Geometry of the Neoproterozoic Continental Break-Up, and Implications for Location of Nevadan Mineral Belts

ABSTRACT
Linear alignments of ore deposits in northern Nevada define
the Battle Mountain-Eureka trend on the southwest and the Carlin
trend on the northeast. Epigenetic sedimentary rock-hosted Au
deposits dominate the belts, but porphyry systems and hot-spring
deposits are also present. The belts are proposed to reflect deepcrustal
fault systems, which have no obvious surficial fault expression
but are marked by subparallel fold hinges at angles to the
regional fold trends. The deep crustal faults cut continental basement
inboard from the North American continental margin, which is
defined by an initial 87Sr/86Sr = 0.706 isopleth and by stratigraphic
facies. The continental margin crosses Nevada with sharp bends in
inferred orientation. A generally NE strike to the margin in southwestern
and northern Nevada is separated by a segment striking NW
through western Nevada. It swings abruptly to northerly strikes in
Idaho, whereas in California it has been offset northward by
Mesozoic and Tertiary strike-slip faults. The eastern edge of
Neoproterozoic terrigenous clastic rocks deposited during rifting
broadly parallels the continental margin. Within those terrigenous
clastic rocks, the isopach defining the eastern edge of significant
thickness of clastic rocks exhibits changes in orientation that
broadly mimic the continental margin, thereby suggesting a structural
relationship. Beneath the clastic rocks in the underlying continental
basement, compositional variations indicated by Pb isotopes
and Pb versus Sr isotopic relations are recorded in Mesozoic and
Tertiary igneous rocks. Based on strongly correlated Pb and Sr isotopic
compositions in the igneous rocks, we propose that a thinned,
transitional continental crust thickens eastward until a boundary
along the Carlin trend, and the strike extensions southeastward, in
eastern Nevada. To the east, Pb and Sr isotopic ratios in plutons are
not strongly correlated and have much greater variability than in
plutons to the west. The discontinuity at the Carlin trend represents
a major crustal compositional change in the basement, which is best
interpreted to be a fault system. The mineral belts are subparallel to
the NW-trending edge of continental crust, but at high angles to the
NE-trending segments. A re-entrant in Pb isopleths corresponds to
the intersection between the NW-striking Battle Mountain-Eureka
trend and the NE-striking continental margin.
The geometric relation between the edge of the continental margin
and the orientation of the mineral belts is reconciled in the context
of a rifted margin during continental break-up in the
Neoproterozoic and Early Cambrian. In the model, the Carlin and
Battle Mountain-Eureka trends are related to crustal-scale normal
fault systems, which accommodated thinning of the continental-margin
crust. The Carlin trend represents the boundary between relatively
intact Proterozoic-Archean crust on the east and thinned, transitional
continental crust on the west. The Battle Mountain-Eureka
trend reflects a major rift fault system within the thinned continental
margin. Implicit in this model is that the currently NE-striking segments
of the continental margin likely represent paleotransform
faults along a rifted margin. In the Phanerozoic, the basement faults,
being fundamental crustal weaknesses, would influence sedimentation,
deformation, and hydrothermal fluid circulation patterns if
appropriately oriented.