Transcript MT TA in region of cryptic accretion, cryptic rifting, potential

MT TA in region of cryptic accretion,
cryptic rifting, potential superplumes
and weak plumes in the cratonic core
Adam Schultz
in collaboration with
Dhananjay Ravat , David P Moecher ,
Hersh J Gilbert
Nd line and TIKL
(A) Middle Proterozoic (1.9-1.6 Ga) crustal provinces in the U.S.: bounded on the north and west by Mojave Province (Mv),
Cheyenne Belt (CB), and continuing up to the Killarney region (K in the northeast) of Ontario. SGR and EGR are Southern
Granite-Rhyolite and Eastern Granite-Rhyolite prov-inces. Dashed line (Nd) represents inferred southeastern limit as defined
by Nd isotopic data which bounds the Middle Proterozoic crustal provinces on its southeastern margin and divides SGR and
EGR provinces (Van Schmus et al., 1996).
(B) Depth integrated magnetic contrasts over the U.S. (Ravat, 2007). Orange/reds are the relative highs in the susceptibility
variation and blue/light greens depict rela-tive susceptibility lows. White continuous and dashed lines (Nd) show the Middle
Proterozoic provinces in the U.S. Mv from part (A) of the figure is excluded from the Middle Proterozoic region because
presently this region has high heat flow related to the Basin and Range province (BR) which has decreased the magnetic
thickness of the crust and hence it does not appear as a magnetic high. R- Rio Grande Rift; CB- Cheyenne Belt; K- Kentucky
magnetic anomaly; T- location of the Oklahoma-Alabama Transform (Thomas, 1991).
Nd line and TIKL
Isostatic residual gravity anomaly map showing primarily the major intracrustal density contrasts in the U.S.
and nearby marine areas (Simpson et al., 1986; Kucks, 1999). Joint inversion of seismic TA and gravity data
will allow us to differentiate better upper and lower crustal source regions in the gravity data and also isolate
mantle density contrasts. (B) Average seismic P-wave velocity variation (km/s) for a portion of North American
continental crust (Braile, 1989). The magnetization variation we observe in part Fig. 1B is also expressed subtly
in the sparse crustal seismic refraction data in the central U.S. A somewhat different picture emerges in the
central U.S. with only a small amount of additional data in the results of Chulick and Mooney (2002).
EarthScope seismic TA results in this project should define the differences across the Nd-boundary better in
both the crust and upper mantle and also define subprovinces within the Paleoproterozoic (>1.55 Ga) and
Mesoproterozoic (<1.55 Ga) Provinces.
Nd line and TIKL
We target a cryptic and enigmatic continent-scale mid-Proterozoic geochemical
boundary, known variously as the Nd-line, the TDM (age) line, and the St. Charles
Lineament, which extends approximately 2000 km from Texas to southwestern
Ontario. The Nd model age line separates magmatic rocks of the same
emplacement/eruption age (ca. 1.4 Ga) that possess different sources in the lower crust
and upper mantle, as indicated by their contrasting Nd model ages.
Rocks southeast of the boundary have TDM ages of <1.55 Ga, and those northwest of
the boundary have TDM ages >1.55 Ga. This difference requires two contrasting
mantles (Van Schmus et al., 1996).
This geochemical boundary coincides nearly perfectly with lower crustal magnetization
contrasts (Ravat, 2007) (Fig. 1B). The degree of coincidence of the Nd model age line
with the magnetic anomaly is remarkable and argues strongly for a fundamental
lithospheric structure not evident from shallow geology or geophysics.
Nd line and TIKL
Specifically, the MT TA we will target:
The structure across the Nd-model age line (1.55 Ga) accretionary boundary, and
the processes responsible for its evolution;
The origin, structure and evolution of Eastern and Southern Granite-Rhyolite
provinces (EGR and SGR), which constitute one of the world’s most widespread
occurrences of anorogenic silicic volcanic and plutonic activity at 1470±30 Ma in
EGR and 1370±30 Ma in SGR (Lidiak, 1996). How did this volume of anorogenic
magmatism occur? Since neither subduction nor hotspots alone lead to such
widespread and voluminous granitic and rhyolitic activity, we will examine and
look for clues for a new idea that the magmatism may have occurred as a result of
intersection of sub-duction/accretion and a hotspot, or a superplume;
Another poorly understood but a major potential field boundary (known variously
as TIKL or SCML+PGL – acronyms defined later), stretching NW-SE for ~400 km
along which the proto-continent may have attempted to separate after
emplacement of EGR and prior to the accretion of the Grenville Province at 1.1
Ga (Ravat, 1985).
Nd line and TIKL
Central U.S. segment of the full-spectrum magnetic anomaly map of the U.S. (Ravat et al., 2009). This new
dataset has corrected, using theoretical bases and not ad-hoc adjustments, erroneous intermediate and longwavelength magnetic anomalies from the U.S. and North American magnetic anomaly map (c. 2002). Warm
colors mark magnetic positive features and cool colors show magnetic negative features. Deep crustal magnetic
field variations seen in Fig. 1B are not apparent in this map because of the strong effect of the short-wavelength
high-amplitude near-surface magnetic sources. This project will detect seismic and geoelectric crustal and upper
mantle structures across the Nd model age boundary in the Eastern Granite-Rhyolite provinces
You “Nd” to TIKL my NELE
before I go OINK!
The black NE-SW trending feature is the approximate location of the presumed Proterozoic convergent
boundary (the Nd line), and the brown NW-SE trending feature is an approximate trend and location of the
TIKL zone. The NW-SE Illinois Basin MT FA profile extends ~380 km within a substantially longer profile
defined by suitably aligned MT transportable array stations. The red triangles and diamonds are locations of
seismic FA stations from the NELE arrays, and the OIINK arrays, respectively.