Locations of Major Thrust Faults on Mercury Point to a Formation Mechanism Associated with Crustal Thickening
Tuesday, 16 December 2014
Large thrust faults on Mercury are the expression of horizontal contraction that resulted from cooling of the planet; their uneven distribution may indicate that other stress fields contributed to their formation and development. We explore the relationship in the northern hemisphere between locations of lobate scarps and high-relief ridges >50 km in length and inferred crustal thickness to understand whether mechanisms that result in crustal thickening and thinning may have also influenced the locations of large thrust faults. These landforms are concentrated in areas with >53 km crustal thickness (at the 99% confidence level, on the basis of a two-sided single proportion test), an extreme end of the normal distribution (with a mean of 40 km) of modeled crustal thickness values. One possible mechanism for thickening crust and concentrating prominent thrust faults is mantle flow; a broad pattern of mantle upwelling and downwelling is compatible with Mercury’s gravity anomaly and topography fields at long (>1000 km) horizontal wavelengths. On Earth (e.g., central Australia), midplate mantle downwelling has been invoked as a mechanism to thicken overlying continental crust and increase compressional stresses in the rigid upper portion of the lithosphere. Downwelling in some areas requires upwelling in others; horizontal divergence could decrease levels of compressive stresses over upwelling regions and might account for the deficiency of large thrust faults in areas of thinnest crust (<27 km, significant at the 95% confidence level). We tested the possibility that this deficiency instead corresponds to burial by thick volcanic flows within areas of smooth plains by excluding the northern plains and Caloris interior plains (the two largest expanses of smooth plains), but there was no significant change from the result for the full hemisphere. If mantle flow contributed to the current patterns of crustal thickness and large thrust fault locations, an implication is that either (i) the large-scale pattern of mantle flow on Mercury was long-lasting and geographically stationary during the period when the planet contracted, or (ii) a shorter episode of mantle flow coincided with the development of many scarps, perhaps because of a relatively high rate of contraction at that time.