Penny Barton

We present a three‐dimensional upper crustal model of the 9°03′N overlapping spreading center (OSC) on the East Pacific Rise that assists in understanding the relationship between melt sills and upper crustal structure at a ridge discontinuity with enhanced melt supply at crustal levels. Our P wave velocity model obtained from tomographic inversion of ∼70,000 crustal first arrival travel times suggests that the geometry of extrusive emplacement are significantly different beneath the overlapping spreading limbs. Extrusive volcanic rocks above the western melt sill are inferred to be thin (∼250 m). More extensive accumulation of extrusives is inferred to the west than to the east of the western melt sill. The extrusive layer inferred above the eastern melt sill thickens from ∼350 (at the neovolcanic axis) to 550 m (to the west of the melt sill). Volcanic construction is likely to be significant in the formation of ridge crest morphology at the OSC, particularly at the tip of the east...

We have developed a pragmatic new processing strategy to enhance seismic information obtained from long-offset multichannel seismic data. The conventional processing approach, which treats data on a sample-by-sample basis, is applied at a coarser scale on groups of samples. Using this approach, a reflected event and its vicinity remain unstretched during the normal moveout correction. Isomoveout curves (lines of equal moveout) in the time-velocity panel are employed to apply a constant moveout correction to selected individual events, leading to a nonstretch stack. A zigzag stacking-velocity function is introduced as a combination of segments of appropriate isomoveout curves. By employing a zigzag velocity function, stretching of key events is avoided and thus information at far offset is preserved in the stack. The method is also computationally cost-effective. However, the zigzag stacking-velocity field must be consistent with target horizons. This method of horizon-consistent non...

In early 2005 the R/V Maurice Ewing conducted a large-scale deep seismic reflection-refraction survey offshore Yucatan, Mexico, to investigate the internal structure of the Chicxulub impact crater, centred on the coastline. Shots from a tuned 20 airgun, 6970 cu in array were recorded on a 6 km streamer and 25 ocean bottom seismometers (OBS). The water is exceptionally shallow to

... GULICK, Sean 1 , BARTON, Penny 2 , CHRISTESON, Gail 3 , MCDONALD, Matthew 4 , MENDOZA, Keren 5 , MORGAN, Joanna 6 , URRUTIA, Jaime 5 , and ... Within the inner ring, theterrace zone slump blocks reach the greatest depth in the northwest part of the crater and ...

In 1996 and 2005 we conducted onshore-offshore seismic experiments to constrain the structure of the Chicxulub impact crater; these datasets can provide constraints on both the crustal and mantle structure of the crater. We inverted more than 50,000 PmP reflection picks ...

We combine numerical modeling with seismic data interpretation to understand the formation of the Chicxulub impact crater, and, ultimately, the impact's role in the K/P mass extinction. Seismic data across the Chicxulub impact crater reveal that the crater structure varies around the offshore portion of the crater. The most striking azimuthal variation is in the position of the Cretaceous sediments

Drilling into the Chicxulub crater provides constraints on how it formed Steady as a rock. We all know what to expect of rock. Rocks deform infinitesimally slowly. Earth scientists get excited at the prospect of “rapid” plate movements that average the same speed at which our fingernails grow. Humans don't make much impact on rocks, except at the most puny of scales. Sometimes nature does experiments for us that we could never do for ourselves: When a large meteorite hits the planet, interactions occur that are far outside our normal experience. The outer surface is deformed with a force and strain rate that we cannot begin to reproduce; rocks flow like fluid, very fast and on a huge scale. On page 878 of this issue, Morgan et al. (1) present results from a drilling expedition into the Chicxulub crater that reveal how the formation of peak rings in large impact craters occurs. Numerical simulations of the impact model the time scale of events: a rim of mountains, higher than the Himalayas, adjacent to a void 25 km deep and about 70 km wide, forming and collapsing within the first three minutes; the central fluidized peak rising and collapsing in minutes 3 to 4; and a shakedown period in minutes 5 to 10, leaving a shallow crater at the surface, an intensely deformed impact zone extending through the thickness of the Earth's crust and beyond, and the world changed forever.

A major UK and international project is investigating how the 3D structure and physical properties of the subduction zone (including the plate boundary) affect earthquake rupture within and between the 2004 and 2005 rupture zones. These earthquake ruptures terminated at clear segment boundaries of the subduction zone and, in part, repeated historic rupture patterns. The project includes marine surveys (seismic

... More recent studies suuest that the two ophioliles have different origins, and the Masirah ophiolite was obducted From the southeast by sinisiral motion alonll the Masirah Fault (Shack-leton and Ri~, I~J0; Mo~cley. 1990). ... Buluch.+tan. Wc~t paLl+tan Bull Am A,~,~. Pet (~ol. ...

Page 1. Non-stretch stacking in the tau-p domain: exploiting long-offset arrivals for sub-basalt imaging Hassan Masoomzadeh and Penny Barton ... This approach potentially applies an exact moveout correction to deep reflection events (Van der Baan, 2003), similar to the shifted ...

We developed a method of moveout correction in the [Formula: see text] domain to tackle some of the problems associated with processing wide-angle seismic reflection data, including residual moveout and normal-moveout stretching. We evaluated the concept of the shifted ellipse in the [Formula: see text] domain as an alternative to the well-known concept of the shifted hyperbola in the [Formula: see text] domain. We used this shifted-ellipse concept to address the problem of residual moveout caused by vertical heterogeneity in the subsurface. We also addressed the stretching problem associated with dynamic corrections by combining selected strips from a set of constant-moveout stacks generated using a shifted-ellipse equation. Application of this method to a wide-angle data set from the Faeroe-Shetland Basin provided an enhanced image of the subbasalt structure.

Chicxulub is the only known impact structure on Earth with a fully preserved peak ring, and it forms an important natural laboratory for the study of large impact structures and understanding of large-scale cratering on Earth and other planets. Seismic data collected in 1996 and 2005 reveal detailed images of the uppermost crater in the central basin at Chicxulub. Seismic reflection profiles show a reflective layer ~1 km beneath the apparent crater floor, topped by upwardly concave reflectors interpreted as saucer-shaped sills. The upper part of this reflective layer is coincident with a thin high-velocity layer identified by analyzing refractions on the 6 km seismic streamer data. The high-velocity layer is almost horizontal and appears to be contained within the peak ring structure. We argue that this reflective layer is the predicted coherent melt sheet formed during impact, and it may be comparable with the unit known as the Sudbury Igneous Complex at the Sudbury impact structure. The Sudbury Igneous Complex, interpreted as a differentiated impact melt sheet, appears to have a similar scale and geometry, and an uppermost lithological sequence consisting of a high velocity layer at the top and a velocity inversion beneath. This comparison suggests that the Chicxulub impact structure also contains a coherent differentiated melt sheet.

Oceanic subduction along the Sunda trench to the west of Sumatra (Indonesia) shows significant along-strike variations in seismicity. For example, the great M9.3 earthquake in 2004 occurred in the forearc basin north of Simeulue island, rupturing the fault predominantly towards the northwest, while the 2005 Nias earthquake nucleated near the Banyak islands, rupturing towards the southeast (Ammon et al., 2005; Ishii et al. 2005). The gap between these two active areas indicates segmentation of the subduction zone, but the cause of the segmentation remains enigmatic. To investigate the apparent barriers to rupture, two 3-D refraction surveys were conducted in 2008, one, the topic of this study, around Simeulue island and the other to the southeast of Nias island. Seismic data were collected using ocean bottom seismometers and a 12-airgun tuned array with a total capacity of 5420 cu. in., together with high resolution bathymetry data and gravity data. 174,515 traveltimes of first refracted arrivals were picked for the study area, and 128,138 of them were inverted for a model of minimum structure required by the data using the ‘FAST’ method (Zelt et.al, 1998). Resolution tests show that the model is resolvable mostly on a scale of >40 km horizontally. The final velocity model produced has two distinct features: i. the subducted oceanic plates (represented by 6 km/s contours) seem to be discontinuous along strike; ii. the subduction dip angle appears to be steeper in the southern part of the survey area than in the north. The geometric variation in the subducted plate appears to coincide with the segment boundary approximately across the centre of Simeulue island, and may perhaps associated with the segmentation of the seismicity noted from the earthquake record. More accurate velocity models will be developed by jointly inverting traveltimes of first and later arrivals as well as normal incidence data using the tomographic inversion program JIVE-3D (Hobro et.al, 2003). Some passive earthquake data may also be available for the inversion for this area. These new results will provide insights into along-strike variations in subsurface structure and/or physical properties within the Sumatra subduction zone, which maybe related to the observed segmentation.

The Chicxulub impact crater includes the only known peak ring structure preserved on the Earth's surface, and as such represents an important natural laboratory for the study of peak ring craters, seen commonly on other planets. Seismic reflection data collected in the offshore part of the crater in 1996 and 2005 includes refracted arrivals recorded on a 6 km multichannel seismic streamer, and the reflection and refraction information together give powerful constraints on the structure of the upper ~2 km of the subsurface, which includes the Cenozoic infill, the crater surface, and about 1 km of underlying material. The refracted travel-time data from the streamer are transformed via tau-p into a detailed velocity map which may be superimposed onto the reflection image in two- way-time or depth. Inside the inner rim of the crater we observe a very consistent pattern, with basin infill of 2.0-3.4 km/s and top crater velocities of 3.4-5.4 km/s. About 1500 m beneath the present day sea level, and 750-1000 m below the crater surface, we detect a high velocity layer, which is seen inside and outside the topographic peak ring interpreted from the reflection profile, but does not appear to run continuously either over the peak ring or beneath it, at least within the depth range of our data. This high velocity feature maps onto a low frequency reflector mapped intermittently on the reflection profiles, and we interpret it as the top of the melt sheet expected in the interior of the crater. The layer is at least three hundred metres thick, and has a seismic velocity of about 5.5- 6.2+ km/s, the maximum resolvable using the given acquisition geometry. We have mapped this surface using the 5.5 km/s velocity contour as a proxy, and will compare this in detail with a map of the low frequency event identified on the reflection profiles. The high velocity layer forms a relatively smooth, though not strongly reflective, surface, possibly modified post-emplacement by hydrothermal alteration. In some cases the feature appears to shallow and possibly finger at the inside edge of the peak ring, and seems to be deeper immediately outside, where it may truncate against the inward-dipping reflectivity observed under the outer edge of the peak ring. Outside the peak ring, this feature remains at a constant depth of about 750 m beneath the crater surface, shallowing with this surface towards the inner rim. Penetration of the melt sheet is one of the key objectives of the proposed ICDP drillhole planned onshore in the Chicxulub crater. We believe that the information from the adjacent offshore surveys provide excellent constraints on the depth, geometry and seismic characteristics of this target.

Chicxulub crater was formed 65 my ago as a result of a meteorite impacting Earth on what nowadays is the Yucatan Peninsula in Mexico. This crater, located half offshore, is the only large-diameter (>150 km) impact structure on Earth that is well preserved due to surrounding weather and tectonic conditions and being subsequently buried by 1 km of carbonates. Since second half of 20th century, Chicxulub impact crater has been surveyed using different geophysical methods to try to define its major characteristics. The most recent seismic survey was carried out during January and February, 2005 aboard the R/V Maurice Ewing. During this experiment marine and land seismic data were acquired. A 24 line grid and 3 radial profiles located in between old 1996 seismic lines provide new images to aid definition of the offshore crater. Previous studies on the Chicxulub impact crater structure interpreted a topographic peak ring at different distances from crater center according to the method used. This recognizable feature in large impact craters is thought to be formed mainly by gravitational forces acting on central uplift material that collapses outward while rim material collapses inward. Preliminary results of processing 2-D seismic reflection profiles from the 2005 survey indicates the outer extent of this peak ring ~40 km distance from center of the crater and could be interpreted as a 10-20 km wide ring in the lines analyzed. Chicxulub peak ring is imaged at a depth range of 500-900 ms two-way-travel time as a deformation of pre-impact lithology. Other major features like slump blocks and the inner and outer ring can also be recognized on the processed reflection seismic data

A seismic refraction survey was conducted as part of the major UK and international project to image the 3-D structures and the seismic velocity of the Sumatra subduction zone. The 3-D seismic refraction tomography mainly focusses on the two segment boundaries identified by the earthquake ruptures in 2004 and 2005. High quality seismic datasets (refraction, reflection, gravity and magnetics) were collected in the two survey areas on the vessel R/V Sonne in 2008. The northern area, around the island of Simeulue, is about 196 km long and 185 km wide. 50 Ocean Bottom Seismometers (OBS) were deployed in this area, and 10462 air-gun shots were fired along 1550 km of profiles. 47 OBSs were then installed near the island of Nias, in an area of 246 km long and 180 km wide, and 9134 shots were fired on 1408 km of profiles. During the OBS deployment, air-gun shooting, and OBS recovery, high resolution swathe bathymetry data were recorded, and XBT data were collected in each OBS deployment location. Gravity data were also recorded during the whole survey and magnetics data collected during the air-gun shooting. The 3-D refraction tomography successfully sampled the two survey areas. Refractions from the oceanic and continental crust are clear and easy to pick, and refractions from the mantle lithosphere are also visible at some locations at an offset up to 150 km, which enables us to image the deeper structures of the Sumatra subduction zone. A tomographic inversion program JIVE-3D (Hobro et al. 2003) will be applied to the refraction/reflection travel times to invert them into a minimum-structure velocity model. The high resolution bathymetry will be smoothed and put into the model as a known interface. The XBT data will be used to calibrate the acoustic velocity in the water. During the shooting period, several earthquakes of magnitude 5.0 and above occurred near the survey area, which also provide extra information for the inversion. The well resolved 3-D structure models obtained will give insight into the possible rupture barriers causing the observed segmentation of the subduction zone.

Formation, release of volatiles, and subsequent collapse of the 65 Ma Chicxulub impact crater are of key interest due to the impact's role in the Cretaceous-Paleocene (K/P) mass extinctions. Seismic data acquired in 1996 and 2005 image the buried and surprisingly asymmetric final crater, and highlight key features that are the target of proposed IODP-ICDP drilling. Gravitational collapse of the transient crater created a terrace zone consisting of faulted slump blocks that reach the greatest depth in the northwest part of the crater. Lying above the terrace zone closer to the center of the crater is the topographic peak ring; a geometry that suggests interaction of the inward slumping terrace zone and the rebounding central uplift is important for the formation of the peak ring. The peak ring rises higher above the crater floor in the west and northwest relative to the east and northeast. A Cenozoic basin overlies the peak ring and crater floor within the inner rim in all imaged azimuths except north and northeast where the inner rim is absent. The K/P surface, defined based on mapped reflections that correlate with the base of the Cenozoic basin, shows significant pre-existing relief on the Cretaceous seafloor. This relief appears to correlate with the observed asymmetries in terrace zone depth, peak ring relief and lack of a crater rim to the north and northeast, and therefore suggests that target heterogeneities strongly influence final crater structure. Reflectivity is present beneath the topographic peak ring along all imaged azimuths that may represent a lithologic base of the peak ring material, or a marker for an extinct hydrothermal system. Asymmetry in the peak ring allows for sampling the lithologies beneath the topographic peak ring at relatively shallow depths in order to explain how the proposed deep crustal material can result in lower velocities and densities than the surrounding impact rocks. Bright, discontinuous reflections to the interior of the topographic peak ring may represent the top of the impact melt sheet that lies beneath potential re-surge deposits and is thought to cap the central uplift observed on seismic and gravity data. A proposed IODP-ICDP drilling transect plans to penetrate and sample the shallowest peak ring and underlying dipping reflectivity in the offshore and the Cenozoic sediments deposits, potential surge deposits, melt sheet, and if possible the central uplift onshore to calibrate existing models for impact crater formation and the mass extinction.

The terrestrial record is the only source of 3-D ground truth observations on the lithological and structural character of natural impact structures. Of the three largest known impact craters on Earth, Chicxulub is the best preserved because of a slow burial on a tectonically quiet carbonate platform. Our proposal is to drill two wells that address fundamental issues about the structure of the Chicxulub impact crater and its environmental effects. CHICX-01A will focus on constraining the environmental effects of the impact. Current emphasis is on the potential effects of vapor species derived from shocked carbonates and sulfates. Chicx-01A will supply a complete litho-stratigraphic section of the offshore sedimentary portion of the target. Anhydrite is likely to be the most lethal component of the target rocks, but estimates of its constituent percentage range between 10 and 40 %. Half of the crater lies offshore, and seismic indicate that the Mesozoic section is > 1-km thicker offshore than onshore. The thicker the sedimentary layer, the greater the volume of potential pollutants released. If we drill Chicx-01A, we will be able to calibrate the marine reflection data, in terms of depth, strata and lithology, and be better able to convert travel-time to depth for the entire marine reflection dataset. Onshore drilling at Yaxcopoil-1 penetrated 600 m of late Cretaceous calcarenite, dolomite and anhydrite rocks. These data are of significant value in establishing the chemistry of the uppermost section of target rock, and will serve as a baseline for onshore-offshore comparisons if Chicx-01A is drilled. CHICX-02A is specifically designed to sample the peak ring and provide information to constrain formational processes. It is widely believed that peak rings form from hydrodynamic collapse in some form of extension of the structural uplift process that leads to central peaks in smaller complex craters. However, annular rings within terrestrial craters correspond to different morphological elements and this diversity, as well as a lack of common understanding as to what constitutes the planetary equivalent of a peak ring, means that there is currently no consensual agreement on the nature of a topographic peak ring. Drilling through the peak ring at Chicxulub will answer this fundamental cratering question. Geophysical property measurements on the core will be used to improve 3D structural models of the central crater. Of particular interest is the source of the short-wavelength magnetic anomalies that appear to track the peak ring, and may represent enhanced hydrothermal circulation. Our high-resolution 3-D seismic survey, shot in early 2005, will place the drill-hole in its correct structural context. Understanding the mechanism for peak-ring formation is fundamental to understanding cratering. When we can model crater formation in detail, we can better use craters as a diagnostic tool for understanding the surface evolution of the other terrestrial planets. Subtle differences in crater morphology between different planetary bodies provide clues to their near-surface rheology.