Revista Mexicana de Ciencias Geológicas ISSN: Universidad Nacional Autónoma de México México - PDF

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Revista Mexicana de Ciencias Gelógicas ISSN: Universidad Nacinal Autónma de Méxic Méxic Reblled-Vieyra, Mari; Urrutia-Fucugauchi, Jaime; López-Lera, Héctr Aermagnetic anmalies

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Revista Mexicana de Ciencias Gelógicas ISSN: Universidad Nacinal Autónma de Méxic Méxic Reblled-Vieyra, Mari; Urrutia-Fucugauchi, Jaime; López-Lera, Héctr Aermagnetic anmalies and structural mdel f the Chicxulub multiring impact crater, Yucatan, Mexic Revista Mexicana de Ciencias Gelógicas, vl. 7, núm. 1, marz, 1, pp Universidad Nacinal Autónma de Méxic Querétar, Méxic Dispnible en: Cóm citar el artícul Númer cmplet Más infrmación del artícul Página de la revista en redalyc.rg Sistema de Infrmación Científica Red de Revistas Científicas de América Latina, el Caribe, España y Prtugal Pryect académic sin fines de lucr, desarrllad baj la iniciativa de acces abiert Revista Mexicana de Ciencias Gelógicas, Aermagnetic v. 7, núm. 1, anmaly 1, p. mdeling, Chicxulub crater 185 Aermagnetic anmalies and structural mdel f the Chicxulub multiring impact crater, Yucatan, Mexic Mari Reblled-Vieyra 1, Jaime Urrutia-Fucugauchi,*, and Héctr López-Lera 3 1 Centr de Investigacines Científicas de Yucatán, CICY, Centr para el Estudi del Agua, Calle 43 N. 13, Clnia Chuburná de Hidalg, 97 Mérida, Yucatán, Mexic. Universidad Nacinal Autónma de Méxic, Institut de Gefísica, Pryect Universitari de Perfracines en Océans y Cntinentes, Circuit Exterir s/n, Cd. Universitaria, Del. Cyacán, 451 Mexic D.F., Mexic. 3 Institut Ptsin de Investigación Científica y Tecnlógica, IPICYT, Camin a la Presa San Jsé 55, Cl. Lmas 4a sección, 7816 San Luis Ptsí, S.L.P., Mexic. * ABSTRACT A structural mdel f the Chicxulub crater is derived frm aermagnetic anmaly mdeling, brehle infrmatin and magnetic mineral data. Magnetic susceptibility measurements frm brehle cres and samples in the crater shw that suevite-like breccias have a variable strng magnetic signature, which is related t basement and melt clasts. The crystalline cmpnent estimated frm clast analyses in the suevite-like breccias has n average higher magnetic susceptibilities (up t SI) than that f impact melt (~5 1-5 SI) and crystalline basement (4 1-5 SI). Reductin t the ple and dwnward analytical cntinuatins shw the discrete cmpsite character f the anmaly, with inverse diplar anmalies. The secnd-derivative f magnetic anmaly depicts five cncentric rings, with the external ring crrelating with the cente ring and marking the surface expressin f crater rim. The analytical signal and the radially averaged spectrum yield an estimate f the averaged depth t the magnetic surces, ranging frm 1 t 6 m. There are three majr magnetic surces within the Chicxulub crater: 1) the melt unit, ) the suevite-like breccia, and 3) the central uplift. Using all these data, including new -D magnetic mdels, a new structural mdel is prpsed. It reveals a system f reginal vertical faults that explain the magnetic signal ver the suthern sectr f the crater, whereas a.5 km deep central uplift and highly magnetized breccia sequences and melt sheet might be the surces f the main magnetic anmalies. Key wrds: Chicxulub crater, magnetic susceptibility, aermagnetic anmaly, structural mdel, uplift, Mexic. RESUMEN En este trabaj presentams un mdel actualizad de la estructura de impact de Chicxulub, utilizand nuevs mdels de la anmalía aermagnética. Estudis de la variación de la susceptibilidad magnética a l larg de la clumna litlógica al interir del cráter revelan que las brechas de tip suevita tienen una firma magnética más fuerte que la unidad fundida (melt). La cmpnente cristalina, estimada a partir del análisis de clasts encntrads en las brechas de tip suevita, tiene una susceptibilidad Reblled-Vieyra, M., Urrutia-Fucugauchi, J., Héctr López-Lera, H., 1, Aermagnetic anmalies and structural mdel f the Chicxulub multiring impact crater, Yucatan, Mexic: Revista Mexicana de Ciencias Gelógicas, v. 7, núm. 1, p 186 Reblled-Vieyra et al. magnética más alta (hasta SI), que el melt (~5 1-5 SI) y ls clasts del basament cristalin (4 1-5 SI). La reducción al pl y la cntinuación hacia abaj, dcumentan el carácter fragmentad de la anmalía. La segunda derivada de la anmalía aermagnética delinea cinc anills cncéntrics al interir del cráter; el últim anill se crrelacina cn el anill de centes, l cual apya la interpretación de que el rigen del anill de centes está ligad cn el cráter. La señal analítica y el espectr radialmente prmediad arrjan una prfundidad estimada a las fuentes magnéticas que va de ls 1 m a ls 6 m. Utilizand ests dats desarrllams nuevs mdels magnétics en -D, ls cuales indican que el carácter fraccinad en la prción nrte del cráter está cntrlad pr un sistema de fallas verticales. La principal anmalía central es prduct de un levantamient estructural, cuya cima se encuentra a ~,5 m de prfundidad a partir del fnd marin, en el área central del cráter. Palabras clave: cráter Chicxulub, magnetmetría, anmalía aermagnética, mdel estructural, levantamient, Méxic. INTRODUCTION Impact cratering is ne f the majr prcesses shaping planetary surfaces in the Slar system. Impact craters are characteristic features in all planets, satellites and asterids, except fr the gaseus giant planets. On Earth, the gelgical recrd f impact craters has been erased by the tectnic, magmatic and ersinal prcesses (Melsh, 1989). Large cmplex multiring craters that are prducts f majr impacts are rare n Earth, with nly three craters dcumented: Vredefrt (Suth Africa), Sudbury (Canada) and Chicxulub (Mexic) (Grieve and Therriault, ; Urrutia-Fucugauchi and Perez-Cruz, 9). Chicxulub is the yungest and best preserved. The impact event is als well knwn since it has been linked t glbal effects n climate and envirnment, resulting in ne f the majr life turnvers marking the transitin frm the Meszic t the Cenzic at the Cretaceus/Palegene (K/Pg) bundary (Hildebrand et al., 1991, 1998; Sharptn et al., 199). Gephysical methds have lng been used t identify and investigate impact craters, which ften shw characteristic circular anmaly patterns (e.g., Pilkingtn and Grieve, 199; Sharptn et al., 1993). Aermagnetic and grund magnetic surveys have been particularly successful in studying the structure and stratigraphy f craters. The changes in physical prperties induced by shck and pressure/thermal prcesses have been als studied and related t petrgraphic and gechemical data and t the impact and cratering prcesses. Recent investigatins n Mars large impact basins, which are characterized by lw amplitude magnetic anmalies, indicate apparent majr demagnetizatin effects n target rcks induced by large impacts (e.g., Acuña et al., 1999). These features have been dcumented by palemagnetic studies f basement rcks frm the Vredefrt crater (Muundjua et al., 7). In general, investigatins n terrestrial impact structures and labratry experiments dcument wide ranges in magnetic prperties (i.e., magnetic susceptibility ranges frm diamagnetic range,. S.I. t SI). In general, it is cnsidered that the shck prduces a drp in the magnetic susceptibility and ften in the remanent magnetizatins (Plad et al., 1999). Lithlgies like the impact melt that cl dwn slwly may acquire a thermremanent magnetizatin (TRM) in the directin f the present magnetic field (e.g., Manicuagan and Chicxulub). In ther cases, like in large cmplex craters, the impact lithlgies may acquire a new remanence by reheating and transient stresses; i.e., the thermremanent and shck remanence (SRM) magnetizatins alng the directin f the Earth s magnetic field at the time f impact. Buried in the Yucatan carbnate platfrm in sutheastern Mexic, the Chicxulub crater was unveiled by gephysical surveys cnducted as part f il explratin prgrams f Pemex (Penfield and Camarg, 1981; Hildebrand et al., 1991; Sharptn et al., 199). In particular, the buried crater shws a circular cncentric Buguer gravity anmaly pattern, which is clearly marked in the lw amplitude reginal gravity pattern. The central zne f the gravity anmaly is marked by high amplitude magnetic anmalies. The magnetic anmaly f Chicxulub crater is an example f a cmplex crater gephysical signature, and in gephysical mdels, it has a semi-circular shape with a large amplitude central anmaly surrunded by smaller inverse diplar anmalies (Figure 1). In the past tw decades, the crater has been intensively investigated using ptential field, electrmagnetic and seismic surveys. Althugh numerus studies and gephysical mdeling have been used t investigate the crater structure (e.g., Hildebrand et al., 1991, 1998; Sharptn et al., 199, 1993; Cnnrs et al., 1996; Mrgan et al., 1997; Mrgan and Warner, 1999; Delgad-Rdríguez et al., 1; Gulick et al., 8; Veermesch and Mrgan, 8), inverse and frward mdels derived frm these data are limited by the lack f direct data n the nature f target lithlgies. Data n physical prperties f target lithlgies and specially f the deep Yucatan basement remain prly cnstrained. Stratigraphic data came mainly frm il explratin brehles and reginal mdels (e.g., Murray and Weide, 196; López-Rams, 1973, 1983; Marshall, 1974; Weidie, 1985, Ward et al., 1995). Recent drilling prgrams that included cntinuus cring have prvided new infrmatin n the subsurface stratigraphy and cnstraints n the basement (Urrutia-Fucugauchi et al., 1996, 4a, 8; Reblled- Aermagnetic anmaly mdeling, Chicxulub crater 187 a) D N 1 N b) A B B Depth (Kunit) C 5 lg(pwer) Aermagnetic Anmaly f the Chicxulub Crater 9 W A 5 5 Kilmeters 89 W C D Radially Averaged Pwer Spectrum Wavenumber (1/Kunit) Estimated Depth Wavenumber (1/Kunit) Aermagnetic Anmaly (nt) 1 lg(pwer) Figure 1. a: Aermagnetic anmaly field ver the Chicxulub crater in the Yucatan peninsula and Gulf f Mexic. Survey flight altitude is 45 m abve sea level. Lcatin f reginal prfiles are shwn: Lines A-A, B-B, C-C and D-D used fr -D mdeling (see Figures 3 and 4). b: Radially averaged spectra f the aermagnetic anmaly, units in the vertical axis are depth in km. Vieyra et al., ). Labratry measurements n physical prperties prvide data needed t cnstraint gephysical mdeling. Samples frm the Chicxulub suevitic breccias and frm PEMEX brehles that prvide infrmatin n the nature f the Yucatan basement are als cnsidered t cnstrain the structural crater mdel. In this wrk, we present new structural mdels f the impact crater derived frm mdeling f the aermagnetic anmaly field (Figure 1). In the analysis, we cnsider previus mdeling f the aermagnetic anmalies (Pilkingtn et al., 1994; Ortiz-Alemán et al., 1), brehle and gephysical lgging infrmatin (Urrutia-Fucugauchi et al., 1996, 4a, 8; Reblled-Vieyra et al., ), palemagnetic, rck magnetic and gechemical data (Urrutia-Fucugauchi et al., 1994, 1996, 4b; Urrutia-Fucugauchi and Pérez-Cruz, 8). First, the magnetic measurements n rck samples 3 Depth (Kunit) are presented. In the fllwing sectin, the prcessing and analysis f the aermagnetic data are analyzed. Then the magnetic mdeling methd is described, and results are linked with the palemagnetic data. Finally, we discuss the cmbined results with regards t the previus mdels and the gelgical cnstraints f the crater basin. PETROPHYSICAL AND GEOPHYSICAL DATA Palemagnetic and rck magnetic measurements Natural remanent magnetizatin (NRM) and lw-field magnetic susceptibility measurements were perfrmed n cre samples frm the UNAM Scientific Drilling Prgram. The NRM intensity and directin (referred t arbitrary azimuth) were determined using a spinner JR-5 magnetmeter. The lw-field magnetic susceptibility was measured with the Bartingtn susceptibility system, using the labratry sensr. We thus btained high-reslutin recrds f the magnetic susceptibility behavir frm three brehles drilled in the suthern sectr f the structure (Reblled-Vieyra and Urrutia-Fucugauchi, 1999). Frm these studies we btained lg data f magnetic susceptibility fr the suevitic breccias, which have mean magnetic susceptibility f SI, but with values as high as SI, fr bunte breccia with a susceptibility mean within the diamagnetic range, fr individual clasts f impact melt, SI, and crystalline basement, SI, and als fr the carbnates and anhydrites, which magnetic susceptibility is als within the diamagnetic range. The 4 Ar/ 39 Ar dates reprted by Sharptn et al. (199) n melt samples recvered frm the PEMEX brehle Chicxulub-1 give an age fr the impact f ~65.5 Ma. The istpic dates agree with the palemagnetic data f the melt samples, which place the impact within reverse plarity gemagnetic chrn C9r. The magnetic plarity f melt, investigated by Urrutia-Fucugauchi et al. (1994) in PEMEX brehle Yucatan-6, Reblled-Vieyra and Urrutia-Fucugauchi (4) in Yaxcpil-1 brehle, and Reblled-Vieyra and Urrutia-Fucugauchi (6) in UNAM brehles give reverse plarity, with mean upward inclinatins f 4 4. Cnsidering this data, we assumed a -41 inclinatin and a declinatin f 163 using the Nrth America plar wander curve fr the Late Cretaceus-early Palegene. Aermagnetic anmaly data The aermagnetic data cme frm a 45 m flight-altitude survey. Data fr the mdeling have been reduced t a regular anmaly grid 171 km by 171 km, with digitized data pints every 1 km. Data prcessing cnsisted in reductin t the ple (Figure a), secnd vertical derivative (Figure b), pseud-gravity (Figure c) and analytical dwnward cntinuatins (Figure d); we als calculated the radial 188 Reblled-Vieyra et al. a) b) N 1 N 5 5 Kilmeters 9 W 89 W nt N 1 N 5 9 W Kilmetrs Kilmeters W nt c) d) N 1 N 9 W 5 5 Kilmeters 89 W nt N 1 N 5 9 W 5 5 Kilmeters W Figure. a) Aermagnetic anmaly field f the Chicxulub crater reduced t magnetic ple. Lcal gemagnetic field inclinatin is 45 and declinatin is 5. b) Secnd derivative f the aermagnetic anmaly field. c) Pseud-gravimetric analysis f aermagnetic anmaly field. Density cntrast is 1.8 g/cm 3. d) Dwnward cntinuatin t a reference surface f 45 m. mgal average spectrum t estimate the depth t surces (Figure e) (e.g., Macled et al., 1993; Pilkingtn and Hidebrand, ). Using this data, we mdeled fur reginal prfiles, riented N-S, E-W, N45 E and N45 W crssing the entire basin (Figure 1). Data prcessing and mdeling was made with the Oasis Mntaj sftware package. Frm this prcess, we cnstrained the principal structures f the crater as being separated int several magnetic dmains. In the analysis, we cnsidered a magnetic dmain as a regin with similar magnetic characteristics in terms f wavelengths and amplitudes. In sme cases, lw amplitude magnetic anmalies, characterized by subdued signals, and null r weak magnetic susceptibility cntrasts have als been separated int distinct dmains. When nticeable cntrasts in anmaly amplitudes, pssibly assciated with magnetic susceptibility cntrasts, are bservable within a dmain, we call them magnetic sub-dmains (Figure ). Analyses f the aermagnetic anmaly field (Figure 1a) suggest separatin f anmaly patterns int three dmains, with characteristic amplitudes and wavelengths. The first is characterized by large amplitude diplar magnetic anmalies lcated at the central part f the crater, extending nshre and ffshre. It has, at least, three magnetic highs, ne f them having an islated maximum and the ther tw having diplar high t lw distances between 7 and 1 km. Tw magnetic sub-dmains can be separated: A sub-dmain assciated with magnetic respnses f lng wavelengths and large amplitudes, and a secnd magnetic sub-dmain lcated SE frm the first sub-dmain and cmpletely nshre. The magnetic cnfiguratin f this Aermagnetic anmaly mdeling, Chicxulub crater 189 anmaly has three anmalus znes, with nrmal diplar behavir; the diplar distances range frm 7 t 8 km. This magnetic sub-dmain is characterized by anmalies with lng wavelengths and intermediate amplitudes. In the ttal magnetic field anmaly map, this dmain has an elngated trend riented N8 W, with a maximum length f 7 km and 4 km wide. The secnd aermagnetic dmain is lcated t the SW, and is characterized by an islated high anmaly with an val shape striking NW, defined by lng wavelengths and large amplitudes; its minimum dimensins are 64 km, striking NW, and 5 km, striking NE. The third magnetic sub-dmain, lcated t the NE, has an elngated shape with mre than 5 km length in either directin. The cnfiguratin reflects presence f majr lineaments striking NE-SW and NW-SE. Within the magnetic dmain znes, secndary lineaments can be bserved, and an E-W lineament that apparently limits the magnetic anmaly frming the first magnetic sub-dmain; its fragmented character can be assciated t faults and/r fractures. T estimate average statistical depths t magnetic surces, we calculated the radially averaged spectrum (Figure 1b). In the depth estimatins, it is assumed that the ejecta blanket within the crater behaves as a cherent and cntinuus depsit, allwing us t estimate the depth t the surces at abut ~6, m in the center f the structure t ~5 m twards the edge f the central anmaly. AEROMAGNETIC DATA PROCESSING AND MODELING Reductin t magnetic ple The cnfiguratin f the magnetic field reduced t the ple permits separatin f three magnetic dmains (Figure a). The first magnetic dmain is lcated in the central part, and it is frmed by fur magnetic anmalies with diplar cnfiguratins. The first anmaly is lcated in the NW within the dmain; it is elngated striking NNE and has a plar distance f 17 km and is assciated t a magnetic respnse f shrt-lng wavelengths and intermediate amplitude. The secnd anmaly is lcated within the NW central prtin f the magnetic dmain and is frmed by a nrmal diplar anmaly, elngated and striking ENE with high-t-lw distance f ~17 km. The anmaly cnsists f tw psitive highs that in the nrth, as in the suth, present diplar cnfiguratin; t the nrth it has nrmal plarity and t the suth it has reverse plarity. A ENE-striking lineament that brders the anmaly dmain divides the tw lbes. These sub-dmains present magnetic anmalies f lng wavelengths and large amplitudes. The third zne f this magnetic dmain is characterized by a nrmal diplar anmaly, elngated, striking N-S, with a plar high-t-lw distance f abut 9 km, assciated t a magnetic respnse f lng wavelengths and intermediate amplitudes. The furth magnetic sub-dmain is frmed by a triangular shaped anmaly that in its central part shws a nrmal diple with a plar high-t-lw distance f abut 17 km. This subdmain is characterized by intermediate wavelengths and amplitudes. The secnd magnetic dmain, lcated SW frm the central zne, is frmed by an anmaly with lng wavelength and large amplitude. The anmaly has an val trend with the lng axis striking NE-SW, and cvers a large area lcated nshre and ffshre. The third magnetic dmain is lcated NE frm the central zne and is characterized by an anmaly assciated t a lng wavelengths and large amplitude signal. T better cnstrain the magnetic characteristics f the zne, it is necessary t have a mre extensive magnetic cverage. Lineaments identified in analyses f magnetic anmaly trends supprt the ccurrence f faulting and fracturing at depth. Lineaments striking NW-SE, NE-SW and N-S brder the anmaly dmain znes. A majr lineament striking NNE-SSW crsses the crater basin, separating the anmaly dmain znes ne and tw f the first dmain. The dimensins estimated in this analysis fr the structure are a maximum lngitude f 167 km striking NW76SE and a perpendicular distance f 16 km. Secnd vertical derivative The secnd vertical derivative yields a magnetic cnfiguratin that emphasizes the circular trends in the magnetic anmaly field (Figure b). Three circular trends are unveiled. The first is frmed by an interir ring f 71.5 km striking NW and 65.5 km striking NE. The secnd is assciated t a circular intermediate ring with a lngitude f 137 km striking NW and 113 km in the NE directin. The third trend is prly defined. Pseud-gravimetry The pseud-gravimetric anmaly, which is the vertical derivative f the ple-reduced magnetic ptential, was calculated. With this analysis (Figure c) fur dmains are identified: the central larger dmain may crrespnd t a brad lw, separated by a high in the middle. The ther three pseud-gravimetric dmains are lcated n the flanks f the brad lw, and are represented by the pseud-gravimetric highs at W, S
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