North Zal Montes, which they suggest the name "At'am Patera" for. What makes the volcano interesting is that it appears to be one of a very rare breed of explosive Ionian volcano. "At'am" erupted between late-June and mid-September 1997, producing a white, Sulfur dioxide-rich plume deposit and a dark-green pyroclastic deposit with a digitate margin. Some of both materials was deposited on North Zal Montes. The digitate appearance is due to the interaction between the pyroclastic flow and the arcuate margin of the western part of North Zal Montes. This morphology may provide clues on how these pyroclastic deposits are formed on Io. Oddly, for an Ionian eruption, no lava flows or thermal emission were observed at this volcano. Also, the central vent is among the smallest paterae found on Io. It is possible that the 1997 eruption could have been the result of an intrusive event, where magma ascends from a deeper chamber, but fails to reach the surface. However, volatiles and other materials, being more buoyant, do make it to the surface.
The paper does touch a bit on the plume seen at Zal last year by New Horizons. This plume is centered on Zal Patera (unlike the plume deposit seen by Galileo starting in Sept. 1997 which surrounds "At'am Patera"). Zal Patera is also the site of fresh surface changes, which include a new dark plume deposit and fresh dark lava flows.
Link: Geologic Mapping of the Zal region of Io [dx.doi.org] (subscription required to view paper)