Jacobs, Joachim

Titanite and apatite fission track (FT) thermochronology on 127 basement and 18 sedimentary rock samples from central and southern Madagascar record a complex cooling and denudation history since the Early Palaeozoic. Titanite FT analyses gave ages ranging between 483 Ma and 266 Ma. Apatite FT ages vary between 460 Ma and 79 Ma. Samples from Late Carboniferous to Jurassic sediments from the Morondava basin gave apatite FT ages ranging between 462 Ma and 184 Ma. FT data argues for reactivation of several Late Neoproterozoic/Early Cambrian ductile shear zones pre-dating the initial opening of the Morondava basin during the Late Carboniferous. Apatite FT data indicate that the subsequent Mesozoic rift evolution was accompanied with an eastward migration of areas of fast cooling. During Mesozoic times the Morondava basin had a greater eastward extension. Detrital apatite FT data derived from the outcropping Sakoa Group imply that after the Late Carboniferous deposition these rocks were covered by 2-4 km of sedimentary rocks. The final separation of Madagascar and East Africa during the Jurassic influenced mainly samples in southwest Madagascar, where basement regions and also former basin regions were exhumed. Combined Titanite FT and structural data argue for a minor tectono-thermal influence of the Marion hot spot during the Madagascar-India separation. However, apatite FT ages and modelled cooling paths indicate that the break-up during the Cretaceous was associated with a denudation amount of ~4 km crustal section along a narrow stripe parallel to the present eastern continental margin.

Results of thermochronometric investigations comprising 40Ar/39Ar hornblende and biotite, titanite and apatite fission track (FT) and apatite (U-Th)/He dating on 102basement rock samples from northern Mozambique record a cooling and denudation history since Early Palaeozoic times. In the north Mozambican sector, these resultsplace new temporal and geometric constraints on the initial rift and subsequent drift configuration during the Gondwana supercontinent dispersal as well as on the postbreak-up evolution of the sheared margin of central East Africa. Furthermore, they highlight the influence exerted by ductile basement structures of Pan-African age on theloci of tectonically active zone and associated denudation since the Late Palaeozoic.40Ar/39Ar hornblende and biotite ages range from c. 542 Ma to 456 Ma and c.448 Ma to 428 Ma, respectively. They record the cooling from the latest Pan-African metamorphic imprint, presumably related to the formation of the Namama Thrust Beltat c. 550-500 Ma, at slow rates of about 11°-7°C/Ma from 525°C to 305°C in Early to Late Ordovician times. Locally, the thermal influences of syn- to post-tectonicgranitoid / pegmatite emplacements at about 500-450 Ma delayed cooling. Widespread basement cooling to < 350°C occurred in Late Ordovician to Early Silurian times.The titanite fission track ages fall into two age groups of c. 378 Ma to 327 Ma and c. 284 Ma to 219 Ma. Very slow cooling since the Late Ordovician/Early Silurian atrates of < 1°C/Ma to below 275 Ã ± 25°C in the Late Devonian/Early Carboniferous is deduced from the older titanite FT ages. It is related to decreasing denudation inresponse to the establishment of pre-Karoo peneplains within central Gondwana. The younger titanite FT ages record the cooling of a denuding and approximately E-Wtrending uplifted rift flank whose formation marks the onset of rifting and incipient Gondwana disintegration in the Early to Late Permian. Associated crustal extensionproceeded obliquely to a NW-SE tensional stress field by brittle reactivation of easterlytrending ductile basement fabrics and presumably, linked to the Zambezi Rift system.Titanite FT results indicate a 9-12 km of crust removal since the Permo-Carboniferous.Apatite FT ages vary between c. 169 Ma and 61 Ma whereby ages of a 100 Ma record complex, and in part multistage cooling and denudation histories that are generallyrestricted to zones of crustal weakness along the Mozambique Belt yielding northerly trending ductile fabrics; along the western Axial Granulite Complex and along thepresent eastern continental margin. Apatite FT data imply that denudation was related to brittle reactivation of ductile basement fabrics by rifting, transtension and/ortranspression along southern Tanganyika-Rukwa-Malawi System and along the Davie Fracture Zone at the eastern margin. Modelled time-temperature paths indicate two periods of more rapid cooling (c. 5Ã °-3Ã °C/Ma) to below c. 110Ã °C in the Early to Late Jurassic and Early to Late Cretaceous. The Early/Middle Jurassic rifting, break-up and subsequent seafloor spreading within the Somali and Mozambique basins triggered the Jurassic periods, which were accompanied by the deposition of up to 2.5 km of volcanic rocks and associated reheating along the eastern margin at about 180-160 Ma. Far field stresses linked to global plate reorganizations due to the opening of the Atlantic and Indian oceans are accounted for the Cretaceous cooling periods. The apatite FT resultsindicate < 6 km and < 7 km of maximum denudation in the Axial Granulite Complex and along the eastern margin since the Jurassic, respectively.....

Jacobs, Joachim

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