Abstract The central peak region of Lyot Crater provides an intriguing case study of Martian gully formation on local topographic highs. To better understand how these gullies formed, we carried out a detailed morphological analysis using a HiRISE stereo image pair and a digital terrain model. Gully lengths range from c. 2.2 to 4.3 km, with maximum depths from 17 to 54 m. Alcove slopes range from c. 20 to 22°, channel slopes range from 12 to 16° and apron slopes range from 10 to 14°. In general, these slopes are much lower than both the angle of repose ( c. 33°) required to initiate dry flows and the apex slope required to keep these flows from depositing (>21°) under Mars gravity. We find that the observed gully morphology and spatial associations are consistent with an origin by liquid flow. Apron volumes are c. 10–40% of the gully volume, which we suggest indicates significant volatile loss (water and/or CO 2 ) in the slope materials in which they formed because apron volumes emplaced by dry gravitational flows would equal or exceed the gully volumes due to their lower packing density. These observations, coupled with the gullies’ unique micro-environmental setting, lead us to favour a fluvial origin. In addition, we find that the gullies formed almost exclusively on the western slope region, which suggests a possible orographic component. Potential water sources may have been supplied locally in the recent geological past by dry winds blowing across an ice-covered lake situated in a low elevation area just west of the central peak or more globally during periods of higher obliquity. In either case, the integration of morphometric and morphological investigations suggests that these gullies probably formed by surface runoff and through-flow from snow- or icepack melting on the central peak of Lyot Crater. Supplementary material: Figures S1–S8 and Tables SI and SII are available at https://doi.org/10.6084/m9.figshare.c.4309382

Abstract: Field analysis shows that fault cores of brittle, extensional faults at a medium to mature stage of development are commonly dominated by lozenge-shaped horses (fault-core lenses) characterized by a variety of lithologies, including intact, mildly to strongly deformed country rock derived from the footwalls and hanging walls, various types of fault rocks of the protocatalasite and breccia series, breccia, fault gouge and clay smear. The lenses are sometimes stacked to form complex duplexes. These structures are commonly separated by high-strain zones of sheared cataclasite, and/or clay smear/clay gouge. The geometry and distribution of clay gouge in high-strain zones sometimes display evidence of intrusion, indicating high fluid pressure. Although the sizes of the horses vary over several orders of magnitude, they frequently display a length:thickness (a:c) ratio of between 1:4 and 1:15. The high-strain zones of fault rocks commonly constitute unbroken, 3D membranes that are likely to constrain fluid communication both across and along the fault zone. There are significant contrasts in fault core architecture that are probably related to processes associated with contrasting fluid pressure, strain intensity and strain hardening/strain softening. Faults associated with strain softening are characterized by less abundant brittle deformation products and are less likely to be conduits for fluid flow compared to those that are affected by strain hardening.

Abstract: Fault inversion has been documented in many basins worldwide, yet the details of how the initial extensional faults impact on the geometry and growth of the reactivated contractional system is often poorly resolved by the available data. Two-dimensional (2D) and 3D seismic reflection, and well data have been used to chart the evolution of inverted faults from the Taranaki Basin, offshore New Zealand. Sedimentary rocks up to 8 km thick record Late Cretaceous–Paleocene normal faults inverted during Miocene and younger shortening. The displacement and length of early normal faults is a key determinant for the reactivation and size of the subsequent reverse faults. All normal faults with maximum vertical displacements ≥600 m and lengths ≥9 km were inverted along their entire length, while smaller faults were not inverted. The proportion of the total basin-wide strain accommodated on each fault is comparable between deformational episodes. The hierarchy of reverse fault lengths was established rapidly, with longer faults accruing a greater proportion of the total strain from an early stage of shortening. The reverse fault system is dominated by inverted normal faults, which accrue displacement at the expense of smaller faults, and utilize the largest crustal-scale elements of the pre-existing system. The size of pre-existing heterogeneities is an important control for the magnitude and spatial extent of elevated stresses during contraction, which, in turn, control the dimensions, locations and displacements of subsequent fault growth.

The four largest well-preserved impact basins in the solar system, Borealis, Hellas, and Utopia on Mars, and South Pole–Aitken on the Moon, are all significantly elongated, with aspect ratios >1.2. This population stands in contrast to experimental studies of impact cratering that predict <1% of craters should be elliptical, and the observation that ~5% of the small crater population on the terrestrial planets is elliptical. Here, we develop a simple geometric model to represent elliptical crater formation and apply it to understanding the observed population of elliptical craters and basins. A projectile impacting the surface at an oblique angle leaves an elongated impact footprint. We assume that the crater expands equally in all directions from the scaled footprint until it reaches the mean diameter predicted by scaling relationships, allowing an estimate of the aspect ratio of the final crater. For projectiles that are large relative to the size of the target planet, the curvature of the planetary surface increases the elongation of the projectile footprint for even moderate impact angles, thus increasing the likelihood of elliptical basin formation. The results suggest that Hellas, Utopia, and South Pole–Aitken were formed by impacts inclined at angles less than ~45° from horizontal, with a probability of occurrence of ~0.5. For the Borealis Basin on Mars, the projectile would likely have been decapitated, with the topmost portion of the projectile on a trajectory that did not intersect with the surface of the planet.