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Single-Plate Rifting Model for Ridge and Trough Terrain on Icy Satellites

Ridge and trough terrains (RTTs) are regions of subparallel ridges and troughs that are observed on Ganymede, Enceladus, Miranda, and Ariel. Most show morphologies indicative of an origin as extensionaltectonic bands shaped by normal faulting and fissure-style volcanism. Terrestrial rift zones are potential analogues to icy satellite RTT, as tension fracturing, normal faulting, fissure-fed volcanism, and massive dike injection occur as extension proceeds in continental rifts, along oceanic rifts, and in Iceland. Extensional-tectonic structures and extrusive fissures in terrestrial rifts are mutually subparallel and constructional volcanic features, where present, are along linear trends. Although terrestrial spreading is sometimes conceptualized as occurring along a planar axis, a "spreading" boundary actually consists of a -10-60 km wide active zone of rifting which resides at least in part above a shallow subsurface magma chamber. Terrestrial-style plate tectonics, involving the physical separation of crustal plates with formation of newer crust in between, is not required for icy satellite rifting; instead a single-plate rifting model is invoked. The single-plate rifting model for RTT is analogous to the plate-boundary style of terrestrial rifting, but with extension concentrated in localized rift zones of a single-plate crust. Spreading does not ensue; instead, older crust is modified by normal faulting, inundated by extrusive volcanism, and replaced by intrusion from below. This model incorporates the processes of tension fracturing, normal faulting, fissure volcanism, and diking which result from extension of the crust over a shallow magma source. The degree and specific style of normal faulting and fissure volcanism are expected to affect the morphology that results from single-plate rifting. The style of normal faulting may range from domino to horst-and-graben, with combinations likely. Fissure volcanism might form broad flows or constructional edifices. Aligned circular-plan volcanoes are not observed on icy satellites; therefore, it is assumed here that where fissure extrusion forms volcanoes or domes on icy satellites, they are linear in plan. Magma composition, viscosity, and effusion rate are all expected to affect the rheology of flows and the morphology of possible volcanos; thus, the potential for linear volcanoes or domes to form during singleplate rifting will not always be realized. In combination or instead, flood volcanism may occur, or there may be no volcanism. In examining the possible ways that normal faulting and fissure volcanism can combine to create RlT, of first-order importance is whether RTT structures are formed directly in (or upon) otherwise undeformed terrain. If so, a RTT is termed primary PI. For example, Ganyrnede's furrows are considered primary R l T in that they formed directly in ancient cratered terrain. If R n instead resides within the confines of major structures such as normal faults, it is termed secondary [Sl. Most of Ganymede's "grooved terrain" can be termed secondary RTT because faulting and volcanism occurred within primary fault-bounded structures. Most RTT thought of as such is of the secondary type. The accompanying figure examines the matrix possible combinations of normal faulting and linearvent extrusion predicted by the single-plate rifting model. Normal faulting can define individual faultbounded structures IN,] or multiple subparallel domino blocks and/or horst-and-graben structures [N,], or a region might not exhibit extensive faulting. Volcanism can be flood-type [FJ or constructional [C]. In the constructional case, if a single major vent is assumed to build each linear volcano or dome, a single active vent [C,] can be distinguished from multiple parallel extrusive vents [Cm]. Where a broad rift zone is resurfaced, it is likely that flood volcanism proceeds from multiple fissures, although this cannot be stated unequivocally. Combinations of the letter designations assigned to these processes are used to indicate combinations of the processes. This means of examining possible combinations of normal faulting and fissure extrusion aids to deconvolve RTT fomation histories. It does not encompass all possible sub-combinations, and other possible endmember processes are not included. Combinations that are marked with an asterisk in the figure do not meet the definition of RTT as a series of subparallel ridges and troughs but do help to establish and identify alternative evolutionary branches related to RTT formation. For example, an individual primary fault structure without volcanism (type PNs) results in an "empty" graben or large-scale rift; the addition of confined flood volcanism creates a resurfaced band (type SF). If multiple secondary normal fault structures are associated with this resurfaced region, a (type SNmF) RTT results. The matrix of potential terrain types that stems from the single-plate rifting model is applicable to icy satellites that display RTT and those which do not. Many of the possible terrains are observed on icy satellites, suggesting a genetic relationship among terrains observed in diverse locations and unifying previously disparate models of RTT formation on icy satellites.