LITHOSPHERE TEARING AND THE FORMATION OF A NEW TRANSFORM PLATE BOUNDARY

Open Access
Author:
Neely, James Scott
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 14, 2016
Committee Members:
  • Kevin Patrick Furlong, Thesis Advisor
  • Charles James Ammon, Committee Member
  • Donald Myron Fisher, Committee Member
Keywords:
  • Earthquake
  • Subduction Zone
  • Solomon Islands
  • Lithosphere Tearing
  • STEP Fault
  • Transform Boundary
  • Plate Boundary
Abstract:
The Solomon Islands-Vanuatu composite subduction zone represents a tectonically complex region along the Pacific-Australia plate boundary in the southwest Pacific Ocean. Here the Australia plate subducts under the Pacific plate in two parts - the South Solomon Trench and the Vanuatu Trench – with the two segments separated by a transform fault produced by a tear in the approaching Australia plate. As a result of the Australia plate tearing, the two subducting sections are offset by the 280 km long San Cristobal Trough (SCT) transform fault, which acts as a Subduction-Transform Edge Propagator (STEP) fault. The formation of this transform fault provides an opportunity to study the mechanics of lithosphere tearing and the evolution of a newly created plate boundary. In 2014, at the western terminus (the tearing zone) of the SCT, two earthquakes (MW 7.4 and MW 7.6) occurred with disparate mechanisms (dominantly thrust and strike-slip respectively), which I interpret to indicate the tearing of the Australia plate as its northern section subducts and southern section translates along the SCT. Analyses of these 2014 events indicate lithosphere tearing is accommodated along a complex network of faults. As distance from the tear increases, both the magnitude and frequency of earthquakes along the transform increase reflecting the coalescence of fault segments into a through-going structure. Over the past several decades, there have been several instances of larger magnitude earthquakes migrating westward along the STEP through a rapid succession of events. A recent May 2015 sequence of MW 6.8, MW 6.9, and MW 6.8 earthquakes followed this pattern, with an east to west migration over three days. However, neither this 2015 sequence, nor a previous 1993 progression, ruptured into or nucleated a large earthquake within the region near the tear. SCT sequence termination outside the region of the newly formed fault happens even though Coulomb Failure Stress analyses reveal that the tear end of the SCT is positively loaded for failure by the earthquake sequence. Changing seismicity patterns along the SCT are also mapped by b-value variations that correspond to the rupture patterns of these propagating sequences. These seismicity pattern changes along the SCT reveal a fault maturation process with strain localization driven by cumulative slip.