Geophysical Investigations of the Lachlan Fold Belt
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Generally the surface geology of the Lachlan tells a fairly complete story of the evolution of this fold and thrust belt. Geochemical data provides some hard evidence for possible subsurface basement types. The geophysical data covering the Lachlan Fold Belt, while limited, helps fill in our understanding of the near and subsurface. Several important features concerning this orogeny have been discerned from geophysical data only: 1) an overall crustal thickness of only 35-40 km, thickening to the east under the island-arc terranes, 2) velocity inversions perhaps due to lower crustal melting and fractionation, 3) the extension of the fold belt under the Sydney and Murray basins and 4) the lack of exotic terranes explained through paleomagnetic experiments.
Refraction studies shot across the Lachlan Fold Belt shed light on crustal thicknesses but do little to solve the question of what basement type is (see geochemical data for this answer). These studies showed that crustal thicknesses, as inferred by the sharp increase from ~7.6 to >8 km/s, across the Lachlan Fold Belt, and in adjacent sedimentary basins that the fold belt is inferred to underlie, vary from 35 km in the far western Murray basin and gradually thicken to 40-45 km in the western and central Lachlan, and in the far eastern Lachlan thicken to slightly more than 50 km. This corresponds nicely with bougeur gravity data that shows similar results. Geologically the thickest crust in the east corresponds to the area of the Lachlan made up of inferred island arc complexes, which still has the highest overall elevation in the Lachlan. Lower crustal velocities, 6.5-7.5 km/s on average, don't lend enough specific evidence to support or detract from the argument that the Lachlan is underlain by oceanic crust. It seems though that these values are on the low side for oceanic crust, which can be reconciled through either delamination of the oceanic basement (Glen 1992) or through bulk remelting and fractionation (Finlayson 1979).
Another interesting feature of the refraction data is the velocity inversions present at mid-crustal depths of 16 to 35 km depending on the locale. These inversions are present underneath the entire fold and thrust belt. They are thought to represent a compositional anomaly in the mid-crust that may be due to the Paleozoic granitic partial melts formed throughout the area (Clemens and Wall 1979) which lowered the crustal density such that seismic velocities decreased. These low velocity zones do correspond to the depth in the crust that the granitic melts in the Lachlan would need to be formed at, but barring further evidence to support this hypothesis I would say that interpretation for the velocity inversions is speculative at best.
Gravity and magnetic surveys over the Lachlan Fold Belt are limited but do help illuminate the subsurface structures. Generally the geophysical data supports the surficial geological definition of the three zones (eastern, central and western Lachlan), with each zone exhibiting unique gravity and magnetic signatures that highlight both near surface and deeper crustal features.
Magnetic data over the Lachlan Fold Belt highlights the near surface features, specifically the igneous intrusions. The eastern Lachlan is characterized by high amplitude (200-500nT) anomalies. These correspond to scattered granitoid plutons and more importantly the great abundance of andesites and basalts that are present from the accreted island arc terranes (Wyatt et al. 1980). These magnetic anomalies tend to be narrow and elliptical and trend in a north-northwest, parallel to the structural grain of the belt in the eastern province. This contrasts to the magnetic signature of the central Lachlan, which is characterized by "isolated, low amplitude (50-100nT) bull's-eye (Wyatt et al. 1980)" patterns and a general regional anomaly trending north-northwest. The bull's-eye pattern correspond directly to granite plutons throughout this region, the general north trending anomalies correspond to the Wagga/Omeo metamorphic zone and the high temperature magnetic resetting of the metamorphosed rocks. Magnetic trends in the the western Lachlan Fold Belt are low amplitude and less disturbed than in the other Lachlan subprovinces. Generally these correspond with "an inferred, relatively uniform thickness (Wyatt et al. 1980)" of folded and thrusted sedimentary rocks and the relative scarcity of granitoid intrusions.
Generally, gravity data highlights several interesting features of the fold belt. One of the most revealing is the exact extent of the belt itself. Eastern and northwestern portions of the belt are actually not exposed on the surface, but extend under large basins, the Sydney Basin to the east, and the Murray Basin to the northwest. Roughly north-northwest trending gravity highs and lows are evident throughout the exposed and unexposed areas and are consistent with the structural grain of the area. Also of interest is the noticeable decrease in the Bouguer anomaly from west to east. This points to increased crustal thickness of 45-50km towards the east in the zones of inferred island arc accretion. The gravity anomaly towards the western Lachlan is indeed almost nothing, suggesting normal continental thickness of approximately 35 km. This is the area of the Lachlan composed of thick turbidite sequences and assumed oceanic crust type basement. Unfortunately gravity data neither supports nor detracts from this supposition, but only points towards overall thicknesses.
Paleomagnetic data is limited at best for the Lachlan Orogeny. This is most likely due to the well constrained evolution of this belt through surficial geological evidence. Paleomagnetic data did provide the result that since the Silurian the eastern Lachlan fold belt has been essentially fixed with respect to the Australian continent. This is important in that it helps preclude any notions of exotic terranes underlying this orogeny, lending further credence to the idea that the extensive turbidite sequences and island arc terranes were off the coast of Gondwana before the compressional phase and merely telescoped in as a result of outside forces (Thrupp et al. 1991).
Given the rather rigid linking of the Lachlan area to eastern Gondwanaland, paleomagnetic data on the unit as a whole provides some insight as to the motions that might be responsible for the compressive forces responsible for the creation of the Lachlan Fold Belt. Paleomagnetic data suggests that during the Late Silurian-Early Devonian Gondwanaland rotated rather abruptly towards the east, creating a compressive environment for the oceanic crust to the east and initiating a subduction zone along the then passive ocean margin (Coney 1992).
Questions? Comments? Please contact fwagner@geo.arizona.edu