Cretaceous to present paleothermal gradients, central Negev, Israel: Constraints from fission track dating

0735-245X/90 $3.00+ .00 Pergamon Press plc

Nucl. Tracks Radiat. Mess., Vol. 17, No. 3, pp. 381-388, 1990 Int. J. Radiat. Appl. Instrum., Part D Printed in Great Britain

CRETACEOUS TO PRESENT PALEOTHERMAL GRADIENTS, CENTRAL NEGEV, ISRAEL: CONSTRAINTS FROM FISSION TRACK DATING B. P. K o ~ , S. FE1NST~Nand M. EYAL Department of Geology, Ben Gurion University, Beer Sheva 84 105, Israel (Received 5 September 1988; in revised form 7 June 1989)

Akm'act--Apatite and zircon fission track ages (FTA), vitrinite reflectance (VR) data and burial history curves were integrated for reconstruction of Early Cretaceous to present maximum thermal gradients in four deep boreholes in the central Negev, Israel. The most complete data set is available from the Ramon 1 borehole. Supplementary data were obtained from Hameishar 1, Makhtesh Qatan 2, and Kurnub 1 boreholes. Between ca. 122-90 Ma the constraints on thermal gradient obtained from apatite FTA overlap with those derived from zircon FT and VR data, restricting them to < 45-50°C km-'. Apatite FTA between 90 and 80 Ma in Ramon I and Hameishar 1 suggest rapid cooling at the time recorded or earlier. Constraints on thermal gradient history derived from these ages are considerably strengthened over a short time span. From 80 Ma to the present, FTA data indicate that gradients had probably decayed to present-day regional levels (ca. 20°C km -I) by Early Tertiary time. Thermal constraints derived from apatite FTA and VR data in Makhtesh Qatan 2 and Kurnub 1 boreholes are consistent with those obtained post-56 Ma for Ramon 1. For pre-56 Ma, only VR data are available and these indicate considerably lower maximum gradients than those obtained for the same time period from Ramon I. This dichotomy reflects different Early Cretaceous-Early Tertiary thermal regimes between the northern and southern parts of the study area.

INTRODUCTION A VAgm'rv of thermally dependent rock properties may be analysed in order to constrain the thermal history of sedimentary basins e.g. fluid inclusions, organic maturation, clay mineralogy and fission track analysis. Recent studies have demonstrated the value of the fission track method for investigation of the thermotectonic evolution of sedimentary basins (e.g. Naeser, 1979a; Gleadow et al., 1983; Duddy and Gleadow, 1984; Naeser, 1986; Feinstein et al., 1988; Green et al., 1988; Naeser et al., 1988). The unique advantage of fission track studies is their potential to constrain paleotemperatures and their variation through time (Gleadow et al., 1983). In this respect two principal types of information may be derived; a specific paleotemperature over a relatively short period of time, or a constraint on maximum paleotemperatures over a longer timespan. The key to such investigations is the variable re-setting of different clocks over time due to the tendency of fission tracks to anneal upon heating (for review of annealing see Naeser, 1979b). The temperature required to anneal tracks is dependent, in part, upon the effective heating time, and is characteristic of the particular mineral under study. Apatite is the most commonly used mineral because temperature conditions for its annealing are relatively well established (e.g. 80-135°C and 60-105°C for heating times of 106 yr and l0 s yr respectively--Naeser, 1981;

Gleadow and Duddy, 1981) and these temperatures fall within ranges commonly encountered in sedimentary basins. Zircon, a mineral commonly encountered in clastic sediments and crystalline rocks, anneals at higher temperatures than apatite, ca. 2 0 0 + 30°C (Naeser, 1979b; Harrison et al., 1979; Zaun and Wagner, 1985; Hurford, 1986). In the present study, we have integrated apatite and zircon fission track data with coal rank and burial history information previously reported from four deep boreholes (Kohn et al., 1988; Feinstein et al., 1988) to derive constraints on the Early Cretaceous to present-day thermal gradient history for the central Negev area, Israel (Fig. 1). The geological history, together with the excellent outcrop and deep borehole distribution, make the study area an ideal setting for thermal history reconstruction.

GEOLOGICAL SETI'ING The Ramon 1, Makhtesh Qatan 2, Kurnub 1 and Hameishar 1 boreholes were sited in three breached anticlines and a monocline respectively in the central Negev, Israel (Fig. 1). The structures form part of a system of NE to ENE trending, elongate, asymmetric, long wavelength folds and monoclines that comprise the eastern part of the Negev-Sinai branch of the S-shaped Syrian Arc (Krenkel, 1924a,b; Bentor and Vroman, 1954; de Sitter, 1962; Freund, 1965). Field 381

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Lang et al., 1988) and >118 Ma based on paleomagnetic measurements (Ron and Baer, 1988). Subsurface igneous rocks in p r e L a t e Jurassic strata have been documented from the studied drill holes (Weissbrod, 1969, 1981; Goldberg, 1970; Druckman, 1974). K-At whole rock dating of five of these magmatics yields ages ranging from Early Cretaceous to Precambrian (Recanati, 1986). The close temporal and spatial relationships between the pronounced intrusive magmatic phase in the Ramon and a period of Early Cretaceous regional uplift and erosion (see Fig. 2) was attributed by Garfunkel and Derin (1988) to the activity of an intraplate hot spot.

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FT DATING CONSTRAINTS ON PALEOTHERMAL GRADIENTS APATITE FISSION TRACK AGE/DEPTH PROFILE IN RAMON 1 In two previous studies (Kohn et al., 1988; Feinstein et aL, 1988) the writers presented fission track and coal rank measurements from the boreholes mentioned above. All apatite fission track ages (FTA) are considerably younger than those of their Precambrian-Permian host strata and the Early Cretaceous magrnatic phase, and are thus interpreted as cooling ages. The most complete data set is available from the Ramon 1 borehole, which penetrated the longest interval of Precambrian section in the central Negev, hence this profile (Fig. 3) serves as a basis for the following discussion. It was proposed by Kohn et al. (1988) that the plot of apparent apatite FTA vs depth in Ramon 1 (Fig. 3) could be described by a three-segmented profile. By analogy to theoretical profiles in earlier works (e.g. Naeser, 1979a) they suggested that the profile revealed preservation of both fossil partial annealing and total annealing zones (PAZ and TAZ respectively) within the present-day total track stability zone (TSZ) (see Fig. 3). The profile recorded thus indicated a considerable amount of cooling. The pronounced inflection point in the profile (marked (X) in Fig. 3) thus represented a paleo PAZ/TAZ boundary and a well-defined cooling event between ca. 90-80Ma B.p. Based on this approach Kohn et al. (1988) estimated total cooling of ca. 580C for rocks at the paleo PAZ/TAZ transition since ca. 90 Ma with most of the cooling (about 40°C) having occurred by ca. 80 Ma B.p. Due to lack of major erosion during this period the rapid cooling was

383

interpreted as indicating a decrease of the thermal gradient possibly related to the onset of Syrian Arc folding. The remaining cooling (ca. 18°C) was mainly attributed to Neogene deep erosion and breaching of the Ramon anticline. Recent studies have shown that annealing of fission tracks actually continues even at ambient surface temperatures (e.g. Green et al., 1986; Duddy et al., 1988; Green, 1988) hence casting doubt on the existence of a true TSZ. Further, Gleadow et aL (1986), Green et aL (1986, 1988) and Duddy et aL (1988) have demonstrated that confined horizontal track length data are crucial for unequivocal interpretation of apatite FTA. To date, no track length measurements have been made on the studied samples. Even though the track annealing rate at low temperatures is very slow, the interpretation by Kohn et al. (1988) of the Ramon 1 profile (based solely on FTA data) using the "three annealing zone concept" is equivocal. This leaves open the possibility that all the FTA measured in Ramon 1 represent partial annealing ages. Hence, the actual timing of the relatively rapid cooling from the paleo TAZ may have been somewhat earlier than the recorded age (see Fig. 3). Nevertheless, even if the latter situation is true, the FTA data can still be used to derive constraints on the maximum thermal gradient history of the studied section. DERIVATION OF CONSTRAINTS ON PALEOTHERMAL GRADIENTS

The effective track retention temperature (or closure temperature) to which a mineral has cooled at the time given by its fission track age varies with the cooling rate (e.g. Wagner and Reimer, 1972; Haack, 1977). The closure temperature approximates to the stage at which about half of the tracks become stable in the PAZ (Wagner and Reimer, 1972). Estimates of the closure temperature for various 1000 minerals have been determined in the laboratory lP Preeerved .4~ e (Naeser, 1979b) and in deep drillholes (e.g. Naeser, -e1981; Gleadow and Duddy, 1981; Gleadow et al., ? Preeerved ] resell TAZ -01983; Naeser, 1986). I "* 2000 The geological history of the study area (see also burial curves presented in Feinstein et aL, 1988), indicates that the duration of heating of the rocks studied during the Early Cretaceous magmatic 300( episode and/or their subsequent phase of burial prior to cooling was of the order of ca. 3-5 × 107 yr. This timespan corresponds to an apatite closure temperature of about 95-100°C, which approximates to a 4001 temperature of about 110-120°C for total annealing (Naeser, 1981; Gleadow and Duddy, 1981; Gleadow et al., 1983). o go ~'o 6o 8'o ,oo' ,2o14o Different types of paleothermometric information AGE ( M e ) may be gleaned from fission track ages depending on FIG. 3. Average apatite fission track age (5: itr) vs depth in whether they represent total or partial annealing ages. Ramon I borehole (modifiedafter Kohn et al., 1988). Point (X) indicates a breakpoint in the profile and possibly marks However, as mentioned above, since annealing conthe boundary between the partial and total annealing zones tinues, albeit very slowly, even at surface tempera( P A Z / T A Z ) during Late Cretaceous time or earlier. tures, pure TAZ apatite ages can rarely be expected /

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in the geologic record. On the other hand, since at low temperatures the rate of annealing is slow, the essential elements of profiles showing pronounced cooling may be preserved with only minor to moderate time shifts. In this situation the measured age will postdate a "meaningful geological event" in proportion to the degree of annealing experienced. Bearing in mind the time span involved in this study (Table 1) and the lack of track length data the F T A recorded cannot be considered to represent cooling ages from the TAZ. Therefore, our interpretation for thermal history reconstruction is restricted to the assumption that all apatite FTA herein represent partial annealing ages. Such an interpretation implies that sample temperatures did not exceed the P A Z / T A Z boundary, at least since the age obtained. For a particular sample, subtraction of surface temperature (here assumed at a constant 20°C) from the constrained temperature divided by the reconstructed variation of depth with time (burial history) provides constraints on paleothermal gradients since the time of the apparent fission track age. Paleotemperature constraints obtained by FTA are specific to the sample studied; however, once converted to thermal gradients, the constraints have general appli-

cation for the section. Paleothermal data derived from a suite of partially annealed F T A in a section can thus be integrated to derive a constraint on the maximum thermal gradient history, but do not define the actual paleothermal gradient. DISCUSSION A set of constraints on paleothermal gradients derived from apatite and zircon FTA (Table l), and vitrinite reflectance in the studied boreholes are presented in Fig. 4. Curve A F T (Fig. 4) defines the maximum constraint on the thermal gradient history constructed by the integration of paleothermal data derived from apatite F T A and burial history in Ramon 1 (see Table l). In addition, thermal gradient data from the three other borehole studies (also presented in Table l) have been plotted in Fig. 4. Most data plot onto the "best fit" curve A F T except for three samples in the shallower part of the Ramon 1 and Hameishar l boreholes, which provide less rigorous constraints. Integration of fission track data for construction of maximum constraint on paleothermal gradients is only effective if samples record the varying ages.

Table 1. Sample data, apatite fission track ages, reconstructed depths and calculated maximum paleothermal gradients

Present depth (m) 548-552 999-1020 1092-1122 1475-1487 1764-1780 1984-1988 2579-2587 3045-3057 3434-3439 660-662 1204-1238 1305-1334 1402-1428 1501-1537

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RAMON 1 Early Cretaceous 121.6 intrusion in Raaf (Triassic) Yamin (Permian) 104.7 Arqov (Permian) 94.7 Zenifim (Preeamb.) 84.1 Zenifim (Preeamb.) 79.2 Zenifim (Precamb.) 81.2 Zenifim (Precamb.) 55.7 Zenifim (Precamb.) 43.0 Zcnifim (Precamb.) 33.7 HAMEISHAR I Intrusion in 110.2 Zafir (Permian) Yam Suf (Cambrian) 94.9 Intrusion in 95.8 Zenifim (Precamb.) Intrusion in 98.6 Zenifim (Precamb.) Zenifim (Precamb.) 86.4

tEstimated depth (m) at time of apatite FT age 880-884

Paleothermal gradient (°C km -I) <50

1399-1420 1502-1532 2279-2291 2568-2584 2788-2792 3383-3391 3849-3861 4238-4243

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FT DATING CONSTRAINTS ON P A L E O T H E R M A L G R A D I E N T S AFT

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FIG. 4. Plot of constraints on paleothermal gradient with time for the central Negev derived from fission track (FT), vitrinite reflectance (VR) and burial history data. Note: (1) Pre-ea. 90 Ma divergence between curve VR 2 and the other curves indicates a lower thermal regime in the northern part of the study area; (2) curve AFT between ca. 90 and 80 Ma (b), indicates a period of relatively rapid cooling in Ramon 1 and Hameishar I wells either at the age recorded or more fikely at a somewhat earlier time; (3) from ca. 56 Ma to present, curves AFT and VR 2 (c) actually coincide at a paleothermal gradient of ca. 20°C km-L In cases where either sample ages do not vary with depth (e,g. in cases of rapid cooling) or where only a discrete age is available, the maximum constraint is provided exclusively by the deepest sample. In the Ramon 1 borehole, zircon FTA are all virtually concordant within analytical errors, thus forming a vertical time/depth profile (Feinstein e t a L , 1988). Curve ZFT in Fig. 4 was constructed by dividing the annealing temperature for zircon (here assumed ca. 200°C) by the depth history of the deepest sample (at 3.43 km) for which zircon FTA is available. Zircon FTA from the Negev boreholes (Feinstein et al., 1988) are probably related to Late Paleozoic uplift and indicate that zircons have not been subjected to temperatures required to cause significant annealing during the Late Cretaceous-present time. Since zircon annealing temperatures are considerably higher than those for apatite, curve ZFT enables a ceiling to be placed on maximum paleothermal gradients. Zircon ceilings for other boreholes studied are not shown in Fig. 4 because their total depths are shallower than for Ramon 1; hence their ceilings plot at higher paleothermal gradietns (Feinstein et al., 1988). NT 17/3~0

In addition to fssion track dating, independent constraints on paleothcrmal gradients may be obtained from other temperature sensitive parameters e.g. vitrinite reflectance 0/R). VR data from central Negcv boreholes indicate that coalification pro-dated the Early Cretaceous unconformity (Feinstein e t aL, 1988). The fact that organic maturation remained "frozen" since that time indicates that post-Early Cretaceous temperatures were lower than those under which coalification evolved. Curves VR in Fig. 4 present maximum possible post-coalification thermal gradients for Ramon 1 (VR-I) and Makhtesh Qatan 2 and Kurnub 1 (VR-2) based on the concept of the LOM model (Hood et al., 1975) together with the burial history, following Feinstein et al. (1988). The constraints on thermal gradient history derived from zircon FTA (ZFT) and vitrinite reflectance (VR-1) in Ramon 1 remain relatively high through the Cretaceous period to present-day. These constraints are of limited value due to the relatively high closure temperature for zircon and the shallow depth history for the samples on which VR was measured.

386

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Combined constraints on the thermal gradient history within the time span covered by the apatite FTA ( < 122 Ma) may be divided into three segments (a, b and c--Fig. 4). In segment (a) (ca. 122-90 Ma), despite the considerable difference in closure temperature between apatite and zircon, constraints derived from apatite FTA overlap with the zircon and VR data (curves ZFT and VR-1 respectively) within the range of ca. 45-50°C km -~ (Fig. 4). This overlap is due to the relatively shallow burial history from which the apatite FTA were measured. Constraints derived from the relatively shallow apatite samples of the Hameishar 1 borehole also fall within this range of paleothermal gradients. An apatite fission track age of ca. 122 _ 13 Ma was determined on an intrusive rock from the depth range 548-552 m within the Triassic section in the Ramon 1 borehole (Kohn et al., 1988). Similar apatite FTA were obtained from two other magmatic bodies in the Ramon area. One from fresh quartz syenite in a drill core in the Gavnunim Valley of Makhtesh Ramon yielded an age of 117 + 8 Ma, whereas a basic sill at Gebel Areif en Naqa, some 25 km SW of Ramon 1, yielded an age of 118 + 5 Ma. These ages are concordant (within experimental error) with K - A r whole rock ages determined on an adjacent intrusion in the Ramon 1 borehole (Recanati, 1986) and with a major pulse of Early Cretaceous magmatism in the Ramon area (Lang et al., 1988). Hence, the 122 Ma apatite age is interpreted as approximating a primary postintrusion cooling age. Preservation of a primary cooling age indicates that the sample has not experienced temperatures higher than the "TSZ"/PAZ boundary (here estimated at ca. 65°C, Gleadow et al., 1983) at a later time. Considering the reconstructed depth of the sample at the time of intrusion implies a maximum thermal gradient of ca. 50°C km -~ (Table 1). By contrast, the constraint derived from three apatite FTA (indicated by arrows in Fig. 4), which plot above the ZFT and VR-I curves in this segment, is less rigorous due to their shallower depth of burial. In segments (b) and (c), from 90 Ma to presentday, apatite FTA provide a stronger constraint than zircon FTA and VR, due to their lower closure temperature and the greater depth from which the samples were measured (see Table 1). For segment (b) (90-80 Ma), paleothermal gradient constraints increase rapidly, due to increased burial depths of samples. If the apatite FTA were related to cooling from the TAZ to TSZ then the ages would date the actual cooling due to gradient decay at that time as suggested by Kohn et al. (1988). But, as mentioned above, the FTA were most likely affected by some degree of partial annealing and therefore the actual timing of the cooling is uncertain. Apatite FTA plotted on segment (c), from 80 Ma to present-day, were obtained from a similar reconstructed depth interval in Ramon 1, Makhtesh

Qatan 2 and Kurnub 1 boreholes (Table 1). The paleothermai gradient derived for these FTA data (Fig. 4) is similar to the present-day average Negev gradient of ca. 19-22°C (Levitte and Olshina, 1985). The constructed curve suggests that in the study area the regional thermal gradient had probably decayed to present day levels by ca. 56 Ma B.P. Constraints on the maximum paleothermal gradient derived from VR in Ramon 1 (VR-1, Fig. 4) are limited due to the relatively shallow post-Jurassic burial history of samples studied. More rigorous constraints may be obtained in Makhtesh Qatan 2 and Kurnub 1 boreholes, where the basal Cretaceous erosion was milder and post-erosion burial was considerably greater (Feinstein et al., 1988). VR constraints for Makhtesh Qatan 2 and Kurnub l boreholes are identical to those obtained from apatite FTA for the ca. 56 Ma to present time span (cf. VR-2 and AFT, Fig. 4). However, VR-2 constraints pre56 Ma obtained for the two boreholes are markedly lower than the paleothermal gradients indicated by apatite FTA for Ramon 1 and Hameishar 1 boreholes. The interpretation of this dichotomy is not entirely clear, but it may reflect a difference in the thermal regime between the northern and southern parts of the study area. VR profiles measured in Ramon l, Makhtesh Qatan 2 and Kurnub 1 boreholes reveal identical thermal history during Jurassic time (Feinstein, 1985; Feinstein et al., 1988). Hence, the difference in the thermal regime observed here is restricted to Early Cretaceous-Early Tertiary time. The extensive Early Cretaceous magmatism observed in the Makhtesh Ramon area probably reflects the existence of a deep seated magmatic chamber. The presence of such a magmatic perturbation further supports the possibility that different thermal regimes existed between the northern and southern parts of the study area during this time. CONCLUSIONS The combination of fission track vitrinite data provides a powerful tool for reconstruction of thermal history. Apatite and zircon F'I'A, VR measurements and burial history data from Ramon 1, Hameishar 1, Makhtesh Qatan 2 and Kurnub 1 boreholes in the Negev, southern Israel, were integrated for reconstruction of Early Cretaceouspresent-day maximum thermal gradient history. Data obtained from Ramon 1, the most complete set available, together with those from Hameishar 1, reveal three paleothermal gradient intervals. Between ca. 122-90Ma, apatite and zircon FTA, and VR measurements, constrain thermal gradient to <45-50°Ckm -t. Apatite FTA from 90Ma to present-day provide stronger constraints on thermal gradients than zircon FTA and VR. Apatite FTA yielding ages between 90-80 Ma in Ramon 1 and Hameishar 1 reflect rapid cooling at the age recorded, or more likely somewhat earlier: The constraints

FT DATING CONSTRAINTS ON PALEOTHERMAL GRADIENTS derived from these apparent ages increase markedly over a relatively short timespan. F r o m 80 M a to present, apatite F T A indicate that by Early Tertiary time, gradients had already decayed to present-day regional levels (ca. 20°C kin-l). Early Tertiary-present thermal constraints derived from apatite F T A and VR data in R a m o n 1 are corroborated by similar data obtained from Makhtesh Qatan 2 and Kurnub 1 boreholes. F o r Cretaceous time only, VR measurements are available in these two boreholes, and these reveal lower maximum gradients than those obtained for the same time period from R a m o n 1. This apparent difference probably reflects different Early Cretaceous-Early Tertiary thermal regimes between the northern and southern parts of the study area. The distribution of Early Cretaceous magmati¢ rocks in the central Negev lend further support to this conclusion. Acknowledgements--Financial support for this study was provided by the Belfer Foundation for Energy Research and the Israel Ministry of Energy and Infrastructure--Administration for Earth Sciences Research. Constructive criticism of an earlier version of this work was provided by two anonymous reviewers.

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