* Dow and S&P 500 rise after seven weeks of gains
* Oil falls after Iran deal, energy shares pressured
* Wal-Mart names new chief executive
* Indexes up: Dow 0.1 pct, S&P 0.2 pct, Nasdaq 0.3 pct
NEW YORK, Nov 25 (Reuters) - U.S. stocks rose modestly on Monday as a deal between world powers and Iran eased tensions in the Middle East, though the agreement also weighed on oil prices and energy stocks.
Trading is expected to be light this week, with markets closed Thursday for Thanksgiving, and closing early on Friday. The light action could increase market volatility. The CBOE Volatility index is down more than 30 percent this year, and is at historically low levels, which could make it vulnerable to a spike.
U.S. crude futures fell 1.3 percent after a breakthrough nuclear deal between world powers and Iran over the weekend eased oil supply fears. Brent crude fell 1.6 percent.
Energy shares were by far the weakest on the day, dropping 0.5 percent. Among the most active names, WPX Energy Inc fell 1.6 percent to $18.64, and Newfield Exploration lost 1.4 percent to $29.60.
Despite the pressure on the energy sector, the deal was viewed as having positive benefits for the market at large.
"Less tension in the Middle East is always a positive, and any drop in gas prices will essentially act as a tax break for consumers going into the holiday shopping season," said Jeff Duncan, chief executive of Duncan Financial Management in St. Louis. "This is a real benefit for the economy."
The Dow Jones industrial average was up 23.12 points, or 0.14 percent, at 16,087.89. The Standard & Poor's 500 Index was up 2.97 points, or 0.16 percent, at 1,807.73. The Nasdaq Composite Index was up 11.81 points, or 0.30 percent, at 4,003.46.
The Dow Jones industrial average and S&P 500 have rallied in recent months, closing out their seventh straight week of gains last week. Both indexes ended at records on Friday, with the S&P closing above 1,800 for the first time.
The gains have largely come on expectations that the U.S. Federal Reserve will maintain its stimulus program through the year-end. While the program is expected to provide a floor under equity prices for as long as it continues, the scale of the rally has some investors looking for a pullback amid few catalysts. The S&P is 1.4 percent above its 14-day moving average, which could serve as support in a correction.
"It won't take much for these gains to fade given the rally we've seen, especially in such a slow week," said Duncan. "The Fed is holding steady, but a lot of investors have become complacent, buying for no real fundamental reason."
Wall Street's rally comes just ahead of December, which has been the best month for both the Dow and the S&P since 1950. If the trend continues, it would add to the S&P's 26.5 percent jump so far in 2013, the best year for the index since 1998.
In company news, Wal-Mart Stores Inc edged 0.5 percent higher to $80.20 after the retailer named Doug McMillon its new chief executive officer, effective Feb. 1, succeeding Mike Duke. McMillon is currently the president and chief executive of the Dow component's international segment.
DaVita HealthCare Partners Inc rose 6.5 percent to $60.25 after giving a 2014 enterprise operating income outlook. The stock was the biggest gainer on the S&P 500, followed by Alcoa Inc, which jumped 4.5 percent to $9.66 after Goldman Sachs upgraded the stock to "buy" from "neutral."