Ligia Pérez-Cruz

The formation of peak rings in large impact craters

Drilling into Chicxulubs formation The Chicxulub impact crater, known for its link to the demise of the dinosaurs, also provides an opportunity to study rocks from a large impact structure. Large impact craters have “peak rings” that define a complex crater morphology. Morgan et al. looked at rocks from a drilling expedition through the peak rings of the Chicxulub impact crater (see the Perspective by Barton). The drill cores have features consistent with a model that postulates that a single over-heightened central peak collapsed into the multiple-peak-ring structure. The validity of this model has implications for far-ranging subjects, from how giant impacts alter the climate on Earth to the morphology of crater-dominated planetary surfaces. Science, this issue p. 878; see also p. 836 Rock samples from IODP/ICDP Expedition 364 support the dynamic collapse model for the formation of the Chicxulub crater. Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

Multiring-forming large bolide impacts and evolution of planetary surfaces

In the past few years, meteoritic and cometary impacts have emerged as a major geological agent in the construction and evolution of planetary surfaces. Formation of complex central ring, peak ring and multiring craters involves excavation and melting of large volumes of crustal material. High-resolution geophysical mapping measuring gravity, magnetics, and topography of the Moon and Mars have recently provided information on the subsurface structure of large basins and aided in identifying buried giant craters. The terrestrial crater record has been significantly erased by tectonic, magmatic, and erosion processes and only a small proportion of impact structures remain. Record of multiring craters is limited to three examples: Vredefort, Sudbury and Chicxulub. Deep geophysical surveys and geochemical and isotopic studies of those craters provide means to evaluate the influence of large impacts on the lithospheric and crustal evolution by providing estimates of excavation depth and volume, amounts of material fragmented, ejected, vaporized and melted, and effects on the crustal stratigraphy and crustal thickness. Analyses on the melt from Vredefort, Sudbury, and Chicxulub indicate andesitic composition derived from lower-crustal material. The melt formed inside the lower transient cavity from lower crustal material that was then redistributed and emplaced in upper-crustal levels, resulting in crustal redistribution. Crystalline basement clasts fragmented and incorporated into the breccias show varying degrees of alteration but no significant thermal effects. Ejecta were deposited locally within the crater region and ballistic material and fine ejecta are globally distributed on the planetary surface. Impacts influence the crust–mantle boundary, with Moho uplift. Material from the mantle was not incorporated into the melt and impact breccias, indicating that the excavation cavities were confined to the lower crust. This is also apparently the case for the giant basins on the Moon, including the 2500 km diameter South Pole-Aitken Basin. Considering the numbers of large multiring basins, possible flux of large impacts, and effects on target surfaces, crustal scale redistribution of material during those large impacts has played a major role in the evolution of planetary surfaces.

A record of Holocene summer-season palaeohydrological changes from the southern margin of Chihuahua Desert (Mexico) and possible forcings

Proxy records of runoff, lake water salinity and aeolian activity, reconstructed by analyzing the concentrations of Ti, Ca and Zr/Ti in the sediments of palaeolake Las Cruces, provide information about millennial-scale summer season palaeohydrological changes at the southern margin of Chihuahua Desert over the last 8.4 cal. kyr BP. The data indicate generally higher-than-average runoff between c. 8.4 and 5 cal. kyr BP, correlative to the early Holocene Thermal Maximum. Except for the c. 2.2–2 cal. kyr BP humid event, runoff was lower than average over the last c. 5 cal. kyr BP. Latitudinal shifts in average position of the Inter-Tropical Convergence Zone (ITCZ) caused by long-term changes in summer insolation was the principal forcing behind the varying summer season precipitation and El Niño Southern Oscillation (ENSO) activity may explain some of the hydrological variability (e.g. higher-than-average summer precipitation during c. 2.2–2 cal. kyr BP). In general, the basin received more runoff during the periods of less frequent and weak ENSO, similar to the modern response to ENSO activity of the region. Significant increases in lake water salinity and aeolian activity over the last 2 cal. kyr BP correspond to stronger and more frequent ENSO during the late Holocene.

Chicxulub and the Exploration of Large Peak-Ring Impact Craters through Scientific Drilling

The Chicxulub crater is the only well-preserved peak-ring crater on Earth and linked, famously, to the K-T or K-Pg mass extinction event. For the first time, geologists have drilled into the peak ring of that crater in the International Ocean Discovery Program and International Continental Scientific Drilling Program (IODP-ICDP) Expedition 364. The Chicxulub impact event, the environmental calamity it produced, and the paleobiological consequences are among the most captivating topics being discussed in the geologic community. Here we focus attention on the geological processes that shaped the ~200-km-wide impact crater responsible for that discussion and the expedition’s first year results.

Heating-induced changes in the anisotropy of magnetic susceptibility of impact breccias, Chicxulub Crater (Mexico)

Initial results of a thermal treatment study on the anisotropy of magnetic susceptibility (AMS) of impact breccias from Chicxulub crater are used to investigate the nature of the magnetic fabrics. Chicxulub impact breccias are heterogeneous materials, with carbonate, basement and melt clasts within carbonate-rich or melt-rich matrix. Samples studied come from the carbonate-rich basal unit Lower Suevite in the Yaxcopoil-1 borehole impactite sequence (core depth interval: 885–895 m). The Lower Suevite is characterized by mixed prolate and oblate ellipsoids with shallow to steep principal susceptibility axes, which had been related to emplacement as an excavation flow with ground-surge components during the early cratering stages. Thermal treatment results in changes in the fabrics with a tendency to oblate fabrics. Stepwise thermal treatment up to 700°C reveals different behaviors for the oblate, neutral and prolate fabrics marked by changes in AMS parameters and principal susceptibility axis orientations. A sample with oblate fabrics and vertical minimum axes showed an increase of magnetic susceptibility at high temperatures, indicating formation of secondary magnetite and fabric enhancement. A sample with neutral ellipsoid showed heating-induced changes towards oblate fabrics and vertical minimum susceptibility axes. Samples characterized by prolate ellipsoids with horizontal maximum axes showed no directional changes. In a sample with apparent intermediate or inverse fabrics, vertical maximum axes showed changes to horizontal inclinations, with the intermediate and maximum axes switching positions. Changes induced by stepwise thermal treatment appear useful to characterize the fabrics of impact lithologies. Further investigation of heating-induced effects in mineralogy, grain size and textural changes is, however, required to relate the different behaviors observed after stepwise thermal treatment with the magnetic mineralogy and emplacement mode of the breccias.

Volcano-sedimentary stratigraphy in the Valsequillo Basin, Central Mexico inferred from electrical resistivity soundings

Initial results of an electrical resistivity survey of the volcano-sedimentary sequence of the Valsequillo basin in central Mexico are presented. The volcano- sedimentary deposits preserve rich paleontological, paleoclimatic and paleoenvironmental records, which include extinct megafauna remains associated with human artifacts. The report of possible 38 ka old human footprint tracks in the Xalnene tuff attracted renewed interest in the basin stratigraphy. We examine the shallow stratigraphic sequence in the Xalnene tuff outcrop plain northwest of Cerro Toluquilla volcano using vertical resistivity soundings (VES). Inversion models of VES soundings show a layered structure of high and low resistivity units, which characterize the Xalnene tuff, lacustrine and fluvial sediments and volcanic rocks. 2-D resistivity cross sections document three major units corresponding to the Xalnene tuff and sediments filling a <30 m deep basin lying on volcanic rocks. Resistivity models provide further support for the association of Xalnene tuff with the Toluquilla volcano and emplacement of the pyroclastic deposits on a shallow lacustrine environment. The resistivity cross sections constrain the thickness of the tuff layers and underlying lacustrine sediments. Observations during the data acquisition field work provide insight on the possible origin of the apparent tracks, which seem to develop from erosion processes acting on quarrying marks. Further analysis and experimental evidence is required to understand the morphology and weathered patterns. The tuff layers are being removed by quarrying operations and the outcrops significantly altered. Adequate conservation measurements should be implemented to preserve the deposits for scientific research.

Planetary Sciences, Geodynamics, Impacts, Mass Extinctions, and Evolution: Developments and Interconnections

Research frontiers in geophysics are being expanded, with development of new fields resulting from technological advances such as the Earth observation satellite network, global positioning system, high pressure-temperature physics, tomographic methods, and big data computing. Planetary missions and enhanced exoplanets detection capabilities, with discovery of a wide range of exoplanets and multiple systems, have renewed attention to models of planetary system formation and planet’s characteristics, Earth’s interior, and geodynamics, highlighting the need to better understand the Earth system, processes, and spatio-temporal scales. Here we review the emerging interconnections resulting from advances in planetary sciences, geodynamics, high pressure-temperature physics, meteorite impacts, and mass extinctions.

Meteorite paleomagnetism - From magnetic domains to planetary fields and core dynamos

Resumen Los meteoritos condriticos representan los registros mas tempranos de la evolucion del Sistema Solar, proveyendo informacion sobre las condiciones, procesos y cronologia de la formacion de los primeros materiales solidos, planetesimales y cuerpos diferenciados. La evidencia sobre los campos magneticos en las etapas tempranas de evolucion del sistema solar se ha obtenido a partir de estudios en meteoritos condriticos. Estos meteoritos se caracterizan por la abundancia de condrulos, que constituyen pequenas esferas de silicatos de tamano milimetrico, formadas a partir del polvo en la nebulosa y que fueron calentadas y enfriadas rapidamente. Los condrulos retienen un registro de magnetizacion remanente, que data del tiempo de calentamiento y enfriamiento durante la formacion de condrulos y su acrecion en planetesimales. Los estudios sobre las diferentes clases de meteoritos, incluyendo a los meteoritos condriticos ordinarios y los meteoritos condriticos carbonaceos han documentado resultados contrastantes con una rango amplio de magnitudes de los campos magneticos en el disco protoplanetario. Ello ha dificultado definir la naturaleza de los campos magneticos en las etapas iniciales de evolucion. Los desarrollos recientes en instrumentacion y tecnicas de analisis de magnetismo de rocas y paleointensidades permiten una mayor precision. Los analisis de micromagnetismo, geoquimica, petrografia y microscopia electronica proveen de una alta resolucion, previamente no disponible, para caracterizar las propiedades magneticas e interacciones a escalas de dominio magnetico. En este trabajo revisamos los estudios en condrulos del meteorito condritico Allende, que revelan relaciones entre los parametros magneticos de histeresis y propiedades fisicas. Los parametros y cocientes de coercitividad, magnetizacion remanente y magnetizacion de saturacion muestran correlaciones con la densidad y tamano de los condrulos, los cuales estan relacionados a la estructura interna, mineralogia, composicion y morfologia. Los condrulos compuestos, fragmentados y con anillos de recubrimiento se caracterizan por propiedades de histeresis magnetica distintas, asociadas a la composicion y arreglos mineralogicos y microestructuras. Los registros de magnetizacion remanente y las estimaciones de paleointensidades derivadas en estudios del Allende y otras condritas carbonaceas apoyan adquisicion de la magnetizacion bajo la influencia de campos magneticos internos dentro de planetesimales. Los resultados apoyan una rapida diferenciacion, siguiendo la formacion de las inclusiones de calcio y aluminio y condrulos para formar los planetesimales. Los planetesimales se caracterizarian por una estructura diferenciada con nucleos metalicos con capacidad de dinamo autosustentable por periodos de varios millones de anos. El meteorito condritico Allende se formo y derivo de un planetesimal parcialmente diferenciado, con un nucleo de hierro capaz de sostener un campo magnetico interno.

Rapid recovery of life at ground zero of the end-Cretaceous mass extinction

The Cretaceous/Palaeogene mass extinction eradicated 76% of species on Earth1,2. It was caused by the impact of an asteroid3,4 on the Yucatán carbonate platform in the southern Gulf of Mexico 66 million years ago5, forming the Chicxulub impact crater6,7. After the mass extinction, the recovery of the global marine ecosystem—measured as primary productivity—was geographically heterogeneous8; export production in the Gulf of Mexico and North Atlantic–western Tethys was slower than in most other regions8–11, taking 300 thousand years (kyr) to return to levels similar to those of the Late Cretaceous period. Delayed recovery of marine productivity closer to the crater implies an impact-related environmental control, such as toxic metal poisoning12, on recovery times. If no such geographic pattern exists, the best explanation for the observed heterogeneity is a combination of ecological factors—trophic interactions13, species incumbency and competitive exclusion by opportunists14—and ‘chance’8,15,16. The question of whether the post-impact recovery of marine productivity was delayed closer to the crater has a bearing on the predictability of future patterns of recovery in anthropogenically perturbed ecosystems. If there is a relationship between the distance from the impact and the recovery of marine productivity, we would expect recovery rates to be slowest in the crater itself. Here we present a record of foraminifera, calcareous nannoplankton, trace fossils and elemental abundance data from within the Chicxulub crater, dated to approximately the first 200 kyr of the Palaeocene. We show that life reappeared in the basin just years after the impact and a high-productivity ecosystem was established within 30 kyr, which indicates that proximity to the impact did not delay recovery and that there was therefore no impact-related environmental control on recovery. Ecological processes probably controlled the recovery of productivity after the Cretaceous/Palaeogene mass extinction and are therefore likely to be important for the response of the ocean ecosystem to other rapid extinction events.Micro- and nannofossil, trace fossil and geochemical evidence from the Chicxulub impact crater demonstrates that proximity to the asteroid impact site did not determine rates of recovery of marine ecosystems after the end-Cretaceous mass extinction.

Magnetic susceptibility logging of Chicxulub proximal impact breccias in the Santa Elena borehole: implications for emplacement mode

Magnetic susceptibility logging is used to study the impact breccias in the Chicxulub crater. The basic premise is that the high contrasts in magnetic properties can be used to characterize the breccias. The Santa Elena borehole was drilled 110 km radial distance from crater center and sampled a 172 m thick sequence of impact breccias, between 332 and 504 m depth. Breccia units are distinguished from differences in composition, size, and relative contents of clasts, type of matrix and textural and lithological assemblages, which can be resolved in the susceptibility logs. The whole-core log shows characteristic variation patterns with high, intermediate and low susceptibilities. High resolution logging of matrix and clasts records the heterogeneous nature of impactites, with higher variability at smaller spatial scales. Measurements confirm that diamagnetic susceptibilities characterize the carbonate clasts, high susceptibilities the basement granitic clasts and intermediate values the silicate melt-rich and silicate-poor matrix. Intermediate variable susceptibilities characterize breccias rich in melt particles. Correlation of matrix and clast logs with whole-core log shows that signal is controlled by the matrix. Logs for clast shows a discrete distribution with peaks of intermediate to high values, which correlate with large clast distributions. The ejecta blanket includes the fallback suevites rich in silicate melt particles and shocked minerals, the high temperature vapor deposits from ejecta curtain collapse and high velocity basal flows, and the carbonate rich deposits from lateral basal flows and secondary cratering. Late fallback suevites record minor turbulent conditions resulting from progressive cooling of the ejecta plume.