Volcanic and nonvolcanic rifted margins of the Red ... - Science Direct

Geochimica et Cosmochimica Acta, Vol. 61, No. 12, pp. 2511-2527, 1997 Copyright © 1997 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/97 $17.00 + .00

Pergamon

P I I S0016-7037(97)00108-7

Volcanic and nonvolcanic rifted margins of the Red Sea and Gulf of Aden: Crustal cooling and margin evolution in Yemen MARTIN MENZIES, 1 KERRY GALLAGHER,2 ANDREW YELLAND,J and ANTHONY J. HURFORD3 JDepartment of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK 2Department of Geology, Imperial College, Prince Consort Road, London SW7, UK 3Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK (Received March 26, 1996; accepted in revised form March 4, 1997)

Abstract--New apatite fission track (AFT) data from the southern Red Sea volcanic and the Gulf of Aden nonvolcanic margins provide important constraints on the timing of crustal cooling relative to periods of volcanism and lithosphere extension. The APT data define several regions of extension immediately adjacent to the Red Sea margin with AFT ages < 25 Ma and track-length distributions consistent with rapid cooling. Elevated Precambrian basement highs on the rift shoulder have AFT ages >~ 100 Ma and track-length distributions indicative of a complex pre-rift history. An intervening area along the Red Sea and Gulf of Aden margins, and inland along the Balhaf graben (Jurassic rift), has AFT ages of 2 5 - 1 0 0 Ma. and track-length distributions indicative of rapid cooling. Elevated Precambrian basement highs are juxtaposed against topographically lower extended coastal terranes with sharp contrasts in AFT ages and track-length distributions, pointing to possible reactivation in the Tertiary of lineaments of Precambrian and Jurassic age. Integration of field observations with AFT data and 4°Ar/ 39Ar data indicates that, on the Red Sea volcanic margin, surface uplift was initiated immediately prior to volcanism and that cooling was synchronous with widespread extension and an apparent hiatus in voluminous volcanic activity. Copyright © 1997 Elsevier Science Ltd 1. INTRODUCTION

ferred cooling histories of the Red Sea and Gulf of Aden margins in Yemen with absolute timing of magmatism through the study of the volcanic stratigraphy and relative timing of extension through an integration of field observations (Menzies et al., 1992, 1994, 1997; Davison et al., 1994) and radiometric dates (Baker et al., 1996a). In general, we expect the cooling histories recorded by the fission track data to reflect denudation, as opposed to transient thermal effects associated with the rift process (e.g., Gallagher et al., 1994). However, denudation may occur in response to both erosional and tectonic processes, and we consider these to be the endmember controls on the cooling history. Thus, erosional denudation reflects the combined role of chemical and physical weathering processes. In particular, erosional denudation rates at passive margins correlate well with local topographic relief contrasts rather than simple regional elevation (Summerfield, 1991). Tectonic denudation describes the effects of low-angle detachment faults which emplaced younger rocks over older, with the associated denudation generally assumed to occur rapidly (e.g., Wheeler and Butler, 1994; Johnson, 1996). Tectonic denudation is effectively controlled by the geometry and rate of crustal extension and, therefore, acts independently of surface processes. However, the development of local fault-block uplift and subsidence during tectonism creates local topographic relief rift mountains which will tend to enhance the surface erosional processes. While it seems reasonable that erosion operates after tectonic denudation, these processes will often be active at the same time, and indeed erosional denudation may exert a control on tectonic exhumation through an influence on the gravitational driving stresses (e.g., Beaumont et al., 1996). Therefore, in practice it is often difficult to isolate these

Nearly 20 years ago, Seng6r and Burke (1978) emphasised the relative timing of surface uplift, magmatism, and extension in understanding whether the mantle was a passive or active participant in rift formation. The passive (platedriven) model predicted a sequence of extension-uplift-magmatism, whilst the active (plume-driven) model implied that uplift predated magmatism and extension. It is still a matter of debate as to whether mantle plumes initiate extension or exploit passive extension (Turcotte and Emerman, 1983; White and McKenzie, 1989; Bott, 1992; Hill, 1991) but clearly the order of tectono-magmatic events is pivotal to defining a rift formation mechanism. At triple junctions where extension is followed by the separation of a failed arm and a two-armed passive margin, the point of inflection between the two passive margins may mark the hotspot location, may be associated with the most intensive pre-break-up uplift and volcanism, and may be the site of the earliest initiation of rifting (Houseman, 1990). The Cenozoic divergence of the Arabian, African, and Somalian continental plates has provided a geologically recent laboratory for the study of surface uplift, plume impingement, magmatism, and the embryonic phases of continental extension (Cloos, 1939; Gass, 1970; Gass et al., 1978; Almond, 1986; Dixon et al., 1989; Girdler, 1991; Davison et al., 1994). In particular, Yemen represents a key locality to establish the sequence and chronology of tectonism and magmatism during the initiation and early phases of ArabianAfrican-Somalian rifting and the subsequent evolution of the Red Sea volcanic and the Gulf of Aden nonvolcanic margins. In this contribution, we report new apatite fission track (AFT) data for Yemen, our aim being to reconcile the in2511

M. Menzies et al.

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Fig. 1. (a) General geology map of Yemen showing major structural provinces and sample locations (open circles) (after Davison et al., 1994; Menzies et al., 1994). (b) Present-day elevation in Yemen. Note elevations in excess of 3000 m on the volcanic margin of the Red Sea. Palaeoenviromental studies of Tertiary sediments (Al'Subbary, 1996) indicates that these elevations did not exist prior to eruption of the volcanic rocks and opening of the Red Sea.

denudational mechanisms. However, the longer a particular region sits around after some tectonic event, the more likely it is that any tectonic denudation signal (i.e., rapid cooling) will be obscured by later erosion and potentially more protracted cooling. Typically, arguments for tectonic denudation rely on identifying relevant structural features, rapid cooling, and the correlation of radiometric/thermochronological ages with some independent constraint on the timing of tectonics (e.g., Wheeler and Butler, 1994). Consequently, we will consider the measured F F data in terms of: ( 1 ) the regional patterns and rates of cooling on the Red Sea volcanic and the Gulf of Aden nonvolcanic margins; (2) the relationship of these cooling trends to the geological, structural, and geomorphological evolution of the area, in particular investigating the possibility of lineament reactivation during rift development; (3) integrating the denudational histories of the rifted margins and the hinterland with the absolute timing of volcanic activity and the relative timing of extension, to provide a composite chronology of events. 2. GEOLOGY OF YEMEN The Republic of Yemen occupies an important position within the Afro-Arabian dome, lying adjacent to the Eritrean-

Ethiopian Large Igneous Province (LIP), including the elevated eastern rift-margin of the southern Red Sea and the northern rift-margin of the Gulf of Aden (Fig. 1 ). Recently several authors have documented stratigraphic, structural, and geological relationships in this region (Bott et al., 1992; Crossley et al., 1992; Mitchell et al., 1992; Davison et al., 1994; Redfern and Jones, 1995; Ellis et al., 1996; Menzies et al., 1997). A simple geological map is provided in Fig. 1 and the regional geology is discussed under three broad headings.

2.1. Western Peneplaned Precambrian Province This region contains flat-lying, mainly fluvial to marine sediments overlying Archaean and late Proterozoic basement exhibiting N-S, NE-SW, NW-SE Proterozoic structural trends (e.g., Davison et al., 1994; Menzies et al., 1994; Ellis et al., 1996). The region has an average elevation in excess of 2 km and is bounded in the west by the plateau region of the Sana'a-Taizz Oligocene LIP (Baker et al., 1996a). Precambrian basement outcrops in a large triangular region (the Aden or Mahfid High) to the east of the LIP on the rift shoulder and in a smaller area in the southwest of the elevated region. In the east the province is bounded by Jurassic grabens (Balhaf, Shabwah), and in the south by the Gulf of

Crustal cooling in the Red Sea rift

2513

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Aden extended coastal plain, where Miocene to recent volcanic rocks abut against the southern margin of the plateau. There is a distinct absence of Mesozoic cover over this region except at the peripheries of the plateau. Thin sequences of sediments directly overlie the basement highs in the south (i.e., the Middle-Upper Jurassic Shukra limestone) and in the west (i.e., Cretaceous-Eocene Tawilah sandstone). Minor post-erosional volcanic fields (i.e., Miocene-recent) are also present near Sana'a, Dhamar, Mareb, and Shuqra. The western and southern borders of the province form the highly dissected margins to the Red Sea and Gulf of Aden.

2.2. Eastern Tertiary Tableland A relatively low-lying region of Cretaceous to Tertiary siliciclastics and carbonates overlying Proterozoic basement is exposed in only a few structurally high erosional windows on the southern border (e.g., Mukalla High). This gently undulating tableland is bordered to the north by the Rub A1 Khali desert and to the south by the extended Gulf of Aden coastal rift-margin.

2.3. Central Mega-Rift Complex This late Jurassic rift basin is thought to have been formed on the Najd strike-slip fault trend which is oriented NW-SE and is believed to be Proterozoic in age. The rift effectively

separates the elevated peneplane from the tableland. The mainly marine-fluvial infill of the grabens (Marib-JaufShabwa and Balhaf) forms a major region for hydrocarbon exploration (Redfern and Jones, 1995; Ellis et al., 1996). Recent studies throughout the three provinces (e.g., Dixon et al., 1989; Menzies et al., 1992, 1997; Davison et al., 1994) have attempted to reconcile predictions about the timing of processes related to rift valley formation with field observations. These studies have illustrated that the potential complexities envisaged by S engor and Burke ( 1978 ) are apparent in both volcanic and nonvolcanic margins (e.g., Dixon et al., 1989). In terms of timing, extension in the Aden-Red Sea-Ethiopian Rift system is inferred to have begun around 35 Ma (early-mid Oligocene; Redfern and Jones, 1995; Watchorn et al., 1996). This timing corresponds to the period of rapid erosion along the flanks of the Red Sea in Saudi Arabia and Egypt (Omar and Steckler, 1995), although a second period of rapid erosion around 21-25 Ma appears to mark the onset of the main phase of extension in this region. On the basis of magnetic anomalies identified in the eastern Gulf of Aden (Sahota et al., 1995), the onset of seafloor spreading in the Gulf of Aden occurred around 20 Ma and younged to the west with onset ages of ~ 3 - 4 Ma around Aden (Cochran, 1981). In contrast, the oldest seafloor spreading anomalies identified in the Red Sea are 5 - 6 Ma.

2514

M. Menzies et al.

Major volcanism (covering ~20,000 km 2 in Yemen) was associated with formation of most of the Gulf of Aden and Red Sea rift systems but was most intense at the Afro-Arabian triple junction in Ethiopia and Yemen. Recent studies of the chronology of the Yemen LIP (AI' Kadasi, 1995; Baker et al., 1996a, 1997) highlighted the problems associated with K-Ar analyses and emphasised the benefits of 4°Ar/39Ar plateau ages for mineral separates as an accurate constraint on the age of volcanism. Precise 4°Ar/39Ar geochronology pointed to a short period of magmatism at 3 1 - 2 6 Ma, that produced ca. 2500 m of basalts and rhyolites/ignimbrites. Significant erosional breaks occur toward high elevations within the volcanic pile and these have been dated, using 4°Ar/39Ar, at between 26 Ma and 19 Ma (Baker et al., 1996a), a period when emplacement of A-type granites may have been at a peak (Blakey et al., 1994). Geochemical data for the Yemen volcanic rocks provides evidence for derivation of the basaltic rocks from a deep mantle source (Chazot and Bertrand, 1993; Baker et al., 1996b). A combination of this information with 4°Ar/39Ar dates allows us to constrain accurately when plume-driven processes were active. We can conclude from these recent data that a hot mantle plume, similar to that presently located beneath Afar, was responsible for the voluminous flood magmatism in western Yemen. This points to plume-related surface activity some 32 m.y. ago. 3. APATITE FISSION TRACK ANALYSIS Fission tracks are discrete, linear zones of damage in a crystal lattice and are formed by the spontaneous fission of 238U, present in trace amounts in minerals such as apatite and zircon. Tracks form at an essentially constant rate, determined by the spontaneous fission decay constant of 238U. Therefore, the number of tracks present in a given apatite crystal depends on both the U concentration and the duration over which tracks have been accumulating. Once the U concentration is determined, the number of tracks intersecting an internal surface of a crystal can be used to determine a fission track age. However, the length of a fission track is a sensitive function of temperature, and tracks shorten, or anneal, as temperature increases. Over timescales of 10 6 - 1 0 s m.y., the fission track age becomes zero at ~120 _+ 10°C (e.g., Gleadow and Duddy, 1981). However, some degree of annealing occurs at all temperatures and the rate of annealing depends primarily on temperature, but also on time and apatite composition (Green et al., 1986). For apatites that are similar in composition to the Durango standard, the temperature interval when significant annealing occurs is ~ 6 0 - 1 1 0 ° in the context of geological timescales. This temperature interval is often referred to as the partial annealing zone (i.e., PAZ) and in an ideal situation represents the depth interval over which the fission track age is reduced from the true age to zero. Brown et al. (1994) and Fitzgerald et al. ( 1995 ) give particularly lucid expositions of the consequences of cooling rocks through, and from within, the PAZ and demonstrate how apatite fission track analysis is particularly well suited to provide information on the cooling histories of surface samples as they pass through the upper few kilometres of

the crust. The important point we reiterate here is that, as each track forms at a different time, they all experience a different proportion of the total thermal history of the host rock. Therefore, for rocks which have cooled from different depths relative to the PAZ, track length data provides information on the temperature variations experienced by a given rock, while the fission track age provides information about timing/duration of these variations. Interpretation of fission track data currently relies on an empirical calibration of the relationship between tracklength, temperature, and beating duration (e.g., Laslett et al., 1987; Carlson, 1990). Consequently, the fission track data may be used to model the thermal histories quantitatively (e.g., Lutz and Omar, 1991; Corrigan, 1991; Gallagher, 1995 ). By making assumptions about the palaeo-geothermal gradient or palaeo-heat flow, the thermal history may, in principle, be converted to depth-time information to reconstruct a denudational chronology for samples presently on the Earth's surface.

3.1. Sampling and Methodology The main basement lithologies sampled for fission track analysis were a variety of Archaean and Proterozoic granitoids, gneisses, and amphibolites found within the Arabian-Nubianshield. Sampling was also undertaken in the Mesozoic-Cenozoicsedimentarycover including the Akbra shales, the Kohlan arkoses, and the Tawilah sandstones. Sufficient apatite for analysis was extracted from >90% of these samples. The external detector method, calibrated using the zeta approach (Hurford and Green, 1982) was used for all age analyses. Samples were irradiated at the well thermalised Riso reactor facility (Cd ratio for Au is 200-400) in Denmark, and lowuranium muscovite external detectors and Corning CN5 dosimeter glasses were used to monitor the neutron flux. Fission track age analyses were carried out on a Zeiss Axioplan microscope at a magnification of ×1250, using a dry (×100) objective. Confined track-length measurements were made using a CRG precision electronics drawing tube and Houston Instruments tablet under the same observational conditions as age analysis.

3.2. Results of Fission Track Analysis The apatite fission track ages and confined track-length data are summarised in Table 1 and full analytical data are available from the authors on request. In this paper, we report central fission track ages, rather than pooled or mean ages. This age estimate is essentially the mean of the log distribution of single grain ages, weighted by individual measurement precision. The central age allows for non-Poissonian variations in the counts and provides a robust measure of the central tendency of the single grain ages. Two measures of uncertainty are considered for a central age: the lc~ error indicates the analytical precision, while the age dispersion (or the spread of the individual crystal data) is given by the relative standard deviation of the central age. Where this dispersion is low ( < 1 0 % ) the data are consistent with a single population, and the central and pooled/mean ages converge. In this case the sample would pass the usual X z test as applied to the mean age. The geographical trends of the data throughout Yemen (Fig. 2) display many features in common with other fission track datasets from rift-flanks throughout the world such as southeast Australia (Moore et al., 1986) and Brazil (Gallagher et al., 1994), amongst others (see Gallagher and

Crustal cooling in the Red Sea rift

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Table 1. Fission track apatite age and confined track length data from the Yemen Red Sea and Gulf of Aden rift-flanks. Field number F4 F12 El3 F28 YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM YEM

6 8 9 10 11 13 14 16 18 19 20 21 24 39 41 49 55 58 60 65 66 68 71 72 75 76 78 79* 85 87 89* 90 91 93 94 95 97 99 101 103 104 106 107 111 112 114 116 133B* 138 ~ 143' 151' 153~ 156 ~

Location

Elevation (m)

Latitude (N)

Longitude (E)

Suq al Hamam Suq al Hamam Suq al Hamam Jihana-Mareb Hajjah Akbra (clast) Akbra (clast) Kohlan (sst) Kohlan (sst) Kohlan (sst) Kohlan (clast) Jihana-Mareb Jihana-Mareb Jihana-Mareb Jihana-Mareb Jihana-Mareb Wadi Habab At Turbah At Turbah (sst) Wadi Dar (sst) Rahidah-Aden AI Mukalla Burum Wadi Hajar Burum Aswad Nusa-Haban sst Nusa-Haban Huban-Ataq Haban-Ataq Wadi Rafad Wadi Rafad Nisab Nisab Nusa-Lawdar Nusa-Lawdar Nusa-Lawdar Lawdar Thirwah scarp Thirwah scarp Thirwah scarp Thirwah scarp Tbirwah scarp Thirwah scarp Thirwah scarp Thirwah scarp Thirwah scarp AI Bayda-Rada AI Bayda-Rada AI Bayda-Rada A1 Bayda-Rada At Turbah At Turbah Wadi Alasan Wadi Bana N. Batays E. Awabih

970 1380 1710 2080 1380 1640 1640 1680 2160 2160 2160 2120 1950 2070 2000 1740 1580 1343 1233 2000 433 95 127 230 130 800 412 444 1304 1323 250 310 1001 1240 659 814 814 1060 1310 1460 1580 1680 1930 1800 2190 2260 2260 1873 1917 1970 2040 1628 1216 512 118 282 1535

15.520 I5.520 15.520 15.279 15.724 15.742 15.742 15.742 15.728 15.728 15.728 15.192 15.217 15.267 15.306 15.308 15.417 13.205 13.245 15.478 13.303 14.582 14.380 14.233 14.341 14.419 13.999 14.009 14.368 14.341 13.782 13.787 14.395 14.395 13.952 13.912 13.913 13.880 13.910 13.910 13.910 13.910 13.910 13.910 13.910 13.910 13.910 14.077 14.240 14.272 14.356 13.161 13.158 13.160 13.395 13.354 13.816

43.584 43.584 43.584 44.710 43.617 43.675 43.675 43.708 43.717 43.717 43.717 44.667 44.722 44.744 44.775 44.783 44.875 44.129 44.017 44.080 44.734 49.156 48.944 48.667 48.944 47.708 46.618 46.661 46.917 46.874 46.658 46.658 46.462 46.452 46.345 46.046 46.047 45.880 45.731 45.731 45.731 45.731 45.731 45.731 45.731 45.731 45.731 45.487 45.351 45.249 45.007 44.114 44.129 44.581 45.254 45.254 44.806

Nc

Spontaneous Ps(N0

Induced PI(N~)

20 6 9 9 20 20 20 15 20 26 17 15 20 17 20 8 20 20 20 13 20 20 20 10 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 16 20 20 17 16 20 10 20 20 13 20 20 20 20 20 15 20 8

0.550 (202) 0.171 (18) 0.154 (81) 2.736 (360) 0.213 (1791 0.141 (239) 0.300 (2141 0.180 11441 0.327 1331) 1.117 (1237) 0.282 (226) 3.934 (1783) 0.768 (934) 1.811 (788) 1.568 11276) 1.751 (432) 3.621 (21431 0.099 (89) 0.195 (172) 0.290 (209) 0.278 (362) 0.840 (377) 0.589 (707) 1.262 (299) 0.545 (446) 0.591 (314) 0.663 (554) 0.343 (540) 0.566 (533) 0.647 (332) 0.461 (385) 0.373 (366) 0.837 (10071 0.697 (616) 0.708 (473) 0.734 (1026) 0.278 (4101 0.385 (400) 0.846 (347) 0.564 (182) 1.032 (366) 0.656 (302) 0.825 (264) 0.627 (198) 0.685 (208) 0.716 (122) 4.595 (1552) 1.606 (1485) 1.405 (960) 2.727 (2823) 0.947 (986) 0.235 (228) 0.109 (139) 0.406 (317) 0.230 (168) 0.786 (451) 4.99 (1731

8.610 (31601 2.556 (269) 2.216 (1163) 1.733 (228) 0.287 (24171 1.379 (2332) 2.972 (2117) 1.782 (1423) 2.433 (2465) 1.750 (19381 2.922 (23811 2.054 (931) 0.420 (5111 1.153 (502) 1.066 (867) 0.985 (243) 2.085 112341 1.037 (937) 2.063 11821) 2.184 (1574) 3.213 (4177) 0.837 (376) 1.574 (1889) 1.051 (249) 1.762 (1443) 3.932 (2087) 3.532 (2952) 1.744 (2743) 1.120 110561 1.455 (746) 2.727 (2279) 2.642 (2596) 1.401 (1686) 1.032 1913) 3.660 (2445) 2.758 (3857) 1.613 (2379) 1.740 (1806) 0.802 (329) 0.626 (202) 1.009 (358) 0.698 (3211 0.612 (196) 0.551 (174) 0.442 (134) 0.698 (119) 3.734 (1257) 1.445 11336) 1.080 (738) 1.990 (2060) 0.753 (784) 2.183 (21181 1.09 (1392) 3.690 (2879) 1.582 (1158) 0.969 (556) 1.47 1518)

Dosimetry P~(N~) 1.309 1.309 1.309 1.309 1.269 1.233 1.269 1.269 1.269 1.170 1.269 1.299 1.233 1.347 1.347 1.233 1.233 1.269 1.233 1.233 1.269 1.380 1.380 1.299 1.380 1.380 1.380 1.299 1.299 1.380 1.178 1.233 1.299 1.380 1.347 1.233 1.299 1.299 1.380 1.380 1.380 1.380 1.380 1.380 1.380 1.380 1.380 1.299 1.233 1.233 1.299 1.130 1.130 1.130 1.360 1.360 1.360

(9068) (9068) (9068) (9068) (8788) 18471) (8788) (8788) (8788) (8107) (8788) (8996) (8471) (9337) (9337) (8471) (84711 (8788) (8471) (8471) (8788) (9551) (95511 (8996) (95511 (9551) (9551) (8996) (8996) (95511 (81611 (8471) (8996) (95511 (9337) (8471) (8996) (8996) 19551) 195511 (95511 (95511 (95511 (95511 (9551) (9551) (95511 (8996) (84711 (84711 (8996) (4700) (4700) (4700) (4700) (4700) (4700)

Relative error % 8 13 #m) tend to be associated with ages 300 Ma) tend to increase as the age increases. As we are dealing solely with surface samples, it is probable that these will all have cooled during denudation, although there may be short-term secondary magmatic influence, on the Red Sea margin, as mentioned earlier. The relationship between FT age and mean track-length can be understood in terms of the maximum palaeotemperature prior to any subsequent cooling episode for each sample. Of course, for apatite fi:;sion track data, we can only infer information about palaeotemperatures experienced since the sample cooled below 120 _+ 10°C, or the time since fission tracks were retained until the present day. In the simplest scenario, this hypothesis about samples residing at different levels in the crust prior to cooling assumes that ( 1 ) a well defined partial annealing zone (PAZ) existed prior to cooling; (2) a cooling episode was initiated at the same time for all samples. Given these conditions, there are three possible situations for a given sample. First is complete resetting. In this situation, a sample initially at temperatures in excess of ~120 _+ 10°C retains fission tracks only after cooling below this temperature. If the rock cools rapidly to temperatures less than about 50°C, then all subsequently formed fission tracks will be relatively long ( > 14 ~tm), forming a unimodal tracklength distribution and the fission-track age approximates the time of cooling (i.e., start of track retention). The second case is a sample not significantly affected by the thermal event, nor any other thermal event over a timescale similar to the measured fission track age. Such an undisturbed sample would initially reside at temperatures of 1 3 - 1 4 /zm, reflecting the contribution of tracks formed prior to cooling as well as those formed afterwards. Tracks shorter than 14 #m can be attributed to earlier thermal events but not of sufficient magnitude to anneal fission tracks totally. The third situation is a sample initially residing within the PAZ (i.e., at tempera-

2519

tures between - 5 0 and -120°C). In this case the tracklength distribution will have a component of shortened tracks formed prior to cooling (i.e., in equilibrium with an elevated temperature) and a component of longer tracks formed after the cooling episode (i.e., in equilibrium with a lower temperature). As a result, the track-length distribution will tend to be bimodal, with a short mean track-length (e.g., 10-12 #m), and the fission track age will be somewhere between the young cooling age and the older undisturbed age. The bimodality of the track length distributions and the value of the mean FT age depend on the proportion of pre- and postcooling tracks. Consequently, the fission track age for such samples will generally not provide direct information about the timing of a single cooling event. To quantify this overall scenario, we use some simple forward model calculations. In these, we specify a thermal history and predict the fission track age and track length distribution. For these calculations, we have used the annealing model of Laslett et al. (1987), which is calibrated for Durango apatite. We have specified two thermal histories commencing at 350-550 Ma with two possible cooling episodes initiated at 25 Ma. In the first the cooling duration is 25 Ma and in the second it is only 5 Ma, in line with our earlier qualitative interpretations. The timing of initiation is essentially the same as that inferred by Omar and Steckler (1995) for the Red Sea margins of Saudi Arabia and Egypt. We model cooling from initial temperatures between 20 and 130°C but all thermal histories end up at 20°C at the present day, a reasonable value for the average near-surface temperature in Yemen. However, this final temperature could vary between 0 and 30°C as the model predictions are not particularly sensitive to a value in this range. Figure 4a illustrates model predictions for the 25 m.y. cooling duration and Fig. 4b for the 5 m.y. durations. The initial temperatures for each sample are indicated on the fi~ures. The most significant difference in the predictions of these two models is primarily reflected in the shape of the track-length distributions and the mean track-lengths for younger predicted ages. The more prolonged cooling episode ( 2 5 - 0 Ma) leads to negatively skewed distributions with shorter means (Fig. 4a). If we compare the predictions to the observed length distributions (Fig. 3), we can see that, at this level, the data are more consistent with the rapid cooling model ( 2 5 - 2 0 Ma, Fig. 4b). However, the youngest predicted ages are - 5 m.y. older than the youngest observed ages, and the initiation of cooling would need to be more recent than 25 Ma to explain these data. Thus, we consider 25 Ma is an upper limit on the timing of the most recent significant cooling episode. While the FT data from the Red Sea margin of Yemen appear to be consistent with a cooling event around 20-25 Ma, the data on the southern Gulf of Aden margin, particularly those east of ca 45°E, may reflect an earlier event >30 Ma, and/or less denudation after 25 Ma. Again this is consistent with the structural and sedimentological history (Watchorn et al., 1996). As mentioned earlier, the data from the Gulf of Aden margin can show large variations in FT parameters over relatively short distances. For example, samples collected NE of Aden and in the Mukalla region show variations in FT age of ~ 150-200 Ma over distances of - 2 0 kin. This

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Fig, 3, The relationship between fission track age and mean track-length, with selected track-length distributions (the sample ID is given at the top left and FT age at the top right). The open and filled circles are data west and east of longtitude 44.8l°E, respectively. The data represented by the open circles most probably reflect the tectonic processes related to formation of a volcanic margin and the eventual opening of the Red Sea. In contrast the data represented by the filled circles record processes associated with the nonvolcanic Gulf of Aden margin.

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Apatite FT age (Ma) Fig. 4. (a) Model results for continuous cooling between the onset of rapid cooling ca 25 Ma and the present day to a final temperature of 20°C. Three sets of results are shown with the temperature history commencing at 350, 450, and 550 Ma and the crosses mark 5°C intervals for the pre-cooling temperature. Selected predicted track-length distributions are shown and labelled with the pre-cooling temperatures. The observed data are labelled as for Fig. 3. (b) As 4a, but with cooling to 20°C between 25 and 20 Ma. The temperature between 20 Ma and the present day was 20°C for all samples.

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Crustal cooling in the Red Sea rift may be explained by differential denudation or different exposure levels on a tilted fault block, as has been noted on this margin (e.g., Watchorn et al., 1996). Small differences in pre-denudation temperature (10-20°C) can result in striking differences in FT age and MTL such as can be inferred from Fig. 4. Using the model results (Fig. 4) we can make an estimate of the pre-denudation temperature for a given combination of fission track age and mean track length. Estimates for the samples from the Gulf of Aden margin (i.e., Yem 58, 60, 65, 66, 151, and 153) indicate a spread in FT parameters that can arise from a temperature difference of 25°C (Fig. 5 ), equivalent to - 1 km of differential denudation or equivalently exposure of ~ 1 km of upper crustal rocks along a fault block. It should be noted that this inferred

2523

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Fig. 6. Approximatepalaeotemperature contours ( i.e., pre-cooling temperature) for the Yemen data, estimated from Fig. 4. The Red Sea margin data generally indicate higher palaeo-temperatures than that from the Gulf of Aden margin. (H-Hodeidah, S-Sana'a, AB-AI Bayda, A-Aden, M-Mukalla).

a FT Age (Ma) 50

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AFT age (Ma)

Fig. 5. (a) AFT data for samples from the eastern Gulf of Aden margin plotted against the pre-cooling temperature (open squares) from Fig. 4, and mean track length (MTL) plotted against temperature (filled circles). The large spread in fissiontrack can be explained by ~25°C variation in the pre-cooling temperatures. (b) AFT age vs. elevation (metres) for the Red Sea volcanic margin (open circles) and the Gulf of Aden nonvolcanic margin and rift shoulder (filled circles). Note that on the Red Sea margin cooling ages are predominantly 2500 metres of subaerial flows with no submarine equivalents. Syn-volcanic denudation and extension is lacking throughout the volcanic stratigraphy. Minor syn-volcanic denudation and sedimentation occurs toward the end of the volcanic activity consitent with a decline in eruption rates and formation of high level magam chambers. Continued surface uplift. 3) 2 6 - 2 0 Ma. Granite-gabbro-syenite intrusives and hypabyssal activity. Unroofing of the Yemen LIP and exposure of the granite-syenite bodies. Extension, cooling, and unroofing of the Red Sea volcanic margin. It cannot be disputed that the change from deposition of monotonous, mature arenites during the Cretaceous-Tertiary to the development of palaeosols immediately beneath the volcanic units indicates a change in base level on the order of tens of metres, which preceded the initiation of plumesupplied magmatism between 3 1 - 2 6 Ma. (Baker et al., 1996b). One can infer that surface uplift occurred between 3 1 - 2 6 Ma. on the basis of evidence for a major denudational/cooling event on the Red Sea volcanic margin at 3 2 Ma), and post-volcanic erosion of a >3000 metre-high rift margin ( < 2 6 Ma). 8. CONCLUSIONS 1 ) FT database. The Yemen fission track dataset displays features in common with other FT datasets from rift-

2)

3)

4)

5)

6)

7)

2525

flanks throughout the world (see Gallagher and Brown, 1996) and allows us to define distinct domains. The youngest apparent ages are generally concentrated on the coastal margins proximal to the regions of active extension and greatest present-day topographic relief. This is particularly striking on the Red Sea margin. Data from lower elevations on the Gulf of Aden margin may reflect differential movement on fault blocks on a relatively local scale. The relatively distal interior plate regions are characterised by older apparent ages and correspond to lower relief, nonrifted crust. As summarised in Fig. 6 these regions correspond to different pre-denudational temperatures for the samples now at the surface. Thermal domains. The FT thermal domains closely match structural, geological, and topographic provinces. In the west of Yemen, the area that was rapidly cooled and extended in the Tertiary coincides with a zone of late Oligocene-early Miocene extension. The products of this unroofing exist as Miocene or younger sediments (ca 3000 m thick) further to the west. To the east, Jurassic terranes reactivated in the Tertiary (e.g., Ellis et al., 1996) coincide with Jurassic grabens full of post-Jurassic sediments to depths in excess of 5 km. that now occupy inverted basin running from the coast to the interior. Basement highs, exhumed in pre-Jurassic times, coincide with structural highs of Archaean-Proterozoic basement that are largely devoid of Jurassic-Tertiary sediments and constitute a peneplaned interior, but which has undergone little Tertiary denudation. Volcanic margin. FT data indicates that rapid cooling on the Red Sea volcanic margin occurred 25 Ma. This is consistent with AFT data that indicate widespread cool-

2526

M. Menzies et al.

ing on the Red Sea margin at < 2 5 Ma. The marine to continental transition within the pre-volcanic ( > 3 1 M a ) sediments and the d e v e l o p m e n t of thick palaeosols is interpreted as the earliest expression of surface uplift. Overall it is apparent that the d e v e l o p m e n t of shallow m a g m a chambers (granite-gabbro-syenite), denudation, and extension largely post-dated the main period of basaltic volcanism ( 3 2 - 2 9 M a ) . The Y e m e n volcanic margin of the Red Sea developed in response to surface uplift and m a g m a t i s m which was followed ca 5 m.y. later by extension and denudation.

Acknowledgments--This work was undertaken with a research grant from the NERC (GR3/8457 Menzies and Hurford) for which we are grateful. We wish also to acknowledge the administrative and logistical support of the Department of Geology, University of Sana'a, Yemen Republic. Fieldwork would have been impossible without the support of our Yemeni counterparts and the skill of Yemeni drivers. We would particularly like to thank Drs. Salah A1-Khirbash, Mohammed Al'Kadasi, and Abdulkarim Al'Subbary. Joel Baker is thanked for his assistance during the 1992 field excursion in Yemen and for discussion of this paper. Colleagues within the Arabian Plate Geological Research Group at Royal Holloway and the Fission Track Research group at UCL are thanked for general comments and discussion. We are indebted to Andy Carter at University College who undertook additional AFT analyses. Thanks to our reviewers, John Garver and Paul Fitzgerald, who expended considerable time and effort on an earlier version of this manuscript and provided lengthy, detailed, and constructive comments. Finally the British Council (Jim McGrath) and the University of Sana'a (Prof. Al'Qirby) are thanked for initiating the Yemen research project in 1989-90. Editorial handling: J. Morris REFERENCES

Al'Kadasi M. (1995) Temporal and spatial evolution of the basal flows of the Yemen Volcanic Group. Doctoral thesis, Royal Holloway Univ. London. Al'Subbary A. A. (1996) The sedimentology and stratigraphy of the Cretaceous-early Tertiary Tawilah group, western Yemen. doctoral thesis, Royal Holloway Univ. London. Al'Thour K. (1992) Stratigraphy, Sedimentology and Diagenesis of the Amran Group (Jurassic) of the region to the west and northwest of Sana'a Yemen Republic. Doctoral thesis, Univ. Birmingham. Almond D.C. (1986) Geological evolution of the Afro-Arabian dome. Tectonophysics 131, 301-332. Baker J., Menzies M., Snee L., and Thirwall M. (1994) Stratigraphy 4°Ar/39Ar geochronology and geochemistry of flood volcanism in Yemen. V. M. Goldschmidt meeting, Edinburgh. (abstr.). Baker J., Snee L., and Menzies M. A. (1996a) A brief Oligocene period of flood volcanism in Yemen: Implications for the duration and rate of continental flood volcanism at the Afro-Arabian triple junction. Earth Planet. Sci. Lett. 138, 39-55. Baker J., Thirlwall M.F., and Menzies M. A. (1996b) Sr-Nd-Pb isotopic evidence for crustal contamination of plume-derived flood basalts: Oligocene flood volcanism in western Yemen. Geochim. Cosmochim. Acta 60, 2559-2581. Baker J., Menzies M. A., Snee L., and Al'Kadasi M. ( 1997 ) Evaluating the usefulness of whole-rock K-Ar dating in a young large igneous province. Geol. Soc. Amer. Mem. (in prep.). Beaumont C., Kamp P. J. J., Hamilton J., and Fullsack P. (1996) The continental collision zone, South Island, New Zealand: Comparison of geodynamical models and observations. J. Geophys. Res. 101, 3333-3359. Beydoun Z. R., Bamahmoud M. O., and Nani A. S. O. (1993) The Qishn Formation, Yemen: Lithofacies and hydrocarbon habitat. Mar. Petrol. Geol. 10, 364-372.

Blakey S., Menzies M. A., and Thirlwall M. F. (1994) Geochemistry of Tertiary within-plate (A-type) granitoids at a passive margin, southern Red Sea. Int. Assoc. Volcano1. Chem. Earth's Interior (abstr). Bohannon R.G., Naeser C.W., Schmidt D.L., and Zimmerman R. A. (1989) The timing of uplift, volcanism and rifting peripheral to the Red Sea: A case for passive rifting. J. Geophys. Res. 94, 1683-1701. Bott M. H. P. (1992) The stress regime associated with continental break-up. In Magmatism and the Causes of Continental BreakUp (ed. B. Storey et al.); Geol. Soc. London Spec. publ. 68, 125136. Bott W. F., Smith B.A., Oakes G., Sikander A.H., and Ibraham A.I. (1992) The tectonic framework and regional hydrocarbon prospectivity of the Gulf of Aden. J. Petrol. Geol. 15, 211-243. Bourman R. P. (1993) Modes of ferricrete genesis: Evidence from southeastern Australia. Z. Geomorph. N.F. 37, 77-101. Brandon M. T. and Vance J. A. (1992) New statistical methods for analysis of fission-track grain-age distributions with applications to detrital zircon ages from the Olympic subduction complex, western Washington State. Amer. J. Sci. 292, 565-636. Brown R. W., Summerfield, M. A., and Gleadow A. J. W. (1994) Apatite fission track analysis: Its potential for the estimation of denudation rates and implications for models of long-term landscape developments. In Process Models and Theoretical Geomorphology (ed. M. J. Kirkby), pp. 23-53. John Wiley and Sons Ltd. Carlson W. D. (1990) Mechanisms and kinetics of apatite fissiontrack annealing. Amer. Mineral. 75, 1120-1139. Cerveny P. F., Naeser N. D., Zeitler P. K., Naeser C. W., and Johnson N.M. (1988) History of uplift and relief of the Himalaya during the past 18 million years: Evidence from fission-track ages of detrital zircons from sandstones of the Siwalik Group. In New Perspectives in Basin Analysis (ed. K. Kleinspehn and C. Paola), pp. 43-61. Springer-Verlag. Chazot G. and Bertrand H. (1993) Mantle sources and magmacontinental crust interactions during early Red Sea-Aden rifting in southern Yemen: Elemental and strontium, neodymium, lead isotopic evidence. J. Geophys. Res. 98, 1819-1835. Cloos H. (1939) Hebung-Spaltung-Vulkanismus. Geol. Rundsch. 30, 405-527. Cochran J. R. ( 1981 ) The Gulf of Aden: structure and evolution of young ocean basin and continental margin. J. Geophys. Res. 86, 263-288. Corrigan J. ( 1991 ) Inversion of fission track data for thermal history information. J. Geophys. Res. 96, 10347-10360. Cox K. G. (1989) The role of mantle plumes in the development of continental drainage patterns, Nature 342, 873-877. Crossley R., Watkins, C., Raven M., Cripps D., Carnell A., and Williams D. (1992) The sedimentary evolution of the Red Sea and Gulf of Aden. J. Petrol. GeoL 15, 157-172. Davison I. et al. (1994) Geological Evolution of the southern Red Sea rift margin - Republic of Yemen. Geol. Soc. Amer. Bull. 106, 1474-1493. Davison I., Tatnell M., Owen L. A., Jenkins G., and Baker J. (1997) Tectonic geomorphology and rates of crustal processes along the Red Sea margin, northwest Yemen. In The Sedimenta~ Record of Rift Basins: The Red Sea-Gulf of Aden Case Stu@ (ed. B. Purser and D. Bosence). Chapman and Hall (in press). Dixon T.H., Ivins E.R., and Franklin B.J. (1989) Topographic and volcanic asymmetry around the Red Sea: Constraints on rift models. Tectonics 8, 1193-1216. Ellis A. C., Kerr H. M., Cornwell C. P., and Williams D. O. (1996) A tectono-stratigraphic framework for Yemen and its implications for hydrocarbon potential. Petrol. Geosci. 2, 29-42. Fitzgerald P. G., Sorkhabi R. B., Redfield T. F., and Stump E. ( 1995 ) Uplift and denudation of the central Alaska Range: A case study in the use of apatite fission track thermo-chronology to determine absolute uplift parameters. J. Geophys. Res. 100, 20175-20191. Galbraith R.F. and Laslett G.M. (1993) Statistical models for mixed fission track ages. Nucl. Tracks Radiat. Meas. 21, 459470. Gallagher K. (1995) Evolving thermal histories from fission track data. Earth Planet. Sci. Lett. 136, 421-435.

Crustal cooling in the Red Sea rift Gall agher K. and Brown R. W. (1996) The onshore record of passive margin evolution. J. Geol. Soc. London Gallagher K., Hawkesworth C., and Mantovani M. (1994) The denudation history of the onshore continental margin of S. E. Brazil inferred from fission track data. J. Geophys. Res. 99, 1811718145. Garver J. I. and Brandon M. T. (1994) Erosional denudation of the British Columbia Coast Ranges as determined from fission-track ages of detrital zircon from the Tofino Basin, Olympic Peninsula, Washington. Geol. Soc. Amer. Bull. 106, 1398-1412. Gass I. G. (1970) Tectonic and magmatic evolution of the AfroArabian dome. In African Magmatism and tectonics (ed. T. E. Clifford and I. G. Gass), pp. 285-300. Oliver and Boyd. Gass I. G., Chapman D. S.. Pollack H. N., and Thorpe R. S. (1978) Geological and geophysical parameters of mid-plate volcanism. Phil. Trans Roy. Soc. London A288, 581-597. Girdler R.W. (1991) The Afro-Arabian rift system-an overview. Tectonophysics 197, 139-153. Gleadow A. J. W. and Duddy I. R. (1981) A long term natural annealing experiment for apatite, Nucl. Tracks Radiat. Meas. 5, 169174. Gleadow A. J. W., Duddy I.R., Green P.F., and Lovering J.F. (1986) Confined fission track lengths in apatite: A diagnostic tool for thermal history analysis. Contrib. Mineral. Petrol. 94, 405415. Green P. F. (1986) On the thermotectonic evolution of Northern England: Evidence from fission track analysis. Geol. Mag. 123, 493 -506. Green P. F., Duddy I. R., Gleadow A. J. W., Tingate P. T., and Laslett G. M. (1986) Thermal annealing of fission track in apatite 1, A qualitative description. Chem. Geol. (lsot. Geosci. Sect.) 59, 237-253. Hill R. I. (1991) Starting plumes and continental break-up. Earth Planet. Sci. Lett. 104, 398-416. Houseman G. A. (1990) The thermal structure of mantle plumes: Axisymmetric or triple junction? Geophys. J. Int. 102, 15-24. Hurford A. J., and Carter A. ( 1991 ) The role of fission track dating in discrimination of provenance. Geol. Soc. London Spec. Pub. 57, 67-78. Hurford A. J., and Green P. F. (1982) A user's guide to fission track dating calibration. Earth Planet. Sci. Lett. 59, 343-354. Laslett G.M., Green P.F., Duddy I.R., and Gleadow A. J. W. ( 1987 ) Thermal annealing of fission tracks in apatite 2. A quantitative analysis. Chem. Geol. (lsot. Geosci. Sect.)65, 1-13. Lutz T. M. and Omar G. ( 1991 ) An inverse method of modelling thermal histories from apatite fission-track data. Earth Planet. Sci. Letts. 104, 181-195. Menzies M. et al. (1992) The timing of crustal magmatism, uplift and crustal extension-preliminary observations from Yemen. In Magmatism and the causes of continental break-up ( ed. B. Storey et al.); Geol. Soc. Lond. Spec. Publ. 68, 293-304. Menzies M. A. et al. (1994) Geology of the Republic of Yemen. In The Geology and Mineral Resources of Yemen: Ministry of Oil and Mineral Resources. Geological Survey and Minerals Exploration

2527

Board, Yemen Mineral Sector Project (ed. D. A. McCombe et al.), Technical Report 21-48. Menzies M. A., Baker J., Chazot G., and Al'Kadasi M. (1997) Evolution of the Red Sea volcanic margin, western Yemen. In Large Igneous Provinces (ed. J. Mahoney), (in press). American Geophysical Union. Mitchell D. J. W., Allen R. B., Salama W., and Abouzakm A. ( 1992 ) Tectonostratigraphic framework and hydrocarbon potential of the Red Sea. J. Petrol. Geol. 15, 187-210. Moore M. E., Gleadow A. J. W., and Lovering J. F. (1986) Thermal evolution of rifted continental margins: New evidence from fission tracks in basement apatites in southeastern Australia. Earth Planet. Sci. Lett. 78, 255-270. Omar G. I. and Steckler M. S. ( 1995 ) Fission track evidence on the initial rifting of the Red Sea: Two pulse, no propagation. Science 270, 1341 - 1344. Overstreet W. C., Stoeser D. B., Overstreet E. F., and Goudarzi G. H. ( 1977 ) Tertiary laterite of the As Sarat Mountains, Asir province, Kingdom of Saudi Arabia. Miner. Res. Bull. Saudi Arabia Dir. Gen. Miner. Res. 21, 1-30. Pollack H. N., Hurter S.R., and Johnson J.R. (1993) Heat flow from the Earth's interior: analysis of the global data set. Rev. Geophys. 31, 267-280. Redfern P. and Jones J.A. (1995) The interior rifts of Y e m e n analysis of basin structure and stratigraphy in a regional plate tectonic context. Basin Res. 7, 337-356. Sahota G., Stules P., and Gerdes K. (1995) Evolution of the Gulf of Aden and implications for the development of the Red Sea. Rift Sedimentation and Tectonics in the Red Sea-Gulf of Aden Region, Sana'a Yemen 56. Sambridge M. S. and Compston W. (1994) Mixture modelling of multi-component data sets with application to ion-probe zircon ages. Earth Planet. Sci. Lett. 128, 373-390. Seng6r A. M. C. and Burke K. (1978) Relative timing of rifting and volcanism on Earth and its tectonic implications. Geophys. Res. Lett. 5, 419-421. Summerfield, M. A. (1991) Subaerial denudation of passive margins: regional elevation vs. local relief models. Earth Planet. Sci. Lett. 102, 460-469. Turcotte D.L. and Emerman S.H. (1983) Mechanisms of active and passive rifting. Tectonophys. 94, 39-50. Watchorn F. ( 1995 ) The role of pre-rifting structures in the Tectonostratigraphic evolution of the Gulf of Aden margin: Hadramaut Province, Yemen. Rift Sedimentation and Tectonics in the Red Sea-Gulf of Aden Region, Sana'a Yemen, October, 79, 1995. Watchorn F., Nichols G.J., and Bosence D. (1996) Rift-related sedimentation and stratigraphy in southern Yemen. In The Sedimentary Record of Rift Basins: The Red Sea-Gulf of Aden Case Stucl~, (ed. B. Purser and D. Bosence). Chapman and Hall (in press ). Wheeler J. and Butler R. W. H. (1994) Criteria for identifying structures related to true crustal extension in orogens. J. Struct. Geol. 16, 1023-1028. White R. and McKenzie D. (1989) Magmatism at rift zones: The generation of volcanic continental margins and flood basalts. J. Geophys. Res. 94, 7685-7729.