unstable northern rifts of the galapagos triple junction

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.