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 Models Time (Ma) Time (Ma) Events -tive+ Scenario 1 Scenario 2 30° N 540 520 500 480 460 440 500 488 476 464 452 440 Cambrian Ordovician Sil Modelling Key 0 Cambrian Ordovician Sil 0 Gzhelian Montagne Noire, South France Correlated Correlated ~303.4 + 0.9 F - Foreland - Stephanian Successions LITHOLOGICAL SYMBOLS Megasequences Megasequences Kasimovian 500 ~307.2 + 1.0 Basin Figure 20: (A) Multiple rifts from the Middle from NW Spain from NW Spain 200 - CONGLOMERATE MARL Moscovian ~311.7 +- 1.1

Cambrian to Late Ordovician displayed by the Pangaea Formation

1000 Activity

Bashkirian SANDSTONE LIMESTONE Olleros Fm E - Foreland Genesis© burial history plot for the southern 400 ~318.1 +- 1.3

Serpukhovian Syn-orogenic Basin, Overfilled DOLOMITE SILTSTONE Montagne Noire 1500 ~328.3 +- 1.6 E - Post Rift 0° Visean D - Foreland SHALY SAND IGNEOUS 600 Carboniferous + Nappe Zone - South Sardinia ~345.3 - 2.1 Development ? L 2000 Tournasian Basin Foreland Basin, Starved

Depth (m) SHALE Figure 20: (B) Multiple rifts from the Late Depth (m) 2500 800 OTHER Cambrian to Late Ordovician displayed by the DEPTH CONTOUR Genesis© burial history plot for the Nappe Zone 3000 1000 D - Foreland Iapetus Ocean UNCONFORMITY E - Post Rift Basin, Starved 30° S of southern Sardinia. A B B B Huergas Fm B 3500 Av Model Summary Sheet Burial History and Identifiable Plate Tectonic Events - CZ Rheic Ocean 60° S 299Ma -200m -200m Formation Lithology Time Topography of Stratigraphic Section Modelled Oville Fm San Pedro Fm Stages Figure 15: Time (Ma) Time (Ma)

Name 20% 60% 100% (Ma) Sea Rhenohercynian 0 1,500 Shows the 500 400 300 200 100 0 500 400 300 200 100 0 Montana Fm modern-day 0 0 C - Passive 1,400 -500 -500 D - Syn Rift Carboniferous San Pedro Fm 1,300 topography -1000 -1000 359.2Ma Margin South Pole 450 -1500 Deposits The Iberian Passive Margin 1,200 of the CZ -1500 Visean highlighting -2000 -2000 G (m) Height 1,100 -2500 -2500

Figure 15 the Variscan Devonian

Figure 13: A palaeogeographical map 1,000 -3000 -3000

Rift-Related Sedimentation Rift-Related Depth (m) Depth 900 Fammenian 0 400 800 1,200 1,600 2,000 and Alpine -3500 (m) Depth -3500 416.0Ma Distance (m) Formigoso Fm highlighting the location of the continents of Activity C - Post Rift

-4000 Ocean Rheic -4000 or Sedimentation Margin Passive 320Ma affects upon A Gondwana

Gondwana, Avalonia, Armorica, Baltica and Portilla Fm -4500 -4500 B Silurian

Cantabrian Zone [10] Time (Ma) it. Subsidence Curves 443.7Ma Supercontinent

Laurentia at the time of the Rheic Ocean closure Huergas Fm Frasnian Sea) Rhenohercynian and Ocean (Rheic ‘Black Shales’ ‘Black 1350 600 500 400 300 200 100 0 generalised relative sea

Figure 19: (A) A subsidence A

Palaeozoic Mesozoic Cenozoic Sedimentation Marine

Givetian curve for the Oville Fm - related to Ordovician -2000 488.3Ma

Glaciation

Hirnantian Eifelian RIFT EVENT 1 Widespread Pre-orogenic 1800 (Rheic Ocean Rifting/ m e g a s e q u e n c e B . ( B ) A 30° N Lochkovian Transgressive Event Late Ordovician Passive Margin) subsidence curve for the San

P e d r o F m - r e l a t e d t o Cambrian B - Rift

curve generated by a facies analysis. Figure 16: B - Syn Rift B 2250 megasequence D. Sedimentation Telychain Regression Transgression Barrios Fm Santa Luc Fm 0 0 Time (Ma)

L Depth(m) 523.5 470.5 417.5 364.5 311.5

Ar 2700 Cambrian Ordovician Silurian Devonian Carboniferous Ocean Iapetus Av Closure of the Rheic Ocean La Vid Gp -2000 0° Unconformity Megasequences (Late Devonian)

The Iberian Passive Margin Unconformity Barrios Fm Ocean) (Rheic 3150 Palibian (Valdemides Event) B - Syn Rift B - Syn Rift 2000 2000 Oville Fm Thermal Subsidence 0 0 Sedimentation Rift-Related San Pedro Fm Unconformity of the Basin(s)

3600 or Sedimentation Margin Passive (Hirnantian Glacial Event) Làncara Fm A - Pre Rift 30° S Depth (m) Formigoso Fm G 4050 Barrios Fm 2000 4000 RIFT EVENT 2 5. Conclusions Láncara Fm (Rheic Ocean Passive Margin/ 4000

60° S Depth (m) Rheno-Hercynian Sea) Variscan Orogeny Alpine Orogeny Burial history modelling in the Cantabrian Zone shows two major periods of rift and thermal subsidence during the Late Cambrian and the Early Devonian 4500 Foreland Basin periods. Burial history results from adjacent sections of the northern Gondwana margin, Montagne Noire and Sardinia also display similar patterns of rift and Development A B C D E/F Figure 14: The generalised modelling parameters for the CZ -4000 thermal subsidence during the Late Cambrian. The subsidence displayed by these models is attributed to passive margin development during the Cambrian Figure 17: A generalised burial history plot for a sedimentological section in the CZ. The show the geological time periods for sediment deposition, Figure 18: This section of the genesis model highlight and Ordovician and the development of a Rhenohercynian sea during the Lower Devonian. The sequence of sediments along the northern Gondwana dates of sediment deposition, lithology % mixes, unconformities (deposited and eroded unconformities in the Middle Cambrian, Lower Ordovician the passive margin formation associated with the Rheic margin are similar in nature and can be divided into one pre-rift, two syn-rift and two post-rift episodes relating to the passive margin. Structural evidence from South Pole amounts), bathymetry and a rift relating to the initiation of the Rheic ocean (~515Ma/6000β of Ocean. Megasequences taken from the sedimentary and Upper Devonian as well as the lithology % making up the [9] northwest Spain have confirmed the two periods of extension at the Gondwana margin at the Cambrian-Ordovician boundary and during the Lower Devonian 2 ) have been applied to the model. individual formations and groups. analysis have been applied to the model. period. The kinematics of these syn-sedimentary faults shows NW-SE to NE-SW extension from south to north around the Ibero-Armorican Arc.

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