Goguitchaichvili Et Al..Fm

Paleomagnetism and Rock Magnetism of the Jurassic La Negra Formation, Northern Chile: Implications for Tectonics and Volcanic Stratigraphy

AVTO GOGUITCHAICHVILI, LUIS M. ALVA-VALDIVIA, AND JAIME URRUTIA-FUCUGAUCHI1 Instituto de Geofísica, Universidad Nacional Autónoma de México, Laboratorio de Paleomagnetismo y Geofisica Nuclear, Ciudad Universitaria, 04510 Mexico D.F. Mexico

Abstract

A detailed paleomagnetic investigation was carried out on the Jurassic La Negra Formation. A total of 32 consecutive lava flows (about 280 standard paleomagnetic cores) were collected. Several rock-magnetic experiments were carried out in order to identify the magnetic carriers and to obtain information about their paleomagnetic stability. Continuous susceptibility measurements with temperature yield in most cases reasonably reversible curves with Curie points close to that of Ti-poor titanomagnetite. Titanomaghemites seem to be the main magnetic carrier in a few cases. Judging from the ratios of hysteresis parameters, it seems that all samples fall in the pseudo-single domain grain-size region, probably indicating a mixture of multidomain and a significant amount of single- domain grains. Twenty-five flows yielded reliable paleomagnetic results. The mean paleodirection obtained in this study is I = –30.7, D = 11.3°, k = 18, α95 = 6.8°, N = 25. Combining all currently available data, including those obtained in previous studies for the La Negra volcanic sequence, we obtained a well-defined mean paleomagnetic direction with I = -33.9°, D = 10.5°, k = 15, α95 = 6.1°, N = 40. These directions are in good agreement with the expected paleodirections at about 180 Ma, which indicates that studied units do not show evidence for significant tectonic rotation. Fifteen sites yield reverse polarity magnetization and 11 flows are normally magnetized. The tentative direct magnetostratigraphic correlation suggests that La Negra volcanics have been emplaced during a relatively large time span of about 5 m.y.

Introduction Carey (1958) and later Isacks (1988) showed that a slight original curvature of the Andean continental BLOCK ROTATIONS ABOUT VERTICAL AXES have been margin was enhanced by differential shortening dur- shown to play an important role in accommodating ing Neogene time, implying rotation of both limbs. lithospheric plate deformations (e.g., Kissel and Laj, The expected rotations are estimated to be less than 1997). Paleomagnetic declinations proved to be a 10° for the southern limb and 10°–15° for the north- good indicator of this kind of rotation (Roperch and ern limb (Arriagada et al., 2002). New paleomag- Carlier, 1992; Roperch et al., 2000; Arriagada et al., netic studies indicate variable amounts of rotation 2000, 2002). predicted by oroclinal bending. The exact origin and Recent paleomagnetic studies have shown that mechanism of rotations, however, is still controver- tectonic rotation is one of the major characteristics sial (Beck, 1998). As emphasized by Arriagada et of the structural evolution of Central Andes (Arria- al. (2000), in most of the published studies the lack gada et al., 2000, 2002). These works demonstrate of detailed geology and restricted paleomagnetic that vertical axis rotations are decisive components sampling impede an understanding of the age of of deformation for the Andes as a whole (Somoza et rotation and the size of rotated blocks. al., 1999; Somoza and Tomlinson, 2002). The most The La Negra Formation, a 3000–10,000 m thick fascinating feature of the entire Andean chain is volcanic sequence, belongs to the coastal Cordil- most likely the Bolivian Orocline, or the change in lera. It is composed mainly of subaerial lava flows trend of the Andes from NW-SE to N-S near 18°S. and occasionally of breccias of basaltic-andesitic composition. The well-established stratigraphic 1Corresponding author; email: [email protected]. position and the easy access of these basalts makes unam.mx them a good target for paleomagnetic studies, with

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FIG. 1. A. Sketch map of northern Chile illustrating the locality of paleomagnetic sampling (modified from Rogers and Hawkesworth, 1989). Legend: 1 = La Negra Formation; 2 = Gatico intrusion (~158 Ma); 3 = Tocopilla intrusion (~155 Ma). B. Simplified stratigraphy of the Antofagasta region, northern Chile, with the principal names of geological formations (redrawn from Arriagada et al., 2002). the purpose of determining to what extent this suc- constraints on paleotectonic evolution of the Central cession has been affected by tectonic rotation about Andes. These data may also provide better con- a vertical axis and thus providing some decisive straints on the age of the rocks. The most serious JURASSIC LA NEGRA FORMATION, NORTHERN CHILE 565

TABLE 1. Site Mean Paleodirections of Cleaned Remanence and Corresponding VGP Positions for La Negra Volcanics1

Site n/N Dec Inc α95 k Pol Plat Plong

NE32 0/6 – – – – ? – – NE31 3/7 4.1 –41.8 13.3 21 N 85.7 48.5 NE30 6/6 354.2 –17.8 5.1 142 N 75.9 265.3 NE29 6/6 355.6 –25.6 4.5 178 N 80.5 263 NE28 6/7 10.3 –38.7 3.4 314 N 80.4 20.2 NE27 1/7 32.3 –48.1 – – N? 60.1 40.2 NE26 6/6 187.3 18.9 5.5 120 R –75.8 140.5 NE25 5/7 203.7 39.8 11.3 29 R –8.1 205.8 NE24 6/6 232.7 46.3 6.5 88 R –2.6 217.7 NE23 5/6 217.2 35.7 8.4 54 R –55.3 203 NE22 7/7 178.6 22.4 5.6 117 R –79.5 102.4 NE21 2/7 346.4 –28.9 – – N? 75.5 224.9 NE20 6/6 5.4 –18.8 5.4 128 N 76.5 313.3 NE19 7/7 11.3 –36.2 6.9 76 N 79.3 11.2 NE18 6/6 358.2 –30.2 8.1 56 N 83.9 273.4 NE17 6/6 185.6 40.9 7.1 75 R –84.7 215.3 NE16 6/8 16.4 –35.6 6.6 91 N 74.5 13.9 NE15 10/11 355.4 –22.4 3.1 386 N 78.6 266.3 NE14 8/8 215.6 –6.3 6.4 89 R –47.2 168.3 NE13 0/6 – – – – ? – – NE12 0/6 – – – – ? – – NE11 6/6 203.4 57.3 4.3 159 R –64.4 143.3 NE10 5/6 188.6 40.3 7.8 98 R –82 207.8 NE09 6/6 174.6 28.5 6.3 128 R –81.4 72.5 NE08 4/6 190.3 –4.7 6.4 126 R –63.4 133.5 NE07 1/6 158.9 19.3 R? –66.4 47.6 NE06 6/7 189.9 43.3 10.3 43 R –80.4 279.6 NE05 5/6 193.7 17.4 8.5 63 R –71.4 156.9 NE04 6/6 193.1 21.5 9.6 45 R –73.4 160.8 NE03 2/6 35.2 –40.6 N? 57.6 28.7 NE02 6/6 14.8 –49.4 5.5 148 N 74.4 54.5 NE01 5/6 11.2 –31.2 8.4 65 N 78.2 355.3

1Abbreviations: N = number of treated samples; n = number of specimens used for calculation; Dec = declination; Inc = inclination; k and α95: = precision parameter and radius of 95% confidence cone of Fisher statistics; Pol = magnetic polarity, Plat/ Plong = latitude/longitude of VGP position. 566 GOGUITCHAICHVILI ET AL.

paleomagnetic study of the La Negra Formation ally over a few tens of meters and in these cases we (Antofagasta region) was conducted by Arriagada et drilled typically 6–11 standard paleomagnetic cores al. (2002). However, only a few sites (8 at Tocopilla per flow. The samples were distributed throughout and 7 at Antofagasta) yielded reliable paleomag- each flow both horizontally and vertically in order to netic results, which may be considered as insuffi- minimize effects of block tilting and lightning. Cores cient; thus, a more robust data set is greatly needed. were obtained with a gasoline-powered portable In this study, we performed a detailed rock-mag- drill and then oriented with both magnetic and sun netic and paleomagnetic study on 32 consecutive La compasses. Negra lava flows. The principal objective of the present study is to increase the amount of available Laboratory Techniques data on that region, in order to obtain more information about its paleotectonic development. A second Rock-magnetic experiments included: (1) mea- objective is to constrain the age of the studied for- surements of the viscosity index; (2) measurements mations, the duration of volcanic activity, and of both low- and high-temperature continuous ther- regional lateral correlation by means of magneto- momagnetic curves (susceptibility versus tempera- stratigraphy. ture); (3) hysteresis experiments; and (4) IRM (isothermal remanent magnetization) acquisition Sampling Details curves.

The La Negra Formation (Figs 1A and 1B) com- Short-term magnetic viscosity measurements prises a thick pile of lava flows, largely of high-K Determination of the viscosity index (Prévot et basaltic andesites, with thin intercalated sediments. al., 1983) makes it possible to estimate the capacity It seems that they erupted in an extensional environ- of samples to acquire a viscous remanent magnetiza- ment (Rogers and Hawkesworth, 1989) and that, in tion, and is therefore useful in obtaining information addition to their chemical similarity to the Puente on their paleomagnetic stability. For this purpose, Piedra volcanics in Peru (Atherton et al., 1983) and we placed the samples during two weeks with one of the lavas at Bustamante Hill in central Chile (Levi their axes aligned with Earth’s magnetic field. After et al., 1982), suggests that they were erupted in an their magnetization (Md) was measured, they were ensialic backarc basin rather than in an actual vol- placed for another two weeks in a field-free space, canic arc. The La Negra Formation is intruded by an and the magnetization (M0) was measured again. elongated batholith of middle Jurassic age that This enabled us to calculate the viscosity index V = yields uniform Rb-Sr whole rock ages between 158 [(Zd – Z0) : Mnrm ] × 100, where Zd and Z0 are the and 154 Ma (Rogers and Hawkesworth, 1989). magnetization components of Md and M0 , respec- Paleomagnetic sampling was conducted near the tively, which are parallel to the magnetizing field. town of Tocopilla (Fig. 1A). Rogers and Hawkes- Mnrm is the intensity of natural remanent magnetiza- worth (1989) proposed a mean age of 187 Ma as the tion. All samples were subjected to these experi- best estimate of the time of emplacement of the ments and although viscosity indexes varied lower La Negra volcanic sequence. It is only regret- between 0.8 and 122.4, most values were lower than table that they do not report the error on their K-Ar 10%. The higher values (above 40%) belong to sites age. We note, however, that the sample they dated NE03, NE12, NE13, NE21, NE27, and NE32. No belongs to site NE06 of our sample (Table 1). reliable paleomagnetic directions were obtained In total, we collected about 280 oriented samples from these sites (Table 1). belonging to 32 consecutive lava flows. Tilt control was based on our field observations and on Arria- Continuous susceptibility curves gada et al. (2002). Both studies are in perfect agree- Low-field susceptibility measurements (k-T ment. Variable bedding attitude parameters exist for curves) under air were carried out using a High- the Antofagasta area—strike varies from 160° to moore susceptibility bridge equipped with furnace 205° and dip varies from 30° to 45° from site to site. in a Mexico City laboratory. One sample from each For the Tocopilla area (our sampling area) a very site was heated to ca. 600°C at a heating rate of simple situation exists. The bedding attitude param- 20°C/min and then cooled at the same rate. The eters are the same for all studied lava flows (strike/ Curie temperature was determined by Prévot et al.’s dip = 0/30). Commonly, the outcrops extend later- (1983) method. Alternatively, low-temperature (from JURASSIC LA NEGRA FORMATION, NORTHERN CHILE 567

FIG. 2. Representative susceptibility versus temperature curves. The arrows indicate the heating and cooling curves (see also text).

about –185°C to room temperature) susceptibility note that we were not able to observe the Curie point was recorded using the same apparatus (Fig. 2). of (titano)hematite on the k-T curves. Two different types of behavior were observed For the remaining samples, a quite well defined during low-T susceptibility experiments (Fig. 2, maximum was detected (sample NE09-055C) at left). More than half of the studied samples exhib- about –63°C. This temperature is substantially dif- ited a rather well-defined peak around –145°C ferent from the Morin transition observed for hema- (sample NE04-024C), indicative of magnetite or Ti- tite (around –15°, after Dunlop and Özdemir, 1997). poor titanomagnetite (Özdemir and Dunlop, 1993). It should be noted that there is a great deficit in The corresponding high-T susceptibility curve indi- rock-magnetic literature about the interpretation of cates the presence of a single magnetic/ferrimag- low-temperature continuous susceptibility curves netic phase with Curie point compatible with from natural samples. Two milestone papers relatively low-Ti titanomagnetite (Fig. 2, right). (Rhadakrishnamurty et al., 1981 and Senanayake, However, the cooling and heating curves are not 1981) yield very contradictory results. More recent perfectly reversible because of the relatively low works (Özdemir and Dunlop, 1993, see also Dunlop signal of initial magnetic susceptibility. Let us also and Özdemir, 1997) show only results from chemi- 568 GOGUITCHAICHVILI ET AL.

FIG. 3. Typical examples of hysteresis loops (corrected for di/paramagnetism) of small chip samples from the studied volcanic flows and associated isothermal remanence acquisition curves. cally well-identified synthetic (titano)magnetites. cal remagnetization by maghemitization have the Corresponding high-T susceptibility curves yield same direction as the original TRM. Consequently, two different thermomagnetic phases during heating paleodirections were most probably unaffected by (Fig. 2). The lower Curie point ranges between 350° alteration and they can be used for determining and 420°C, while the highest one is ca. 580°C. The magnetic polarities and mean paleodirections. cooling curve shows only a single phase, with a Curie temperature close to that of magnetite. Such Hysteresis properties irreversible k-T curves can be explained by tita- Hysteresis measurements at room temperature nomaghemite, which probably was transformed into were made on small chip specimens. These experi- magnetite (Readman and O’Reilly, 1972; Ozdemir, ments were carried out in fields of up to 1.3 Tesla 1987) during heating. It is possible that these sam- (Fig. 3). The hysteresis parameters (saturation rema- ples have chemical remanent magnetization. How- nent magnetization Jrs, saturation magnetization Js, ever, both experimental and theoretical studies and coercive force Hc) were calculated after correc- (Heider and Dunlop, 1987; Nishitani and Kono, tion for the paramagnetic contribution. Coercivity of 1989; Ozdemir and Dunlop, 1989) show that chemi- remanence (Hcr) was determined by applying a JURASSIC LA NEGRA FORMATION, NORTHERN CHILE 569

171A, NE25-182A). The secondary remanences were removed by applying 40 mT alternating magnetic field. The extreme case is shown in Figure 5 (sample NE25-182A), when the application of a 60– 70 mT field was necessary to obtain the characteristic remanent magnetization. Essentially single and stable magnetization components were observed occasionally (Figure 5, sample NE2-011C and NE31-217A). The median destructive fields (MDF) range mostly from 30 to 50 mT, suggesting pseudo- single domain grains as remanent magnetization carriers (Dunlop and Özdemir, 1997).

Results and Discussion The average flow paleodirections are well determined overall (Table 1, Fig. 6A). Almost all α95 values are less than 10°, except the flows NE31 and FIG. 4. Day plot (Day et al., 1977) with hysteresis parameter ratios. NE25. Due to the unstable paleomagnetic behavior, paleodirections could not be obtained from sites NE12, NE13, and NE32. Additionally, the paleodi- progressively increasing backfield after saturation. rections from flows NE03, NE07, NE21, and NE27 IRM (isothermal remanent magnetization) curves are based only on one to two samples. These sites show (Fig. 3, right) that saturation is reached in were rejected to calculate La Negra mean paleodi- moderate fields (150–200 milli-teslas [mT]), which rections. Thus, 25 of the 32 flows studied yielded points to some spinel phases (titanomagnetites or reliable paleomagnetic results. These directions are titanomaghemites) as predominant magnetic miner- considered to be of primary origin. It is quite als. Judging from the ratios of hysteresis parameters, unusual that the studied La Negra Formation lavas it seems that the magnetic particles fall in the small are remagnetized by intruding batholiths because: pseudo-single-domain grain size (Fig. 4; Day et al., (1) our sampling locality is rather distant from the 1977). This probably indicates a mixture of multido- intrusions; (2) the remanent magnetizations of main and a significant amount of single-domain batholith bodies are insignificant with respect to (SD) grains. strong thermoremanent magnetization of La Negra lava flows. Moreover, the primary character of Paleodirection determination remanent magnetization is supported by the occur- Remanence measurements were made using rence of almost antipodal normal and reversed both JR5a and JR6 spinner magnetometers (sensi- polarities. Thermomagnetic investigations reveal tivity ca. 10–9 Am2). Stepwise alternating field (AF) that the remanence is carried in most cases by Ti- demagnetization used a Molspin AF demagnetizer poor titanomagnetites, resulting of oxy-exsolution of providing fields up to 100 mT. A characteristic mag- original titanomagnetite during the initial flow cool- netization was determined by the least squares ing. Thus the nature of magnetic carriers most prob- method (Kirschvink, 1980), with 3 to 10 points used ably indicates a thermoremanent origin for the for this determination. The directions were averaged primary magnetization. In addition, unblocking tem- by flow and the statistical parameters calculated perature spectra and relatively high coercivities assuming a Fisherian distribution. A tilt correction point to “small” pseudo-single domain magnetic was applied to the directions assuming a tectonic tilt structure grains as responsible for remanent magne- of 30° in the 360° direction (Arriagada et al., 2002). tization. The secondary magnetizations were suc- All samples (i.e., one specimen per sample) were cessfully removed in most cases using the progressively demagnetized. Typically, 6 to 11 sam- alternating field demagnetization technique. These ples per flow were subjected to AF demagnetization. factors definitively attest to the fact that remanence Most samples carry multicomponent magnetization is primary (i.e., synchronous with emplacement of (Figure 5, samples NE1-001B, NE17-129A, NE24- the studied lava flows) and can definitively be used 570 GOGUITCHAICHVILI ET AL.

FIG. 5. Orthogonal vector plots of stepwise alternating field demagnetization (stratigraphic coordinates). The numbers refer to peak alternating fields in mT. Symbols: • = projections into the horizontal plane, x = projections onto the vertical plane.

FIG. 6. A. Equal-area projections of the flow mean characteristic paleodirections for the La Negra Formation. Circles and crosses denote the negative/positive inclination. B. Equal-area projection of the mean directions for Antofagasta- Tocopilla volcanic units, as indicated in Table 2 (see also text). JURASSIC LA NEGRA FORMATION, NORTHERN CHILE 571

TABLE 2. Average Paleodirections for the La Negra Volcanic Units

La Negra N Inc Dec a95 k Reference

Normal 11 –31.8 5.6 6.7 47 This study Reverse 14 –29.7 15.9 11.6 13 This study All1 25 –30.7 11.3 6.9 18 This study Tocopilla 8 38.3 5.6 20.4 8 Arraigada et al., 2001 Antofagsta 7 –40 12.9 15.9 15 Arraigada et al., 2001 All2 15 –39.2 9.1 12 11 Arraigada et al., 2001 Total 40 –33.9 10.5 6.1 15

1All sites studied in present work. 2All sites studied in Arriagada et al., 2002.

for tectonics, volcanic stratigraphy, and any other geologic and geomagnetic applications. The mean paleodirection obtained in this study is I = –30.7, D = 11.3°, k = 18, α95 = 6.8°, N = 25 (Table 2 and Fig. 6B). A previous paleomagnetic study provided basically similar directions with I = –39.2°, D = 9.1°, k = 11, α95 = 12°, N = 15 (Arria- gada et al., 2002). By combining both sets of data we obtained a well-defined mean paleomagnetic direction with I = –33.9°, D = 10.5°, k = 15, α95 = 6.1°, and N = 40. These directions are in good agreement with the expected paleodirections at about 180 Ma (Roperch and Carlier, 1992; Fig. 6B). The differ- ence between observed and expected magnetic declinations is insignificant—less than 5°, while the confidence cones of Fisher statistics (α95 in Table 2) vary from 6.1° to 20.5°. In this context, any attempt to determine possible vertical axis rotation should be considered as pure speculation. Thus, our data, in conjunction with Arriagda et al.’s (2002) results, indicate that the La Negra Formation does not show evidence for significant tectonic rotation, suggesting that the development of the Bolivian orocline during the upper Neogene and the formation of the Altipl- ano-Puna plateau cannot be explained by simple bending of the whole margin. As suggested by Arria- gada et al. (2002), the fore-arc probably acted as a translating rigid block during the Neogene development of the Altiplano-Puna plateau. Further paleomagnetic studies are required to better constrain the FIG. 7. Tentative magnetostratigraphic correlation (two paleotectonic regime of northern Chile. options) between La Negra volcanic units and reference geo- Fifteen sites yield reverse polarity magnetization magnetic polarity time scale (retrieved from Berrgreen et al., and 11 flows are normally magnetized (Table 1, Fig. 1995). 6). No intermediate polarity magnetization was 572 GOGUITCHAICHVILI ET AL.

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