unstable northern rifts of the galapagos triple junction
TRANSCRIPT
Unstable Northern Rifts of the Galapagos Triple
Junction
How Fracture Mechanics Helps us to Reconstruct Seafloor Morphology
Project Proponents• Hans Schouten, WHOI• Deborah Smith, WHOI• Laurent Montesi,
University of Maryland• Emily Klein, Duke
University (“The Duchess”)
• US National Science Foundation
A Glance at Seafloor
Galapagos Triple Junction
• Ridge-Ridge-Ridge (RRR) triple junction• Cocos, Nazca, and Pacific Plates separated by
the Galapagos microplate
Evolution of the Galapagos TJ• Succession of Rifts
– the East Pacifc Rise (EPR)
– Incipient Rift (2˚40’N) – Cocos-Nazca Rift– Dietz Deep Rift
(Galapagos-Nazca Rift, 1˚10’N)
• Dietz Deep Rift ~1.5 Ma, propagated northeast approaching Cocos-Nazca Rift near 101˚W (~1 Ma), the Galapagos microplate formed
• Role of Incipient Rift in theGalapagos TJ kinematics
Incipient
Dietz Deep
Cocos-NazcaEPR
Cruise to Incipient Rift (2º40’N)
• Chief Scientists: Emily Klein (Duke U.) & Debbie Smith (WHOI), 8/5-9/2/2002
Incipient Rift Morphology• Multi-beam Sonar (seafloor bathymetry)• Dreges and wax cores (geochemistry)• Magnetometer (near surface magnetic)• Underwater Camera Tow (detailed morphology)• MAPR (hydrothermal venting plume particles)
Multibeam Bathymetry Data• multiple volcanic
ridges changing to graben along their strike
• Western-end volcanism blankets and abuts the N-S abyssal hills
• Graben widths vary
• Volcanism: intercepts EPR, triple junction
• Graben width: duration of triple junction volcanism
• >20 Rifts from IR to ER and farther east• IR: the latest• ER: the largest?• Succession of Triple Junctions moving south-and westward in past ~4-5 Ma
Sequence of Triple Junctions?
Propagation of Mode I Crack
• Rectangular Cocos & Nazca Plates, cut perpendicular to and stop at D from EPR (represent Cocos-Nazca Rift)• EPR stress free, north/south of cut at spreading rate• Off-axis tensile stress maxima develop at 1.4D
Graben Bounded by Normal Faults• Mastin and Pollard, 1988; Pollard and Holzhausen, 1979
(Figure 1.8, Pollard and Fletcher, 2006)
Where are the Southern Rifts?
N S
Coco
s-Na
zca
Spre
adin
g ce
nter
Diet
z De
ep R
ift
Inci
pien
t Rift
Extin
ct R
ift
East Pacific Rise (EPR)
• Mirror-image of rifts south of Cocos-Zazca Spreading Center; • Extinct Rift and the incipient proto Dietz Deep Rift, 1.4D;• Similar jumping TJ south of the Cocos-Nazca rift prior to ~1.5 Ma when the last active boundary became Dietz Deep Rift;• Transition to rotating microplate result of seamount formation
Incipient Rift Trajectory
Evolution of Galapagos TJ
Conclusion• Simple fracture mechanics model predicts a zone of
reduced tension ahead of a propagating mode I crack and tensile stress maxima develop along the free surface at ±1.4D from the rift tip;
• Based on the results of our model, we interpret that the NW-SE rifts and graben north of the Cocos-Nazca Rift each initiates as a crack at the EPR;
• A steady distance D from Cocos-Nazca Rift tip to the EPR results in a stable triple junction at 1.4D along the EPR;
• Through time, the triple junction migrates north relative to the Cocos-Nazca Rift tip; Eventually the location of the triple junction will be unfavorable for continued opening and a new crack will initiate to the south (at 1.4D) forming a new triple junction;
• The sizes of the rifts may reflect the change in distance from Cocos-Nazca Rift tip to the EPR.