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Back rotation of individual faults, accompanied by palinsepastic reconstruction of fault positions and accommodation for slippage along the northeastern- oriented Neogene faults of the shear zone, produced the most probable pre-Neogene configuration for many of these east-west late Precambrian faults. Such reconstruction resulted into a northwest-trending set of Precambrian faults which form along-strike continuation of the Najd Fault System faults. Therefore, we interpret the Precambrian faults in southwestern Jordan and northwestern Saudi

Arabia as segments of the Najd Fault System which were subjected to rotation in response to major drag along the Agabah Gulf - Dead Sea transform shear zone.

Our conclusion is that the structural features of the Precambrian faults in southwest Jordan and northwest Saudi Arabia suggest that the faults of the Najd Fault System propagate into this region, and that its seeming absence is only a manifestation of the later disruption induced by the structural overprinting of Neogene rotational strain.

Gondwana Research, V 4, No. 2, pp . 165-1 68. 0 2001 International Association for Gondwana Research, Japan. ISSN: 1342-937X

Tectonic Evolution of the Itremo Region (Central Madagascar) and Implications for Gondwana Assembly

Alain Fernandezl, Sarah Huberl, Guido Schreursl, Igor Villa2 and Michel Rak~tondrazafy~

Geologisckes lnstitut, Universitat Bern, Baltzerstr. 2,3022 Bern, Germany Mineralogisck-Petrograpkisckes Institut, Universitat Bern, Baltzerstr. 1,3012 Bern, Germany Universite' d'Antananarivo, BP 906 Ankatso, Madagascar

The East African orogen (or Pan-African Mozambique belt) is the result of Neoproterozoic and early Paleozoic plate interactions that led to the closure of the Mozambique Ocean and subsequent assembly of Gondwana (e.g. Stern, 1994). The exact geographic extent, timing and nature of this major orogenic belt is still a matter of debate. Remnants of the East African orogen have been documented in the Arabian-Nubian shield, eastern Africa, Madagascar, India, Sri Lanka and Antarctica. The Itremo Region in central Madagascar occupied a central location within the East African Orogen and holds a key position for a better understanding of the timing and nature of events that led to Gondwana assembly.

The Precambrian basement of central Madagascar is dominated by strongly deformed, high-grade para- and orthogneisses (upper amphibolite or granulite facies metamorphism). The Itxemo Region in central Madagascar,

however, also comprises a sequence of lower grade (greenschist to lower amphibolite grade) metasedimentary rocks that are referred to as SQD-series (Moine, 1974) or Itremo Group (Cox et al., 1998). They consist of strongly deformed quartzites, metapelites and dolomitic carbonate rocks that have been interpreted as a former shallow continental shelf or platform environment (Moine, 1974; Cox et al., 1998). U-Pb geochronology of intrusive rocks in the Itremo Region and its immediate surroundings has revealed two major intrusive events at ca. 796-776 Ma and 570-540 Ma (Handke et al., 1999). However, these ages have not been related to the complex tectono- metamorphic evolution of the Itremo Group. Therefore, many questions remain about the age of structures and the relationships between structures and metamorphism.

Our structural, petrographical and geochronological data of the Itremo Region differ from previous studies, with most important distinctions related to the geographic

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19'40's-

Magmatic Rocks Post:kinematic intrusive granite (435 Ma ?)

Syn to post-kinematic magmatic rocks (790 Ma)

Late Proterozoic or Early paleozoic low grade metasediments (Itremo Group

Dolomitic carbonate rocks

Micaschists

0 Quartzites

Precambrian basement Migmatites. migmatitic gneisses and undifferentiated gneisses

Sillimanitc quartzite

Undetenninated units

20"25'S-

5'E

Fig. 1. Geological map of Itremo Region (modified from 1/100,000 geological maps, data fields and remote sensing analysis) with inside simplified ztruatural map of Ampozana area showing recumbent fold, facing downward, refolded by north-south fold (from 1999 field campaign).

extent of the Itremo Group (Fig. l), its depositional age, metamorphic grade, tectonic transport direction and timing of deformation. The main structures of the Itremo Region have been determined by extensive field work and by remote sensing studies using over 250 aerial photographs (scale 1: 75,000), and both Landsat TM and

SPOT satellite data. Mosaics of aerial photographs and satellite images were geometrically corrected in order to accurately portray large-scale geological structures. The presence of numerous stratigraphic polarity criteria in quartzites (e.g. cross-bedding, ripple marks) and carbonates (stromatolites) of the Itremo Group allowed

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Younging direction

Facing of F2 folds

Facing of F1 folds

Axial plane of F, fold

Axial plane of Fz fold

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the determination of structural facing directions. Overprinting relationships and structural facing directions clearly indicate that two main deformation phases affected the Itremo Group. During the first deformation phase, W to SW-directed thrusting and associated major recumbent folding of Itremo Group sediments (and possibly basement slices) occurred at low-grade (greenschist to lower amphibolite facies) metamorphic conditions. This deformation resulted in a penetrative axial plane foliation in suitable lithologies. A second phase of (probably transpressional) deformation affected the Itremo Group and resulted in the formation of steep shear zones and large-scale roughly N-S trending folds with steeply dipping axial planes. Depending on the initial orientation of first-phase structures in Itremo Group metasediments or pre-existing structures in high-grade basement rocks, second-phase overprinting resulted in type 1 dome-and-basin structures (e.g. in high-grade basement rocks north and west of the Itremo Group), and type 2 or type 3 interference patterns in metasediments of the Itremo Group (cf. inset in Fig. 1). Spectral analysis of Landsat TM images allowed us to distinguish between high-grade sillimanite-bearing quartzites and low-grade Itremo Group quartzites. The contact between these two units in the western, southern and northern part of the Itremo Region is a thrust contact along which low-grade Itremo Group sediments were transported west to southwestward on top of high-grade basement rocks. This tectonic contact is deformed by the second phase (with respect to deformation of the Itremo Group) N-S trending folds and shear zones. In the eastern part of the Itremo Region, the tectonic contact between low-grade Itremo Group metasediments and high-grade basement rocks to the east has an overall westward dip and seems to postdate contractional deformation within the Itremo Group in the hangingwall. This contact has been interpreted by Collins et al. (2000) as part of a major N-S trending extensional

Age 540 Ma Muscovite

440 il = = = =: Sample A18

- Sample A8 -

0 20 40 60 80 100 %39Ar

Fig. 2. Ar-Ar dating of white mica in Itremo metasediments, yield plateau ages of 490 Ma, which are associated with first-phase penetrative foliation (Huber, 2000).

detachment (Betsileo shear zone) associated with orogenic collapse of the East African Orogen in central Madagascar.

Although previous workers have proposed a westward increase in metamorphic grade from greenschist to granulite facies in the Itremo Region (Moine, 1974; Windley et al., 1994; Collins et al., 2000), we find no evidence for such a major gradient within the Itremo Group. High-grade metamorphic rocks surround the low- grade Itremo Group metasediments in map view and occur in tectonic windows, structurally overlain by the Itremo Group. Deformed gabbros and granites (magmatic age between 804 and 776 Ma; Handke et al., 1999) have been folded by the late N-S trending upright folds. These plutons are restricted to the high-grade basement, and do not intrude the low-grade metasediments of the Itremo Group. Therefore, their age does not provide a minimum depositional age for Itremo Group metasediments as previously postulated by Handke et al. (1999).

The age of the first-phase penetrative schistosity in metapelites of the Itremo Group was dated by Ar-Ar analysis at 492 Ma (Fig. 2; Huber, 2000; Fernandez et al., 2000). Chemical Th-U-Pb microprobe dating of newly grown outer rims of zoned monazites (elongated and aligned within the first-phase foliation) confirm this age, however with a larger error: 484k42 Ma and 481+55 Ma. Microprobe dating of the detritic cores of zoned monazites reveal minimum ages of 1680+34 Ma. These geochronologic data indicate that depositional age of the Itremo Group metasediments lies between 492 Ma and 1680k 34 Ma. Undeformed, post-tectonic intrusions which occur throughout the Itremo Region must be younger than 490 Ma. This upper limit is concordant with a 455+34 Ma age for a post-tectonic syenite complex in the central Itremo Region (Andriamampihantona, 1992).

Our investigations in the Itremo Region indicate that contractional deformation of the Itremo Group metasediments commenced in early Paleozoic times at about 500 Ma. The timing of deformation in the high- grade basement rocks of the Itremo Region is not known. 590-530 Ma deformation event at granulite facies metamorphic conditions has been reported for Precambrian basement rocks immediately south of the Itremo Region. The lack of such a high-grade event in the Itremo Group metasediments implies that they were either at higher crustal levels during this time interval or not yet deposited. Although the depositional age of the Itremo Group remains very poorly constrained, we speculate that sediments of the Itremo Group were possibly deposited in an early Paleozoic foreland basin (between about 530 and 500 Ma). They were subsequently folded and thrusted, and overprinted by N-S trending folds and shear zones during late Cambrian-Ordovician times. In case, the

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tectonic contact along the eastern margin of the Itremo Group is related to orogenic collapse, as proposed by Collins et al. (2000), this event must be younger than 500 Ma. The Itremo Region provides evidence that final assembly of Gondwana in central Madagascar continued into late Cambrian-Ordovician times.

References Andriamampihantona, M.J. (1992) Contribution a l’etude du

complexe alcalin d’ambatofinandrahana et de ses min6ralisations A lanthanides (Region centrale de Madagascar). Thitse de Doctorat, UniversitC Joseph Fourier, Grenoble I.

Collins, A.S., Razakamanana, T. and Windley, B.F. (2000) Neoproterozoic extensional detachment in central Madagascar: implications for the collapse of the East African orogen. Geol. Mag., v. 137, pp. 39-51.

Cox, R., Armstrong, R. and Ashwal, L. (1998) Sedimentology, geochronology and provenance of the Proterozoic Itremo Group, central Madagascar, and implications for pre- Gondwana palaeogeography. J. Geol. SOC. London, v. 155, pp. 1009-1024.

Fernandez, A., Huber, S. and Schreurs, G. (2000) Evidence for late Cambrian-Ordovician final assembly of Gondwana in central Madagascar. In: Geol. SOC. Amer. Annual Meeting, Reno, Nevada.

Handke, M., Ncker, R. and Ashwal, L. (1999) Neoproterozoic continental arc magmatism in west-central Madagascar. Geology v. 27, pp. 351-354.

Huber, S. (2000) Geologie und Geochronologie der Itremo Sedimente im Gebiet des Mont Ibity (Zentral Madagaskar). M.Sc. thesis, University of Bern, Switzerland, 97p.

Moine, B. (1974) Charactkres de sedimentation et de metamorphisme de series precambriennes epizonales a catazonales du centre de Madagascar. Sci. de la Terre Memoire, v. 31, 293p.

Stern, R.J. (1994) Arc assembly and continental collision in the Neoproterozoic East African orogen: implications for the consolidation of Gondwanaland. Ann. Rev. Earth Planet. Sci.,

Windley, B.F., Razafiniparany, A., Razakamanana, T. and Ackermand, D. (1994) Tectonic framework of the Precambrian of Madagascar and its Gondwana connections: a review and reappraisal. Geol. Rundsch., v. 83, pp.

V. 22, pp. 319-351.

642-659.

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Tectono-Thermal History of Eastern Eritrea: The Eastern Margin of West Gondwana Woldai Ghebreab

Department of Earth Sciences, University of Asmara, P.O. Box 1220, Asmara, Eritrea, E-mail: Woldai@earth.uoa.edu.er

Neoproterozoic rocks of the East African Orogen (EAO) in eastern Eritrea comprise two lithotectonic domains, Ghedem and Bizen. The Ghedem domain consists of high- grade metamorphic rocks with low angle tectonic grains dominant. The Bizen domain consists of low-grade metamorphic rocks with steep tectonic grain dominant, though sub-horizontal collapse structures are also common. These structures include flat-lying folds and low- angle detachments with top-to-west/northwest senses of shear. A major westward dipping low-angle discontinuity separates the two domains with the Bizen domain in the hanging wall. Along parts of this discontinuity pyrite, chalcopyrite (with or without malachite stains),

ferruginization, silicification and chloritization, invite future detailed investigation towards mineral exploration.

Three Pan-African D,., phases of deformation are recognized. D, deformation is characterized by a steep penetrative foliation S, which is axial planar to upright F, folds. F, folds were distorted by low-angle shear zones and F, recumbent folds during D,. D, resulted in steep N-S/NNE-SSW strike-slip shear zones and a steep S, spaced cleavage axial planar to open upright NNW trending F, folds.

Garnet rim-matrix and inclusion geothermobarometry indicate that rocks of the Ghedem domain experienced syn-metamorphic deformation and were exhumed from

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