Regional exhumation history of brittle crust during subduction initiation, Fiordland, southwest New Zealand, and implications for thermochronologic sampling and analysis strategies

Abstract

We analyze spatial trends and statistical properties of 410 apatite and zircon fi ssion track and (U-Th)/He ages, and implement a weighted least-squares regression scheme to obtain the regional rock uplift history associated with subduction initiation beneath Fiordland, New Zealand. We observe the onset of rapid exhumation at 25-15 Ma in southwest Fiordland, immediately following a time of signifi cant change in regional plate motions. During the period 15-5 Ma, the locus of rapid exhumation broadened and migrated toward the northeast at ~30% of the plate motion rate, but exhumation remained localized along the northwest margin. Since 5 Ma, the zone of rapid exhumation has become broader, and the present highamplitude gravity and topographic anomalies are spatially associated with the most tightly folded part of the subducted slab. We suggest that the pattern of exhumation tracks the along-strike and downdip development of the subducted slab, which requires tectonic erosion of mantle lithosphere of the overriding plate. Based upon local patterns of age variability, we hypothesize that brittle faults have displaced and rotated equal-cooling-age surfaces, and that there is short-wavelength (<10 km) spatial correlation between faults and topographic features. Our regression method allows us to simultaneously consider implications of all age data, evaluate "geological noise" introduced by brittle faults, and make cooling age predictions at any point in the region. The residuals from our regression indicate that, on average, mountain tops in Fiordland have undergone slightly greater rock uplift than adjacent valleys, even though our data are too sparse to identify specifi c faults. We suggest that sampling programs in active tectonic settings such as Fiordland must be suffi ciently dense to determine both mean exhumation history and regional geological variability associated with faults.

Dynamics of subduction initiation with different evolutionary pathways

“…For example, the Puysegur subduction zone, i.e., the northern segment of the Macquarie Ridge Complex between the Australian and Pacific plates, has undergone convergence since $15 Ma. Although the downdip tip of the slab has reached 150 km depth [Sutherland et al, 2009], only small isolated sea mounts are known on the Pacific plate, most notably Solander Island. In addition, no backarc spreading is observed on the overriding Pacific plate either where the Australian plate subducts beneath oceanic crust along the Puysegur Ridge or below continental crust in the Fiordland region of New Zealand's South Island.…”

Quantifying bedrock-fracture patterns within the shallow subsurface: Implications for rock mass strength, bedrock landslides, and erodibility

“…In the Southern Alps, CLARKE AND BURBANK: QUANTIFYING BEDROCK-FRACTURE PATTERNS F04009 F04009 rates attain 7-11 mm/yr on the rain-soaked West Coast and decrease sharply across the divide to the east Tippett and Kamp, 1993;Herman and Braun, 2006;Herman et al, 2007Herman et al, , 2009. Although less well-constrained, rock-uplift rates in Fiordland range between ∼0.2 and ∼1 mm/yr [House et al, 2002[House et al, , 2005Sutherland et al, 2009]: ∼5 to 20 times slower than those of the Southern Alps. Despite these contrasts in uplift rates and rock type, both ranges are characterized by similar modal slopes of ∼32° [Clarke and Burbank, 2010].…”

A Faulted Thin‐Sheet Model of Plate Boundary Deformation That Fits Observations

“…The South Island plate boundary has east dipping subduction of Australian Plate to its southwest at the Puysegur Trench and west dipping subduction of Pacific Plate to its northeast at the Hikurangi Trough (Figure 2; Sutherland et al, 2009;Wallace et al, 2009). The Alpine Fault has formed over the last 30 Ma by progressive northeastward displacement of an Eocene passive margin into the oblique continental collision zone, and the Alpine Fault now accommodates c. 70% of plate motion (Richard J.…”