Postseismic Relaxation Following the 2019 Ridgecrest, California, Earthquake Sequence

ABSTRACT The 2019 Ridgecrest, California, earthquake sequence involved predominantly right-lateral strike slip on a northwest–southeast-trending subvertical fault in the 6 July M 7.1 mainshock, preceded by left-lateral strike slip on a northeast–southwest-trending subvertical fault in the 4 July M 6.4 foreshock. To characterize the postseismic deformation, we assemble displacements measured by Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar. The geodetic measurements illuminate vigorous postseismic deformation for at least 21 months following the earthquake sequence. The postseismic transient deformation is particularly well constrained from survey-mode GPS (sGPS) in the epicentral region carried out during the weeks after the mainshock. We interpret these observations with mechanical models including afterslip and viscoelastic relaxation of the lower crust and mantle asthenosphere. During the first 21 months, up to several centimeters of horizontal motions are measured at continuous GPS and sGPS sites, with amplitude that diminishes slowly with distance from the mainshock rupture, suggestive of deeper afterslip or viscoelastic relaxation. We find that although afterslip involving right-lateral strike slip along the mainshock fault traces and their deeper extensions reach a few decimeters, most postseismic deformation is attributable to viscoelastic relaxation of the lower crust and mantle. Within the Basin and Range crust and mantle, we infer a transient lower crust viscosity several times that of the mantle asthenosphere. The transient mantle asthenosphere viscosity is ∼1.3×1017 Pa s, and the adjacent Central Valley transient mantle asthenosphere viscosity is ∼7×1017 Pa s, about five times higher and consistent with an asymmetry in postseismic horizontal motions across the mainshock surface rupture.

Weak, Seismogenic Faults Inherited From Mesozoic Rifts Control Mountain Building in the Andean Foreland

New earthquake focal mechanism and centroid depth estimates show that the deformation style in the forelands of the Andes is spatially correlated with rift systems that stretched the South American lithosphere in the Mesozoic. Where the rifts trend sub-parallel to the Andean range front, normal faults inherited from the rifts are being reactivated as reverse faults, causing the 30--45 km thick seismogenic layer to break up. Where the rift systems are absent from beneath the range front, the seismogenic layer is bending and being thrust beneath the Andes like a rigid plate. Force-balance calculations show that the faults inerhited from former rift zones have an effective coefficient of static friction < 0.2. In order for these frictionally-weak faults to remain seismogenic in the lower crust, their wall rocks are likely to be formed of dry granulite. Xenolith data support this view, and suggest that parts of the lower crust are now mostly metastable, having experienced temperatures at least 75--250 degrees hotter than present. The conditions in the lower crust make it unlikely that highly-pressurised free water, or networks of intrinsically-weak phyllosilicate minerals, are the cause of their low effective friction, as, at such high temperatures, both mechanisms would cause the faults to deform through viscous creep and not frictional slip. Therefore pre-existing faults in the Andean forelands have remained weak and seismogenic after reactivation, and have influenced the style of mountain building in South America. However, the controls on their mechanical properties in the lower crust remain unclear.

Crystal Specific Constraints on Subvolcanic Processes Preceding Eruptions at Mt Taranaki, New Zealand

<p>Andesitic magmas are the product of a complex interplay of processes including fractional crystallisation, crystal accumulation, magma mixing and crustal assimilation. Recent studies have suggested that andesitic rocks are in many cases a complex mixture of a crystal cargo and melts with more silicic compositions than andesite. In situ glass- and mineral-specific geochemical techniques are therefore key to unravelling the processes and timescales over which andesitic magmas are produced, assembled and transported to the surface. To this end, this thesis presents a detailed in situ glass- and mineral-specific study of six Holocene eruptions (Kaupokonui, Maketawa, Inglewood a and b, and Korito) at Mt Taranaki to investigate the petrogenetic processes responsible for producing these sub-plinian eruptions at this long-lived (130 000 yr) andesitic volcano. Mt Taranaki is an andesitic stratovolcano located on the west coast of New Zealand’s North Island and as such it is distinct from the main subduction related volcanism. Crystal-specific major and trace element data were combined with textural analysis and quantitative modelling of intensive magmatic parameters and crystal residence times to identify distinct mineral populations and constrain the magmatic histories of the crystal populations. Least-squares mixing modelling of glass and phenocryst compositions demonstrates that the andesitic compositions of bulk rock Mt Taranaki eruptives results from mixing of a daciticrhyolitic melt and a complex crystal cargo (plagioclase, pyroxene, amphibole) that crystallised from multiple melts under a wide range of crustal conditions. Magma mixing of compositionally similar end members that mix efficiently also occurred beneath Mt Taranaki, and as such only produced prominent disequilibrium textures in a small proportion of the minerals in the crystal cargo. The chemistry of the earliest crystallising amphibole indicates crystallisation from an andesitic-dacitic melt at depths of ca. 20-25 km, within the lower crust. Magmas then ascended through the crust relatively slowly via a complex magmatic plumbing system. However, most of the crystal cargo formed by decompression-driven crystallisation at depth so 6-10 km, as is indicated by the dominance of oscillatory zoning and the equilibrium obtained between mineral rims and the host glasses. Taranaki magmas recharge on timescales of 1000-2000 yrs. The eruptions investigated here provide a snapshot of the end of one cycle and the beginning of another. The younger Kaupokonui and Maketawa eruptions (ca. 2890 - <1950 yr BP) are the least evolved magmas, record a stronger mixing signal in the crystal cargo, and are volumetrically smaller than the earlier Inglewood a and b and Korito eruptions (ca. 4150-3580 yr BP). The Kaupokonui and Maketawa eruptions may reflect arrival of a new pulse of magma from the lower crust, or that these are early eruptions within a recharge sequence, which have not had as much time to further differentiate and evolve as the earlier Inglewood a and b and Korito eruptions that represent the end of a magma recharge cycle. One of the six investigated eruptions was identified to come from Fantham’s Peak on the basis of its distinctive glass and mineral chemistry and petrology. Glass trace element data indicate that this eurption’s magmatic system was distinct from that of the other main vent Holocene eruptions investigated in this study. Crystal residence times were investigated using Fe-Mg interdiffusion in clinopyroxene and indicate that magma bodies stall in upper crustal storage chambers for timescales of a few months to years. The younger eruptions of the least evolved magmas with the strongest mixing signal return the shortest residence times, which may indicate that magma mixing events occurring a few months before eruption may have been the trigger for these eruptions at Mt Taranaki. Amphibole geospeedometry for these eruptives reveal rapid magma transport from depths of 6-10 km to the surface on timescales of < 1 week.</p>

Crystal Specific Constraints on Subvolcanic Processes Preceding Eruptions at Mt Taranaki, New Zealand

<p>Andesitic magmas are the product of a complex interplay of processes including fractional crystallisation, crystal accumulation, magma mixing and crustal assimilation. Recent studies have suggested that andesitic rocks are in many cases a complex mixture of a crystal cargo and melts with more silicic compositions than andesite. In situ glass- and mineral-specific geochemical techniques are therefore key to unravelling the processes and timescales over which andesitic magmas are produced, assembled and transported to the surface. To this end, this thesis presents a detailed in situ glass- and mineral-specific study of six Holocene eruptions (Kaupokonui, Maketawa, Inglewood a and b, and Korito) at Mt Taranaki to investigate the petrogenetic processes responsible for producing these sub-plinian eruptions at this long-lived (130 000 yr) andesitic volcano. Mt Taranaki is an andesitic stratovolcano located on the west coast of New Zealand’s North Island and as such it is distinct from the main subduction related volcanism. Crystal-specific major and trace element data were combined with textural analysis and quantitative modelling of intensive magmatic parameters and crystal residence times to identify distinct mineral populations and constrain the magmatic histories of the crystal populations. Least-squares mixing modelling of glass and phenocryst compositions demonstrates that the andesitic compositions of bulk rock Mt Taranaki eruptives results from mixing of a daciticrhyolitic melt and a complex crystal cargo (plagioclase, pyroxene, amphibole) that crystallised from multiple melts under a wide range of crustal conditions. Magma mixing of compositionally similar end members that mix efficiently also occurred beneath Mt Taranaki, and as such only produced prominent disequilibrium textures in a small proportion of the minerals in the crystal cargo. The chemistry of the earliest crystallising amphibole indicates crystallisation from an andesitic-dacitic melt at depths of ca. 20-25 km, within the lower crust. Magmas then ascended through the crust relatively slowly via a complex magmatic plumbing system. However, most of the crystal cargo formed by decompression-driven crystallisation at depth so 6-10 km, as is indicated by the dominance of oscillatory zoning and the equilibrium obtained between mineral rims and the host glasses. Taranaki magmas recharge on timescales of 1000-2000 yrs. The eruptions investigated here provide a snapshot of the end of one cycle and the beginning of another. The younger Kaupokonui and Maketawa eruptions (ca. 2890 - <1950 yr BP) are the least evolved magmas, record a stronger mixing signal in the crystal cargo, and are volumetrically smaller than the earlier Inglewood a and b and Korito eruptions (ca. 4150-3580 yr BP). The Kaupokonui and Maketawa eruptions may reflect arrival of a new pulse of magma from the lower crust, or that these are early eruptions within a recharge sequence, which have not had as much time to further differentiate and evolve as the earlier Inglewood a and b and Korito eruptions that represent the end of a magma recharge cycle. One of the six investigated eruptions was identified to come from Fantham’s Peak on the basis of its distinctive glass and mineral chemistry and petrology. Glass trace element data indicate that this eurption’s magmatic system was distinct from that of the other main vent Holocene eruptions investigated in this study. Crystal residence times were investigated using Fe-Mg interdiffusion in clinopyroxene and indicate that magma bodies stall in upper crustal storage chambers for timescales of a few months to years. The younger eruptions of the least evolved magmas with the strongest mixing signal return the shortest residence times, which may indicate that magma mixing events occurring a few months before eruption may have been the trigger for these eruptions at Mt Taranaki. Amphibole geospeedometry for these eruptives reveal rapid magma transport from depths of 6-10 km to the surface on timescales of < 1 week.</p>

Long-lived Paleoproterozoic eclogitic lower crust

The nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences. Transformation of lower crustal rocks into eclogite facies is usually expected to result in lower crustal delamination. Here we provide compelling evidence for long-lasting presence of lower crustal eclogite below the seismic Moho. Our new wide-angle seismic data from the Paleoproterozoic Fennoscandian Shield identify a 6–8 km thick body with extremely high velocity (Vp ~ 8.5–8.6 km/s) and high density (>3.4 g/cm3) immediately beneath equally thinned high-velocity (Vp ~ 7.3–7.4 km/s) lowermost crust, which extends over >350 km distance. We relate this observed structure to partial (50–70%) transformation of part of the mafic lowermost crustal layer into eclogite facies during Paleoproterozoic orogeny without later delamination. Our findings challenge conventional models for the role of lower crustal eclogitization and delamination in lithosphere evolution and for the long-term stability of cratonic crust.

Lithospheric Shortening and Ductile Deformation in a Back-Arc Setting: South Wanganui Basin, New Zealand

<p>South Wanganui Basin (SWB), New Zealand, is located behind the southern end of the Hikurangi subduction system. One of the most marked geophysical characteristics of the basin is the -150 mGal Bouguer/isostatic gravity anomaly. Sediment fill can only partly explain this anomaly. 3-D gravity models show that the gravity anomaly associated with the basin is generally consistent with a downwarp model of the entire crust. However, the downwarp of the Moho has to be 3-4 times larger than the downwarp of the sediment-basement interface to fit the observed gravity anomaly. Hence a model of lithospheric shortening where ductile thickening of the crust increases with depth is proposed. Finite element modelling demonstrates that the crust, in order to produce the ductile downwarp, is best modelled with at least two distinct different layers. The model requires the top 15-20 km of the crust to behave purely elastic and the lower part (10 km thick) to be viscoelastic with a viscosity of 10[to the power of 21 pascal-seconds]. The existence of this ductile lower continental crust can be explained due to fluids released from the subducting slab accumulating in the lower crust. This is supported by receiver function analysis results. These results propose a 10+/-2 km thick low S-wave velocity layer in the lower crust. The vertical loading necessary to create the basin is high (up to 200MPa) and is difficult to explain by slab pull forces transmitted via a strongly coupled subduction interface alone. An additional driving mechanism proposed is a thickened mantle lithosphere inducing normal forces on the base of the crust. However, the exact origin of the basin remains a puzzling aspect. Receiver function analysis shows that the crust of the subducting Pacific plate underneath the mainland in the lower North Island is abnormally thick ([approximates]10 km) for oceanic crust. This matches with results from the 3-D gravity modelling. Further features discovered with the receiver function analysis are an up to 6 km thick low-velocity layer on top of the slab, which is interpreted as a zone of crushed crustal material with subducted sediments. Furthermore, a deep Moho (39.5+/-1.5 km) is proposed underneath the northern tip of theMarlborough sounds. Shallow seismic and gravity investigations of the southeastern corner of the SWB reveal a complex faulting regime with high-angle normal and reverse faults as well as a component of strike slip. The overall style of faulting in the SWB changes from the west to the east. There are the low-angle thrust faults of the Taranaki Fault zone in the west, the high-angle mostly reverse faults in the eastern part of the basin and the strike slip faults, with a component of vertical movement, at the eastern boundary within the Tararua Ranges.</p>

Lithospheric Shortening and Ductile Deformation in a Back-Arc Setting: South Wanganui Basin, New Zealand

<p>South Wanganui Basin (SWB), New Zealand, is located behind the southern end of the Hikurangi subduction system. One of the most marked geophysical characteristics of the basin is the -150 mGal Bouguer/isostatic gravity anomaly. Sediment fill can only partly explain this anomaly. 3-D gravity models show that the gravity anomaly associated with the basin is generally consistent with a downwarp model of the entire crust. However, the downwarp of the Moho has to be 3-4 times larger than the downwarp of the sediment-basement interface to fit the observed gravity anomaly. Hence a model of lithospheric shortening where ductile thickening of the crust increases with depth is proposed. Finite element modelling demonstrates that the crust, in order to produce the ductile downwarp, is best modelled with at least two distinct different layers. The model requires the top 15-20 km of the crust to behave purely elastic and the lower part (10 km thick) to be viscoelastic with a viscosity of 10[to the power of 21 pascal-seconds]. The existence of this ductile lower continental crust can be explained due to fluids released from the subducting slab accumulating in the lower crust. This is supported by receiver function analysis results. These results propose a 10+/-2 km thick low S-wave velocity layer in the lower crust. The vertical loading necessary to create the basin is high (up to 200MPa) and is difficult to explain by slab pull forces transmitted via a strongly coupled subduction interface alone. An additional driving mechanism proposed is a thickened mantle lithosphere inducing normal forces on the base of the crust. However, the exact origin of the basin remains a puzzling aspect. Receiver function analysis shows that the crust of the subducting Pacific plate underneath the mainland in the lower North Island is abnormally thick ([approximates]10 km) for oceanic crust. This matches with results from the 3-D gravity modelling. Further features discovered with the receiver function analysis are an up to 6 km thick low-velocity layer on top of the slab, which is interpreted as a zone of crushed crustal material with subducted sediments. Furthermore, a deep Moho (39.5+/-1.5 km) is proposed underneath the northern tip of theMarlborough sounds. Shallow seismic and gravity investigations of the southeastern corner of the SWB reveal a complex faulting regime with high-angle normal and reverse faults as well as a component of strike slip. The overall style of faulting in the SWB changes from the west to the east. There are the low-angle thrust faults of the Taranaki Fault zone in the west, the high-angle mostly reverse faults in the eastern part of the basin and the strike slip faults, with a component of vertical movement, at the eastern boundary within the Tararua Ranges.</p>