Using paleomagnetism to test rolling hinge behaviour of an active low angle normal fault, Papua New Guinea

10.26686/wgtn.17136617 ◽

2021 ◽

Author(s):

◽

Emma Watson

Keyword(s):

Papua New Guinea ◽

Geomagnetic Field ◽

Remanent Magnetization ◽

Normal Component ◽

Normal Fault ◽

Blocking Temperature ◽

Metamorphic Core Complexes ◽

Large Rotations ◽

Fault Trace ◽

Metamorphic Core

Metamorphic core complexes (MCC) are widespread in extensional tectonic environments. Despite their significant contribution to extension in rifts, little is known about the origin and evolution of metamorphic core complexes. Particular controversy regards the origin of the typically shallowly dipping (<30°) detachment fault that bounds the footwall core of metamorphic rocks. According to Andersonian faulting theory, normal faults should initiate at a dip of ~60° and frictionally lock up and stop slipping at dips of <30°. One possible solution to this problem is a rolling hinge evolution for the fault. In this scenario the fault initiates at a steep dip of ~60° and evolves to a shallower dip during slip due to the rebound of the footwall in response to progressive unloading as the hangingwall is removed (Wernicke & Axen, 1988; Buck, 1988; Hamilton, 1988). Large rotations of the footwall, indicative of rolling hinge style deformation, may conceivably be measured by comparing the remanent paleomagnetic vector of the footwall rocks with the expected direction of the geomagnetic field at the site where the remanent magnetization was acquired. Using these techniques, large rotations of footwall rocks consistent with rolling hinge style deformation have been demonstrated for the footwalls of oceanic core complexes (Garcés & Gee, 2006; Zhao & Tominaga, 2009; Morris et al., 2009; MacLeod et al., 2011), but not for continental MCCs. In this study we attempt to test, using the remanent magnetization of the footwall rocks, whether rolling hinge style rotations have affected the footwall of the Mai’iu fault, Papua New Guinea. The Mai’iu fault, located in the continental Woodlark Rift, is a rapidly slipping (~1 cm/yr) (Wallace et al., 2014; Webber et al., 2018), shallowly-dipping (<22° at the surface) normal fault (Spencer, 2010; Little et al., 2019) responsible for the Pliocene-Recent exhumation of the domed Suckling-Dayman MCC, which is comprised mostly of Goropu Metabasalt. The remanent magnetization of forty-four samples of footwall Goropu Metabasalt were measured for this study. Close to the fault trace (<1.5 km) a moderately inclined, northerly trending, normal component of magnetic remanence is preserved (Dec: 351.1°, Inc: -35.7°, α₉₅: 6.8°, N= 18 sites). Farther to the south, and up-dip of the fault trace (>1.5 km to 10 km from the fault trace) a normal component is observed in the lower blocking temperature range (Dec: 347.2°, Inc: -41.7°, α₉₅: 9.4°, N= 7 sites) (up to 300-400°C) that we interpret to be equivalent to the normal component present in samples closer to the fault trace. The maximum (un)blocking temperature to which the normal component is carried decreases with increasing distance up-dip and away from the fault trace. In the higher blocking temperature range a southerly trending, reversed component of magnetization is preserved that is more steeply inclined than the component mentioned above (Dec: 177.2°, Inc: 57.1°, α₉₅: 7.3°, N= 8 sites). We interpret the moderately-inclined normal component in both regions to be a recent component of magnetization to have been acquired during the exhumation of the Goropu Metabasalt over the last 780,000 years (Brunhes chron). The origin of the older, reversed component is less clear; however, we prefer the interpretation that this component is also an exhumational overprint that was acquired between 2,600,000-780,000 years ago during the Matuyama chron. Comparison of the direction of the average normal component of both Group 1 and Group 2 samples (Dec: 350.6°, Inc: -37.1°, α₉₅: 5.4°, N= 25 sites) with the expected direction of the geomagnetic field at the paleomagnetic sampling locality indicates that 23.9 ± 2.6° (1σ) of back-rotation about a sub-horizontal axis sub-parallel to fault strike has affected the footwall of the Mai’iu fault. Taking into account the known dip of the fault at the surface of <20-22°, this rotation value implies an original fault dip at depth of 41.3-48.5° that is inherited from a paleo-subduction zone. This result is remarkably consistent with other estimates of the original fault dip: for example, geologically observed fault-bedding cut-off angles on an upper plate imbricate (rider) block imply an original fault dip of ~40-49° (Little et al., 2019). Also, microseismicity between 10-25 km depth implies a modern dip there of 30-40° (Eilon et al., 2015; Abers et al., 2016). This study is the first of its kind to use paleomagnetism to demonstrate that substantial rolling hinge style rotations have affected the footwall of a continental MCC.

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The thermo-tectonic evolution of the Suckling-Dayman metamorphic core complex, southeastern Papua New Guinea

10.26686/wgtn.17136578 ◽

2021 ◽

Author(s):

◽

Jürgen Österle

Keyword(s):

Papua New Guinea ◽

Slip Rate ◽

Carbonaceous Material ◽

Metamorphic Core Complex ◽

Calc Alkaline ◽

Core Complex ◽

Igneous Complex ◽

Fault Trace ◽

Initial Dip ◽

Metamorphic Core

The Suckling-Dayman metamorphic core complex (SDMCC) in the Woodlark Rift of southeastern Papua New Guinea is being exhumed along the Mai’iu Fault, an active low-angle normal fault dipping ~20-22° northwards at the surface. The spectacularly smooth topography of the Mai’iu Fault footwall clearly is expressive of geologically recent uplift. The precise timing and rates of the exhumation of this continental metamorphic core complex (MCC) have, however, never been studied in detail. This thesis provides the first systematic set of U-Pb, fission track (FT), (U-Th[-Sm])/He and ²⁶Al/¹⁰Be ages from metaigneous and metasedimentary rocks of the footwall of the SDMCC, clasts and a tephra deposit contained within syn-tectonic conglomerates (the Gwoira Conglomerate) in a rider block, and modern stream sediments in the footwall and hanging wall of the Mai’iu Fault. The ages are complemented by whole-rock compositional and thermobarometric data (Al-in-amphibole, Al-in-biotite, Raman spectroscopy of carbonaceous material). Based on these data, the timing of the onset of extension along the Mai’iu Fault, its long-term dip-slip rate and its initial dip were constrained. These data are presented in the context of the evolution of the SDMCC from the Cretaceous to the present. The dominant lithology of the SDMCC, the Goropu Metabasalt, formed in a marginal basin to the northeast of the Australian continent. Two zircon U-Pb ages of 103.0 ± 5.7 and 71.6 ± 3.3 Ma, indicative of maximum depositional ages, from metasedimentary intercalations (the Bonenau Schist) in the Goropu Metabasalt, suggest formation of the oceanic protolith in the Late Cretaceous. Between 60.4 ± 2.5 and 56.6 ± 2.3 Ma (zircon U-Pb), tholeiitic to mildly calc-alkaline gabbroic to tonalitic rocks of the Yau Igneous Complex intruded the Goropu Metabasalt. The age of the Yau Igneous Complex overlaps with the known timing of north-directed subduction of the oceanic lithosphere along the Owen Stanley Fault (OSF) beneath the Cape Vogel Arc and provides a minimum age for the oceanic protolith. A second phase of magmatism, consisting of peraluminous-metaluminous calc-alkaline (Suckling Granite) and high-K (Mai’iu Monzonite, Bonua Porphyry) granitoids and basaltic andesite dikes that were cut by the Mai’iu Fault, was associated with the tectonic inversion of the OSF. Zircons from these syn-extensional intrusions suggest crystallization between 3.8 ± 0.2 and 2.0 ± 0.1 Ma. The oldest age of this range is inferred to mark the time by which the OSF had been re-activated as an extensional structure, the Mai’iu Fault. Al-in-amphibole and -biotite thermobarometry suggests crystallization of the Suckling Granite and Mai’iu Monzonite in a relatively shallow crust (~2-8 km depth) at pressures of ~0.4-2.3 kbar. Inherited zircons in the Plio-Pleistocene granitoids indicate that the Goropu Metabasalt carapace of the SDMCC is underlain by Australian-derived Cretaceous crustal material that is inferred to be the continuation of the Kagi Metamorphics in the central Papuan Peninsula. Further constraints of the timing of unroofing of the SDMCC were determined from three quartz clasts in the Gwoira Conglomerate. ²⁶Al/¹⁰Be burial ages of these samples indicate deposition in the Pliocene between 4.6 ± 2.9 and 3.4 ± 2.1 Ma. A tephra in the upper section of the exposed conglomerates was dated employing U-Pb methods on zircon, combined with apatite, zircon and magnetite (U-Th[-Sm])/He chronometers, yielding a complex age spectrum. An eruption age of 0.6 ± 0.4 Ma was extrapolated for this tephra. FT and (U-Th[-Sm])/He low-temperature thermochronometry details a young (≤3 Ma) and rapid exhumation history. Based on the crystallization ages of the syn-extensional granitoids, the depositional age of the Gwoira Conglomerate, the extensional cooling recorded by low-temperature thermochronometry, and the backwards projection of the published Holocene dip-slip rate of the Mai’iu Fault, the timing of the onset of extension is estimated at ~4 Ma. A minimum dip-slip rate of 8.1 ± 1.3 km/myr has been calculated from the inverse slope of zircon (U-Th)/He (ZHe) ages with slip-parallel distance from Mai’iu Fault trace. This is slightly lower than the >12 km/myr required to restore the intrusion depths (2-8 km) of the syn-extensional granitoids, now exposed 20-25 km south of the Mai’iu Fault trace at elevations up to 3.4 km. Collectively, these constraints suggest that the Mai’iu Fault has moved at cm-per-year rates since ~3 Ma. Evidence for both a fossil zircon FT (ZFT) partial annealing zone (PAZ) and a ZHe partial retention zone (PRZ) on the footwall of the SDMCC is presented. Combining paleo-temperature estimates from the inferred bases of the zircon PAZ and PRZ, peak-metamorphic temperatures inferred from Raman spectroscopy of carbonaceous material (RSCM), and published peak-metamorphic temperature constraints on the extensional shear zone mylonites near the Mai’iu Fault trace, a minimum slip-parallel, down-dip paleo-temperature gradient of 9.7 ± 2.2°C/km has been estimated for the exhumed Mai’iu Fault plane. Assuming that the modern regional geothermal gradient in the Woodlark Rift is a maximum estimate of that which existed prior to extensional exhumation of the SDMCC, the paleo-temperature gradient was used to estimate an average initial dip of the Mai’iu Fault of ~44° for pre-extensional geothermal gradients ranging between 10 to 20°C/km. Presently dipping 20-22° at the surface, the constraints on the initial dip suggest that the Mai’iu Fault may have been back-rotated by >20° since the onset of extension, consistent with a rolling hinge-style evolution of this continental MCC.

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The thermo-tectonic evolution of the Suckling-Dayman metamorphic core complex, southeastern Papua New Guinea

10.26686/wgtn.17136578.v1 ◽

2021 ◽

Author(s):

◽

Jürgen Österle

Keyword(s):

Papua New Guinea ◽

Slip Rate ◽

Carbonaceous Material ◽

Metamorphic Core Complex ◽

Calc Alkaline ◽

Core Complex ◽

Igneous Complex ◽

Fault Trace ◽

Initial Dip ◽

Metamorphic Core

The Suckling-Dayman metamorphic core complex (SDMCC) in the Woodlark Rift of southeastern Papua New Guinea is being exhumed along the Mai’iu Fault, an active low-angle normal fault dipping ~20-22° northwards at the surface. The spectacularly smooth topography of the Mai’iu Fault footwall clearly is expressive of geologically recent uplift. The precise timing and rates of the exhumation of this continental metamorphic core complex (MCC) have, however, never been studied in detail. This thesis provides the first systematic set of U-Pb, fission track (FT), (U-Th[-Sm])/He and ²⁶Al/¹⁰Be ages from metaigneous and metasedimentary rocks of the footwall of the SDMCC, clasts and a tephra deposit contained within syn-tectonic conglomerates (the Gwoira Conglomerate) in a rider block, and modern stream sediments in the footwall and hanging wall of the Mai’iu Fault. The ages are complemented by whole-rock compositional and thermobarometric data (Al-in-amphibole, Al-in-biotite, Raman spectroscopy of carbonaceous material). Based on these data, the timing of the onset of extension along the Mai’iu Fault, its long-term dip-slip rate and its initial dip were constrained. These data are presented in the context of the evolution of the SDMCC from the Cretaceous to the present. The dominant lithology of the SDMCC, the Goropu Metabasalt, formed in a marginal basin to the northeast of the Australian continent. Two zircon U-Pb ages of 103.0 ± 5.7 and 71.6 ± 3.3 Ma, indicative of maximum depositional ages, from metasedimentary intercalations (the Bonenau Schist) in the Goropu Metabasalt, suggest formation of the oceanic protolith in the Late Cretaceous. Between 60.4 ± 2.5 and 56.6 ± 2.3 Ma (zircon U-Pb), tholeiitic to mildly calc-alkaline gabbroic to tonalitic rocks of the Yau Igneous Complex intruded the Goropu Metabasalt. The age of the Yau Igneous Complex overlaps with the known timing of north-directed subduction of the oceanic lithosphere along the Owen Stanley Fault (OSF) beneath the Cape Vogel Arc and provides a minimum age for the oceanic protolith. A second phase of magmatism, consisting of peraluminous-metaluminous calc-alkaline (Suckling Granite) and high-K (Mai’iu Monzonite, Bonua Porphyry) granitoids and basaltic andesite dikes that were cut by the Mai’iu Fault, was associated with the tectonic inversion of the OSF. Zircons from these syn-extensional intrusions suggest crystallization between 3.8 ± 0.2 and 2.0 ± 0.1 Ma. The oldest age of this range is inferred to mark the time by which the OSF had been re-activated as an extensional structure, the Mai’iu Fault. Al-in-amphibole and -biotite thermobarometry suggests crystallization of the Suckling Granite and Mai’iu Monzonite in a relatively shallow crust (~2-8 km depth) at pressures of ~0.4-2.3 kbar. Inherited zircons in the Plio-Pleistocene granitoids indicate that the Goropu Metabasalt carapace of the SDMCC is underlain by Australian-derived Cretaceous crustal material that is inferred to be the continuation of the Kagi Metamorphics in the central Papuan Peninsula. Further constraints of the timing of unroofing of the SDMCC were determined from three quartz clasts in the Gwoira Conglomerate. ²⁶Al/¹⁰Be burial ages of these samples indicate deposition in the Pliocene between 4.6 ± 2.9 and 3.4 ± 2.1 Ma. A tephra in the upper section of the exposed conglomerates was dated employing U-Pb methods on zircon, combined with apatite, zircon and magnetite (U-Th[-Sm])/He chronometers, yielding a complex age spectrum. An eruption age of 0.6 ± 0.4 Ma was extrapolated for this tephra. FT and (U-Th[-Sm])/He low-temperature thermochronometry details a young (≤3 Ma) and rapid exhumation history. Based on the crystallization ages of the syn-extensional granitoids, the depositional age of the Gwoira Conglomerate, the extensional cooling recorded by low-temperature thermochronometry, and the backwards projection of the published Holocene dip-slip rate of the Mai’iu Fault, the timing of the onset of extension is estimated at ~4 Ma. A minimum dip-slip rate of 8.1 ± 1.3 km/myr has been calculated from the inverse slope of zircon (U-Th)/He (ZHe) ages with slip-parallel distance from Mai’iu Fault trace. This is slightly lower than the >12 km/myr required to restore the intrusion depths (2-8 km) of the syn-extensional granitoids, now exposed 20-25 km south of the Mai’iu Fault trace at elevations up to 3.4 km. Collectively, these constraints suggest that the Mai’iu Fault has moved at cm-per-year rates since ~3 Ma. Evidence for both a fossil zircon FT (ZFT) partial annealing zone (PAZ) and a ZHe partial retention zone (PRZ) on the footwall of the SDMCC is presented. Combining paleo-temperature estimates from the inferred bases of the zircon PAZ and PRZ, peak-metamorphic temperatures inferred from Raman spectroscopy of carbonaceous material (RSCM), and published peak-metamorphic temperature constraints on the extensional shear zone mylonites near the Mai’iu Fault trace, a minimum slip-parallel, down-dip paleo-temperature gradient of 9.7 ± 2.2°C/km has been estimated for the exhumed Mai’iu Fault plane. Assuming that the modern regional geothermal gradient in the Woodlark Rift is a maximum estimate of that which existed prior to extensional exhumation of the SDMCC, the paleo-temperature gradient was used to estimate an average initial dip of the Mai’iu Fault of ~44° for pre-extensional geothermal gradients ranging between 10 to 20°C/km. Presently dipping 20-22° at the surface, the constraints on the initial dip suggest that the Mai’iu Fault may have been back-rotated by >20° since the onset of extension, consistent with a rolling hinge-style evolution of this continental MCC.

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