Michelle Markley

Oblique plate motion and continental tectonics

Three-dimensional deformation is necessarily associated with oblique plate convergence and commonly results in partitioning of deformation between contractional and transcurrent components along plate margins. Kinematic models of strike-slip partitioning for transpression and transtension allow the exact relation among three critical parameters—plate motion, instantaneous strain axes, and degree of strike-slip partitioning—to be calculated. Application to two end-member tectonic environments characterized by a low (South Island, New Zealand) and high (central California) degree of strike-slip partitioning demonstrates a remarkable consistency among the three parameters, suggesting that strike-slip partitioned transpression is a valid model for deformation in these regions. The extreme degree of strike-slip partitioning in wrench-dominated systems, such as central California, is tentatively associated with a fundamental misorientation of finite and instantaneous strain axes.

Alpine deformation and 40Ar/39Ar geochronology of synkinematic white mica in the Siviez-Mischabel Nappe, western Pennine Alps, Switzerland

We explore the timing of deformation and exhumation of the Siviez-Mischabel Nappe (western Swiss Alps), which has been considered a classic example of a midcrustal crystalline nappe since the studies of Argand [1916]. This study presents 40Ar/39Ar ages obtained on both synkinematic white mica from Permo-Triassic cover sediments and more complex white mica populations from basement gneisses of the Siviez-Mischabel and middle Pennine Nappes. Primary foliation developed in cover units by nucleation, growth, and rigid rotation of mica grains during noncoaxial Alpine deformation. Although some samples show a crenulation of this primary foliation, mica growth appears to have occurred only during the development of primary foliation, the main phase of greenschist facies deformation related to imbrication of the Siviez-Mischabel Nappe and other middle Pennine Nappes. Good agreement exists between independent estimates of the timing of deformation and reported 40Ar/39Ar white mica ages from cover units of the central and southern Siviez-Mischabel Nappe. In cover units from the central and southern Siviez-Mischabel regions of the study area, 40Ar/39Ar ages appear to date synkinematic white mica growth. Results suggest that the Siviez-Mischabel Nappe was emplaced and developed foliation during a 5 m.y. period from 41 to 36 Ma. In cover units from the eastern Siviez-Mischabel, however, 40Ar/39Ar white mica ages appear to date postkinematic thermal events. These thermal events may be related to Oligocene magmatic activity in the lower Pennine Nappes or to Miocene development of the Simplon fault zone. Variations in the relation between Alpine age and grain size for cover samples from the central, eastern, and southern Siviez-Mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamorphism. When considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western Swiss Alps, these data support arguments that the relative timing of events such as thrusting and back thrusting of crystalline nappes in hinterland units and exhumation of high-pressure units in the suture zone of the western Alps are intimately related and synchronous on the scale of a few million years.

Aragonite pseudomorphs in high-pressure marbles of Syros, Greece

Numerous rod-shaped calcite crystals occur in the blueschist to eclogite facies marbles of Syros, Greece. The rods show a shape-preferred orientation, and the long axes of the rods are oriented at a large angle to foliation. The crystals also have a crystallographic-preferred orientation: calcite c-axes are oriented parallel to the long axes of the rods. Based on their chemical composition, shape, and occurrence in high-pressure marbles, these calcite crystals are interpreted as topotactic pseudomorphs after aragonite that developed a crystallographic-preferred orientation during peak metamorphism. This interpretation is consistent with deformation of aragonite by dislocation creep, which has been observed in laboratory experiments but has not been previously reported on the basis of field evidence. Subsequent to the high-pressure deformation of the aragonite marbles, the aragonite recrystallized statically into coarse rod-shaped crystals, maintaining the crystallographic orientation developed during deformation. During later exhumation, aragonite reverted to calcite, and the marbles experienced little further deformation, at least in the pseudomorph-rich layers. Some shearing of pseudomorph-bearing marble layers did occur and is indicated by twinning of calcite and by a variable inclination of the pseudomorphs relative to foliation.

Lithospheric and crustal reactivation of an ancient plate boundary: the assembly and disassembly of the Salmon River suture zone, Idaho, USA

Abstract The Salmon River suture zone, western Idaho, is a fundamental lithospheric boundary between the North American craton and the accreted terranes of the Cordilleran margin. The initial juxtaposition along this north-south-oriented structure occurred during Early Cretaceous time. This zone was potentially reactivated twice by subsequent tectonism, once during Cretaceous time and once during Miocene time. The Late Cretaceous western Idaho shear zone formed along the Salmon River suture zone, as denoted by a sharp gradient in the isotopic signature of the granitoids that intruded the lithospheric boundary zone. The reconstructed Late Cretaceous orientation of the western Idaho shear zone contains subvertical fabrics (lineation, foliation). The same boundary also acted as a locus for subsequent Miocene Basin and Range extensional deformation. Domino-style normal faulting and deep (2100 m) basin formation accommodated the motion between the extending accreted terranes to the west and the unextended Idaho batholith to the east. Whereas either the mantle boundary or a crustal-scale structuring controls the regional extent of the extensionally reactivated zone, locally crustal basement faults and lithological contacts control the orientation and precise location of faults that accommodate reactivation. The multiple reactivation of the Salmon River suture zone is critical for several reasons. The Early Cretaceous suture zone apparently created a fundamental lithospheric flaw, which was reactivated after terrane accretion. Whether this zone was a fracture or a shear zone, the fabric in the mantle lithosphere was apparently not ‘healed’ during orogenesis. Thus, juxtaposition of mantle lithosphere, which is inferred to occur by faulting in the uppermost mantle, acts as a weakness during later tectonism. Second, the paucity of strike-slip plate boundaries in the geological record makes sense in the context of reactivation. The vertical, lithospheric-scale nature of these structures makes them particularly susceptible to lithospheric-scale reactivation during both transcurrent and/or extensional deformation. These reactivations both overprint the earlier deformation and modify the original geometry. Steeply dipping fabrics, rather than vertical fabrics, may be the general signature of major, ancient strike-slip faults.

The role of material anisotropy in the neotectonic extension of the western Idaho shear zone, McCall, Idaho

Basin and Range normal faulting in west-central Idaho reactivates preexisting Cretaceous structures, resulting in a series of normal fault–bound basins. The most intense extensional deformation is concentrated in the Late Cretaceous western Idaho shear zone, resulting in the development of the Long Valley basin. The area affected by the western Idaho shear zone displays two orientations of steep faults: one set of normal faults strikes north-south and is parallel to fabrics within the western Idaho shear zone; the other set strikes east-west and accommodates components of both normal and strike-slip movement. Areas within the Idaho Batholith that do not have strong Cretaceous fabrics (i.e., outside of the western Idaho shear zone) are characterized by a strong preferred north-northeast orientation of faults. From this preferred orientation we infer that the maximum infinitesimal stretch is oriented at 110/290° in this part of the Idaho Batholith. Gravity inversion indicates that the north end of Long Valley is an asymmetric basin about one kilometer deep, with the largest basin-bounding normal fault on the west side of the Long Valley. Paleomagnetic analysis of the Columbia River basalts indicates that the north-south elongate fault blocks within the western Idaho shear zone have not rotated. One block, located just to the west of the western Idaho shear zone, may have rotated counterclockwise. The lack of rotation of north-south oriented fault blocks, in combination with the fault orientations of the Idaho Batholith, indicate that the regional neotectonic deformation within and east of the western Idaho shear zone is characterized by dextral transtension with a divergence vector oriented 130/310°. The extensional reactivation of the western Idaho shear zone demonstrates the effect of material anisotropy at local and regional scales. On a local scale, the mylonitic foliation of the western Idaho shear zone is reactivated as normal faults, even though the regional flow field is oblique to the foliation. On a regional scale, the possible counterclockwise fault block rotation recorded west of the western Idaho shear zone is inconsistent with dextral transtension, suggesting that extensional deformation has reactivated the western edge of the arc-craton boundary as a kinematic domain boundary. We conclude that the preservation of initial features in vertical shear zones and/or plate boundaries is unlikely, due the tendency for well-developed, subvertical fabrics to reactivate, particularly in extension.

Vertical-axis rotation of rigid crustal blocks driven by mantle flow

Abstract Vertical-axis rotation of rigid crustal blocks occurs in a variety of obliquely convergent and divergent plate boundaries. We quantify the rotation of these blocks using models of transpressional and transtensional kinematics, and corroborate our results using physical models where rigid blocks rotate in response to flow of a ductile substrate. Consequently, one can explicitly demonstrate a relationship between the amount of rotation of a rigid crustal block and strain recorded in ductile substrate. This strain should be reflected directly by the orientation of rock fabrics, such as those measured by shear-wave splitting in the in situ upper mantle.s We apply this approach to southern California and New Zealand by using previously documented palaeomagnetic rotations and plate motion vectors, and calculate the strain recorded by the material below rigid blocks. These strain calculations are compared to shear-wave splitting data, which record upper mantle fabric, from the same region. Our model results suggest that similar deformation is recorded by the upper crust and lithospheric mantle. A bottom-driven flow, in which mantle deformation drives upper crustal rotations, is most consistent with these observations.

Geometry of the folded Otago peneplain surface beneath Ida valley, Central Otago, New Zealand, from gravity observations

Abstract We present new Bouguer gravity data and detailed cross‐sections of the geometry of the deformed Otago peneplain surface beneath the Ida valley, a northeast‐southwest elongate basin in Central Otago, New Zealand. Taking the strong regional gravity gradient into consideration, a‐15 mgal Bouguer anomaly results from the density contrast between Cenozoic Ida Basin sediments and Mesozoic Otago Schist basement. Using gravity modelling and geologic constraints, we present a series of northwest‐southeast cross‐sections through the basin. Underneath the southeast side of the Ida Basin, adjacent to the flank of North Rough Ridge, the peneplain surface dips gently. Underneath the northwest side of the Ida Basin, adjacent to the flank of Blackstone Hill, the peneplain surface is strongly deformed by a fault‐propagation fold associated with the tip of the Blackstone Fault. Along this margin, the orientation of the peneplain surface varies dramatically along‐strike. In the north, the peneplain surface is overturned, and the fault‐propagation fold is a tight and gently inclined anticline‐syncline pair. In the south, the peneplain surface is upright, and the fault‐propagation fold is an open and moderately inclined anticline‐syncline pair. Three possible explanations for the along‐strike variation in fold geometry are: (1) the dip of the fault varies dramatically along‐strike, (2) the fault tip at the northeast end of the study area has an unexpectedly shallow depth, and (3) mechanical anisotropy of the basement schists exerts a strong influence on fold shape.

The [Geo]Scientific Method; Hypothesis Testing and Geoscience Proposal Writing for Students

Most undergraduate-level geoscience texts offer a paltry introduction to the nuanced approach to hypothesis testing that geoscientists use when conducting research and writing proposals. Fortunately, there are a handful of excellent papers that are accessible to geoscience undergraduates. Two historical papers by the eminent American geologists G. K. Gilbert and T. C. Chamberlin (Gilbert, 1886; Chamberlin, 1897) were the first to fully articulate and explore the method of multiple working hypotheses. Both papers still make for inspirational reading. A long essay on the scientific method by Johnson (1933) presents both a recipe for rigorous scientific thinking and a traditional but detailed articulation of linear hypothesis testing using geologic examples. More recently, papers by Frodeman (1995) about the fundamentally non-linear nature of interpretation and reasoning in the geosciences and Cleland (2001) about a “smoking gun” approach to validating hypotheses are helpful articulations of the geoscientific method, i.e. a shared understanding of how geoscientists articulate, frame, and tackle research questions.