Interplay between tectonics and magmatism during the last stages of continental breakup

Transcription

1 R. Pik, P. Burnard, A. Williams, C. Vye, L. France, D. Ayalew, G. Yirgu Interplay between tectonics and magmatism during the last stages of continental breakup The Manda Hararo rift segment, Afar, Ethiopia Sarah Medynski Magmatic rifting and active volcanism conference Addis Ababa, 11th January 2012

2 Topographic response to 2005 dike intrusion registered by INSAR The Afar triple junction: Deformation already focused along rift segments Emplacement of new crust driven by dike injection the best location to observe the Continental- Oceanic Transition The 2005 event: provided information at the scale of a single rifting event: Dike injections induce a topographic response by fault reactivation Grandin et al Ebinger et al Involve complex interactions between magma bodies along the rift axis 2 Grandin et al. 2010

3 Aim of the work Are the current observations representative of the entire rift segment history? How to create and maintain a rift segment at the magmatic cycle scale? (10 to 100 ka) Magma supply -At least 2 magma chambers -Stability through time? -Replenishment recurrence? - Relationship tectonics / topography Require an accurate chronology of volcanic and tectonic events 3

4 How to date magmatic and tectonic events? Good arguments to use the cosmonuclides method for dating Minimal erosion Surface exposure age dating Applicable on flow surfaces and fault scarps In volcanic environment: 3 He, 36 Cl Top of the atmosphere Cosmic rays Sampling implications: Chain of nuclear reactions 1 mm Rock surface 0 Depth (m) 3 He cos -4 0 [ 3 He cos ] 1 mm Preserved surfaces Mineral phases which retain 3 He cos (olivines & pyroxenes) t = 3 He cos / P 4

5 Sample sites Lidar capture motion courtesy of BGS 10km Dabbahu Baddi Aa flows Pahoehoe flows 5

6 O l d e r Y o u n g e r Old rift floor Fissure fed flows Massive lavas from Dabbahu Recent fissural lavas emitted at the axis Pahoehoe porphyritic basalts Aa porphyritic basalts Aa aphyric trachy-andesites 6

7 Dabbahu Major elements Geochemistry Different partial melt fraction from a common source? Trace elements ratio Baddi Partial Melt Fraction Garnet source content Mid Segment Magma Chamber 2 distinct trace elements signatures Allows low partial melt fraction, off-axis products to be distinguished from those issued from the Mid Segment Magma Chamber area. Ferguson Thesis, 2011 More details see D. Pyle PM: Sun & McDonough 1989 DM: Workman 2004 Mantle mineralogy: Winter Partition coefficient: McDade et al.2002

8 37k a Chronology 90 Axial zone - N Baddi Dabbahu S ern flanks Axial zone - S 20ka 35-40ka 72ka 2ka 40ka 3 He cos exposure age, Ka ka Rift floor oldest outcropping pahoehoe lavas 40ka 40 ka Baddi refilling 52 ka 42 ka 40 ka Baddi and Dabbahu flanks aa lavas differentiation trend 37 ka ka 10 New fissure pahoehoe lavas at the rift axis< 2.5 ka Oldest outcopping rift floor emplacement around 72ka Recovering starts ~ 40ka Intense Dabbahu activity at 40ka central and fissure volcanism 8 Dabbahu contributed to the building of rift topography

9 Dabbahu Chronology Periodic replenishment of the Mid-Segment magma chamber? PAxis P off-axis Baddi Baddi Mid Segment Magma Chamber MgO (wt %) 8 6 4??? Common Baddi & Dabbahu differentiation trend?? 2 End of Dabbahu previous magmatic cycle? Age (ka) Off-axis replenishment beneath Baddi around 50 ka Possible connexion at depth between Baddi & Dabbahu 40ka-long magmatic cycle beneath Dabbahu? Not many data in the Southern part: replenishment around 30ka? 9

10 Dating fault activity Bottom of the scarps: last tectonic event 2.6 ka ± 1.7 kyr 3 kyr 2.6 ka ± 1.7 kyr 3 kyr 70 kyr ±1.5 Top of a major scarp: initiation of the tectonic Altitude (m) from top of the scarp Altitude (m) from top of the scarp Base of the scarp - last tectonic event in 2005 Refreshed scarp Main axial fault 70 kyr ±1.5 Mean slipping rate = 1.35 m ka -1 ± 0.25 Base of the scarp - last tectonic event in Exposure Age Flow emplacement Main axial fault Mean slipping rate = 1.35 m ka -1 ± 0.25 Refreshed scarp Flow emplacement Long term displacement rates: Short term displacement: Vh = 15 mm/yr (GPS data ) L vertical L horizontal Vv = 1.35 mm/yr Exposure Age Discrepancy between long term history and single rifting event Site Gab A 10

11 72 40 ka Old rift floor at 72 ka associated with minor scarps Conclusions -Baddi & Dabbahu: off-axis partial melting fraction - Higher partial melting fraction at the axis, consistent with axial melt zone ka Major magmatic episode around 40ka at Dabbahu - Dabbahu: strongly implied in the acquisition of rift topography 20ka - <2 ka Major topographic construction phase started around 30ka -Recent axis flows: - from the Mid-Segment Magma Chamber 11

12 Thank you for your attention 12