60th Annual Report on Research 2015

Purpose of Study.  The geometry of faults and their associated damage zones controls fault-rock properties, permeability, and fluid flow; these factors strongly influence their potential as seals or migration pathways in conventional petroleum reservoirs. They also control the strength of basement faults and their potential for reactivation, which can influence reservoir architecture. Faults accumulate displacement at variable rates spanning several orders of magnitude, encompassing a continuum between slow aseismic creep and standard earthquake slip accommodating high-velocity dynamic rupture.  A significant component of the accumulated displacement and damage is gained from propagation of the latter; however, little is known of the internal geometry of high-velocity dynamic ruptures within fault zones or what geometric conditions favor their development. The goal of this project is to document the internal structure, kinematics, and dynamics of fault-zone segments that hosted high-velocity dynamic ruptures. We focus on a unique record of rupture recorded by pseudotachylyte (solidified frictional melt) preserved in a fault zone in western Greenland.  The ruptures are developed in foliated, high-grade metamorphic rocks that are characteristically layered into 1–10-m-thick bands that emulate the geometry of sedimentary beds.  Since pseudotachylyte is the only clear record of high-velocity rupture that is mappable at the fault-system scale, this field site serves as a unique analog for study of rupture propagation in sedimentary strata. 

Year 1 Accomplishments.  Pseudotachylytes from the Ikertooq thrust zone in western Greenland were first characterized and mapped by Grocott (1981, J. Structural Geol.) as dextral and sinistral strike-slip faults concentrated within three 4–5-km-long brittle fault zones. The work introduced the classic concept of the pseudotachylyte generation zone as a system of paired shears bounding a complex damage zone.  The Ikertooq exposures were subsequently utilized as a model for sidewall ripouts in strike-slip faults (Swanson, 1989, J. Structural Geol.). Our ongoing mapping of these faults on Sarfannguit and outlying islands and skerries in the Ikertooq fjord and Davis Strait has: (1) Extended the known length of the pseudotachylyte system from a few km to more than 13 km, (2) defined a previously unknown fourth fault zone southeast of the main system, and (3) led to a revised kinematic model.

In the 2015 field season, two undergraduates and the PI mapped the southern three of the four fault zones. Our early results demonstrate that the Ikertooq brittle faults are not a strike-slip system.  Instead, offset dikes and slickenlines with steep rakes preserved in pseudotachylyte show that the faults primarily developed as dextral oblique reverse faults with top-to-southeast displacement.  These faults are concordant with straight-banded foliation in Archean and Paleoproterozoic host gneisses (mean strike/dip 240/50 NW).  In some vertical exposures, we observed meter-scale, subhorizontal fault ramps that cut foliation and confirm kinematic relationships. The NW-dipping faults are commonly linked by 1-4-m-long, subvertical, dextral strike-slip relay faults that strike 280 degrees and accommodate a right-stepping fault geometry. Displacement on these faults controls the distribution of thick (4–15 cm) pseudotachylyte fault veins and spectacular 10–80-cm-thick pseudotachylyte breccia zones (Fig. 1).

The pseudotachylyte-bearing faults are dispersed across the width of each of the 100–250-m-wide fault zones with a typical density of 1–5 faults/10 m (Fig. 2). Many faults are localized at the margins of 0.2–3-m-thick Paleoproterozoic mafic dikes that are subparallel with gneissic foliation. On a broader scale, the four fault zones are localized within high-strain zones that strike 240 degrees and are characterized by transposed foliation and mesoscopic isoclinal and sheath folds with moderately west-plunging axes. Gneisses between the high strain zones strike 220–230 degrees with locally shallower dips (40 degrees). We also documented subordinate veins of older mylonitic pseudotachylyte indicating a long history of localization of seismicity in the Ikertooq zone.

The two undergraduates who participated in the Greenland field mapping during summer 2015 will continue to work on the project during their senior year (2015-16).  Our 2015 results will be presented at the Geological Society of America annual meeting in Baltimore, Maryland. 

Figure 1. Hand sample from a thick pseudotachylyte breccia zone formed during a single rupture event; 15 cm protractor for scale.  Detailed geologic mapping in 2015 demonstrated that the breccia zones were systematically distributed across the fault zones. They are commonly found in 2- to 10-m-long segments in top-to-south reverse faults and are terminated at strike-slip fault stepovers that serve to link reverse-fault segments.  Ongoing work will focus on further developing a mechanical model to explain their origin, as they either represent major episodes of frictional melting as a result of large slip, or fluid ponding in an over-pressured fault.

Figure 2. Cross section of one of the three major fault zones mapped in 2015.