Heat flow pattern at the Chicxulub impact crater, northern Yucatan, Mexico

    Heat flow pattern at the chicxulub impact crater, northern yucatan, mexico J.M. Espinosa-Carde˜na, J.O. Campos-Enr´ıquez, M. Unsworth PII: DOI: Reference:

S0377-0273(15)00419-9 doi: 10.1016/j.jvolgeores.2015.12.013 VOLGEO 5731

To appear in:

Journal of Volcanology and Geothermal Research

Received date: Accepted date:

18 August 2014 27 December 2015

Please cite this article as: Espinosa-Carde˜ na, J.M., Campos-Enr´ıquez, J.O., Unsworth, M., Heat flow pattern at the chicxulub impact crater, northern yucatan, mexico, Journal of Volcanology and Geothermal Research (2016), doi: 10.1016/j.jvolgeores.2015.12.013

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HEAT FLOW PATTERN AT THE CHICXULUB IMPACT CRATER, NORTHERN YUCATAN, MEXICO

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Espinosa-Cardeña, J.M.1, Campos-Enríquez, J.O.2, and M. Unsworth3. 1

División de Ciencias de la Tierra, CICESE, Ensenada, Baja California, México.

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Instituto de Geofísica, Universidad Nacional Autónoma de México, D.F. México, [email protected], tel. (52)-5556224147, Fax: (52)-55552486. 3

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University of Alberta, Canada-

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Corresponding author

ACCEPTED MANUSCRIPT ABSTRACT Along an east-west profile crossing the Chicxulub impact structure in northern Yucatán, México, Curie depths were obtained from statistical-spectral analysis of a grid of aeromagnetic

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data (256 km wide and 600 km long). These depths were corrected for flight height and depth

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to the sea floor to determine the geothermal gradient, assuming a temperature of 580 °C for the Curie temperature. Heat flow was then calculated from the geothermal gradients using a value of 2.67 W/m-K for the mean crustal thermal conductivity. The results show a

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conspicuous heat flow high above on the impact basin. In this location, the heat flow is 80 mW/m2 approximately. Available offshore estimates of the depth to the crustal magnetic

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source bases, on the northern Yucatán platform, and onshore heat flow determination on 8 shallow bore holes, and in a 1,511 m deep one, support the existence of this major high heat

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flow anomaly associated with the impact crater. This high heat flow might be related to the

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impact through: a) an uplift of the crystalline basement rocks in the center of the crater; b)

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impact induced radioactive element concentration into the crust below the impact structure. Higher thermal conductivities at the lower crust might also play a key role. Available

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seismological and thermal properties data are compatible with these mechanisms.

INTRODUCTION A complex, 180-200 km (Morgan et al., 1997; Gulick et al., 2008), multi-ring impact structure was excavated, 65 Ma ago, in northern Yucatán (México) (Figure 1). The Chicxulub crater is related with the mass extinction that occurs at the Cretaceous-Paleogene boundary (K-Pg) (Alvarez et al., 1980; Schulte et al., 2010). A cover of Tertiary sediments, between 0.1 and 2 km (Gulick et al., 2008; Gulick et al., 2013) have preserved the structure of the crater and recorded many of the processes that took place in its formation. Thus its study allows detailed insight of

ACCEPTED MANUSCRIPT the cratering processes as well as the environmental and climatic, and biological effects of the

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impact.

The Chicxulub impact structure has been the subject of geochemical, geological, and geophysical studies in the recent years. Geophysical investigations conducted at Chicxulub

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include gravity (Figure 1) and magnetic (Figures 2 and 3b) (Sharpton et al., 1993; Pilkington et al., 1994a; Espindola et al., 1995; Hildebrand et al., 1995; Campos-Enríquez et al., 1998;

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Pilkington and Hildebrand, 2000; Ebbing et al., 2001; Vermeesch and Morgan, 2008; Vermeesch et al., 2009; Ortiz-Alemán and Urrutia-Fucugauchi, 2010; Gulick et al., 2013) rock

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magnetism studies (Urrutia-Fucugauchi et al., 1994), magnetotellurics (Figure 1) (Unsworth et

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al., 2002; Campos-Enríquez et al., 2004) heat-flow (Figure 4) (Matsui et al., 1998; Flores-

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Márquez et al., 1999; Wilhelm et al., 2004), and offshore seismic reflection (Figure 1) (Camargo-Zanoguera and Suárez-Reynoso, 1994; Morgan et al., 1997; Maguire et al., 1998; Snyder and Hobbs, 1999; Snyder et al., 1999; Brittan et al., 1999; Christeson et al., 1999;

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Morgan and Warner, 1999a,b; Morgan et al., 2000; Mackenzie et al., 2001; Morgan et al., 2002a; Bell et al., 2004; Barton et al., 2010; Morgan et al., 2011; Whalen et al., 2013; Gulick et al., 2013), and refraction (Figure 1) (Brittan et al., 1999; Christeson et al., 1999; Christeson et al., 2001; Mackenzie et al., 2001; Morgan et al., 2002b; Vermeesch and Morgan, 2008; Christeson et al., 2009; Vemeesch et al., 2009; Morgan et al., 2011; Gulick et al., 2013). These studies helped to establish the major features of the subsurface structure of the impact structure. Drilling (Figure 4) (Lopez-Ramos, 1975; Hildebrand et al., 1991; Sharpton et al., 1992; Ward et al., 1995; Sharpton et al., 1996; Urrutia-Fucugauchi et al., 1996; Rebolledo-Vieyra et al., 2000; Dressler et al., 2004; Stöffler et al., 2004), radiometric and paleomagnetic dating (Krogh et al.,

ACCEPTED MANUSCRIPT 1993; Urrutia-Fucugauchi et al., 1994) helped to establish its K-Pg age. Petrological and geochemical studies (Smit and Klaver, 1981; Montanari et al., 1983; Smit and Kyte, 1984;

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1980; Trinquier et al., 2006) proved the impact nature of the crater.

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Sigurdsson et al., 1991; Blum et al., 1993; Koeberl, 1993; Koeberl et al., 1994; ; Alvarez et al.,

Heat flow determinations have been made by Matsui et al. (1998), Flores-Márquez et al.

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(1999) and Wilhelm et al. (2004). Matsui et al. (1998) determined a heat flow value of approximately 60 mW/m2 near the structure centre, and values fading out to the rims (20

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mW/m2). They based their determinations on shallow boreholes (Figure 4). Flores-Márquez et al. (1999) made temperature measurements in eight boreholes, drilled by

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the Universidad Nacional Autónoma de México. They studied the heat flow in the southern

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and southeastern regions of the crater (Figure 4). They delineate a heat flow high of about 60

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mW/m2 with a NE-SW orientation flanked by two heat flow lows (a minimal heat flow of 52 mW/m2). Wilhelm et al. (2004) conducted high resolution temperature measurements in the Chicxulub Scientific Drilling Project Yaxcopil-1 borehole, Y-1 (Figure 4). Measurements were

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made to a depth of 858 m to avoid the convective effects due to shallow ground water circulation. They established that heat flow was in the range from 65 to 80 mW/m2.

Flores-Márquez et al.´s (1999) conduction-convection modeling of heat and mass transfer in the impact structure found that the groundwater flow is controlled by the structure and modifies the thermal state. Wilhelm et al.´s (2004) high resolution temperature measurements showed that convection had significant influence on the heat flow in the upper 250 m. These effects have been further analyzed by Šafanda et al. (2005) and Šafanda et al. (2007). This

ACCEPTED MANUSCRIPT influence has to be taken into account in considering the heat flow values of Matsui et al.´s

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(1998) and Flores-Márquez et al.´s (1999) made on shallow bore-holes (Figure 4).

The study of the present thermal regime is underway. Thermal conductivity and related

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2004; Mayr et al., 2008; Popov et al., 2011).

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properties were measured from samples of the scientific drilling Yaxcopoil-1 (i.e., Popov et al.,

No deep enough boreholes have been drilled north of 21° of latitude (including the offshore

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area). The available boreholes drilled in the Chicxulub impact structure are not sufficient to

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map the heat flow variation across it.

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The depth to the Curie isotherm provides a proxy of the thermal regime at depth, and can be used to infer the heat flow from the mantle. The depth to the Curie isotherm is obtained from spectral analysis of aeromagnetic data, which provides an alternate and cheaper way to

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measure regional heat flow than through expensive drilling.

This method has been used to estimate the depth to the Curie isotherm at Yelowstone (Bhattacharyya and Leu, 1975a, b; Smith et al., 1974), and at the Vale-Owyhee, in Oregon State (Boler, 1978). At intermediate spatial scales, we mention regions of Utah and Wyoming (Shuey et al., 1977), in northern and central Arizona (Byerly and Stolt, 1977). At a regional scale, it has been applied in the Cascade Range province of Oregon (Couch, 1978; Couch et al., 1981; Connard and Gemperle, 1983), in Nevada (Blakely, 1988), at the island of Kyushu (Japan) (Okubo et al., 1985), eastern and southeastern Asia (Tanaka et al., 1999), California (Ross et al.,

ACCEPTED MANUSCRIPT 2006), and more recently in western USA (Boulingand et al., 2009). At a local scale in Cerro Prieto geothermal field Espinosa-Cardeña and Campos-Enríquez (2008) determined the depth

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to the Curie isotherm.

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Limitations of these spectral methods have been analized (i.e., Shuey et al., 1977) and recently

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addressed (i.e., Ross et al., 2006; Ravat et al., 2007; Boulingand et al., 2009). In particular, the presence of linear regional magnetic anomalies can affect depth determinations based in these spectral techniques (i.e., Shuey et al., 1977). In our study area this aspect must be taken into

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account because besides the sub-circular, 60-80 km wide magnetic anomaly associated with

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site of regional anomalies (Figure 2).

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the breccia and melt sheet of the Chicxulub impact crater, northern Yucatán platform is the

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García-Abdeslem and Ness (1994ab) estimated depths to the bottom of crustal magnetic

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sources of part of these anomalies located on the platform of the northern Yucatán peninsula (Figure 2). Their study area was limited to the north by the Campeche escarpment (see Figure 2) and the shallow waters of the Yucatán platform devoid of marine magnetic data. This area

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comprises regional magnetic anomalies (Figure 2). They modeled by inversion the radial averaged power-density spectrum of marine magnetic data acquired on the platform of Yucatán. Accordingly, depths to the base of the crustal sources range between 18 and 35 km (Figure 2). They mapped shallow crustal sources at the western portion of their study area in association with some of the regional magnetic anomalies featuring the Yucatán platform. This data set indicates the existence of a high heat flow zone at the western portion of the Chicxulub crater. The rest of the crater was not covered by this study.

ACCEPTED MANUSCRIPT Application of spectral techniques to the magnetic data from the North American Magnetic Anomaly Map (NMAG, 2002) (Figures 2 and 3) needs to be evaluated because of the quality of

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the data themselves (i.e., presence or regional components in the NAMAG data), and the

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limitations of the spectral methods (i.e., spectra distortion by regional linear anomalies). In our

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case, the existence of independent data can help to verify the applications of the spectral methods to data from the NAMAG map. The independent data comprise: 1) the depths to the base of magnetic sources of part of some of the regional lineal anomalies present around the

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Chicxulub impact crater (García-Abdeslem and Ness, 1994ab), 2) Heat flow direct measurements on the southern onshore part of this impact structure (Flores-Márquez et al.,

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1999¸ Wilhelm et al., 2004). In this context, we conducted a pilot study to test the feasibility of the spectral techniques to infer the heat flow across the Chicxulub impact structure. In this

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work we took into account: 1) the limitations of the spectral techniques recently analyzed (i.e.,

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Shuey et al., 1977; Maus et al., 1997; Ravat et al., 2007, Boulingand et al., 2009), 2) the effect

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due to the existence of regional lineal anomalies no directly related with the breccias and melt sheet from the impact structure.

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Spectral methods provide best results when applied to homogenous zones. Because of this we limited our study to the area comprising only the regional magnetic anomalies of the Yucatán platform (continental crust) and excluding marine magnetic anomalies of the Gulf of Mexico basin (oceanic crust). Thus, we analyzed aeromagnetic data from an area 256 km wide and 600 km long on northern Yucatán, to calculate the depth to the Curie isotherm. The results are referred to an east-west profile crossing the impact structure and also the regional magnetic anomalies from the Yucatán platform. Here we report the methods used and the heat flow variations across the Chicxulub structure. Results are correlated with the above mentioned independent determinations of depths to the base of the magnetic sources of the regional magnetic anomalies. This correlation also includes direct heat flow determination made on the southern crater portion as a way to test our results.

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MAGNETIC CRUSTAL NATURE AND THE CHICXULUB IMPACT STRUCTURE

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Figure 2 indicates the presence of different regional magnetic anomalies around the magnetic

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signature of the impact crater. The magnetic anomaly due to the impact structure has been

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analyzed (Pilkington and Hildebrand, 2000). Accordingly its high frequency part (wavelengths < 40 km) is due to the melt sheet located at a mean depth of 2 km. The low frequency portion (wavelengths > 40 km) is associated with the structural high and topography of the basement

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(with a mean depth of 5 km). In a recent forward modelling Ortiz-Alemán and Urrutia-

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Fucugauchi (2010), locate the deeper sources at 8 km.

Several magnetic anomalies with larger dimensions surround the 40 to 60 km in radius

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Chicxulub sub-circular anomaly (Figure 2). Immediately to the west there is a NW-SE linear

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regional anomaly (M1 in Figure 2, 360 long and 60 km wide) tangential to the crater to the southwest. 110 km to the east, there is another regional NE-SW magnetic anomaly (M2)

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(about 350 long and 60 km wide). To the north of the impact structure, there is a series of three major anomalies (M3, M4 and M5) feature the Yucatán platform.

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A portion of the above mentioned NW-SE trending linear regional M1 magnetic anomaly tangential to the crater was included in the 3-D inversion based magnetic model of Pilkington and Hildebrand (2000) and resulted in a large basement topographic relief. Pilkington and Hildebrand (2000), themselves consider it unrealistic since drill-hole data point to a gentler basement. They suggested that this anomaly is probably associated with an increased basement magnetization. These anomalies have wavelengths larger than that of the Chicxulub impact structure magnetic anomaly. Their source is related to the Yucatán crust nature that has been considered of a Gondwanan affinity (i.e., Weber and Kohler, 1999; Weber et al., 2006).

ACCEPTED MANUSCRIPT Mesozoic and Cenozoic sediments cover the basement in the Yucatán peninsula. Only in southern Yucatán, in the Maya mountains, crystalline rocks (diorite-granodiorite-granite) crop

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out (Steiner and Walker, 1996). Quartzite, quartz-mica schist, felsic-intermediate granitic

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gneiss, volcanic arc granitoids (quartz-diorite, granodiorite, and tonalite), and mafic arc

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volcanic rocks (basaltic andesite and olivine tholeiite have been identified in the breccia samples recuperated from the Yaxcopoil-1, and Yucatán-6 boreholes (Vera-Sánchez, 2000; Kring et al., 2004). This types of rocks would indicate a granitic nature for the subcrater

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basement.

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Kettrup et al. (2000) studied the precursors of the crater melt lithologies. 1) They concluded that the geology of the Chicxulub area is more complex than originally assumed. 2) The melt

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lithologies from the crater cannot be explained by a mixture from carbonaceous rocks and a

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homogenous granitic basement, but a mafic-to-intermediate component is necessary. This mafic-to-intermediate precursor component of the Chicxulub impact melt samples from

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Chicxulub-1 and Yucatán-6 boreholes is accounted by the presence of mafic rocks in the Yucatán basement (Kettrup et al., 2000). Similarly, Kring and Boyton (1992) proposed the

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presence in the crust of diabase, piroxenites, or amphibolite. Pan-African crystalline rocks (gneisses, phyllites and amphibolites) have been reported in the Deep Sea Drilling Project cores from leg 77 (sites 537 and 538) in southeastern Gulf of Mexico (i.e., Schlager et al., 1984). Recently Keppie and Keppie (2013) correlated the major NW-SE linear M1 magnetic anomaly with the Wichita gravity and magnetics highs and lows that coincide with Cambrian Wichita rift-related igneous rocks (Hanson et al., 2011). According to Keppie and Keppie (2013), the NESW easternmost linear M2 magnetic anomaly seems to continue into southern Laurentian in the Blue Ridge where highs and lows and magnetic highs correspond to Grenvillian mafic rocks (Hatcher et al.; 2004, Hatcher, 2005). The northern magnetic highs might continue into the

ACCEPTED MANUSCRIPT Reelfoot rift (Cambrian mafic rocks of the Mississippi Valley graben) high and magnetic low anomalies. Also according to Keppie and Keppie (2013) easternmost Yucatán is of Gondwanan

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affinity, while the western part is of Laurentia nature.

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METHOD

Spector and Grant (1970), by applying mechanical statistical principles, to the analysis of

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aeromagnetic data developed a method to establish the mean depth to the top of an ensemble of magnetic sources. When the parameters describing the magnetic sources of the

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ensemble are random independent variables obeying probabilistic distributions common to

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the entire set (i.e., uniformly distributed), then the radially averaged power spectrum can be

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expressed as:

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 ERP (k )   F (T (k ) RP

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  Cm

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4 2 M 2  e2 hk  (1  etk )2  S 2 (k , a, b) 

………………………………………………………………………………………..(1) The mean depth to the ensemble top is represented by h, its mean thickness by t, k represents the radial wavenumber (cycles/km), Cm = 10-7 to express equation (1) in SI units (Blakely, 1995). The mathematical expectation is expressed by

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S 2 (k , a, b)

represents

the effect of the body’s horizontal dimensions. M represents the effect of magnetization, its orientation and the regional field. Subscript RP express reduction to the pole, although Zhou

ACCEPTED MANUSCRIPT and Thybo (1998) found that the radial power spectra with and without reduction to the pole

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(RP) are virtually identical.

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. Under this

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For k less than 1/t, the following approximation holds

e2 hk  e2 hk

assumption, the contribution of the factor involving the thicknesses can be ignored. Finally if one also ignores the effect of the horizontal dimensions, then this exponential represents the

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major contribution to the power spectrum, and in a logarithmic representation, this factor approximates a straight line with slope mt, thereby providing a simple means to obtain the

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mean depth to the ensemble top.

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Ht = - mt /2π………………………………………(2)

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Okubo et al. (1985) proposed to modify the spectrum to obtain the mean depth to the

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centroid, hc:

1 H (k )  2 [

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 

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1 F (k , ) d k ]

(3)

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2

The average depth to the centroid bears a similar relationship with the slope of this modified spectrum (i.e., in a similar way this depth can be obtained). Finally, it is straightforward to calculate the mean depth to the bottom, hb, of the ensamble with the following relationship between these different depths (Battacharryya and Leu, 1977; Okubo et al., 1985):

hb  2hc  ht

(4)

ACCEPTED MANUSCRIPT 2 In reality the factor S (k , a, b) deforms the spectrum (Spector and Grant, 1970; Pedersen,

1978; Ruotoistenmaki, 1983; García-Abdeslem and Ness 1994a¸ Fedi et al., 1997), and it must

Ln ERP (k ) k 

(5)

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Ec (k ) 

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be corrected according to (Fedi et al., 1997):

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The above method, assumes that the magnetization is uncorrelated. The underlying model is a

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white noise one. However, it has been shown that the crustal magnetization has a fractal structure (i.e., Pilkington et al., 1994b). To account for this self similarity structure the spectra

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needs to be corrected by dividing the power spectrum by k-α .

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where α represents the scaling factor for a power-law decay rate of the power magnetic spectra due to a fractal magnetization, as has been shown with susceptibility logs (Pilkington and Todoeschuck, 1995), magnetic maps (Gregotski et al 1991; Pilkington and Todoeschuck,

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1993; Maus and Dimri, 1995, 1996; Pilkington et al., 1994b). The corresponding depth can be obtained from the slope of the logarithm of the corrected spectrum. A value α of 4 is considered for the lower crust (Maus et al., 1997). This last correction and that associated with the effect of the horizontal dimensions of the magnetic sources are similar. According to Fedi et al. (1997) it is difficult to know which one applies.

Maus and Dimri (1994) analyzed the relationship between the fractal natures of potential fields (i.e., gravity and magnetics) to that of density and magnetic susceptibility. Maus and

ACCEPTED MANUSCRIPT Dimri (1995) developed a half-space model of fractal magnetization. Finally, Maus and Dimri, (1996) proposed a slab model of fractal magnetization, and applied it to obtain the Curie depth

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(Maus et al., 1997). The model theoretical curve is fitted to that of the real data power spectre.

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al. (2009) refines this model and applies it to western USA.

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This model does not require the data be reduced to the pole (Maus et al., 1997). Bouligand et

Other methods are those of Ross et al. (2005) and García-Abdeslem and Ness (1994ab). Ross et al. (2006) fit manually, to real data spectrums, the theoretical curve underlying the Spector

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and Grant (1970)-Okubo et al. (1985) method. They change the depths to the top and bottom

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of the magnetic source ensemble until a fit is obtained. García-Abdeslem and Ness (1994ab) adjusted by a damped least square inversion procedure a theoretical model considering

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normal and Gaussian distributions of the body parameters of equation (1).

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Limitations of the spectral techniques are being analyzed. Shuey et al. (1997) already recommended avoid using windows where the autocorrelation function is not approximately circular. This effect might affect our depth determinations due to the presence of regional

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linear anomalies in the Yucatán platform. Ravat et al. (2007) recommended to use magnetic maps compiled from spherical harmonic degree 13 Earth´s main field models such as recent IGRF´s or Comprehensive models (Sabaka et al., 2012) These magnetic maps not include components from the core magnetic field. Ravat et al. (2007) also recommends no to use the spectrum very low portion in self-similarity, stratified magnetization cases. These aspects are taken into account below, in order to analyze their effects on our results.

A general recommendation is to use windows at least 10 times the depth to the bottom (i.e., Ravat et al., 2007; Boulingand et al., 2009). It is advised to start with large windows and

ACCEPTED MANUSCRIPT progressively to reduce the windows sizes to get a larger resolution (i.e., Ross et al., 2006;

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Ravat et al., 2007; Boulingand et al., 2009).

DATA AND PROCESSING

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We used an E-W, 256 km wide, and 600 km long grid of the Magnetic Anomaly Map of North America including the Chicxulub crater (North American Magnetic Anomaly Group (NAMAG,

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2002) (Figure 2). Marine and airbone magnetic surveys with different flight heights, line spacing, and processed with different IGRF models were used to elaborate on this map. Figure

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3a shows the tracks of the marine surveys covering our study area. The airbone magnetic survey covers the eastern two thirds of the study area (Figure 3b). It constitutes a good quality

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survey flown at heights ranging from 450 to 3,350 m, line spacings between 2 and 6 km, flown

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by PEMEX, N-S tie lines every 10 km.

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These different surveys were upward or downwards continued to a common height of 305 m above ground (i.e., sea level) NAMAG(2002). Ravat et al. (2007) analyzed the use of regional maps compiled from different magnetic surveys. He recommended using magnetic maps

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compiled from spherical harmonic degree 13 Earth´s main field models such as recent IGRF´s or Comprehensive models (Sabaka et al., 2002), to avoid having long wavelength residual components from the Earth´s core magnetic field. NAMAG map was compiled including spherical harmonic degree 10 Earth´s main field IGRF models. However, magnetic data from the CHAMP satellite were used to correct the data to avoid wavelengths larger than 500 km. NAMAG (2002) gives details of the compilation of this map. Working in south central USA, Ravat et al. (2007) found no large differences between a magnetic map reduced with a comprehensive model and the North America compilation. Boulingand et al. (2009), from tests conducted on a comprehensive-based magnetic map of Nevada and on the North America compilation points out that the North America Magnetic

ACCEPTED MANUSCRIPT Map can provide reliable determinations of the depths to the bottom of magnetic sources. They avoided wavelengths shorter than 1.4 rad/km. Manea and Manea (2010) used it to

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estimate depths to the Curie isotherm in central Mexico respectively with good results.

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We scanned this band with squared sub-grids of different sizes (100, 200, 250 km). For each window, the magnetic data were detrended and tapered to eliminate discontinuities between

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the magnetic values along the borders. In this way Gibbs effects were avoided. We did not

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filter the data as not to distort depths determinations as recommended by Ravat et al. (2007).

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Power spectrum were obtained by means of the Fast Fourier Transform (FFT) (i.e., Blakely,

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1995), then radially averaged. We analyzed the FFT and the data covariance (i.e., autocorrelation) from windows to check how much power spectrum distortion is caused by the

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linear regional anomalies of the Yucatán platform. Figure 5 shows the autocorrelation of 250 times 250 km and 200 times 200 km windows and containing in particular portions of the

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regional magnetic anomalies above mentioned. As can be seen the central, low frequency portions of the corresponding power spectrums are quite circular, i.e., they are not significantly distorted by the regional anomalies. Thus its central, low-to middle frequency portions can be used to estimate depths to the bottom and top of the sources of these regional anomalies. Figure 6 shows (filled circles) Fourier and the frequency scaled Fourier spectrums (i.e., according to Eq. (3)) corresponding to 200 km squared windows overlapped 50 % (i.e., 50 km), and covering the study area from west to east. As can be noted, towards the crater the power of the low-frequencies decreases. At the crater, the high frequency content increases. Qualitatively this indicates the magnetic sources are shallower beneath the crater.

ACCEPTED MANUSCRIPT In a first step, depths to the top of the magnetic sources were obtained by means of the Spector and Grant’s (1970) method. In the crater central portion(Figures 6d, 6e and 6f) depths

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range between 3 and 10 km in agreement with depth location of the magnetic sources (i.e.,

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impact breccia, melt sheet and deeper magnetic sources) as established by forward and

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inverse modelling (e.g., Pilkington and Hildebrand, 2000; Ortíz-Alemán and Urrutia-Fucugauchi, 2010). The first point is located to the west of the Campeche escarpment. This first estimation comprises the westernmost regional magnetic anomalies. Across the escarpment the long

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wavelength content decreases as can be observed in Figure 6. Respectively the depths to the top as well as to the bottom become shallower as we will below. The estimated depths were

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assigned to the central square points resulting in an east-west profile. Figure 2 shows such

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points located each 100 km.

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To estimate depths to the bottom of the magnetic sources, we first applied Okubo’s et al.´ (1985) centroid method (open circles on Figures 6a to 6g). Mean depths to the centroid (hc)

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and bottom (hb) of the ensemble of magnetic sources are indicated. Depths were also estimated with squared windows of 256, 128 and 100 km lengths.

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To consider effects of the magnetization self-similarity we estimated also depths according to Bansal et al.’s (2011) modified equations of the traditional centroid method. Figures 6a to 6g (stars) also shows these respective power spectrums and respective centroid depth estimations. Also the slab model of Maus et al. (1997) refined by Bouligand et al. (2009) was applied, in a forward modeling basis, to the Fourier power spectrum. Figures 7a to 7g. show the modeling results.

RESULTS AND DISCUSSIONS

Depth to the Curie isotherm

ACCEPTED MANUSCRIPT Curie depth along the resulting E-W profile cutting across the Chicxulub impact basin were constructed based on these respective depths (Figure 8). The different applied methods

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indicate the presence of a high beneath the impact crater. Depths to this high range from 10 to

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crater flanks the maximal depths vary between 20 and 40 km.

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30 km. To the east and west, this high is delimited by marked horizontal gradients. At the

Depths according to Okubo et al.s’ (1985) modified centroid method (i.e., according to Bansal et al., 2011) provides the most shallow depths. Somewhat larger depths are obtained by

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forward modelling according to Bouligan et al. (2009). Depths based on Okubo et al.’s (1985)

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centroid method with 256 times 256 km windows are the largest but of a lesser horizontal resolution. Windows of 200 times 200 km provide intermediate depths. Depths to the centroid

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from windows 100 times 100 km are unreliable indicating they do not contain information

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about the depth to the magnetic source bottoms. Depths based on Okubos et al.’s (1985) centroid method with 256 times 256 and 200 times 200 km windows range from 28 to 41 km.

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Beneath the crater, windows of 256, 200, and 128 km lengths, all provided shallow depths to

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the bottom of magnetic sources.

To the east of the central high, the horizontal gradient is sharp, at about 500 km, the isotherm falls from 10-20 km depths to 30-40 km (Figure 8). On the western side the gradient is smoother, the Curie isotherm climbs from 23 to about 10 km over a distance of approximately 200 km. The large horizontal gradients delimiting it coincide with the limits of the impact structure. This high correlates fairly well with the impact basin. Figure 8 also shows, in the westernmost portion, the depths to the bottom of crustal magnetic sources obtained by Garcia-Abdeslem and Ness (1984ab) which in general range between 8 and 11 km. García-Abdeslem and Ness (1984ab) used marine magnetic data, windows of 128 times 128 km, and an independent spectral method. In the common portion of their study and

ACCEPTED MANUSCRIPT our profile (i.e. Figure 2) the western gradient, and part of the high are also observed in GarciaAbdeslem and Ness (1984ab) data. Thus a very good correlation is observed between these

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two independent data sets.

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Outside the crater, depths to the Curie isotherm correlate with mantle depths as determined

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by seismic studies (i.e., Christeson et al., 2009; Christeson et al., 2001; Vermeesch and Morgan, 2008; Gulick et al., 2013). In the crater area, the Curie depths are located at the base of the

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upper crust and upper lower crust.

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Heat flow

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To estimate the heat flow, depths to the Curie isotherm were corrected for flight height, and

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depth to sea floor to obtain the geothermal gradients. A Curie temperature of 580 °C,

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corresponding to magnetite, was used in these calculations. Accordingly, beneath the centre of the impact basin, geothermal gradients between 58 and 19 °C/km are obtained, and if a thermal conductivity value of 2.67 W/m-K is assumed as

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representative of the mean crustal thermal conductivity, corresponding heat flow values ranging from 156 to 51 mW/m2 are inferred beneath the impact structure (Figure 9). The heat flow varies between 77 and 38 mW/m2 to the east and to the west. Heat flow corresponding to a 10 km deep Curie isotherm is too high and not supported by direct measurements (i.e., Matsui et al., 1989; Flores-Márquez et al. 1999; Wilhelm et al., 2004). On this base we discard the shallower Curie depths. A heat flow around 75 mW/m2 obtained from Curie depths of about 20 km correlates with the values reported by Wilhelm et al. (2004) (between 65 and 80 mW/m2 ), but are somewhat higher than the heat flows reported by Matsui et al. (1998) and Flores-Márquez et al. (1999) (about 60 mW/m2 ). We must recall that the heat flow values estimated from Curie depth

ACCEPTED MANUSCRIPT represent the heat flow coming out from the mantle and are of regional nature (i.e., averages values). In contrast, surface measurements such as those of Matsui et al. (1998), FloresMárquez et al. (1009) and Wilhelm et al. (2004) also include the crustal radioactive

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contribution to the surface heat flow, which can be as large as 30 mW/m2.

Depths to the bottom of crustal magnetic sources, reported by Garcia-Abdeslem and Ness´s

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(1994ab), that we interpret as depths to the Curie isotherm, were converted into heat flow values (Figure 9). A heat flow value of 86 mW/m2 is obtained for the northwestern crater

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portion. The western portion of the heat flow high is also delimited by García-Abdeslem and Ness (1994b). Again a very good qualitative correlation is observed for that portion common to

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our profile.

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Figure 4 shows the heat flow map obtained from the values listed in Table 3 of Flores-Márquez et al. (1999) and the value of Wilhelm et al. (2004). Accordingly, about 20 km south of crater center, there are heat flow values of about 80 mW/m2. Southwards, the heat flow values

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decrease and constitute a belt shaped low along the cenote ring. These low values were obtained from shallow bore-holes and certainly affected by circulation of groundwater associated with the cenote ring. The pattern of the heat flow values based in García-Abdeslem and Ness´s (1994b) data and the direct determinations based heat flow configuration of Figure 4 do not match very well. This can be due to their different nature already commented. However both of these independent data sets point out to the existence of a heat flow high at the northwestern (offshore) and southeastern (onshore) portions of the impact structure, and thus lend support to our inference a high heat flow encompassing the impact structure.

ACCEPTED MANUSCRIPT Let us note that only a rough correlation of the heat flow determined by García-Abdeslem and Ness (1994ab) and in this study with gravity and magnetic data is possible because spectral

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methods provide mean values and not punctual determinations. However, as can be observed

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in Figure 4 this heat flow high correlates with the northwestern crater basin. It is located

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between regional linear magnetic anomalies M1, M2, and M3. It correlates fairly well with a zone featured by low magnetic values. It is located approximately above the northwestern gravity high (see Figure 1). In the study of García-Abdeslem and Ness (1994b), lack of magnetic

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data did not enable to study the shallow portion of the Yucatan platform, and the configuration is open to the southeast. However, this study indicates the heat flow might

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extend across the crater center. This heat flow high is in agreement with the depths established through forward and inverse modeling of the sub-circular magnetic anomaly

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located in the crater center (i.e., , Pilkington and Hildebrand, 2000;

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HEAT FLOW CRUSTAL CONTRIBUTION

A roughly estimation of the heat production due to the crust radioactivity can be obtained

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using the method of Čermak et al. (1991). This estimate was based on a linear velocity gradient between the depth of 10 km and the Moho, where Christensen et al. (2001) report seismic velocities of 5.6 and 6.8 km/s respectively. According to Čermak et al. (1991) a velocity of 5.6 km/s at a depth of 10 km would correspond a radioactive heat generation of 1.0 μ mW/m3. A velocity of 6.8 km/s at a depth of about 32 km, corresponds to a radioactive heat generation of 0.2 μ mW/m3.According to Poisson´s equation, for the case of a constant radioactive heat flow generation, and heat flows values of 50 and 80, the mean crustal radioactive heat generation has respectively values of 0.9 to 1.5 μ mW/m3. For example, in order to match an estimation of 20 km deep Curie isotherm with a surficial heat flow of 80 mW/m2, the heat generation should be about 1.5 μ mW/m3. Figure 10 shows the geotherm corresponding to a constant crustal

ACCEPTED MANUSCRIPT heat generation and surficial heat flow values of 45, 70 and 80 mW/m2, and respective radiogenic heat generations of 1.5 μ mW/m3. The geotherm for a heat flow of 45 mW/m2 corresponds to the estimated heat flow immediately outside the impact structure.

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.

Accordingly, the heat flow produced, in the impact structure, due to radiogenic heat sources

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located between 10 km depth and the crust base amounts approximately to 13 mW/m2. These values are similar to those observed at the Vredefort structure (Hart et al., 1981). This value is

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to be added to our estimate of the heat flow coming out from the mantle, and we obtain heat flow values ranging from 169 to 64 mW/m2 a fair correlation with the high precision heat flow

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from Wilhelm et al. (2004). The correlation is very good with a Curie depth of about 20 km.

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The difference observed between the possible geotherms inside the crater and those outside it indicate different rheologic properties. The sharp decrease in heat flow at the eastern portion

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of the impact structure would imply lower temperatures in the crust in relation to its western portion where higher crustal temperatures are associated with higher heat flow values. The difference in thermal regime correlates with the easternmost Yucatán being of Gondwanan affinity, while the western part being of Laurentian nature (Keppie and Keppie, 2013). Accordingly, we would have lower crustal temperatures at relatively greater depths to the east of the crater than to the west. Correspondingly these different thermal states would partially favor a brittle rheology to the east, and a ductile regime to the west. In this way, it can be accounted for the presence, to the west, of numerous faults that sole in a horizontal surface as reported by Gulick et al. (2008; 2013), resembling a decollement (i.e., a

ACCEPTED MANUSCRIPT ductile regime). On the east a cooler crust would enable the brittle regime to extend deeper

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(i.e., sub-vertical normal faults) as reported by Gulick et al. (2008:2013).

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ORIGIN OF THE HEAT FLOW HIGH

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A number of factors could be contributing to the elevated heat flow. The source of the anomaly might be associated with one or a combination of the following mechanisms; 1) an uplift of the mantle lithosphere, 2) distribution of radiogenic elements in the crust and upper

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mantle, 3) an increase of thermal conductivity at deeper levels.

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Crust and mantle uplift.

Seismological studies provided the first images of the central uplift (Christeson et al., 2009;

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Christeson et al., 2001; Vermeesch and Morgan, 2008; Gulick et al., 2013). In particular, Christeson et al. (2001) reported a minimum crustal uplift of 10 km. Seismic tomography

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(Vermeesch and Morgan, 2008) corroborates the existence of such a crustal 10 km uplift.

Unsworth et al. (2002) and Campos-Enríquez et al. (2004) reported resistivity images of the crust and upper mantle (Figures 1 and 10) along three MT profiles. Accordingly, the rocks beneath the impact crater are featured by low-resistivity rocks (Figures 1 and 11). At present the crust is a 30 km thick normal continental crust and locally the mantle is as shallow as 28 km (i.e., along BIRPS seismic lines B and F and A-A1) (Figure 1) (i.e., Christeson et al., 2009; Christeson et al. 2001). This small scale mantle upheaval might contribute to focus the mantle heat flow in the impact structure through the structural high. This lithosphere upheaval would be an expression of the root of the central structural high that formed in the

ACCEPTED MANUSCRIPT first seconds of the impact itself, an expression of the lithosphere rebound. It is thought that the shallow portion of this structural high may have collapsed afterwards (Melosh, 1989;

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Collins et al., 2008; Gulick et al., 2008).

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Distribution of radiogenic elements in the crust and upper mantle.

Interpretation of the velocity distribution around the central structural high, indicates that the

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upper uplift varies laterally: lower crust-middle crust rocks are surrounded by upper crustal rocks (Vermeesch and Morgan 2008; Vermeesch et al., 2009). This upturning and overturning

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would be similar to that observed in the updomed Precambriam basement of the Vredefort

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structure (Hart et al., 1981). Where middle and upper Precambrian crust (corresponding to a depth profile of about 15 km) is exposed to view, and had enabled to study the distribution

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with depth of heat producing elements (Hart et al., 1981). Thus, it is possible that the impact might have recycled radioactive elements in the crust and

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upper mantle and concentrated them in the central uplift underneath the impact structure. The here estimated high heat generation values indicate that the lithospheric root of this central uplift might concentrate radiogenic elements, and contribute to the existence of this heat flow high.

It is expected that meanwhile the peninsula should have recovered or approached its isostatic equilibrium. Nevertheless incomplete isostatic equilibrium of the Yucatán peninsula is indicated by the admittance function along a NW-SE gravity profile across the impact structure (Campos-Enríquez et al., 1996). Since a conspicuous crustal root has not been observed

ACCEPTED MANUSCRIPT beneath the impact structure, the source of the isostatic decompensation might be in the crust. This above mentioned mass distribution might account partially to the concentration of

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heat producing radioactive elements but also to the inferred isostatic decompensation.

Thermal conductivity

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Thermic properties determined at Puchezh-Katunk impact structure indicate that thermal metamorphism tend to decrease the thermal conductivity (Popov et al., 1998). But at the same

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time, thermal conductivity shows a general increase with depth. At this impact structure, from 3,300 m and downwards, this metamorphism reducing effect gradually diminishes.

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Nevertheless, in general, conductivity remains somewhat lower than that of rocks outside the

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impact structure. For example, at the deepest levels, the gneiss conductivity remains somewhat lower relative to normal metamorphic rocks. Worth noting that due to the 5 km

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uplift, lithologicaly, the Vorotilovo borehole at Puchez-Katunk cuts Archean rocks otherwise

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emplaced at 10-11 km depth.

This reducing effect is also observed at Ries (Popov et al., 2003) and at Chicxulub craters. At Chicxulub, petrophysical studies (Mayr et al., 2008; Popov et al., 2011) indicate the shockthermal metamorphism tends to diminish the thermal conductivity of the suevite and brecciated impact melt, but not of the lower suevite (Mayr et al., 2008; Popov et al., 2011). Contrastingly, no impact effects were measured on the thermal conductivity at the preimpact target rocks (i.e., Cretaceous mega blocks) (Mayr et al., 2008; Popov et al., 2011).

ACCEPTED MANUSCRIPT Similar to the observations at Puchez-Katunk impact structure (Popov et al., 1998) and at Ries crater (Popov et al., 2003), the metamorphism reduced thermal conductivity of rocks cut by

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the Yaxcopoil-1 borehole also presents a general increase, between 1.6 and 5.0 W/m-K, with

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depth (in the 404-1300 m depth range). If this depth trend is sustained, we might expect that

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the metamorphism effects disappear at the base of the upper crust, and that higher thermal conductivities in lower crust-upper mantle might be focusing the lithospheric heat flow into

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the impact structure.

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CONCLUSIONS

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We infer the existence of a high heat flow anomaly centered on the Chicxulub impact structure. First we estimated the depth to the Curie isotherm along an east west, 660 km long

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profile across the impact structure. These depths were corrected for flight height, and depth to sea floor to obtain the thermal gradient assuming a 580 °C Curie temperature. Finally, heat

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flow was obtained by considering a mean crustal thermal conductivity of 2.67 W/m-K. The heat flow amounts to approximately 80 mW/m2. Available heat flow measurements done in boreholes, and depths to the base of magnetic sources in the Yucatán platform determined previously, support the existence of this heat flow high. Among the factors that might be contributing to this high heat flow is a 10 km of the crust, and a mantle uplift beneath the crater, and an impact associated heat generating radioactive element concentration beneath the crater. A higher thermal conductivity is also expected at deeper levels. Available seismological and thermal properties data are compatible with these mechanisms.

ACCEPTED MANUSCRIPT ACKNOWLEDGMENTS

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We acknowledge two anonymous reviewers for their critical reviews that helped to improve

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this paper. Ramón-Márquez, V.M and Cristobal López-Capir elaborated the figures.

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Figure 1.- Location of study area in northern Yucatan Peninsula. The background represents the Bouguer anomaly. The red line indicates the location of the E-W profile analyzed in this study (red dots indicate the points where the depth to the Curie isotherm was estimated). Green and yellow dotted lines onshore represents location of MT line 1, and 2 of Unsworth et al. (2002), and of Campos-Enríquez et al. (2004). White dotted lines indicate location of offshore ocean bottom seismometers and onshore PASSCAL receivers along the offshore and onshore seismic lines (i.e., Morgan and Warner, 1999a). Offshore, the seismic lines are indicated by straight black lines. Blue lines indicate location of exterior ring and inner rim of the impact structure (after Gulick et al., 2013). Black squares indicate location of boreholes drilled by PEMEX: S1: Sacapu-1, C1: Chicxulub-1, T1: Ticul-1, Y1: Yucatan-1, Y2: Yucatán-2, Y4: Yucatan-4, Y5A: Yucatán-5A, Y6: Yucatán-6.

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Figure 2.- The study area and the Magnetic Anomaly Map of Gulf of Mexico and surrounding area (NAMAG, 2002). Dotted line indicates location of the E-W profile analyzed in this study. (red dots indicate the points where the depth to the Curie isotherm was estimated). Dots are 100 km apart. Blue rectangle indicate limits of the magnetic area analyzed in this study. M1, M2, M3, and M4 represent regional magnetic anomalies referred to in the text. Isolines indicate depth (in km) to the bottom of crustal magnetic sources as determined by GarcíaAbdeslem and Ness (1994b). White lines indicate location of exterior ring and inner rim of the impact structure (after Gulick et al., 2013). Iso-baths of 200 and 300 m indicates the Campeche escarpment. Figure 3.- a) The lines inside the white rectangle represent track-lines of magnetic marine studies contributing to the Magnetic Anomaly Map of the study area (NAMAG, 2002). Regional magnetic anomalies are labeled M1, M2, M3, M4 and M5 as referred to it in the text. B) aeromagnetic survey of northern Yucatan also contributing to the Magnetic Anomaly Map (NAMAG, 2002). Limits of the airborne magnetic survey is indicated with a yellow line in Figure 3a).

Figure 4.- Isolines offshore represent heat flow (in mW/m2) obtained from the depth to bottom of magnetic sources of García-Abdeslem and Ness (1994b). Isolines onshore represent heat flow values as obtained from available direct heat flow determinations in boreholes. Y1: Yaxcopoil-1 (Wilhelm et al., 2004), and by Flores-Márquez et al., 1999) in wells U1: UNAM-1, U2: UNAM-2, U3: UNAM-3, U4: UNAM-4, U5: UNAM-5, U6: UNAM-6, U7: UNAM-7, U8: UNAM8. Matsui et al. (1998) determined heat flow at wells A0, A1, A2, A4 and A6. The analyzed profile is also indicated in red. (dots are 100 km apart and indicate the points where the depth

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to the Curie isotherm was estimated). Dots are separated 100 km. White lines indicate location of exterior ring and inner rim of the impact structure (after Gulick et al., 2013). Iso-baths of 200 and 300 m indicates the Campeche escarpment.

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Figure 5.-Autocorrelation of magnetic anomalies included in square windows centered at points of Figure 1. Sizes of windows are 250 km (left) and 200 km (right). Upper panel corresponds to westernmost position (i.e., a is located at 0 km). Bottom panel corresponds to easternmost position (g is located at 600 km).

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Figure 6.- Fourier (normal) power spectrum is indicated by filled circles. Power spectrum modified according to Okubo et al. (1985) is represented by stars. Power spectrum modified according to Bansal et al. (2011) is represented by open circles. Segments used in the depth determinations and depth (in km) to the top (hT), centroid centroid (ho , hC) and bottom (hB) of the magnetic sources are indicated.

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Figure 7.- Application of the method of Bouligand et al. (2009). Fitting to the low wave length portion of the normal power spectrum is shown.

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Figure 8.- Depth to the Curie isotherm along the studied profile. Location in Figures 1 and 2. Depths obtained according to Bouligand et al. (2009) are represented by squares, Okubomodified according to Bansal et al. (2011) (triangles), García-Abdeslem and Ness (1994b (stars), Okubo with a 200 km square window (filled circles), Okubo with a 250 km square window (rombedrals).

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Figure 9.- Heat flow along the studied profile. Location in Figures 1 and 2. Inferred from depths obtained according to Bouligand et al. (2009) are represented by squares, Okubo-modified according to Bansal et al. (2011) (triangles), García-Abdeslem and Ness (1994b (stars), Okubo with a 200 km square window (filled circles), Okubo with a 250 km square window (rombedrals).

Figure 10.- Geotherms obtained for constant radiogenic heat generation of μ 1.5 mW/m3, and surficial heat flows of 45 (green), 70 (blue) and 80 (yellow) mW/m2. A mean crustal thermal conductivity of 2.67 W/m-K was used. The geotherm corresponding to a heat flow of 45 is representative of the exterior of the impact structure. Figure11.- Electrical resistivity distribution in the crust and upper mantle according to the MT study of Unsworth et al. (2002) and Campos-Enríquez et al. (2004). Top: MT line 1 of Unsworth et al. (2002); Middle: MT line 2 of Unsworth et al. (2002); Bottom: MT line of Campos-Enríquez et al. (2004). Locations are indicated in Figures 1 and 6. On the structural high in each of these MT profiles is indicated the top and southeastern limit of the structural high imaged by seismic studies (Christeson et al., 2001; Veermesch and Morgan , 2008).

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Highlights

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A high heat flow is inferred at the Chicxulub impact structure (northern Yucatan) based on a spectral analysis of aeromagnetic data.