Geologia Colombian a No. 18, 1993 Steeply Dipping Basement Faults and Associated Structures of the Santander Massif, Eastern Cordillera, Colombian Andes ANDREAS KAMMER Departamento de Geocienciss, Universidad Nacional de Colombia, Apartado 14490, Bogota, Colombia KAMMER, A. (1993): Steeply Dipping Basement Faults and Associated Structures of the Santander Massif, Eastern Cordillera, Colombian Andes.- Geologia Colombiana, 18, pp. 47 - 64,12 figs, 2 Tablas, Bogota. RESUMEN vollzogen, an denen Relikte des Sedimentmantels eingekeilt wurden und sich Halbqraben bildeten. Das EI Macizo de Santander expone ampliamente el Bruchmuster wurde weitgehend von einer jurassischen basamento andino "viejo" y estil Iimitado hacia sus Dehnungstektonik ererbt. Die Neogenen cuencas de "antepais" par fallas inversas mayores. Abschiebungen kormen einer vertikalen Fallas mas Internas, aunque con buzamientos fuertes, Hebungstektonik zugeordnet werden, bei der, bedingt pueden clasificarsecomo faUasnormales que hundieron durch die facherartige Anordnung der BrOche, zentrale la cobertera sedimentaria a 10largo de semi - grabenes. Teile der Aufwolbung des Massivs absinken mussten. EI patron de estas fallas estil heredado en gran medida Ein leicht abgeandertes Modell wird an einem schmalen, a partir de una tectcnlca [uraeica de extension. La kompressiven Grundgebirgshorst erprobt, WO, wahrend tectonlca extensiva Ne6gena puede integrarse a un der Hebung Randbereiche direkt Ober den inversen modelo de levantamiento vertical, que par el arreglo en BrOchen einbrachen und sich charakteristische abanico de sus fallas indujo un componente de Synklinalen bildeten. dilatacion lateral. Este modelo estil modificado para un estrecho antiforme fallado, el alto de Malaga, en donde 1. INTRODUCTION el espacio Iimitado no permitio el hundimiento de su parte central, pero en dondeel ajuste del bloque colgante Section modelling of the Eastern Cordillera has become a las fallas inversas divergentes se hizo por el colapso increasingly popular in the past years and different attempts de sinclinales marginales. have been made to reconcile modern thin - skinned concepts with the well established basement involved structures, as ABSTRACT described by Julivert (1970). Difficulties which have to be overcome when interpreting the geometry and kinematic The santander Massif is a broad basement uplift, evolution of the Eastern Cordillera concern its doubly vergent limited on both flanks by two major reverse faults. More structure which rules out stacking by a simple forward internal faults, however, though very steeply dipping, breaking thrust sequence; the recognition of old Jurassic are normal faults and limit sedimentary half· grabens. normal faults which, where inverted, supposedly give rise to Much of the Neogene fault pattern is inherited from a high angle faults; the differentiation between anticlinal stacks Jurassic extensional event. The Neogene extensional and simple basement uplifts; the question of how to consume deformation regime of the massif's internal parts is excess lengths of basement, if thin - skinned thrusting is accounted for by a vertical uplift with an outward assumed and finally, along strike integration of geologic directed spread component, due to the fan array of its data which may involve sudden changes in fold or fault faults. This model is modified for a narrow, elongated vergence, fault displacement, etc. uplift at Malaga, where the space problems created by With these uncertainties, restoration may become highly the heave of a rock volume above downward curved interpretative and gives at best guesses of possible orogenic faults have been overcome by the collapse of marginal shortening values. Things turn worse, if one recognizes that synclines. much of the Cordillera's internal deformations clearly implies horizontal extensions. KURZFASSUNG In this report the basement involved tectonics of the southern Santander Massif are analyzed. Emphasis will be Das santandermassif besteht aus einer weitlaufigen put on faulted uplifts; unfaulted, basement cored structures Aufwolbung des andinen Grundgebirges und wird gegen will be treated in a later analysis. Vertical uplift models will seine Vorlandbecken durch steilfallende be examined which may imply a zero orogenic shortening Aufschiebungen abgetrennt. An BrOchen des and represent an alternative to the thin - or thick - skinned Massiflnnern haben sich dagegen Abschiebungen scenarios which require as much as 150 km lateral shortening 47 Kammer: Basement Faults, Santander Massif elongated, sheet like plutons and trends N-S. It 1/ '\. is somewhat oblique to the massif's NWoriented axis. : . ;......J-\..\.. II' '\\ 0 This basement is unconformably overlain by a J / I;, :' " 1r>. ; 100km fine grained clastic sequence, the Devonian \ o 0 \ • / \\ {.....{ 0 'io== ........b=~;,;,;;;. Floresta formation which is overprinted by a yet little studied pre - Carboniferous low grade metamorphism (Ward et aI., 1973). In the {':~\\' '\," southern Santander Massif this predominantly ," ~ ,:" f 1:;0/ /,' 0 metapelitic sequence may be affected by an intense folding and an incipient slaty cleavage . \. -""ao \~.) • / / .... / Curuto In the Malaga area, these structures maintain a 0 constant easterly vergence across the Neogene ·o~ 0 ...\ BF. t t 1/:/ - . ~.. t tJ \·:...·-<i··..~ uplift and are, thus, clearly unrelated to Neogene ) J \ ~ t \ 1\: faulting and folding. This Devonian sequence is unconformably g 'e;-' 0 l"-<\ Fi ·2 j~~.~:,~:,0 overlain by Carboniferous and Permian clastic .... )~~~~Fi~li~ I:~ : I J \ LF 0 ;\'. \ ,I': \ V:/ \' \ to calcareous sediments (Villarroel & Mojica, r G 1987; Ward et al., 1973; Carrillo, 1982). Due to \' ,i·~\t;\I~ -/\- Q,"~rnl section pre - Cretaceous erosional events, these Paleozoic sequences are, however, restricted B","~m7';/"··{'~f.:)::;{~~I";'~ CF • in their occurence. o . r : .... ,~,,,,/..\.. .~ Post - Paleozoic (?) sediments include the / ...r: .\ :·.{.: ·····t·,,,.....:.. Bocas and the Jordan formations, the latter / :' , : . ':'7 ...,.: r-. \ -.. -,- 0 consisting of fine grained red beds. They are / ....~I . I .:I ';-1 MQl~g.Q \ \ \ \ ... Suarez \~ _I _:.J : . separated from the Jurassic red bed sequence l: ....I Fault ~"" \ .... ; :\ \ ~ of the Giron group by an anqutar disconformity J :' n ~.: \ 1 ~ \ ..', I I which amounts up to 30° in the Bucaramanga . II rj It.; 1 i r » plateaus (Cediel, 1968). This sequence reaches a minimum thickness of 4650 m west of Buca- fl.;," ~.0 \ / 10 / ramanga (Cediel, 1968, Ward et al., 1973) and / ~o~ ( '/./ J II.~:...:/.... I;i / may be completely absent below the Cretaceous / / "..... ." :11 ....: / transgressive sediments. It records a rifting I ,.,._..<; :i:.·· .... ,: . 0 event which reactivated in the Maracaibo region / / .:<~/:../ ,.~-;/ / /I a Triassic rift trend (Bartok, 1993) and affected / / /./ / / from there on the area of the present Eastern / / / I I Cordillera. Although a regional synthesis of these Jurassic tectonics remains to be done, in ~ Terciary continental sediments M- "'olalla Uplift the Santander region some important points 1--d Cretac.us marine sediment V- Vetol Dome can be outlined: ~ Jurassic continental sediment '.F. -Bucoromonllo fault 1. In the southern continuation of the Bucaramanga plateaus, in the Aratoca region, 1 1 "'etomorphic and illneus bosemet l.F" Loboteco fault an unreactivated half - graben with a downthrown western block is preserved (Julivert & Tellez, Fig. 1. Map for locations of the southern santander Massif and 1963). The minimum thickness of the graben's southerly more tectonic units.- V: Vetas high; M: Malaga uplift; infill amounts to 800 m. B.F.: Bucaramanga fault; L.F.: Labateca faUlt; C.F.: Chucarima 2. In the Surata valley of the western flank of fault. the Santander Massif, a half - graben with an original down-to-the-Wthrow has been partially reactivated as a normal fault of opposed dip. (Dengo & Covey, 1992; Colletta et aI., 1990). This down-to-the-W polarity is displayed by another reactivated Jurassic fault, west of the 2. GEOLOGIC SETIING town of Pamplona. 3.ln the eastern flank of Santander Massif, two The southern Santander Massif exposes for its most part "old" (Precambrian ?) metamorphic basement and igneous, acid to ancient faults with a down-to-the-E polarity have been identified which underwent dip reversal intermediate intrusive rocks, whose emplacements record Caledonian during Andean orogeny and were reactivated to Jurassic orogenic events (Forero, 1990; Boinet et al., 1985; Dorr et both as reverse and normal faults. al., 1993). The structural grain of this basement is clearly outlined by 48 Geologia Colombiana No. 18, 1993 Evidence for an activity of the Jurassic faults during the The Malaga uplift (figs. 8 and 9) is a doubly faulted Cretaceous depositional history has not' been found so far. anticline, cored by Jurassic red beds and bordered on either Instead, the red beds of the grabens are capped by an side by two synclines which contain Cretaceous and Tertiary unconformity which affected part of and in some cases even sediments. The limiting faults are tfle Baraya faults on its the whole column of the Jurassic graben - fills and its western, and the Servita fault on its eastern margin. These underlying sediments. In extreme cases, the basal faults are reverse faults which converge toward the interior Cretaceous sediments overly the Devonian Floresta of this uplift, defining a fan - structure or a compressional formation within eroded Jurassic half grabens. On the horst