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The Rheic Ocean Gondwana Passive Margin: Insights from a Structural Analysis and a Sedimentary Response H

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The Rheic Ocean Gondwana Passive Margin: Insights from a Structural Analysis and a Sedimentary Response H The Rheic Ocean Gondwana passive margin: insights from a structural analysis and a sedimentary response H. M. DOHERTY* & G. D. WILLIAMS Earth Sciences and Geography, Keele University, Keele, Staffordshire, St5 5BG, United Kingdom - *Email [email protected] Palaeostrain Analysis Passive Continental Margins 2. Structural Analysis 1. Introduction CZ & WALZ: North CZ & WALZ: South CZ & WALZ: South East Extension Fault Analysis in NW Spain 0° 0° 0° The Cantabrian and West Asturian-Leonese Iberian Zones are Ibero-Armorican Arc 1 N 6a. Playa de Aguilar situated in the northwest of Spain (Figure 1). These Zones 0° Figure 1: The Cantabrian Zone, NW Spain Continental Shelf A B 6 consist of stratigraphical successions spanning from the Submarine Canyon Cambrian (~542.0Ma) to the Carboniferous (~299.0Ma) and are 270° 90° 270° 90° 270° 90° WALZ CZ cross-cut by large and small scale extensional and thrust faults. Continental Slope 270° 90° These sedimentary successions remain variable in thickness Marine Shallow W across thrusted sections of these Zones. In addition, numerous Continental Crust ater Sediments unconformities are identifiable within these successions, the 180° 180° 180° most notable during the Late Ordovician (Hirnantian ~445.6Ma Equal Area Projection, Lower Hemisphere Equal Area Projection, Lower Hemisphere Equal Area Projection, Lower Hemisphere 180° n=2 Narcea Equal Area Projection, Lower Hemisphere to ~443.7Ma) as a result of widespread glaciation and eustatic Iberian Figure 7: The contoured Antiform Figure 8: The contoured Figure 9: The contoured sea level fall. Overall these zones illustrate the Palaeozoic Normal Faults Massif stereonet above displays stereonet above displays stereonet above displays history of the northern Gondwana margin and its passive margin Outer Ridge Ocean Crust 6b. Playa de Aguilar palaeostrain data from East palaeostrain data from North palaeostrain data from North Nonmarine 0° associated with the Rheic Ocean. Sediments and Viodo (1) and Playa de Aguilar and South Barrios (2 and 3) Aralla (4) and Villamanin (5). Intrusions and Extrusions Extrusives Mantle (2). In total 12 normal faults and Villablino (7). In total 39 In total 43 faults were (syn-sediments) (planar data) were measured normal faults were measured measured and 10 movement Geological Settings 2 and 5 movement vectors and 8 movement vectors vectors determined. The data 250km 270° 90° SYN-RIFT 4 5 (linear data) determined. The determined. The data infers indicates Middle Cambrian to MEGASEQUENCE Key: 3 Long. 000° Long. E 006° d a t a i n d i c a t e s M i d d l e Middle Cambrian to Middle middle Devonian extension to Long. W 005° Long. 000° B & D Cantabrian Zone (1. East Viodo, 2. North 7 POST Cambrian to Middle Devonian Devonian extension to be in be in mainly an E-W direction CANTABRIAN ZONE MEGASEQUENCE-RIFT Barrios, 3. South Barrios, 4. North Aralla WEST AUSTRIAN LEONESE ZONE n=2 extension to be in NW-SE and NW-SE, N-S and NE-SW as well as NE-SW and N-S. and 5. Villamanin) 180° Lat. N 43° C & E Equal Area Projection, Lower Hemisphere W-E directions. Lat. N 41° West Asturian Leonese Zone (6. Playa directions. de Aguilar and 7. South Villablino) Figure 6: (A) The main tectonostratigraphical SPAIN FRANCE ITALY [8] Normal Fault Case Study N Lat. N 40° Central Iberian Zone/Central Armorican Zone zones of the Iberian Peninsula all of which, 4a. North Aralla Figure 10: Extension faults identified within the NAPPE ZONE 0° except the SPZ, lay on the northern margin of S Figure 10 San Pedro Fm. N of the village of Aralla, NW Spain FORELAND ZONE PRE-RIFT Ossa-Morena Zone/North Armorican Zone MONTAGNE Long. E 009° MEGASEQUENCE Gondwana during Cambrian-Devonian times; S (GPS: 0269463/4754613). NOIRE South Portuguese Zone Ÿ A (B)The main tectonostratigraphical units of the The structural data (fault) collated: Bedding Lat. N 36° 200km 200km 150km Long. E 012° 286/86°/N, Fault 272/30°/E. Lat. N 42° Cantabrian Zone 270° 90° Figures 2, 3 and 4: Areas of interest highlighted in NW Spain, S France and Sardinia All of the faults measured at this location shared similar attributes - fluvially deposited, haematitc, Figure 5: A generalised diagram of the passive margin associated with the development of the Rheic The stereonets in Figure 6C show a portion of the structural data, in siliciclastic sediments, thickened in places with The geology of NW Spain (Figure 2) contains one of the most diverse and complete Palaeozoic Ocean at the northern Gondwana margin. The highlighted segment shows the pre-rift, syn-rift and post- particular normal faults, from Lower-Middle Cambrian up to Devonian Figure 6C: [5 and 6] n=2 the growth of the fault(s). Indicating a syn- 180° sedimentary successions in SW Europe. The Cantabrian Zone belongs to the northern part of rift megasequences identified in the Cantabrian and West Asturian Leonese Zones of NW Spain. strata during the initial field season to the NW Iberian Massif, Movement Vectors Equal Area Projection, Lower Hemisphere sedimentary response either the passive margin the Iberian Massif and exhibits thin-skinned Variscan structures complicated by the Ibero-Armorican particularly the CZ and the WALZ. Normal faults were abundant within Primary and Secondary 0.5m extension associated with the Rheic Ocean or the the siliciclastic Oville (Middle Cambrian) and San Pedro (Lower-Middle Values of data collated Arc. The Cantabrian Zone includes a relatively thin Cambrian-Ordovician succession in comparison within the field from the N Rhenohercynian Sea. [2] 4b. North Aralla Devonian) formations. The locations where these simple and Somiedo-Correcilla Unit 0° with other parts of the Iberian Massif , and segments of modern-day France and Italy (Figures 3 and 1 1 Passive margins (Figure 5) are created as continents rift apart to form new ocean basin(s) and do conjugate extensional faults were extracted from is shown in Figure (Figure 6B) Figure 11: Schematic diagrams of (A) Conjugate 4). Exposed structures, sediments and fauna from NW Spain, S France and S Sardinia (particularly [3] A B not coincide with the boundary of a lithospheric plate. They originate at divergent plate Fault (B) Quadrimodal Fault . Possible the Nappe and Foreland Zones) reflect a unique Palaeozoic palaeogeography (Figures 12 and 13) 6B. The sediments from which these structural readings were taken 6a. 29° 274 boundaries, but as spreading proceeds and the ocean basin(s) expand they end up in a midplate from showed no movement vector information, deformation along the explanation for the variable palaeostrain data in influenced by various passive margin and collisional systems. This research takes into account 6b. 20° 113 90° the CZ and the WALZ of NW Spain. position. Nearly all passive margins share a fundamental morphological pattern – the continental faults was ductile. As a result, a palaeostrain approach has been used 270° 3 structural and stratigraphical data from these areas in order to gain a greater understanding of the 3 shelf, slope and rise as a result of similar erosional and depositional styles. However, the overall to determine the movement direction(s) of the faults. The data plotted 4a. 05° 175 The structural data collated during the first field geodynamic pattern for the northern Gondwana margin after the Cadomian orogeny. dimensions and shape of these physiographical provinces vary between margins [4]. has been rotated back to horizontal where required and rotated season of the normal faults shows in the N of the 4b. 11° 086 counter clockwise by ~35°to take into consideration the Aptian rotation CZ and the WALZ a NW-SE trend in the S of the CZ n=2 2 [1] 180° and the WALZ a N-S, NE-SW trend and in the SE of Iberia as a result of the opening of the northern Bay of Biscay . Equal Area Projection, Lower Hemisphere of the CZ and the WALZ an E-W trend. 2 Figure 11 Burial history modelling of stratigraphical data interpreted from measured sections and published log data from the Cantabrian Zone, the Montagne Noire and Sardinia provides a unique insight into the passive 3. Burial History margin development and its associated stratigraphy (Between the time periods highlighted by the palaeogeographical maps in Figures 12 and 13). For each of the respective stratigraphical units lithology, 4. Sedimentological Response thicknesses, predicted erosion and deposition, relative sea level and thermal parameters were input into burial history modelling software in order to analyse the stratigraphical development of the passive margin. Figure 12: A palaeogeographical map highlighting the location of the continents of Burial history modelling in the Cantabrian Zone shows two major periods of rift and thermal subsidence during the Late Cambrian and the Early Devonian. Burial history results from adjacent sections of the Megasequence Response - Cantabrian Zone 480MaGondwana, Avalonia, Baltica and Laurentia at the northern Gondwana margin, the Montagne Noire and Sardinia also display similar patterns of rift and thermal subsidence during the Late Cambrian. The subsidence displayed by these models is attributed to the [10] time of the Rheic Ocean origination growth of the Rheic Ocean and passive margin development during the Cambrian and Ordovician and the development of a Rhenohercynian seaway during the Lower Devonian (Figures 14 to 20). Regional Relative Geological Stratigraphic Chart of the Tectono-Sedimentary Major Major Megasequences Geological Sea-Level Time Southern Cantabrian Mountains Episodes Continents Oceans [7] N Gondwana Margin
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