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In Central Africa: a Kinematic Approach to Proterozoic Tectonics

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In Central Africa: a Kinematic Approach to Proterozoic Tectonics C ru stal Sh ear Z o n es in C en tral A frica ： a K inem atic A pproach to P ro tero zo ic T ecto n ics m ore intense deform ation w ithin them . C rustal plates or 妙 M ichael C. D aly terranes m ay thus appear as discrete geological units bounded by high strain shear zones, the displacem ent along w hich m ay be of dip, oblique or strike-slip type. T he nature A n understanding of the displacem ent patterns (kinem atics) of these bounding shear zones is useful in constraining the of orogenic belts is essential in order to analyze com plex kinem atic evolution of very com plex orogenic terranes. regional structural relationships and the tectonic processes W ith such data a beginning can be m ade in m odelling the in vo lved . T his a rtic le o utline s so m e results fro m kin e m a tic tectonic processes that fashioned Precam brian and Pbanero- studies carried out during the past decade on shear and zoic o rogenic belts, in addition to the m ore classical thrust zones associated w ith P roterozoic orogenic belts in approaches to orogenic analysis using regional cross-cutting C e n tral A fric a . T h e se d a ta ind ica te th at P ro te ro zo ic relationships and radiom etric dating pioneered by H olm es orogeny is characterized by m ajor horizontal displacem ents (1951) and C ahen and others (1984). and that previous ideas of dom inantly vertical tectonics are C entral A frica contains a large num ber of shea r zones that not supported. range w idely in size and im age. T hey form ed at a variety of S he a r Z o ne s a nd T e cto nics crustal levels and differ greatly in their tectonic signifi- cance and displacem ent pattern. T h e r e h a s b e e n c o n s id e r - Shear zones are one of the m ost prom inent defo rm ation able debate as to the role of large horizontal displacem ents features of orogenic belts, both in the Phanerozoic and in ．t 卜e form ation of the co m plex netw ork of A frican Precam brian. In m any places they define the m ajor bound- orogenic belts. R eports of very little offset of pre-existing aries of deeply eroded orogenic belts, as w ell as zones of s t ru c t u re s a e r O S S the Za m bezi, K ibaran and Irum ide Proter- ozoic orogenic b elts i n C entral A frica (Shackleton, 1973), and Ra n g e o f po te ntial apparently supporting paleom ag- O b liqu e L a te r a l tra nsp ort d ire ctio ns netic evidence (B riden, 1976), led to A B 言irnegclteio pno言sfible for an ind ivid u al tra n sp o rt ram p orogenic m odels involving vertical tectonics (K r6ner, 1977). It was w ith the ai巾 o f p r o viding so m e k ine m atic co nstra ints on th e se m o d e ls t h a t t h e w o rk s u m m a riz e d he re w a s u nde rta k en. T h is artic le discusses concepts of orogenic kine- m a tic s a nd o u tlines the tec to nic E xten s io n lin ea tio n setting and kinem atic patterns of r fa u lt 1) three regional shear zones exposed in Za m bia, Zim babw e, M alaw i and T a n z a n ia . s t c u lm in a tio n O rogenic Kinem atics 口Decreases as T he shear zones that bound m a ny s t ra in in c r e a s e s orogenic belts appear analogous in g eom etry to hig her level struc- S h ea r d ire ctio n E x ten s ion a lon g tures developed in thin-skinned E Pre-shear P o st- sh ea r F s h ea r p la n e thrust zones. C ontem porary con- cepts of thrust tectonics have developed from foreland thrust belts Fo ld y ． 匕 萝 g e lin es X Fig u re 1 : S o m e s t 工u c t u r e s M a in s h ea r z on e fa b ric t yp i ca l of f o r e l a n d t h r u s t b e l t s . S e e te xt f o r exp la n a - t i o n . E PISO D E S, Vol. 11, No. 1, M arch 1988 such as the R ocky M ountains and are based on field, w ell, and seism ic reflection data (Bally et al., 1966; D ahlstrom , 三夕 Foliation trend 1970; B oyer and Elliott, 1982). L uo ng o z T h ru st s are com pressional structures that shorten a n d b e d s a n d thic ke n strata, characteristically duplicating 1 0 - S m id e b e lt cutting-up section in the direction o f m o ve m en t. In w e ll- L u ffl ia n a rc bedded rocks, thrusts often follow a staircase trajectory, a k 日 M a l 日胃 1 分 ～ r ． l ． Z dcroelicvmekbslo.in pgin Tg rh afelmas tpes praa rcamrloples sl tsot hr ubece d tbuderidnegdsi ningt re aainssy f esmlrio pr edho i srcpiolzamo cnpesem,t ebunt ⋯ 、 M w e 从 曰 brtahemetw pteh)e,r unpsa ter atslrylaen ls pl(lioaprt teh rdoairlri ezrcoatnmisop na), n( oFdri gm o. abIlyAiq )su.terliyke (o nbolirqmuael r a(fmropn)t atol 曰 介 Z IM B A B W E A single ram p structure has a potential transport direction N c o n t a in p d w ith in A i li n o - Thus, the recognition of 50 0 krn several ram p orientations w ill a llo w the constraining of 3 0 * E ． I thrust transport direction (Fig. IB ). in fron t o f an d a bo ve frontal and oblique ram ps, beds a r e duplicated and the Figure 3 : 5 力e a r z o n e s o f Cen tr al A f ri ca , s h ow in g in to a 【1 a ntifo rm a l struc tu re . overlying strata a re bulged up the lo ca ti on of th e c r os s s e ct ion sh o wn in F ig ur e 7. W here the ram p develop m ent is irregula r the thrust p rofile mdiareyc taipopne.a rT choisrr ulgeadtesd t ion tsheect dioenvse lnooprmmeanlt t oo ft hae sterarniesspo ortf broad, flat-topped anticlines offset across the lateral ram ps, T he shear zones m ay be associated w ith dip, oblique or resulting in a series of culm inations and dep ressions strike-slip displacem ents and vary in attitude from flat to (F ig. IC , after D ahlstrom , 1970). vertical. T hey approxim ate in their deform ation m echanism to zones of sim ple shear (R am say, 1980), in w hich the initial A geom etry sim ilar to that of foreland thrust zones m ay be planar deform ation fabric develops at 450 to the shear detected in the m ore ductile regim e of the m iddle and low er plane. W ith increasing shear strain (show n by the solid crust. D eep displacem ent zones developing as broad 旦uc丝些． ellipses in Fig. ID ) this fabric rotates into parallelism w ith shear zones occur frequently in the basem ent gneiss ter- the shear plane. Sim ilarly, the extension direction M , 而蔽刃石f万口rica. These shear zones are characterized by m arked as a linear m ineral fabric on the resultant foliation, grain-size reduction, m ylonitic foliation and a w ell devel- rotates tow ards parallelism w ith the m ovem ent direction in oped preferred orientation of m inerals. Figure 2 sho w s the the shear plane. This fabric results in the typical L-S type of rock fabrics found in shear zones from the Irum ide m ylonitic fabrics that characterize m ajor shear zones. A ny belt of Zam bia, w hich are typical of the structures discussed passive m arkers, such as pre-existing fold structures, w ill here. The m ontage show s an undeform ed porp hyritic granite rotate tow ards this bulk transport direction w ith increasing (F ig. 2A ) and its highly sheared equivalent in a m ajor shear ·strain. The rotation m ay result in folds (Fig. 1E ) w ith zone. F igure 2B show s a w ell-developed m ylonitic foliation ’curvilinear fold hinge lines of sheath-like geo m etry (Cobbold and sm eared-out porphyroclasts, and F igure 2C a foliation and Q uinquis, 1980). surface, w hich contains a strong m ineral lineation thought Shear zones often develop secondary shea rs at an oblique to define the displacem ent direction during shearing.
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