The southward propagation of the southern Main Ethiopian Rift (sMER) and the northward propagation of the Kenya Rift have generated the Broadly Rifted Zone (BRZ), a ~40-km-wide region of extensional overlap between the Chew Bahir Basin-Gofa Province and the sMER. However, the tectonic interaction between these propagating rifts is not well-understood. We present new paleomagnetic and geochronologic data from Eo–Oligocene (45–35 Ma) and Miocene (18–11 Ma) volcanic and sedimentary rocks from the BRZ. Rock magnetic, alternating field and thermal demagnetization experiments indicate simple titanomagnetite mineralogies carrying a characteristic remanent magnetization from which straightforward magnetization directions were obtained. Site-mean paleomagnetic directions obtained from the analyzed samples reflect stable normal and reversed polarity directions. A comparison of the mean directions obtained for the Eo–Oligocene and Miocene rocks relative to the pole for stable South Africa at the corresponding ages reveals a significant counterclockwise (CCW) rotation of ~11.1° ± 6.4° and insignificant CCW rotation of ~3.2° ± 11.5°, respectively, reflecting a decrease in the extent of block rotations through time. Our results are consistent with the regional migration patterns of deformation during rifting. In the context of the regional tectonic evolution toward a narrow zone of extension, much of the deformation associated with block rotations probably occurred prior to the final stages of the emplacement of the Miocene volcanic flows. In light of the structural fabrics in the basement rocks exposed in the sMER, the observed CCW block rotations were likely accompanied and aided by the reactivation of NW-SE-striking basement heterogeneities, supporting the notion that inherited crustal-scale structures play a significant role during rifting across the BRZ.

The Sierra Pie de Palo (W-Argentina) is part of an Ordovician collisional wedge between the Cuyania microcontinent and the Ordovician Famatinian arc along the west Gondwana protomargin. The flatly E-dipping collisional wedge formed by polyphase shearing. The lower plate consists of Neoproterozoic-Cambrian metasedimentary rocks and Mesoproterozoic mafic-ultramafic basement of eastern Cuyania. The Famatinian forearc represents the upper plate composed of Mesoproterozoic metasedimentary rocks intruded by Mesoproterozoic and Ordovician plutons and overlying Neoproterozoic metasedimentary rocks. Pseudosection modeling on 20 samples distributed over the entire wedge yields a range of recorded maximum PT conditions of 7-14 kbar/ 480-590 degrees C along metamorphic gradients of 10-20 degrees C/km. Clockwise PT-paths are common starting from a midcrustal level with gradients of 20-30 degrees C to maximum depth followed by thermal relaxation. Local anticlockwise PT-paths are observed (1) in high-grade basement klippen of the overriding Famatinian arc (8-10 kbar/700-765 degrees C) caused by late emplacement onto the nappe stack and (2) within late upper crustal ductile zones with normal shear sense. Isotopic ages of (re)crystallisation of metamorphic minerals and assemblages comprise: 429 +/- 2 to 434 +/- 7 Ma (Lu-Hf; garnet), 460 +/- 6 Ma (U-Pb monazite in garnet), 404 +/- 7 to 422 +/- 8 Ma (Rb-Sr; white mica) and 403 +/- 4 to 440 +/- 8 Ma (40Ar/39Ar white mica). Including previous data, ages cluster around major peaks at 464 +/- 1 Ma, 436 +/- 3 Ma, 427 +/- 3 Ma, 415 +/- 2 Ma and 405 +/- 1 Ma related to deformation during three orogenic stages: (1) subduction of the lower plate (eastern Cuyania) to the basis of the wedge at 470-455 Ma and later burial of the upper plate (Famatinian forearc) to similar depths at 440-415 Ma, a characteristic of a "hard" collision; (2) exhumation mainly by erosion and (3) late dissection of the wedge by upper crustal thrusts and some normal faulting at the top of the wedge during arrival of the colliding microcontinent Chilenia (405-360 Ma) at the new Devonian leading margin.

ABSTRACT The Baie Verte Line in western Newfoundland marks a suture zone between (1) an upper plate represented by suprasubduction zone oceanic crust (Baie Verte oceanic tract) and the trailing continental Notre Dame arc, with related upper-plate rocks built upon the Dashwoods terrane; and (2) a lower plate of Laurentian margin metasedimentary rocks with an adjoining ocean-continent transition zone (Birchy Complex). The Baie Verte oceanic tract formed during closure of the Taconic seaway in a forearc position and started to be obducted onto the Laurentian margin between ca. 485 and 476 Ma (early Taconic event), whereas the Birchy Complex, at the leading edge of the Laurentian margin, was subducted to maximum depths as calculated by pseudosection techniques (6.7–11.2 kbar, 315–560 °C) by ca. 467–460 Ma, during the culmination of the Taconic collision between the trailing Notre Dame arc and Laurentia, and it cooled isobarically to 9.2–10.0 kbar and 360–450 °C by 454–449 Ma (M1). This collisional wedge progressively incorporated upper-plate Baie Verte oceanic tract rocks, with remnants preserved in M1 high-pressure, low-temperature greenschist-facies rocks (4.8–8.0 kbar, 270–340 °C) recording typical low metamorphic gradients (10–14 °C/km). Subsequently, the early Taconic collisional wedge was redeformed and metamorphosed during the final stages of the Taconic cycle. We relate existing and new 40Ar/39Ar ages between 454 and 439 Ma to a late Taconic reactivation of the structurally weak suture zone. The Taconic wedge on both sides of the Baie Verte suture zone was subsequently strongly shortened (D2), metamorphosed (M2), and intruded by a voluminous suite of plutons during the Salinic orogenic cycle. Calculated low- to medium-pressure, low-temperature M2 conditions in the Baie Verte oceanic tract varied at 3.0–5.0 kbar and 275–340 °C, with increased metamorphic gradients of ~17–25 °C/km during activity of the Notre Dame arc, and correlate with M2 assemblages in the Birchy Complex. These conditions are associated with existing Salinic S2 white mica 40Ar/39Ar ages of ca. 432 Ma in a D2 transpressional shear zone and synkinematic intrusions of comparable age. A third metamorphic event (M3) was recorded during the Devonian with calculated low-pressure, low-temperature conditions of 3.2–3.8 kbar and 315–330 °C under the highest metamorphic gradients (23–30 °C/km) and associated with Devonian–early Carboniferous isotopic ages as young as 356 ± 5 Ma. The youngest ages are related to localized extension associated with a large-scale transtensional zone, which reused parts of the Baie Verte Line suture zone. Extension culminated in the formation of a Middle to Late Devonian Neoacadian metamorphic core complex in upper- and lower-plate rocks by reactivation of Baie Verte Line tectonites formed during the Taconic and Salinic cycles. The Baie Verte Line suture zone is a collisional complex subjected to repeated, episodic structural reactivation during the Late Ordovician Taconic 3, Silurian Salinic, and Early–Late Devonian Acadian/Neoacadian orogenic cycles. Deformation appears to have been progressively localized in major fault zones associated with earlier suturing. This emphasizes the importance of existing zones of structural weakness, where reactivation took place in the hinterland during successive collision events.

Abstract Magmatic continental rifts often constitute nascent plate boundaries, yet long‐term extension rates and transient rate changes associated with these early stages of continental breakup remain difficult to determine. Here, we derive a time‐averaged minimum extension rate for the inner graben of the Northern Kenya Rift (NKR) of the East African Rift System for the last 0.5 m.y. We use the TanDEM‐X science digital elevation model to evaluate fault‐scarp geometries and determine fault throws across the volcano‐tectonic axis of the inner graben of the NKR. Along rift‐perpendicular profiles, amounts of cumulative extension are determined, and by integrating four new 40Ar/39Ar radiometric dates for the Silali volcano into the existing geochronology of the faulted volcanic units, time‐averaged extension rates are calculated. This study reveals that in the inner graben of the NKR, the long‐term extension rate based on mid‐Pleistocene to recent brittle deformation has minimum values of 1.0–1.6 mm yr−1, locally with values up to 2.0 mm yr−1. A comparison with the decadal, geodetically determined extension rate reveals that at least 65% of the extension must be accommodated within a narrow, 20‐km‐wide zone of the inner rift. In light of virtually inactive border faults of the NKR, we show that extension is focused in the region of the active volcano‐tectonic axis in the inner graben, thus highlighting the maturing of continental rifting in the NKR.

A multidisciplinary approach to the study of collisional orogenic belts can improve our knowledge of their geodynamic evolution and may suggest new tectonic models, especially for (U)HP rocks inside the accretionary wedge. In the Western Alps, wherein nappes of different origin are stacked, having recorded different metamorphic peaks at different stages of the orogenic evolution. This study focuses on the External (EPZ) and Internal (IPZ) ophiolitic units of the Piedmont Zone (Susa Valley, Western Alps), which were deformed throughout four tectonometamorphic phases (D1 to D4), developing different foliations and cleavages (S1 to S4) at different metamorphic conditions. The IPZ and EPZ are separated by a shear zone (i.e. the Susa Shear Zone (SSZ)) during which a related mylonitic foliation (SM) developed. S1 developed at high pressure conditions (Epidote-eclogite vs. Lawsonite-blueschist facies conditions for IPZ and EPZ, respectively), as suggested by the composition of white mica (i.e. phengite), whereas S2 developed at low pressure conditions (Epidote-greenschist facies conditions in both IPZ and EPZ) and is defined by muscovite. White mica defining the SM mylonitic foliation (T1) is mostly defined by phengite, while the T2-related disjunctive cleavage is defined by fine-grained muscovite. The relative chronology inferred from meso-and micro-structural observations suggests that T1 was near-coeval with respect to the D2, while T2 developed during D4. A new set of radiometric ages of the main metamorphic foliations were obtained by in situ Ar/Ar dating on white mica. Different generations of white mica defining S1 and S2 foliations in both the IPZ and EPZ and SM in the SSZ, were dated and two main groups of ages were obtained. In both IPZ and EPZ, S1 foliation developed at-46-41 Ma, while S2 foliation developed at-40-36 Ma and was nearly coeval with the SM mylonitic foliation (-39-36 Ma). Comparison between structural, petrological and geochronological data allows to define time of coupling of the different units and consequently to infer new tectonic implications for the exhumation of meta-ophiolites of the Piedmont Zone within axial sector of the Western Alps.

The Red Indian Line (RIL) in central Newfoundland is the suture, where the main tract of the Iapetus Ocean was closed at similar to 452 Ma during accretion of the peri-Gondwanan Victoria arc with the composite active Laurentian margin. The protracted deformation history of this soft collision started at similar to 471 Ma with accretion of oceanic terranes to the active composite Laurentian margin. After Iapetus closure both colliding active margins were progressively deformed and metamorphosed during Silurian and Devonian (Salinic, Acadian and Neoacadian orogenic cycles). Peak conditions of the very low- to medium-grade, heterogeneously distributed metamorphism were determined by pseudosection techniques within the range of 2-7 kbar, 230-450 degrees C during increase of the metamorphic field gradient from similar to 12 degrees C/km to similar to 32 degrees C/km over time. Multiple metamorphic crystallisation stages were dated by white mica 40Ar/39Ar spot and plateau ages, additional Rb-Sr mineral isochrons involving white mica and one U/Pb age of titanite. All resulting ages between 439 +/- 4 Ma and 356 +/- 16 Ma postdate the closure of Iapetus. Results differ along two transects: The oldest ages of 443-421 Ma (Salinic orogenic cycle) were observed along the northern transect through the RIL zone with minimal younger overprint. Hence low temperature, intermediate to high pressure conditions (4.0-7.0 kbar, 230-340 degrees C) achieved during Taconic-Salinic underthrusting are well preserved. During Acadian dextral transpression the Taconic-Salinic structural wedge was tilted subvertically. In contrast, rocks along the southern transect through the RIL zone mainly show Acadian ages of 408-390 Ma with local preservation of older ages. Acadian deformation occurred under low temperature/low pressure conditions (similar to 250-450 degrees C, 2.5-4.6 kbar). Also Silurian terrestrial cover rocks were buried under these conditions. Acadian-Neoacadian deformation (393-340 Ma) becomes younger towards the northwest and progressively localized in transcurrent fault zones. This final foreland deformation at shallow crustal level established the Acadian/Neoacadian orogenic front in central Newfoundland slightly northwest of the RIL. The Red Indian Line (RIL) in central Newfoundland is the suture, where the main tract of the Iapetus Ocean was closed at similar to 452 Ma during accretion of the peri-Gondwanan Victoria arc with the composite active Laurentian margin. The protracted deformation history of this soft collision started at similar to 471 Ma with accretion of oceanic terranes to the active composite Laurentian margin. After Iapetus closure both colliding active margins were progressively deformed and metamorphosed during Silurian and Devonian (Salinic, Acadian and Neoacadian orogenic cycles). Peak conditions of the very low- to medium-grade, heterogeneously distributed metamorphism were determined by pseudosection techniques within the range of 2-7 kbar, 230-450 degrees C during increase of the metamorphic field gradient from similar to 12 degrees C/km to similar to 32 degrees C/km over time. Multiple metamorphic crystallisation stages were dated by white mica 40Ar/39Ar spot and plateau ages, additional Rb-Sr mineral isochrons involving white mica and one U/Pb age of titanite. All resulting ages between 439 +/- 4 Ma and 356 +/- 16 Ma postdate the closure of Iapetus. Results differ along two transects: The oldest ages of 443-421 Ma (Salinic orogenic cycle) were observed along the northern transect through the RIL zone with minimal younger overprint. Hence low temperature, intermediate to high pressure conditions (4.0-7.0 kbar, 230-340 degrees C) achieved during Taconic-Salinic underthrusting are well preserved. During Acadian dextral transpression the Taconic-Salinic structural wedge was tilted subvertically. In contrast, rocks along the southern transect through the RIL zone mainly show Acadian ages of 408-390 Ma with local preservation of older ages. Acadian deformation occurred under low temperature/low pressure conditions (similar to 250-450 degrees C, 2.5-4.6 kbar). Also Silurian terrestrial cover rocks were buried under these conditions. Acadian-Neoacadian deformation (393-340 Ma) becomes younger towards the northwest and progressively localized in transcurrent fault zones. This final foreland deformation at shallow crustal level established the Acadian/Neoacadian orogenic front in central Newfoundland slightly northwest of the RIL. The Red Indian Line (RIL) in central Newfoundland is the suture, where the main tract of the Iapetus Ocean was closed at similar to 452 Ma during accretion of the peri-Gondwanan Victoria arc with the composite active Laurentian margin. The protracted deformation history of this soft collision started at similar to 471 Ma with accretion of oceanic terranes to the active composite Laurentian margin. After Iapetus closure both colliding active margins were progressively deformed and metamorphosed during Silurian and Devonian (Salinic, Acadian and Neoacadian orogenic cycles). Peak conditions of the very low- to medium-grade, heterogeneously distributed metamorphism were determined by pseudosection techniques within the range of 2-7 kbar, 230-450 degrees C during increase of the metamorphic field gradient from similar to 12 degrees C/km to similar to 32 degrees C/km over time. Multiple metamorphic crystallisation stages were dated by white mica 40Ar/39Ar spot and plateau ages, additional Rb-Sr mineral isochrons involving white mica and one U/Pb age of titanite. All resulting ages between 439 +/- 4 Ma and 356 +/- 16 Ma postdate the closure of Iapetus. Results differ along two transects: The oldest ages of 443-421 Ma (Salinic orogenic cycle) were observed along the northern transect through the RIL zone with minimal younger overprint. Hence low temperature, intermediate to high pressure conditions (4.0-7.0 kbar, 230-340 degrees C) achieved during Taconic-Salinic underthrusting are well preserved. During Acadian dextral transpression the Taconic-Salinic structural wedge was tilted subvertically. In contrast, rocks along the southern transect through the RIL zone mainly show Acadian ages of 408-390 Ma with local preservation of older ages. Acadian deformation occurred under low temperature/low pressure conditions (similar to 250-450 degrees C, 2.5-4.6 kbar). Also Silurian terrestrial cover rocks were buried under these conditions. Acadian-Neoacadian deformation (393-340 Ma) becomes younger towards the northwest and progressively localized in transcurrent fault zones. This final foreland deformation at shallow crustal level established the Acadian/Neoacadian orogenic front in central Newfoundland slightly northwest of the RIL. The Red Indian Line (RIL) in central Newfoundland is the suture, where the main tract of the Iapetus Ocean was closed at similar to 452 Ma during accretion of the peri-Gondwanan Victoria arc with the composite active Laurentian margin. The protracted deformation history of this soft collision started at similar to 471 Ma with accretion of oceanic terranes to the active composite Laurentian margin. After Iapetus closure both colliding active margins were progressively deformed and metamorphosed during Silurian and Devonian (Salinic, Acadian and Neoacadian orogenic cycles). Peak conditions of the very low- to medium-grade, heterogeneously distributed metamorphism were determined by pseudosection techniques within the range of 2-7 kbar, 230-450 degrees C during increase of the metamorphic field gradient from similar to 12 degrees C/km to similar to 32 degrees C/km over time. Multiple metamorphic crystallisation stages were dated by white mica 40Ar/39Ar spot and plateau ages, additional Rb-Sr mineral isochrons involving white mica and one U/Pb age of titanite. All resulting ages between 439 +/- 4 Ma and 356 +/- 16 Ma postdate the closure of Iapetus. Results differ along two transects: The oldest ages of 443-421 Ma (Salinic orogenic cycle) were observed along the northern transect through the RIL zone with minimal younger overprint. Hence low temperature, intermediate to high pressure conditions (4.0-7.0 kbar, 230-340 degrees C) achieved during Taconic-Salinic underthrusting are well preserved. During Acadian dextral transpression the Taconic-Salinic structural wedge was tilted subvertically. In contrast, rocks along the southern transect through the RIL zone mainly show Acadian ages of 408-390 Ma with local preservation of older ages. Acadian deformation occurred under low temperature/low pressure conditions (similar to 250-450 degrees C, 2.5-4.6 kbar). Also Silurian terrestrial cover rocks were buried under these conditions. Acadian-Neoacadian deformation (393-340 Ma) becomes younger towards the northwest and progressively localized in transcurrent fault zones. This final foreland deformation at shallow crustal level established the Acadian/Neoacadian orogenic front in central Newfoundland slightly northwest of the RIL.