Alter und Genese des Nicoya-Komplexes, einer ozeanischen Paläokruste (Oberjura bis Eozän) im südlichen Zentralamerika

Zusammenfassung

Der mehrere Kilometer mächtige, im wesentlichen aus ozeanischen Basalten aufgebaute Komplex von Nicoya, der den Sockel der zahlreichen pazifischen Halbinseln von Costa Rica und Panama bildet, wird biostratigraphisch zum ersten Mal in sechs, altersmäßig sehr verschiedene Teilkomplexe (Subkomplexe) gegliedert (Abb. 33). Methodisch ist dies durch Extrapolation der markierenden Alterswerte möglich, wobei die in die Basalte eingeschlossenen sedimentären Xenolithe aufgrund ihrer Mikrofauna ein Maximalalter, die normal lagernden hangenden Sedimente über den Basalten ein Minimalalter des vulkanischen Ereignisses liefern. Damit wird bewiesen, daß sich der Komplex, im Gegensatz zu den bisherigen Vorstellungen, sehr diskontinuierlich in der Zeit vom Oberjura bis in das Eozän gebildet hat. Die Teilkomplexe werden als submarine, abyssische bis bathyale Deckenergüsse im Bereich ozeanischer Kruste gedeutet. Bei diesen Vorgängen wurden die inzwischen auf den liegenden Eruptiva abgelagerten Sedimente aufgearbeitet und in Form von Xenolithen innerhalb des sich neu bildenden Eruptionskörpers in eine völlig ortsfremde Umgebung verschleppt. Es wird angenommen, daß sich die primäre, also an einem mittelozeanischen Rücken gebildete, ozeanische Kruste erst im Liegenden dieser Teilkomplexe befindet.

Abschließend wird ein geotektonisches Modell (Abb. 34) vorgestellt, das im Einklang mit den vorliegenden Daten steht. Der Komplex von Nicoya wird als der nicht subduzierte Rest eines aseismischen Schwellensystems angesehen, der durch plate boundary jumping, also unter Verlagerung der Plattengrenze (Mittelamerika-Graben) nach W, an den südzentralamerikanischen Inselbogen angefügt wurde.

Abstract

The Nicoya Complex, built by thick sequences of mainly oceanic basalts and exposed on the numerous pacific peninsulas of Costa Rica and Panama (Fig. 1), is, for the first time, divided by means of biostratigraphic and facies methods. This classification is not only possible by means of biostratigraphic dating of the overlying sediments but also of occasional inclusions of sedimentary silicious and calcareous exotic blocks within the basaltic sequences. This method shows that the formation of the Nicoya Complex, contrary to the hitherto idea of a uniphase and short development, not only took up very long periods of geological time but was also very discontinuously built. The Nicoya Complex can now be divided into six subcomplexes of different age which, in part, may already be regionally differentiated (Fig. 33). These subcomplexes have to be interpreted as being submarine, abyssal to bathyal sheet flows of mainly tholeiitic basalts up to several hundred meters in thickness. During these effusions of short time duration, the sedimentary cover, in the meantime deposited on the underlying effusiva, was more or less intensively reworked by these processes and deported within the new effusion body in form of exotic blocks (xenolithes, volcanic mélange) where contactmetamorphic changes partly occurred.

The following subcomplexes, named after their type localities, are differentiated:

• 1. The subcomplex of Brasilito contains radiolarite-, respectively jaspilite-xenolithes (contactmetamorphosis!) of early Lower Cretaceous and especially Upper Jurassic. TheSphaerostylus lanceola- zone is proved by rich radiolarian faunas. In these radiolarites, developed below the CCD, partly workable manganese nodule deposits are occurring. Hitherto these were explained as being hydrothermal replacements of basic volcanites and sediments (Webber 1942,Roberts 1944) now compared with similar occurrences in the Olympic Peninsula (Roy 1976).

• 2. Because the xenolithes of the subcomplex of Junquillal have suffered stronger contactmetamorphosis, their biostratigraphic classification is not entirely proved.

• 3. The subcomplex of Murcielago on the Nicoya peninsula is widespread. Xenolithes, so far, were less useful here. But this subcomplex possesses well-developed sedimentary overlying units which are not noticed in the older ones. They start with radiolarian rocks of the older Campanian and succeed with pelagic limestones during the Upper Campanian, followed by mostly thick tuffitic sediments.

• 4. The subcomplex of Golfito contains numerous xenolithes from silicious limestones of the Upper Campanian in which, besides planktonic foraminifers, well-preserved radiolarians are met.

• 5. The subcomplex of Garza contains xenolithes of pelagic limestones rich of faunas of the Middle Maastrichtian planktonic foraminifers and is superposed by pelagic limestone of the uppermost Maastrichtian.

• 6. The subcomplex of Quepos contains xenolithes of Paleocene age (planktonic foraminifers and macro-foraminifers), partly developed as classical mélange in the sense of HSÜ, and is overlain by sediments of the Lower Eocene.

Consequently, the age of these volcanic occurrences can be classified biostratigraphically as follows:

Late Lower Cretaceous (1); post-Cenomanian but pre-Campanian (2); about Santonian or Lowest Campanian (3); post-Campanian, probably at the beginning of the Maastrichtian (4); Upper Maastrichtian (5) and presumably boundary of Paleocene/Eocene (6).

As these subcomplexes represent abyssal to bathyal volcanic lava sheets — adjoining and younger plutonic intrusions have to be neglected — they cannot be interpreted as being a primary oceanic crust built on a mid-oceanic ridge. Therefore, it can be assumed that such rocks are present in the underlying basal part of the older effusion bodies. In fact, pyrite deposits of massive layer-bounded sulphides, typical for the top of such ophiolithic sequences, are occurring in pillow basalts near Punta Gorda below the Brasilito subcomplex as far as can be geologically recognized (E.Kuypers 1977: personal communication).

Using these and further data, the geotectonical development may be reconstructed (Fig. 34). An aseismic rise “Nicoya-Azuero” which “far away” in the Pacific is still developing since the end of the Jurassic (the palaeomagnetic test is still outstanding) and, since the formation of a subduction zone in the southern part of Central America, is moving into the direction of this zone — there is biostratigraphic proof of an island arc “Nicaragua-Panama” from about the Cenomanian onwards. In the Campanian, at least parts of this “Nicoya-Azuero rise” possess an authentic submarine relief of more than 4 km. Finally, during the Campanian, this rise slowly comes into contact with the subduction zone: Removing of the subduction to the E takes place [in the sense ofVogt et al. (1976)] leading to a plugging of the subduction zone by drifting of rise sections (cf. with the present geotectonical relation between the Cocos/Coiba rise and the Central American isthmus); and finally, during this course of movements, there is an advancing echelon shearing by parts of the rise into a lateral direction and an attachment to the island arc (cf.Lowrie 1978). During this time strong vertical movements also occur which, at times, lead to a considerable isostatic uplift of the southern Central America - (faunal exchange of short duration from N to S America! - cf. present situation). After the last oceanic effusions at the boundary Paleocene/Eocene, this development is finally completed during the Eocene, whilst the Central American subduction zone has finally shifted W in front of the “ruins” of the previous “Nicoya-Azuero rise” (plate boundary jumping). Since the Upper Eocene the Nicoya Complex tectonically shows platform characteristics and is continentally orientated.

Résumé

Une classification biostratigraphique du complex de Nicoya est proposée. Il est composé essentiellement de basaltes océaniques d'une épaisseur de plusieurs kilomètres formant le socle de nombreuses îles et presqu'îles de la bordure pacifique de Costa Rica et du Panama. La classification comprend six subdivisions d'âge très différent (fig. 33). Elle est rendue possible par extrapolation des âges indiqués, étant donné que les xénolithes sédimentaires renfermés dans les basaltes indiquent l'âge maximum et que les sédiments du toit en position normale donnent l'âge minimum de l'évènement volcanique. Ceci prouve, contrairement aux idées publiées jusqu'à présent, une évolution très discontinue du Jurassique supérieur à l'Eocène. Les six évènements volcaniques (subcomplexes) représent des épanchements basaltiques sous-marins abyssaux et bathyaux dans la région de la croûte océanique. Les couches sédimentaires déposées après une éruption sont détruites par la suivante et transportées sous forme de xénolithes dans un environnement complètement différent de celui de son origine. On adment que la croûte océanique primaire, c'est -à-dire cella qui s'est formée sur une dorsale océanique médiane, se trouve au mur de ces complexes partiels.

Un modèle géotectonique en accord avec les résiltats de ces recherches est présenté. Le complexe de Nicoya est considéré comme le reste de sous-charriages d'un système de rides asismiques accolé à l'arc insulaire de l'Amérique Centrale méridionale par »plate boundary jumping«, c.à.d. par migration de la limite de la plaque (= le fossé d'Amérique Centrale) vers l'Ouest.

Краткое содержание

Впервые провели биос тратиграфические по дразделения комплекса Nicoya построен ного из океанических база льтов в несколько км м ощности и образующего цоколь м ногочисленных полуостровов Пацифи ки от Коста-Рики до Пан амы, на 6 подкомплексов разли чного возраста (рис. 33). Методи чески это была экстра поляция возрастных отметок, п ричем осадочные ксенолиты, заключенн ые в базальты, дали — по их микрофауне — максима льный возраст, а обычные отл ожения осадочных пор од кровли над базальтами — мини мальный возраст вулканическ ого события. Т.о. устано вили, что этот комплекс — во пре ки существующему мнени ю — образовывался с пе рерывами в период от верхней юры до эоцена. Субкомплексы рассматривают, как по кровы, образовавшиеся в сле дствие подводных, абиссиальных и батиа льных излияний в райо не океанической коры. Во время этих процессов осадочные породы, отложившиеся с эруптивными породам и, оказывались мобили зованными и были внедрены в виде ксенолитов в новообр азующиеся эруптивные тела, т.е. в чуждое им ок ружение. Считают, что п ервичная образовавшаяся на ок еаническом хребте кора находитс я в подошве этой части комплекса.

В заключении предста влена геотектоничес кая модель (рис. 34), согласующаяся с имеющимися данными. Комплекс Nicoya ра ссматривают, как не по ддающийся субдукции остаток асейсмическ ой пороговой системы, который в результате plate boundary jumpling при перемещении гран ицы глыбы на запад был перенесен к южному краю централь ноамериканской островной дуги.