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. The angle to the m ain foliation, but w ith the sam e m o v e m e n t photom icrograph (Fig. 2D ) sho ws both brittle and ductile sense (Fig. IF ). T hese secondary features ha ve th e effe c t deform ation features w ithin these shear zones, w here brittle of extending the shear zone and have been variously fracturing of feldspar porp hy roclasts has occurred. described as an extensional crenulation cleavage (Platt and Vissers, 1980) or shear bands (W hite et al., 1986). They m ay Figure 2 : R ock fabrics in the Irumide belt of appear as both brittle or ductile features and characterist- Z amb i a . D i s 2 cm wi d e . S ee te xt for exp l an a tio n . ically occur perpendicular to the extension lineation. T hey

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6 E PIS O D E S, Vol. 11, No. 1, M arch 1988 0 巨 2 日 3 2 . T he exam ples that follow rely on both regionally consistent 10 - S extension lineations and lateral ram p structures to deduce Z A IR E the displacem ents outlined. M A L A W I T he M w e m b eshi: A T ran sfo r m S h ea r Z o ne A m ajor crustal scale shear zone transe c ts C entral A frica irn a lir o (Daly, 1986a). T he central part of this EN E-trending zone is ／ B e lt know n as the M w em beshi D islocation Z one (de Sw ardt et al., 1965), w hich occurs betw een the tw o Late Proterozoic orogenic belts of the Lufilian a re and Zam bezi belt (F ig. 3).

M O Z A M B IQ U E Several other dislocations, involving both lateral and verti- w a B rid g e cal displacem ents are associated w ith the M w em beshi zone.

M w e rn b e s h i The tectonics and relative age of these thrust belts have Z o n e been discussed elsew here (D aly et al., 1984; C ow a rd a nd D aly, 1984; D aly, 1986a) and w ill only be sum m arized here. The L ufilian are com prises the copper bearing sedim enta ry 曰 异 ko nd o C o } e r rocks of the K atangan Supergroup. These have bee n folded 图 1 6 ' and thrust to define a m ajor arcuate structure, largely P a n - A fric a n ro c k s a n d fo lia t io n inherited from the original K atangan basin m orp hology. In 口 Z IM P re - P a n - A tric a n the southeast pa rt of this arc the m ajority of the structures

＂ C raton face E N E, and the rocks a re of low m etam orphic grade. K 1 e x te n s ion lin e at io n Thrust geom etries define a num ber of E N E-trending lateral 夕 七＝二二二二- 10 0 k . 怜 K 2 e x te n s io n lin e at io n 了尸 ram p and culm ination structures, w hereas basem ent shear r C h im an im an i zones are characterized by a SW to W SW -plunging linear fabric (Fig. 4). These features indicate an E N E tectonic Fi g ure 4 : Sk e tc h m ap o f t h e M w e m b e s h i s h e a r z o n e transport direction fo r the m ain Copperbelt structu res. a n d a s s o c i a t e d t h r u s t b e l t s sh o win g th e d i s t r i b u t i o n T o the south of the C opperbelt a series of shear zone s are of s tr e tch in g (e x t e n s i o n ) l i n e a t i o n s . developed at a high angle to the trend of the arcuate thrust belt. In this region the sole thrust of the L ufilia n thrust system appears to have clim bed section to the base of the are thus useful as displacem ent indicators and also dem on- K atangan m eta-sedim ents (C ow a川 and D aly, 1984). H ere strate shear sense. A range of other m eso- and m icroscopi c the K atangan basem ent contact is intensely deform ed and indicators in sheared rocks have been discussed by W hite and has clearly been a significant m ovem ent zone. Southwards, others (1986). These shear sense indicators, how ever, m ay the L ufilian arc term inates against the M w em beshi zone. only reflect a part of the strain path, and the gross geo- T h e Z a m be zi b elt lie s to the so uth o f th e L u filia n a re a n d is m etric effects of the shea r zone (i.e. crustal thickening, separated from it by the M w em beshi Zone. The Zam bezi crustal thinning o r lateral translation) m ust also be con- structures face to the SW and SSW (Fig. 4) and involve deep sidered w hen assessing its tectonic significance. ' crustal m etam orp hic rocks w ith high pressure m ineral From the criteria outlined, the tectonic transport direction ass}讯blages (Barr, 1974) and eclogitic intrusions (Vrana et of both high-level, foreland thrust belts and m iddle to low er al., 1975). D espite their dissim ilarity in m etam orphic grade, c rusta l shea r zo n es ca n be de term ined a nd th eir k ine m atic the m e ta -se dim e nts o f the Z a m b ez i be lt ha v e be e n c or- relationships deduced. T he orientation of lateral ra m ps and related on lith ostr atigraphic grounds w ith the low er g rade tear faults, developed at all crustal levels, are probably the K atangan sedim ents of the L ufilian a rc (Sm ith, 1963). m ost reliable indicators of displacem ent direction. Linear T he SW and southern boundary of the Zam bezi be lt is a fabrics of shear zones m ay be affected by p re-existing m ajor shear zone that im bricates M id-Late P roterozoic fabrics and local perturbations in the strain pattern, but a m etasedim entary sequences w ith Proterozoic and A rchaean test of their reliability lies in their regional consistency. basem ent aneisses of the Zim babw e craton. T o the east, the

E PISO D ES, Vol. 11, No. 1, M arch 1988 7 ， ， 夕 户 front of the Z am bezi belt sw ings southw ards to define the northerly trending , eastern edge of the Zim babw e craton. A

in e nta l low -grade m etasedim entary sequence know n a s the

o n e U m kondo G roup is preserved here in the frontal part of the belt. F rom thrust geom etry and stretching lineations Z a m be zi (Fig. 4) in low -grade m ylonite zones it is apparent that d e fo rm a tio n the se sed im e nts w e re th ru st to the W S W o ve r the A rc hea n c ra to n .

g io n al In the east-w est part of the Zam bezi belt, extension line- m b e zi ations and thrust geom etries indicate tw o directions of In te r- c o n t rm a tio n s he a r zo n e U run gw e tectonic transport: an early W SW -directed thrusting k lip p e follow ed by, or broadly contem poraneous w ith, a southerly － 1 0 0 k m directed event. Figure 4 show s the added com plexity of the north-p lunging lineations in the east-w est arm of the Fi g u re 5 : Te ct on ic s e t tin g o f t h e M w e m b e s h i s h e a r Zam bezi belt and the northern part of the Lufilian arc.

z o n e , a s s o c i a t e d th r u s t b e l t s a n d in t r a - c on t i n e n t a l D aly (1986a) interpreted the tw o distinct lineatio n direc- sp la ys . tions as representing tw o separate kinem atic events: K I (E N E-W S W directed) and K 2 (north-south directed). Ilow - 尸q 。 ever, the sequential develop m ent of the tw o orientations is ChT H R U S T S not pro ven, and a coeval relationship with tw o displacem ent s C hirnim bi com ponents (W S W and north-south) of a single transpressive M B a h a ti o M a n s a deform ation is also possible. C uo ill 2 0 1 8 T h e M w e m b e s h i Z o n e m a rk s the boundary betw een the L u filia n a rc a n d Z a m b e z i b e lt 4 9 (Figs. 4, 5), trending approxi- ／ ZO m ately E N E/W SW . In the w est it is a 10 km -w ide shear zone, and eastw ards the zone passes below the L uano rift a nd is co rre la ted w ith the Ilind a a nd C hin ko m be sh e a r zo ne s of the e a ste rn Z a m b ia and the C h im a liro b elt o f M a la w i (D aly, 1986b).

10 k rn T h e de fo rm a ti o n fab rics w ithin th e M w e m bes hi zo n e h a ve a （ I 孟 舀 sub-horizontal, EN E or W SW -plunging extension lineation, 丫 止 B I,7/ 6}' indicating a sim ilar tectonic transport direction to both the 阵 一 Lufilian (EN E-directed) and Za m bezi (W SW -directed) 口 d e fo rm ation s. T he de form a tio n fa b ric o f the Z a m b ez i belt 鬓Lua pula b eds s沙ings into the M w em beshi zone, m aintaining the consistent M p oro ko so lineation trend. These features suggest that the Lufilian 巨g ro u p arc, Zam bezi belt and M w em beshi zone are coeval and G ranite kinem atically equivalent structures. Thus, the M w em beshi 了 V o lc a n ic s zone acted as a m ajor continental transform for the opposed S tre tching lin e a tio n 2}} thrust belts of the L ufilian and Zam bezi. C ow ard and Daly 了 (1984) suggested that this transform r epresented the boundary betw een the Congo and K alahari plates.

F igure 6: The L u on go sh ea r z o n e . Th e m a -i 刀 p o rt ion sh ow s th e d i s t r i b u t i o n o f the m a j or th r ust s T h e ea stw a rd c on tin ua tio n of the M w e m b e shi zo n e is also a n d l i n e a r f a b r i c s i n th e s ou the rn p ar t o f th e sh ea r m ylonitic and characterized by dom inantly E N E-W SW 2 0 月e s plunging linear fabrics (H ickm an, 1978). H ow ever, here the

L U O N G O Z O N E T H E IR U M ID E B E LT

F o re la n d s h e a r z o n e Fo rela n d fo ld b e lt S E facing sNtWru cftaucrieng st ruc tu re

N W S E

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5 0 k m 二 口 Basem ent ！巫］M uva sedim ents v = h

F ig u re 7 : R eg ion a l cr o ss s e c t i o n sh o wi n g th e proposed tecton ic setting of the Luongo s h e a r z o n e l i n k e d t o th e f o r e l a n d f o l d a n d t h r u s t b e l t o f th e Ir um ide b el t . S e ct ion is l oc a ted on Fi g ur e 3 .

8 E P ISO D ES, Vol. 11, N o. 1, M arch 1988 zone is one of a series of late to post-Irum ide shear zones of dextral strike-slip displacem ent and is not thought to T E R R A N E D O M IN A N T L IN E A T IO N represent a m ajor plate boundary. T his intra-continental L IT H O L O G Y T R E N D continuation, or splay, links into the M w em beshi zone in the area of the Luano rift valley (Fig. 5). A n irrip lication of the in tra-co ntin en ta l a nd in te r- co n tine ntal na ture o f the U pang w a M eta-a no rtho site N W -S E 丽丽百nbeshi and associated一s hear zones is that the Congo- M b o z i Kalahari plate boundary m ust diverge eastwards from the M eta-b asite s a nd in term ed ia te N E - S W M w em beshi zone. T he location of this boundary is uncer- granulites w ith quartzites tain, but it m ust lie som ew here w ithin the penetrative d efo rm a tio n zo n e o f the Z a m be zi belt. L upa M eta-v o lc an ic s N o d a ta

N 四 -S E Clearly, therefore, an understanding of the kinem atic Ufipa G neissic granite evolution of the Late Proterozoic orogenic belts and shear Nyika C ordierite granulites E - W zones in C entral A frica clarifies the regional structural relationships of orogenic belts of w idely differing trends and U b e n d e M e t a -b a s ite s E N E - W S W m etam orp hic grade. W ith such an internally consistent kinem atic m odel it is now possible to incorporate other W a k o le A lu m in o-silica te sc h ists N o n e geological data and begin to understand the tectonic pro- cesses that fashioned these com plex Proterozoic terranes. Im portantly, the regionally consistent horizontal m ovem ent indica tors sho w tha t the L a te P rotero zo ic te cto nics o f T A BL E 1: Do m inant lithologies of the U bendian terranes C entral A frica w ere do m inated by horizontal displacem ents and the regional trend of their m ineral extension lineations as opposed to p revious m od els that favoured vertical tectonics (K r6ner, 1977). W ithin the m ylonitic basem ent shear zones and the intensely T he Luongo Zone: A D iscrete Foreland Shear Z one deform ed zones of the sedim ents, a strong L-S fabric occurs, generally dipping to the ESE to SE. The linear elem ent of this fabric is a dow n-dip stretching lineation, A discrete, arcuate shear zone occurs in the Bang w eulu w hich together w ith the N W to W -vergence of the fo lded B lock of Zam bia (Fig. 3), foreland to the Irum ide orogenic and thrust quartzites ind icates tectonic transport w as belt (D aly, 1986b). T he shear zone is exposed in the south as directed approxim ately northw estw ards. A sim ilar a series of steep, N N E-striking, brittle to ductile shears, displacem ent direction is obtained if one applies to the are w hich im bricate the basem ent granite and volcanics as a w hole the bow and arrow rule of Elliott (1976), w hich (Fig. 6). T o the north, this narrow zone of foreland states that the bisector of a sim ple thrust belt arc is d e fo rm a tio n fo lds a nd thrusts a se ries o f M id -P ro te ro zo ic parallel to regional displacem ent. cover quartzites, the M porokoso G roup. Locally, the basem ent is thrust over the quartzites and m ajor recum bent T he - displacem ent on this shear zone suggests a kinem atic folding m arks the eastern-m ost basem ent cover contact. reliitionship betw een the shea r zo ne an d the N W 一 ire cted fo ld a n d t h r u s t b e l t of the Irum iAde belt, N so m e 2 0 0 k m t o t h e so u t h e a st . c o e v a l L A K E by relationship is also supported ti K /A r d eterm ina tio ns c a rrie d o u t o n b io fr o m w ithin the Luongo sh ea r zo ne , t h e ｛ un d eform ed B lo c k a n d t h e Bangw eulu Iru m id e b e lt (D aly 1986b). Sim ilar d a te s ．10OKm. a ro u n d 1 0 0 0 M a w e re o b ta in e d fr o m b o t h the Luongo s h e a r z o n e a n d t h e Iru m id e Belt, indicating that both had a s i m ila r cooling age. How ever, th e und e fo rm e d T A N Z A N IA N C R A T O N ／ PBraontge兰wroezuoluic dBaltoec.k.r e t a in e d older, Early T h . “；一” k i n e m atics and age of ． ／可尸尸IR IN G A cooling lead to the interp retation that the Luongo zone and the Irum ide B elt form ：及 Z-Z}+L' + part of a linked thrust system , activated during the crustal shortening recorded in 一姗 ＋ P O S T 1 8 0 0 M a S E D IM E N T S MB荔 ＋ the Irum ide B elt, as show n in Figure 7. 户 ＋ C IR C A 18 0O M a G R A NIT E 蔑义双 、 U nderstanding the direction of tectonic 日 争 十 乡笼 ：万2 弥1： M A P A N D A / transport associated w ith the deform ation K A T U M A T U N b U M A of the Luongo shear zone constrains the W A K O L E 十J regional orogenic relationships of this LIB EN D E is o la t e d s h e a r z o n e a n d d e m o n st ra t e s its 1J F IP A U B EN DIA N a sso ciation to h o rizo ntal tec tonic s a nd T ER R A N E S M B O Z I regional crustal shortening during the L ate 一T E R R A N 住 B O U N D A R Y L U P A S H E A R Z O N E P roterozoic, in contrast again to the ideas N Y IK A ++ U P A N G W A 一 of no crustal shortening previously I N F E R R E D N Y IK A 丫 popularized for the Irum ide belt. TA N Z A N IA N C RA T O N F R O N T A L T H R U S 丫 B E L T L A K E M A L A W I > 24 0 O M a

F ig ur e 8 S ke tc h m ap o f t h e U b e n d i a n t e r r a n e s a n d t h e i r bo un d in g sh ea r 2 0 口 e s ．

E PISO D ES, Vol. 11, N o. 1, M arch 1988 9 sh ea r zo ne s are ofte n d iffic ult to I ／ 一 — 一气二卜 、、 interpret because of later reac- 月己 －．J z 、 T A N Z A N IA N C R A T O N ／ ／＼ 气 、 tivation. They do have a con- ，‘ ，‘ 少咨兮＼ sistent sub一orizontal plunge, 、、 but it is rarely possible to be 月口卜 ＼ ，‘ 5户匕 c ertain a s to w h ic h d efo rm a tio n ＼ th is re fle c t s . 气＼ ，‘ ／ 产 癖 ，弓 ＼ J 口‘ 二鱼‘！ 、＼ Figure 9 presents a tectonic 例映烤资软 ： 构＼ ＋矛夸苍 m odel for the juxtaposition of 盯-/111 the U bendian terranes. It sug- 鄂 gests the terranes w ere accreted U B E N D IA N 一T E R R A N E S adjacent to the T anzanian craton during the Early Proterozoic U sagaran orogeny to the east. O n the basis of sim ilar kine- m atic data to that used here, Shackleton and R eis (1984) concluded tha t the U saga ran consists o f a series of N W - 鬓 UPANGWA 洲 M B O Z I ． U F IP A 口 L U P A directed thrust sheets, overthrust onto the Ta nzania n craton. T he U bendian terranes probably developed as a series of tectonic slivers, accreted laterally to the craton Figure 9: T e c t o n i c m o d e l f o r t h e e v o l u t i o n o f th e during a late stage in this thrusting event, the internal fabrics of the terranes being developed du ring the Usagaran U b e n d i a n b e l 亡 t e r r a n e s . S h e a r z o n e f a b r i c s a r e orogeny. The sub-horizontal linear fabrics in the inter- shown schema tically on the steep m argins of t h e t e r r a n e s . terrane shear zones together w ith the internal folding suggests this occurred under a strike-slip to transpiressive tectonic regim e.

T h e U be nd ia n B elt: S hea r Z o nes an d A c c re ted T erra n es Large sub-horizontal displacem ents are, again, im plied but the m echanism and tectonic setting of this accretion aw aits The U bendian belt is a Late A rchaean-Early P roterozoic a better understanding of the terranes them selves. H ow - orogenic belt that defines the SW m argin of the A rchaean ever, it does seem clear that the terranes w ere juxtaposed Tanza nian craton (F ig. 4). M cC onnell (1950) recognized a along a N W -SE trending, strike-slip, transpressive craton series of bloc ks w ithin the northern U bendian belt, w hich he m argin, at broadly the sam e tim e as thrust tecto nics w ere believed to represent separate geological entities. Sutton generating the Usagaran belt to the east. Such a m echanism and others (1954) rem arked on the intensity of deform ation is re m in isc en t of the C o rdille ra n tec to n ic s o f th e no 此h - w ithin the belt and on the abundance of m ylonitic rocks. w estern U .S.A . and w estern C anada (Coney et al., 1980). They interp reted the belt as a deep seated zone of strike-slip m ovem ent. F o ld ＆ t h ru s t b e lt T he occurrence in the Ubendian belt of lithologically and E re ．I on lev e l A) Contraction In . . ny structurally distinct blocks or teeranes has been supported P ro te ro z o ic by m ore recent w ork. D aly and others (1985) established on o r气o go n io b e lts 、 lithological and structural grounds the presence of eight 1 T h ic k e n e d c ru s t terranes (Table 1), strongly elongated in a N W -SE direction S he ar z one w ide ns and laterally persistent for up to 400 kin . Figure 8 outlines w ithwe depth the distribution of the terranes and show s the m ajor shear H alf- g rab en ba sin system ｜ zo n es th at d e fine the ir bo un da ries. j 厂J Internally the terranes are heterogenously deform ed w ith extensive tracts of foliated m ylonitic and ultra-m ylonitic B) Extension gneisses. In all terranes the m ain foliation is folded, generally about axes sub-parallel to the elongation of the terrane. Tectonite fabrics w ithin the terranes com m only have a strong L一 fabric. In the Upangw a and U fipa Fi g ur e 1 0 : S c h e m a t i c r el a tion ship s b e twe en s h e a r terranes the extension lineation plung es gently to the N W or z o n e s a n d f a u l t s . A 一 Li nk be tw ee n th e d eep -c r us ta l SE, parallel to the terrane elongation. How eve r, in the s 力e a r z o n e s a n d h i gh 一 ev el thr us t b el ts . B 一 A intervening M bozi terrane the lineation plunges to the EN E- s i m i l a r g e o m e t r y s u gge s te d f or ex te n si on a l s h e a r N E, and in the U bende an E to E N E plunge is characteristic, z o n e s o r "detachments " and their high l e v e l both at a high angle to the terrane elongation. T his varia- exp r es s ion 一 as ym m e tri c ba s in s . tion in finite strain orientation com plim ents the lithological distinction and em phasizes the differences of the terranes. This variation in strain pattern m ay reflect a com ponent of C o n clusio ns rotation during em placem ent of the terranes. In several of A nalysis of ductile shear zones and of the geom etry of the terranes thrust repetitions are recognized, their m ove- frontal thrust belts has generated a kinem atic data set for m ent direction being parallel to the displacem ent indicated the evolution of the Proterozoic orogenic belts of C entral by the linear fa brics. A frica. These data independently constrain the inter- The U bendian terranes a re bounded by a series of m ajor, relationship betw een orogenic segm ents a nd, w hen com bined steep shear zones, w hich persist up to 600 kin along strike. w ith the tools of radiom etric dating and conventional T he rectilinear trace of the shear zones together w ith their structural and stratigraphic m apping greatly im pro ve our steep attitude at outcrop suggests that they have a signif- understanding of P recam brian orogenic sequences. This in ic a n t v e r t ic a l d im e n sio n . T h e se z o n e s a r e th e st r u c t u r e s turn allo w s a better understanding of the tectonic processes along w hich the disparate terranes of the U bendian belt in v o lv e d . w ere juxtaposed. 'The shear zones com p rise a m ixture of The w ork review ed here strengthens the conclusion that ductile and brittle fault rocks, varying from ultra-m ylonites large horizontal displacem ents dom inated the Proterozoic to goug e, testifying to a long-lived and com plex structural tectonic evolution of C entral A frica. These displacem ents history. Stretching lineations from these inter-terrane occurred on m ajor shear zones that link deform ation at

1 0 E P IS O D E S , V ol. 1 1, N o . 1. M a rc h 1 98 8 m id dle a nd lo w e r c rusta l lev els to that in high-level foreland A cknowledgenients t h ru st b e lt s (Fig.。 1I O A ). T h e le v e l o f e ro sion in m o st P ro te ro zoic orog en ic belts results in the extensive exposure The Luongo w ork w as carried out w hile the author w as o f sh ea re d g n e is se s an d a c o m m o n a bse nc e of fo re la n d em ployed by the Geological Survey of Z am bia, and later supported by the N atural Environm ent Research C ouncil of tsheirsumstic b erletfsl.e cStiuocnh p greoofimleest rfirco mm odtehlesr a rreeg isounpsp worhteerde bcyru dsetaelp- the U .K . T he M w enbeshi and U bendian studies result from scale ram p-flat geom etries have been im aged to link high- participation in U nesco's "G eology for D evelopm ent" p ro- and low -level deform ation zones (A ndo et al., 1984). gram in Zam bia and Tanzania. The author acknow ledges discussions w ith num erous colleagues in A frica and E urope, T he se C e n tral A frica n shea r zo ne s are a sso c iate d w ith c o m - notably M .P. C ow ard, J. K lerkx and J.T. N anyaro. This pressional or strike-slip tectonics. A sim ilar relationship paper is published by kind perm ission of the B ritish and geom etry (Fig. IOB ) has been proposed fo r m id-c ru st Petroleum C om pany. extensional shear zones and high-level basin system s in the Basin and R ange Province of the w estern U .S.A . (W ernicke and Bu rchfield, 1982). H ow ever, to date there is no con- vincing exam ple of a regionally developed extensional P re c a m b ria n s h e a r z o n e .

The horizontal displacem ents dem onstrated above im ply D r. M .C . D aly is a m em ber of the BP Structural Studies G roup (B P that tectonic m odels involving dom inantly vertical displace- International, B ritannic H ouse, m ents are not supported. H ow ever, the re still rem a ins a M oor Lane, London EC 2Y 9B U , m ajor gap betw een the recognition of regionally im portant U .K .), w here he w orks on basin h o rizo ntal tec to nic s a nd the tectonic process d riving them . evolution in term s of regional and T he obvious analogue is w ith plate tectonics, recognition of deep crustal plate boundaries plate tectonics. A form er regional assem blages c ha ra c te ristic of them is not easy, geologist w ith the G eological rarely are P rote ro zo ic Survey of Zam bia, he has plate tectonic interp retations participated in several U nesco- c o n v in c ing (H offm an, 1980；R ies et al., 1983). How ever, k in em a tic st u d ie s s u c n a s tho se revie w e d h ere o ffe r a sponsored research projects i n su b-S a ha ra n A frica . significant constraint to understanding the tectonic pro- cesses of both Proterozoic and A rchaean orogeny.

R e fe ren ces Pla tt, J.P . and Vissers, R .L .M ., 1 980. E xtensional A nd o, C .J., C zu chra, B .L ., X lem pe re r, S.L ., B ro w n, 罗n3a3d2i-a4n0厂etroleum Geology Bulletin,“18, no. 3, structure s in an isotro pic roc ks. Jo urnal of S tructu ral L.D ., C h eadle , M .J., C ook , F.A ., O live r, J.E ., K a uf- G eo logy, v. 2, no. 4 , p . 39 7-4 10. m an, S., W alsh, T., T h om pson, J .B., Jr., 勿 on s, J.B . D aly, M .C ., 19 86a. C ru stal shear z ones a nd thrust a nd R osenfeld, J.L ., 198 4. C rusta l pro file o f m o un - b elts: their g eom e try a nd co ntinuity in c en tral R a m say, J.G ., 198 0. Shea r zone g eom etry: a re vie w . tain belts: C O C O R P de ep seism ic reflec tion p ro - Jo urna l o f S tructu ra l G e ology , v. 2, no . 1- 2, filing in N ew En gland A ppa lachia ns a nd im plication s T ransac tio ns Se rie s A , v. 31 7, no . 1539, p . 111-12 8. p. 83-10 0. for a rchitec tu re o f co nve rge nt m oun tain c ha in s. A m e rican A ssoc ia tio n o f P etro leum G eology B ull- D aly, M .C ., 19 86b. T he tec to nic a nd the rm al evo- R ie s, A .C ., Shac kleton, R .M ., G ra ham , A .H . a nd e tin , v. 68, no . 7, p . 8 19-83 7. lution o f the Iru m id e. be lt, Za m bia . U nive rsity of Fitc hes, W .R ., 1983. Pa n-A frican struc tu res, oph i- L ee ds, E ngland , P h.D . thesis. olite s a nd m ela ng e in the Eastern D ese rt o f Egy pt; a B ally , A .W ., G o rdy, P .L . a nd Ste w a rt, G .A ., 1966. traverse at 26*N. Journal of the G eological Society Struc ture, seism ic da te a nd o rog enic e volution o f D aly, M .C., Cbakraborty, S.K., Kasolo, P., M usiwa, o f Lo ndon, v. 140, no. 1, p . 75-9 6. sou th ern C anad ian R ocky M oun tains. C anad ia n M ．，M um ba, P., N aidu, B ., N a m ateba , C ., N g a m bi, Petroleum G eology Bulletin, v. 14, no. 3, p. 337- 0 . and C o w a rd, M .P ., 1984. T he Lu filian are and S hac kleton, R .M ．，19 73. C orrela tio n of struc tu res 3 8 1 . Irum ide belt of Za m bia : results of a g eotra verse a cross P rec am brian orog enic belts in A frica. In: ac ross their interse ctio n. Jo urnal o f A frica n E arth T a rling , D .H . a nd R unco rn , S .K . (ed s.), Im p lications B arr, M .W .C ., 19 74. T he p re-H a rroo G eolo gy o f th e Science s, v. 2, no . 4 , p . 311-31 8. o f C ontinen tal D rift to the Ea rth S cien ces, v. 2, R ufunsa A rea , Z a m bia, w ith Spec ia l R e fe ren ce to p art 10. A cad em ic Press, L o nd on , p . 109 1- 1095. Structure a nd M eta m o rp hism . U npublish ed P h .D . D a ly, M .C ., K lerkx, J. a nd N an yaro, J.T ., 198 5. thesis, U n iversity of L eeds. S hackle to n, R .M ., an d R ies, A .C ., 1984. T he rela- Eacacrlryet iPorno tienr otzhoeic U ebexnodtiac。n teberrltan oesf saonudthw stersitk eT-salnip- tionship betw een regio nally con sista nt stretc hing B oy er, S.E. a nd Elliott, D ., 19 82. T hru st system s. zarna . (A bstra ct) T erra C og nita , v. 5, no . 2-3, linea tio ns and p late m otions. Jou rna l o f Stru ctura l A m erican A ssociation o f P etro leum G eo lo lg ists p . 25 7. G eology , v. 6, no. 1/2, P . 1 11- 120. B ulle tin, v . 66, no. 9, p. 1196-23 0. E llio tt, D ., 197 6. T he ene rgy balanc e and de fo rm a - S m ith, A .G ., 1963. T he G eology o f the C ou ntry B rid en, J.C ., 1976. A pplica tio n of palae ornag netism tion m echanism s of thrust sheets. Royal Society of around M aza buka and K afue : E xpla nation o f D egree to P ro te ro zoic tecton ics. R oy al S ocie ty o f Lo ndo n, Lo ndo n, P hiloso phica l Transac tion s Series A , V. 283, S heets 152 7, S E Q uarter a nd 1 528, S W Q ua rter. P hilOSOD hica l T ransaction s Se rie s A . v. 2 80. no. 13 12. D. 289-3 12. G e ologic al S urvey of N o rthe rn R hod esia R ep ort, no . 1298, p . 4 05-416 . no. 2, 32p .

C a hen , L., Snellin g, N .J ., D elhal, Jr. a nd V ail, J.R ., Hicllksm Aanre,a A: .ECx.Jp.l,a n1a9t7i8o.n T ohfe DGegolroeegy S ohfe eth eN Bo.u lo1n4g30 Sutto n, J., W atson} J. a nd Ja m es, T .C ., 1954 . A 19 84. T h e G e ochron ology a nd E volution of A frica . N W Q ua rter. G eologic al S urvey o f Z am bia R epo rt, study of the m eta m o rph ic roc ks of K a re m a a nd C la re nd o n P re ss, O x fo rd , U .K ．，5 12p . n o . 6 9 . K u ng w e B ay, w estern T ang any ika. G eolog ica l S urvey o f Ta ngan yika B ulle tin, no . 22, 7 0p . C o bbo ld , P .R . and Q u inqu is, H ., 198 0. D ev elop m ent H offm a n, P .F ., 1980. W op m ay o rog ern A W ilso n of s he ath folds in shea r reg im es. Jo urna l of Struc- cy cle o f ea rly Protero zoic a ge in the n orth w est of d e S w a rdt, A .N'-J-, G a rra rd , P . a nd Sim p so n, J .G ., tural Geology, v. 2, no. 1/2, p. 119-126. the C a nad ian Shield . G eo lo gical A ssoc iatio n of 196 5. M ajo r zo nes o f tra nscurrent d isloc atio n a nd C anad a, Sp ecial Pap er 20, p. 523 -5 49. sup erpo sitio n of o rog enic be lts in pa rts o f c en tral C o ney , P .J., Jo nes, D .L. a nd M o ng e r, J.W .H ., 198 0. A frica. Geological Society of A m erica Bulletin, C o rdille ran susp ec t terra nes. N ature , L o nd on , H olm es, A ., 195 1. Th e seq uenc e of P re-c am bria n v . 7 6 . n o . 1 . D . 8 9 - 1 0 2 . v. 288 . D . 329-333. oroge nic belts in so uth an d c entra l A frica. l8th Internationa l G eologica l Co ng ress L ondo n, G rea t Vrana , S ., P rasad, A . a nd F ediu kova, E ., 1 975. C ow a rd , M .P., 19 80. S hear zo nes in the P recam bria n B ritain , 194 8, p art 1 4, p . 254-269 . M eta m orp hic kyan ite eclog ites in the Lu ffl ia n are of crust of southern A frica . Jo urn al of Structura l Z a m bia. C o ntributions to M inera log y a nd Pe trology , G eolog y, v. 2, no . 1/2, p . 19-27. K r-6n er, A ., 19 77. P re cam brian m ob ile b elts o f v . 51, no. 2, p . 139-16 0. so uthern a nd e aste rn A frica , anc ient suture s or sites C o w a rd , M .P . a nd D a ly , M .G ., 1984. C rustal line s- of ensia lic m ob ility ? A case fo r c rusta l e volution W e rnic ke, B ., B u rc hfiel, B .C . a nd W illia m s, P .F ., to w a rds plate te cto nics. T ec to noph ysic s, V . 4 0, 1982. M o des of ex te nsio nal tec ton ic s. Jo urnal o f m e nts and shear zon es in A frica; their relationship no. 1-2, p. 1 01-135. Struc tu ra l G eology, v. 4. n o. 2v p . 105-15.

tno．. pl1a,欲2m7o-v4e5m.ents. Precambrian Research, v.’‘， W hite, S.H ., B reta n, P .G . a nd R utte r, E A L , 198 6. M cC onn ell, R .B ., 195 0. O utlin e o f the G e ology o f F ault zo ne reac tiva tio n: kinem a tic s and m e cha n- L)a hlstro m , C .D ., 1 970. S tructura l geo logy in the Ufipa and Ubende. G eological Survey of Tanganyika eastern m a rg in o f the C a nadia n R oc ky M oun tain s. ism s. R oyal S oc iety o f Lo ndo n, P hilosop hical T ran - sac tio ns S eries A , v. 31 7, no. 153 9, p . 81-92.

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