Craters

The Dalgaranga meteorite crater, 100 km northeast of Yalgoo, Western Australia, was the first impact structure identified in Australia, the smallest isolated crater found in Australia, and the only confirmed crater in the world associated with a mesosiderite projectile. 17 years passed before the Dalgaranga meteorites were described in the scientific literature and nearly 40 years passed before a survey of the structure was published. The reasons for the time-gap were never explained and a number of factual errors about the discovery and early history remain uncorrected in the scientific literature. Using historical and archival documents and accounts from people involved in early Dalgaranga research, we provide a detailed history of the discovery and study of the crater and its associated meteorites. We then discuss discrepancies in the age estimates of the impact and use pi-scaling laws to estimate the mass of the projectile.

Closed depressions at the northern Golan volcanic plateau were attributed in the past to various processes without clear evidence and conclusions. We analyze the morphology, origin, and deformation of the depressions using geologic and speleologic techniques, assisted by air photos and satellite imaging. No ejecta, rim deposits, or lava tubes were found in association with the studied depressions. Comparison with similar recent features at Hawaii provides an explanation to their formation and subsequent definition as pit craters. They probably relate to the last, late Pleistocene volcanic phase of the northern Golan. The magma has apparently been channelled through faults and fractures associated with the Dead Sea Transform, without reaching the surface. The pit craters indicate that inflation by pressurized magmatic intrusion could cause extension along the unstable flanks of the deep Hula basin, while during the deflation stage the evacuated magma left voids which reached the surface by consecutive roof collapse. Since collapse reached the surface, recent erosion and sedimentation have modified the original morphology of the pit craters, commonly forming smooth and shallow bowl-shaped depressions. The youngest pit crater in the Golan breached the surface during the Holocene, still demonstrating the typical vertical pit morphology, similar to recent pit craters in Hawaii.

Chicxulub Crater, formed ~66Ma ago by an asteroid impact on the southern Gulf of Mexico, is the best preserved of the three large multi-ring basins in the terrestrial record. The crater structure is characterized by a semi-circular concentric ring pattern, marking the crater basin, peak ring, terrace zone and basement uplift. Analysis of a grid of 19 seismic reflection profiles using seismic attributes, marker horizons, contour surfaces and 3-D views is used to investigate the stratigraphy of the central zone. We used interactive software and routine applications to map the impact breccias, breccia-carbonate contact and post-impact carbonates. Four horizons marked by high-amplitude reflectors representing high-impedance contrasts were identified and laterally correlated in the seismic images. Complex trace attribute analysis was applied for petrophysical characterization. Surface contour maps of base and top of stratigraphic packages were constructed, which mapped the impactites and post-and pre-impact carbonate stratigraphy. Basin floor, marked by the contact between the impact breccias and overlying carbonates is shown by laterally discontinuous high-amplitude reflectors. Discontinuous scattered reflectors interpreted as the upper breccias beneath the crater floor, have an average thickness of ~300msm. The Paleogene sedimentary units are characterized by multiple reflectors with lateral continuity, which contrast with the seismic response of underlying breccias. The basal Paleocene sediments follow the basin floor relief. Upwards in the section, the carbonate strata are characterized by horizontal reflectors, which are interrupted by a regional unconformity. Onlap/downlap packages over the unconformity record a period of sea level change.

We explore the origin of a ~280 m wide, heavily eroded circular depression in Palm Valley, Northern Territory, Australia using gravity, morphological, and mineralogical data collected from a field survey in September 2009. From the analysis of the survey, we debate probable formation processes, namely erosion and impact, as no evidence of volcanism is found in the region or reported in the literature. We argue that the depression was not formed by erosion and consider an impact origin, although we acknowledge that diagnostics required to identify it as such (e.g. meteorite fragments, shatter cones, shocked quartz) are lacking, leaving the formation process uncertain. We encourage further discussion of the depression's origin and stress a need to develop recognition criteria that can help identify small, ancient impact craters. We also encourage systematic searches for impact craters in Central Australia as it is probable that many more remain to be discovered.

This study presents a detailed analysis and interpretation of the seismicity that occurred on July 2–7 and August 22, 2000, during a ground uplift episode which started on March 2000 at Solfatara crater, Campi Flegrei. Earthquakes are located using a probabilistic grid-search procedure acting on a 3-D heterogeneous earth structure. The mainshock of the July swarm depicts a spectrum characterized by a few narrow peaks spanning the 1–5-Hz frequency band. For this event, we hypothesize a direct involvement of magmatic fluids in the source process. Conversely, the spectra of the August events are typical of shear failure. For these latter events, we evaluate the source properties from P- and S-wave displacement spectra. Results for the most energetic shocks (Md around 2) yield a source radius in the order of 100 m and stress drop around 10 bars, in agreement with most of the earthquakes that occurred during the 1982–1984 bradyseismic crises. For the August swarm we identify two clusters of similar earthquakes. Application of high-resolution relative location techniques to these events allows for the recognition of two parallel alignments trending NE–SW. The relationship among source dimension and relative location evidences overlapping of sources. This may be interpreted in terms of either a heterogeneous stress field or a lubrication process acting over the fault surface. For a selected subset of the August events, we also analyze the splitting of the shear waves: results are indicative of wave propagation through a densely fractured medium characterized by a distribution of cracks oriented NE–SW. The pattern of faulting suggested by relative locations and shear-wave splitting is not consistent with the surface trace of NW–SE striking faults. However, a detailed mesostructural analysis carried out over the Solfatara area indicated the occurrence of two main crack systems striking NW–SE and NE–SW. This latter system shows a strike consistent with that derived from seismic evidence. Results from a stress analysis of the crack systems indicate that a fluid overpressure within the NW–SE-striking faults is able to form NE–SW cracks. We found that the pressure of fluids Pf required to activate the NW–SE faults is less than σHmin, while the Pf value required to open the NE–SW cracks is higher than σHmax. Our main conclusions are: (a) the Solfatara area is affected by two orthogonal fracture systems, and the fluid pathway during the 2000 crisis mainly occurred along the NNE–SSW/NE–SW-striking crack system; (b) the July seismicity is associated to the upward migration of a pressure front triggered by an excess of fluid pressure from a small-size magmatic intrusion; conversely, the August events are associated to the brittle readjustment of the inflated system occurring along some lubricated structures.