Tectonic and volcanic evolution of dark terrain and its implications for the internal structure and evolution of Ganymede

Abstract

Furrows in ancient dark terrain on Ganymede form three systems that are each hemispheric or greater in scale. The oldest of the systems, designated system III, is dominated by approximately concentric troughs centered on about 60°N, 50°W. System I, in the anti‐Jovian hemisphere, contains concentric and subradial furrows arrayed around a large, degraded palimpsest centered at 15°S, 165°W. Furrows in each system formed on and locally are buried by dark volcanic materials that embay and infill preexisting topographic features; they crosscut extremely few well preserved older craters; and they occur on surfaces having significantly different relative crater ages. System II, also in the anti‐Jovian hemisphere, contains widely spaced, radially arrayed furrows commonly 500–2000 km in length, which are organized around a large area of dark smooth resurfacing material, intense dark terrain fracturing, and some of the globally oldest light material. The total thickness of dark terrain resurfacing, estimated using the stratigraphy of different dark material deposits and the rim heights of the largest crater size class whose density was depleted by each deposit, is probably in the range of 3–8 km. Multiple models of the origin of each furrow system were tested using observed geologic features and patterns. Systems I and III were found to be most consistent with reactivation of impact‐generated, multiringed structures by endogenic global extension, during a period of widespread dark material volcanism that obliterated a dense, ancient crater population. System II was found to be most consistent with fracturing of a single, circular, isostatic uplift covering an entire hemisphere. On the basis of geologic observations and interpretations and theoretical models of convection in spheres, it is hypothesized that the uplift developed by long‐term warming of the upwelling current of a single axisymmetric convection cell in an initially cooler, undifferentiated interior. Such wanning would also have created global expansion and supplied the tensional stress inferred to have formed systems I and III. This hypothesis is supported by the concentration around the center of system II of intense fracturing and relatively young dark volcanic deposits, suggestive of high lithospheric heat flow, lithospheric thinning, and stress concentration. An observed long‐term decrease in the width of extensional tectonic features interpreted to be of endogenic origin is also consistent with lithospheric thinning due to warming of a cool interior.