Jaime Urrutia-Fucugauchi

The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary

The Fall of the Dinosaurs According to the fossil record, the rule of dinosaurs came to an abrupt end ∼65 million years ago, when all nonavian dinosaurs and flying reptiles disappeared. Several possible mechanisms have been suggested for this mass extinction, including a large asteroid impact and major flood volcanism. Schulte et al. (p. 1214) review how the occurrence and global distribution of a global iridium-rich deposit and impact ejecta support the hypothesis that a single asteroid impact at Chicxulub, Mexico, triggered the extinction event. Such an impact would have instantly caused devastating shock waves, a large heat pulse, and tsunamis around the globe. Moreover, the release of high quantities of dust, debris, and gases would have resulted in a prolonged cooling of Earths surface, low light levels, and ocean acidification that would have decimated primary producers including phytoplankton and algae, as well as those species reliant upon them. The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

Chicxulub Multiring Impact Basin: Size and Other Characteristics Derived from Gravity Analysis

The buried Chicxulub impact structure in Mexico, which is linked to the Cretaceous- Tertiary (K-T) boundary layer, may be significantly larger than previously suspected. Reprocessed gravity data over Northern Yucatan reveal three major rings and parts of a fourth ring, spaced similarly to those observed at multiring impact basins on other planets. The outer ring, probably corresponding to the basins topographic rim, is almost 300 kilometers in diameter, indicating that Chicxulub may be one of the largest impact structures produced in the inner solar system since the period of early bombardment ended nearly 4 billion years ago.

Continental boundaries of the Jalisco block and their influence in the Pliocene-Quaternary kinematics of western Mexico

Extensional faulting observed in southwestern Mexico has been related to the incipient rifting of the Jalisco block from the Mexican mainland since the Pliocene. On the basis of new structural and geophysical data, we propose that (1) the continental boundaries of the Jalisco block are ancient structures reactivated since the Pliocene at a low (<1 mm/yr) rate of deformation, and (2) Pliocene-Quaternary extensional faulting at the edges of Jalisco block is a basement-controlled intraplate deformation related to plate boundary forces rather than to active continental rifting. The Jalisco block boundaries first developed in response to the uplift of the Puerto Vallarta batholith in pre-Neogene time and underwent a complex contractile deformation before the Pliocene. During Pliocene-Quaternary times north-northeast extension reactivated the northern boundary, forming the Tepic-Zacoalco rift, whereas east-southeast extension formed the northern Colima rift. South of the Colima volcano, active extension is found only west of the so-called southern Colima rift and partly reactivates old northeast-trending basement faults. The parallelism between the subducted Rivera-Cocos plate boundary zone and the eastern neotectonic boundary of the Jalisco block supports east-southeastward motion of the southern Mexican blocks induced by the differential motion and oblique subduction of the Cocos and Rivera plates. On the other hand, we envisage Pliocene-Quaternary extension along the northern boundary as an upper-plate response to the low convergence rate and the steep subduction angle of the Rivera plate.

The subducted Rivera‐Cocos Plate Boundary: Where is it, what is it, and what is its relationship to the Colima Rift?

Gravity data, the geometry of the Wadati-Benioff zone beneath western Mexico, and the seafloor morphology of the Rivera-Cocos plate boundary west of the Middle America trench suggest that the subducted part of this boundary lies directly beneath and is oriented parallel to the Southern Colima rift. Thus, the Southern Colima rift likely formed in response to divergence between the subducting Rivera and Cocos plates due to direct coupling between these two plates and the overriding North American plate. In contrast, the subducted plate boundary lies east of and oblique to the Northern Colima and Central Colima grabens. East of the Central Colima graben a low density zone overlies the boundary and underlies surface exposures of Cretaceous granitoids and associated thermal springs and shallow focus earthquakes; characteristics that are explained by thermal convection induced in the upper mantle by divergence between the subducted Rivera and Cocos plates. These characteristics along with the adjacent locations of the low density upper mantle and the Central Colima graben are consistent with crustal extension produced by the uniform-sense normal simple shear mechanism.

Bouguer Gravity Anomalies and Regional Crustal Structure in Central Mexico

Analysis of the Bouguer gravity anomalies for central Mexico, between 18° and 21° N and 96° and 106° W, is used to document the regional crustal structure beneath the active magmatic arc, the continental margin, and the high-elevation provinces north of the arc. A variogram analysis was performed first to investigate the statistical properties of the gravity anomaly map and to define an optimum sampling scheme for application of spectral and iterative modeling methods. A sampling rate of 6′ (11.11 km) was selected. Bouguer gravity is characterized by large wavelength negative anomalies of −200 to −250 mgal over the magmatic arc and a smooth increase of about 2 mgal/km to positive values toward the Pacific and Gulf margins. Short-wavelength anomalies 20 to 50 km in size correlate with major structural features. Crustal models were derived for 11 N-S profiles spaced one degree apart and three long E-W profiles, from spectral analysis and two different iterative modeling schemes that model the crust-mantle i...

UNAM Scientific Drilling Program of Chicxulub Impact Structure‐Evidence for a 300 kilometer crater diameter

As part of the UNAM drilling program at the Chicxulub structure, two 700 m deep continuously cored boreholes were completed between April and July, 1995. The Peto UNAM-6 and Tekax UNAM-7 drilling sites are ∼150 km and 125 km, respectively, SSE of Chicxulub Puerto, near the craters center. Core samples from both sites show a sequence of post-crater carbonates on top of a thick impact breccia pile covering the disturbed Mesozoic platform rocks. At UNAM-7, two impact breccia units were encountered: (1) an upper breccia, mean magnetic susceptibility is high (∼55 × 10−6 SI units), indicating a large component of silicate basement has been incorporated into this breccia, and (2) an evaporite-rich, low susceptibility impact breccia similar in character to the evaporite-rich breccias observed at the PEMEX drill sites further out. The upper breccia was encountered at ∼226 m below the surface and is ∼125 m thick; the lower breccia is immediately subjacent and is >240 m thick. This two-breccia sequence is typical of the suevite-Bunte breccia sequence found within other well preserved impact craters. The suevitic upper unit is not present at UNAM-6. Instead, a >240 m thick evaporite-rich breccia unit, similar to the lower breccia at UNAM-7, was encountered at a depth of ∼280 m. The absence of an upper breccia equivalent at UNAM-6 suggests some portion of the breccia sequence has been removed by erosion. This is consistent with interpretations that place the high-standing crater rim at 130–150 km from the center. Consequently, the stratigraphic observations and magnetic susceptibiity records on the upper and lower breccias (depth and thickness) support a ∼300 km diameter crater model.

Mapping Chicxulub crater structure with gravity and seismic reflection data

Abstract Aside from its significance in establishing the impact-mass extinction paradigm, the Chicxulub crater will probably come to exemplify the structure of large complex craters. Much of Chicxulub’s structure may be ‘mapped’ by tying its gravity expression to seismic-reflection profiles revealing an ∼180 km diameter for the now-buried crater. The distribution of karst topography aids in outlining the peripheral crater structure as also revealed by the horizontal gradient of the gravity anomaly. The fracturing inferred to control groundwater flow is apparently related to subsidence of the crater fill. Modelling the crater’s gravity expression based on a schematic structural model reveals that the crater fill is also responsible for the majority of the negative anomaly. The crater’s melt sheet and central structural uplift are the other significant contributors to its gravity expression. The Chicxulub impact released ∼1.2 × 1031 ergs based on the observed collapsed disruption cavity of ∼86 km diameter reconstructed to an apparent disruption cavity (Dad) of ∼94 km diameter (equivalent to the excavation cavity) and an apparent transient cavity (Dat) of ∼80 km diameter. This impact energy, together with the observed ∼2 × 1011 g global Ir fluence in the Cretaceous-Tertiary (K-T) fireball layer indicates that the impactor was a comet estimated as massing ∼1.8 × 1018 g of ∼16.5 km diameter assuming a 0.6 gcm−3 density. Dust-induced darkness and cold, wind, giant waves, thermal pulses from the impact fireball and re-entering ejecta, acid rain, ozone-layer depletion, cooling from stratospheric aerosols, H2O greenhouse, CO2 greenhouse, poisons and mutagens, and oscillatory climate have been proposed as deleterious environmental effects of the Chicxulub impact with durations ranging from a few minutes to a million years. This succession of effects defines a temperature curve that is characteristic of large impacts. Although some patterns may be recognized in the K-T extinctions, and the survivorship rules changed across the boundary, relating specific environmental effects to species’ extinctions is not yet possible. Geochemical records across the boundary support the occurrence a prompt thermal pulse, acid rain and a ∼5000 year-long greenhouse. The period of extinctions seems to extend into the earliest Tertiary.

Stratigraphy and Tectonics of the Guadalajara Region and Triple-Junction Area, Western Mexico

In the region between Guadalajara and the triple junction of the Tepic-Zacoalco, Chapala, and Colima rifts, the late Miocene to Quaternary volcanics of the Mexican Volcanic Belt (MVB) largely conceal the boundaries between the basement domains of the Sierra Madre Occidental (SMO) to the north and the Jalisco and Michoacan blocks to the south. Integrating previous work with new geologic mapping and isotopic age determinations, we propose a comprehensive regional stratigraphy for Guadalajara and the triple-junction area and attempt to define the boundaries between these basement domains. In the study area, the silicic succession of the SMO is restricted to the north of the Santa Rosa-Cinco Minas fault and the northern boundary faults of Lake Chapala. The succession of the MVB began with widespread mafic volcanism, and lacustrine sedimentation occurred at 11 to 8 Ma in several tectonic basins developed along the Tepic-Zacoalco and the Chapala rifts, suggesting that the extensional reactivation of the block b...

Paleomagnetism of the Chiapas Massif, southern Mexico: Evidence for rotation of the Maya Block and implications for the opening of the Gulf of Mexico

We have analyzed 180 oriented samples of the batholithic complex and overlying red beds of the Chiapas Massif in southern Mexico for a paleomagnetic study. Both stepwise alternating-field and thermal demagnetization procedures revealed dual-polarity characteristic magnetizations above 10-30 mT or 200-400 °C in about 60% of the samples demagnetized. Three distinct and ancient paleomagnetic directions have been identified. Late Permian intrusions (256 ± 10 Ma) yield nearly equatorial southern hemisphere paleolatitudes and a paleopole (13.4°S,176.1°E; 12 sites, K = 15.4, A95 = 11.4°) rotated ∼75°, counterclockwise, with respect to the North American reference pole. A selected set of Late Triassic(?)-Jurassic red beds of the Todos Santos Formation yields a paleopole at 22.6°N, 170.6°E (4 sites, K = 56.8, A95 = 12.3°), which implies ∼60° counterclockwise rotation, and a near equatorial northern hemisphere paleolatitude. Middle-Late Jurassic dikes and granites yield a paleopole at 77.4°N, 97.4°E (6 sites, K = 269.2, A95 = 4.1°) near the Mid-Jurassic segment of the North American apparent polar wander path. These data are consistent with a pre-rift configuration in which the Maya Block is rotated to a position off the coast of Texas and Louisiana in the present-day area of the Gulf of Mexico. A further implication is that by Oxfordian times, Chiapas (and by inference the Maya Block) was essentially in its current position with respect to cratonic North America.

Mammoth bones embedded in a late Pleistocene lahar from Popocatépetl volcano, near Tocuila, central México

The accidental discovery in July 1996 of the Tocuila mammoth site in the eastern Valley of Mexico was followed by intensive scientific excavation, which yielded remains of at least seven individuals of Mammuthus columbi in an area measuring only 28 m 2 . The high density of mostly disarticulated mammoth bones identified 3 m below the surface in a massive, 130-cm-thick lahar deposit of sandy ash and rounded pumice fragments is unusual and suggests a high-energy depositional environment. Samples of charcoal found within this deposit yielded radiocarbon ages ranging between 10 220 ± 75 and 12 615 ± 95 yr B.P. The lahar was studied in greater detail in order to establish its provenance. Determination of grain-size distribution and magnetic properties of the deposit, as well as petrographical, mineralogical, and geochemical analyses of pumice clasts, all indicate that the lahar was derived from volcanic fallout deposits that originated at Popocatepetl volcano, located 50 km south-southeast of the site. Popocatepetl underwent a major phreato-Plinian eruption, dated ca. 14 000 yr B.P., at the end of the last major glaciation. Dispersal of the fallout was mostly toward the northwest and north; deposits were found on the slopes of Tlaloc mountain (4150 m), 20 km southeast of Tocuila. At the beginning of deglaciation and the onset of more humid conditions ∼3000 yr later, lahars derived from the fallout deposits picked up the bones of already dead animals and then deposited them on the shores of Lake Texcoco and/or simply covered bones without major displacement.