The Barringer Meteorite Crater, or Meteor Crater, is the prime example of a young, well-preserved and well-documented simple impact crater. As simple craters are among the most common morphological features on planetary surfaces in the solar system, understanding Meteor Crater is of major importance. While the outline of most simple craters is circular, the shape of Meteor Crater strongly deviates from a circle and resembles a quadrangle (Figure 1). The quadrangular shape is generally attributed to the occurrence of joint sets running through the diagonals of the crater, although, so far, little research has been performed on the details of the proposed process. Ground truth data are needed to better understand how the preimpact target structure can affect the shape and internal deformation of the crater wall and crater rim. This knowledge in turn can be used to enhance our understanding of the cratering process and the implications crater morphology and structure have on the configuration and composition of the surface and subsurface of other solid bodies in the solar system.
1.1. Meteor Crater
 Meteor Crater was formed ∼50,000 years ago in flat-lying sedimentary rocks of the Southern Colorado Plateau in Arizona by the iron Canyon Diablo meteorite. With a diameter of ∼1.2 km, Meteor Crater is a simple, bowl-shaped crater that is surprisingly well preserved by terrestrial standards. Shoemaker and Kieffer  estimate 15–20 m of erosion in the rim crest, while the crater floor shows ∼30 m of postimpact sedimentation of lake sediments and alluvium. Talus covers the lower segments of the inner crater wall, leaving 80–100 m of the stratigraphic sequence of the Southern Colorado Plateau exposed (Figure 1). The lowest unit visible, the Permian Coconino sandstone, is only exposed in small areas of the crater wall, revealing white, finegrained quartzose sandstone layers. It is overlain by 3 m of Toroweap Formation, consisting mainly of white to yellowish-brown calcareous sandstones. The main unit visible in the crater wall is the 80 m thick Kaibab Formation, with the older gamma member (white-yellowish dolomite) and beta member (yellow, massive dolomite) overlain by yellow, well-bedded dolomite and sandstone interbeds of the alpha member. The top unit of this sequence is the 8.5 m thick Permo-Triassic Moenkopi Formation, which follows unconformably. It is subdivided into the reddish-brown, massive sandstone of the Wupatki Member and the reddish-brown, fissile siltstone of the Moqui Member, which both form a marked contrast to the underlying yellow Kaibab units and are easily traced in the crater wall. The inverted sequence of these rocks can be observed in the hinge zone and overturned flap and further outward in the ejecta blanket (Figure 1b). For a detailed review of the geology of Meteor Crater, see Kring .
1.2. Previous Structural Research in Meteor Crater
 Structural aspects of Meteor Crater were first observed and published in the early 20th century by Barringer , who described uplift of the crater rim and speculated that uplift is caused by crushed rock thrust into the crater walls. In their research on Meteor Crater, Shoemaker [1960, 1963] and Shoemaker and Kieffer  also briefly focused on several structural aspects of the crater rim, noting (1) that joint sets coincide with the diagonals of the crater's roughly squareshaped outline, (2) that the joint sets additionally enable large-scale vertical faulting with “scissors-type displacement” in the crater wall, and (3) that coherent rock material has been emplaced in horizontal zones of weakness, resulting in thrusting and horizontal faulting in several areas of the crater rim. Roddy  was the first to quantify structural features of the crater rim and showed how closely the crater diagonals are related to the orientation of joint sets. Until recently, no comprehensive quantitative structural data of the crater wall other than Roddy's concise data set of joints were available in peer-reviewed literature. Kumar and Kring  measured over 1700 fractures. They were able to differentiate between impactinduced, outward dipping “conical fractures” and preimpact radial and concentric fractures. They conclude that a combination of faulting along joints oriented diagonally and “fracture-controlled motion” along the crater walls led to the crater's square shape. It should be stated that Kumar and Kring's study was coordinated with our study. While Kumar and Kring's focus was on the orientation and generation of fractures, our study investigated kinematic processes of cratering on the basis of bedding and GPS measurements.
 The effect of target structure, especially jointing, on the shape of craters has been discussed in several papers. Fulmer and Roberts  concluded, from lunar observations and experiments with explosion cratering, that joint sets determine whether polygonal impact craters (PICs) are formed or not. They believe that PICs are not formed if an uppermost, unconsolidated target layer is at least 1/4 as thick as the (transient) crater depth or if the joint spacing is too large. Gault  report squareshaped and hexagonal craters formed during hypervelocity impact experiments into “jointed” targets with a rather large spacing of 1/5 crater diameter but do not quantify the experimental outcome in detail (e.g., orientation of crater walls to joints). Eppler et al.  propose a model that differentiates between the crater shapes of simple and complex craters and is based mainly on observations in Meteor Crater and publications of other authors. They suggest that in simple craters, excavation is more efficient parallel to joint sets, thus forming corners, while in complex craters, the dominant effect is slumping along joints during crater modification, resulting in walls that are parallel to the to the joint sets, not to the diagonals in simple craters. Interestingly, while tectonic regimes and structures have been inferred from PICs on the Moon and other planets [e.g., Elston et al., 1971; Öhman et al., 2006; Aittola et al., 2007], to the best of our knowledge there are only two terrestrial PICs in which joints have been correlated with crater shape: Söderfjärden Crater (an eroded, complex crater) and Meteor Crater.
 This paper provides new structural data obtained from the crater rim of Meteor Crater. On the basis of these new data the effects of target heterogeneities, as well as possible effects of oblique incidence, are analyzed and discussed.
1.3. Can the Impact Vector Be Derived in Simple Craters? A Working Hypothesis
 On the basis of the probability P of an impact occurring below a certain angle θ above the horizon given as P = sin2 θ [Shoemaker, 1962], one out of two impacts occurs below 45°, and one out three impacts occurs below 35°. Only highly oblique impacts of less than 10° incidence from the horizontal create elliptical craters. Therefore, the crater shape cannot normally be used as an indicator of an impact direction. The shape of the ejecta blanket, on the other hand, has proven its use in determining the impact vector in numerous studies if the impact angle is less than ∼35° from the horizontal [e.g., Gault and Wedekind, 1978; Herrick and Hessen, 2006; McDonald et al., 2008]. The offset position of the blanket in the downrange direction and the v-shaped uprange forbidden zone are two indicators of the presence of the nonradial aspects that exist during oblique cratering (Figure 2). This nonradial behavior of ejecta has been analyzed in hypervelocity impact experiments by Anderson et al.  in which the vectors of ejected particles were imaged. A strong, nonradial, bilaterally symmetric signature can be seen in their data, which weakens as cratering progresses but is still present in images of the late stages. On Earth the ejecta blankets are rarely preserved, and thus, no craters have been found with an ejecta blanket capable of giving unambiguous indicators for an impact direction. While distal ejecta with its asymmetric features is lacking in terrestrial craters, the most proximal parts of the ejecta blanket, the overturned flap and hinge zone of the crater rim, are preserved in a number of craters on Earth. The possibility that these proximal parts may preserve nonradial symmetry was explored by Poelchau and Kenkmann  at Wolfe Creek Crater. We refer to that paper for a comprehensive description of the so-called “two corners” model.
 Poelchau and Kenkmann's  two corners model is based on the observation that the proximal and distal ejecta blankets of oblique craters on the Moon and Mars display deviations from pure radial flow (Figure 2). When extrapolated backward into the crater, striations on the ejecta blankets do not meet in the crater center but focus along a line running from the uprange section of the crater to its center, suggesting nonradial, outward flow from a moving source of ejection, as also suggested from experiments [e.g., Anderson et al., 2003]. While these striations are a late-stage cratering phenomenon and on Mars may possibly indicate atmospheric interaction, the nonradial pattern the striations describe is believed to reflect early cratering asymmetries.
 It is expected that the bedding orientation of blocks in the ejecta blanket is too chaotic to be used to derive any deviation from radial ejection patterns. However, near the crater rim in the hinge zone and within the overturned flap, strata are often coherent over large distances. In these settings it can be assumed that strata strike is perpendicular to the excavation flow direction (for originally horizontal bedding) (Figure 2, inset). Thus, deviations from radial flow should lead to a measurable deviation in strike from a concentric direction. The expected pattern of strike should be bilaterally symmetric to the direction of impact and, on the basis of the analysis of Tooting Crater on Mars and Wolfe Creek, Australia, have two “corners” between the uprange and crossrange sector, in which an abrupt change in strike orientation occurs (Figure 2).
 In this paper we present new structural data of Meteor Crater and provide a kinematic model that is consistent with structural observations, in particular, the quadrangular crater shape. We quantify the characteristic structural asymmetries of this crater and address the question as to whether these asymmetries could be exclusively caused by target heterogeneities or if they are additionally related to an oblique impact scenario. Structural data are presented in section 4. Section 5 critically reviews arguments in favor of an oblique impact scenario and applies the two corners model [Poelchau and Kenkmann, 2008] to Meteor Crater in order to derive a possible impact vector. In section 4 phenomenological models are presented that bring rim uplift and shape into a causal context.